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Composition of phenolic and antioxidant activity of water chestnut peel during digestion in vitro as affected by blanching time

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Abstract

Water chestnut peels have good antioxidant activity. The effects of simulated gastric fluid (SGF) and intestinal fluid (SIF) fluid digestion in vitro on the active substance and antioxidant activity of water chestnut peels that were pre-treated using different boiling times (P10 and P30) were investigated. The results showed that the SGF obviously increased both the total phenolic content and total flavonoid content of water chestnut peels. The SGF digestion significantly enhanced both the FRAP and ABTS antioxidant capacity of water chestnut peels only blanched with P10. However, the SIF digestion significantly increased the FRAP and ABTS antioxidant capacity of all water chestnut peels regardless of whether these were pre-treated or not. The HPLC results showed that the simulated digestion in vitro enhanced the flavonoids content of the peels. Water chestnut peels could be used as an inexpensive source of natural functional food ingredients.
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International Journal of Food Properties
ISSN: 1094-2912 (Print) 1532-2386 (Online) Journal homepage: https://www.tandfonline.com/loi/ljfp20
Composition of phenolic and antioxidant activity
of water chestnut peel during digestion in vitro as
affected by blanching time
Yang Yuan, Jie Li, Shan He, Qingzhu Zeng, Lihong Dong, Ruifen Zhang,
Dongxiao Su & Mingwei Zhang
To cite this article: Yang Yuan, Jie Li, Shan He, Qingzhu Zeng, Lihong Dong, Ruifen Zhang,
Dongxiao Su & Mingwei Zhang (2019) Composition of phenolic and antioxidant activity of water
chestnut peel during digestion in�vitro as affected by blanching time, International Journal of Food
Properties, 22:1, 71-83, DOI: 10.1080/10942912.2019.1573255
To link to this article: https://doi.org/10.1080/10942912.2019.1573255
© 2019 Yang Yuan, Jie Li, Shan He, Qingzhu
Zeng, Lihong Dong, Ruifen Zhang, Dongxiao
Su, and Mingwei Zhang. Published with
license by Taylor & Francis.
Published online: 07 Feb 2019.
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Composition of phenolic and antioxidant activity of water
chestnut peel during digestion in vitro as affected by blanching
time
Yang Yuan
#
a
, Jie Li
#
b,c
, Shan He
a
, Qingzhu Zeng
a
, Lihong Dong
b
, Ruifen Zhang
b
,
Dongxiao Su
a
, and Mingwei Zhang
b
a
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, P.R. China;
b
Guangdong Key
Laboratory of Agricultural Products Processing, Sericultural & Agri-Food Research Institute, Guangdong Academy of
Agricultural Sciences, Guangzhou, P.R. China;
c
College of Food Science, Fujian Agriculture and Forestry University,
Fuzhou, P.R. China
ABSTRACT
Water chestnut peels have good antioxidant activity. The effects of simu-
lated gastric fluid (SGF) and intestinal fluid (SIF) fluid digestion in vitro on
the active substance and antioxidant activity of water chestnut peels that
were pre-treated using different boiling times (P10 and P30) were investi-
gated. The results showed that the SGF obviously increased both the total
phenolic content and total flavonoid content of water chestnut peels. The
SGF digestion significantly enhanced both the FRAP and ABTS antioxidant
capacity of water chestnut peels only blanched with P10. However, the SIF
digestion significantly increased the FRAP and ABTS antioxidant capacity of
all water chestnut peels regardless of whether these were pre-treated or
not. The HPLC results showed that the simulated digestion in vitro
enhanced the flavonoids content of the peels. Water chestnut peels could
be used as an inexpensive source of natural functional food ingredients.
ARTICLE HISTORY
Received 6 September 2018
Revised 3 January 2019
Accepted 15 January 2019
KEYWORDS
Water chestnut; simulated
digestion; phenolics; HPLC;
antioxidant activity
Introduction
Water chestnut, which is also known as Eleocharis dulcis, belongs to the sedge family and grows in
wet farmlands or pool districts. Furthermore, it is a subterranean bulbous of sedum.
