A MODEL OF AEROBIC FUNGAL GROWTH IN SILAGE .1. MICROBIAL CHARACTERISTICS

The components of a simulation model of mesophilic and thermophilic yeast and mould growth in aerobically deteriorating silage are presented. The purpose of the model is to examine the relative roles of yeasts and moulds in deterioration and the factors affecting aerobic stability. Information for the model was based on literature studies. Growth of the fungi was assumed to be affected by temperature, pH, water activity, and lactic and acetic acid concentrations. The substrates utilized in the model, in order of preference, were water-soluble carbohydrates, ethanol, lactic acid and acetic acid. Gas movement and heat transfer were not considered. Consequently, oxygen and carbon dioxide concentrations were assumed to be those for open air, and the heat of respiration retained in the silage was set as a constant percentage of that released by fungal growth. Based on the relationships developed for the model, pH affects yeast growth minimally and does not affect mould growth at all. Water activity over the range in silage affects yeast growth more than mould growth, but affects neither one strongly. Undissociated lactic and acetic acids decrease growth rate and may be important factors in silage stability. Compared with moulds, yeast growth rates are less affected by suboptimum temperatures but are more sensitive to temperatures in excess of the optimum. A succeeding study compares the model with published aerobic deterioration studies and examines the predicted succession of microbial groups and the variation in silage stability as affected by silage parameters, initial temperature, and initial microbial populations.