Aerobic co-cultures of Kluyveromyces marxianus and Candida utilis utilizing multiple substrates from whey bioconversion waste streams

Lorenzo Saliceti-Piazza, Purdue University

Abstract

Several processes have been suggested for the bioconversion of lactose from whey to other useful chemicals. However, these whey bioconversion processes can generate aqueous waste streams with a high chemical oxygen demand (COD) level because of their residual amounts of lactose and other metabolic by-products. There is a need for a simple treatment process that will reduce the pollution level of the fermented whey waste streams. This work investigates the production of yeast single-cell protein (YSCP) from these wastes. The objectives were to: (i) define an appropriate mixed yeast culture, (ii) determine the model and parameters that describe the culture growth, and (iii) compare batch and continuous culture operating modes for the reduction of COD. Objective (i) was achieved by identifying the mixed yeast culture consisting of Kluyveromyces marxianus and Candida utilis strains. The selection was based in higher growth rates, simultaneous utilization of lactose, lactic acid, glycerol and ethanol by the two yeasts, and faster COD reduction rates. There was no apparent interaction between the two species in the cultures. Objective (ii) was completed by selecting the cybernetic model described by Kompala et al., (1986), and performing the individual substrate/yeast strain kinetics to determine the growth parameters needed. The most interesting case was K. marxianus and lactose, where aerofermentative growth can occur and the model was adjusted to account for this phenomenon. Objective (iii) was fulfilled by performing and modeling the yeast co-cultures in media containing the mixed substrates. No significant difference was observed in the COD reduction levels (80%) between batch and CSTR. The batch culture produced a final yeast biomass with a proportion of each strain similar to the inoculum proportion. In CSTR, C. utilis dominated (90%) because of its lower nutrient requirements at the low dilution rates investigated. The growth rates were faster under non oxygen-limiting conditions, obtained when the total substrate concentration did not exceed 15 g/L. The model predicted very well the substrate utilization and COD reduction profiles in batch cultures. The results were excellent when non oxygen-limiting conditions persisted in the cultures, but showed some discontinuities in the dissolved oxygen profiles if the culture switched from excess oxygen to oxygen-limiting conditions.

Degree

Ph.D.

Advisors

Okos, Purdue University.

Subject Area

Chemical engineering

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