Microorganisms. Saccharomyces yeasts 1400 and genetically engineered 1400(pLNH33) were used in the experimental work. The yeast 1400 (pLNH33) was obtained from Dr. Nancy Ho at LORRE.

The yeast strain 1400 (77) is a protoplast fusion product of Saccharomyces diastaticus and Saccharomyces uvarum.

Culture Conditions. The recombinant yeast 1400 (pLNH33) was maintained on YEPX seed cultures. The composition of the seed culture media per liter of distilled water is as follows: D-xylose 20g, yeast extract lOg, Bactopeptone 20g. The yeast was grown to an OD of 400-450 Klett Unit (measured by a Klett-Summerson colorimeter) and then maintained at 4°C. The medium for the preparation of the inoculum was the YEPX medium described above. 1 ml of the seed culture was added to a sterilized 250 ml Erlenmeyer flask with silicone sponge closure, containing 50 ml of medium. The inoculum was incubated at 30°C in a floor shaker at 150-200 rpm for 18-20 hours (when the cells were in the late exponential phase) before being used to inoculate the fementation medium.

The parent yeast strain 1400 was maintained on YEPD agar plates. The composition of the plate media per liter of distilled water is as follows: glucose 20g, yeast extract lOg, Bactopeptone 20g and agar 20g. The yeast was inoculated on the plate medium at 30°C for 48 hours and then maintained at 4°C. For the preparation of the inoculum, yeast extract-Bactopeptone medium with 20 g/L glucose was used. A loopful of yeast cells were transferred from the agar plate into 50 ml sterilized medium in a 250 ml Erlenmeyer flask.

Fermentation Conditions. The fermentation was performed in 250 ml Erlenmeyer flasks with silicone sponge closures, containing 100 ml sterilized medium. The fermentation medium consisted of 20 g/L Bactopeptone, 10 g/L yeast extract and appropriate concentrations of glucose and/or xylose. The inoculum sizes used were in the range of 0.1 g/L-2.5 g/L. The fermentation conditions were same as those indicated earlier for the inoculum preparation.

Pretreatment and Hydrolysis of Corn Fiber. Com fiber (com hull from A. E. Staley, Lafayette, IN) was pretreated with 0.5% dilute hydrochloric acid at 120°C for 45 minutes. Enzymatic hydrolysis of the pretreated com fiber was performed at 45 °С using Iogen cellulase having an activity of 154 FPU/ml. The pretreated com fiber was thoroughly washed, following which cellulase was added to the glucose free medium.

Simultaneous Saccharification and Fermentation (SSF) of Corn Fiber. Dry

pretreated com fiber (25% w/v) was added into a 250 ml side-arm Erlenmeyer flask. Yeast extract (10 g/L) and Bactopeptone (20 g/L) were also added and the pH was adjusted to 5. The yeast was inoculated from the seed culture to give an initial cell concentration of 0.5 g/L. Iogen cellulase was added to the medium to give an activity of about 5-10 FPU per gram of cellulose from com fiber. This fermentation medium was diluted using deionized water to give the desired solid fraction. The SSF of pretreated com fiber was conducted at 30°C in a shaker at 150-200 rpm.