Laboratory Production of Alcohol from Apple Cider
Klaus Dölle *
Department of Chemical Engineering, College of Environmental Science and Forestry (ESF), State University of New York (SUNY), One Forestry Drive, Syracuse, NY 13210, USA.
Lingkun Li
Department of Chemical Engineering, College of Environmental Science and Forestry (ESF), State University of New York (SUNY), One Forestry Drive, Syracuse, NY 13210, USA.
Botao Zhao
Department of Chemical Engineering, College of Environmental Science and Forestry (ESF), State University of New York (SUNY), One Forestry Drive, Syracuse, NY 13210, USA.
*Author to whom correspondence should be addressed.
Abstract
Apple cider (AC) is inexpensive, widely available, and does not compete directly with the food supply. It is also rich in fermentable sugars, making it an attractive feedstock for second-generation bioethanol. This study examined how aeration affects ethanol yield when fresh-pressed AC is fermented by the yeasts and bacteria naturally present in the juice. AC from an orchard near Syracuse, New York (solids content 10.46%) was fermented in 300 mL laboratory batches at a nominal temperature of 30 °C (actual range 30.3–34.7 °C) with magnetic stirring at 125 rpm. One batch received a continuous air stream of 16.9 mL/min, whereas the control batch was not aerated. Sugar content, alcohol content, and pH were measured approximately every 24 h over 192 h. Without aeration, sugar content fell from 13.2% to 0.5% within 77 h. Alcohol content peaked at 6.6% (v/v), approximately 80% of the theoretical yield, and the pH remained between 3.35 and 3.55 during active fermentation. With aeration, sugar was consumed more slowly and less completely, levelling off at 2.5%, while alcohol peaked at 5.7% (v/v) at 96 h. Aeration diverted part of the sugar towards aerobic respiration and yeast growth rather than ethanol production. It also raised the pH to 5.5, compared with a maximum of 4.5 without aeration, thereby increasing the risk of microbial spoilage. In both trials, alcohol content declined after reaching its peak, which was attributed to ethanol evaporation, oxidative metabolism, and acetic acid formation. The apparent late increase in measured sugar was attributed to ethanol interference with refractometer readings and to evaporation concentrating the remaining solids. Aeration lowered the maximum ethanol yield from AC by approximately 14% and reduced product stability. Ethanol production from AC should therefore be conducted without aeration, and fermentation should be stopped or ethanol recovered at 72–96 h, when alcohol content peaks. Future work should use closed systems, sugar-analysis methods unaffected by ethanol, and replicate trials.
Keywords: Apple cider, Bioethanol, aerobic fermentation, alcoholic fermentation, aeration, native yeast, crabtree effect, ethanol yield