[1]
Water
chestnut, which is native to China and India, is cultivated primarily in the southern Yangtze River
provinces of China.
[2]
It has a long history of cultivation in China and is widely planted.
[3]
The
Chinese water chestnut peels are by-products that constitute approximately 20% (w/w) of the whole
fruit, which were often discarded, resulting in wasted resources and environmental pollution.
[4]
Water chestnut peels are rich in brown pigments, which are an excellent water-soluble natural food
coloring, and its main components are flavonoids (such as flavonoids, flavonols, flavonoids).
[5]
It
was reported that the extracts of Eleocharis dulcis peels showed strong bactericidal and antioxidant
activities.
[5,6]
Organic solvent extraction is usually carried out to determine the content of phenolics and
flavonoids of fruit extracts as well as their antioxidant activity.
[7,8]
However, the digestion fluid is
significantly different from organic solvents. The digestion of fruit phenolics in the gastrointestinal
tract is quite complex as it is affected by digestive enzymes, pH, inorganic salts and other physio-
logical factors.
[911]
It may interact with pepsin or trypsin to form complexes that can change their
CONTACT Dongxiao Su dongxsu@126.com School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou
510006, P.R. China; Mingwei Zhang mwzhh@vip.tom.com Sericultural & Agri-Food Research Institute, Guangdong Academy of
Agricultural Sciences, Guangzhou 510610, P.R. China
#
Yang Yuan and Jie Li should be considered joint first author.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ljfp.
© 2019 Yang Yuan,Jie Li, Shan He, Qingzhu Zeng, Lihong Dong, Ruifen Zhang, DongxiaoSu, and Mingwei Zhang.Published with license by Taylor & Francis.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by/4.0/), which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
INTERNATIONAL JOURNAL OF FOOD PROPERTIES
2019, VOL. 22, NO. 1, 7183
https://doi.org/10.1080/10942912.2019.1573255
molecular structural features, functions and nutritional properties, which can increase or decrease
their antioxidant activity.
[12]
Compared with the method of traditional chemical extraction, the
method of gastrointestinal digestion in vitro to evaluate the biological activity of fruit phenolics can
better reflect the actual physiological digestion.
A previous study had confirmed that the water chestnut peel has good antioxidant activity.
[6]
However, there has not been a report on the rules determining the release of phenolic substances in
water chestnut peels after gastrointestinal digestion. Herein, the present study aimed to explore the
release of phenolic substances from the water chestnut peels that were pre-treated using boiling
water after being digested with simulated gastric or intestinal fluid in vitro as well as the changes in
the antioxidant activity of FRAP and ABTS.
Materials and methods
Chemicals and reagents
Water chestnut was purchased from local agricultural product markets and washed with deionized
water to remove the dirt prior to being peeled by hand. After this, the peels were collected and
washed with deionized water again. The water chestnut peels were divided into three groups: the first
group was dried with hot air drying (P0+ HAD) (DHG9023A, Shanghai Jinghong Laboratory
Instrument Co., Ltd, China); the second group was dried with hot air after a blanching treatment
at 95°C (P10+ HAD) for 10 min; and the third group was dried with hot air after a blanching
treatment at 95°C (P30+ HAD) for 30 min. All samples were dried at 60°C for 24 h. After this, the
dried samples were ground by a universal grinder (FW100, Tianjing Hengrui Science & Teaching
Instrument Co., Ltd) and filtered over 80 mesh sieves, before being packed into a sealed bag and
stored at 20°C in a refrigerator for further analysis.
Pepsin, trypsin, gallic acid, catechin, and Trolox (6-hydroxy-2,5,7,8-tetramethy lchroman-2-car-
boxylic acid) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Ferrisulfas (GR), Foline-
phenol, ABTS (2,2ʹ-Azinobis-(3-ethylbenzthiazoline-6-sulfonate)) and TPTZ (2,4,6-tripyridyl-s-triazine)
were obtained from Xiya Reagent Company (Chendu, China). All other reagents were of analytical grade
and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Simulated digestion in vitro
The simulated gastric fluid treatment and the simulated intestinal fluid digestion followed the
method previously described by Fu et al.
