Yeast encapsulation for bioethanol production.
The bioethanol production industry is faced with hurdles such as uncovering cheap and abundantly available fermentation substrates, as well as yeast strains possessing high ethanol tolerance properties. Cane molasses is a substrate that sufficiently fits the aforementioned description and this has catapulted its use in bioethanol production. However, the downside to employing cane molasses as a fermentation substrate under laboratory conditions for comparative fermentation studies is nutrient composition variability in different batches. This has prompted the development of a standardised chemically defined molasses medium that facilitates the generation of more consistent and reliable fermentation data. In the first aspect of this study, a chemically defined molasses medium was formulated based on nutrient composition data of 10 different cane molasses batches as provided by the South African Sugar Research Institute (SASRI). The ability of laboratory and industrial Saccharomyces cerevisiae strains to ferment chemically defined molasses medium and industrially-derived cane molasses sourced from the Amatikulu, Felixton and Gledhow South African-based sugar mills was evaluated. The batch fermentation of the chemically defined molasses medium supplemented with yeast extract by BY4743 (laboratory strain), dry yeast (baker’s yeast), Angel and cream yeast (distiller’s yeast) were similar to those attained in batch fermentations of cane molasses in terms of fermentation kinetic profiles (sugar conversion, ethanol titer, yield and productivities). It was also observed irrespective of the fermentation substrate involved that cream yeast produced the highest ethanol output followed by angel yeast, dry yeast and then BY4743. These results seem to suggest that the chemically defined molasses medium containing yeast extract can be employed as a standardised laboratory medium. Increased bioethanol yield is commercially attractive to relevant fermentation-based industries. In this regard, the immobilization of yeast by cell encapsulation has been touted as a tool which may increase the yeast’s tolerance to higher ethanol levels. In this study, a strategy was developed in which the better performing Angel and cream yeast strains were immobilized in calcium alginate, alginate-chitosan, and low melting point agarose capsules. The fermentation efficiency in terms of ethanol production of encapsulated cells versus their free-suspended yeast cell counterparts was evaluated. The reusability of the capsules for more than one fermentation cycle was also investigated. The fermentation of Amatikulu and chemically defined molasses medium containing 10 g/L of yeast extract by Angel yeast encapsulated in low melting point agarose resulted in a 10% increase in bioethanol yields in comparison to their free-suspended Angel yeast counterparts. However, it was also observed that cream yeast fermentations with free-suspended and encapsulated cells generated similar fermentation profiles and bioethanol yields. Only alginate-chitosan and low melting point agarose were used in the investigation of capsule reusability because of their superior stability over calcium alginate capsules. The low melting point agarose capsules remained stable and active for the three consecutive batch fermentations of Amatikulu cane molasses and synthetic CDM-YE molasses. The alginate-chitosan capsules remained active and stable for two cycles of fermentation and only showed signs of breakage during the third fermentation cycle. Fermentations with encapsulated Angel and cream yeast resulted in sustained ethanol outputs for the three fermentation cycles. The data seem to suggest that the cell encapsulation strategy may be beneficial to the bioethanol industry in that lower ethanol tolerant distillers yeast and other yet to be used strains which produce significantly less undesirable by-products such as acetic acid, ethyl aldehyde, n-propanol and methanol can be improved in terms of their bioethanol yield to meet the requirement of industry.