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 ºC 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 ºC. Purified yeast cultures were kept on YPD agar slants (1% yeast extract, 2% peptone, 2% glucose and 2% agar)
and stored at 8 ºC. Thermotolerant yeast strains were selected based on their growth performances at 40 and 45 ºC 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.