Thermophilic lactic acid starters, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, were produced by controlled mixed cultures. The total bacterial concentration and the population ratio at the end of fermentation were fixed by the operating conditions (pH and temperature) and by the initial percentage of S. thermophilus in the inoculum. The suitability of these mixed starters for yoghurt manufacture was determined from their acidification activity. This was not significantly different from that obtained with equivalent mixing of pure cultures. During preservation, the mixed starters were slightly less resistant, both in frozen and freeze-dried forms, than starters obtained from pure cultures. The productivity of the mixed cultures was twice from that obtained with the traditional starter production process, whatever the final population ratio. Finally, the ability to control the mixed starter cultures, the increased productivity and their properties (acidification and preservation) proved their interest for industrial applications. (C) Inra/Elsevier, Paris.
