Multiscale structural changes and retrogradation effects of addition of sodium alginate to fermented and native wheat starch

Abstract

This work investigated the changes in multi-scale structure and retrogradation properties of native wheat starches (NS) modified by sodium alginate (AG) with and without fermentation. AG adhered on the surface of NS granules and fermentation promoted the adhesions. Compared with the addition of AG alone, dual modification by fermentation and AG together showed a greater effect to increase the weight-average molecular weight and reduce the relative crystallinity and double helix degree of NS. Small angle X-ray diffraction results showed a significant increase in amorphous region with dual modification compared with AG alone. Additionally, dual modification greatly slowed the increase of relative crystallinity and the enthalpy (ΔH) of NS paste during storage. The results of this study suggest that dual modification is a more effective approach to modify structures and properties of wheat starch than single AG treatment, and suggest its potential industrial application in starch-based foods.

Introduction

Starch is a polymeric carbohydrate mainly found in cereal, tubers, roots, and legumes, and is used in different food products to improve texture and mouth sensation [[1], [2], [3]]. When heated, starch absorbs water, causing it to swell and form a paste, affecting the characteristics of starch-based products and allowing its wide use as a thickening agent, stabilizer, and texturizer in food and non-food industries [4]. However, native starch has many innate disadvantages, such as syneresis, retrogradation, and textural change [5]. Retrogradation of starch can occur due to amylose reassociation (short-term retrogradation) and amylopectin crystallization (long-term retrogradation) during storage of native starch paste. Many studies have examined the modification of the structure and physicochemical properties of starch to delay retrogradation [6].

Many methods such as chemical or physical modifications and enzymatic treatments have been used to delay or decrease starch retrogradation [7]. Additives including sodium alginate, gum, hydrocolloids, inulin, sodium carboxymethyl cellulose, pullulan, oligosaccharides, fatty acid, salt and plasma protein, can retard starch retrogradation and enhance the quality of starch-based foods by changing the structural and gelatinization properties of starch [3,[8], [9], [10]]. Sodium alginate (AG), a hydrocolloid, is widely used as a thickener, emulsifier, and stabilizer, and can effectively retard starch aging [11,12]. The addition of AG changed the freeze-thaw stability of sweet potato and AG affected the gelatinization and retrogradation properties of high-amylose cornstarch [13,14]. Sodium alginate has also been used in fermented food such as steamed bread for resistance to staling during storage [15]. Additionally, sodium alginate showed a more effective retardant effect on starch retrogradation compared with other additives [14]. Most recent studies of the effect of AG to retard starch retrogradation have mainly examined the effect of using AG alone or using AG combined with other additive. However, there have been few studies on dual modification by AG and the use of methods such as fermentation to modify starch properties and delay retrogradation.

Fermentation is one of the oldest food processing methods, and is widely used in preparation of foods such as bread, steamed bread, and rice noodles [16]. The study of structural changes and retrogradation modification of fermented starch is attracting researchers' interest as a way to improve the quality of fermented starch-based food [17]. Some researchers have reported that fermentation can erode the surfaces of starch granules, alter the amorphous region of starch granules, reduce the content of amylose and the average molecular weight of starch, and endow modified starch with distinct pasting properties and higher gel strength [16,17]. In our previous study, we found that fermentation could decrease the weight-average molecular weight of wheat starch and erode the surface of starch granules. Additionally, fermented wheat starch showed lower amylase content, lower peak viscosity, and lower setback than native starch [16], which indicated that natural fermentation can affect short-term retrogradation. However, to the best of our knowledge, the effect of natural fermentation on long-term retrogradation of wheat starch is poorly understood, with no information available on changes in the multiscale structure of fermented wheat starch combined with AG. Therefore, in this work, the granular morphology, molecular structure, crystallinity, paste property, gel firmness, and retrogradation enthalpy were investigated for native and fermented starch prepared with different concentrations of AG. The results of this work help to probe the mechanism of how the use of AG combined with fermentation can affect the multiscale structure of wheat starch. Additionally, this work provides the theoretical basis for the application of AG in fermented starch-based foods for quality improvement.