Effects of Auricularia auricula-judae polysaccharide on pasting, gelatinization, rheology, structural properties and in vitro digestibility of kidney bean starch

Highlights

• AP greatly influenced the pasting properties of KBST.

• AP increased the viscoelasticity and cold storage stability of KBST.

• AP treated gels have higher ordered structures and thicker cell walls.

• AP significantly decreased KBST digestibility in vitro.

• The G′ and DO values of AP/KBST were positively correlated with the SDS and RS contents.

Abstract

Auricularia auricula-judae polysaccharide (AP) has unique molecular structures and multiple bioactivities with excellent gel-forming property and thermal tolerance. However, few researches focus on the interactions between AP and legume starches. In this study, the effects of AP on the pasting, gelatinization, rheology, microstructure, and in vitro digestibility of kidney bean starch (KBST) were evaluated. The pasting, gelling and structural properties of AP-KBST mixtures were characterized by rapid visco analyzer, rheometry, texture analyzer, laser particle analyzer, low-field nuclear magnetic resonance, Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. And an in vitro method was employed to measure the digestibility of AP-KBST composites. The pasting viscosity, swelling degree of starch granules, viscoelasticity, gel strength, cold storage stability and water-retention capacity of KBST were enhanced with increasing AP concentration. The combination of AP and KBST exhibited a higher short-range ordered and a firmer and denser structure than that of KBST alone. Moreover, AP increased the contents of resistant starch and slowly digestible starch, which were positively correlated with the storage modulus and the degree of order, thereby suggesting that the formation of strong and ordered gel network structure by synergistic interactions between AP and KBST was responsible for the reduced starch digestibility.

Introduction

Starch, the primary form of carbohydrate stored in plants, is a major source of glucose and energy in human daily life, which has been extensively applied in food industry [1], [2]. However, native starch is limited in the industrial application due to its inherent drawbacks during food processing and storage, such as heating and shearing instability, pH sensitivity, syneresis and prone-ageing or easy retrogradation [3], [4]. Moreover, consumption of high glycemic index (GI) starch becomes an increasing concern for certain populations with Impaired Glucose Tolerance (IGT) and especially Type 2 Diabetes (T2D) [1], [5]. Therefore, many starch modification methods were developed to overcome these shortcomings and improve the overall starch qualities [6]. Among various modification approaches, such as physical, chemical and biological methods, the physical blending of starch and non-starch polysaccharides (NSPs) was recognized as a safer, more convenient, efficient and economical as well as environment friendly method for native starches, which has been explored extensively [1], [2], [3], [4], [5], [7], [8], [9].

Auricularia auricula-judae, a large ear-shaped edible colloidal fungus, is extremely popular and massively planted in East Asia, including China, Korea, Vietnam and Indonesia, which has been used both as healthy food and traditional drug for more than 1000 years in China [10]. As an important bioactive component, Auricularia auricula-judae polysaccharide (AP), a fungus-based non-glycaemic polysaccharide, has unique molecular structures and excellent physiological functions. Xu et al. [11] firstly reported that the comb-branched structure of a water-soluble AP mainly comprises a β-(1 → 3)-D-glucan backbone accompanied by two β-(1 → 6)-D-glucosyl residues, which displayed a stiff chain conformation with good thickening property and favourable stability below 150 °C in water [12]. In addition, AP possesses abundant hydrophilic side groups, which renders it with good water solubility; whereas a relatively hydrophobic backbone and the fully extending chains of AP in aqueous solutions allow the molecules easily align in parallel and self-assemble into ordered hollow nanofibers through hydrophobic interactions and hydrogen bonding effects [13]. Moreover, it has already been proved that AP exhibits multiple beneficial physiological activities, including hypoglycemic activity [14], antioxidant [15], antitumor [16], immunomodulatory [17], hypolipidemic [18] and anti-inflammatory [19]. In our previous study, we obtained a water-soluble AP through hot water extraction [20]. The reports of chemical and rheological experiments revealed that the total carbohydrate, protein and uronic acid contents of AP were 72.07%, 8.63% and 5.27%, respectively; the monosaccharide composition of AP mainly included glucose (61.65%), mannose (32.94%) and galactose (5.40%); meanwhile, AP exhibited excellent gel-forming ability and thermal stability with high apparent viscosity and strong shear-thinning characteristics. Moreover, the AP gelation mechanism study revealed that the carboxyl groups of the uronic acids on polysaccharide chains was in favor of inter/intra hydrogen bonding formation by which the spatial network structure was maintained [21]. Furthermore, the synergistic interactions between AP and yam starch have also been studied in our previous research [22], [23]. And we found that AP could effectively facilitate the expansion of yam starch granules and the leaching of amylose, and significantly increased the viscosity and elasticity, and strengthened the network structure of the complex as well as improved the overall textural properties and water holding capacity during cold storage. Notably, AP could remarkably reduce the digestibility of yam starch.

Kidney beans (Phaseolus vulgaris L.), one of the most valuable leguminous crops, are extensively grown worldwide and consumed in traditional foods such as bakery products, canned food and salads. Kidney beans are rich in resistant starch (RS) and fibers and thus bean products have been considered as low GI foods [24]. Kidney bean starch (KBST) has higher amylose contents with C-type crystallinity, stronger interactions between amylose, and hence lower digestibility in comparison with potato or cereal starches, leading to its lower glycemic and insulinemic postprandial responses [25]. The health benefits of resistant starch or low GI foods are well known for the control of T2D, obesity, for lowering the incidence rate of cardiovascular diseases and for preventing colon cancer [24]. However, to our best knowledge, the potential synergistic interactions between AP and leguminous starches have not been reported.

Therefore, this study aimed to assess the effects of AP on the pasting, gelatinization, rheology, structural properties and in vitro starch digestibility of kidney bean starch, and to further explore the correlation between in vitro digestibility and the structural and gelling characteristic parameters. The research conclusions will provide new insights into the interaction mechanism between AP and KBST, and the innovative utilization of Auricularia auricula-judae polysaccharide in the development of bean starch-based or bean flour-based products, especially semi-solid leguminous foods, with favourable quality characteristics and health benefits.