Isolation and characterization of a new low-diacetyl-producing yeast for fermentation of rice beer using high- and low-gravity wort

Abstract

Diacetyl is a buttery aromatic compound that is formed during beer fermentation as a by-product of amino acid synthesis. In particular, high-adjunct wort contains a low valine concentration that can stimulate diacetyl synthesis. The objective of this research was the selection of a low-diacetyl-producing yeast for brewing rice beer. Ninety-two yeast isolates were isolated from 55 yeast sources and were screened for their ability to produce ethanol in the rice wort. Three isolates (DG201, BNS04 and LPST01) were chosen as the maximum ethanol producers. LPST01 produced diacetyl (mean ± SE; l8.32 ± 1.63 mg/L in all-malt beer and 9.63 ± 0.78 mg/L in 70% (weight per weight; w/w) rice grit beer), and the levels were comparable to the values for the S-23 commercial yeast strain (7.80 ± 4.07 and 11.97 ± 4.08 mg/L, respectively). Sequence analysis of the divergence in the variable D1/D2 domain of the large subunit (26S) ribosomal DNA revealed that LPST01 was Sacccharomyces cerevisiae, with a similarity of 100%. The strain was characterized for optimal fermentation temperature, flocculation property and diacetyl production in three worts prepared from three percentages of rice grit (30%, 50% and 70% (w/w)) under normal- and high-gravity conditions. At 20ºC, LPST01 had the highest specific growth rate (0.0626 ± 0.00 per hour) and was characterized as a non-flocculent yeast. LPST01 tolerated high-gravity worts. The diacetyl production and yield in high-gravity fermentations was slightly lower than that under normal gravity. These results suggested LPST01 was a promising strain for economical beer production, and that it could be used in high-gravity beer fermentation.