EVALUATING THE EFFECT OF SELECTED SOAKING PRETREATMENTS ON THE COLOR QUALITY AND PHENOLIC CONTENT OF PURPLE POTATO CHIPS THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By KAI ZHANG Graduate Program in Food Science and Technology The Ohio State University 2017 Master’s Examination Committee: Dr. M. Monica Giusti, Advisor Dr. Christopher T. Simons Dr. Lynn C. Knipe
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EVALUATING THE EFFECT OF SELECTED SOAKING PRETREATMENTS ON
THE COLOR QUALITY AND PHENOLIC CONTENT OF PURPLE POTATO CHIPS
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in
the Graduate School of The Ohio State University
By
KAI ZHANG
Graduate Program in Food Science and Technology
The Ohio State University
2017
Master’s Examination Committee:
Dr. M. Monica Giusti, Advisor
Dr. Christopher T. Simons
Dr. Lynn C. Knipe
Copyrighted by
Kai Zhang
2017
ii
Abstract
Purple potato chips made from natural potato varieties with purple-bluish flesh are a
novel alternative product to traditional potato chips. However, color loss and anthocyanin
degradation occur during the processing of purple chips due to high frying temperature
and Maillard browning. The objective of this study was to evaluate the influence of
pretreatment soaking solutions on the color formation and phenolic content of purple
potato chips.
Purple potatoes (Purple majesty) were obtained from a local market in Columbus (OH,
USA), and cut into slices of 2±0.2 mm. After washing in running water, slices were
soaked in solutions of citric acid, acetic acid, sodium chloride (NaCl) or calcium chloride
(CaCl2) using four different concentrations (0.1%, 0.2%, 1%, 2%) for 10 minutes,
superficially dried and fried for 3 minutes in vegetable oil at 170 °C. Color (CIELab),
monomeric anthocyanins, and total phenolics were monitored on the final product.
Potato slices pretreated with 1% and 2% citric acid solutions had brighter purple-bluish
color (L*=39.1±0.4, h*=317.9°±3.6°) with less browning compared to samples pretreated
with other solutions (L*=22-33, h*=306˚-329˚). Potato chips pretreated with 1% and 2%
2.3.1 Color Formation – Maillard Reaction.............................................................................................5
2.3.2 Color Parameters....................................................................................................................................6
2.4 Process Variables Affecting Chips Color................................................................................7
was used to collect the results. A reverse-phase symmetry C18 column with 5 µm particle
size, 100Å pore size and 150 x 4.6 mm column size (Phenomenex®, Torrance, CA, USA),
was used to separate anthocyanins, with a C18 column guard (Phenomenex®, Torrance,
CA, USA). Samples were purified using C18 column, filtered through 0.25 µm, 15 mm
membrane syringe filter (Phenomenex®, Torrance, CA, USA), and a 75μL sample
extract was injected . Separation of the anthocyanins was achieved at 25 °C using two
solvents. The solvents were phase A, 4.5% formic acid in H2O, and phase B, acetonitrile.
A binary gradient was used for solvent B with a flow rate of 0.8 mL/min: 0-2 min for 7%,
2-30 min for 7-20%, 30-36 min for 20-60% and 36-50 min for 60-7%; spectral data were
obtained from 200-700 nm, and the measurement of anthocyanin eluted was at 520 nm.
3.11 Storage Stability Analysis
20 grams of purple potato chips pretreated with 1% and 2% citric acid solutions were
vacuum packed in plastic bags to prevent oxidation by air, covered with aluminum foil to
avoid light exposure, and stored for stability analysis. CIELab color of chips were
measured every two weeks as well as total monomeric anthocyanin content, percent
polymeric color and total phenolics following extraction and purification.
37
3.12 Statistical Analysis
Microsoft Excel 2013 was used to process and analyze data of colorimetric measurements,
reducing sugar amount, monomeric anthocyanins, percent polymeric color, and total
phenolics. Analysis of variance (ANOVA) was completed by using SPSS 24.0, and
significance of difference was identified by Tukey’s post hoc test (α=0.05).
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Chapter 4 Results and Discussion
4.1 Colorimetric Evaluation
Color is an important attribute affecting product quality and customers’ willingness to
purchase. CIELab color parameters of all samples, purple potato flesh as well as freshly-
made chips and stored chips were measured using a HunterLab benchtop
spectrophotometer.