[13]
, with slight modifications. For the simulated gastric
fluid, pepsin (1.60 g) and sodium chloride (1.00 g) (BR, 3000 USP u/mg activity units, Yuanye
biotechnology Co., Ltd, Shanghai, China) were placed into a beaker before 475 mL of double distilled
water and 3.5 mL of concentrated hydrochloric acid were added to the beaker. Following this, the pH
was adjusted to 1.2 using concentrated hydrochloric acid. After this, the fluid was transferred into
a 500-mL volumetric flask and double distilled water was added to reach a volume of 500 mL. For
the samples treated by the simulated gastric fluid, three water chestnut peel samples that underwent
different pre-treatments all weighed 1.0 g and were placed into three centrifuge tubes. After this, the
overnight prepared simulated gastric fluid was added to the three centrifuge tubes at a ratio of 1:15
(g/mL). The samples were subsequently put into a desktop thermostat oscillator (IS-RDD3,
Shanghai, China) at 37°C with a speed of 120 rpm/min for 60 min. After that, the samples were
centrifuged at 5000 rpm/min for 10 min, before the supernatant was collected into a 1.5-mL
centrifuge tube and placed in a refrigerator at 20°C.
For the simulated intestinal fluid, 3.4 g of potassium dihydrogen phosphate was dissolved in 125 mL
of deionized water in a beaker. After this, 95 mL of 0.2 mol/L NaOH and 200 mL of deionized water
were added in before the mixtures were shaken evenly. A total of 5.00 g of trypsin (BR, 4000 USPu/mg
activity units, Yuanye biotechnology Co., Ltd, Shanghai, China) was subsequently added in. The pH of
72 Y. YUAN ET AL.
the solution was adjusted to 7.5 ± 0.1 using NaOH or HCl. Finally, the fluid was transferred into a 500-
mL volumetric flask and double steamed water was added to reach a volume of 500 mL. For the
samples treated by simulated intestinal fluid, the intestinal fluid was added to 1.0 g of three water
chestnut peel samples that underwent different pre-treatments in centrifuge tubes at a ratio of 1:15 (g/
mL). After this, the sample was mixed thoroughly before being put into a desktop thermostat oscillator
at 37°C with a speed of 120 rpm/min for 120 min. After that, the samples were centrifuged at
5000 rpm/min for 10 min, before the supernatant was collected into a 1.5-mL centrifuge tube and
placed in a refrigerator at 20°C.
Deionized water extraction
Deionized water was added to 1.0 g of three water chestnut peel samples that underwent
different pre-treatments in centrifuge tubes at a ratio of 1:15 (g/mL). After this, the samples
weremixedthoroughlybeforebeingputintoadesktopthermostatoscillatorat37°Cataspeed
of 120 rpm/min for 120 min. After that, the samples were centrifuged at 5000 rpm/min for
10 min, before the supernatant was collected into a 1.5-mL centrifuge tube and placed in
a refrigerator at 20°C.
Organic solvent extraction
Methanol was added to 1.0 g of three water chestnut peel samples that underwent different pre-
treatments in centrifuge tubes at a ratio of 1:15 (g/mL). After this, the samples were mixed
thoroughly and extracted in an ultrasonic cleaner (KQ100DE, Xinzhi Bioscience Co., Ltd) at 30°C
under an ultrasonic power of 600 W for 15 min, before the supernatant was collected. After that, the
samples were treated as described above for one more time, before the supernatants were merged
with the above, split into a 1.5-mL centrifuge tube and placed in a refrigerator at 20°C.
Total phenolic content
The total phenolic content (TPC) of water chestnut peel samples was determined by the Folin
Ciocalteu (FC) assay following the procedure described by Su et al.
[14]
with slight modifications.
