Rapid quantification of probiotic cultures using microcalorimetry
The potential of using microcalorimetry as a rapid method for the quantification of probiotic micro-organisms was investigated. The technique was based on two potential mechanisms: the metabolism of specific carbohydrate and the ability to metabolise glucose in the presence of inhibitors. In preliminary experiments, heat production from the probiotic cultures indicated that a lot of exothermic reactions occurred together. For the expression of results, the average amount of heat produced from a culture over a specific period of time was calculated. The average of heat was related to specific bacterial counts with high regression coefficients. This technique worked for the quantification of both L. acidophilus La-5 and B. lactis Bb-12 despite the fact that they reacted differently to variation in composition of the medium. Results indicated that salicin and maltose can be used as specific carbohydrates for the enumeration of pure cultures of L. acidophilus La-5 and B. lactis Bb-12 in a milk-free model system. A model system containing milk supplemented with dicloxacillin (2.0 µg.ml-1) or oxgall associated with gentamicin (147.8 µg.ml-1 and 29.9 µg.ml-1) supported the differentiation of the probiotic cultures from traditional starter cultures. Results were obtained in less than four hours. The developed method was assessed to explore the feasibility of using microcalorimetry for the selective enumeration of live probiotic cultures in commercial fermented milk products. The results indicated that yoghurt samples can be analysed directly with minimum sample preparation, that heat outputs produced from the commercial yoghurts were consistent with the viable probiotic cell content and results were obtained in less than four hours.