Color of fresh purple potato flesh and chips are shown in Table 2 and Figure 7,
respectively. Purple potato flesh had values of lightness (L*) of 36.2 ± 2.9, hue angle (h*)
of 330.7 ± 2.0, as well as a*= 7.7 ± 1.1 and b* = -4.3 ± 0.6, indicating a bright purple
reddish pigment of the flesh. By contrast, all purple potato chips pretreated with 0.1%
and 0.2% citric acid solution, as well as those pre-treated with the different
concentrations of acetic acid, NaCl, CaCl2 soaking solutions had darker colors with L*
ranging from 21.6 to 32.9 and h* from 306° to 329°. Potato slices pre-soaked in 1% and
2% citric acid solutions had lighter color with average values of 39.1 in lightness and
317.9°in hue angle. Color of purple potato extract at pH 3 to 10 are presented in Figure 8
to help perceive the color based on CIELab data. Significant differences between chips
soaked in 1%, 2% citric acid and other solutions were identified in lightness (p ≤ 0.003)
while there was no significant differences in hue angles (p ≥ 0.772). Chips colors of most
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of all samples were darkened mainly as a result of melanoidins, brown-colored
compounds produced by Maillard reaction occurred under high temperature (Kita et al.
2015; Hofmann 1998). Besides that, discoloration due to enzymatic browning could be
prevented by dipping peeled potatoes in citric acid solution with low pH to inactivate the
polyphenoloxidase (Sapers & Miller 1995).
Table 2. Color characteristics of raw purple-fleshed potato and purple potato chips with 1% and 2% citric acid pretreatments using CIELab color
Sample Time (week) L* a* b* L* C* h*(°) ∆E*
Raw purple potato 0 36.2
(2.9) 7.7
(1.1) -4.3 (0.6)
36.2 (2.9)
8.8 (1.2)
330.7 (2.0) ---
Chips soaked in 1% citric
acid
0 38.8 (0.4)
3.3 (0.3)
-3.0 (0.1)
38.8 (0.4)
4.5 (0.2)
317.9 (3.6) ---
2 39.3 (0.7)
3.7 (0.1)
-4.5 (0.3)
39.3 (0.7)
5.9 (0.2)
309.7 (1.2)
1.8 (0.5)
4 39.3 (0.8)
3.7 (0.3)
-3.8 (0.3)
39.3 (0.8)
5.3 (0.4)
314.4 (1.8)
1.4 (0.2)
6 39.3 (0.3)
3.7 (0.2)
-4.0 (0.3)
39.3 (0.3)
5.5 (0.3)
312.7 (2.0)
1.3 (0.4)
8 40.5 (0.8)
4.1 (0.4)
-4.2 (0.4)
40.5 (0.8)
5.8 (0.5)
314.1 (1.4)
2.3 (0.6)
10 40.3 (0.2)
4.1 (0.3)
-4.9 (0.2)
40.3 (0.2)
6.4 (0.3)
310.1 (1.6)
2.5 (0.3)
Chips soaked in 2% citric
acid
0 39.3 (1.5)
4.2 (0.3)
-4.3 (0.5)
39.3 (1.5)
6.0 (0.5)
314.3 (1.8) ---
2 39.6 (0.3)
4.7 (0.2)
-4.5 (0.3)
39.6 (0.3)
6.5 (0.3)
316.3 (1.5)
1.4 (0.5)
4 39.3 (0.3)
4.3 (0.2)
-4.4 (0.5)
39.3 (0.3)
6.1 (0.5)
314.8 (2.6)
1.4 (0.3)
6 39.4 (0.5)
4.3 (0.3)
-3.5 (0.2)
39.4 (0.5)
5.6 (0.3)
321.3 (1.0)
1.7 (0.5)
8 39.9 (0.3)
4.4 (0.2)
-4.1 (0.3)
39.9 (0.3)
6.0 (0.4)
317.4 (1.4)
1.4 (0.9)
10 39.1 (0.2)
4.3 (0.3)
-4.0 (0.5)
39.1 (0.2)
5.9 (0.5)
317.3 (2.2)
1.4 (0.4)
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Figure 7. Lightness (L*) and hue angle (h*) of purple potato chips pretreated with different soaking solutions. Columns with the same letters are not significantly different from each other at p > 0.05; columns with different letters are significantly different from each other at p < 0.05.
