Protein–phenolic interactions in lentil and wheat crackers with onion skin phenolics: effects of processing and in vitro gastrointestinal digestion
Food &
Function
PAPER
Cite this: Food Funct., 2023, 14, 3538
Received 27th September 2022,
Accepted 20th March 2023
DOI: 10.1039/d2fo02885a
rsc.li/food-function
Protein–phenolic interactions in lentil and wheat
crackers with onion skin phenolics: effects of
processing and in vitro gastrointestinal digestion†
Deniz Günal-Köroğlu,
a
Semra Turan
b
and Esra Capanoglu *
a
This study aimed to evaluate the protein–phenolic interaction in functional crackers made of wheat/lentil
flour with onion skin phenolics (onion skin powder: OSP, onion skin phenolic extract: OSE, or quercetin:
Q) after in vitro gastrointestinal digestion. Phenolic/antioxidant recovery in crackers was lower with higher
levels of phenolic addition. In vitro gastrointestinal digestion procedure was applied for crackers prepared/
cooked with onion skin phenolics (functional crackers) or crackers consumed with onion skin phenolics
(co-digestion). Functional crackers had similar nutritional attributes (p> 0.05), however they had lower L*
values, and higher a* values. A higher concentration of OSP/OSE caused a decrease in the b* value while
it was increased with the quercetin addition. Phenolic/antioxidant recovery in functional crackers was
decreased by increasing the ratio of phenolic supplements. The amount of quercetin 7,4-diglucoside was
lower than the theoretical value whereas the amount of quercetin was higher in functional crackers. The
phenolic bioavailability index (BI
P
) of co-digested crackers was higher than that of functional crackers,
whereas antioxidant bioavailability index (BI
A
) was mostly similar. Quercetin was only identified in func-
tional wheat/lentil crackers with OSE. After digestion (1) TCA-precipitated peptides of the wheat crackers
could not be identified, whereas that of co-digested lentil crackers was more abundant, (2) level of free
amino groups of co-digested/functional crackers were lower than the control except for the co-digested
sample of lentil cracker with quercetin.
Introduction
Phenolic compounds are secondary plant metabolites with
thousands of compounds, from simple to complex, including
glycosylated or acylated compounds containing hydroxyl,
methoxyl, or glycosyl groups. Onion skin, used as a phenolic
source in this study, contains mostly flavonols, predominantly
glycoside derivatives of quercetin and kaempferol.
1
Functional foods can be defined as foods that contain
ingredients such as phenolic extracts, which provide
additional health benefits to the nutritional value of tra-
ditional foods.
2
Therefore, the bioavailability of food ingredi-
ents in functional foods is of great importance. Bioavailability
is a term used to describe bioaccessibility and absorption.
3
The bioavailability of phenolic compounds primarily depends
on the food matrix, the processing method of the food,
4
and
also protein–phenolic interactions.
5
Liquid foods pass through the stomach faster than the
solid foods due to their lower viscosity, which reduces the
possibility of the interaction of proteins with polyphenols.
Although the availability of polyphenols in solid foods (bread,
cookies, etc.), which had a more complex matrix, is delayed,
protein–phenolic interactions can stabilize these compounds
or increase their absorption by protecting against other reac-
tions.
3
Thus, it is thought that phenolics are also protected
from oxidation as a result of interaction with proteins during
digestion.
6
Conversely, several studies in the literature reported
that protein–phenolic complexes cause not only suppression
of antioxidant effects of phenolic compounds during digestion
but also a decrease in the digestibility of proteins in processed
products such as wheat bread.
7,8
In the literature, changes in the physicochemical and struc-
tural properties of wheat-based bakery products as a result of
protein–phenolic interaction have been investigated.
9
It was
reported that quercetin decomposed rapidly in baked bread
and cookies, which were exposed to direct heat.
10,11
On the
other hand, to the best of our knowledge, the protein–phenolic
interactions have never been investigated neither in legume-
†Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d2fo02885a
a
Department of Food Engineering, Faculty of Chemical and Metallurgical
Engineering, Istanbul Technical University, Istanbul, Turkey.
E-mail: capanogl@itu.edu.tr
b
Department of Food Engineering, Faculty of Engineering, Bolu Abant Izzet Baysal
University, Bolu, Turkey
3538 |Food Funct.,2023,14,3538–3551 This journal is © The Royal Society of Chemistry 2023
Published on 21 March 2023. Downloaded on 6/26/2024 3:20:35 AM.
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based bakery products nor in these foods after in vitro gastro-
intestinal digestion.
In this study, different forms of phenolics (onion skin
powder: OSP, onion skin phenolic extract: OSE, or quercetin)
were added to wheat/lentil crackers at two different ratios
(functional crackers). The aim was to investigate the protein–
phenolic interactions in functional crackers or crackers con-
sumed with phenolics (co-digestion) after in vitro gastrointesti-
nal digestion by determining the total phenolic content and
antioxidant activity values.
