Master Thesis The effect of heating processes on milk whey protein denaturation and rennet coagulation properties Effekt af varmebehandling af mælk på valleprotein denaturering og koaguleringsegenskaber med chymosin _______________Marije Akkerman__________ Department of Food Science, Aarhus University Student number 20092385
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Master Thesis
The effect of heating processes on milk whey protein
denaturation and rennet coagulation properties
Effekt af varmebehandling af mælk på valleprotein denaturering og koaguleringsegenskaber med chymosin
_______________Marije Akkerman__________
Department of Food Science, Aarhus University
Student number 20092385
Preface
The present master thesis, “The effect of heating processes on milk whey protein denaturation and rennet
coagulation properties” of 60 ECTS was part of the education “Molecular Nutrition and Food Technology”
at Aarhus University and was performed in the period October 2013 to October2014. The thesis was carried
out at Arla Strategic Innovation Centre, Brabrand, and department of Food science, Aarhus University. The
project was done with supervision from Lotte Bach Larsen from Aarhus University, and Mette Christensen
from Arla Foods.
Acknowledgement
First, I would like to thank my supervisors Lotte Bach Larsen and Mette Christensen for good and help-
ful supervision. Furthermore, I would like to thank Dairy technician Kent Matzen for assistance in the
pilot plant and especially Lene Buhelt Johansen, Per N. Andersen, Betina Hansen and Hanne Sønder-
gaard for experimental guidance in the laboratories at Arla Strategic Innovation Centre, Brabrand, and
Department of Food Science, Aarhus University, and for help with data analysis.
l would also like to thank Valentin Rauh for help with data analysis and good discussions of results
throughout the project and Eva Hansen for professional revision of the final report.
At last, special thanks to my family and friends for moral support.
Aarhus University, Department of Food Science, October 22. 2014
Abstract Whey protein denaturation as a cause of heat treatment has been investigated by various authors with the
study by Dannenberg and Kessler (1988) being the most acknowledged. Skim milk with various contents of
whey proteins and caseins were heat treated using three different heating processes, namely Plate Heat
Exchanger (PHE), Tubular Heat Exchanger (THE) and Direct Steam Injection (DSI) to provide further insight
into how heat treatment affects whey protein denaturation and rennet coagulation. The samples were
subjected to heating from 80 °C to 145 °C and holding times from 2s to300s. The milk samples were ana-
lysed on liquid chromatography (LC) to analyse the degree of whey protein denaturation, while rennet co-
agulation analysis were made with ReoRox rheometry. Heat induced aggregates were analysed at 1D- and
2D gel electrophoresis and size exclusion chromatography.
Heat treatment increased the degree of whey protein denaturation as the holding time increases for all
temperatures. Heat treatment using DSI gave the smallest increase in denaturation at all temperatures,
with a degree of denaturation of 40 % of β-Lactoglobulin B (β-Lg B) heating at 145 °C, while heating using
THE and PHE showed denaturation degrees above 95 % heating at 130 °C. Temperatures below 100 °C re-
sulted in higher degree of denaturation for heat treatment using THE compared to PHE, while at 130 °C
more than 90 % of β-Lg B was denatured for both methods. A reaction order of 1.5 was found for β-Lg and
1 for α-Lactalbumin, with reaction kinetics showing similar pattern compared to previous findings. The
variations are caused by different heating systems and heating profiles which have great impact on the
whey protein denaturation.
Rennet coagulation properties were impaired as the holding time increases for all temperatures. Heating
using DSI resulted in the least impairments. Heat treatment using PHE gave better rennet coagulation
properties when heating at temperatures below 100 °C, compared to THE, while THE reached Rennet co-
agulation time(RCT) within two hours at temperatures of 130 °C which was not observed for heat treat-
ment using PHE. The differences in heating systems using PHE and THE were primarily caused by variations
in formation of heat induced aggregates. THE was found to result in a higher degree of large whey protein
and κ-casein complexes, while a higher proportion of smaller whey protein aggregates were found when
heating using PHE.
Milk with reduced whey protein content showed improved rennet coagulation properties for all heating
methods compared to skim milk. Increasing the casein level and reducing the whey protein content gave
further improvements of rennet coagulation properties. Heating these at high temperatures, however,
resulted in impairments of rennet coagulation properties which can be caused by a change in mineral solu-
bility, casein dissociation and degradation of lactose which decrease the ability for rennet cleavage.
Resume Denaturering af valleproteiner pga. varmebehandling er blevet undersøgt af forskellige forskere, hvor de
mest anerkendte undersøgelser er foretaget af Dannenberg og Kessler (1988). For at få indsigt i forskellige
varmebehandlingers påvirkning af denaturering af valleprotein og koaguleringsegenskaberne med chymo-
sin, blev skummetmælk med forskellige indhold at valleproteiner og kaseiner udsat for tre typer varmebe-
control skim milks from three different batches from three different weeks, analysed on LC-MS. The peaks
on the chromatogram were analysed by use of the MS data for each fraction. From figure 10A It is clear
that there are small variations in the protein content, especially in the casein composition, which are ob-
served in fraction 1-8. The total protein content measured on the Milkoscan also showed these variations,
respectively 3.471 %, 3.598 % and 3,663 % protein for samples from sampling week 2, 14 and 21.
Figure 10B shows analysis of total protein of control samples of the three milk types, from the same trail
week but from different batches. Small variations were found for the caseins fractions, which are seen in
fraction 1-8. The content for casein is higher in the MCI milk, while MCIc and skim milk are equal in casein
content. There are only small amounts of whey protein present in MCI and MCIc compared to skim milk.
The amount of β-Lg B, β-Lg A and α-La, shown in fraction 9-11, represent 2-4 % of the total protein content
in MCI while it was around 2 % of total protein content for MCIc. Due to this low content for whey proteins
in MCI and MCIc, the denaturation degree of whey protein was only analysed for skim milk samples.
Variations in the milk composition also gave rise to variations in enzymatic coagulation. Table 4 shows the
average relative standard deviation, minimum and maximum for the four measured parameters from 38
coagulation analyses on control skim milk from 19 different trial days. A large variation was found in the
measured properties between the control skim milk samples. A variation of RCT from 11.6 min to 19.6 min
and CFR varying from 10 Pa/min to 24 Pa/min had great influence on gel strength at 45 min and also on the
time reaching gel strength of 200 Pa. The relative change in coagulation properties for heat treated samples
according to the control sample was therefore calculated and used throughout the report, unless otherwise
it stated.
Table 4 Rennet coagulation properties of 38 control skim milk samples. Rennet coagulation time, curd firming rate, gel
strength at 45 min and time at gel strength of 200 Pa is shown.
Rennet
coagulation
time (min)
Curd firming
rate (Pa/min)
Gel strength,
45 min (Pa)
Time at 200 Pa
gel strength
(min)
Average 15.37 14.76 398.06 29.56
Relative standard
deviation (%)
11.51 23.63 21.64 13.84
Minimum 11.60 10.10 284.00 20.20
Maximum 19.60 24.40 641.30 35.90
- 33 -
4.2 Effect of indirect heating on skim milk For each skim milk sample, pH 4.5 soluble protein fractions were analysed on LC –MS in duplicates and
integration of peak areas of each whey protein fraction was used to calculate the percentage of whey pro-
tein in the heat treated samples compared to the control sample. The denaturation degree is defined as the
percentage of whey protein not appearing in pH 4.5 soluble protein analysis compared to the control sam-
ple, which is stated to have a native whey protein percentage of 100 % and thereby no denaturation.
Heat treatment using THE and PHE were compared according to temperature and holding time to investi-
gate differences in denaturation degree of whey proteins and coagulation properties between the two
heating methods. Figure 11 shows the denaturation degree of β-Lg B and α-La in skim milk heat treated
using PHE and THE. The denaturation pattern of β-Lg A is shown in Appendix 1 which shows similar results
as denaturation of β-Lg B. The denaturation of β-Lg B is shown in figure 11A, for heating temperatures
Figure 11. Denaturation degrees of β-Lg B and α-La for skim milk heated at PHE and THE.
A: Denaturation of β-Lg B heated at temperatures below 100 °C. B: Denaturation of β-Lg B heated at temperatures above 100 °C.
C: Denaturation of α-La heated at temperatures below 100 °C D: Denaturation of α-La heated at temperatures above 100 °C
Different letters indicate significant differences were found between heating methods for each given temperature (p<0.05).
