Comparison of Two l-Arabinose Isomerases for Multienzymatic Conversion of Lactose in Skim Milk Permeate at Neutral and Acidic pH.

The sweetness of dairy byproducts can be increased by hydrolysis of lactose to d-glucose and d-galactose and subsequent isomerization to d-fructose and d-tagatose. The isomerization of d-galactose to d-tagatose in skim milk ultrafiltration permeate at pH 4.5 and 6.5 was investigated here, comparing the l-arabinose isomerase from Lentilactobacillus parakefiri (l-AI-Lp) with l-AI-N13, a protein-engineered variant of l-AI from Geobacillus stearothermophilus. l-AI-Lp combined with a commercial β-galactosidase and xylose isomerase converted 95 g/L of lactose to d-glucose, d-galactose, d-fructose, and d-tagatose (23-24 g/L each) at pH 6.5. At pH 4.5, lactose (100 g/L) was converted by a commercial β-galactosidase and l-AI-Lp to 46 g/L d-glucose, 22 g/L d-galactose, and 23 g/L d-tagatose, while no activity was measured for l-AI-N13 at this pH. This is the first study that demonstrates the enzymatic conversion of lactose in a dairy byproduct to d-glucose, d-galactose, and d-tagatose at an acidic pH.