Effect Of Freeze-thaw Cycle On Physicochemical Properties Of Brown Rice

In this study,“Chuanyou” indica rice and “Panjin” japonica rice were used to investigate the effect of freeze-thaw cycle(FTC)treatment on the quality of brown rice(BR).The parameters were as follows: soaking water content(14.8%,27.3% and39.8%),number of freeze-thaw cycles(0,1,3 and 5).The advantages of FTC treatment were analyzed from the aspects of cooking properties,textural properties,antioxidant activity and microstructure compared with high pressure processing brown rice(HPPBR),germination-parboiling brown rice(GPBR),untreated BR and white rice.Results were shown as follows:1.The cooking time of BR decreased with the increase of water content and number of freeze-thaw cycles,decreased by up to 27.2%.Freezing and thawing for 1time at high water content(39.8%)is similar to freezing and thawing for 5 times at medium moisture content(27.3%).The water absorption index(WAI)and volume expansion index(VEI)increased with the increase of soaking water content and number of freeze-thaw cycles.The WAI and VEI of “Chuanyou” BR 39.8%-5 treatment group was close to white rice level.FTC treatment under high treatment strength conditions can increase the iodine-blue value of rice soup.The results of iodine staining showed that the FTCBR had more fluffy structure.FTC treatment can make the rice bran layer more susceptible to breaking and increase the micro-crack in endosperm.During the cooking process,the endosperm starch that breaks through the bran layer is in full contact with water,thereby decreasing the cooking time and increasing the WAI as well as the rice soup solid content and the iodine-blue value.2.With the increase of water content and the number of freeze-thaw cycles,the hardness of rice decreased significantly.The softening effect of FTC treatment on the texture of “Chuanyou” BR is particularly obvious.After freezing and thawing for 1time at 39.8% soaking water content,BR can reach the softness of white rice.Except for the very few “Panjin” BR group,the FTC processing had no significant effect on the cohesiveness,springiness,gumminess and chewiness of brown rice.3.FTC processing can retain or even increase phenolic content(97.2%-106.6%),flavonoid content(96.3%-106.2%)and ABTS radical scavenging capacity(89.8%-108.7%)in BR,can reach up to 3.0,3.1 and 4.6 times of white rice,respectively.The results showed that soaking had significantly effect on antioxidant activity,while the number of freeze-thaw cycles not.4.FTCBR,HPPBR and GPBR displayed in an A-type starch XRD,these three treatments increase,maintain and reduce relative crystallinity,respectively.The results of scanning electron microscopy(SEM)showed that the surface of the FTCBR was everted and some aleurone particles disappeared.The HPP treatment partially denatured the protein in the aleurone,and destroyed the boundary between the aleurone layer and the endosperm.The GP treatment made the peel and the seed layers loose,the protein in the aleurone was partially denatured,the boundary of the endosperm starch became blurred,and the primary micro-crack healed.The microstructure of the rice showed that the network structure of the HPPBR endosperm was small and uniform.The structure of GPBR network partially collapsed.The collapsed in FTCBR was more severely,wide and long cracks appeared.5.The effect of FTC treatment on the cooking properties and the hardness of BR is much better than that of HPP and GP treatment.The retention rate of antioxidant substances in FTCBR is the same as that of HPPBR,and higher than that of GPBR.