- 34 -
below 100 °C and in figure11B for heating temperatures above 100 °C. For all temperatures, an increase in
denaturation degree was observed as the holding time increases. However, the greatest effect was seen by
increase in temperature, especially when temperatures above 85 °C are used. A heating temperature of 85
°C resulted in denaturation degree of 25 % of β-Lg B with a holding time of 5 s, while heating at 130 °C and
140 °C resulted in denaturation degrees above 90 % even at very low holding times.
This was applicable for both heating methods. Comparing the denaturation degrees for heating tempera-
tures at 80°C and 95 °C using PHE and THE from figure 11A, it is clear that THE results in significant higher
denaturation degree than PHE for holding times above 10 s for 80 °C (p<0.04) and for holding times below
120 s for 95 °C (p< 0.009) as both methods have denaturation degrees above 90 % at holding times of 120 s
and 300 s for heating temperature of 95 °C. Figure 11B shows heating temperatures above 115 °C, which
results in denaturation degrees of β-Lg B above 90 % for both heating methods.
The denaturation of α-La is shown in figure 11C and figure 11D. It is observed that the denaturation of α-La
is also affected by temperature and holding time. The degree of denaturation of α-La is however lower than
the degree of denaturation of β-Lg B, which is shown in figure11 A and figure 11B. Figure 11C shows the
denaturation degree of α-La for temperatures below 100 °C. From this, it is observed that the denaturation
degree does not exceed 50 % at any of the investigated holding times for both PHE and THE. No significant
difference in denaturation of α-La was found when heating at 80 °C between methods. Heating tempera-
ture at 95 °C results in significant higher denaturation degree was found for heat treatment using THE,
compared to PHE, with a holding time of 60 s (p<0.002). Figure 11D shows the denaturation degree of α-La
for heating at temperatures above 100 °C. Heating at 130 °C and 140 °C gave the highest denaturation de-
gree, but no denaturation degrees above 90 % were found for any of the investigated temperature and
holding time combinations. Heating temperatures of 130 °C results in significant higher denaturation de-
gree for heat treatment using THE compared to PHE for all holding times (p<0.001).
Rennet coagulation analysis of skim milk heat treated using PHE and THE are shown in figure 12. Figure 12A
and figure 12B shows the relative RCT of heat treated skim milk samples, heat treated below and above
100 °C, respectively. From figure 12A, it is observed that heat treatment at temperatures below 85 C only
have slight increase in coagulation time at all holding times. Heating temperatures of 95 °C had a significant
increase in relative RCT, even at the short holding times for both heating methods (p< 0.006). Comparing
the two methods, there is a tendency towards PHE having less increase in relative RCT for temperatures of
80 °C and 95 °C, but there is no significantly difference between the two methods at these temperatures
for all holding time (p = [0.52-0.86]). Heating at temperatures above 115 °C shown in figure 12B, resulted in
very long RCT and samples heated at 140 °C using PHE did not reach RCT within 2 hours for any of the in-
- 35 -
vestigated holding times. Samples heated at 130 °C resulted in large increase in relative RCT and there is a
significant difference between the methods for all holding times (p <0.03]). THE had less increase in relative
RCT compared to PHE and RCT was detected within two hours with use of holding times of 30 and 60 s,
which was not observed for heat treatment using PHE.
The relative CFR for skim milk samples heated at PHE and THE is shown in figure 12C and figure 12D. The
relative CFR decreases with increasing temperature and holding time. Comparing relative CFR for the two
heating methods at temperatures below 100 °C from figure 12C, there is a significantly lower CFR for THE
heated at 80 °C with holding times above 120 s (p<0.025). This is also observed for heating at 95 °C with
holding time less than 60 s (p<0.048). Holding times above 60 s results in very low relative CFR for both
methods, with a CFR corresponding to less than 5% of control samples. Comparing heating at 80 °C and
85 °C at PHE, small differences in relative RCT was found in figure 12A, but when comparing CFR, the de-
Figure 12. The relative Rennet coagulation time and relative curd firming rate for skim milk samples heated at PHE and
THE.
A: Relative RCT of skim milk heated at PHE and THE at temperatures below 100 °C. B: Relative RCT of skim milk heated at
PHE and THE at temperatures above 100 °C. Graphical lines continuing out of the visualized graph indicates that the RCT is
not reached within two hours of measurements. C: Relative CFR for skim milk heated at PHE and THE for temperatures be-
low 100 °C. D: Relative CFR for skim milk heated at PHE and THE for temperatures above 100 °C. Graphical lines going out of
the visual range indicates that no CFR was detected within two hours of measurement.
Different letters indicate significant differences were found between heating methods for each given temperature, at a given
holding time (p <0.05).
- 36 -
crease in relative CFR is significantly lower for 85 °C compared to 80 °C, at holding times of 30 s (p=0.002)
and 60 s (p=0.003).
Heating at temperatures above 100 °C is shown in figure 12D had strong pronounced decrease in relative
CFR, with only low CFR detected, even though the RCT was reached within two hours of measurement, as
observed in figure 12B. The UHT treated samples did not reach RCT within two hours, and therefore no
curd firming rate was detected. PHE had significant lower CFR when heating at 130 °C for all holding times
(p<0.03), compared to THE.
4.3 Effect of indirect and direct heating systems on heat treatment of skim milk To investigate how different heating method affects the milk properties , skim milk samples heated at DSI,
PHE and THE were compared. Milk samples heated using DSI were only heated with one holding time,
namely 4s. Milk samples heated indirectly used a holding time of 5 s. When comparing the three methods,
the variation in holding time should be kept in mind.
Figure 13A shows the denaturation degree of β-Lg B in skim milk heated at DSI, PHE and THE at various
temperatures. Heating at 105 °C, no denaturation was found when heating at DSI while heating at PHE was
significantly higher (p<0.0001), which has a denaturation degree of 70 %. For indirect heating, a denatura-
tion degree above 90 % was observed for heating temperatures of 115 °C or more. There is significant dif-
ference between the two indirect heating methods and DSI (p<0.002) with DSI having lowest denaturation
degree for all comparable temperatures. The denaturation of α-La is seen in figure 13B. No denaturation
Figure 13. Denaturation degrees of whey protein in skim milk heated with DSI for 4 s and skim milk heated with PHE and THE
for 5 s. A: denaturation degree of β-Lg B. B: denaturation degree of α-La. Significant differences were found between heating
methods for each given temperature. Different letters indicate significant differences were found between heating methods for
each given temperature (p <0.05).
- 37 -
was detected for heating at 105 °C using DSI and a low increase in denaturation is observed as temperature
increases. The denaturation of α-La is significantly lower for DSI compared to the indirect heating at all
temperatures (p<0.0001). These variations in denaturation degrees between indirect heating and DSI are
large, which makes it reasonable to say these large differences would also be present if the indirect heating
samples had a holding time of 4 s.
The rennet coagulation properties were compared for the three heating methods. Figure 14A shows the
relative RCT for skim milk samples using PHE and THE heated for 5 s and DSI heated for 4 s. There is a clear
significant difference between PHE and DSI for all temperatures (p<0.002), with DSI having the lowest in-
crease in RCT. Skim milk was heated at 140 °C using PHE and this did not reach RCT within two hours. DSI
samples heated at 145 °C reached RCT within two hours and the relative increase in RCT was lower than
samples heated at 130 °C using PHE. THE and DSI can only be compared at heating of 130 °C but here is the
difference also significant.
In figure 14B, the relative CFR is shown. Both PHE and THE have significantly lower CFR for all temperatures
compared to DSI (p<0.024). The DSI sample heated at 145 °C reached RCT within two hours, but no CFR was
detected. The differences in RCT and CFR between the indirect and direct methods are large, which makes
it reasonable to say that there would still be a significant difference between the methods if they had the
same holding time.
Figure 14. Relative RCT and CFR of skim milk heated with PHE for 5 s and DSI for 4 s.
A: relative RCT for skim milk heated at DSI, PHE and THE. PHE heated 140 °C is not within two hours, which is indicted with a bar
going out of scaled area. B: relative CFR for skim milk heated at DSI, PHE and THE. Samples with negative CFR indicate that RCT was
not detected within two hours of measurement. Different letters indicate significant differences were found between heating methods for
each given temperature (p <0.05).
- 38 -
4.4 Effect of whey protein denaturation on rennet coagulation
The relative RCT and denaturation degree of β-Lg B of heated skim milk was compared to investigate the
correlations between degree of denaturation and relative RCT. Figure15 shows the relative RCT as a func-
tion of denaturation degree of β-Lg B for skim milk samples heat treated using PHE, THE and DSI.
It is observed that the denaturation degree of β-Lg B in some extent explain the increase in relative RCT.
