The aim of the work is to produce D-Tagatose by direct addition of alginate immobilized Lactobacillus plantarum cells to lactose hydrolysed whey permeate. The cells were untreated and immobilized (UIC), permeabilized and immobilized (PIC) and the relative activities were compared with purified L-arabinose isomerase (L-AI) for D-galactose isomerization. Successive lactose hydrolysis by beta-galactosidase from Escherichia coli and D-galactose isomerization using L-AI from Lactobacillus plantarum was performed to investigate the in vivo production of D-tagatose in whey permeate. In whey permeate, maximum conversion of 38% and 33% (w/w) D-galactose isomerization by PIC and UIC has been obtained. 162 mg/g and 141 mg/g of D-tagatose production was recorded in a 48 h reaction time at 50 degrees C, pH 7.0 with 5 mM Mn2+ ion concentration in the initial substrate mixture. (C) 2017 Elsevier Ltd. All rights reserved.