Effect of drying Condition on the physicochemical properties of oil extracted from two varieties of tiger nuts from Northern Nigeria. ABSTRACT Two varieties of tiger nuts grown in Northern Nigerian were studied to determine the effect of drying temperatures on the physicochemical properties of oil extracted from the nuts; to compare the parameters and evaluate the qualities and quantities of the oil at the drying conditions and also determine varietals and geographical impacts on those parameters. The drying protocol used were sun drying, and oven drying at 60 0 C, 120 0 C, and 180 0 C. The following physicochemical parameters were evaluated: Free fatty acid,
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Effect of drying Condition on the physicochemical properties of oil
extracted from two varieties of tiger nuts from Northern Nigeria.
ABSTRACT
Two varieties of tiger nuts grown in Northern Nigerian were studied to
determine the effect of drying temperatures on the physicochemical
properties of oil extracted from the nuts; to compare the parameters and
evaluate the qualities and quantities of the oil at the drying conditions and
also determine varietals and geographical impacts on those parameters. The
drying protocol used were sun drying, and oven drying at 600C, 1200C, and
1800C. The following physicochemical parameters were evaluated: Free fatty
acid, peroxide value, Iodine value, percentage oil yield, specific gravity and
smoke point. Results from this study revealed that the free fatty acid,
peroxide value, Iodine value, percentage oil yield, specific gravity and smoke
point of oil from the two varieties ranged from (0.01-0.03%) as oleic, (0.63-
value (131 wijs) reported by Alasela (2006). However, the values were higher than values
reported by Shaker et al; (2009) and Ezebor et al; (2005). The high values obtained suggest the
presence of unsaturated fatty acid and this places the oil in the drying groups (Nzikou et al;
2010). Also, the values were above the range of codex standard values (80 – 106 Wijs) for
groundnut oil however, the values were within the codex standard
values (124 – 139 wijs) for crude soybean oil (FAO\WHO, 1993). Statistically, all the samples
were significantly different at (P<0.05) at the end of the drying. Table 1 also reveal that t
he iodine values of oil samples from brown variety tiger nut tuber varied from 125.716 – 131.299
wijs. The result shows that the iodine value decreased as the temperature was increased,
however, sample BDs had the highest iodine value of 131.299 wijs. This suggest that
temperature of the sun has little if any effect at all on the iodine value of Brown variety tiger nut
oil. Also, sample oven dried at 600c,1200c, and 1800c had the same trend of decrease in iodine
value as the temperature was increased which was in agreement with the report from Nzikou and
co-worker; (2010) which suggest loss of unsaturation in the fatty acids of the triacylglycerols.
However, the values were higher than the value (104.2 wijs) reported by Ezebor et al; (2005) but
were in agreement with the value (131 wijs) reported by Alasela (2006). The values equally
conformed to the codex standard value (124- 139 wijs) for crude soybean oil (FAO/WHO, 1993).
The high iodine values obtained in the oil samples suggest the presence of unsaturated fatty acid
and this places the oil in the drying groups (Nzikou et al; 2010). Statistically, all the oil samples
from the brown variety tiger nut tuber differed significantly at (P<0.05) at the end of the drying.
4.1.4 PERCENTAGE OIL YIELD
This can be seen as the quantity of extractable oil present in a given
quantity of an oil seed expressed in percentage. From table 1, the percentage
oil yield ranges from 9.267 – 10.900%. The values did not follow a particular
linear trend which could be traced to method of oil extraction (cold
extraction method) adopted. According to fellows (2009), solid-liquid
extraction involves the removal of a desired component (the solute)from a
food sample (oil seed) using a liquid (i.e. suitable solvent like Hexane,
ethanol, petroleum ether, methanol etc.) which is able to dissolve the solute.
Further studies by fellow (2009) indicates that extraction rate (% oil yield) is
dependent on the temperature of extraction, the surface area of solid exposed
to the solvent, the viscosity of the solvent and finally the flow rate of the
solvent. Report by Ezebor et al; (2005) shows that yellow variety tiger nut
tuber had a percentage oil yield of 22.3% after the solvent extraction. Which
was higher than the values in table 1. This suggests that soxhlet extraction
method is more efficient than cold extraction method since there were a
significant difference between the percentage oil yields from both methods.
Also, the values were not in agreement with the specification of codex
Alimentarius commission of 20% oil content for edible oil (Pearson 1976).
Since the percentage oil yield from the cold extraction method is low, it may
limit the utilization of the method in vegetable oil industries. Statistically, all
the samples were significantly different at (P<0.05) at the end of drying.
