High-Yield Bioethanol Production from Unfit Oranges for Consumption

Abstract

With their very high fermentable sugar content, rotten oranges stand out as one of the raw materials of choice to maximize bioethanol production. This work was deliberately aimed to evaluate the efficiency of the technology for producing bioethanol from musts prepared from rotten oranges. To achieve this goal, four types of musts (16 ˚Brix, 20 ˚Brix, 24 ˚Brix, and 28 ˚Brix) were prepared from raw juice (9.5 ˚Brix) extracted by mechanical pressing of the oranges. Sucrose was used as the reference fermentable sugar. Sodium glutamate, added to the musts, played the role of increasing yeast cells to effectively boost their enzymatic catalysis. Ethanol fermentation in batch mode and fed-batch mode was adopted. The ethanol fermentation reaction was monitored by refractometric measurement, while the alcoholic strength of the fermented musts was determined by the pycnometric method. The results revealed that the ethanol content (% vol.) produced from the must of 28 ˚Brix from rotten oranges, fermented in batch mode, reached a maximum value of 16.51 ± 0.21 with the addition of sodium glutamate SG (2 g/L), whereas with the same must, only a content (% vol.) of 14.72 ± 0.05 was obtained in semi-continuous mode. However, without SG, an ethanol content (% vol.) of 4.93 ± 0.33 was only produced from the raw juice (9.5 ˚Brix), by mechanical pressing of rotten oranges. In contrast to the results obtained with sucrose, the musts of rotten oranges without SG supplementation produced higher ethanol contents than those supplemented with SG (2 g/L). Finally, batch fermentation of rotten orange musts was more efficient than fed-batch fermentation. However, the addition of SG (2 g/L) in the rotten orange musts fermented by fed-batch mode enhanced the ethanol content, while in batch mode, this addition was only beneficial for the rotten orange musts of 28 ˚Brix and 9.5 ˚Brix. The valorization of rotten oranges into bioethanol therefore fits into local circular economy approaches to promote the sustainable use of natural resources.