Studies in the Continuous Tower Fermentation of Alcohol

Abstract

The introduction of continuous tower fermentation for the production of beer and industrial alcohol has posed the questions; which type of yeast is most suited to the system and what conditions produce maximum performance? This study is an attempt to provide some answers. A method of selecting yeasts was devised based upon a sedimentation test and assessment in a laboratory tower fermenter, designed to simulate an industrial unit. Pure culture yeasts of the species Saccharomyces cerevisiae, Saccharomyces uvarum and Saccharomyces diastaticus were cultivated in a tower fermenter. From this work, cultures of each were selected as suitable for further investigation. It has been suggested that the prolonged continuous cultivation of yeast can result in undesirable changes which are due to mutation. Studies were made to determine whether mutation would present a serious hazard in continuous tower fermentation. Fifty cultures of a brewery yeast (Saccharomyces cerevisiae) were isolated before and after 134 days* continuous fermentation in a laboratory tower fermenter. A decrease in flocculence was noted in two isolates but the population as a whole showed an increase in flocculence. A change in giant colony morphology was noted in two isolates but no changes were noted in any of the other characteristics examined, i.e. cell shape, spore formation, melibiose fermentation. Similar and additional tests (fermentation rate, yeast yield and attenuation gravity) were applied to the same strain of yeast which had been cultivated in a production scale fermenter for one year. Five isolates showed unusual giant colony characteristics and a slight decrease in flocculence was noted for the population as a whole. Eight isolates failed to produce ascospores. There were no changes in other characteristics. Because of the limited nature of the changes, it was concluded that mutation does not present a serious practical hazard to the operation of a tower fermenter for prolonged periods. The continuous tower fermentation of lager beer has been complicated by the presence of high concentrations of vicinal diketones in the product. These compounds are normally found in beer in concentrations up to 0.1 ppm but above 0.2 ppm the flavour of the beer is impaired. Fourteen strains of lager yeasts were assessed against criteria of flocculence and amounts of vicinal diketones in the finished beer. Four yeasts were tested in the laboratory tower fermenter and of these two were found to be suitable for further investigation. The selected cultures were cultivated in the fermenter and operational parameters of wort flow rate, temperature and aeration altered to determine the most suitable conditions. It was concluded that the fermenter must be operated at slow wort flow rates with a relatively high temperature in the top half of the fermenter. Previously the tower fermenter has not been applied to the continuous fermentation of molasses mashes for industrial alcohol. A strain of Saccharomyces diastaticus was selected and operated in the tower fermenter. It was shown that mashes containing up to 20 g% w/v sugar could be successfully fermented. The parameters influencing the rate of fermentation were investigated. It was concluded that the production of industrial alcohol is an application in which the tower fermenter may be successfully employed. The value of this work in relation to industrial requirements and the role of the laboratory fermenter in assessing flocculence has been discussed.