Table 1 also reveal that the percentage oil yield from the brown variety tiger
nut tuber ranges form 8.333-14. 767%. The decrease in percentage oil yield
from the tiger nut tuber did not follow a particular linear trend also which
could be attributed to the inefficiency of the cold extraction method adopted.
However, sample BDs recorded the highest percentage oil yield of 14.767%.
This suggests that sun drying increases the percentage oil yield than over
drying in the brown variety tiger nut tuber. The percentage oil yield from the
entire sample fell below the codex alimentarius commission specification of
20% oil content for edible vegetable oil (Pearson, 1976). The yields were also
below the value (20.4%0 reported for brown variety tiger nut tuber by Arubi
(2009). This implies that cold extraction method is not a reliable means of oil
extraction to be adopted by commercial oil processors. Fellow (2009)
reported that percentage oil yield depends on the temperature of extraction,
the surface area of the solid exposed to the solvent, the viscosity of the
solvent as well as the flow rate of the solvent statistically, the result obtained
from the percentage oil yield of the brown variety tiger nut tuber shows that
all the samples differed significantly at (P < 0.05) at the end of the drying
process.
4.1.5 SPECIFIC GRAVITY
This is the ratio of the density of a substance to the density of a
reference substance otherwise know as relative density. Table 1 shows that
the relative density of the oil samples varied from 0.863 – 0.853. The values
decreased as the temperature of the oven was increased however, when the
tuber was subjected to a temperature of 1800c, there was an increase in the
specific gravity of the oil. This suggests that there was no direct relationship
between the temperature and specific gravity of oil extracted from yellow
variety of tiger nut tuber. The values also suggests that the oil is less dense
than water. According to Codex Alimentarius Commission, specific gravity
of 0.919 – 0.925 at 200c have been recommended for soybean oil
(FAO/WHO, 1993). However, the values were lower than the above
specification but is within the range 0.86g/ml reported for water melon seed
oil by Taiwo and co-workers(2008). This shows that the oil contains lower
molecular weight of fatty acid (Mowla et al; 1990, Ching Kilang cho 2000).
According to Hawley (1981), compounds containing several functional
groups especially those groups that promote association have a specific
gravity more than 1.0. Since all the oil samples had specific gravity less than
1.0 it implies that the samples were made up of fewer functional groups
within the triglyceride structures. Statistically, there was no significant
difference at (P>0.05) among the samples at the end of drying. Table 1
equally shows that the relative density of the oil samples from the brown
variety tiger nut tuber varies from 0.860- 0.883. The result revealed that there
was no particular pattern or trend followed by the oil sample as the
temperature was increased. This suggest that temperature has little or no
direct relationship with the specific gravity of oil samples extracted from
brown variety of tiger nut tuber. However, there was an increase in specific
gravity as the temperature was increase to 600C, 1200C but later deceased
when the tiger nut tuber was dried at 1800C. t The results from table 1 were
equally below the codex alimentarius commission specification (0.919-0.925
at 200C) for soybean oil (FAO/WHO, 1993). This suggest that the brown
variety tiger nut oil contains low molecular weight of fatty acids (Mowla et
al; 1990, ching Kuang Cho 2000). This also indicates the possible use of the
oil in soap manufacture (Arubi 2009). Statistically, sample BDs and BD60
differed significantly at (P < 0.05) while sample BD120 and BD180 did not
differ significantly at (P > 0.05) at the end drying processes.
4.1.6 SMOKE POINT
When a fat or oil is heated to a certain temperature, it starts to
decompose producing a blue haze or smoke and a characteristics acrid smell.
The temperature at which this occurs is known as smoke point (Gaman &
Sherrington, 2001). The smoke points of different oil samples in table 1
ranges from 240.000 – 263.0000c. The smoke point of all the samples
decreases with increase in the oven temperature. According to Onwuka
(2005), the smoke point is used in determining the thermal stability of the oil.
A good quality palm oil will have a smoke point at least 215 – 3330c when
fresh but this can be lowered by the free fatty acid present. The values of the
smoke points of the oil samples were in agreement with this report.
