Abstract
The potential of two zygomycetes fungi, Mucor indicus and Rhizopus oryzae, in assimilating citrus waste free sugars (CWFS) and producing fungal chitosan, oil, and protein as well as ethanol was investigated. Extraction of free sugars from citrus waste can reduce its environmental impact by decreasing the possibility of wild microorganisms growth and formation of bad odors, a typical problem facing the citrus industries. A total sugar concentration of 25.1 g/L was obtained by water extraction of citrus waste at room temperature, used for fungal cultivation in shake flasks and airlift bioreactor with no additional nutrients. In shake flasks cultivations, the fungi were only able to assimilate glucose, while fructose remained almost intact. In contrast, the cultivation of M. indicus and R. oryzae in the four-liter airlift bioreactor resulted in the consumption of almost all sugars and production of 250 and 280 g fungal biomass per kg of consumed sugar, respectively. These biomasses correspondingly contained 40% and 51% protein and 9.8% and 4.4% oil. Furthermore, the fungal cell walls, obtained after removing the alkali soluble fraction of the fungi, contained 0.61 and 0.69 g chitin and chitosan per g of cell wall for M. indicus and R. oryzae, respectively. Moreover, the maximum ethanol yield of 36% and 18% was obtained from M. indicus and R. oryzae, respectively. Furthermore, that M. indicus grew as clump mycelia in the airlift bioreactor, while R. oryzae formed spherical suspended pellets, is a promising feature towards industrialization of the process.
Highlights
The world production of citrus fruits in 2010/11 was over 115 million tons, of which almost 70 million tons was orange [1]
The presence of these free sugars in citrus wastes (CW) can cause problems associated with the extracted chemicals purity, e.g., pectin, and wastewater treatment of the extraction processes
Fungal biomass and ethanol production from citrus wastes (CW) with minimal auxiliary nutrients usage was the main focus of the present study
Summary
The world production of citrus fruits in 2010/11 was over 115 million tons, of which almost 70 million tons was orange [1]. Over 23 million tons of orange was utilized for processing, producing 50%–60% organic waste [1]. Considering the potential environmental problems caused by this huge quantity of organic chemicals, efficient citrus waste treatment is vital for citrus industries. A minor part of citrus wastes (CW) is used to industrially produce fiber and food ingredients, e.g., pectin, mucilage, and flavonoids [2]. CW contain considerable amounts of free mono- and di-saccharides, which remain intact after juice extraction. The presence of these free sugars in CW can cause problems associated with the extracted chemicals purity, e.g., pectin, and wastewater treatment of the extraction processes. The free sugars can be extracted and used as carbon- and nutrient-rich medium for fungal cultivation under controlled conditions
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