Rheological, structural, and water-immobilizing properties of mung bean protein-based fermentation-induced gels: Effect of pH-shifting and oil imbedment

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Food Hydrocolloids 129 (2022) 107607

Available online 21 February 2022

Rheological, structural, and water-immobilizing properties of mung bean

protein-based fermentation-induced gels: Effect of pH-shifting and

oil imbedment

Yunqing Nie

, Yuanfa Liu

, Jiang Jiang

, Youling L. Xiong

, Xiangzhong Zhao

School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu, 214122, China

Department of Animal and Food Sciences, University of Kentucky, Lexington, KY, 40546, USA

Qilu University of Technology (Shandong Academy of Sciences), Jinan, Shandong, 250014, China

ARTICLE INFO

Keywords:

pH-shifting

Oil imbedment

Plant-based fermentation-induced gels

Rheological properties

Structure

ABSTRACT

The objective of this study was to investigate the impact of pH-shifting and oil inclusion on the textural prop-

erties of plant protein-based fermentation-induced gels resembling dairy yogurt in consistency. Gels were formed

by pH

-treated or native mung bean protein (MBP) with or without emulsied coconut oil (3% w/v) and

transglutaminase (0.1% w/w protein) during fermentation at 43 ◦C for 8 h. Quinoa our hydrolysate was used to

modulate the MBP gel network. Creep-recovery and viscoelasticity (Gʹ/Gʹʹ) tests showed that gels prepared with

treated MBP were less deformable and stiffer than native MBP gels. The pH

treated MBP gels also

exhibited superior hardness and water-holding capacity over the native MBP gels. Consistently, the pH

gels

displayed more compact and denser network structures, and oil emulsion droplets as well as transglutaminase

further contributed to such packing effect.

H-LF-NMR conrmed less mobility of bulk water in pH

gels

compared with native gels.

1. Introduction

Greek yogurt is one of the most widely consumed dairy products that

has gained immense popularity due to its higher nutritional value and

purported health benets compared with traditional yogurt (Gyawali,

2017). Greek yogurt is dened as a strained yogurt which is concen-

trated by removing acidic whey from the solid part, making it denser and

creamier in texture (Uduwerella, Chandrapala, & Vasiljevic, 2018). In

recent years, with the increasing emphasis on sustainable food produc-

tion, animal welfare, and milk allergies, the demand for plant-based

dairy alternatives is rapidly growing. Meanwhile, the potential bene-

ts of preventing chronic diseases by increasing the intake of

plant-based foods also intensify this new initiative (Greis et al., 2020;

Pandey, Ritz, & Perez-Cueto, 2021).

As dairy substitutes, plant-based milk and yogurt have received

considerable attention. This reects an overall trend of research to

partially substitute legume proteins for animal-based proteins (Grasso,

Alonso-Miravalles, & O’Mahony, 2020; Sim, Xin, & Henry, 2020).

Among plant-based sources used for yogurt production, soybean has

been most widely employed due to its protein quantity, quality, and

functional properties (Cheng, Thompson, & Brittin, 1990; Rusmarilin,

Nurhasanah, & Andayani, 2018). However, beany off-avors inherent to

soybean are a signicant hurdle to alternative yogurt production. With

pea protein as the base for yogurt production, less such off-avors are

perceived (Youssef et al., 2020). Yet, pea-based yogurt lacks the texture

density when compared to Greek style yogurt (Klost & Drusch, 2019).

Indeed, syneresis and granular texture are two common phenomena for

legume-based fermentation-induced gel products intended as alterna-

tives for dairy yogurt (Sodini, Remeuf, Haddad, & Corrieu, 2004;

Youssef et al., 2020). As proteins are the key gelling component in all

fermentation-induced acid gels, the interactions between protein mol-

ecules or protein aggregates are essential to the gel structure formation

(Nguyen et al., 2018; Pang, Xu, Zhu, Bansal, & Liu, 2019). To aid in

protein gel matrix formation, the cross-linking enzyme transglutaminase

has been suggested for milk as well as plant-based yogurt preparations

(Gharibzahedi & Chronakis, 2018; Ziarno & Zaręba, 2020). On the other

hand, the presence of nonprotein materials and the pH could either

promote or inhibit the cross-linking of protein aggregates during gel

* Corresponding author.

** Corresponding author.

E-mail addresses: jiangjiang@jiangnan.edu.cn (J. Jiang), ylxiong@uky.edu (Y.L. Xiong).

