Nutritional Compositions, Polyphenolic Profiles, Modulation of Gut Microbiota, Glycemic Index and Some Biological Properties of Pigmented Rice Varieties and Adlay Influenced by Soaking and Germination Conditions

Abstract

Pigmented rice varieties are gaining interest due to their superior nutritional and phenolic properties compared to white rice varieties. With the rising risk of diabetes, cereals with higher protein contents should be consumed with rice in order to reduce the risk of diabetes. Adlay is one of the underutilized crops with higher protein contents and other functional properties. The germination process is one of the easy and innovative techniques for improving functional properties in seeds. Therefore, the aim of this study was to evaluate the effects of soaking and germination on the physicochemical properties, polyphenolic profiles and antioxidant activities of pigmented rice varieties and adlay. Two pigmented rice varieties and adlay seeds were soaked (24 h) and germinated for 12, 24, 36, and 48 h. The physicochemical properties, polyphenolic profiles, and the antioxidant activities were evaluated. Purple rice (PR) demonstrated the highest value of polyphenolic content, with 9 compounds detected for antioxidant activities. There were 6 phenolic compounds detected for red rice (RR). The adlay seeds had the least concentrations of phenolic compounds, with 6 compounds and higher nutritional properties identified. The following pheolic compounds were synthesized after soaking and germination, namely; gallic acid in purple rice; gallic acid, catechin and caffeic acid in red rice; vanillin in adlay, respectively. Among the red and purple rice samples, 24 h soaking gave the best results for phenolic and antioxidant properties, with 24 h germination in adlay seeds. Germination at 48 h yielded higher nutritional values in all the samples. The present study demonstrates how the process of soaking is a cheap and less time-consuming process of improving bioactive compounds and antioxidant activities in pigmented rice varieties and adlay seeds. Bioactive components and anti-oxidative properties of purple rice after 24 h soaking and 36 h germination periods were investigated. The in vitro starch digestibility and glycemic index (GI) of soaked and germinated purple rice samples were determined. In addition, their prebiotic properties on the gut microbiota and their metabolites were investigated via batch culture fecal fermentation. The anti-inflammatory properties of the methanolic extracts of the purple rice were also evaluated on macrophage cells, stimulated with lipopolysaccharide. These conditions significantly increased (P < 0.05) the bioactive components and anti-oxidative properties. Total anthocyanin contents, total carotenoid contents and gamma-aminobutyric acid (GABA) were increased more than twofold in the soaked sample. All the extracts led to cell proliferation and inhibited nitric oxide (NO) production. Twenty-four hour soaking of purple rice exhibited highest bioactive compounds, anti-oxidative properties and showed greater NO inhibitory effects with IC50 of 234.00 ± 0.01 μg mL. In addition to established information on germination, soaking greatly improved the bioactive components and anti-inflammatory properties of the purple rice variety. Whole grain rice is known to contain fiber, phytochemicals and nutrients that could improve human health. The results from the study revealed that soaking and germination conditions significantly reduced (P < 0.05) the GI (72.64 ± 0.75 to 61.52 + 3.11), but increased the total starch contents (58.1 ± 0.06 to 63.9 ± 0.09%). Lowest values for the GI were recorded in the soaked purple rice sample. All the samples evaluated in this study affected the gut microbiota in fecal batch culture. Interestingly, the soaked purple rice selectively increased the amount of bifidobacteria (log 9.62 to 10.21) and lactobacilli (log 8.55 to 9.30) groups and at 48 h fermentation and significantly inhibited (P < 0.05) the growth of clostridia and bacteroides groups. This correlates with the increasing production and highest concentration of acetate (35.27 ± 0.09 mM) in this purple rice after 48 h fermentation. It also exhibited a prebiotic index (PI) score of 0.79, thus indicating its potential gut microbiota modulation. The phenolic metabolites identified in the fecal fermented medium include 3-(4-hydroxyphenyl) propionic acid, pyrocatechol, trans-cinnamic acid and salicylic acid. Therefore, the soaked purple rice could be a potential functional hypoglycemic food products in food and nutraceuticals industry for gut microbiota modulation and reducing risks of colon cancer. In conclusion, the best of our research knowledge obtained from this study would be about the process of 'soaking' which is an easy, innovative and economic technique of increasing bioactive components of rice with improved anti-inflammatory properties, lower glycemic index and gut microbiota modulation potential that could confer more health benefits to humans.