Streptococcus thermophilus (S. thermophilus) is a common starter in yogurt production and plays an important role in the dairy industry. In this study, a galactose-positive (Gal+) mutant strain, IMAU20246Y, was produced using the chemical mutagen N-methyl-N'-nitro-N-nitrosoguanidine (NTG) from wild type S. thermophilus IMAU20246 which was known to have good fermentation characteristics. The sugar content of milk fermented by either the mutant or the wild type was determined using high performance liquid chromatography (HPLC); metabolism of lactose and galactose was significantly increased in the mutant strain. In addition, we used the response surface methodology to optimize components of the basic medium M17 for survival ratio of the mutant strain. Under these optimal conditions, the viable counts of mutant S. thermophilus IMAU20246Y reached 4.15 × 108 cfu/mL and following freeze drying in the medium retained cell viability of up to 67.42%. The above results are conducive to production of a high vitality starter culture and development of 'low sugar, high sweetness' dairy products.
Keywords: N-methyl-N'-nitro-N-nitrosoguanidine; Streptococcus thermophilus; galactose-positive (Gal(+)) mutant; response surface methodology.
The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).