Evolutions of rheology, microstructure and digestibility of parboiled rice during simulated semi-dynamic gastrointestinal digestion

Physicochemical properties, phenolic content and in vitro digestion profile of Chinese black rice (Oryza sativa L.)

Yangxian black rice, as one of the ancient Chinese black rice varieties, is widely planted in the Yangxian area of China. This study investigated the physicochemical properties, phenolic content and in vitro digestion profile of Chinese black rice under gradient milling treatment. The chemical composition, color, pasting and thermal properties of black rice with different milling degrees were comprehensively compared. In vitro digestion analysis indicated that cooked rice flour had higher rapidly digestible starch (RDS) and lower resistant starch (RS) contents compared with the uncooked one. Besides, all cooked black rice samples exhibited high predicted glycemic index (pGI) value and whole black rice showed a lower pGI than refined rice. The microstructure and the abundance of phenolic compounds in the solid matrix during different treatments or digestion stages were observed by CLSM. Furthermore, a total of 102 phenolic constituents were absolutely quantified by targeted metabolomics techniques. Methanol extraction and moderate cooking treatment contributed to the release of phenolic compounds from the solid matrix of whole black rice. Besides, compared to the gastric digestion stage, the transition in the intestinal environment caused a decrease in the majority of the analyzed polyphenols. Identifying the phenolic constituents was favorable for a better elucidation of the chemical basis of the function and nutritional value of Chinese black rice.

The surface mechanics of cooked rice as influenced by gastric fluids measured using a micro texture analyzer

In this study, a micro texture analyzer (MTA) was employed to explore the texture characteristics of the surface of an individual steamed rice (SR) and fried rice (FR) grain exhibited in four simulated digestion environments in vitro. The elastic modulus, hardness and elastic index of the single cooked rice particle were measured using the MTA. The hardness of SR particles decreased by 66, 81, 89.1, and 95% after simulated digestion in distilled water, HCl, simulated gastric fluid (SGF), and simulated salivary and gastric fluid (SSF + SGF), respectively. This is in line with the most significant volume expansion and structure ruptures when digested in SSF + SGF. Similar mechanical and structural behaviors were shown for FR, but the hardness and elastic modulus decreased less than those of SR under the same digestion conditions. The different surface mechanics are consistent with the reduced expansion and more compact structure with smaller voids in FR during in vitro digestion. This could be attributed to the encapsulation by frying oil on the surface that would retard the diffusion of digestive fluids into the rice kernels. A weak negative correlation was found between the elastic modulus and the moisture content of the cooked rice. The present study has quantitatively assessed the surface mechanics of cooked rice as influenced by gastric fluids using the MTA. This is practically meaningful for gaining an in-depth understanding of the influence of textural modifications on disintegration of solid foods and release of nutrients during digestion.

Physicochemical and Digestible Properties of Parboiled Black Rice With Different Amylose Contents

The varieties of black rice with different amylose contents (waxy; medium-amylose: 16.03%; high-amylose: 27.14%) were parboiled and then evaluated for physicochemical and digestible properties. The color, crystalline structure, and swelling property of parboiled rice were analyzed, and the water molecular mobility, texture, and starch digestibility of cooked parboiled rice were determined. The color of black rice was only slightly changed after the parboiling treatment. The crystalline structures of waxy and medium-amylose rice were severely damaged by the parboiling treatment, while the highly crystalline structure was retained in parboiled high-amylose rice. During heating in water, parboiled high-amylose rice had a lower water absorption ratio (WAR) and volume expansion ratio (VER) than the other two varieties. After cooking, parboiled high-amylose rice had higher water molecular mobility and harder texture compared with the other two varieties. Cooked parboiled high-amylose rice contained higher content of resistant starch than cooked parboiled waxy and medium-amylose rice.

A Review of In Vitro Methods for Measuring the Glycemic Index of Single Foods: Understanding the Interaction of Mass Transfer and Reaction Engineering by Dimensional Analysis

The Glycemic Index (GI) has been described by an official method ISO (International Organization for Standardization) 26642:2010 for labeling purposes. The development of in vitro methods for GI measurement has faced significant challenges. Mass transfer and reaction engineering theory may assist in providing a quantitative understanding of in vitro starch digestion and glycemic response from an engineering point of view. We suggest that in vitro GI measurements should consider the mouth and the stomach in terms of fluid mechanics, mass transfer, length scale changes, and food-solvent reactions, and might consider a significant role for the intestine as an absorption system for the glucose that is generated before the intestine. Applying mass transfer and reaction engineering theory may be useful to understand quantitative studies of in vitro GI measurements. The relative importance of reactions and mass-transfer has been estimated from literature measurements through estimating the Damköhler numbers (Da), and the values estimated of this dimensionless group (0.04–2.9) suggest that both mass transfer and chemical reaction are important aspects to consider.