INTERACTIVE FERMENTATION OF MILK BY MEANS OF A MEMBRANE DIALYSIS FERMENTER - YOGURT

The yoghurt bacteria Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus were cultivated separately in the compartments of a fermenter between which exchange of low-molecular-weight compounds was possible through a dialysis membrane module. Mutually stimulated growth of both species was obtained in this way, expressed in an increase of growth rate and a faster acidification compared to separately growing cultures. If L. delbrueckii subsp. bulgaricus was cultured batchwise, exhaustion of the formed substrates for growth of S. thermophilus resulted in a decrease of the dilution rate. The dilution rate obtained in the S. thermophilus compartment was related to the concentration of free amino acids, in particular of a limited number of essential amino acids. Under optimal conditions of dialysis against a culture of L. delbrueckii subsp. bulgaricus, moderately high acetaldehyde concentrations were found, and when S. thermophilus was cultured continuously in milk, a product was obtained with a smooth structure similar to that of conventionally prepared stirred yoghurt. Without or with limited dialysis the product contained lumps and had a rough appearance. Yoghurt manufactured by dialysis fermentation was mildly acid and showed a significantly slower acidification during storage as compared to the traditional product.