2. Methods
2.1. Isolation and selection of thermotolerant yeast strains
Yeasts were isolated from soil and water samples from
sugar cane plantations and sugar factories in four provinces,
namely Phra Nakhon Si Ayutthaya, Ratchaburi,
Suphanburi and Uthaithani, Thailand. Isolation was carried
out at 35 C by an enrichment technique using sugar
juice media containing sugar cane juice (5% or 8% total
sugars), 0.05% (NH4)2SO4 and 4% (v/v) ethanol and with
(pH 4.5) and without pH adjustment (pH 3.8). After inoculation,
cultures were incubated for 3 days in a rotary shaker
(Gallenkamp Orbital Incubator, Leicester, UK) at a
predetermined temperature with shaking speed of
170 rpm. Enriched cultures were then streaked on agar
plates containing the same medium and inoculated at
35 C. Purified yeast cultures were kept on YPD agar slants
(1% yeast extract, 2% peptone, 2% glucose and 2% agar)
and stored at 8 C.
Thermotolerant yeast strains were selected based on
their growth performances at 40 and 45 C in sugar cane
juice broth containing 8% total sugars and 0.05%
(NH4)2SO4 and supplemented with 4% (v/v) ethanol. Successful
cultures were collected and screened further for
their ethanol production efficiency at elevated temperature.