2.2. Microorganisms and media
Saccharomyces cerevisiae TSH1 (thereafter TSH1 for brevity) was used as fermentation strain in the demonstration study. TSH1 was stored in yeast-peptone-dextrose (YPD) medium containing 20% glycerol at −20 °C or 70 °C for long-term storage. Regular YPD medium was used to revive the strain from frozen stocks at 30 °C for 4–6 h, and the culture was used as initial inoculum to prepare the seed cultures. YPD was used as medium for seed preparation at the bench scales. The wort from malt saccharification was diluted to the final total sugar concentration of 30 g/L, which was then used as medium to prepare the seeds at the pilot scales. The procedure of malt saccharification was described in Section 2.4. The preparation, inoculation, and process control were described in the section on fermentation control.
2.3. Analytical methods
2.3.1. Dry cell weight of yeast
The correlation between optical density and dry mass weight of cells was determined with a standard protocol [12].
2.3.2. Sugar analysis
Total and reducing sugar contents were analyzed by Fehling titration method. Approximately 50 g of sweet sorghum particles was extracted with 500 g distilled water at 80 °C for 60 min. The liquid part was divided into two parts: the first part was used in directly reducing the sugar assay, and the second part was hydrolyzed with 6 mol/L hydrochloric acid at 70 °C for 15 min. After neutralization with 10 mol/L sodium hydroxide solution, the sample was utilized for the total sugar assay [13].
2.3.3. Ethanol content
A 20 g fermented residue was mixed thoroughly with 100 mL distilled water for ethanol distillation. The ethanol content in the 100 mL distilled liquid was determined with a gas chromatograph (SHIMADZU GC-14C) equipped with a flame ionization detector and column. Propanol was used as internal standard to determine the standard curve.
2.4. Malt saccharification
Malt was obtained from Yongchang Jiuyuan Malt Company, Gansu, China. The grist was mixed with water with a ratio of 1:4 (w/w) in a tank equipped with a stirring and heating system. Process conditions were altered sequentially in the following manner. (1) The tank was heated to 40 °C for 20 min and then to 50 °C for 30 min at a pH range of 5.0–5.2. (2) The temperature of the tank was increased and maintained at 62 °C (pH range of 5.3–5.5) until the liquid part turned into orange-red after addition of drops of iodine solution. The system was then cooled down to 58 °C. (3) α-1,4-glucan-glucohydrolase TH-G-10 (Noao Sci. & Technol. Development Ltd. Tianjin, China) was added into the tank and then reacted at 58 °C and pH 4.4–4.6 for 1 h. Enzyme loading utilized 5 mg of enzyme powder with 1 g malt. (4) The saccharified samples were filtered to remove the solid residues, and the total sugar concentration in the liquid part was diluted to 3% (w/w), which was then used as the medium for the seed culture.