In batch fermentations of brewers' wort using different strains of Saccharomyces, the declining specific rate of utilisation of carbohydrates by the yeast paralled the declining specific growth rate. During the growth phase, the coefficient relating utilisation of carbohydrate to accumulation of yeast mass remained approximately constant. When mass production ceased the utilisation of carbohydrate required for maintenance of the yeast population was relatively insignificant compared with that associated with growth. To achieve satisfactory attenuation, the extent of growth must be increased so that little fermentable carbohydrate remains to be metabolised by yeast in the maintenance phase. Strains of Saccharomyces diastaticus excrete amylo-beta-1,4-glucosidase (AMG), which hydrolyses the "normally" non-fermentable dextrins of wort. AMG is synthesised constitutively by all dextrin-utilising Saccharomyces strains examined. Prior to excretion into the medium, AMG is located extra-cytoplasmically where it cannot hydrolyse dextrins. A single nuclear gene (CDX 1), unlinked to DEX 1, determines that AMG is not excreted into the medium until carbon-catabolite repression is relieved. Excretion of AMG by strains carrying CDX 1 was greatest, and initiated earliest, when their requirement for oxygen or unsaturated lipids was incompletely satisfied. Strains defective in, or lacking, CDX 1 excrete AMG constitutively. The presence/absence of CDX 1 does not correlate with resistance/sensitivity of strains to non-metabolisable analogues of glucose (i.e., 2-deoxyglucose and D-glucosamine), nor does escape of AMG excretion from carbon-catabolite repression co-incide with the derepression of production of invertase. In the diploid state, AMG excretion is blocked even in the absence of CDX 1; this effect is over-ridden when carbon-catabolite repression is relieved. AMG activities from strains of Saccharo-myces diastaticus have pH optima over the range 4.2 to 5.4, share an optimum temperature of 55°C and are destroyed rapidly at temperatures of ca. 60°C. Strains which excrete AMG alone are capable of only limited attack on wort dextrins while others hydrolyse at least 66% (w/w). This improved dextrin hydrolysis has been attributed to excretion of AMG and an additional debranching enzyme, probably of the amylo-beta-1,6-glucosidase/transferase type.
|Date of Award||1982|