Slowly digestible starch in fully gelatinized material is structurally driven by molecular size and A and B1 chain lengths

Highlights

• Structure-digestion relationships of fully gelatinized material were investigated

• The digestion rate during storage was reduced depending on the starch source

• Long external A and B1 chains are prone to forming slowly digestible assemblies

• Acid-converted maize starch exhibited slowly digestible intermolecular associations

• Sago starch resulted in cake crumb with twice SDS as the maize counterpart

Abstract

The objective of this study was to obtain structure-digestion relationships of fully gelatinized starch. Twelve starch samples with marked fine structural differences (HPLC-SEC) were studied for their retrogradation behavior (thermal and rheological properties of starch gels) and in vitro digestibility. A reduction in the digestion rate during storage for 7 days was observed in all samples and, interestingly, this reduction was particularly evident in sago (64.7%), potato (57.3%), pea (55.1%) and acid-converted maize (ACM, 51.6–51.8 %) starches. Results indicated two potential interactions that may result in slowly digestible supramolecular structures: 1) double helices between external A and B1 chains of DP at peak maximum ≥ 15.5 Glucose Units (perhaps involving internal long chains) that also are prone to forming intermolecular associations [high relative drop in the storage modulus (G’) during heating of 7 days-stored gels] and; 2) interactions of small molecular size acid-hydrolyzed starch molecules that may be more mobile and easily aligned.

Introduction

Starch, being the principal component in cereals, tubers and pulses, is the major polysaccharide related to postprandial glycaemia and glycemic index of foods. Glucose response is frequently described by its glycemic index (GI) or glycemic response, i.e., rapid increase of the postprandial blood glucose, and many studies have linked low GI diets with reduced risk of developing type 2 diabetes and cardiovascular disease (Jenkins et al., 2002). In humans, starch is successively hydrolyzed by salivary and pancreatic α-amylase in the mouth and small intestine, and then to glucose by the mucosal α-glucosidases, and differences in the glycemic response depend highly on the rate and extent to which starch is digested.

Native starch naturally exists in the form of starch granules with different properties, including surface porosity and semi-crystalline structure. Based on their X-ray diffraction pattern, starches can be classified into three types: A-, B- and C- (combination of A and B-allomorphs) type. A-type starches encompass those from cereals, and many possess surface pores and channels, whereas such pores do not exist in B-type starches, which include those from tubers. This macrostructural difference is important during digestion as digestive enzymes enter the channels and digest starches gradually in an inside-out manner. In this way, A-type starches are slowly digestible (Zhang, Ao, & Hamaker, 2006; Zhang, Venkatachalam, & Hamaker, 2006) and B-type starches are inherently resistant. This native starch macromolecular structure, acting as a natural physical barrier to enzyme digestion, is lost in many foods because of hydrothermal processes, such as cooking, boiling and baking, among others, which lead to partial or complete starch gelatinization (Martinez, Roman, & Gomez, 2018; Varriano-Marston, Ke, Huang, & Ponte, 1980). An exception to this is high amylose starch (HAMS), which have been reported to retain its native structure (assessed by X-ray diffraction pattern) during baking (Hoebler, Karinthi, Chiron, Champ, & Barry, 1999). The amorphous structure of gelatinized starch results in far greater availability of α-amylase binding sites, which makes the substrate more susceptible to enzyme hydrolysis (Baldwin et al., 2015) and results in a wide array of food products with high GI (Foster-Powell, Holt, & Brand-Miller, 2002).

A great deal of effort has been put forth to decrease starch bioaccessibility using a variety of methods, although many entail the use of expensive enzymes (Ao et al., 2007; Guraya, James, & Champagne, 2001; Han et al., 2006; Shi, Cui, Birkett, & Thatcher, 2005; Shin et al., 2004), or physical modifications such as heat-moisture treatment (Lee & Moon, 2015), where the positive effects on digestion are often lost during further thermal processing unless using HAMS (Hoebler et al., 1999).

Starch retrogradation, defined as the re-association of amylose and amylopectin chains into ordered structures that are different from the original, is known to result in a reduction of the rate and extension of starch digestion, depending on the main constituent involved. Amylose double helices (retrograded amylose) are known to be enzymatically resistant and yield resistant starch (RS) (Haralampu, 2000; Patel et al., 2017), whereas retrograded amylopectin has been attributed to the formation of slowly digestible starch (SDS) [Cui & Oates, 1997; Farhat et al., 2001; Zhang, Sofyan, & Hamaker, 2008; Borah, Deka, and Duary, (2017). In a study on rice starch digestibility and amylopectin fine structure parameters, Benmoussa, Moldenhauer, and Hamaker, (2007) showed that the lowest content of rapidly digestible starch (RDS) was found in rice cultivars with high proportions of amylopectin with long chains. Zhang et al. (2008) found that higher content of amylopectin with longer chains promoted formation of SDS through retrogradation in starches from maize mutants. Several authors have reported, albeit with no digestion analysis, that longer amylopectin branches are more prone to form intra- and intermolecular associations during cooling (Jane & Chen, 1992; Kohyama, Matsuki, Yasui, & Sasaki, 2004; Matalanis, Campanella, & Hamaker, 2009). However, the combined roles and synergies of multiple starch molecular features, including amylopectin fine structure and amylose length, for controlling amylopectin and amylose retrogradation and the rate of digestion are not evident. For example, Jane and Chen (1992), working with reconstituted slurries of different amylopectins and amyloses, found that mixtures of amylopectin with amylose of intermediate length (667 DP) resulted in the greatest gel strength; yet, the structure-digestibility relationship was not investigated.

Despite extensive research conducted on starch structure-digestion relationships, a mechanistic understanding of how SDS structurally forms in a fully gelatinized material is still unclear. We hypothesized that certain starches found in nature inherently, or after acid-conversion commonly used in some food applications, lead to the formation of SDS after gelatinization. In this study, 12 starches with different structural features served as a basis to elucidate structure-re-association relationships and their effect on starch digestibility. The slow digestion property of these starches was investigated after full gelatinization as determined by DSC, except for high amylose maize (HAM) starches, whose slow digestion property could also be derived from ungelatinized material. HAM starches have been reported to have conclusion temperatures of starch gelatinization (T_c) up to 130 °C (Jane et al., 1999), which is not attainable with the conventional RVA used in this study. Then, select starches were tested in a cake system to demonstrate this effect in a real product.