Salmonellae continue to be a major concern for the dairy industry because these bacteria have caused recent outbreaks of illness and have been isolated from various dairy products in the market place. Salmonellae are generally not heat resistant and normally grow at 35 to 37 degrees C, but they can grow at much lower temperatures, provided that the incubation time is suitably extended. To minimize problems, foods should be held at or below 2 to 5 degrees C at all times. Both conventional and rapid methods are available to isolate salmonellae from dairy foods and to identify the bacteria. Salmonellae behave differently in different kinds of cheese: they survived in ripening Cheddar cheese for up to 7 mo at 13 degrees C and for 10 mo at 7 degrees C; in coldpack cheese food for several weeks, depending on the pH and preservative used; and in Domiati cheese 13 to 36 d, depending on the manufacturing process used. When Mozzarella cheese was made, temperatures of stretching and molding (60 degrees C) killed all salmonellae present, but, in cottage cheese, survival of the pathogen depended on the cooking temperature of curd. Spray drying of skim milk killed substantial numbers of salmonellae, but some survivors remained. Butter readily supported growth of salmonellae at room temperature, and neither freezing nor refrigeration for brief periods eliminated salmonellae from butter. Use of appropriate hygienic procedures, e.g., Hazard Analysis Critical Control Point system, during processing should reduce the likelihood of salmonellosis outbreaks associated with dairy foods.
Ultrafiltration and reverse osmosis processes can be useful in the dairy foods industry. When milk is processed, milk fat and casein are rejected fully (e.g., are in retentate) and thus are concentrated by ultrafiltration and reverse osmosis membranes. Lactic cultures are slow to reduce the pH of retentate because of its increased buffering capacity since concentrated calcium phosphate and proteins are present. Conditions for growth of pathogenic microorganisms and inhibition of such bacteria in ultrafiltered milk differ from those of unfiltered milk. The principal advantage of using ultrafiltered milk for conversion into such cheeses as Cheddar, cottage, Havarti, Feta, brick, Colby, and Domiati is an increase in yield of product. Additional benefits claimed for use of ultrafiltered milk in cheese making include reduction in costs of energy, equipment, and labor; improved consistency of cheese flavor; and possible production of new byproducts.
Tyndallized samples of unfiltered skim milk and retentate (concentrated fivefold or twofold by volume) and permeate from UF skim milk were inoculated with 5.5 x 10(3) to 1.5 x 10(5) cfu/ml of Listeria monocytogenes strains California or V7 together with 4 x 10(7) to 2.3 x 10(8) cfu/ml of mesophilic lactic acid bacteria. Numbers of L. monocytogenes (McBride Listeria agar) and lactic acid bacteria (all purpose Tween agar) were determined after 0, 6, 12, 24, 30, and 36 h of incubation at 30 degrees C. Lactic acid bacteria significantly inhibited or inactivated L. monocytogenes in all three products. Inactivation was greater in permeate (6.77 orders of magnitude) than in unfiltered skim milk (3.67 orders of magnitude) or in retentate (4.21 orders of magnitude). Degree of inactivation in retentate was related to the extent of concentration. Inactivation was not complete, and L. monocytogenes survived in these products during fermentation for up to 36 h. When fermented products were refrigerated (4 degrees C), L. monocytogenes survived for 4 to 6 wk in skim milk, 3 to 5 wk in retentate, and 1 wk in permeate. At refrigeration temperature, length of survival was dependent on type of product and strain of the pathogen.
Experiments were done to determine effects of different concentrations of acetic or propionic acid in a glucose-yeast extract-salts medium with an initial pH value of 4,5 or 5.5 on growth and aflatoxin production by Aspergillus parasiticus NRRL 2999. Amounts of aflatoxin were measured with reversed-phase high-performance liquid chromatography. The maximum concentration of acetic or propionic acid that permitted growth at an initial pH of 5.5 was 1% after 7 d of incubation and 0.25% after 3 d of incubation, respectively. When the initial pH of the medium was 4.5, the maximum concentration of acetic or propionic acid that permitted growth was 0.25 or 0.1%, respectively. There was no significant difference (p>0.05) in amount of mycelial (dry weight) produced by cultures in the presence of 0.0, 0.25, 0.50 or 0.75% acetic acid. Amounts of aflatoxin B1 and G1 produced decreased with an increasing concentration of acetic acid. Increasing concentrations of propionic acid caused a decrease in the amount of mycelial dry weight and aflatoxin produced by cultures growing in the medium with an initial pH of 5.5. At an initial pH of 4.5 mycelial growth was slow and at 3 d of incubation amounts of aflatoxin B1 and G1 produced were reduced as concentrations of acetic acid increased. This also was true for propionic acid in the medium with an initial pH of 4.5. Cultures with an extended lag phase in the presence of acetic or propionic acid overcame this and then produced large amounts of aflatoxin B1 and G1 at 7 and 10 d of incubation.
Sodium benzoate, 0.0, 0.1, 0.2, 0.3 or 0.4%, was added to a glucose-yeast-salts medium which was inoculated with 1 ml of a spore suspension containing 108 conidia of Aspergillus parasiticus NRRL 2999 and then was incubated at 28°C. Cultures were analyzed after 3, 7 and 10 d for mycelial dry weight, pH and accumulation of aflatoxin B1 and G1. Amounts of aflatoxin produced were determined using reversed-phase high performance liquid chromatography (HPLC). The percentage of inhibition or stimulation by the additive was used to make comparisons between treatments and control. Generally, increasing the concentration of sodium benzoate increased the percentage of inhibition at the end of incubation (10 d). However, the average accumulation of mycelial dry weight was greater in the presence of benzoate than in its absence, with the greatest increase occurring when the medium contained 0.3% sodium benzoate.
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