Effect of lipase addition on inhibition of starch retrogradation in rice

We investigated whether the production of the free fatty acids (FFAs) by the addition of lipase during rice cooking inhibits starch retrogradation, and to examine whether proteins are involved in this process. Rice milled with milling ratios of 90%, 80%, 70%, and 60% and alkaline-soaked raw rice (AS) were prepared to obtain samples with different protein content. The protein content decreased from 8.24% to 5.83% as the milling ratio decreased from 90% to 60%, and AS contains 4.58% of protein. The prepared rice was cooked with and without lipase, and then treated with two freeze-thaw cycles to accelerate starch retrogradation. The addition of lipase increased the FFAs content and the peroxide value form 1.65 meq/kg to 4.75 meq/kg and decreased the free thiol (SH) content in cooked rice. The degree of swelling was reduced by 0.14 during cooking with the addition of lipase in cooked rice with a milling ratio of 90%. The degree of starch retrogradation (Dr) tended to increase as the milling ratios decreased from 90% to 60% and AS showed Dr similar to milling ratio of 60%. The degree of amylose-lipid complexes formation, which is involved in the inhibition of starch retrogradation, did not change with or without lipase, suggesting that the inhibitory effect of lipase became smaller as protein decreased. These results suggest that the FFAs produced by lipase converted the free SH of the protein into disulfide bonds, which altered the swelling characteristics of the starch and eventually inhibited the starch retrogradation.