Protein–phenolic interactions in lentil and wheat crackers with onion skin phenolics: effects of processing and in vitro gastrointestinal digestion

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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,

Semra Turan

and Esra Capanoglu *

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

) of co-digested crackers was higher than that of functional crackers,

whereas antioxidant bioavailability index (BI

) 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.

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.

Therefore, the bioavailability of food ingredi-

ents in functional foods is of great importance. Bioavailability

is a term used to describe bioaccessibility and absorption.

The bioavailability of phenolic compounds primarily depends

on the food matrix, the processing method of the food,

and

also protein–phenolic interactions.

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.

Thus, it is thought that phenolics are also protected

from oxidation as a result of interaction with proteins during

digestion.

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.

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

Department of Food Engineering, Faculty of Chemical and Metallurgical

Engineering, Istanbul Technical University, Istanbul, Turkey.

E-mail: capanogl@itu.edu.tr

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.,

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.,

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.

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.

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

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

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

Published on 21 March 2023. Downloaded on 6/26/2024 3:20:35 AM.

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