A total of 125 μL of the diluted samples or standards was pipetted into a centrifuge tube, before
0.5 mL of deionized water and 125 μL of FolinCiocalteu reagent were added in. The mixtures were
thoroughly shaken and left to stand for 6 min, avoiding light. Following this, 1.25 mL of the sodium
carbonate solution (m:v = 7%) and 1.00 mL of deionized water were added in sequence, before the
samples were shaken and placed in the dark for 90 min. The absorbance was measured at 760 nm
using a spectrophotometer (TU-1900, Beijing General Analytical Instruments Co., Ltd.). The total
phenolic content was based on the amount of gallic acid and expressed as the gallic acid equivalent
(GAE) per 100 g of water chestnut peel.
Total flavonoid content
The total flavonoid content (TFC) in water chestnut peel samples was determined by aluminum
chloridesodium nitrite colorimetry according to the procedures described by Bouayed et al.
[15]
with
slight modifications. A total of 300 μL of diluted samples or standards were pipetted into a centrifuge
tube before 1.5 mL of deionized water and 90 μL of 5% sodium nitrite solution were added in. After
that, the mixtures were thoroughly shaken and left to stand for 6 min. Following this, 180 μL of the
sodium carbonate solution (m:v = 10%) was added in sequence, before the samples were shaken and
left to stand for 5 min. After that, 0.6 mL of 1 mol/L sodium hydroxide solution and deionized water
were added to make up a volume of 3.00 mL. Finally, the absorbance value of the samples was
INTERNATIONAL JOURNAL OF FOOD PROPERTIES 73
measured with a UV-visible spectrophotometer at 510 nm. The total flavonoid content was based on
the amount of catechin and expressed as catechin equivalent (CE) per 100 g of water chestnut peel.
Antioxidant capacity
The ferric reducing antioxidant power (FRAP) assay was performed according to the method described
by Thaipong et al.
[16]
with slight modifications. For the preparation of the FRAP working fluid,
10 mmol/L TPTZ, 20 mmol/L hexahydrate ferric chloride and 300 mmol/L sodium acetate buffer
solution were mixed thoroughly at a ratio of 1:1:10 and placed in a 37°C water bath. After that, 0.3 mL of
the samples or standards was added to 2.7 mL of the FRAP working liquid and thoroughly shaken,
before the sample was stored in a dark environment for 30 min. Finally, the absorbance value of the
samples was measured with a UV-visible spectrophotometer at 593 nm. The FRAP antioxidant capacity
value of water chestnut peel samples that underwent different pre-treatments was expressed in terms of
ferrous ion equivalent per gram of water chestnut peel (mg FeE/g).
The ABTS
+
·radical scavenging capacity assay was determined by the method reported by
Thaipong, Boonprakob
[16]
with slight modifications. For the preparation of the ABTS working
fluid, 2.6 mmol/L high potassium sulfate solution and 7.4 mmol/L ABTS solution were mixed
thoroughly at a ratio of 1:1 and placed in the dark at room temperature for 12 h. The mixture
was diluted by deionized water until the absorbance of ABTS working fluid was 0.7 ± 0.02 at 734 nm.
After that, 2.4 mL of the above-mentioned working fluid was added to 0.6 mL of water chestnut peel
samples or standards (Trolox). The mixtures were shaken evenly and left to stand for 6 min before
the absorbance was measured at 734 nm. The ABTS antioxidant activity values were expressed as
micromole Trolox equivalents (TE) per g water chestnut peel.
Flavonoid composition by HPLC
The phenolic substances in the water chestnut peels were determined using the HPLCDAD method
previously described by Su, Li.
[14]
HPLCDAD analysis was performed on a ZorboxSB-C18 column at
30°C with a flow rate of 1.0 mL/min, the injection volume of 20 uL and detection wavelength of 280 nm.
The mobile phases were 10% glacial acetic acid in ultrapure water (A) and acetonitrile (B). The binary
gradient elution program was as follows: 012 min, 9575% A; 1217 min, 75% A; 1720 min, 7550% A;
2030 min, 5025% A; 3035 min, 255% A; and 3545 min, 595% A,followed by a 5-min equilibration
period with 95% A. The chromatographic peak preliminary identities were determined based on the
retention time of the standard compounds (catechin, epicatechin, and catechin gallate) and qualitatively
analyzed by the peak area. The results were expressed in terms of mg/g of water chestnut peel.
