Raw milk products are a safety concern for consumers, according to a recent study.

Cheese made with unpasteurized milk is a safety concern because of possible contamination with foodborne pathogens, the study by the U.S. Food and Drug Administration, Division of Food Processing Science and Technology, and the Institute for Food Safety and Health, Illinois Institute of Technology found.

Listeria monocytogenes and Escherichia coli O157:H7 have been the cause of several outbreaks and recalls linked to Gouda cheese made with unpasteurized milk. In the past year, there have been multiple outbreaks linked to cheese made from raw milk — Listeria in Canada and E. coli in France.

The goal of the study

The goal of this study was to assess the population dynamics of L. monocytogenes and E. coli O157:H7 during the aging of Gouda cheese in unpasteurized milk.

Listeria and E. Coli were compared with the native microbial populations throughout manufacture and aging. The types of native microflora assessed included Enterobacteriaceae, lactic acid bacteria, mesophilic bacteria, and yeasts and molds.

The testing method

Unpasteurized milk was inoculated with L. monocytogenes at 1 or 3 log CFU/mL or with E. coli O157:H7 at 1 log CFU/mL, and Gouda cheese was manufactured in laboratory-scale or pilot plant-scale settings. 

Cheeses were stored at 10°C for at least 90 days, and some cheeses were stored up to 163 days.

The findings 

Initial native microflora populations in unpasteurized milk did not differ significantly from laboratory-scale or pilot plant-scale trials, and population dynamics trended similarly throughout cheese manufacture and aging.

During manufacture, approximately 81% of the total L. monocytogenes and E. coli O157:H7 populations were found in the curd samples. 

At an inoculation level of 1 log CFU/mL, L. monocytogenes survived in the cheese beyond 60 days in four of five trials.

E. coli O157:H7 was detected beyond 60 days in only one trial. 

At the higher 3-log inoculation level, the population of L. monocytogenes increased significantly from 3.96 ± 0.07 log CFU/g at the beginning of aging to 6.00 ± 0.73 log CFU/g after 150 days, corresponding to a growth rate of 0.04 ± 0.02 log CFU/g/day. 

Lactic acid and mesophilic bacterial populations remained consistent at approximately 8 to 9 log CFU/g during aging, wheras yeast and mold populations steadily increased. 

This study will contribute to knowledge about the survival of these pathogens during Gouda cheese production. It will also help researchers assess the safety risks of the consumption of Gouda cheese made with unpasteurized milk.