Screening of yeast isolates. Wild yeast strains were
selected for their tolerance to a high concentration of glucose
using YPD agar plates containing 40% dextrose.
Strains forming large colonies on plates incubated at 30o
C
for 48 hr were selected and tested further by culturing in
YPD broth containing 30, 40, or 50% dextrose at 30o
C for
24 hr with agitation (200 rpm) and compared at OD600
using a spectrophotometer (Ultrospec 2000; Pharmacia
Biotech, Upsala, Sweden). The strains were also examined
for their tolerance to high alcohol content by culturing
them on YPD agar plates containing 15 or 30%
alcohol. Those strains that could grow on the alcohol plates
were screened further by culturing them in YPD broth
containing 5~15% alcohol under the conditions described
above and then comparing the growth at OD600. The ability of the isolates to utilize maltose was screened using
YP agar or broth containing 2% maltose (YPM). Strains
forming larger colonies on YPM plates incubated at 30o
C
for 48 hr were selected and tested further by culturing
them in YPM broth at 30o
C for 24 hr with agitation and
comparing the growth at OD600. Yeast isolates producing
α-amylase were screened on YP agar plates supplemented
with 1% soluble starch (YPS) and by staining with iodine
solution (1% KI, 0.1% KI2). Two commercial strains of S.
cerevisiae, Lv001 (Denmark) and La Parisienne (The
Netherlands), were used as controls