D-Tagatose production in the presence of borate by resting <i>Lactococcus lactis</i> cells harboring<i> Bifidobacterium longum</i> 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