The biochemistry of brewery fermentation is complex and many of its aspects remain to be fully elucidated. The reactions - which underpin the growth of yeast during fermentation and the concomitant conversion of wort to beer - touch on almost every facet of cellular metabolism. Many of the pathways involved are those which may be found described in any standard biochemistry text and to duplicate this information here is both needless and beyond the scope of this book. Instead the intention is to concentrate on those aspects which are peculiar to brewery fermentation. In particular, the factors which control the flow of carbon and other nutrients between biomass formation, ethanologenesis and the formation of metabolites that contribute to beer flavour.
Three general areas of discussion may be considered which influence the biochemistry of fermentation. These are wort composition, the genotype of the yeast strain and phenotypic expression of the genotype as influenced by fermentation practice.
Wort composition is complex and it follows that, as with growth of microorganisms on any uncharacterised medium in a batch culture, the biochemical reactions which contribute to the assimilation and metabolism of its individual components will be equally convoluted. Wort provides a complete growth medium for yeast. Indeed, it should be remembered that brewery fermentation is nothing more than a manifestation of yeast growth and beer is merely the by-product of that activity. The yeast strain used will have been chosen because it has desirable fermentation properties and has the potential to produce beer with a suitable composition. The object of fermentation management and control is to regulate conditions such that the by-products of yeast growth and metabolism are produced in desired quantities and within an acceptable time.
The precise composition of wort is unknown, although provided similar materials and methods are used in its preparation the gross analysis should be relatively constant. All malt wort prepared with brewing liquor with an appropriate ionic composition provides a medium with the potential to produce new yeast biomass, ethanol and flavour components in balanced and desired quantities. To realise this potential it is necessary to control other parameters such as temperature and yeast inoculation (pitching) rate. Although wort composition is somewhat uncharacterised, it is possible to establish relationships between the assimilation by yeast of the major classes of wort nutrients and the formation of products of yeast metabolism. This equilibrium can be disturbed if wort composition is modified by, for example, changing the ratio of carbon to nitrogen in the wort by increasing the fermentable extract with sugar syrup adjuncts. Conversely, manipulation of wort composition can be a deliberate strategy for producing desired changes in fermentation.
The genotype of the particular strain of yeast used is critical to the outcome of fermentation. For example, the spectrum of flavour-active metabolites produced is as much determined by the yeast as by the conditions established during fermentation. Of fundamental significance is the response of brewing yeast strains to sugars and oxygen. All brewing yeast strains have limited respiratory capacity and are subject to carbon catabolite repression. In a brewery fermentation, irrespective of the presence of oxygen, metabolism is always fermentative and derepressed physiology never develops (see Section 4.3.1 for further discussion). Thus, the major products of sugar catabolism are inevitably ethanol and carbon dioxide. Respiration, in the true sense of complete oxidation of sugars to carbon dioxide and water, coupled to ATP generation via oxidative phosphorylation does not occur. The maximum ethanol concentration that may be generated during fermentation is also determined by the yeast genotype. In this regard strain-specific tolerances to ethanol and reduced water activity are important. Flavour considerations apart, this parameter may serve to limit the maximum concentration of sugar that can be used with no detrimental effect to the yeast.
Phenotypic expression of the genotype of the yeast is modulated by the conditions experienced during fermentation. In large vessels, in particular, the yeast is subjected to multiple stresses such as high hydrostatic pressure, elevated carbon dioxide concentration, low pH and reduced water activity. All of these have the potential to elicit specific biochemical responses by the yeast. In this regard channelling of a proportion of wort carbohydrate into accumulation of the disaccharide trehalose (see Section 184.108.40.206) may be implicated in the ability of yeast to withstand stress.
Aspects of yeast handling peculiar to the brewing industry are crucial to understanding the biochemistry of the process. In many fermentation industries it is normal practice to use an inoculum, which has been specifically cultivated for the purpose. In order to ensure rapid onset, starter cultures are often employed in the exponential phase of growth. This has the additional twin benefits of providing a means for introducing an inoculum which is of consistent physiological condition and which may be relatively small compared to the total new biomass generated during the fermentation.
In traditional brewing fermentations, such as Belgian Iambic and many native (ethnic) beers, the inoculum may arise via contamination from the environment. However, most often and certainly in the modern process it is taken from yeast cropped from a previous fermentation. In consequence, the inoculum consists of stationary phase cells which are depleted in membrane lipids, sterols and unsaturated fatty acids. Replenishment of the pools of these essential lipids and restoration of membrane function is dependent on the supply of oxygen provided with wort at the start of fermentation. Commencement of fermentation and movement of yeast cells from stationary to growth phases is therefore associated with simultaneous assimilation of wort nutrients and a transition from aerobic to anaerobic conditions.
The practice of serial cropping and re-pitching requires that relatively high inoculation rates are used and this together with control of wort oxygen concentration limits subsequent yeast growth to modest levels. An inherent part of this practice is that the cells in the inoculum form a significant proportion of the total population in the fermenter, and, furthermore, there is an opportunity for these cells to persist through several generations of serially cropped and re-inoculated fermentation. Unlike bacteria, yeast cells have a finite life span and undergo an ageing process. Like any other mortal cell, both phenotypic and genotypic modifications are possible due to the effects of ageing and the onset of senescence (see Section 220.127.116.11).
The practice of serial fermentation necessitates having facilities for yeast handling during the interval between cropping and re-pitching. In consequence there is an opportunity for further modification to physiology depending on the time the yeast is held and the conditions under which it is stored. A feature of this aspect of fermentation management is that the yeast is subjected to periods of growth in fermenter interspersed with intervals of starvation during storage. It follows that whilst the yeast is in fermenter it must achieve a physiological condition in which it is capable of withstanding starvation during storage. In this respect regulation of carbon flow during fermentation between glycolysis and gluconeogenesis is of significance. Thus, carbohydrate reserves accumulated during fermentation provide a source of maintenance energy during the storage phase and possibly carbon for lipid synthesis during the aerobic phase of fermentation when utilisation of exogenous carbon is limited by lack of membrane function. The ability of yeast to channel carbon into gluconeogenic pathways during fermentation is favoured by the high ratio of carbohydrate to other wort components.
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