Combination of parallel and sequential digestion kinetics reveals the nature of digestive characteristics of short-term retrograded rice starches

Combined effects of starch fine molecular structures and water content on starch digestibility of cooked white rice

Although the starch digestibility of cooked white rice has been investigated with regard to its relation to starch structure, it is not yet clear how starch molecular structure and water content affect its digestion rate. To investigate this, the in vitro starch digestibility and molecular structure of 10 rice varieties with a range of rice-to-water cooking ratios were investigated. As expected, starch digestibility varied with different conditions. Typically, a higher amylose content resulted in a lower maximum digestion extent for a given water content. Having relatively more and longer amylopectin intermediate chains caused a slower starch digestion rate, but only with rice-to-water ratios between 1:1 and 1:1.2. These results could prove useful to find combinations of starch fine molecular structures and water contents to produce cooked rice with low glycemic index.

Relations between starch fine molecular structures with gelatinization property under different moisture content

Although gelatinization property has been intensively investigated with its relation to starch structures, how a combination of starch molecular structures and moisture content affect the gelatinization remains unclear. The gelatinization of six rice starches with a wide range of amylose content was investigated under different moisture content in this study. Results showed that starch gelatinization temperatures increased and biphasic endothermic peaks appeared over the decreased moisture content. For the first time, amylose content was shown to have a parabolic relationship with gelatinization temperatures. Distinct linear relations among starch fine molecular structures with gelatinization parameters were observed under different moisture contents, which suggested that amylose short chains were involved in the first endothermic peak, while interactions among amylose intermediate chains and relatively shorter amylopectin trans-lamellar chains dominantly contributed to the second endothermic peak when gelatinized under limited moisture content. These results help in better understanding of starch structure-gelatinization relation.

Microwave Cooking Enriches the Nanoscale and Short/Long-Range Orders of the Resulting indica Rice Starch Undergoing Storage

The chain reorganization of cooked starch during storage plays an important role in the performance of starchy products such as rice foods. Here, different analytical techniques (such as small-angle X-ray scattering) were used to reveal how microwave cooking influences the chain assembly of cooked indica rice starch undergoing storage for 0, 24, or 48 h. While stored, more short-range double helices, long-range crystallites, and nanoscale orders emerged for the microwave-cooked starch than for its conventionally cooked counterpart. For instance, after storage for 24 h, the microwave-cooked starch contained 46.8% of double helices, while its conventionally cooked counterpart possessed 34.3% of double helices. This could be related to the fact that the microwave field caused high-frequency movements of polar groups such as hydroxyls, which strengthened the interactions between starch chains and water molecules and eventually their assembly into double helices, crystallites, and nanoscale orders. This work provides further insights into the chain reassembly of microwave-cooked starch undergoing storage, which is closely related to the quality attributes of starch-based products.

Consecutive reaction kinetics model reveals the nature of long-term rice amylopectin retrogradation characteristics

Starch retrogradation involves nucleation and crystal growth steps, while their relative contribution to the overall retrogradation kinetics and relations with starch fine molecular structures have not been elucidated. In this study, a consecutive reaction kinetics model (CRK) was developed to fit long-term retrogradation kinetics curves for 10 rice starches with distinct molecular structures. Starch chain-length distributions (CLDs) and melting enthalpy kinetics curves for these starches were obtained from our published data. It was shown that these melting enthalpy kinetics curves can be satisfactorily deconvoluted by the CRK model into a combination of nucleation and crystal growth curves. Correlation analysis between CRK model-fitted parameters with starch CLDs showed that starch retrogradation nucleation and crystal growth steps were controlled by distinct starch fine molecular structures. These results have practical applications, as it enables a separate regulation of nucleation and crystal growth steps during retrogradation process of starch-based foods for desirable nutritional properties.

Mutual interactions between α‑amylase and amyloglucosidase on the digestion of starch with distinct chain-length distributions at fully gelatinized state

Amyloglucosidase (AMG) and α-amylase (AMY) are involved in the human small intestine for starch digestion, whereas their mutual interactions with starch molecules of distinct structures are still unknown. In current...

