METABOLIC INTERACTION BETWEEN STREPTOCOCCUS-THERMOPHILUS AND LACTOBACILLUS-BULGARICUS IN SINGLE AND MIXED STARTER YOGURTS

Yoghurt was manufactured with mixed starter of S. thermophilus and L. bulgaricus or a single starter of either in the presence or absence of 600 ppm free amino acids mixture (glutamic acid, histidine, cysteine, valine, methionine and tyrosine) and/or 600 ppm of sodium formate. Results showed that the acidity, total volatile fatty acids (TVFA), NPN, total free amino acids and soluble tyrosine contents of different yoghurt samples increased gradually during storage at 5 - 8-degrees-C for 7 days especially in the mixed starter yoghurt fortified with free amino acids mixture and Na formate. Organoleptic evaluation showed that the flavour of single strain starter yoghurt with free amino acids mixture of Na formate gave nearly the flavour of control yoghurt samples. However, the mixed starter yoghurt fortified with free amino acids mixture and Na formate gave more acceptable flavour. It could be concluded that addition of free amino acids mixture and/or Na formate to yoghurt stimulated the activity of starter bacteria. Also manufacture of yoghurt using a single starter of S. thermophilus or L. bulgaricus should be associated with addition of free amino acids mixture or Na formate to yoghurt milk respectively.