Statistical analysis
All the measurements of each indicator were repeated 3 times and the results were expressed as the
mean ± standard deviation. SPSS 24.0 statistical software was used for the single factor analysis of
variance, while SNK was used to identify differences. The significance level was p< 0.05, while the
significant differences among different treatments were expressed with different letters.
Results
Effects of simulated digestion in vitro on total phenolic content in water chestnut peels that
underwent different pre-treatments
The total phenolic contents in water chestnut peels that underwent different pre-treatments were
significantly affected by the simulated digestion treatment, which is shown in Figure 1. The result of
the Met group showed that the total phenolic contents significantly increased (17.01%) after short-
74 Y. YUAN ET AL.
time heat treatment (10 min). However, prolonged heat treatment (30 min) would cause the
decomposition of heat-sensitive substances and the total phenolic content was significantly
(p< 0.05) reduced (24.43%) compared to P0+ HAD. Furthermore, the total phenolic content of
the water chestnut peels in the methanol extracts was significantly higher than that in deionized
water (DW). The total phenolic contents in the water chestnut peels under different pre-treatment
times had a similar change trend in the extraction of deionized water and simulated gastric or
intestinal fluid, which were significantly increased only in the shortened pre-heat treatment (10 min)
compared to P0+ HAD. The total phenolic content of water chestnut peels could not be increased
with increases in the pre-treatment time in all deionized water extractions and simulated gastric or
intestinal digestion. The P0+ HAD and P30+ HAD treatments had a similar TPC (p> 0.05) in the
above three groups. It has been found that there was a significant difference (p< 0.05) among the
simulated gastric fluid group, the simulated intestinal fluid group and the deionized water treatment
group after further analysis of the effects of simulated digestion treatment on the total phenolic
content of the water chestnut peels under the same treatment. The order of TPC was: SIF > SGF >
DW, which showed that the total phenolic content of water chestnut peels was increased after the
gastric fluid digestion (increased 33.70% for P10+ HAD) and intestinal digestion treatment
(increased 70.72% for P10+ HAD) compared to deionized water treatment. The effect of simulated
intestinal fluid digestion was greater than the effect of simulated gastric fluid digestion. These added
phenolic substances might be the phenolic compounds that are covalently or noncovalently bound to
pepsin or trypsin-hydrolyzed proteins.
Effects of simulated digestion in vitro on total flavonoid content of water chestnut peels that
underwent different pre-treatments
The effect of simulated gastric fluid and simulatedintestinal fluid on the total flavonoid content
of in water chestnut peels that underwent different pre-treatments is shown in Figure 2.The
results showed that the TFC variation trends inwaterchestnutpeelsunderdifferentheat
Figure 1. Effects of simulated digestion on the total phenolic content of water chestnut peels that underwent different pre-
treatments (mean ± SD, n = 3). Bars with no letters in common within one extraction method are significantly different (p< 0.05).
DW: distilled water extraction; SGF: simulated gastric fluid extraction; SIF simulated intestinal fluid extraction.
INTERNATIONAL JOURNAL OF FOOD PROPERTIES 75
treatment times were similar to those of TPC. The total flavonoid content of P10+ HAD was the
highest, followed by P0+ HAD and P30+ HAD in all treatments. The TFC of water chestnut
peels in the methanol solution was significantly higher than that in the corresponding samples of
deionized water, gastric fluid, and intestinal fluid digestion groups (p< 0.05). The effects of
simulatedgastricfluiddigestionandsimulated intestinal fluid digestion on the TFC were
different to those on TPC. The TFC of P10+ HAD after simulated gastric fluid treatment
(7.33 mg RE/g) was significantly higher than that after simulated intestinal fluid digestion
(6.35 mg RE/g) and deionized water (6.26 mg RE/g) extraction (p<0.05).Therewasno
significant difference of the TFC in the water chestnut peels that underwent P0+ HAD and
P30+ HAD treatments. The above results indicated that the TFC of water chestnut peels, which
underwent different pre-treatments, was increased by SGF treatment, while there was no
significant change in TFC after simulated intestinalfluiddigestion.Thisshowedthatthe
flavonoids were mainly released after the simulated gastric fluid digestion.
