![]() ![]() They are produced during fermentation by the nonenzymatic degradation of acetolactate and acetohydroxybutyrate, which are metabolic intermediates in pathways of amino acid synthesis that leak out into fermenting wort. Of especial significance are the VDKs, diacetyl and pentanedione, which afford a buttery or honey-like character that is undesirable for most beers ( 123). Two effects may be at play, namely, the increased resulting tendency of yeast to be moved through the fermenter and the impact that this has on lowering dissolved CO 2 levels in the wort from inhibitory concentrations ( 94). The presence of insoluble particles in wort (which are derived in the brewhouse and are present at a level in inverse proportion to the extent that they are removed in clarification stages prior to fermentation) promotes yeast action by their ability to nucleate carbon dioxide, thereby releasing bubbles ( 93). To a first approximation, this seems to be a reasonable situation on an experiential basis, although there are two variables that many brewers do seek to regulate more closely, i.e., the clarity of the wort and the concentration of zinc ions ( 89, 90), although other additions to promote fermentations, particularly those with higher-strength wort, may be employed ( 91, 92). Most brewers simply regulate the strength of the wort (degrees Plato) and pitch on that basis, assuming that the relative balance of the diverse nutrients within the feedstock is consistent and modulated by the malt selection and how that malt is processed in the brewhouse. One major variable that perhaps receives less detailed analysis and control than others in fermenter control is actually the wort composition ( 87, 88). There is extensive use of high-gravity brewing in commercial brewing ( 86), with the attendant osmotic and alcohol stresses. A review of all the stresses likely to be encountered by brewing yeast has been provided by Gibson et al. Accordingly, there is interest in the development of yeast strains with greater tolerance of high-gravity conditions ( 84). On the other hand, oxygen represents one of the stress factors encountered by yeast ( 82), while others include ethanol, which limits the practical alcohol concentrations that can be achieved in brewery fermentations ( 83). Ensuring contact of all yeast cells with oxygen when yeast is present at a high density is important ( 81). Traditionally, the oxygen is introduced to the wort, although there have been proposals to pitch unaerated wort with yeast that has been supplied directly with oxygen ( 80). ![]() While it has long been recognized that a proportion of oxygen is needed by all yeast cells to support the production of the sterols and unsaturated fatty acid components of the cell membranes ( 77, 78), there is a less-than-clear appreciation of why different yeast strains vary considerably in the amount that they demand ( 32, 79). ![]()
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