Starch molecular fine structure is associated with protein composition in chickpea seed

Chickpea (Cicer arietinum L.) seed is a nutritional food high in starch and protein. This study aims to find the relationships between the molecular fine structure of starch and the composition of storage proteins and metabolic enzymes, using different chickpea varieties. It is found that storage proteins and starch biosynthetic enzymes influence each other. The initial formation of amylopectin molecules is affected by storage proteins, as suggested by the positive correlation (p < 0.01) between the average molecular size of amylopectin and total protein content. In addition, a higher amount of seed globulin could be an indication of higher amylose content and more short - medium amylose chains (degree of polymerization, DP, 118-2000). This study might assist selection of chickpea varieties with desirable qualities, such as low starch digestibility.

Structural factors governing starch digestion and glycemic responses and how they can be modified by enzymatic approaches: A review and a guide

Starch is the most abundant glycemic carbohydrate in the human diet. Consumption of starch-rich food products that elicit high glycemic responses has been linked to the occurrence of noncommunicable diseases such as cardiovascular disease and diabetes mellitus type II. Understanding the structural features that govern starch digestibility is a prerequisite for developing strategies to mitigate any negative health implications it may have. Here, we review the aspects of the fine molecular structure that in native, gelatinized, and gelled/retrograded starch directly impact its digestibility and thus human health. We next provide an informed guidance for lowering its digestibility by using specific enzymes tailoring its molecular and three-dimensional supramolecular structure. We finally discuss in vivo studies of the glycemic responses to enzymatically modified starches and relevant food applications. Overall, structure-digestibility relationships provide opportunities for targeted modification of starch during food production and improving the nutritional profile of starchy foods.

Effects of degree of milling on the starch digestibility of cooked rice during (in vitro) small intestine digestion

Effects of degree of milling on starch digestibility of cooked rice during (in vitro) small intestine digestion were investigated. By fitting starch digestograms to the logarithm of slope plot and combination of parallel and sequential digestion kinetics model, two starch fractions with distinct digestion rate constants were identified. Results from scanning electronic microscope and confocal laser scanning microscope showed that the rapidly digestible starch fraction (RDF) was mainly composed of gelatinized starch, while the slowly digestible starch fraction (SDF) was consisted of relatively intact starch granules, protein matrix encapsulated starch and starch-protein binary complex. The cooked rice with milling treatment had more loosely packed and larger network cells compared to that for brown rice. Consequentially, the RDF content was decreased, while that for SDF was increased by the milling treatment. These results could help the rice processing industry to develop healthy rice products with desirable starch digestibility.

Application of first-order kinetics modeling to reveal the nature of starch digestion characteristics

Mathematical modeling of in vitro starch digestograms is essential to understand starch structure-digestibility relationships as it covers all detailed information of the starch digestograms with only a few kinetics-based parameters. However, many assumptions exist for these mathematical models, which are frequently overlooked by researchers and lead to inappropriate or even wrong interpretations of the fitted parameters. This review presents a critical evaluation of four mostly applied empirical first-order kinetics models including single first-order kinetics (SK), logarithm of slope (LOS) transformed kinetics, parallel first-order kinetics (PK) and the combination of parallel and sequential (CPS) kinetics models. For homogeneous food systems, the SK model is perfectly suitable, whereas the LOS, PK and CPS models were suitably developed for food systems containing multiple digestible fractions. For the digestion of starch containing multiple digestible fractions, the LOS model assumed a sequential digestion pattern, whereas the PK model assumed a parallel pattern. In the current review, there is also emphasis on the recently developed CPS model, which is able to differentiate the sequential and parallel digestion patterns for different starch digestible fractions existing in food systems. Understanding these assumptions enables a better selection of an appropriate mathematical model for improving the understanding of in vitro starch digestion characteristics. This review meets the growing interest of the food industry in terms of developing a new generation of foods with slower starch digestibility.

