Growth and viability of yogurt starter organisms in honey-sweetened skimmed milk

Lactic acid bacteria ( LAB): Streptococcus thermophilus TA 040 and Lactobacillus delbrueckii spp. bulgaricus Lb 340, were cultured in reconstituted (10%, w/ v) skimmed milk with 5 or 10% (w/v) polyfloral or unifloral honey. Inoculated samples were incubated aerobically at 42 C until milk coagulation. Samples were collected at 2 h intervals and examined for biomass and pH changes. Cell viability and post-acidifying activity of both strains during 28 days of storage at 4 C were also measured. A higher increase (P < 0.05) in growth and acidifying activity of S. thermophilus monocultures was observed when 10% honey was added. However, L. bulgaricus did not show such a marked increase in its growth capacity. In associated cultures, LAB growth was slightly inhibited, whereas curdling time was prolonged by an hour when 10% honey was added and yogurt acidity was moderate. Cell viability improved by 5 to 6.6% for S. thermophilus and 10% for L. bulgaricus in pure honey-sweetened cultures over 28 days of refrigerated storage. This protective effect of honey on LAB cell viability was also observed in associated cultures ( 10 to 12% comparatively to the control).