H-T. Lin, T-H.D. Ho, A-I. Yeh
National Taiwan University,
Keywords: amylomaltase, freeze-thaw stability, chain length distribution, molecular weight
Summary:Amylomaltase (AM), a glucanotransferase, can hydrolyze and transfer fragments of α-glucan chains from one α-glucan molecule to another by α-1,4 linkage. Recent reports showed that the action of amylomaltase creates and improves some starch functionalities, such as thermoreversible gelling properties, slow digestibility, freeze-thaw stability and creaminess properties. In this study, the amylomaltase from Thermus Thermophilus expressed in Escherichia coli BL21 (DE3) was used to modify potato starches with various processing conditions (0.1-10 U/g starch for 10 min) by Brabender Viscoamylograph. One unit (U) of AM activity is defined as enzymes that produce 1 μmole of glucose per minute from maltotriose at 70oC. The freeze-thaw stability and structural properties of potato starches have been determined and compared with control groups. Repeated freezing/thawing led to a significant increase in syneresis. The syneresis percentage (62.31%) of control group was higher than AM treated after 5 times of freezing/thawing cycles, especially AM01U (33.29%) and AM1U group (38.96%). The data showed that AM treated starches have better freeze-thaw stability. Chain length distribution of starches were analyzed by SEC-MALS-RI detection after debranching by isoamylase. The DPw (weight average of degree of polymerization) of Fr Ⅰ (amylose fraction) increased from 3655 to 6616 after 0.1 U amount of AM treatment, but decreased to 3151 for 1 U treatment and 232 for 10 U treatment. The DPw of Fr Ⅱ (longer chain of amylopectin, B2+B3+B4 chain) and Fr Ⅲ (shorter chain of amylopectin A+B1 chains) were reduced with AM treatments. The lower amount of AM treatment, especially 0.1 U treatment, the greater is the chain length and polydispersity. The more AM treatment (greater than 1U /g starch), the lower is the chain length and molecular weight, which would be contributed to higher hydrolysis activity of Amylomaltase at high dose.