The role of starch granule-associated proteins in enhancing the strength of indica rice starch gels

The gel properties of indica rice starch strongly influence the quality of foods in which it is used. This study investigated the effects of chymotrypsin treatment on the rheological and pasting properties of starch prepared by different treatment methods. The results demonstrated that all the starches contained starch granule-associated proteins (SGAPs). Specifically, chymotrypsin-treated fermented extracted starch (E-FS) and chymotrypsin-treated alkali extracted starch (E-AS) exhibited a storage modulus (Gʹ) about 4 times higher than that of fermented extracted starch (FS) and alkali-extracted starch (AS). The flow consistency coefficients (K), loss moduli (Gʺ) and setback values tended to increase, while the characteristic rheological indices (n) and loss tangent (tan δ) tended to decrease after chymotrypsin treatment. This suggested that the starch gel was significantly enhanced by the proteinase modification. The protein composition of AS and E-AS only contained the 66 kDa of granule-bound starch synthase I (GBSS I) subunit. Therefore, the significant increase in gel strength was attributed to the exclusive modification effects of GBSS I. Microstructural analysis also confirmed that the three-dimensional gel network of indica rice starch was formed by a backbone of SGAP polymers filled with starch granules. The mechanism underlying GBSS I-enhanced gel was also elucidated based on the sequence and structure of GBSS I. The gel network formed by SGAP polymers of FS and AS was mainly caused by disulfide bonds between GBSS Is, while that of E-FS and E-AS was mainly caused by hydrophobic interactions.