Some research was done on the use of mixed cultures of different microorganisms to improve the utilization of nutrients in stillage and to obtain better yields of biomass. Combinations of different yeasts, yeasts and bacteria, yeasts and molds or different molds were studied. Biomass yield of the mixed cultures can be substantially higher than that of pure cultures. A two-step fermentation proved to be very effective. Yeast was used in the first step to produce large amounts of biomass, and a filamentous fungus in the second step markedly reduced the COD of the effluent (Azzam and Heikel 1989; Bottaro Castilla et al. 1984; Nudel et al. 1987). In malt whisky spent wash a three-membered mixed culture of G. candidum, Hansenula anomala, and Candida krusei constituted a stable population. The relative proportions of the three organisms were dependent on the dilution rate during continuous cultivation (Barker et al. 1982). Thirteen different yeast species, mainly Candia spp. and Trichosporon spp., were used in a mixed culture for treating the vinasse of beet molasses (Malnou et al. 1987). Mixed cultures of two ascomycetous fungi, A. awamori and Trichoderma reesei, were tested in apple slops; they expressed the positive properties of both genera: Aspergillus improved the filtration time and COD reduction whereas Trichoderma decreased fiber and increased protein content in the biomass (Friedrich et al. 1987).
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