Denaturation degrees of 50 % or less gives only small changes in relative RCT for the two indirect heating
methods, while it is seen that THE has lower relative RCT as the denaturation degrees of β-Lg B exceeds
70 %. Heat treatment using PHE at temperatures of 130 °C and 140 °C did not reach RCT within two hours
of measurement and these samples had denaturation degrees of 92 % or more. These are imagined in
figure 15 by exceeding relative RCT of 800 %. Heat treatment using THE can be described as a two phased
linear correlation with a break at denaturation degrees around 95 %, while heat treatment using PHE can
be explained exponentially.
Heat treatment using DSI is also shown in figure 15. It is seen that a denaturation degree for β-Lg B around
40 % resulted in significantly longer relative RCT compared to samples heated indirectly with same denatu-
ration degree (p<0.001). From section 4.3, it is clear that this denaturation degree is observed at low tem-
peratures for heat treatment using PHE and THE, while DSI was heated at 145 °C to obtain same denatura-
tion degree. This indicates that it is not only the denaturation degree of whey proteins that has an impact
Figure 15. The denaturation degree of β-Lg B shown as a function of relative RCT of skim milk heated at DSI, PHE and THE.
Data points exceeding relative RCT of 800 % did not reach RCT within two hours of measurement.
- 39 -
on the RCT, but that the temperature has a great impact, as it gives rise to other chemical changes in the
milk which has an impact on coagulation.
4.5 Effect of heat treatment of MCI milk samples on rennet coagulation The rennet coagulation properties of MCI milk samples heat treated using PHE and THE were analysed and
compared to investigate the effect of heating method. Only one trial day was used to collect MCI samples
heated using THE and thereby only temperatures with holding times of 5 and 10 s, and for 80 °C also one
sample with a holding time of 120 s, were collected. The data points for heat treatment using THE are
therefore an average of two rennet coagulation analysis while milk heated at PHE is an average of four ren-
net coagulation analysis from two trial days. The relative RCT and relative CFR for MCI samples heat treated
using PHE and THE are shown in figure 16.
Figure 16. RCT and relative CFR for MCI samples heated at PHE and THE. A: Relative RCT of MCI samples heated at PHE and THE
at temperatures below 100 °C. B: Relative RCT of MCI samples heated at PHE and THE at temperatures above 100 °C. C: Relative
CFR for MCI samples heated at PHE and THE below 100 °C. D: Relative CFR for MCI samples heated at PHE and THE above 100 °C.
Graphical lines going out of the visual range indicates that no CFR was detected within two hours of measurement.
Different letters indicate significant differences were found between heating methods for each given temperature, at a given
holding time (p <0.05).
- 40 -
Figure 16A shows the relative RCT for MCI samples heated at temperatures below 100 °C. The relative RCT
follows a parable formation for heat treatment using PHE at temperatures 100 °C. The relative RCT in-
creases for holding times up to 60s and then decreases when a holding time of 120 s is used. Here, the rela-
tive RCT was less than RCT of control samples. As not all holding times were measured for heat treatment
using THE, it is not possible to say if heat treatment using THE follows the same pattern. The relative RCT is
significantly larger for heating at THE than PHE when comparing each holding time between the methods
(p<0.03). MCI samples heat treated at temperatures above 100 °C are shown in figure 16B. Increases in
relative RCT was observed for all temperatures and holding time combination, but all measured samples
reached RCT within two hours of measurement. Heating at PHE was observed to give significant lower rela-
tive RCT compared to heating at THE (p<0.04).
Figure 16C and figure 16D shows the relative CFR for MCI samples heat treated at temperatures below and
above 100 °C, respectively. A decrease in relative CFR is observed for all MCI samples heat treated using
PHE at temperatures below 115 °C with a holding time of 5s, but the relative CFR then increases as the
holding time is increased from 5s up to 30 s. Holding times above 60s resulted in a decrease in the relative
CFR. Heat treatment using PHE has higher relative CFR compared to the corresponding control sample.
Comparing the two heating methods, there is significant lower relative CFR for heat treatment using THE
compared to PHE at temperatures below 100 °C (p<0.03). Heating at temperatures of 130 °C and 140 °C
resulted in a fast decrease in relative CFR for heat treatment using PHE, but a CFR was detected for all MCI
samples measured. No CFR was detected for heat treatment at 130 °C using THE.
Table 5. Relative RCT and relative CFR for MCI samples heated at PHE, 5s and THE, 5 s and DSI, 4s. PHE samples are heated at
140 °C and not at 145 °C ad DSI. *: No CFR was detected within two hours of measurement. Different letters indicate significant
differences were found between heating methods for each given temperature (p <0.05).
Relative RCT (%) Relative CFR (%)
Temperature (°C) DSI, 4 s PHE, 5 s THE, 5 s DSI, 4 s PHE, 5 s THE, 5 s
105 103.23 108.24
115 111.69 120.76 105.88a 78.60c
130 114.11a 138.87b 168.44c 93.82a 44.82b 0*
140 168.82 32.73
145 124.19 64.85
- 41 -
The two indirect heating methods with a holding time of 5 s were compared with direct heat treatment
using DSI with a holding time of 4s, which is shown in table 5.
It is observed that the relative RCT increases slightly as the temperature increases for heat treatment using
DSI. Only two temperatures could be compared directly between the heating methods, namely tempera-
tures of 115 °C and 130 °C using PHE and 130 °C using THE.
There is a significantly difference between DSI and the two indirect methods for heat treatments at 130 °C
(p<0.002). MCI samples were heated at 140 °C using PHE while samples using DSI were heated at 145 °C.
Comparing the relative RCT of these, there are significant differences between PHE at 140 °C and DSI at
145 °C (p<0,003) and it is therefore reasonable to state that a MCI sample heat treated at 145 °C using PHE
would have an even longer relative RCT and thereby also be significant different from MCI samples heat
treated using DSI.
The relative CFR for MCI heated using DSI decreased slightly as temperature increased. Heating at 145 °C
had a relative CFR of 64.85 % while heating at 130 °C using THE, no CFR was detected within two hours of
measurement. There is significant larger decrease in CFR for the indirect methods compared to DSI for all
comparable temperatures (p<0.02).
4.6 Effect of milk type on rennet coagulation Control milk samples from the three milk types were compared to investigate the differences in coagulation
in relation to milk type. The three milk samples are from the same trial week, but from different milk
batches, to avoid seasonal variation. From figure 17A, it is observed that the RCT for the three milk types
were similar and no significant variation was found, even though skim milk as a tendency to have a longer
RCT. Figure 17B shows the curd firming rate. From this, is it clear that MCI, with a casein content of 3.5 %,
had faster gel formation compared to skim milk and MCIc which have casein contents of 3.05 % (p<0.04).
The CFR of MCIc milk is larger than skim milk but this difference was not found to be significant.
These results indicate that the removal of whey proteins from low pasteurized skim milk gives a slight
faster gel formation, while removal of whey proteins and increasing the casein content increases the gel
formation significantly. This is important to have in mind, as the relative difference of control samples can
be equal for the milk types, but it does not consider MCI having faster curd firming rate.
- 42 -
The effects of heat treatment on the three milk types at PHE were analysed. Heat treatment of MCIc milk
samples was only performed using PHE and each temperature and holding time combination was only
measured once due to lack of time. These data points are therefore only the average of two ReoRox meas-
urements while skim milk and MCI samples are an average of four ReoRox measurements from two trial
days. Figure 17A and figure 17B shows the relative RCT for skim milk, MCI and MCIc samples heat treated
using PHE. Significant variations were observed between the three milk types heated at 80 °C (p<0.001).
The MCI and MCIc heated at temperatures of 115 °C and 130 °C reached RCT within two hours, while skim
milk samples with a holding time exceeding 30 s did not reach RCT within two hours for the same tempera-
tures. No significant difference was found between MCI and MCIc but skim milk increased significantly in
relative RCT compared to MCI and MCIc heat treated at temperatures of 115 °C and 130 °C (p<0.03).
The relative CFR of heat treatment of the three milk types are shown in figure 18C and figure 18D. MCIc
heated at 80 °C resulted in large increase in CFR with a holding time of 5s and afterwards decreased until a
holding time of 30 s from where it kept fairly constant. MCI and skim milk decreased in CFR with holding
times below 10 s while a slight increase was observed for longer holding times. Overall, significant differ-
ences in relative CFR between all three milk types was found (p<0.03).
