Soluble Dietary Fiber Fractions in Wheat Bran and Their Interactions

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Soluble Dietary Fiber Fractions in Wheat Bran and Their Interactions with Wheat Gluten Have Impacts on Dough Properties Qian Li, Rui Liu, Tao Wu, and Min Zhang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b03451 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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Journal of Agricultural and Food Chemistry

Title: Soluble Dietary Fiber Fractions in Wheat Bran and Their Interactions with Wheat Gluten Have Impacts on Dough Properties Authors: Qian Li1, Rui Liu1,2, Tao Wu1, Min Zhang1,2,*

Affiliation: 1

Key Laboratory of Food Nutrition and Safety (Tianjin University of Science & Technology),

Ministry of Education, Tianjin 300457, China 2

Tianjin Food Safety & Low Carbon Manufacturing Collaborative Innovation Center,

Tianjin 300457, China

Corresponding Author: Min Zhang Key Laboratory of Food Nutrition and Safety (Tianjin University of Science & Technology), Ministry of Education, Tianjin 300457, China Telephone: +86-22-60912343 Fax: 86-22-60912343 E-mail address: [email protected]

KEYWORDS Soluble dietary fiber; wheat bran; fermentation; structural analysis; dough quality

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1 ABSTRACT 2 Six soluble dietary fiber (SDF) fractions were prepared via stepwise ethanol precipitation from natural 3 and fermented wheat bran. The chemical composition, molecular weight distribution, glycosidic linkage 4 and substitution pattern of the SDF fractions were elucidated by sugar analysis, periodate oxidation and 1

5 Smith degradation, molecular determination, and H nuclear magnetic resonance (NMR) analysis. The 6 impacts of SDF fractions on the rheological properties and morphologies of doughs were investigated by 7 farinography, rheometry and scanning electron microscopy (SEM) to clarify the relationship between the 8 microstructural features of SDF fractions and the macroscopic properties of SDF-containing doughs. The 9 interactions between SDF fractions and wheat glutens in doughs were further studied by confocal laser 10 scanning microscopy (CLSM). The experimental results indicated that the SDF fraction with an 11 intermediate molecular weight but a higher substitution degree and a larger di-substitution ratio was most 12 compatible with the dough network and beneficial to dough quality.

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13 INTRODUCTION 14 Dietary fiber (DF) is considered a valuable constituent for health-promoting foods and possesses 15 advantageous nutritional and functional benefits. On the basis of their water solubility, DFs are 16 categorized into soluble dietary fiber (SDF) and insoluble dietary fiber (IDF). SDF is considered 17 to perform more significant physiological functions due to its hydrophilic characteristics and 18 availability to beneficial microorganisms.

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The inclusion of SDF in the diet is beneficial to

19 glucose homeostasis-related diseases, particularly type 2 diabetes, by slowing the release of 20 glucose in foods. 21 during digestion.

2

3

SDF can reduce blood cholesterol levels by binding cholesterol to the stool SDF also plays an important role in reducing the risk of colorectal cancer by

22 modulating gut flora.

4,5

In addition, SDF-enriched foods enhance the feeling of fullness and

23 stimulate the release of appetite-suppressing hormones, which may be the reason SDF-containing 24 foods aid in weight loss.

6,7

Thus, consuming foods with high SDF content is a good choice for the

25 improvement of health conditions. 26

By-products from grain milling have been proven to be important sources of DF and have good

27 application prospects in the preparation of high-fiber and low-calorie flour products.

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However,

28 directly incorporating these by-products is detrimental to the appearance, taste and texture of flour 9

29 products, which is one of the technological obstacles to obtaining DF-enriched foods. Therefore, 30 concentrated SDF, with a capacity to efficiently avoid undesirable properties, has received 31 considerable attention and is being added to flour products as an alternative source of DF.

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Many

32 methods have been proposed by researchers to prepare SDF, which are capable of increasing SDF 33 yield and improving processing adaptability or dietary function. These include water extraction, 34 alkali extraction, physical assistant extraction, enzymatic extraction and biological fermentation

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35 methods, etc.

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Solid-state fermentation has emerged as a potential means of SDF production

36 due to its numerous advantages in the processing of agro-industrial residues, such as its lower 37 energy requirements, reduced wastewater production and environmentally friendly process.

