Abstract

Background: One of the most important parameters in a commercial fermented milk drink containing probiotics is the viability of the microorganisms.

Aims and Objectives: This study described the viability kinetics of Lactobacillus casei Shirota strain in a commercial fermented milk drink during refrigerated storage using nonlinear models.

Materials and Methods: Viability of L. casei Shirota strain was monitored during refrigerated storage using standard bacterial plate count method. Several nonlinear mathematical models such as the zero-order, first-order, and second-order kinetic equations were employed in describing the population dynamics. Best-fit models were selected based on prescribed criteria including sum of squares of the error, p-value, and coefficient of determination.

Results: The viable counts of L. casei Shirota strain in the fermented milk samples stored under refrigerated conditions decreased from 3.73 × 108 colony forming units per ml (CFU/ml) to 2.70 × 108 CFU/ml when monitored every 3-4 days interval over 10 different monitoring points within the product’s indicated shelf life. The counts significantly differed between the monitoring points that were at least 14 days apart. The lowest viable count was still within the recommended therapeutic dose. The decrease in the bacterial population behaved under a second-order kinetic relationship.

Conclusion: The viability of L. casei Shirota strain in a commercial fermented milk product during refrigerated storage is governed in accordance with the second-order kinetic mechanism.

Key words: Viability Kinetics; Lactobacillus casei; Probiotics; Second-order Kinetics