Abstract:
The microbial ratio of the bulk yoghurt starter cultures determines the strength of proto cooperation among the lactic acid
bacteria which can build up the proper chemical and physical structure of the yoghurt and control the post-acidification effect.
Therefore, this attempt is to find out the best microbial ratio of the yoghurt starter culture which can produce good quality yoghurts
and control post acidification. The isolated L. bulgaricus and S. thermophilus strains from bulk cultures were used to prepare
new starter cultures by changing L. bulgaricus, S. thermophilus ratios as 1:1, 1:1.5 and 1.5:1 and then S. thermophilus and L.
bulgaricus growth pattern during the yoghurt incubation period was determined and compared that with the typical lactic acid
fermentation growth curve. Then each culture added yoghurt sample was stored under the cooling conditions for 15 days and then
checked pH, acidity like chemical changes, physical changes of the yoghurt and sensory properties to select the best microbial
ratio. The results revealed when L. bulgaricus and S. thermophilus ratio was 1:1, their growth patterns are similar to the typical
lactic acid growth curve as two exponential phases separated from stationery phase which reveals the efficient proto cooperation
among two bacteria and higher metabolic potentials of them which is able to release metabolites into the fermenting medium in
right time to develop standard yoghurt qualities as texture, color, taste and pH. Furthermore, the results revealed end of the 15
days pH of yoghurt was 4.4 which are suitable to maintain the good quality of yoghurt with high shelf life. But when L. bulgaricus
population is high in the yoghurt starter culture, post acidification was high, and it deforms the yoghurt structure due to syneresis.
And, when S. thermophilus population is high in the yoghurt starter culture, sensory qualities of the yoghurt were not up to the
standards.