Color stability of chips pretreated with 1% and 2% citric acid during the storage was
completed in the 10-week study, and CIELab color characteristics were measured every
two weeks and presented in Table 2. Color data from week 0 (one day after frying) were
Figure 8. Color of purple potato extract at pH 3 to 10
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considered as the control for 1% and 2% citric acid-treated chips separately. In 1% citric
acid-treated chips, average color values were maintained almost unchanged, with
lightness (L*) ~ 40, hue angle (h*) ~ 313°, a* ~ 3.8 and b* ~ -4.0. As a comparison, color
data from 2% citric acid-treated chips were measured as of L* 39.4, h* 316.9º, a* 4.4
and b* -4.1 in average. As the study progressed, significant differences of blueness (-b*)
and color intensity (C*) were found in chips made with 1% citric acid (p ≤ 0.004) while
lightness (L*) and redness (a*) started to show significance of difference since week 8 (p
≤0.006). In chips soaked in 2% citric acid solutions, there was no significant change in all
color parameters (L*, a*, b*, C*, h*) during the 10-week storage (p ≥ 0.109). Total color
difference (△E) was included in the color scale to quantify the overall difference of one
sample and one standard on the three dimensional color space (△L*, △a*, △b*) of
CIELab. Nearly all △E values in color of both pre-treated purple potato chips were
below 2, indicating that there were very small difference identified; however, color
difference from week 8 and 10 of 1% citric acid-treated chips were above 2 but below 3.5,
indicating a medium difference in total color. △E value less than 2 is expected to be only
noticed by trained eyes, and the color changes of purple potato chips during storage are
not obvious and not able to be detected by consumers, especially the chips treated with 2%
citric acid.
4.2 Analytical Testing
Reducing sugars, together with amino acids, are the major reactants of the Maillard
browning reaction, and the reduction in the amount of reducing sugar has been used as a
means to inhibit or limit the extent of this reaction (Márquez & Añón 1986). Sugars in
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potatoes consist of glucose and fructose as free reducing sugars, as well as sucrose (Zhu
et al 2010). Reducing sugar content of raw purple potatoes was 3.38±0.03 g/100g in dry
weight (DW); reducing sugar in potato slices washed but without soaking (Control 1) and
soaked in distilled water (Control 2) decreased, with values of 2.63±0.03 g and 2.06±0.02
g per 100 g (DW), respectively. Compared to Control 1 and Control 2, the amount of
reducing sugar of purple potato slices soaked in citric acid, acetic acid and NaCl, were all
significantly reduced (p ≤ 0.005), while no significant changes were identified in
reducing sugar content of samples treated with CaCl2 solutions (p = 0.641). Citric acid
solutions decreased the reducing sugar content of the potato slices the most, followed by
NaCl and acetic acid solutions (Figure 9).
Figure 9. Content of reducing sugar in potato slices pretreated with soaking solutions. Columns with the same letters are not significantly different from each other at p > 0.05; columns with different letters are significantly different from each other at p < 0.05.
43
Soaking in lower pH solutions such as citric acid would not only help lower the pH of
potatoes but also leach out reducing sugars and asparagine from the surface layer of
potato slices (Jung et al. 2003). In this study, soaking in distilled water did not induce pH
reduction in potato slices; however, soaking solutions of citric acid and acetic acid
possessed higher acidity values, with pH between 2 and 3, which helped with the removal
of reducing sugar content and explained the larger reduction of reducing sugar content
during soaking.
Once potato slices were soaked in NaCl solutions, concentration differences formed
between cell membranes of potato tissues and hypertonic NaCl solutions. As previously
reported (Bunger et al. 2003; Califano & Calvelo 1988), soaking potato strips into NaCl
solution prior to frying could favor osmotic dehydration to help with the removal of
solutes (organic acids, reducing sugars etc.) present in potato tissues by using water as the
carrier, which removed the precursors of the Maillard reaction in potatoes.
