Low-water activity foods: increased concern as vehicles of foodborne pathogens.

Foods and food ingredients with low water activity (a(w)) have been implicated with increased frequency in recent years as vehicles for pathogens that have caused outbreaks of illnesses. Some of these foodborne pathogens can survive for several months, even years, in low-a(w) foods and in dry food processing and preparation environments. Foodborne pathogens in low-a(w) foods often exhibit an increased tolerance to heat and other treatments that are lethal to cells in high-a(w) environments. It is virtually impossible to eliminate these pathogens in many dry foods or dry food ingredients without impairing organoleptic quality. Control measures should therefore focus on preventing contamination, which is often a much greater challenge than designing efficient control measures for high-a(w) foods. The most efficient approaches to prevent contamination are based on hygienic design, zoning, and implementation of efficient cleaning and sanitation procedures in the food processing environment. Methodologies to improve the sensitivity and speed of assays to resuscitate desiccated cells of foodborne pathogens and to detect them when present in dry foods in very low numbers should be developed. The goal should be to advance our knowledge of the behavior of foodborne pathogens in low-a(w) foods and food ingredients, with the ultimate aim of developing and implementing interventions that will reduce foodborne illness associated with this food category. Presented here are some observations on survival and persistence of foodborne pathogens in low-a(w) foods, selected outbreaks of illnesses associated with consumption of these foods, and approaches to minimize safety risks.

Isolation of Tyrosine Decarboxylaseless Mutants of a Bacteriocin-Producing Enterococcus faecalis Strain and Their Application in Cheese.

Enterococcus faecalis INIA 4, a bacteriocin-producing strain, was exposed to the mutagenic agent ethyl methanesulfonate (EMS). A modified differential plating medium permitted the isolation of mutants INIA 4-01 to 4-09 with reduced tyrosine decarboxylase activity. All mutant strains displayed rapid growth in milk, while bacteriocin production ranged from 15 to 125% as compared to the parental strain. Enterococcus faecalis INIA 4, INIA 4-07 (less than 0.1% residual tyrosine decarboxylase activity) and INIA 4-03 (10% residual activity) were used in cheese-making experiments. In cheese made from pasteurized milk, using INIA 4-07 as adjunct starter, tyramine was not produced (< 5 mg/kg of cheese after 4 months of ripening). Suppression of Listeria innocua in raw-milk cheeses made using INIA 4-03 and INIA 4-07 was equal to that observed in the cheese made using the parental strain INIA 4. After 2 months of ripening, no free tyrosine and 195 mg/kg of tyramine were detected in the cheese with INIA 4. The tyrosine content of the cheeses made with INIA 4-03 and INIA 4-07 and of the control cheese was approximately 95 mg/kg. The cheeses with INIA 4-03 and 4-07 contained 116 and 120 mg/kg of tyramine respectively, values comparable to the level of 101 mg/kg detected in a control raw-milk cheese to which no enterococci were added. The relatively high tyramine formation in the cheeses made with INIA 4-03 and INIA 4-07 and in the control cheese was probably caused by the decarboxylating activity of the indigenous flora of the raw milk.