Studies from Ezigbo (2009), indicates that smoke point vary with the chain
length of free fatty acid. Hence sample YDs and YD60 with smoke points
(263.0000c and 247.0000c) respectively had higher free fatty acid values
which is in agreement with the report. However, the degree of unsaturation of
oil has little, if any effect on its smoke point (Hui, 1996). Sample YDs and
YD60 were significantly different at (P<0.05) while there was no significant
difference at (P>0.05) between sample YD120 and YD180. Table 1 also shows
that the smoke point of oil of oil
Samples extracted from brown variety tiger nut tuber ranges form 240.333-
252.6670C. The result also reveal that the smoke points of the oil samples
Decreased as the temperature was increased. However, sample BDs had the
highest smoke point of 252.6670C. According to report from Ezigbo (2009),
smoke point vary with the chain length of the free fatty acid which was in
agreement with the result in table 1. The values were equally within the range
(215-3330C) reported by Onwuka (2005) for a good quality palm oil.
Statistically, sample BDs and BD60 differed significantly at (P <0.05) while
there was not significant difference between sample BD120 and BD180 at (P>
0.05).
FFA as %
oleic
PV meq/g IV Wijs’ % oil yield Specific
gravity
Smoke point
Y B Y B Y B Y B Y B Y
0.034a
0.019b
0.015c
0.012c
0.013d
0.0010
d
0.009e
0.007e
2.833a
2.267b
1.867c
1.807cd
1.873c
1.473e
0.793f
0.633g
129.861
b
129.523
cd
128.930
de
124.024
h
131.299
a
129.607
bc
128.846
ef
125.716
g
10.900
b
9.267e
10.767
c
9.433d
14.767
a
8.200h
8.833f
8.333g
0.863c
0.857cd
0.853df
0.863c
0.860cd
0.877ab
0.883a
0.877ab
263.000
a
247.333
c
241.667
ef
240.000f
g
Table 2. Relationship between drying condition and variety
The values are mean of triplicate determination. Means in the same column
with different superscript are significant difference at (P < 0.05)
Where y = yellow variety tiger nut tuber
B = Brown variety tiger nut tuber
Ds = Sun dried
D60 = Oven dried at 600C
D120 = Oven dried at 1200C
D180 = Oven dried at 1800C
4.3.1 FREE Fatty Acid
Free fatty acid is one of the products of odour and rancid flavour in fat
and oils especially when they are more of short-chain length
(Norman and Hatchikiss, 2007, Ogundele et al; 2006). Thus, it is a measure
of hydrolytic randicty of an oil (Arawande 2008, Ihekoronye & Ngoddy
1985).
Table 2 shows the relationship between drying conditions and variety on the
physicochemical properties of tiger nut oil from two varieties of tiger nut
tuber. The data obtained for free fatty acid of the two varieties indicate that
the free fatty acid varies form 0.007-0.034%. The results indicate that oil
from sun dried tiger nut tuber had highest free fatty acid content among the
samples but the FFA of the oil from sun dried yellow variety tuber (0.034%
oleic) was higher than the oil form sun dried brown variety tuber (0.013%
oleic). Generally, the rest of other oil samples maintained a linear trend of
decrease as the temperature was increased. This suggest that oils form brown
variety tiger nut tuber are more hydrolytically stable than the yellow variety
and thus will have a higher shelf life (Oyedeji et al; 2006). The difference in
the results between the oil from the two varieties could be probably traced
from the difference in their fatty acid composition, tocopherol content, soil
type as well as agronomic practices. The data obtained were below the values
(0.3 and 0.4% oleic) reported by Arubi (2009) for oil from yellow and brown
variety tiger nut tuber respectively. This is probably attributed to difference
in conditions of manufacture, age and storage (Morris 1999). Statistically, oil
samples YDs and YD60 differed significantly at (P < 0.05) while oil samples
(YD120 and YD180, BDs and BD60, BD120 and BD180) did not differ significantly
at (P > 0.05) respectively.
4.3.2 PEROXIDE VALUE
Is the measure of primary product of lipid oxidation (oxidative
rancidity) (Rossel, 1994). Seed oil or nuts are known to deteriorate when
processed inadequately with the principal decomposition reaction being
oxidation which occur by free radical mechanism, initially characterized by
the emergence of a sweetish and unpleasant odour which becomes
progressively worse until it attains a characteristic smell of rancid fat
(Grouveia et al; 2004). Data obtained in table 2 reveal that peroxide value of
oil form the yellow and brown variety tiger nut tuber varied from 0.63-2.83
meg/kg. The result also indicates that the oil from the yellow variety tiger nut
tuber had higher peroxide values (1.81-2.83 meq/kg) than the oil from brown
variety tiger nut tuber which had peroxide values (0.63-1.87Meq/kg).