By contrast with other soaking pretreatments, CaCl2 solutions did not significantly lower
the reducing sugar amounts in the soaked potato slices. This could be explained because
adjacent chains of pectin polymers can be bridged by Ca2+ and the formation of cross
links between pectin polymers in potato tissues could have trapped the sugars in (Ralet et
al. 2001; Thakur et al. 2009; Andersson 1994).
The pH values of pretreatment solutions along with purple potatoes were measured
before and after soaking process (Table 3). The pH of purple potatoes was around 6.4,
and a pH value of 5.8 – 6.0 was found in distilled water. Citric acid and acetic acid made
soaking solutions below pH 4, especially approximately pH 2 in 1% and 2% citric acid
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solutions. Soaking solutions of NaCl and CaCl2 had pH values around 6, closer to the pH
level of purple potatoes. The soaking process slightly increased the pH of each solution.
Table 3. Changes in pH of pretreatment solutions after soaking purple potato slices
Soaking solutions pH of soaking solutions with different concentrations
Control 1 No soaking 6.3-6.5
Control 2 before 5.8-6.0 after 6.3-6.4
0.1% 0.2% 1% 2%
Citric acid before 2.7-2.9 2.5-2.7 2.1-2.3 1.9-2.1 after 2.9-3.0 2.7-2.8 2.2-2.3 2.0-2.1
Acetic acid before 3.2-3.3 3.0-3.2 2.7-2.8 2.5-2.7 after 3.7-3.8 3.4-3.6 2.9-3.0 2.8-2.9
NaCl before 5.7-5.9 5.6-5.9 5.6-5.7 5.8-6.1 after 6.0-6.1 6.0-6.1 5.8-6.0 5.8-5.9
CaCl2 before 5.7-5.9 5.8-5.9 5.9-6.0 6.1-6.2 after 5.9-6.0 5.8-5.9 5.7-5.8 5.5-5.6
Average of monomeric, polymeric anthocyanins and total phenolics content in raw purple
potatoes and chips are presented in Table 4. Content of monomeric anthocyanins and
total phenolics was expressed as mg of Cyanidin-3-glucoside equivalents/100g (mg Cy-3-
gluc eq./100g) and mg of gallic acid equivalents/100g (mg GAE/100g), respectively.
Freeze-dried purple potato powder was determined to contain 150.6±1.8 mg Cy-3-glu
eq./100g monomeric anthocyanins and 634.2±3.9 mg GAE/100g total phenolics with
11.6% polymeric color. Anthocyanins concentration in some purple-fleshed tubers was
estimated and reported as 5.5 – 17.1 mg/100g FW by Brown et al. (2003), while Kita et al.
(2015) reported a range from 37.20-57.18 mg/100g DW in some purple flesh potato
tubers. Nemś et al. (2015) stated that the amount of total anthocyanins in different raw
purple potato varieties ranged from 38.7-93.7 mg/100g DW. Total polyphenols content in
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some purple-fleshed potatoes was quantified as 455-481 mg GAE /100g dry weight
(Lachman et al. 2008). The content of mono- and polymeric anthocyanins differed among
publications due to different cultivars and harvest time of purple potatoes.
Compared to raw potatoes, the contents of monomeric anthocyanins and total phenolics
in all chips samples decreased significantly (p ~ 0.000) while the percentage of polymeric
color increased to a great extent. A loss of 57.7-79.7% monomeric anthocyanins and
12.5-36.8% loss of polymeric anthocyanins were found in all purple potato chips samples.
Loss of total phenolics was found up to 64% in potato chips made with some purple-
fleshed cultivars (Kita et al. 2015).
High temperature is critical in the chips production to achieve dehydration and final
product with appealing color and crunchy texture; however, high temperatures can also
be detrimental to pigments in purple potatoes during the chips frying because of the
thermal sensitivity of anthocyanins, constituting a primary caused of degradation
(Wrolstad 2000; Kita et al. 2015). Similarly, high frying temperature could also degrade
polyphenols and transform polyphenols of different groups (Bąkowska et al. 2003). In
addition, enzymes like polyphenoloxidase (PPO) contained in potato slices soaked in
non-acidic solutions (NaCl, CaCl2) were not inactivated during the sample preparation,
which would also contribute to the loss of phenolics.