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38 Unsurprisingly, the quality of flour products is significantly affected by the SDF content and by 39 the interactions between SDF and food components, particularly wheat gluten protein. However, 40 structural studies of SDF are currently insufficient, partly because other cell wall components in 41 grains, as well as their by-products, covalently or non-covalently associate with SDF, resulting in 42 the formation of a complex network whose molecular features are technically difficult to analyze. 43 44

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Wheat flour, as a global staple, provides an ideal matrix for SDF delivery to the consumer in

45 acceptable food. The processing adaptability of wheat dough during mixing and kneading is 46 largely dependent on its rheological behavior, which is among the main macroscopic indexes for 47 predicting the complex interactions of natural ingredients or additives in dough. Many studies 48 have reported the effects of SDF addition on the rheological properties of dough,

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suggesting

49 that SDF addition can either positively or negatively impact dough properties. These contradictory 50 conclusions are often interpreted in terms of the molecular architecture of SDF, such as 51 composition, molecular weight distribution, substitution pattern (substitution degree and 52 substitution position, etc.) and spatial density.

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For instance, cereal arabinoxylans (AXs) with

53 arabinofuranose (Araf) and other substituents as steric hindrances can prevent the aggregation of 54 β-(1,4)-D-xylan chains, thereby leading to the formation of an extended and asymmetrical 55 polysaccharide chain structure, as well as corresponding functions. Thus, aside from the structure 56 of SDF, further detailed studies on the interactions between SDF and wheat dough are needed to

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57 facilitate research on SDF and applications of SDF-containing flour products. 58

The purpose of the present work is to study the structural features and physicochemical

59 properties of SDF preparations, sub-fractionated via stepwise ethanol precipitation from natural 60 and fermented SDF. The influence of different SDF fractions on the rheological properties of 61 dough were also investigated to clarify the microstructural characteristics of SDF fractions in 62 relation to the macroscopic properties of SDF-containing dough. 63 MATERIALS AND METHODS 64

Materials. Wheat bran was purchased from Public Grain and Oil Food Co., LTD (Henan

65 Province, China). The wheat bran was finely ground to pass through a 0.8-mm screen for further 66 use. The composition (%, w/w) of the wheat bran was 53.5 total DF, 24.3 starch, 15.7 protein, 67 3.86 lipids and 2.64 ash on a dry-weight basis. Wheat flour was acquired from the Tianjin Rakem 68 Grain and Oil Co., LTD (Hebei District, Tianjin, China). The wheat flour was composed of 11.2% 69 (w/w) protein, 73.9% carbohydrates, 1.80% lipids and 13.1% moisture. All standard chemicals 70 and other chemical reagents were of analytical grade and were purchased from Sigma Chemical 71 Co. (Beijing, China). 72

SDF Fractionation. The moisture content of the wheat bran was adjusted to 60% before

73 sterilization. The fermented wheat bran was prepared by inoculating 20 g of sterilized bran with 74 pre-cultured Rhizopus oryzae pellets (10%, v/w, a species from biology laboratory of culture 75 collection in Tianjin University of Science and Technology) and then incubating at 30 ± 2 °C for 5 76 days. SDF was extracted from natural and fermented wheat brans by water three times at 100 °C 77 for 2 h according to previous work.

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The SDF extracts from natural and fermented wheat brans

78 were further treated with α-amylase and protease E (7892 U/mL and 1020 U/mL, Novozymes

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79 (China) Biotechnology Co., Ltd., Tianjin, China), and amyloglucosidase (1176 U/g, Tianjin Noao 80 Science & Technology Development Co., Ltd, Tianjin, China) to remove starch and protein, 81 according to a previously described method.

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To study structural differences in the SDF present

82 in wheat bran, SDF fractions from natural and fermented wheat bran were isolated via stepwise 83 ethanol precipitation. The natural or fermented SDF (250 mg) was dissolved in 50 mL of distilled 84 water at room temperature under constant stirring. Then, 5 mL of 95% ethanol was slowly added 85 to SDF solutions and kept at 4°C for 2 h. This procedure was repeated until the total ethanol 86 volume was 900 mL. The SDF samples that precipitated at each ethanol concentration were 87 recovered by filtration and sequentially washed with 75% ethanol, 95% ethanol, acetone, and 88 distilled water, followed by dialysis and lyophilization. All of the precipitated SDF fractions were 89 analyzed to determine their molecular weight distribution (Figure S1 in Supplementary Materials). 90 The stepwise precipitation curve was obtained by plotting the ethanol concentration on the X-axis 91 and the mass fraction of the SDF precipitation on the Y-axis (Figure S2 in Supplementary 92 Materials). According to the yield and the molecular weight distribution of SDF, the optimal 93 concentration was determined from the test. Supplementary Figure S2 shows that the yield of the 94 SDF fractions precipitated with 40% and 60% ethanol for the natural wheat bran first increased 95 then decreased, with a significant decrease of SDF fractions precipitated with 80% ethanol for the 96 natural wheat bran. SDF fractions precipitated with 40%–60% ethanol, as well as 60%–80%, 97 showed similar molecular weight distribution with a main spectral peak (Figure S3 in 98 Supplementary Materials). Thus, the SDF fractions from the natural wheat bran that precipitated 99 with

Soluble Dietary Fiber Fractions in Wheat Bran and Their Interactions

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