Effects of simulated digestion in vitro on the FRAP antioxidant activity of water chestnut
peels that underwent different pre-treatments
The effect of different digestion treatments on the FRAP antioxidant capacity of water chestnut peels
that underwent different pre-treatments is shown in Figure 3. The FRAP antioxidant activity of
water chestnut peels treated with P10+ HAD could be significantly improved after solvent extraction
and simulated digestion treatment. Among all groups, the FRAP antioxidant activity of water
chestnut peels, blanched for different times after organic solvent (methanol) extraction, was higher
than other groups. Compared with the deionized water group, there was a significant increase in the
FRAP antioxidant activity in the P10+ HAD sample after simulated gastric fluid digestion (increased
28.06%) treatment (p> 0.05) but no significant increase in P0+ HAD. The FRAP antioxidant
activities of water chestnut peels of all three samples treated with simulated intestinal fluid were
significantly higher than that of the sample treated with deionized water. The FRAP antioxidant
activity of water chestnut peels was in the order of SIF > SGF > DW. The above results showed that
pepsin and trypsin had different effects on the FRAP antioxidant activity of water chestnut peels. The
Figure 2. Effects of simulated digestion on the total flavonoid content of water chestnut peels that underwent different pre-
treatments (mean ± SD, n = 3). Bars with no letters in common within one extraction method are significantly different (p< 0.05).
76 Y. YUAN ET AL.
FRAP antioxidant activity of water chestnut peels blanched for different times was significantly
increased after being treated with simulated intestinal fluid.
Effects of simulated digestion in vitro on the ABTS antioxidant capacity of water chestnut
peels that underwent different pre-treatments
The effect of different digestion treatments on the ABTS antioxidant capacity of water chestnut peels
that underwent different pre-treatments is shown in Figure 4. The ABTS antioxidant capacity of the
water chestnut peels treated for different pre-treatment times was similar to that of the FRAP
antioxidant capacity. We found that the antioxidant activity of water chestnut peels treated with
P10+ HAD was highest after both solvent extraction and simulated digestion compared to P0+ HAD
and P10+ HAD. The antioxidant activity of water chestnut peels in the organic solvent methanol
extraction group was higher than that of other groups. Furthermore, compared with the deionized
water group, the ABTS antioxidant activity of water chestnut peels treated with P10+ HAD was
significantly increased after both simulated gastric fluid digestion (increased 60.94%) and simulated
intestinal fluid digestion (increased 200.82%). The antioxidant capacity of the water chestnut peel
treated with P10+ HAD was higher than that of both the P30+ HAD and P0+ HAD groups. The
ABTS antioxidant capacity of water chestnut peels decreased in the order of SIF > SGF > DW. The
above results indicated that the ABTS antioxidant capacity of water chestnut peels that underwent
different pre-treatments could be significantly improved after simulated intestinal fluid digestion.
Effects of simulated digestion in vitro on the phenolic composition of water chestnut peels
The effect of simulated digestion in vitro on the phenolic composition of the water chestnut peels is
shown in Figure 5. We found that the peak areas of the five peaks (P1, P2, P3, P4, and P5) in the
water chestnut peels changed under the wavelength of 280 nm from the HPLC results. There were
significant changes in the peak area of the samples treated with P10+ HAD after simulated gastric
fluid digestion. We found that P1 peak area was decreased, while the P3, P4, and P5 peak areas were
increased. In particular, the P4 peak area was significantly higher than that of the deionized water
Figure 3. Effects of simulated digestion on the FRAP antioxidant activity of water chestnut peels that underwent different pre-
treatments (mean ± SD, n = 3). Bars with no letters in common within one extraction method are significantly different (p< 0.05).
INTERNATIONAL JOURNAL OF FOOD PROPERTIES 77
Figure 4. Effects of simulated digestion on the ABTS antioxidant capacity of water chestnut peels that underwent different pre-
treatments (mean ± SD, n = 3). Bars with no letters in common within one extraction method are significantly different (p< 0.05).
