By-products from potato processing plants were liquefied and partially saccharified using three commercially available enzymes, followed by ethanol fermentation by yeast and then hydrolysis by glucoamylase. From 12% (w/w) of fresh potato peel, about 20 mg/mL ethanol was formed in the supernatant via amylase, pectinase, or an enzyme complex; this yield was slightly increased with combinations of these enzymes. Supplementation of substandard mash to potato peel (1:1) as a raw material enhanced the amount of ethanol formation, increasing it to approximately 50 mg/mL by the mixture of all three enzymes. After the addition of yeast and glucoamylase to the partially saccharified material, ethanol formation proceeded gradually and slowed in 12 h with the consumption of fermentable sugars. Galacturonic acid derived from pectin was not fermented and remained in the fermented broth. In the by-products, the conversion rate of sugars to ethanol was estimated to be 42.5%.

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Ethanol production from a mixture of wheat flour or potato tubers and cheese whey was examined using the flex yeast Kluyveromyces marxianus KD-15, a 2-deoxyglucose-resistant mutant of strain NBRC 1963 that can produce ethanol from sugar beet thick juice diluted with whey. Strain KD-15 simultaneously converted glucose and lactose to ethanol within 48 h, in media containing 10.0% to 15.0% (w/v) total sugars from saccharified filtrate and whey. For efficient production of ethanol from 15.0% (w/v) total sugars, KD-15 cells were collected after fermentation and inoculated into fresh media. Batch fermentation was successfully repeated at least ten times in medium composed of saccharified potato tubers mixed with whey. Yeast cells began to convert all the sugars to ethanol within 24 h, accompanied by cell propagation after the third batch.

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