d -Tagatose production in the presence of borate by resting Lactococcus lactis cells harboring Bifidobacterium longum l -arabinose isomerase

Bifidobacterium longum NRRL B-41409 l -arabinose isomerase ( l -AI) was overexpressed in Lactococcus lactis using a phosphate depletion inducible expression system. The resting L. lactis cells harboring the B. longum l -AI were used for production of d -tagatose from d -galactose in the presence of borate buffer. Multivariable analysis suggested that high pH, temperature and borate concentration favoured the conversion of d -galactose to d -tagatose. Almost quantitative conversion (92 %) was achieved at 20 g L −1 substrate and at 37.5 °C after 5 days. The d -tagatose production rate of 185 g L −1 day −1 was obtained at 300 g L −1 galactose, at 1.15 M borate, and at 41 °C during 10 days when the production medium was changed every 24 h. There was no significant loss in productivity during ten sequential 24 h batches. The initial d -tagatose production rate was 290 g L −1 day −1 under these conditions.