Contents lists available at ScienceDirect

Food Hydrocolloids

journal homepage: www.elsevier.com/locate/foodhyd

https://doi.org/10.1016/j.foodhyd.2022.107607

Received 15 July 2021; Received in revised form 13 January 2022; Accepted 18 February 2022

Food Hydrocolloids 129 (2022) 107607

network formation (Nguyen, Chassenieux, Nicolai, & Schmitt, 2017).

Mung bean is a leguminous plant widely cultivated in many parts of

the world (Du et al., 2018). Mung bean is rich in sugar, protein, trace

elements, and essential vitamins, and its protein content ranges from 19

to 33% making it one of the best protein sources for new food product

development (Ebert, Chang, Yan, & Yang, 2017). Mung bean protein

(MBP) consists of globulin, albumin, prolamin, and gluten fractions, and

is rich in lysine, one of the principal essential amino acids (Lee & Chin,

2013). Furthermore, MBP has several reported physiologically benets,

including antioxidative potential, antiproliferative capacity, and anti-

hypertension activity (Gupta, Srivastava, & Bhagyawant, 2018). In Asia,

MBP is considered as a value-added by-product of mung bean starch

processing. Nevertheless, the application of such legume proteins in the

food industry has been limited for its poor functional properties,

including solubility, gelling capacity, and emulsifying activity. Avail-

able processing technologies that show promise to modify plant protein

structures, solubility, and functionality characteristics have been

described by Akharume, Aluko, and Adedeji (2021). In particular,

pH-shifting treatment, which induces structural modication of pro-

teins, i.e., an increased exposure of hydrophobic groups and enhanced

molecular exibility, has been successfully utilized to modulate func-

tional properties of legume seed proteins (Jiang, Wang, & Xiong, 2018).

This technique would conceivably promote the structure-forming ca-

pacity in legume-based acid gels.

In the preparation of fermentation-induced foods, it is desirable to

include fermentable ingredients for nutritional and functional benets.

Quinoa (Chenopodium quinoa Willd.), which consists of plentiful starch

(32%–69%), protein (15%), balanced amino acids, and abundant min-

erals as well as vitamins, has remarkable nutritional properties (James,

2009). As natural stabilizers in acidic protein gel system, starch could

reduce syneresis and improve the texture of acid gels, thus, meeting the

increasing “clean label” demand (Wang, Kristo, & LaPointe, 2020). In

addition, protein in quinoa could interact with MBP in the network

matrix formation to improve the texture properties of the gel.

In the present study, fermentation-induced (43 ◦C for 8 h) gels were

prepared from the pH

-treated (2%) or native (2%) MBP mixed with 3%

w/v coconut oil (CNO). The amount of oil added was correspondent to a

typical commercial full-fat yogurt (Gharibzahedi & Chronakis, 2018);

and 0.1% (w/w protein) transglutaminase was applied to promote

protein aggregation as described above. On the other hand, alkaline

pH-shifting treatment was done to modify the protein structure to

improve emulsifying, cross-linking, and gelling potential of the

fermentation-induced gels. Quinoa our hydrolysate as a ller was used

to modulate the gel network. Furthermore, the relationship between

protein structure, oil imbedment, and fermentation-induced gel char-

acter was elucidated.

2. Materials and methods

2.1. Materials

Quinoa (Chenopodium quinoa Willd.) seeds, imported from Bolivia,

were purchased from a local market. Quinoa was crushed with a micro

multifunctional grinder (CS–1000Y, Wuyi Haina Appliances Co. Ltd.,

Zhejiang, China) and then sieved through 60 mesh to obtain quinoa

our. MBP was donated by Shuangta Food Co., Ltd (Yantai city, Shan-

dong, China). Coconut oil was obtained from a local store.

-Amylase (5

U/mg) was purchased from Sigma-Aldrich (St. Louis, MO, USA), and

β-Amylase (32270 SBAB/g) was purchased from Genencor Bio-Products

Co. Ltd. (Wuxi, China). Neutral protease (154,438 U/g) was obtained

from Nanning Pangbo Biological Engineering Co. LTD (Guangxi, China).

Transglutaminase was donated by Ajinomoto Co., Inc. (Kawasaki,

Japan). Starter cultures (a blend of L. bulgaricus, S. thermophilus, Ker

microora, B. lactis, B. longum, and B. infantis) was purchased from

Kunshan Baishengyou Biological Technology Co. Ltd. (Jiangsu, China).

