The Effect of Cheese Brine Concentrations on Survival of Listeria monocytogenes

The salting process is an important step in the manufacture of most cheese varieties. The salt in cheese has different functions, such as reduction of curd moisture, suppression of unwanted microorganisms, modification of flavour and texture and contribution to cheese ripening (Ibáñez et al. 1993; Mulvihill and McCarthy 1994; Laborda and Rubiolo 1999; Mulet et al. 1999). Therefore, in cheesemaking processes of some traditional cheese varieties, a high salt content in brine is essential for controlling microflora, preventing growth of pathogens and controling enzyme activities during storage (Abd El-Salam et al. 1993). Although storage in brine is thought to cause a decrease in the populations of undesirable contaminants, there is great concern that the brine can also serve as a reservoir of certain salt-tolerant pathogens. There have been a number of cases where especially brined cheeses have acted as carriers of foodborne infections (Keceli and Robinson 1997), because poorly maintained brine tanks can become ready sources of contamination. Since Listeria monocytogenes can survive after the pasteurization process (Doyle et al. 1987), they can easily be carried into commercial cheese brines. Because commercial cheese brines are mostly used repeatedly, the proteins and other nutrients from cheese are accumulated in brines, which make the brine a nutrient-rich environment. Larson et al. (1999) reported that L. monocytogenes survived for 118 days in fresh feta cheese brines (65 g/L NaCl and pH 6.8) at 4 °C and 12 °C; moreover, it has been shown that L. monocytogenes can grow in salt solutions of up to 60 g/L NaCl. Papageorgiou and Marth (1989) studied the fate of in salted whey. They found that the pathogen was able to grow in salted whey (60 g/L NaCl, pH 5.65), but was inhibited by a salt concentration of 120 g/L NaCl in the whey (pH 5.50); large variation in salt tolerance between strains was observed. One of the potential difficulties to control L. Abstract: In this study, inactivation of Listeria monocytogenes at different salt concentrations and times were studied to gain a better understanding of the response of the bacterium in the brines. Cheese brines containing 13%, 15% and 19% NaCl were inoculated with 10 3 L. monocytogenes (serotype 4b, RSKK 475) CFU/mL and stored at 4 °C for 90 days. Population of the pathogen in 13% brine decreased significantly (P<0.05) during first 30 days of storage compared to population in initial brine. However, L. monocytogenes was able to survive in 13% brine during 90 days of storage. Whereas the population of L. monocytogenes in the 15% brine decreased significantly between days 0 to 15 of storage so that direct planting at 30, 60 and 90 days gave negative results, but the same samples gave positive results after enrichment. Numbers of L. monocytogenes in the 19% brine decreased faster than mentioned above for other salt concentration and the pathogen was not detected in brine after 15 days of storage by both the direct planting and enrichment. As a result, it was suggested that brined cheeses should be stored in brines containing 19% or more NaCl for at least 15 days to prevent survival of L. monocytogenes.