Heat treatments at temperatures of 130 °C resulted in significant difference between the three milk types
(p<0.02). MCI had the least decrease in relative CFR which was significantly lower than for MCIc, even
though CFR was detected for both milk types within two hours of measurement. CFR was only observed for
heating at 130 °C for 5 s for skim milk and this is significantly different from MCI and MCIc (p<0,001). Sig-
Figure 17. RCT and CFR of control milk samples for the three milk types, from the same trial week.
A: RCT of skim milk, MCI and MCIc. Skim milk has the greatest RCT, but no significant differences between the three milk types.
B: CFR of skim milk, MCI and MCIc. MCI has the CFR rate, but no significant differences between skim milk and MCIc. Different
letters indicate significantly different values (p<0.05)
- 43 -
nificant difference between the milk types heated at 115 °C was found, with skim milk having the most
intense decrease in CFR and MCI having the least decrease (p<0,01).
4.6.1 Formation of protein network
The formation of protein network induced by rennet was investigated for six samples – three skim milk and
three MCI samples. The skim milk samples are from same batch, which is also valid for the MCI samples.
Figure 19 shows images of the protein structure of the six samples captured with CLSM. FTIC is bound to
the proteins and this gives the green emission of the proteins. The heat treated samples were all heated
using PHE with a holding time of 10 s. For all MCI samples and for control skim milk, it is seen that a strong
and compact protein network was formed, which indicates that the majority of the caseins are bound in the
network. This can be seen by the clear separation of the proteins coloured green and the dark background.
The dark background indicates that the most protein detected on the images, are bound in protein net-
work.
Figure 18. Relative rennet coagulation time and curd firming rate for skim milk, MCI and MCIc samples heated at PHE.
A: Relative RCT for the three milk types heated at 80°C. B: Relative RCT for the three milk types heated at 115 °C and 130 °C.
Graphical lines continuing out of the visualized graph indicates that the RCT is not reached within two hours of measurements
C. Relative CFR for the three milk samples heated at 80°C. D: Relative CFR for the three milk samples heated at 115 °C and
130 °C. Graphical lines going out of the visual range indicates that no CFR was detected within two hours of measurement.
Different letters indicate significant differences were found between the milk types for each given temperature (p <0.05).
- 44 -
The heat treated MCI samples shown in figure 19B and figure 19C form dense networks compared with the
control sample, shown in figure 19A, having very compact and strong network. This is observed, as strong
networks are compact and little space is found in between the protein networks and less protein not bound
in the network. The skim milk sample heated at 80 °C for 10 s shown in figure 19E, has no large contrast
between the green colour between the network and background. This indicates that there are proteins not
bound in the network. Comparing the sample heated treated at a temperature of 80 °C with the corre-
sponding control sample, shown in figure 19D, t is clear that the control sample has more compact net-
work. The skim milk sample heated at 130 °C, shown in figure 19F, contains many small networks, but the
formation of one large protein gel has not occurred yet. This can also be seen as the solution surrounding
the small networks contains large amounts of proteins due to the background is quite green. These results
are consistent with the results shown in figure 17 and figure 18. An increase in heating temperature gives
decrease in RCT and CFR, which therefore gives prolonged protein network formation. This is more pro-
nounced in skim milk compared to MCI.
Figure 19. CLSM images of the protein network in three skim milk samples and three MCI samples heated at PHE. The
proteins are colored green. A: MCI control sample. B: MCI sample heated at 80 °C, 10 s. C: MCI sample heat at 130 °C, 10 s.
D: Skim milk control sample. E: skim milk sample heated 80 °C, 10 s. F: skim milk sample heated 130 °C, 10 s.
- 45 -
4.7 Heat induced protein aggregation Protein aggregation was investigated with three different analytical techniques, namely 1DGE and 2DGE
and SEC. The main focus is on the skim milk samples, investigating the aggregate size and composition,
mainly for indirect heating.
4.7.1 1-DGE
Two 1-D gels with four skim milk samples and two MCI samples, analysed reduced and non-reduced,
are shown in figure 20. The milk samples were analysed under reducing conditions to disrupt the S-S
bridges within the proteins structure and between various proteins. Milk samples analysed under
non-reducing conditions have intact S-S bridges and differences between the reduced and non-
reduced analysis can be used to identify protein bands containing S-S bridges in the structure. The
protein bands were identified according to (Souza et al., 2000).
The whey protein bands of heat treated skim milk, which are observed at the low molecular mass
range of the gels in figure 20 are of low intensity, both under reduced and non-reduced conditions.
These bands are less intensive than the whey protein bands for the control skim milk. Decreases in
intensity of these bands were most pronounced in heat treated samples using PHE while heat treat-
ment using DSI having the least reduction. This is consistent with results presented in section 4.2 and
section 4.3. No protein bands were found in the top of the gel, which indicates that there are no
large protein aggregates present in the reduced samples.
For all non-reduced fractions, there is a clear band at the top of the gels above the marker band of
300 kDa. This indicates there are protein complexes that do not migrate on the gel. These are various
large aggregates bound together with disulphide bonds as these do not appear in the reduced sam-
ples. The non-reduced samples of skim milk heated using the three different methods, shown in
figure 20B, indicate that there are more complexes reaching the gel in the sample heat treated using
DSI compared to the indirect methods, while there is a tendency toward more protein complexes
that are not migrating at the gel for the indirect methods. The band for κ-CN is not very pronounced
in the non-reduced samples for all milk samples but becomes clearer in the reduced samples for all
six samples. The same tendency is seen for β-Lg even though the amount of β-Lg is low in the heat
treated samples due to denaturation. This indicates that κ-CN and β-Lg is present in the large com-
plexes. For the heat treatment of the MCI sample, shown in figure 20A, the complexes not migrating
on the gel in the non-reduced sample may also contain other caseins as the fraction of whey protein
is very small and it seems like the casein bands become slightly more intense.
- 46 -
Figure 20. 1DGE of six milk samples in reduced and non-reduced form, visualized by colloid Coomassie Brilliant Blue
G-250 staining. The molar masses of the marker used are given, as well as the amount of protein loaded in each well. The
most intense bands are identified. A: three samples; control skim milk, control MCI and MCI heated at 130 °C, 5 s using
PHE are shown in a reduced and non-reduced form. B: three skim milk samples; 130 °C, 5 s at PHE, 130 °C, 5 s using THE
and 130 °C, 4 s using DSI are shown in a reduced and non-reduced form.
- 47 -
4.7.2 2DGE
2DGE was performed on same samples as for 1D gel electrophoresis, section 4.7.1. The protein sport on the
gel were identified according to Jensen et al. (2012b) and Larsen et al, (2010). Four 2D gels are shown for in
figure 21. These gels contain skim milk samples heat treated using PHE and THE, and analysed under reduc-
ing in both dimensions and under non-reducing conditions in both dimensions. The 2D gels that are non-
reduced in both dimensions were less clear in the separation between protein bands compared to the re-
cued 2D gels. It is observed for both milk samples that the spots of β-Lg and α-La were moved towards a
higher pI in the non-reduced samples compared to the reduced samples. This can be due to refolding of the
denatured protein into a non-native structure and thereby changing the pI. β-casein was not affected by
electrophoresis method, while some αs1-casein multimers (sport 6) and αs2-casein dimmers (spot 7) were
observed on the non-reduced 2D gels. The 2D gels of the remaining milk samples are shown in Appendix 2.
For the skim milk sample heat treated using PHE, figure 21A and figure 21B, there is a clear difference in
Figure 21. 2D gel electrophoresis on skim milk samples heated with PHE and THE run with and without DTE in both dimen-
sions. A: Reduced skim milk sample heated with PHE at 130 °C for 5 s. B: Non-reduced skim milk sample heated with PHE at
130 °C for 5 s. C: Reduced skim milk sample heated with THE at 130 °C for 5 s. D: Non-reduced skim milk sample heated with
THE at 130 °C for 5 s. 1: genetic variants of κ-CN. 2:β-Lg. 3: α-La. 4: β-CN. 5: αs1-CN. 6: αs1-CN aggregates. 7: αs2-CN dimers.
- 48 -
intensity of the whey protein bands and also κ-casein bands at the non-reduced and reduced 2D gels. The
κ-casein bands are not visible on the non-reduced 2D gel and the whey protein spots are weak. This indi-
cates that the whey proteins and κ-casein are bound in various complexes which are not seen on the non-
reduced 2D gels, while these complexes are broken down in reduced 2DGE analysis where each protein
fragment is seen individually on the gel. Furthermore can the low intensity of κ-casein be caused by
κ-casein fund as multimers in milk.