Generally, all the oil samples from the two varieties recorded a linear trend of
decrease in peroxide value as the temperature was increased but was more
pronounced in the oil from sun dried tubers which had peroxide values (2.833
and1.873meq/g) respectively. This suggest that tiger nut oil is more prone to
hydrolytic rancidity than oxidative rancidity since there was a decrease in the
peroxide values of oil from other samples as the temperature was increased.
The high peroxide values recorded in the oil from the sun dried tubers
samples could be attributed to prolong period of sun drying which must have
promoted hydrolytic rancidity in the tiger nut tuber. It was also discovered
from the result in table 2 that the oil form the brown variety tiger nut tuber
were thermally and hydrolytically more stable than the oil from the yellow
variety tiger nut tuber since all the oil samples from the yellow variety tiger
nut tuber had a peroxide values higher than the oil from the brown variety
tiger nut tuber. The low peroxide value recorded in oil samples from brown
variety tiger nut tubers as the temperature was increased also indicates slow
oxidation of the oil samples according to Damain (1990). It also suggests that
the oil will have a high induction period than the yellow variety. The
variance could be probably attributed to their differences in fatty acid
composition, vitamin E content (Tocopherol), soil type as well as agronomic
practices. Nevertheless, all the oil samples from the two variety tiger nut
tubers had peroxide values below the codex standard (10meq/kg) for freshly
refined vegetable oil. This suggests the edibility and freshness of tiger nut oil
even without refining (Tiger nut Traders, 2008). The result of the statistical
analysis revealed that there was no significant difference among the oil
samples (YD120, YD180 and BDs) at (P >0.05) while oil samples (YDS, YD60,
BD120 and BD180) differed significantly at (P < 0.05)
4.3.3 IODINE VALUE
This is the measure of the degree of unsaturation in oil and it is an
identity characteristic of native oil which is an indicatives of the degree of
unsaturation in the fatty acid of triacylglycerol which can be used to quantify
the amount of double bonds present in an oil and evaluate the susceptibility
of oil to oxidation (Nzikou et al; 2010 ). Result from table 2, indicates that
the iodine values of oil form the two varieties
tiger nut tuber varies from 124.024-131.299 wijs. The result from the two
varieties were comparable however, the oil from the brown variety tiger nut
tuber recorded higher iodine values than the oil from the yellow variety tiger
nut tuber. Also, there was a linear trend of decrease in the iodine value as the
temperature was increased. This suggests the loss of degree of unsaturation in
the fatty acids of the triacylglycerols (Nzikou 2010). According to Arawande
and Ademulegun (2009) report, the increase in Iodine value is always
accompanied with decrease in peroxide value owing to more C = C
unsaturated double bond that are present in the oil that is left to be oxidized
therefore leaving more C =C unsaturated double bond in the oil for iodination
reaction during Iodine value determination. The results in table 2, is not in
agreement with this report probably because oil from tiger nut tuber is
hydrolytically and thermally stable. The difference in the results of Iodine
values of tiger nut oil from the two varieties could be attributed to varietals
differences as well as difference in agronomic practices. From statistical
analysis, sample (YDs, BDs, YD180 and BD180) different significantly at (p <
0.05) while sample (YD60, YD120, BD60, and BD120) were significantly the
same at (P > 0.05).
4.3.4 PERCENTAGE OIL YIELD
Apart from the use of hydraulic pressing machine, use of solvent
extraction method which involves the process of leaching out soluble
constituent (non-polar) present as a solid or liquid from a solid or from a
liquid by means of a solvent (Richardson 1993). According to Mcclement
(2003), solvent extraction technique is one of the most commonly used
methods of isolating lipids from food samples and of determining the total
lipids content (percentage oil yield). Table 2 shows that the percentage oil
yield of oil from two varieties of tiger nut tuber ranges from 8.333-14.767%.
The sun dried samples from both variety (YDs and BDs) had the highest % oil
yield of (10.9000 and 14.767 %) respectively while oven dried samples
recorded a low % oil yield when the tubers were dried at 600C. However,
when the tubers were dried at 1200C, there was an increase in % oil yield
which later decreased again when the tubers were dried at 1800C. This
indicates that there was no linear trend in the percentage oil yield as the
temperature was increased. However, the result from the two varieties
followed the same pattern of rising and falling as the temperature was
increased. All the data generated from the two varieties tiger nut oil fell
below the codex alimentarius commission standard of 20% oil content
(percentage oil yield) for edible vegetable oil (Pearson 1976). The results also
fell below the values reported by Ezebor and co-worker (2005), Arubi (2009).