However, purple potato chips samples pretreated with 1% and 2% citric acid showed
better color quality and retention of phenolics after frying. Higher monomeric
anthocyanin (50.3-63.7 vs 30.6-44.6 mg Cy-3-glu eq/100g) and less polymeric color
(25.4-35.7% vs 51.4-69.4%) were also found in 1% and 2% citric acid samples compared
with other chips, as well as lightly more total phenolics (538.5-554.7 vs 400.7-519.1 mg
46
GAE/100g). Significant differences in mono-, polymeric anthocyanins, and total
phenolics were identified with p ≤ 0.015. The 1% and 2% citric acid soaking solutions
had pH values around 2, in which anthocyanins are mostly present as the flavylium cation
form, showing bright red color with the most stability (Jackman et al. 1987; Giusti &
Wrolstad 2001). pH value is an important factor in Maillard browning, and pH reduction
would inhibit the reaction by blocking the addition of asparagine with a carbonyl
compound and preventing the formation of a key intermediate in the Maillard reaction,
the Schiff base (Kita et al. 2004; Rydberg et al. 2003). Soaking potato slices in acidic
solutions like citric acid and acetic acid, is able to not only lower the non-enzymatic
browning by reducing pH of potatoes and leaching out reducing sugars, but also inhibit
the enzymatic browning by inactivating polyphenoloxidases (Mestdagh et al. 2008b;
Sapers & Miller 1995; Jung et al. 2003). Polyphenols in purple potatoes were protected
from enzymatic reaction due to the inactivation by low pH of 1% and 2% citric acid
soaking solutions. Thus, the acidity of 1% and 2% citric acid soaking stabilized
anthocyanin in potatoes, resulting in brighter purple-reddish chips with (Table 3, Figure
8).
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Table 4. Average of monomeric, polymeric anthocyanins and total phenolics content in dried raw purple potatoes and chips. Monomeric anthocyanin expressed as mg of cy-3-glu equivalents / 100 grams, total phenolics expressed as mg of gallic acid equivalents /
100 grams.
Monomeric
anthocyanins (mg/100g)
Polymeric color (%)
Total phenolics (mg/100g)
Raw purple potato (dry basis) 150.6 (1.8) a 11.6 (2.9) a 634.2 (3.9) a Control 1 No soaking 36.9 (0.3) b 64.6 (2.8) b 400.7 (4.1) b Control 2 Distilled water 36.8 (1.5) b 51.4 (0.6) c 432.5 (4.5) c
Citric acid
0.1% 40.8 (0.7) c 53.8 (4.4) c 489.8 (3.8) d 0.2% 38.3 (0.8) c 51.9 (5.1) c 519.1 (5.0) e 1% 50.3 (1.7) d 35.7 (2.6) d 538.5 (4.8) f 2% 63.7 (1.0) e 25.4 (1.3) e 554.7 (4.2) g
Acetic acid 0.1% 37.2 (0.4) b 60.8 (2.9) b 443.6 (3.0) h 0.2% 40.8 (0.5) c 63.8 (4.4) b 415.7 (3.3) i 1% 39.6 (0.6) c 66.5 (0.8) b 449.4 (4.3) h 2% 40.3 (0.7) c 63.2 (4.3) b 469.5 (3.9) j
NaCl 0.1% 42.7 (1.8) f 67.1 (2.4) b 423.4 (5.1) c 0.2% 40.3 (0.9) c 62.1 (3.5) b 463.7 (3.5) j 1% 41.1 (0.5) c 65.1 (2.6) b 435.7 (2.9) c 2% 44.6 (1.5) f 62.1 (3.7) b 510.1 (4.7) e
CaCl2 0.1% 30.6 (0.4) g 69.4 (4.6) b 420.6 (3.9) i 0.2% 37.4 (0.9) b 64.0 (1.8) b 439.1 (3.7) c 1% 36.4 (1.4) b 66.1 (3.1) b 417.6 (4.1) i 2% 37.8 (1.8) b 68.1 (1.7) b 445.1 (4.9) h
Within each column, data with the same letters are not significantly different from each other at p > 0.05 while data with different letters are significantly different from each other at p < 0.05.