Figure 5. Effects of simulated digestion in vitro on the phenolic composition of water chestnut peels, determined by HPLC. Bars
with no letters in common within one extraction method are significantly different (p< 0.05).
78 Y. YUAN ET AL.
group. The peak area of the five peaks also changed after simulated intestinal fluid treatment as the
peak areas of P1, P2, and P4 were significantly higher than that of the deionized water group. Peak 4
was identified as catechin after comparison to the standard. The effect of simulated digestion on the
catechin of water chestnut peels that were pre-treated using different boiling times is shown in
Figure 6. The results show that the catechin content of P10+ HAD water chestnut peels was
significantly higher than that of P0+ HAD and P30+ HAD (p< 0.05) in all treatments. The catechin
contents of P10+ HAD water chestnut peels were SIF > SGF > DW. The HPLC results were
consistent with the results of the total phenolics content of P10+ HAD.
Discussion
Foods, such as fruits, vegetables, and grains, are rich in phenolics, which have been reported to have
good antioxidant activity. Positive correlations were found between phenolic contents and antiox-
idant activities.
[17]
The digestibility of food could be increased after heat treatment as it makes active
substances that are more easily eluted but the active substances might be degraded or polymerized
after prolonged heat treatment.
[18,19]
There would be a great change in the content and composition
of the phenolic substances after the foods were digested by the gastrointestinal tract, which directly
affects its biological activity.
[20]
It has been reported that both free phenolics and bound phenolics
existed in food.
[21]
Free phenolics might be polymerized or degraded during heat treatment, while
bound phenolics might be released during digestion. The present study found that the total phenolic
content of water chestnut peels that underwent different pre-treatments could be increased after
simulated gastric fluid digestion or intestinal fluid digestion compared with the extraction of
deionized water. Furthermore, the TPC in the SIF group was higher than that in the SGF group,
which might be related to the type of protein contained in the sample and pancreatic proteolysis.
Pancreatic enzymes hydrolyzed the ester bonds that were formed by the phenolic and proteins inside
and outside the cell, releasing bound phenolic. In addition, it has been reported that the release of
phenolic could be promoted under an alkaline (higher pH) environment so that the TPC was
increased.
[22,23]
The effects of simulated gastric fluid or simulated intestinal fluid digestion on the
TFC of water chestnut peels that were pre-treated using different boiling times were different from
Figure 6. Effects of simulated digestion in vitro on the content of catechin of water chestnut peels, determined by HPLC. Bars with
no letters in common within one extraction method are significantly different (p< 0.05).
INTERNATIONAL JOURNAL OF FOOD PROPERTIES 79
that of TPC. The TFC in water chestnut peels that underwent different pre-treatments could be
increased after simulated gastric fluid digestion, while the TFC in water chestnut peels that under-
went different pre-treatments was not significantly changed after simulated intestinal fluid digestion.
We speculated that the flavonoids of water chestnut peels were mainly released in the stomach
digestion stage. The simulated digestion of apple
[24]
, grape
[25]
, and blueberry
[26]
has been
previously studied. These studies found that the flavonoids-based polyphenols were released during
simulated gastric fluid digestion and most phenolics were stable under simulated gastric fluid
conditions in an acidic (lower pH) environment. Based on this speculation, in the present study,
the reason for the increase in flavonoids in the simulated gastric fluid may be that the phenolics,
which were covalently or non-covalently bound to the protein, are released after the proteins are
hydrolyzed by pepsin. This results in a higher TFC compared to that of the deionized water group.
However, the TFC in the water chestnut peels cannot be significantly altered after simulated
intestinal fluid digestion as flavonoids and proteases could interact to form flavonoidprotease
complexes in addition to hydrolysis. It has been reported that phenolic compounds may covalently
or non-covalently bind to proteases.
[27,28]
This study found that the TPC was also affected by the heat pre-treatment time. The TPC in the
water chestnut peels treated with P10+ HAD was significantly higher than that in HAD or P30+ HAD.
