Optimization of overproducing S-adenosyl-Lmethionine <i>Saccharomyces cerevisiae</i> S-W55 mutant utilizing unpolished rice from aging paddy by feeding Lmethionine

Abstract

The medium and fed-batch fermentation by Saccharomyces cerevisiae S-W55 were optimized. The unpolished rice from aging paddy was used as major nutrient source to reduce the raw material cost of SAM fermentation. The unpolished rice saccharificate (URS) and yeast extract were employed as carbon source and nitrogen source, respectively. The dosages of URS and yeast extract in the medium compositions were optimized by response surface methodology (RSM). As a result, when the fermentation was carried out under the optimal conditions for URS (51.4 g/L) and yeast extract (4.74 g/L), the SAM yield reached 2.61 g/L. Some fed-batch processes by adding L-methionine (MET) were investigated. Adding MET into the fermentation broth at one time at the time of high cell density reaching 80 g/L could get better results with the optimal SAM concentration of 5.3 g/L and biomass yield of 89.1 g/L. By feeding MET into the fermentation broth at a feeding rate of 2 g/h for 5 h at the time of high cell density reaching 80 g/L, the optimal results were reached and the maximal SAM yield and biomass density were 5.82 and 90.2 g/L, respectively. It indicated that, the fed-batch at high cell density would be more propitious to the SAM biosynthesis.

Key words: Unpolished rice saccharificate, S-adenosyl-L-methionine, fermentation optimization, fed-batch fermentation, Saccharomyces cerevisiae.