Effects of fatty acids with various chain lengths and degrees of unsaturation on the structure, physicochemical properties and digestibility of maize starch-fatty acid complexes

Highlights

• Fatty acid structure affected the digestibility of starch–fatty acid complexes.

• Complex behavior was affected by chain length and unsaturation degree of fatty acids.

• Complexes with 14-carbon fatty acids showed the highest degrees of structural order.

• The complexes with 14-carbon fatty acids had the highest resistant starch content.

• The complexes with various FA structures displayed different V-type polymorphs.

Abstract

In this study, we investigated the effects of chain length and degree of unsaturation of fatty acid (FA) on the structure, physicochemical properties, and in vitro digestibility of maize starch (MS)–FA complexes. Differential scanning calorimetry, X-ray diffraction, and Raman spectroscopy revealed that crystalline structure with V-type was formed between MS and FA. Nuclear magnetic resonance spectroscopy confirmed that the complexes with various FA structures displayed different V-type polymorphs. The thermal stability of the complexes with 14-carbon and 16-carbon FAs was better than that of other complexes. The complexes with 12-carbon (21.77%) and 14-carbon (26.31%) FAs displayed high resistant starch (RS) content, whereas those with 10-carbon FAs exhibited high slowly digestible starch (SDS) content (28.99%). Additionally, the FAs with a chain length of 10–14 carbons produced more ordered structures and showed stronger intermolecular interactions with MS than other FAs. For the 18-carbon FAs, the complexes with unsaturated FAs displayed higher RS and SDS content than those with saturated FAs. Moreover, the unsaturated FAs showed stronger intermolecular interactions with MS than the saturated FAs. These results indicated that FA structures significantly affected the structures of complexes, further affecting their physicochemical and digestive properties.

Introduction

Starch is a macro-constituent of many foods, and has a significant influence on the quality attributes of processed foods (Copeland, Blazek, Salman, & Tang, 2009; Wang, Wang, Yu, & Wang, 2016). The increasing occurrence of diet-related chronic diseases has attracted increased attention to the digestibility of starch (Wang & Copeland, 2013; Zheng et al., 2018). Lipids are commonly used in food systems to improve the processing characteristics and quality of products (Chao, Yu, Wang, Copeland, & Wang, 2018; Wang et al., 2016). When starch undergoes gelatinization and retrogradation, its linear amylose can particularly interact with endogenous or exogenous lipids to form V-type single-helix inclusion complexes, which have been studied extensively at various molecular scale (Chao et al., 2018; Kawai, Takato, Sasaki, & Kajiwara, 2012; Wang et al., 2016; Wang, Zheng, Yu, Wang, & Copeland, 2017; Zhang, Huang, Luo, & Fu, 2012). It is generally assumed that the hydrocarbon chains of lipid molecules can enter the hydrophobic cavity of the amylose helices through hydrophobic interactions, primarily resulting from conformational changes in amylose (Hay et al., 2017; Lu, Shi, Zhu, Li, & Huang, 2019; Putseys, Lamberts, & Delcour, 2010; Wang, Zhan, Jin, & Tian, 2017). As a result, the cavity is strongly hydrophobic within the helix, whereas the surface is strongly hydrophilic outside the helix (Hay et al., 2017; Wang, Zhan, et al., 2017).

Previous studies showed that complexation between starch and lipid could decrease the susceptibility to enzymatic breakdown (Ai, Hasjim, & Jane, 2013; Copeland et al., 2009; Wang & Copeland, 2013). Thus, a novel resistant starch has been developed through complexing starch with lipids to decrease the incidence of diet-related diseases (Ai et al., 2013; Chang, He, Fu, Huang, & Jane, 2014; Zheng et al., 2018). Starch–lipid complexes possess a compact structure, which hinders the formation of enzyme–substrate complexes (Panyoo & Emmambux, 2017; Putseys, Derde, et al., 2010). Hence, the mechanism underlying the enhanced resistance of starch to enzymatic hydrolysis by complexation with lipids is mainly due to restrictions of accessibility of amylase to its substrate (Chen, He, Fu, Zhang, & Huang, 2017; Farooq, Dhital, Li, Zhang, & Huang, 2018). However, the susceptibility of starch to enzymatic breakdown in the starch–lipid complexes is still not fully understood. Therefore, the effects of complexation between starch and lipids on the digestibility of starch systems are of great interest to the food industry and human nutrition (Chao et al., 2018; Panyoo & Emmambux, 2017; Zheng et al., 2018).

Numerous studies have suggested that the digestibility of starch is closely connected with the formation, structures and physicochemical properties of starch–lipid complexes (Ai et al., 2013; Chen et al., 2018; Kawai et al., 2012; Sun et al., 2019a, 2019b; Wang et al., 2020). Lipid structures, especially chain length and degree of unsaturation of fatty acid (FA), are crucial determinants for the formation, structures and functional properties of resulting complexes (Ai et al., 2013; Meng, Ma, Cui, & Sun, 2014; Putseys, Lamberts, & Delcour, 2010; Tang & Copeland, 2007; Wang et al., 2020; Zheng et al., 2018). Thus, starch digestion in formed complexes could be considerably affected by lipid structures. However, lipid structures showed various effects on the digestibility of starch according to previous studies, possibly resulting from various experimental conditions or materials (Ai et al., 2013; Chao et al., 2018; Wang et al., 2020). Some studies indicated that complexes with shorter chain FAs displayed lower digestibility, probably due to the higher complexing degree and formation of more complexes (Kawai et al., 2012; Meng et al., 2014). Contrary results were reported containing longer chain FAs, presumably attributing to their stable crystalline structures (Ai et al., 2013; Qin et al., 2019; Wang et al., 2016). According to Zheng et al. (2018), chain length and degree of unsaturation of FAs showed little effect on the digestibility of starch, possibly resulting from the small amount of starch–FA complexes formed during rapid viscosity analysis. Additionally, complexes containing unsaturated FAs could also strengthen resistance of starch to enzymatic breakdown (Kawai et al., 2012; Meng et al., 2014). However, the effects of chain length and unsaturation degree of FAs on the fine structures and digestibility of the complexes were not comprehensively investigated in these studies. Besides, on the basis of previous studies about such starch–FA complexes, little information concerning the relationship among intermolecular interactions, fine structures and digestibility of starch–FA complexes containing various FA structures is available. Therefore, it is essential to understand the interactions between starch and FAs with various structures and their effects on the formation, molecular structures, and functional properties of starch–FA complexes.

In this study, seven kinds of FAs with different chain lengths and unsaturation degrees were chosen to prepare maize starch (MS)–FA complexes. The objectives of the present study were to systematically investigate the effects of FA structures on the formation mechanisms, structures, physicochemical properties, and digestibility of model MS–FA complexes. Moreover, the differences in digestibility of MS–FA complexes were elucidated by their fine structures and physicochemical properties. This information will enhance our understanding of the interactions between starch and lipids in food processing, and this is beneficial to control the quality characteristics of foods.