The skim milk sample heat treated using THE, shown in figure 21C and figure 21D, shows the same ten-
dency as the skim milk samples heated using PHE. One major difference though, is that THE skim milk sam-
ple has visible κ-casein bands in the non-reduced sample but the same intensity pattern for whey protein
bands. This indicates that the complexes made of THE skim milk contain less κ-casein but the same amount
of whey protein, as the protein content is the two samples are similar, shown in table 3, and it is therefore
reasonable to believe that β-Lg forms aggregates with other β-Lg proteins.
4.7.3 Size exclusion chromatography
Size exclusion chromatography was performed on milk samples in reduced and non-reduced form to inves-
tigate the amount of aggregates formed in each sample. Figure 22 shows the chromatograms of the re-
duced and non-reduced samples. The reduced samples in figure 22A results in peaks observed at retention
time 7-9 min for all skim milk samples. The MCI samples have very low intensity of this peak indicating that
there are some aggregates in this area containing whey proteins which are not S-S bound.
For all samples under non-reduced conditions samples shown in figure 22B, large peaks are observed with
retention times of 4.5-7 min. This indicates a high amount of disulphide bound aggregates which contain
caseins are present in this area. Compared to the reduced samples, no peaks at retention time 12-14 min
can be observed in the non-reduced samples.
The control skim milk has a higher overall absorbance in the non-reduced form, figure 22B, compared to
the heat treated samples which could indicate that there are even larger aggregates present in the heat
treated milk which cannot be detected by the used column.
Comparing the three heat treated skim milk samples, it is seen that THE has larger amount of the large ag-
gregates and also a lower amount of the intermediate size proteins for reducing and non-reducing condi-
tions while the amount of small peptides are equal for all heating methods. DSI has more intermediate
aggregates, retention time 7-9 min, compared heat treatment using PHE and THE.
- 49 -
4.7.3.1 Identification of aggregates
For two of the samples heat treated using PHE and THE at 130 °C for 5 s, the large aggregates were col-
lected and analysed to identify the content of the aggregates containing disulphide bonds.
Figure 23 shows the chromatogram from SEC analysis. The total protein content measured on Milkoscan
was 3.64% and 3.66% for the heat treated sample using THE and PHE, respectively.
The chromatograms for the two samples show similar pattern, but differences in intensities. Heat treat-
ment using PHE had higher absorbance and more protein is thereby detected by the use of the specific
column compared to heat treatment using THE. This could indicate that there is a higher content of large
protein complexes that cannot be detected with the used column for heat treatment using THE, as the total
protein content in the two samples are similar. The used column separates molecules in the weight range
from 5kDa to 1200 kDa. From the results obtained from 1DGE shown in section 4.7.1, could it be concluded
that protein aggregates above 300 kDa were present in heat treated milk. Results presented in this section
show that protein aggregates exceeding a molecular mass 1200 kDa are present.
Figure 22. Chromatograms for milk samples analyzed with SEC under reduced and non-reduced conditions. A: Reduced
milk samples analyzed on SEC. B: non-reduced milk samples analyzed on SEC.
- 50 -
Figure 24. Total protein analysis of skim milk fractions collected at SEC. Peak 1-3: κ-casein, peak 4: αs2-
Figure 23. Chromatograms for skim milk samples analysed with SEC in non-reduced
conditions. The peak area from 8-12 min was collected into four fragments for
further analysis.
- 51 -
The protein aggregates appeared on the chromatogram with RT 8-12 min were analysed on LC-MC.
Figure 24 shows the chromatogram of UV detection from LC analysis of the four fractions collected for both
heat treatments. The four fractions all show similar pattern but are of different intensities. This is caused by
the amount of protein in each fragment is not equal due to differences in intensity of the SEC analysis of
which the four fragments were collected from. Comparing the chromatograms of the two heating methods,
there were found variations in the protein content in the aggregates. The aggregates formed by heat
treatment using PHE, contained large amounts of glycosylated κ-casein (peak 1-3), β-casein (peak 6) and
whey proteins (peak 7-9). The aggregates formed by heat treatment using THE, contained large amount of
αs1-casein (peak 5) while almost no κ-casein and whey proteins are present compared to heating using PHE.
- 52 -
4.8 Kinetics of denaturation of whey proteins
The effect of heating temperatures and holding times were investigated to determine the rate of denatura-
tion of β-Lg and α-La. The order of reaction used for β-Lg was 1.5 and for α-La it was 1 according to previ-
ous studies (Dannenberg and Kessler, 1988; Kessler and Beyer, 1991; Oldfield et al., 1998a; Zúñiga et al.,
2010), using the rate equations
, for n = 1.5
and
, for n = 1
Figure 25. Denaturation degree of β-Lg B and α-la for skim milk samples heat treated using PHE at temperatures from
80 °C to 140°C at various holding times. For each temperature, a linear regression is fitted and these data are shown in
table 5. A: denaturation of β-Lg with a reaction order of 1.5. B: Denaturation of α-La with a reaction order of 1.
- 53 -
Figure 25A shows graphical representation of the denaturation of β-Lg B and figure 25B shows denatura-
tion of α-La for skim milk samples heated at PHE. For each temperature, the best fitted straight line was
plotted. This line indicates the kinetic fit which is used to calculate the rate constants shown in table 6. β-Lg
A follows same pattern as β-Lg B and is therefore not shown graphically.
The rate constant k was calculated from the slope of regression from the best fitted straight line for each
temperature. The slope is , for a reaction order of 1.5 and –k, for a reaction order of 1.
Table 6. Rate ate constant k and correlation coefficient of reaction kinetics on denaturation of β-Lg B, β-Lg A and
α-La in skim milk heated at PHE and THE. The values for β-Lg B and α-La heated at PHE are obtained from figure
25. Values for β-Lg A heated with PHE and all values heated at THE are obtained from graphical analysis, which are
not shown.
Tempera-ture (°C)
k 103 (S-1
) R2 k 103
(S-1)
R2
PHE THE β-Lg B n = 1.5 80 1.64 0.90 3.00 0.94
85 9.39 0.88
95 47.08 0.99 89.46 0.93 105 117.18 0.94
115 160.52 0.93
130 213.96 0.86 226.06 0.80
140 396.08 0.90
β-Lg A
n = 1.5 80 1.29 0.85 2.23 0.95
85 5.28 0.99
95 33.70 0.99 55.07 0.93
105 89.97 0,95
115 162.93 0.91
130 251.16 0.91 304.62 0.90
140 454.68 0.94
α-la
n = 1 80 0.77 0.84 0.71 0.90
85 1.92 0.83
95 2.40 0.93 6.82 0.83
105 7.87 0.92
115 9.72 0.94
130 27.75 0.84 28.13 0.89
140 42.08 0.85
- 54 -
Table shows the achieved data from linear regression and the correlation coefficient for each temperature
of β-Lg B and α-La from figure 25The values for β-Lg A heated at PHE and all values for heat treatment using
THE were obtained in similar way. The reaction order of 1.5 for β-Lg B and β-Lg A and reaction order of 1
for α-La obtained good correlation for all measured temperatures, with R2 from 0.80-0.99. Comparing the
reaction constants for heating at PHE and THE, it is observed that the reaction constant is larger for β-Lg B
and β-Lg A when heating using THE which indicates that the denaturation of β-Lg B and β-Lg A is faster
when heating using THE.
The obtained rate constants were plotted against the reciprocal of the absolute temperature. Figure 26
shows the effect of temperature on the rate constant of denaturation of β-Lg B, β-Lg A and α-La for skim
milk samples heated at PHE.
For β-Lg B and β-Lg A heated at PHE, it is possible to make linear regression in the temperature range from
80 °C to 95 °C and again from 95 °C to 140 °C. For α-La, this break is found at 85 °C. From the linear regres-
sion of each temperature range, the activation energy is calculated from the Arrhenius equation
which is shown in table 7.
Figure 26. The effect of temperature on rate constant for denaturation of β-Lg B, β-Lg A and α-La for skim milk heated at PHE.
Linear regressions are made by fitting rate constants for heating at PHE.
- 55 -
The activation energy for β-Lg at heating temperatures below 95 °C obtained activation energies of 250
,
while heating temperatures above 95 °C obtained activation energies of 55-72
.similar shift in activation
energy is observed for α-La, in the temperature ranges 80-85 °C and 85-140 °C.
As can be seen in figure 26, the measured rate constants are fitted to a linear regression, which is stated in
table 7. The calculation of reaction kinetics for α-La at the low temperature range, only two rate constants
were available and thereby could the uncertainty of the fit not be given. It was not possible to calculate the
reaction kinetics for heat treatment using THE as only three heating temperatures were used and the ob-
tained kinetic result would be very uncertain.
