Study On The Effect Of High Pressure On Main Physicochemical Properties Of Brown Rice

Brown rice has been widely concerned because of its high nutritional value. However, its consumption is limited by its poor palatability and long cooking time. High pressure is an emerging nonthermal processing technology, whose effect on modification of starch has been reported by many studies. Our previous research has confirmed that high pressure processing can well maintain nutritional quality of brown rice, shorten the cooking time and improve its favor as well as taste. However, mechanism of high pressure effect onbrown rice remains unclear.The effect of high pressure treatment on the composition of brown rice including moisture content, crude protein, crude fat and ash was studied. High pressure influence on drying and sorption properties and equilibrium moisture content of brown rice were compared. The water distribution of brown rice treated with different high pressure conditions was analyzed. Combining the changes in thermal and rheological properties, the mechanism of high pressure effect on main physicochemical properties of brown rice is dicussed. The condition of high pressure treatment is 100-500 MPa, and the holding time is 5,10 and 15 min, and the results are as follows:1. The moisture content of brown rice was between 11.86-12.80%, the crude protein content was about 8.30%, the crude fat content of brown rice was between 2.15%-2.60%, the ash content was about 1.06%. Except high pressure (lower pressure 100,300 MPa, holding time of 5 min) slight influences on crude fat, high pressure has no significant effect on these composition.2. Brown rice starch granules remain particle shape in microstructure after high pressure processing. However, the surface is crack, aleurone layer becomes thinner and with increasing high pressure, damage on structure is more severe.3. The moisture content of high pressure treated brown rice decreased slightly in the drying process and weight changed slowly after 40-50min soaking. The water absorption capacity of brown rice was poorer after high pressure processing. With the increase of the level of high pressure, the swelling power of brown rice increased significantly, but there was little difference among different pressures.4. The moisture sorption isotherm of brown rice is S type. The higher the temperature is, the lower the equilibrium moisture content of brown rice at 20,30, 40℃. The pressure enhances the temperature effect and 300 MPa sample acquried the lowest equilibrium moisture content of all samples. The absorption and desorption curves of brown rice form a hysteresis loop, and the hysteresis area of 300 MPa sample is the largest. The low field nuclear magnetic resonance spectra of the brown rice samples under different equilibrium relative humidity were different. T2b1 or Tab accounted for 72-89% of the water in the sample. T2b of 300 MPa treated sample has significant difference with other high pressure processing conditions, indicating the decline in mobility.5. With the increase of the pressure and holding time, the gelatinization parameters showed a decreasing trend, and the gelatinization degree of brown rice increased. The high pressure sample had significant difference with control. The effect of pressure on gelatinization was not continuous.6.The pasting parameters of the samples after high pressure treatment showed significant (P<0.05) decrease, and the 300 MPa treated samples showed the lowest peak viscosity. The brown rice paste is a typical non Newtonian and weak gel structure. After high pressure treatment, the apparent viscosity and shear stress decreased, rigidity and elasticity of the gel increased.