Study of Listeria monocytogenes in Gouda Cheeses

Abstract

Listeria monocytogenes is a foodborne pathogen, which has been associated with several outbreaks of human listeriosis especially in high risk groups. L. monocytogenes could present in Gouda cheeses because it was influenced by various factors, including unsatisfactory pasteurization treatment, contamination after heat treatment, improper sanitizing procedures and ability of this bacteria to forming biofilms. In addition, this bacteria could survive during the process of cheese making and ripening. The purpose of this study were to examine the presence of L. monocytogenes in Gouda cheeses and to provide an overview of Gouda cheese safety of in regards with L. monocytogenes. The samples of this study consisted of 15 local Gouda cheese and 15 imported Gouda cheese which were sold in supermarkets. The samples were taken with considering production code and expired date. The method to detect presence of L. monocytogenes which referred to the Bacteriological Analytical Manual, US Food and Drug Administration and Bergey's Manual of Determinative Bacteriology. The first step was enrichment of samples in Listeria enrichment broth, then isolated in oxford agar and identification step using catalase test, 3% KOH test, Gram stain, carbohydrate fermentation test (mannitol, rhamnose, xylose), motility test and CAMP test. The results of this study showed that L. monocytogenes was not found in 15 samples of local Gouda cheese (0%) and 15 samples of imported Gouda cheese (0%). The absence of L. monocytogenes in local and imported Gouda cheeses was related to combination of preservative factors in Gouda cheese such as pasteurization of milk as raw material for making of Gouda cheese, decrease in pH was caused by the addition of lactic acid bacteria starter cultures, and decline in water activity during the ripening process of Gouda cheeses. The combination of these preservative factors could inhibit the growth of L. monocytogenes could even inactivate L. monocytogenes. In addition, there was not contamination of L. monocytogenes after the pasteurization and during processing of milk to become cheeses. Another factor that could affect the absence of L. monocytogenes in Gouda cheese samples was the number of L. monocytogenes in the samples below the detection limit of conventional method which used in this study. The study could be concluded that all of samples of Gouda cheese were not found the existence of L. monocytogenes and relatively safe from contamination of L. monocytogenes and fulfilled the standard that determined by national standard of Indonesia 7388:2009 about Maximum Limit of Microbial Contamination in Food.