Table 7. Reaction kinetic for denaturation of β-Lg B, β-Lg A and α-La for skim milk heated at PHE. The values
are calculated from data obtained from figure 26.
Order (n) Temperature
range °C
ln(k0) Ea
R2
β-Lg B 1.,5 80-95 73.25 247.66 0.96
95-110 14.42 56.14 0.94
β-Lg A 1.,5 80-95 72.05 245.07 0.99
95-140 19.03 72.19 0.96
α-La 1 80-85 58.11 203.57 1
85-140 18.01 77.15 0.97
- 56 -
5 Discussion In this study, three different types of heat treatments were performed on milk with various whey protein
and casein content, and these were examined for coagulation properties, whey protein denaturation and
formation of heat induced aggregates.
The denaturation degree of whey proteins in skim milk increased with increasing holding time for all inves-
tigated temperatures. β-Lg B showed a higher degree of denaturation compared to β-Lg A, while α-La had
the least denaturation at all measured temperature and holding time combinations and heating methods.
This is consistent with theory, as β-Lg A has a slightly lower denaturation temperature, but is less reactive
due to a higher negative charge compared to β-Lg (O’Connell and Fox, 2011). α-La has a tendency to reform
into native structure at low temperatures and it higher temperatures are required to form aggregates with
other proteins which can explain the lower degree of denaturation observed for α-La.
The reaction kinetics of denaturation of whey proteins was extensively investigated by Dannenberg and
Kessler (1988) and their results are widely accepted and commonly used as reference for the effect of heat
treatment on whey protein denaturation. Since the publication of their study, various different heating
systems and other analytical methods have been used to analyse the denaturation degrees of whey pro-
tein.
Figure 27. Effect of heat treatment with PHE on the denaturation of β-Lg B in skim milk. The lines represent the calcu-lated rate of denaturation measured by Dannenberg and Kessler (1988) and each point represents the measured dena-turation degrees in the present study.
- 57 -
From an industrial perspective, it is of great interest to know how the denaturation degree of whey pro-
teins is affected in pilot scale heating systems. This allows mimicking the industrial used heating systems
and thereby enables the transfer of knowledge achieved in the laboratory to the production sites.
Figure 27 shows a comparison of obtained denaturation degrees of β-Lg B of skim milk heated at PHE from
the present study and the denaturation degrees observed by Dannenberg and Kessler (1988). From
figure 27 it observed that the denaturation degrees obtained in the present study for heating temperatures
below 90 °C show similar tendencies as the results by Dannenberg and Kessler (1988). Heating at tempera-
tures of 90 °C and above, the degree of β-Lg B differs substantially. Heating at a temperature of 140 °C for
5s gave a denaturation degree of 94 % for β-lg B in the present study, while Dannenberg and Kessler (1988)
only achieved a denaturation degree slight above 60 %. These variations were also found when comparing
heating using THE in the present study compared to Dannenberg and Kessler (1988). The same tendency
can be found comparing denaturation degrees of β-lg A and α-la with results from Dannenberg and Kessler
(1988).
The calculated kinetic parameters for denaturation of whey proteins in skim milk, presented in section 4.8,
were calculated on the basis of previous findings and compared with these (Dannenberg and Kessler, 1988;
Kessler and Beyer, 1991; Oldfield et al., 1998a; Zúñiga et al., 2010). The reaction order for β-Lg of 1.5 and 1
for α-La gave good correlation for the calculation of the rate constant, with R2 ranging from 0.8 and 0.99 for
all temperatures measured. All previous studies and the present study show similar patterns in the activa-
tion energies, with large activation energy for temperatures below 95 °C and low activation energies above
95 °C. The denaturation degree and reaction kinetics detected are observed to vary according to heating
system and analytical method used and also according to the milk type used (Anema and McKenna, 1996;
Dannenberg and Kessler, 1988; Oldfield et al., 1998a; Tolkach and Kulozik, 2007). The activation energy
found in the present study was lower at temperatures below 95 °C and higher for temperatures above
95 °C compared to previous studies (Corredig and Dalgleish, 1996b; Donato et al., 2007; Singh and Latham,
1993). This implies that less energy is required to unfold the whey proteins; while more energy is needed to
form aggregates containing these unfolded whey proteins.
The variations in degree of denaturation of whey proteins can be caused by differences in the heating sys-
tems used. A preheating period is often not used in existing publications in the area of research and often
small amount of skim was heated in tubes in water baths. Dannenberg and Kessler (1988) used a very small
pilot plant tubular heat exchanger with extremely short heating and cooling time and very small tubes for
heating (Dannenberg and Kessler, 1986).
The heating system in the present study contained a preheating step with preheating temperatures of 60 °C
or 75 °C which was estimated to be reached within the first minute of heat treatment. Based on the results
- 58 -
obtained by Dannenberg and Kessler (1988) approximately 5 % of the total β-lg content was denatured
during the preheating period. This means that the preheating process contributed to the denaturation of
whey proteins, but it could not explain the large variations in denaturation when heating at temperatures
above 90 °C.
By using a preheating section and three cooling sections, the total heating profile and heating period is
prolonged compared to previous studies. In this project, a small pilot plant heat exchanger was used which
is similar to a large industrial scale heat exchanger system. The heating and cooling period for used in the
present study was longer than the holding time at a desired temperature and this plays a significant role in
the denaturation. The overall heating profile is thereby larger compared to Dannenberg and Kessler (1988)
and which could affect the milk properties.
The desired temperatures in the present study were reached within approximately 1.5 min, while Dannen-
berg and Kessler (1988) reached the desired temperature in 0.3s. This short heating profile could favour
refolding of denatured protein in a non-native structure, which can be difficult to separate from non-
denatured whey proteins (Oldfield et al., 1998b). Aggregation of denatured whey proteins with other pro-
teins is expected to be more pronounced by using a larger heating profile and longer total heating time. An
important factor to take into account is the size of the heating system. The results made on small pilot sys-
tems could be difficult to reproduce in large scale production, as the heating process will for example have
different flow rates, heat transfer rates and turbulence. It could be argued that the heat transfer and heat
distribution is better in the present study due to higher milk flow and corrugations of the plates and tubes.
Another cause of differences in denaturation degrees are the methods for analysing the denaturation de-
gree of the whey proteins. Zúñiga et al. (2010) used SDS-Page and HPLC for analysing β-Lg denaturation and
detected a lower degree of denaturation when analysing heated β-Lg dispersions. Dannenberg and Kessler
(1988) used gel electrophoresis with isoelectric focusing which also resulted in lower denaturation degrees,
as shown in figure 27. This could be caused by gel electrophoresis providing a more limited separation than
HPLC, as denatured whey protein with closely related structures and no change in charge will migrate to
the same extent on the gel and will not be separated. The same insensitivity can be found for HPLC analysis
if there is no change in hydrophobicity if the whey proteins are denatured.
Furthermore, the integration and quantification of protein bands on gel electrophoresis can be difficult if
the protein concentrations are either high due to broadening of the protein band and interference with
other protein bands, or too low at which the background noise can be hard to separate from the protein
bands. This can be solved by using various concentrations of proteins in each sample. This was also neces-
sary in the present analysis of whey protein denaturation. For Control milk samples and milk samples which
were heated at temperatures below 100 °C, a lower injection volume was used due to the amount of pH
- 59 -
4.5 soluble proteins was higher in these samples. The integration of the peaks from these samples had to
be adjusted to be able to compare all samples. This means that small variations found for integration of
peak areas can lead to large variations when multiplying. These problems could also have affected the re-
sults of previous studies since the protein content changes in the soluble phase at pH 4.6 when the milk has
received heat treatment.
One major advantage of LC compared to gel electrophoresis are the reduced working hours, due to auto
sampling, auto cleaning and also the data analysis afterwards.
The effect of heat treatment of milk and how this affects rennet coagulation properties have been investi-
gated (Anema et al., 2011; Blecker et al., 2012; Waungana et al., 1996b) It was found that a denaturation
degree below 60 % did not affect the rennet coagulation time, while curd firming rates were much more
sensitive to denaturation. The present results are in agreement with these results. In the present study,
rennet coagulation properties for skim milk samples was impaired as the holding time increased for all
temperatures and all three heating methods. The rennet coagulation properties of skim milk did not show
any significant difference between the methods at temperatures for heat treatment using PHE and THE
below 100 °C. At temperatures above 100 °C, heating using THE resulted in worse rennet coagulation prop-
erties compared to PHE. It is generally accepted that at temperatures above 100 °C, most whey proteins are
instantly unfolded and the limiting step of irreversible denaturation is the aggregation of unfolded whey
proteins. At temperatures below100 °C, while it is the unfolding step that is the limiting factor shown by a
large activation energy (Dannenberg and Kessler, 1988; Oldfield et al., 1998a; Tolkach and Kulozik, 2007). It
can be speculated that heat treatment of skim milk using THE gives larger conformation changes of caseins
which could lead to heat induced aggregates of whey proteins not attached to casein in the casein micelle,
but with free caseins, and these complexes are therefore not as big a hindrance for rennet cleavage of
κ-casein on the casein micelle structure (Anema et al., 2007).
