Extraction of defatted rice bran protein by solid-state fermentation and characterization of the protein

Abstract

Defatted rice bran (DRB) is an abundant by-product of rice bran oil industries and a potential source of nutritious and hypoallergenic protein content. The most common alkaline extraction of defatted rice bran protein (DRBP) possed undesirable characteristics, making nutritional and functional compromises. However, enzymatic extraction could extract protein with improved protein properties despite being expensive. Therefore, this thesis exploited solid-state fermentation (SSF), which is an economically sustainable and natural approach to facilitate the extraction of DRBP using two fermentation starters, Loog-pang (Thai wine starter) and Koji.

The feasibility of two fermentation starters, Loog-pang and Koji, in producing enzymes and their efficacy in extracting protein from DRB were investigated. SSF of DRB was carried out at different times and Loog-pang and Koji fermented DRB obtained was further hydrolyzed for 24 h to enhance the protein extraction. The results obtained indicated that both the fermentation starters could secrete cellulase and protease (acid and neutral) enzymes with almost similar effectiveness in DRBP extraction. The fermented DRB (72 h) followed by hydrolysis (24 h) process revealed maximum protein extraction with both the fermentation starters. Loog-pang and Koji fermentation could extract protein of 65.66 and 65.67 g/100 g DRB, respectively. However, the defatted rice bran protein hydrolysate (DRBPH) presented a significant quantity of ash content which impaired the protein purity (27.83 and 29.36 g/100 g DRBPH with Loog-pang and Koji, respectively). Therefore, the DRB was dephytinized at pH 2 to obtain dephytinized defatted rice bran (DDRB) and SSF was carried out with a Loog-pang fermentation starter to evaluate the protein extraction. The result revealed that 48 h of SSF followed by 24 h hydrolysis could extract maximum protein content of 59.44 g/100 g DDRB. The DDRBPH prepared contained very low ash content (2.73 g/100 g DDRBPH) with better protein purity (37.23 g/100 g DDRBPH).

The SDS-PAGE protein profile showed that 72 h fermented DRB for Loog-pang and Koji obtained protein of diverse molecular weights ranging from 10-100 KDa. These polypeptides were later hydrolyzed and fragmented into smaller molecular weight peptides during 24 h hydrolysis and enhanced protein solubility. Glutamic acid followed by aspartic acid, leucine, arginine, alanine, and glycine were the most abundant amino acid present in both non-fermented and fermented DRBP. SSF assisted to extract protein without altering the amino acid profiles to that of non-fermented DRBP. This indicated the effectiveness of SSF on DRBP extraction which helped to improve protein yield and maintained the amino acid profile.

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