Fermentation of reconstituted milk by Streptococcus thermophilus: Effect of irradiation on skim milk powder

The growth of Streptococcus thermophilus ST12 (ST12) in milk reconstituted from non-irradiated and irradiated at 10 kGy low-heat skim milk powders (RSM and irrRSM, respectively) at 40 °C was monitored by microcalorimetry. Statistically significant differences of the growth patterns of ST12 in RSM and irrRSM were found. Distinctively diauxic growth curves in RSM were replaced by one-stage growth curves in irrRSM. The final pH in RSM was 5.56 while in irrRSM samples it was 4.41. The time of initiation of gel formation was about 36 min shorter; however, the gels were considerably weaker in irrRSM than in RSM. The front-face fluorescence spectra were also used to characterise the differences in acidification processes. The microcalorimetric data together with the concentrations of metabolites determined during fermentation, rheological and fluorescence measurements indicated the substantial changes in the growth of ST12 in irrRSM in comparison with RSM.