As can be seen in the analysis on composition of aggregates shown in section 4.7.3, it becomes clear that
there are great differences in the aggregates formed with heating using PHE and THE. Heating using THE
resulted in large protein complexes with a size above 1200 kDa which were not possible to detect on the
current method used in this study, while heating using PHE resulted in a larger content of smaller com-
plexes that were possible to detect with the used column. As the complexes found in skim milk heated us-
ing THE did not contain large amounts of whey protein and κ-casein and the pH 4,5 soluble analysis showed
low content of native whey proteins, it could be argued that the whey proteins are bound in large com-
plexes exceeding 1200 kDa. This is supported by the findings of Guyomarc’h et al., (2003), who investigated
protein aggregation in reconstituted skim milk and found whey protein-κ-casein complexes in the size
- 60 -
range of 3500-5000 kDa.
In order to obtain knowledge on the size and content of the heat induced complexes formed by heating
using THE, a SEC column which can separate even larger aggregates would be preferable. Furthermore, the
amount of protein in each collected fraction was fairly low (section 4.7.3.1) and it would have been a good
idea to collect a larger amount of each fraction to be able to obtain a better LC analysis of these.
The RCT and denaturation degrees of the differently heat treated milks were compared. As shown in
figure 15, only slight changes in RCT were seen for denaturation degrees up to 60 % when using indirect
heating. This is consistent with previous studies (Singh and Waungana, 2001; Waungana et al., 1996a). This
indicates that when more than 50 % of the whey proteins are denatured, the amount of whey protein
bound to the casein micelle is high enough to result in a steric hindrance for rennet cleavage. The correla-
tion between rennet coagulation time and denaturation degree is not similar for the three heating meth-
ods. Denaturation degrees above 70 % resulted in lower increase in relative RCT was found for heat treat-
ments using THE compared to PHE with the same denaturation degree. It seems like it is not only the de-
gree of whey protein denaturation that is important, but also how these denatured proteins interact with
other proteins.
Heat treatment of skim milk is shown to lead to formation of aggregates containing k-casein and whey pro-
teins, which is also shown by various authors (Graveland-Bikker and Anema, 2003; Tran Le et al., 2008). The
unfolding of β-lg exposes the reactive thiol groups which can bind the para κ-casein region of κ-casein
through disulphide linkages (Jean et al., 2006). As the formation of these complexes are altered by a large
heating profile with slow heating, the formation of the heat induced aggregates would be expected to be
more pronounced in heat treatments using PHE and THE compared to DSI. Dalgleish (1990) observed an
increase in size of heat induced aggregates as the denaturation of whey proteins increased. Formation of
large whey protein complexes could also in some extent be bound to the casein micelle in a different way
or not bound at all due to size, and they are thereby thought to give less steric hindrance while smaller
complexes might bind easier to the casein micelle.
The attachment of these heat induced aggregates on the casein micellar surface creates large steric hin-
drance for rennet induced cleavage. When the RCT is eventually reached for high heat treatment, the curd
firming process was slow for milks with a high denaturation degree. This could be caused by the attach-
ment of β-Lg to the k-casein which occurs on the para κ-casein region and this complex there stays on the
casein micelle after cleavage (Anema et al., 2007, 2011).
Differences in whey protein denaturation and formation of aggregates are caused by differences in the
heating method. The heating profile for heating using THE and PHE are similar when the same milk flow
- 61 -
was used, with THE having a slight longer preheating and cooling period. The heat transfer is expected to be
better in PHE due to corrugation of the plates which gives greater turbulence. The twisted shape of the
tubes decreases the heat transfer and turbulence and thereby resulted in a more unequal heat distribution,
compared to heating using PHE. These observations in differences in heating methods are consistent with
previous literature (Deeth and Datta, 2002). This could indicate that the high heat transfer in PHE and
slightly shorter heating profile leads to formation of smaller aggregates compared to THE. Fouling was ob-
served when the UHT plant was using PHE at high temperatures for several hours without cleaning, which
was not seen when heating with THE. Fouling on the surface of the plates mainly consists of whey protein
aggregates and calcium phosphate particles (Visser and Jeurnink, 1997). Formation of a fouling deposit on
the surface of the plates thereby binds denatured whey proteins which are not present in the milk any
longer. Although fouling was no large problem in this study, it could be expected that this could have an
effect on the content of denatured whey protein and thereby also an effect on the content of whey protein
that can form large aggregates.
The DSI heating system was found to be very different from the two indirect heating methods. DSI has
shown to have a significant lower denaturation degree of β-Lg and α-La and the rennet coagulation proper-
ties were also significantly less affected by the heat treatment compared to indirect heating. The heating
profile of DSI is different from the two indirect methods by having a very short heating time from preheat
temperature to the desired temperature and a flash cooling to 65 °C before reaching the three plate cool-
ing sections. The injection of steam into the preheated milk gives a good heat transfer due to mixing of milk
and steam, but this also induces more stress to the milk. This can be seen when heating at temperatures of
145 °C, where DSI had a low denaturation degree compared to skim milk samples heated using PHE and
THE, but this gave rise to a three times higher relative RCT compared to samples heated indirectly with
same denaturation degree. The increase in relative RCT can thereby not be the only explained by the dena-
turation of whey proteins, but the temperature gives rise to other changes milk, such as mineral precipita-
tion and lactose degradations (Lewis and Deeth, 2008).
No whey protein denaturation was observed upon heating skim milk at 105 °C for 4s using DSI. As the pre
heating and cooling sections are PHE, it could be expected that whey protein denatured to some extent.
The fast heating and flash cooling times gives less time for formation of aggregates and denatured whey
proteins could refold into non-native structures as most whey protein are instantly unfolded at tempera-
tures above 100 °C.
The flow rate of milk through the system had a great impact on the heating profile in the same extent as
the length of the holding section. The general flow rate in this project was 20 L/h. Heating using PHE and
THE, with holding times of 10 s and 30 s, operated with flow rates of 30L/h and 40 L/h, respectively. This
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gave a faster heating and cooling time and thereby a different total heating profile, even though the hold-
ing time at the desired heating temperature was increased. These variations in heating profile could affect
the whey protein denaturation. This can in some extent be seen in figure 11, where the denaturation of β-
lg B and α-la, heated at temperatures of 95 °C or below, show a decrease in denaturation compared to the
heat treatment with a holding time of 5s. This can also in some extent explain why the relative RCT for the
three milk types varies at these holding times. This is shown in figure 12. Holding times of 30 s results in
decrease in the relative RCT while the relative CFR increases compared to heating with a holding time of 10
s and 60s.
Variations in whey protein denaturation and rennet coagulation properties between skim milk samples
with the same heat treatment combination and heating method were found between different trial days.
This was caused by variations in milk batches on various trail days. Even though there were only small varia-
tions in the milk composition analysis of total protein on LC-MS and Milkoscan, Law and Leaver (1997) have
shown that the level of denaturation decreases as the total protein content decreases and reverse. The
ratio between casein and whey protein also has a great impact on the denaturation of whey protein. Re-
moving casein from the milk and keeping the whey protein content constant, less denaturation of whey
protein was found, but these whey proteins formed large aggregates in the milk as they could not aggre-
gate with caseins in the casein micelle.
The content of casein and whey protein and the ratio between these is also important for coagulation
properties. The casein:whey protein ratio of 80:20 in skim milk had a prolonged RCT and lower CFR com-
pared to MCIc and MCI with a casein:α-la, β-lg ratio of 96:4. This was seen for all samples, both control and
heat treated samples. Skim milk and MCIc had the same casein content, with MCIc having lower total pro-
tein content. Still, MCIc showed slightly better coagulation properties when comparing control samples,
which indicates that whey proteins had an impact on CFR, even on low pasteurized (72 °C, 15s) skim milk.
Since 85 % of all κ-casein has to be hydrolysed to initiate casein micelle aggregation, it could be thought
that MCI would have longer RCT as the content of casein is higher and thereby more κ-casein has to be
cleaved. This was not seen and the results therefore indicate that the amount of Chymosin added to the
milk in these analyses was enough even though the casein content is 13 % higher in MCI compared to skim
milk. Removal of whey proteins from low pasteurized skim milk and increasing the casein content gave
significant improvements of the rennet coagulation properties.
