Rice Bran Stabilization by Improved Internal and External Heating Methods.

Abstract

Rice bran stabilization was conducted by internal (microwave), external (oven drying and steam retorting) heating and non-heating ($\sp{60}$Co irradiation) treatments to improve currently used methods. Two commercial rice varieties cultivated in Louisiana, one long grain (Lemont) and a medium grain (Nato), were milled to collect bran samples. The bran was analyzed for its proximate composition and various physical and thermal characteristics. The proximate composition for long and medium grain bran, respectively, was: moisture, 11.2% and 10.8%; protein, 16.07% and 16.20%; fat, 19.20% and 21.97%; crude fiber, 8.49% and 8.41%; ash, 9.23% and 9.46%; and nitrogen-free extract, 47.01% and 44.07%. Physical and thermal properties for corresponding brans, respectively, were: bulk density, 18.20 lb/ft$\sp3$ and 17.17 lb/ft$\sp3$; porosity, 73.0% and 72.1%; particle density, 67.60 lb/ft$\sp3$ and 61.20 lb/ft$\sp3$; specific heat, 0.41 Btu/lb $\sp{\rm o}$F and 0.41 Btu/lb $\sp{\rm o}$F; thermal diffusivity, 5.17 $\times$ 10$\sp{-3}$ ft$\sp2$/hr and 5.29 $\times$ 10$\sp{-3}$ ft$\sp2$/hr; and thermal conductivity, 3.85 $\times$ 10$\sp{-2}$ Btu/hr ft $\sp{\rm o}$F and 3.72 $\times$ 10$\sp{-2}$ Btu/hr ft $\sp{\rm o}$F. The rice bran stabilized by four methods was packed in brown paper bags and stored in an extremely unfavorable environment (33 $\pm$ 2$\sp\circ$C, 75 $\pm$ 5% relative humidity) to test the efficacy of each of the four stabilization methods. Bran treated by microwave heating was found to be very stable. At the end of the four week storage period, free fatty acids (FFA) increased only from 4.0% to 4.9% in long grain bran and 4.6% to 6.2% in medium grain bran, while in untreated bran it increased from 4.0% to 68% and 4.6% to 56.8%, respectively. Bran stabilized by microwave heating was as well as steam retorted bran in terms of FFA and better in color and nutrition. Oven dried bran was found to be unstable at high temperature and under high humidity conditions, and so was the bran stabilized by irradiation. Heat transfer efficiency analysis revealed that microwave heating is superior to either external heating methods for bran stabilization. Multiple regression analysis indicated that the initial moisture content, weight of the rice bran and microwave heating time are three important variables for the inactivation of bran peroxidase and lipase. Given the microwave heating strength constant, optimum stabilization effect was obtained with 21% initial moisture content in rice bran.