Survival of Listeria monocytogenes ATCC 7644 in Fermented Milk and Mahewu

Introduction: Listeria monocytogenes has been isolated from a wide range of food products including fermented products. While their growth and survival have been determined in popular fermented products, there is no reported work on locally fermented foods such as mahewu and locally fermented milk.

Purpose: The aim of this work is to evaluate the survival of Listeria monocytogenes in fermented milk and mahewu, and to also determine the physicochemical properties of these products under different storage conditions.

Methods: Milk and maize were fermented with a mixed starter culture. Fermented milk and maize were each inoculated with 10⁸ CFU/ml of Listeria monocytogenes ATCC 7644 and stored at 37°C, room temperature (22°C) and 4°C for five days. After which Listeria monocytogenes counts as well as the physicochemical properties were determined after every 24 h.  

Results: L. monocytogenes counts taken showed that fermented milk stored at 4°C after 120 h of storage had a 2.60-log reduction, while at 37°C there was a 0.70-log reduction and at room temperature (22°C) there was a 0.90-log reduction. Mahewu stored at 4°C stored for 120 h had a 2.99-log reduction, 37°C had 1.00-log reduction and at room temperature (22°C) there was a 1.50-log reduction. Statistical analysis showed that temperature had a significant effect on the survival of L. monocytogenes. However there was no significant effects of storage time or temperature on the physiochemical products of mahewu and fermented milk.

Significance: This research indicated that L. monocytogenes can survive in the two fermented foods however storage at 4°C reduced the numbers of the pathogens compared to storage at 37°C or room temperature (22°C). The importance of good manufacturing practices and good hygiene to reduce bacterial contamination can therefore not be overemphasized.