Prediction of Streptococcus salivarius subsp. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus populations in yoghurt by Curie point pyrolysis-mass spectrometry

Abstract

We evaluated the potential of pyrolysis-mass spectrometry (PyMS) for quantifying the binary mixed population of Streptococcus salivarius subsp. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in yoghurt. For this purpose, a new analytical approach was developed. The yoghurt was transparised and its total bacterial population was recovered by centrifugation and estimated by turbidimetric measurement. The quantity of each population ( L. bulgaricus, S. thermophilus) was then estimated in the pellet by PyMS, and the data were analysed by artificial neural networks (ANNs). In parallel, streptococci and lactobacilli were numerated on SYL agar and these data were used as reference values to predict the bacterial counts of each population by PyMS. A close correlation was established between the streptococci and the lactobacilli counts on SYL agar and PyMS measurements ( r 2=0.98 for S. thermophilus and r 2=0.96 for L. bulgaricus). Combined turbidimetric measurement and PyMS/ANNs seemed to be a powerful method for obtaining rapid counts of binary mixtures of bacteria in yoghurt.