Factors affecting the activity of baker's compressed and active dry yeast : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Biotechnology at Massey University

Abstract

Factors affecting the Activity of Baker's Compressed and Active Dry Yeast. Parameters important in the production of Baker's Yeast were correlated with the product's final activity. Activity was a measure of the gas evolved in a fermenting dough, expressed as mMCO2/hr/g yeast solids. The drying of Compressed Yeast to Active Dry Yeast was optimised in terms of the drying air. A tunnel tray drier was used to dry yeast to a 9% moisture content (dry weight). At 40°C. the optimum drying humidity was found to be 30-32% relative humidity. The leavening ability of yeast dried at 17% and 45% relative humidity decreased. A drying additive, 2% glyceryl monostearate, halved the drying time to 4 hours. Equations were developed to describe these observations as a function of relative humidity, drying time and additive concentration. The equilibrium relative humidity of stored dried yeast was found to be 32% at 20°C. Fermentation parameters were correlated with the activity of Compressed Yeast using an experimental design. Growth temperatures varied from 28°C. to 37°C., initial pH from 4 to 6, glucose concentrations from 0.5% to 3%, nitrogen concentrations from 0.3% to 1.2% and dissolved oxygen varied as either agitated or standing cultures. Factors significantly affecting cell yield and yeast activity were growth temperature, dissolved oxygen and glucose concentrations. Maximal yeast activity occurred at 0.5% glucose concentration, 28°C. and non-agitated conditions. A model was developed to describe yeast activity as a function of these variables. The observed optimal conditions for cell yield were similar to those for yeast activity except for the dissolved oxygen level. Maximum yeast activity of Compressed Yeast occurred in non-agitated fermentations, compared with cell yield which required agitated conditions to achieve the greatest cell yield. A rapid screening test for evaluating dried yeast was incorporated into the yeast activity analysis. This involved monitoring foam production during rehydration.