Optimization of compound enzymes solution pretreatment for improving germinated brown rice yield

Abstract: The purpose is to solve the problems about long soak time of germinated brown rice as well as low production efficiency in traditional production process. A novel processing technology is developed using compound enzyme solution pretreatment with cellulase and xylanase rather than distilled water soak brown rice during the process of producing germinated brown rice. The brown rice's tough cortex is mainly caused by the coarse fiber structure, whose major components are cellulose and araboxylan. The xylan layer with its covalent linkage to lignin and its noncovalent interaction with cellulose may play a significant role in maintaining the integrity of the cellulose in situ and protecting the fibers against degradation by cellulases. Using compound enzyme pretreatment to degrade some cortex resulted in a significant increase in water absorption rate over brown rice. What's more, the brown rice obtained a suitable water content to sprout in a much shorter time. By shortening the soak time, it could improve the efficiency of germinated brown rice production. Brown rice degraded some cortex during soaking, which thereby reduced the block of cortex to water. Compared with soaking methods, the new method not only reduced the soak time but also improved the germinated rate. Beyond that, a central composite rotatable orthogonal experimental design of response surface methodology with 4 factors and 5 levels was employed. Taking raw material of brown rice, the influences of the 4 parameters including enzyme treatment time, enzyme treatment temperature, concentration of compound enzyme solution, percentage of cellulase quality on the germinated brown rice yield as well as the optimal parameters of this process were investigated. Additionally, mathematical models for the influences of various parameters on the germinated brown rice yield were set up. As shown, the mathematical models for the germinated brown rice yield were extremely significant (P<0.01). Meanwhile, the optimum process parameters for maximizing the germinated brown rice yield were 0.57 g/L of enzyme concentration, 35 ℃ of enzyme treatment temperature, 135 min of enzyme treatment time and 1.86:1 of cellulose-to-xylanase mass ratio, respectively. Moreover, the germinated brown rice yield and the content of γ-aminobutyric acid (GABA) under the optimum parameters were 3.90% and 3.86 mg/(100 g), which was higher than that of germinated brown rice produced by soaking method. Compared with conventional soaking method, compound enzyme pretreatment method significantly shortened the time of water absorption (135 min), in which soak time was reduced by 62.5%. As a result, it greatly improved the efficiency of germinated brown rice production. Through enzymatic soaking pretreatment, seed germination can be promoted. Brown rice has the characteristic of seed. Compound enzymatic soaking pretreatment will not only wake up the dormancy of brown rice but also improve the activity of endogenous enzyme to bud. GABA in germinated brown rice is produced by glutamic acid, and it removes carboxyl under the effect of glutamate decarboxylase (GAD). Moreover, the activity of GAD is closely related to the accumulation of GABA. The optimum pH of GAD is acidity. The brown rice soaked in acidic environment of compound enzyme solution increases the activity of GAD, which will be conducive to generation of GABA. Surface structure of the germinated brown rice degraded by enzyme was studied by Scanning Electron Microscopy. Microstructure of brown rice cortex confirmed that part of the cortex degraded because the brown rice soaked in compound enzymes solution. Furthermore, hydration characteristics of brown rice under different soaking conditions showed that the water absorption rate of brown rice soaked in compound enzyme solution was significantly faster than that soaked in distilled water. Starch in the endosperm was easier to combine with water molecules, and brown rice with a suitable water content to sprout in a much shorter time was obtained. By utilizing the technology described in this paper, the research results can provide a reference to improve industrial production of germinated brown rice.