Abstract
Free radical-induced oxidative stress is the root cause of many diseases, such as diabetes, stress and cardiovascular diseases. The objective of this research was to screen GABA levels, antioxidant activities and bioactive compounds in brown rice. In this study, we first fermented brown rice with different lactic acid bacteria (LABs), and the best LAB was selected based on the levels of GABA in the fermentate. Lactobacillus reuterii generated the highest levels of GABA after fermentation. To ascertain whether germination can improve the GABA levels of brown rice, we compared the levels of GABA in raw brown rice (Raw), germinated brown rice (Germ), fermented brown rice (Ferm) and fermented-germinated brown rice (G+F) to identify the best approach. Then, antioxidant activities were investigated for Raw BR, Germ BR, Ferm BR and G+F BR. Antioxidant activity was calculated using a 2,2-diphenyl-1-picryl hydrazile radical assay, 2,2-azino-bis-(3-ethylene benzothiozoline-6-sulfonic acid) radical assay and ferric-reducing antioxidant power. In Ferm BR, DPPH (114.40 ± 0.66), ABTS (130.52 ± 0.97) and FRAP (111.16 ± 1.83) mg Trolox equivalent 100 g, dry weight (DW), were observed as the highest among all samples. Total phenolic content (97.13 ± 0.59) and total flavonoids contents (79.62 ± 1.33) mg GAE/100 g and catechin equivalent/100 g, DW, were also found to be highest in fermented BR. Furthermore, an untargeted metabolomics approach using ultra-high-performance liquid tandem chromatography quadrupole time of flight mass spectrometry revealed the abundance of bioactive compounds in fermented BR, such as GABA, tryptophan, coumaric acid, L-ascorbic acid, linoleic acid, β-carotenol, eugenol, 6-gingerol, etc., as well as bioactive peptides which could contribute to the health-promoting properties of L. reuterii fermented brown rice.
Highlights
Rice (Oryza sativa) is one of the most important and stable human food crops in the world
These results show L. reuterii as the most efficient bacteria in Gamma-Aminobutyric Acid (GABA) production during fermentation among all other
We identified that fermented brown rice exhibited high antioxidant activities as well as a significant amount of phenolic compounds and flavonoids, as compared to raw, germination and germination combined with fermentation samples
Summary
Rice (Oryza sativa) is one of the most important and stable human food crops in the world. Rice grain is a major source of carbohydrate and protein, as well as other important nutrients, for billions of people worldwide, in developing countries [1]. Brown rice (BR) has gained growing attention due to its obvious advantages over white rice for the content of bioactive compounds [2], as it is known that the wrong diet may lead to an imbalance between free radical formation and radical scavenging ability, causing oxidative stress [3]. A lower reactive oxygen species (ROS) concentration is important for normal cellular signaling, higher concentrations and long-term exposure to ROS cause damage to cellular macromolecules such as DNA, lipids and proteins, eventually leading to necrosis. It is well known that the brain absorbs a significant amount of oxygen and is very high in lipid content and is vulnerable to oxidative stress [5]. Neuronal membranes are found to be rich in polyunsaturated fatty acids, which are susceptible to ROS [5]
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