Stability tests were carried out over 10 weeks of storage, and measurements were
completed every two weeks. In chips samples with 1% citric acid pretreatment,
monomeric anthocyanin content decreased from 50.3 to 29.0 Cy-3-glu eq. mg/100g, and
polymeric anthocyanins percentage increased from 22.5% to 36.3% during the first 8
weeks of storage (Figure 10), likely due to the polymerization of monomeric
anthocyanins. After week 8, the amount of monomeric anthocyanins increased to 38.6
48
Cy-3-glu eq. mg/100g while percent polymeric color fell down to 22.4% at the same time
as the polymers started to precipitate due to lower solubility. Compared with the fresh
chips, stored samples did not show significant difference in the monomeric anthocyanins
change until week 6 (p ~ 0.000). Similar trends presented on 2% citric acid-treated chips
(monomeric anthocyanins: from 63.7 to 36.7 to 42.9 Cy-3-glu eq. mg/100g; percent
polymeric color: from 18.2% to 33.4% to 22.4%).
Figure 10. Changes in monomeric anthocyanin content and percent polymeric color of purple potato chips during storage. Columns with the same letters are not significantly different from each other at p > 0.05; columns with different letters are significantly different from each other at p < 0.05.
49
Total phenolics of purple potato chips were also monitored over the 10-week storage
period. Changes in the phenolic content of chips were not largely affected as mono- and
polymeric anthocyanins, and it remained around 520 GAE mg/100g DW.
4.3 Anthocyanins in Raw Purple Potatoes and Potato Chips
The major anthocyanin (peak 2) in both, extracts of purple raw potatoes and purple chips,
eluted in approximately 15.2 minutes and were identified as the same compound,
Figure 11. Anthocyanins from purple potato and purple potato chips extracts
50
This indicated that the type of major anthocyanin in raw purple potatoes did not change
during the production of chips. Other anthocyanins were also identified, including
Petunidin-3-(caffeoyl)-rutinoside-5-glucoside (peak 1) and Malvidin-3-(p-coumaroyl)-
rutinoside-5-glucoside (peak 3). Petunidin-3-(p-coumaroyl)-rutinoside-5-glucoside, as the
dominant anthocyanin found in the Purple Majesty, were also reported by other
researchers (Stushnoff et al. 2008; Nayak et al. 2011). In addition to Petunidin-3-(p-
coumaroyl)-rutinoside-5-glucoside, minor anthocyanins like Petunidin-3-(caffeoyl)-
rutinoside-5-glucoside and Malvidin-3-(p-coumaroyl)-rutinoside-5-glucoside were
tentatively identified, which was in agreement with the identification of predominant
pigment in Purple Majesty cultivar by Li et al. (2012).
51
Chapter 5 Conclusion
Soaking purple potato slices in selected solutions as pretreatments before frying had an
effect on the color quality and phenolic content of purple potato chips. However, the
results obtained differed depending on the type and concentration of the chemicals used
for soaking. Compared to other solutions (acetic acid, NaCl, CaCl2), soaking purple
potato slices in citric acid solutions of 1% and 2% resulted in chips of appealing brighter
purple-reddish color and higher retention of monomeric anthocyanins and total phenolics.
Citric acid solutions of 1% and 2% had acidity around pH 2, which not only helped lower
the pH level of soaked purple potato slices but also suppress the Maillard reaction and
enzymatic browning, resulting in less brown color development and brighter purple-
reddish chip color. In the meantime, soaking potato slices in 1% and 2% citric acid also
made chips of higher monomeric anthocyanins and total phenolics as well as less
polymeric color due to the inhibition of both enzymatic and non-enzymatic reaction. The
frying process influenced the color appearance and phenolics content of purple potato
chips, but the type of major anthocyanin in purple potatoes did not change, which was
identified as Petunidin-3-(p-coumaroyl)-rutinoside-5-glucoside. Chips made with 2%
citric acid solutions showed better color stability with no significance (p ≥ 0.109) of color
changes during the storage test. Long storage period of chips would accumulate the
52
polymerization of anthocyanins from fresh purple potato chips. Total phenolic content in
chips were not largely changed as much as anthocyanins.
Citric acid’s ability to function as an acidulant resulted in greater benefit as compared to
other pretreatment chemicals tested. Soaking potato slices in citric acid at certain
concentrations could be used as pretreatment methods to enhance the color and phenolic
quality of fried purple potato chips, which could bring potential benefits to the potato
chips industry.
53
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