Chris Boulton

Growth and Metabolism of Mannitol by Strains of Saccharomyces cerevisiae

Of 40 polyploid strains of Saccharomyces cerevisiae screened for growth on D-mannitol (5%, w/v), half grew well (5-20 mg dry biomass ml-1). Certain of these strains were unable to grow on low concentrations of mannitol (1-2%, w/v) and others, initially unable to grow on mannitol, exhibited long-term adaptation to growth. An NAD+-dependent D-mannitol dehydrogenase (EC 1.1.1.67) was detected in mannitol-grown yeast. Growth was dependent on mitochondrial function and was obligately aerobic. Measurement of products of metabolism and respiratory activity indicated that growth on mannitol allows catabolite derepression.

Fermentation of beer

Publisher Summary The modern brewing industry is dominated by a small number of large companies. These operate on an international stage and produce national or global brands, typically pale pilsner-type lager beers. This market is highly competitive. The need to sustain or grow these global brands within this highly competitive marketplace has provided much of the impetus for the introduction of new fermentation technologies and changes in fermentation practice. Undoubtedly, the major driver for changes in fermentation practice and the underlying technology is cost. Since fermentation is frequently the rate-determining step in the brewing process, much effort has been devoted to identifying ways of increasing productivity. Batch sizes may be increased by using ever bigger fermenters; however, approaches that increase the productivity of existing vessels are particularly attractive since they avoid capital expenditure. Such strategies include the use of ultra-high gravity brewing, ensuring high efficiencies of conversion of sugar to alcohol and developing methods for reducing cycle times. Good fermentation practice requires precise control of the variables that are influential on the progress and outcome of the process. This is especially important in the case of global brands produced simultaneously at several sites. Precise control of fermentation is an essential prerequisite to ensuring that different product streams generate consistent beer. In recent years, advances have been made in improving the ability to monitor and control fermentation progress. These developments have been underpinned by a better understanding of the processes that occur during fermentation. In particular, much progress has been made in unraveling the complex interrelationships that exist between wort composition, yeast physiology, fermenter design, fermentation management and beer quality. This chapter presents a review and discussion of these developments.