All other reagents and solvents were of analytical grade.

2.2. pH-shifting treatment of MBP

Structurally modied protein was prepared by alkaline pH treatment

as described previously (Jiang, Chen, & Xiong, 2009). For the purpose of

ensuring adequate protein unfolding, pH 12 was chosen as the modi-

fying condition. Briey, MBP solution (60 mg/mL, pH 7.0) was titrated

to pH 12 with 2 M NaOH at room temperature, held at this pH for 0.5 h

to induce partial unfolding, and then titrated back to 7.0 with 2 M HCl to

allow refolding. The residual salt was negligible (<0.03 M in ionic

strength) and therefore was not removed before fermentation.

2.3. Enzymatic pretreatment of quinoa our

Quinoa our was pretreated with mixed hydrolytic enzymes to

obtain hydrolysate (QFH). Briey, 25% (w/w) quinoa our was sus-

pended in deionized water, stirred for 2 h at room temperature (23 ◦C)

and then heated at 100 ◦C for 1 h to inactivate endogenous enzymes and

pregelatinize the starch. After cooling to room temperature,

-Amylase

(0.1% w/w) and β-Amylase (0.1% w/w) were added. The mixture was

adjusted to pH 5.0 and then incubated at 55 ◦C for 2 h to allow starch

hydrolysis. Thereafter, the pH was adjusted to 7.0 and neutral protease

(0.1%) was then added. After incubation at 50 ◦C for 1 h to initiate

proteolysis, the hydrolysate was heated at 100 ◦C for 15 min to inacti-

vate the enzymes. The resulting QFH contained 4.5 g/100 mL of

reducing sugar as measured by the 3,5-dinitrosalicylic acid (DNS)

method (Deshavath, Mukherjee, Goud, Veeranki, & Sastri, 2020) and

10.6 g/100 mL of total solutes as determined by a digital Brix refrac-

tometer (PAL-1, ATAGO, Tokyo, Japan). Furthermore, the viscosity,

measured with a viscometer (Brookeld Asset Management Inc., Tor-

onto, Canada), was found to be 54 mPa⋅s.

2.4. Preparation of the fermentation-induced gels

A predetermined volume of MBP (60 mg/mL) was mixed with QFH

to obtain a nal mixture of a 30 mg/mL protein where MBP and QFH

accounted for 2/3 and 1/3 of this nal protein content. For treatment

gels with emulsied oil, coconut oil was added at this time (see below).

The mixture was stirred at room temperature for 30 min and then

blended at 13,600 rpm for 3 min using an Ultra-Turrax blender (T18,

IKA, Staufen, Germany), followed by a two-stage high pressure ho-

mogenization (180 bar/30 bar) using an AH-2010 homogenizer (ATS

Engineering Inc., Ontario, Canada). After that, the mixture was heated at

95 ◦C for 10 min and then rapidly chilled to 40 ◦C in an ice water bath.

Starter cultures (0.6% w/w) were added to initiate fermentation (43 ◦C

for 8 h). The pH of the mixture decreased from 7.0 to 4.0–4.5 during the

fermentation time (data not shown) and a fermentation-induced acid

gel, designated “Con”, was formed. For treatment gels, coconut oil (3%

w/w) was added before the above described blending (homogenization)

to obtain the gel named “+Oil” and the size of dispersed oil droplets was

measured by dynamic light scattering (DLS) using a Nano Brook Omni

(Brookhaven Instruments, Holtsville, New York, USA) as described in a

previous study (Jiang, Jin, et al., 2018). Transglutaminase (TG, 0.1%

w/w protein) was added with starter cultures to obtain the gel named

“+TG”. Finally, both coconut oil (3% w/w, added before blending) and

TG (0.1% w/w protein, added with starter cultures) to obtain the gel

named “+Oil +TG”. All four types of gels were ripened for 12 h at 4 ◦C

before being subjected to the following analyses.

2.5. Rheological tests

Rheological measurement was conducted using a stress-controlled

DHR-3 rheometer (TA Instruments, New Castle, Delaware, USA) with

a parallel plate (40 mm diameter and 1 mm gap). Composite

fermentation-induced gel samples were thinly sliced and carefully

loaded in the rheometer. Samples were allowed to rest for 2 min to

release the stress before measurements. The linear viscoelastic region

Y. Nie et al.

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Rheological, structural, and water-immobilizing properties of mung bean protein-based fermentation-induced gels: Effect of pH-shifting and oil imbedment

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