The coagulation properties for heat treated MCI by the indirect heating methods showed the opposite of
heating of skim milk. For skim milk, heat treatment using THE improved the rennet coagulation properties
than PHE at high temperatures to the milk, but the opposite was found for heating of MCI. For tempera-
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tures below 100 °C, PHE was better for both milk types. This could indicate that the heating system of THE
with larger flow area and lower heat transfer results in larger changes of the casein micelles compared to
heat treatment using PHE which is seen to affect the rennet coagulation properties. In order to make a
proper conclusion, more heat treatment combinations using THE are necessary. Analysis of the aggregate
content in heat treated MCI samples have to be analysed for all heat treatment systems, which could give
indications of which heat induced casein aggregates are formed by heat treatment of MCI using the differ-
ent heating systems.
The rennet coagulation properties of MCI and MCIc change during heat treatment which indicates that the
heat treatment results in structural changes of the casein micelle, even though the caseins and casein mi-
celle are stated to be quite heat stable. The CFR was increased when heating MCI and MCIc at tempera-
tures below 100 °C. Heat treatment above 100 °C was shown to have a negative effect on the coagulation
properties. As only 4 % of the total protein content was β-Lg and α-La, these are not the only explanations
for the decrease in coagulation properties. Heat treatments above 100 °C increases the amount of soluble
casein in solution and k-casein can be removed from the micellar surface at normal pH. The amount of
soluble phosphate and calcium decreases when increasing temperature (O’Connell and Fox, 2003; Sauer
and Moraru, 2012). This exposes the calcium sensitive caseins in casein micelle which likely could form
small aggregates. These aggregates have structures different from the casein micelle and could be less af-
fected by rennet cleavage. Mohammad and Fox (1987) also reported precipitation of calcium phosphate on
the surface layer of casein micelles, which hinders the rennet coagulation when heating at 140 °C.
Bulca and Kulozik, (2004) found crosslinking between casein and also dissociation of caseins from the mi-
celle when heating whey protein free casein milk solutions at high temperatures.
The degradation of lactose and binding of lactose to casein occurs in a higher degree than maillard reac-
tions. The binding of lactose to caseins on the casein micellar structure can form steric hindrance for rennet
cleavage of κ-casein. Furthermore is degradation of lactose responsible for formations of formic acids
which can give small decreases the pH in milk and reduce the micellar calcium phosphate (Martinez-Castro
et al., 1986; Turner et al., 1978).
These structural changes also appear in skim milk, as the only difference between the milks used is the
protein content, and this can help explaining why high heated treated skim milk with equal denaturation
degrees have variations in coagulation properties, even though the whey protein denaturation and forma-
tion of heat induced aggregates containing these are the main cause of impairments of rennet coagulation
properties of skim milk.
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6 Conclusion The aim of this study was to investigate the effect of three different heating systems used in the dairy in-
dustry for heat treatment of milk, and how denaturation of whey proteins and rennet coagulation proper-
ties of skim milk were affected by the heating process. Further on, the effect of heat treatment on rennet
coagulation properties were analysed in skim milk with reduced content of whey proteins and various ca-
sein concentrations. The content of β-Lg and α-La in MCI and MCIc was less than 4 % of the total protein
content, and it was therefore not possible to make proper calculations on the denaturation of an already
small content of whey proteins.
The degree of denaturation of whey proteins in skim milk increased when increasing holding time for all
temperatures measured and heating methods used. Heat treatment using indirect heating at temperatures
of 115 °C or more resulted in denaturation degrees above 90 % for β-Lg for all holding times, while the de-
naturation degree of α-La did not exceed 90 % at any temperature and holding time combination. Heating
using THE gave rise to significantly higher degrees of denaturation at temperatures below 100 °C, com-
pared to PHE, while when heating at 130 °C, most β-Lg was denatured in both indirect heating methods.
Heating using DSI is gentle when comparing the denaturation degree with the indirect heating methods. A
denaturation degree of 40 % was found for β-Lg B when heating at 145 °C, while the indirect methods
showed denaturation degree of >95 % when heating at 140 °C.
The reaction kinetics of denaturation of whey proteins in skim milk was found to follow reaction order 1.5
for β-Lg and 1 for α-La. The denaturation degrees in this study were found to be higher than what was
found in previous studies, and the activation energy was found to be lower at temperatures below 95 °C
and higher at temperatures above 95 °C. This was caused by using a different heating system and a
generally longer and larger heating profile which gives rise to a larger extent of denaturation.
The rennet coagulation properties of skim were impaired when increasing holding time for all temperatures
measured for all heating methods, which is linked to the whey protein denaturation. An increase in degree
of denaturation gave impairments in rennet coagulation properties. Heating skim milk using DSI gave sig-
nificantly lower decrease in rennet coagulation properties compared to the indirect heating methods, and
RCT was reached within two hours when heating at 145 °C. Heating at PHE gave better rennet coagulation
properties when heating at temperatures below 100 °C, compared to THE, while THE reached RCT within
two hours at temperatures of 130 °C which heating at PHE did not. The removal of whey proteins from milk
improved the rennet coagulation properties compared to skimmed milk. MCI, with casein content equal to
total protein content of skim milk, showed the largest improvements in rennet coagulation properties at all
temperatures, but MCIc, with a casein content equal to skim milk, was also significantly better than skim
milk. When heating using PHE with temperatures below 100 °C with holding times below 60 s, the RCT and
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CFR were improved compared to the control milk for MCI and MCIc. Rennet coagulation properties de-
creased when heating above 100 °C, but RCT and CFR were detected for all measured temperature and
holding time combinations. Heating MCI using DSI was also a gentle heating method compared to the indi-
rect heating. Heating using PHE shows a tendency towards improved rennet coagulation properties com-
pared to THE.
To investigate why THE and PHE showed similar degree of denaturation when heating at 130 °C, in spite of
THE having significantly better rennet coagulation properties, the content of heat induced aggregates in
skim milk were analysed. THE caused formation of a high level of large whey protein and κ-casein aggre-
gates, the sizes of which was larger than the column could detect, while heating using PHE resulted in lar-
ger amounts of smaller aggregates of whey protein and κ-casein, which could be detected.
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7 Perspectives Heat treatments used in this project are performed on heating systems that are supposed to mimic the
large heating systems used in the dairy industry in the preheating and cooling sections. The results on how
the three heating methods affect milk proteins can thereby easily be transferred to large scale productions
without experiencing large changes in these parameters which can occur when upscaling productions.
Heat treatment using temperatures above those used in traditional pasteurization (72 °C for 15 s) is nor-
mally not used in cheese production due to the negative effect on whey protein denaturation, although
these temperatures are not sufficient to inactivate all undesirable microorganisms (Belitz et al., 2004). The
rennet coagulation properties of milk with low content of whey proteins, following high heat treatment can
be useful if the milk has to receive severe heat treatment to destroy higher contents of bacteria and spores
and still be able to make a hard or semi hard cheese.
The three methods used and their resulting differences in denaturation of whey protein and rennet coagu-
lation can be used in production of products with special characteristics which can be improved when using
a more specialized heating. This can be cheeses with certain hardness, incorporation of whey proteins in
the cheese without large change in functionality of the cheese, or decreasing the heating time in the proc-
essing of yogurt.
7.1 Future research Future research in this area could be to look more into the particle size of the heat induced protein aggre-
gates for various temperatures and also for the various milk types, to make a full picture of the content of
the aggregates. It could also be interesting to look into the casein micellar structure to see which changes
happen in the various milk types at the different heating methods and relate this with rennet coagulation.
Furthermore, more heat treatments could be made for heat treatment using THE and additionally also us-
ing direct steam injection for all milk types, to make at full picture of the effect of each heating system on
heat treatment of milk.
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Appendix 1 Denaturation degree of β-Lg A in heat treated skim milk at PHE and THE, shown for temperatures below
100 °C and above 100 °C.
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Appendix 2 2DGE for milk samples under non-reduced and reduced conditions.
A: control skim milk, non-reduced. B: control skim milk, reduced. C: skim milk heated at DSI, 130 °C for 4s, non-reduced.
D: skim milk heated at DSI, 130 °C for 4s, reduced. E: control MCI, non-reduced. F: control MCI, reduced. G: MCI heated at
PHE 130 °C for 5s, non-reduced. H: MCI heated at PHE 130 °C for 5s, reduced.