This could be attributed to the difference in methods of oil extraction used.
The results also indicate that cold extraction method is not a reliable method
of extracting oil from tiger nut tubers. Sample BDs was closer to the value
(15% oil content) reported by Tiger nut Traders (2008) for mechanically
pressed tiger nut oil. From statistical analysis, all the oil samples from the
two varieties were significantly different at (P < 0.05).
4.3.5 SPECIFC GRAVITY
Is one of the physical analyses used in predicting the quality of oil
extracted from an oil seed or nut. It has a linear relationship with the
saponification value of oil and can be used in predicting the suitability of an
oil for soap and shampoo manufacture (Karim 2009). Data obtained in table 2
revealed that the specific gravity of oil samples extracted from the two
varieties of tiger nut tubers varied from 0.853-0.883. the result of the specific
gravity of oil from the two varieties were comparable however there was no
specific pattern of increase or decreases as the temperature was increased
among the samples. Nevertheless, when the tiger nut tuber from the brown
variety was oven dried at 600C and 1200c, there was an increase in specific
gravity of the oil while sample from the yellow variety decrease when
subjected to the same temperatures. But when the temperature was increased
to 1800C sample from the brown variety decreased while sample from the
yellow variety increase in the specific gravity of their oil. Also, all the
specific gravities of all the oil samples from the two varieties did not fall
within the codex specification (0.919-9.25) for soybean (FAO/WHO, 1993) .
the differences could be attributed to temperature difference, geographical
location, as well as differences in agronomic practices.
However, the values were similar to the values (0.86) reported by Taiwo and
co-workers (2008) for water melon seed oil. Sample (YDs, YD60, YD120,
YD180 and BDs) and sample (BD60, BD120 and BD180) were significantly the
same at (P > 0.05).
4.3.6 SMOKE POINT
This is one of the factors used in the selection of oil for deep-frying
application. Oil smoke as a result of the decomposition of volatile
compounds from the oil followed by the production of a blue haze or smoke
and a characteristic burnt odour usually at a temperature above 2000C
(Gaman and sherrington, 1977). Table 2 shows the comparison of the smoke
points of oil extracted from the two varieties of tiger nut tubers subjected to
different drying temperatures. The result revealed that the smoke point of the
oil samples from the two varieties were within the ranges 240.000-263.0000C.
The values from the two varieties were comparable.
The data from the two varieties for smoke point also showed a linear
trend of decrease in their smoke points as the temperature was increased
which is in agreement with the report by Ezigbo (2009). This implies that the
smoke point of the oil samples from the two varieties tiger nut tubers varies
with their chain length of free fatty acid. The results were equally within the
range (215-3330C) reported by Onwuka (2005) for a good quality palm oil.
This suggests that the oil samples from the two varieties were thermally
stable and could be used for deep –frying operations (Gaman and Sherrington
1977). The difference among the smoke points of the oil from the two
varieties could be attributed to their differences in fatty acid composition,
Varietal differences, geographical difference, differences in soil types as well
as agronomic differences. Oil samples from (YDs, BDs, YD60 and BD60) were
significantly different at (P < 0.05) however, there was no significant
difference among sample (YD120, BD120, YD180 and BD180 at (P > 0.05).
CONCLUSION AND RECOMMENDATION
This study showed that oven drying tiger nut tubers especially at 1800C
is the best temperature for drying tiger nut tubers for oil extraction since the
results revealed that at this temperature, oil samples from the two variety
had the lowest value of peroxide value and free fatty acid which are
important variable in considering the quality of an oil because the lower the
values ( PV and FFA) the better the quality of the oil. The varietal differences
as well as differences in geographical locations had the least significant effect
on the quality of the oil samples from the two varieties at that temperature
also. The oil samples from the brown variety tiger nut tuber are preferred
because of its low PV and FFA values.
This implies generally, that the tiger nut oil from both varieties can be
rated as one of the best oil suitable for deep-trying, long term storage, soap
making and other industrial applications since they are hydrolytically and
thermally stable.
RECOMMENDATION
Based on findings from this work, the use of oven drying instead of sun
drying should be encouraged as a preparatory step in processing of tiger nut
tubers for oil extraction.
Further studies should be carried out to determine the effect of drying
temperatures and time on the induction time, chemical kinetics, fatty acid
composition and vitamin E content of tiger nut oil from tiger nut tubers so as
to evaluate the overall stability and behaviours of the oil at different
temperatures and times.
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