Impact of physical and biochemical pre-treatments on dry fractionation of rice bran proteins by air classification: Master's thesis

Rice bran is an underutilized by-product of rice milling industry which is mainly used as feed or in energy production but could provide a feasible plant based raw material for food products when further processed. From nutritional viewpoint, rice bran contains 13.1-17.3% protein which can be seen superior to other cereal proteins due to its hypoallergenic and gluten-free properties and high lysine content. The aim of this master's thesis was to study the effect of different pre-treatments on dry fractionation of rice bran proteins by air classification. The objective was to elucidate the effect of fat extraction, particle size reduction, air classification and sieving on the raw material structure, composition and protein enrichment by air classification. Effect of carbohydrate hydrolysing enzyme treatments in low moisture content followed by washing and dehydration with ethanol was also studied in terms of protein enrichment during air classification. Moreover, protease-aided solubilisation of the liberated rice bran proteins was studied. Fat extraction and milling were found out to be the most essential pre-treatment steps allowing protein enrichment from rice bran by air classification. When utilizing one-step air classification after fat extraction and milling, the protein content could be increased 8.2 percentage points. However, the more the protein content increased, the more the total protein yield decreased. Approaches utilizing two-step fractionations showed that by air classifying the raw material prior to milling the protein fractionation could be enhanced. Especially, by milling and further air classifying the coarse fraction accounting for 70% of the raw material, a fine fraction with even a higher protein content compared to the one-step approach was obtained. This protein content was the highest achieved in this work, and the most critical factor restricting further enrichment was suggested to be the intensive particle size reduction of the non-protein structures and the inefficient detachment of the residual proteins from other bran components. Additionally, further air classification of the fine fraction accounting for 30% of the raw material enabled almost total removal of pericarp structures from fine fraction. Pre-treatment with cell wall degrading enzymes did not improve the protein fractionation efficacy by air classification. Protease treatment of the liberated proteins was able to solubilise 75% of the raw material protein suggesting that some interactions took place in the residue between the proteins and other bran components thus restricting the solubility.