Materials and methods
Materials
Yellow onion skins were collected from local onion suppliers.
Red lentil flour (carbohydrate: 41.9, protein: 25.8, oil: 1.6 g per
100 g flour), wheat flour (carbohydrate: 71.9, protein: 10.5, oil:
1.1 g per 100 g flour), corn starch, salt and butter were pur-
chased from a local market. α-Amylase from Aspergillus oryzae
(A9857, 1500 U L
−1
), pepsin from porcine gastric mucosa
(P7000, ≥250Umg
−1
), bile extract porcine (B8631), pancreatin
from porcine pancreas (P7545, 8 × USP), DL-dithiothreitol
(D9163), sodium dodecyl sulfate (SDS), L-serine (S4500),
albumin from bovine serum (BSA, A2153), Trolox ((±)-6-
hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid), DPPH
(2,2-diphenyl-1-picrylhydrazyl) and HPLC grade standards
(protocatechuic acid, quercetin, quercetin 3-glucoside, and
kaempferol) were purchased from Sigma-Aldrich (St. Louis,
Missouri, ABD). Other reagents were purchased from Merck
(Darmstadt, Germany).
Preparation of onion skin Powder (OSP) and onion skin
phenolic extract (OSE)
Yellow onion skins were separated from impurities (stem,
adventitious roots, shoots, etc.), washed in cold water, and
dried at room temperature. The dried onion skins were then
ground in a coffee grinder and passed through a household-
type flour sieve to obtain onion skin powder (OSP).
Phenolic extracts of onion skins (OSE) were prepared
according to Günal-Köroğlu et al.,
12
The phenolic profile of
OSP and OSE in 80% methanol : water (v/v) was also deter-
mined by HPLC. OSP and OSE were stored at −18 °C.
Formulation and preparation of crackers
Functional crackers. Each phenolic source (OSP, OSE, quer-
cetin) at two different ratios was added by replacing with
wheat/lentil flour (weight basis). The amount of OSE and quer-
cetin was calculated to be equivalent to the amount of OSP by
considering the extraction yield of OSE (6.88%, w/w), and the
TPC of OSE (412.50 ± 6.45 mg QE per g extract), respectively
and the basic formulation of cracker dough (control) was pre-
pared according to Han et al.,
13
with some modifications
(Table 1).
Dry and liquid ingredients were mixed, and the dough was
kneaded for 3 minutes before resting for 10 minutes. A house-
hold pasta-making machine was first adjusted to 6 mm thick-
ness and the dough was passed twice. The machine was
adjusted to 2 mm thickness and the dough was passed for the
last time. After the dough was shaped (1.8 × 4 cm) with a
cracker mold, the wheat and lentil dough were baked for 30
and 20 minutes in an oven at 175 °C, respectively. The cooked
crackers were then left to cool down before storing at −18 °C.
Preparation of samples for in vitro gastrointestinal diges-
tion. Co-digested crackers were also prepared to understand
the matrix effect of consuming phenolic sources and crackers
together.
14
Functional crackers were the final products that are
baked after adding the phenolic source to the dough and were
powdered before the application of digestion protocol. Each
functional cracker, phenolic source, and co-digested sample
was weighed in separate flasks and in vitro gastrointestinal
digestion procedure was applied. Representations of samples
are given in Table 2.
Nutritional value, sensory analysis, and color of functional
crackers
Moisture (945.39A), ash (923.03), crude fat (920.39C), and
crude protein content by the Kjeldahl method (960.52) were
determined according to AOCS.
15
Chromameter (Konica Minolta, CR-400, Japan) was used to
determine CIE-L* (lightness/darkness), a* (redness/greenness),
and b* (yellowness/blueness) color parameters by three
different readings on five different crackers of the same
Table 1 Formulation of crackers
Ingredients (g) Control S1 S2 E1 E2 Q1 Q2
Flour 80 79.20 76.80 79.95 79.78 79.98 79.92
Corn starch 20 20 20 20 20 20 20
Clarified butter 20 20 20 20 20 20 20
Water
a
45/40 45/40 45/40 45/40 45/40 45/40 45/40
Salt 1 1 1 1 1 1 1
Baking powder 0.6 0.6 0.6 0.6 0.6 0.6 0.6
Phenolic source –– 0.80 3.20 0.06 0.22 0.02 0.08
a
45 g and 40 g for wheat and lentil flour, respectively. Control: plain crackers without phenolic source, S1: 1% onion skin powder, S2: 4% onion
skin powder, E1: 0.075% onion skin phenolic extract, E2: 0.27% onion skin phenolic extract, Q1: 0.025% quercetin, Q2: 0.1% quercetin was
replaced with wheat/lentil flour in the dough.
Food & Function Paper
This journal is © The Royal Society of Chemistry 2023 Food Funct.,2023,14,3538–3551 | 3539
Published on 21 March 2023. Downloaded on 6/26/2024 3:20:35 AM.
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