The TFC after simulated gastric fluid digestion in the water chestnut peels treated with P10+ HAD pre-
treatment was significantly higher than that in HAD or P30+ HAD groups, which indicated that the
P10+ HAD treatment could increase the amount of released phenolic from the water chestnut peels,
while lengthening the preprocessing time of P30+ HAD treatment would reduce the phenolic. Heat
treatment time could affect both TPC and TFC. Heating for a short period of time could increase both
TPC and TFC, which might be due to the cell wall of the samples being damaged during this short
time, facilitating the release of bound phenolic being more easily to be released. It was also possible
that the endogenous polyphenol enzyme was inactivated by heating for a short time to prevent the
oxidation loss of the polyphenol material. However, TPC and TFC were reduced after a prolonged heat
treatment time, which might be due to the prolonged heat treatment causing the oxidation or
decomposition of phenolic compounds, which ultimately reduces their content.
[2932]
The flavonoids
composition and content variation in the water chestnut peels of different simulated digestion and
solvent extraction were determined by HPLC. The results showed that the phenolic content of water
chestnut peels could be increased after simulated digestion. Compared with the deionized water group,
the SGF treatment mainly affected P4, which had the highest content. The P4 with the largest peak area
increased, while the P1 and P2 contents increased by SIF digestion. These indicated that simulated
digestion could change the content of phenolic compounds although the types of substances produced
need to be further analyzed and identified by liquid chromatography.
The active components in the water chestnut peels had better FRAP iron ion reducing ability and
ABTS free radical scavenging activity. The antioxidant activity of the extracts from water chestnut
peels that had different heat treatment times was different. The antioxidant activity of P10+ HAD
after solvent extraction and simulated digestion was higher than that of P30+ HAD and HAD, which
might be related to the content of phenolic compounds. This is because the total phenolic content of
water chestnut peels treated with P10+ HAD was higher than that of P30+ HAD and HAD after the
same treatment. It has been reported that FRAP and ABTS antioxidant activity was positively
correlated with the phenolic content.
[33,34]
There was a series of complex changes in the phenolic
substances of water chestnut peels after simulated digestion, such as the release of phenolic
compounds and the possible complexes formed by phenolic compounds and proteases. SIF digestion
treatment for different periods of time could significantly improve the antioxidant capacity of FRAP
and ABTS. The antioxidant capacity of FRAP and ABTS of water chestnut peels that were pre-
treated using different times could be significantly improved after SIF digestion. The TPC of the
water chestnut peels could also be significantly increased after SIF digestion, indicating that the
antioxidant activity of the water chestnut peels was related to the total phenolic content. The above
80 Y. YUAN ET AL.
results were consistent with the findings of Koehnlein et al., who discovered that the simulation of
intestinal fluid digestion improved the antioxidant activities of 36 popular Brazilian food items.
[19]
Conclusion
We found that the TPC and TFC in the water chestnut peels, as well as the antioxidant activity of
FRAP and ABTS, were changed after simulated gastric fluid and simulated intestinal fluid digestion.
The TPC and TFC of the water chestnut peels with different heat pre-treatment times could be
significantly increased after SGF digestion. The TPC of the water chestnut peels could be signifi-
cantly increased after SIF digestion, but the TFC could not be significantly increased. Compared with
the simulated gastric fluid digestion, the TPC of water chestnut peels increased more after SIF
digestion. Both the FRAP and ABTS antioxidant capacity of water chestnut peels could be improved
after SGF and SIF digestion, while the antioxidant ability improved more after SIF digestion.
Acknowledgments
The project supported by the National Natural Science Foundation of China (31601469; 31601420), Scientific Research
Foundation for Hundred-Talent Program of Guangzhou University (69-18ZX10011), Natural Science Foundation of
Guangdong Province (2017A030313205), Science and Technology Program of Guangzhou (201604020089). There is
no conflict of interest.
Funding
This work was supported by the National Natural Science Foundation of China [31601469, 31601420]; Natural Science
Foundationof Guangdong Province [2017A030313205]; Science and Technology Program of Guangzhou [201604020089].
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INTERNATIONAL JOURNAL OF FOOD PROPERTIES 83
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