Potato starch modified by Streptococcus thermophilus GtfB enzyme has low viscoelastic and slowly digestible properties.

Potato starch with high viscosity and digestibility cannot be added into some foods. To address this issue, a novel starch-acting enzyme 4,6-α-glucosyltransferase from Streptococcus thermophilus (StGtfB) was used. StGtfB decreased the iodine affinity and the molecular weight, but increased the degree of branching of starch at a mode quite different from glycogen 1,4-α-glucan branching enzyme (GBE). StGtfB at 5 U/g substrate mainly introduced DP 1-7 into amylose (AMY) or DP 1-12 branches into amylopectin (AMP), and increased the ratio of short- to long-branches from 0.32 to 2.22 or from 0.41 to 2.50. The DP 3 branch chain was the most abundant in both StGtfB-modified AMY and StGtfB-modified AMP. The DP < 6 branch chain contents in StGtfB-modified AMY were 42.68%, much higher than those of GBE-modified AMY. StGtfB significantly decreased viscoelasticity but still kept pseudoplasticity of starch. The modifications also slowed down the glucose generation rate of products at the mammalian mucosal α-glucosidase level. The slowly digestible fraction in potato starch increased from 34.29% to 53.22% using StGtfB of 5 U/g starch. This low viscoelastic and slowly digestible potato starch had great potential with respect to low and stable postprandial blood glucose.

A cold-active 1,4-α-glucan branching enzyme from Bifidobacterium longum reduces the retrogradation and enhances the slow digestibility of wheat starch.

To develop a 1,4-α-glucan branching enzyme (BE) without homology to known allergens, the glgB gene from Bifidobacterium longum was overexpressed under the control of BLMA promoter in Escherichia coli. B. longum BE (BlBE) had a molecular weight of 86.1 kDa and a specific activity of more than 18.5U/mg protein at 25-35 °C and pH 5.5-7.0, and exhibited 30% of the maximum activity at 10 °C. The cold-active BlBE preferred to transfer maltohexaose and introduced DP 4-36 branches into amylose. BlBE also increased the proportion of DP 2-10 branches in amylopectin and decreased its Mw from 1.39 × 106 to 1.16 × 105 g/mol. As the BlBE concentration increased from 0.0 to 0.5U/mg substrate, the retrogradation enthalpy of BlBE-modified wheat starch decreased from 4.50 to 1.83 J/g (p < 0.05) at day 14 and the slowly digestible starch content increased from 2.10% to 17.39% (p < 0.05).