A comprehensive review of the factors influencing the formation of retrograded starch

The retrogradation of starch is an inevitable change that occurs in starchy food during processing and storage, in which gelatinized starch rearranges into an ordered state. The chain length, proportion and structure of amylose and amylopectin vary in different types of starch granules, and the process is affected by the genes and growth environment of plants. The internal factors play a significant role in the formation of retrograded starch, while the external factors have a direct impact on its structural rearrangement, and the creation of suitable conditions enables food components to affect the rearrangement of starch. Interestingly, water not only directly affects the gelatinization and retrogradation of starch, but also serves as a bridge to deliver the influence of other components that influence retrogradation. Moreover, there are three mechanisms responsible for forming retrograded starch: the migration of starch molecular chains in the starch-water mixed system, the redistribution of water molecules, and the recrystallization kinetics of gelatinized starch. In this paper, the effects of internal factors (amylose, amylopectin, food ingredients) and external factors (processing conditions) on the formation of retrograded starch and the mechanism controlling these effects are reviewed.

Chemical components and chain-length distributions affecting quinoa starch digestibility and gel viscoelasticity after germination treatment

A germination treatment was explored in this study as a green strategy to reduce the in vitro starch digestibility of cooked quinoa. The alterations of chemical compositions, starch chain-length distributions (CLDs) and rheological characteristics of quinoa flours after the germination treatment were characterized. Results showed that a significant alteration of amylose CLDs and the starch digestibility was observed for cooked quinoa flours after different germination times. By fitting starch digestograms to the logarithm of slop (LOS) plot and the combination of parallel and sequential kinetics model (CPS), two starch digestible fractions with distinct rate constants were identified. Pearson correlation analysis further found that the observed starch digestive characteristics could be largely explained by the alterations of amylose CLDs caused by the germination treatment. More specifically, the rapidly digestible starch fraction mainly consisted of amorphous amylopectin molecules and amylose intermolecular crystallites. On the other hand, the slowly digestible starch fraction was largely formed by intramolecular interactions among amylose short chains (degree of polymerization (DP) < 500). These results suggest that germination may be a promising way to develop cereal products with slower starch digestibility.

Effects of different storage temperatures on the intra- and intermolecular retrogradation and digestibility of sago starch

Three different storage temperatures including room temperature (RT), 4 °C and -20 °C were investigated in this study, with respects to their effects on the retrogradation property and in vitro digestibility of gelatinized sago starch. Storage at -20 °C resulted in the highest amount of both intra- and intermolecular double helices and a fracture-like structure under scanning electron microscopy (SEM). These crystallites were more homogenous while less thermally stable than that from RT and 4 °C storage conditions. Storage at RT significantly increased the stability and heterogeneity of the formed crystallites, resulting in a sponge-like structure under SEM. Causally, the digestion rate of retrograded sago starch by α-amylase was significantly lowered after storage at -20 °C compared to that at RT and 4 °C. The crystallite heterogeneity, thermal stability, and ratio of inter- to intramolecular double helices were possibly the main driven factors for the observed digestion rates instead of the amount and micro-morphology of the crystallites. These results supply potential tools for the manufacture of food products with slower starch digestibility.

Effects of acid hydrolysis on the evolution of starch fine molecular structures and gelatinization properties

Effects of acid hydrolysis on amylose molecular structures and their relations to starch gelatinization properties were investigated. First-order kinetics models were applied to fit the evolution curve of starch chain-length and molecular size by acid hydrolysis treatment. Results showed that a single hydrolysis phase was involved in the degradation of waxy maize starch chains, while two distinct phases existed for the degradation of maize, high amylose maize and sago starch chains. The fast hydrolysis phase involved degradation of amylose chains with DP > ~300 and amylopectin long intra-cluster branches, while amylose chains with DP < ~300 was involved in the slow hydrolysis phase. Amylose molecules with DP ~ 300 were proposed to impact starch gelatinization properties by interaction with cut-off amylopectin double helices and formation of amylose crystallites/entanglements. This study could help food industry precisely control amylose molecular structures by acid hydrolysis treatment to develop starchy foods with desirable properties.

Causal relations among starch fine molecular structure, lamellar/crystalline structure and in vitro digestion kinetics of native rice starch

Causal relations among starch fine molecular structures, lamellar/crystalline structures, and the in vitro digestion kinetics of native rice starches.