Fate of Listeria innocua in a Probiotic Dairy Dessert

Introduction: Members of genus Listeria are able to survive to a wide range of stress conditions. Listeria is a ubiquitous microorganism, prevailing in raw materials and with known ability to persist in environments of processing, further contaminating processed foods. Listeria innocua has been used as a nonpathogenic surrogate for L. monocytogenes in survival and growth studies in foods.

Purpose: The objective of this study was to determine the fate of L. innocua in a probiotic dairy dessert supplemented with Lactobacillus acidophilus during chilling storage.

Methods: Three different formulations of dairy desserts were processed: F1 – control, inoculated with 10⁴ CFU/g of L. innocua LH 475 only; F2 – added of 10⁸ CFU/g of L. acidophilus only; and F3 – addition of both L. acidophilus (10⁸ CFU/g) and L. innocua (10⁴ CFU/g). Microbiological (L. acidophilus and L. innocua) and pH analysis were performed at time 0, 7, 14, 21 and 28 days of storage. L. innocua was enumerated using Oxford agar supplemented with antibiotics, while L. acidophilus was enumerated using MRS agar supplemented with 0.15% bile salts. Statistical difference was assessed through analysis of variance and Tukey’s test using the XLSTAT for Window 2011.

Results: The pH of probiotic dairy desserts varied between 5.57 (F1) to 7.12 (F2) thoroughout the shelf life (28 days). Both time of storage and formulations significantly influenced the pH values (P < 0.001). The populations of L. innocua increased over the storage period independently of the formulation studied (F1 or F3) (P < 0.001). The populations of L. acidophilus increased expressively in F3 (from 7.04 to 9.01 log CFU/g), while in F2, the populations of this microorganism decreased from 7.78 to 6.59 log CFU/g.

Significance: It has been observed that L. innocua is not inhibited by the presence of probiotic culture in the dairy dessert.