Microbial Survey of Pennsylvania Surface Water Used for Irrigating Produce Crops.

Recent produce-associated foodborne illness outbreaks have been attributed to contaminated irrigation water. This study examined microbial levels in Pennsylvania surface waters used for irrigation, relationships between microbial indicator organisms and water physicochemical characteristics, and the potential use of indicators for predicting the presence of human pathogens. A total of 153 samples taken from surface water sources used for irrigation in southeastern Pennsylvania were collected from 39 farms over a 2-year period. Samples were analyzed for six microbial indicator organisms (aerobic plate count, Enterobacteriaceae, coliform, fecal coliforms, Escherichia coli, and enterococci), two human pathogens (Salmonella and E. coli O157), and seven physical and environmental characteristics (pH, conductivity, turbidity, air and water temperature, and sampling day and 3-day-accumulated precipitation levels). Indicator populations were highly variable and not predicted by water and environmental characteristics. Only five samples were confirmed positive for Salmonella, and no E. coli O157 was detected in any samples. Predictive relationships between microbial indicators and the occurrence of pathogens could therefore not be determined.

Decontamination of hard cheeses by pulsed UV light.

Cheese is a ready-to-eat food that may be contaminated on the surface by undesirable spoilage and pathogenic microorganisms during production, packaging, and postpackaging processes. Penicillium roqueforti is commonly found on cheese surfaces at refrigeration temperatures and is one of the most common spoilage fungal species. Consumption of cheese contaminated with Listeria monocytogenes can result in foodborne listeriosis. Therefore, cheese should be decontaminated at postprocessing stages. Pulsed UV light is a nonthermal method for food preservation that involves the use of intense short pulses to ensure microbial decontamination on the surface of foods or packaging materials. In this study, the efficacy of pulsed UV light for inactivation of P. roqueforti and L. monocytogenes inoculated onto packaged and unpackaged hard cheeses was investigated. Treatment times and the distance from the UV strobe were evaluated to determine optimum treatment conditions. Packaged and unpackaged cheeses were treated at distances of 5, 8, and 13 cm for up to 60 s. For P. roqueforti, maximum reduction after 40 s at 5 cm was 1.32 log CFU/cm(2) on unpackaged cheese and 1.24 log CFU/cm(2) on packaged cheese. Reductions of L. monocytogenes under the same treatment conditions were about 2.9 and 2.8 log CFU/cm(2) on packaged and unpackaged cheeses, respectively. The temperature changes and total energy increases were directly proportional to treatment time and inversely proportional to distance between the UV lamp and the samples. The changes in color and lipid oxidation were determined at mild (5 s at 13 cm), moderate (30 s at 8 cm), and extreme (40 s at 5 cm) treatments. The color and chemical quality of cheeses were not significantly different after mild treatments (P > 0.05). The mechanical properties of the plastic packaging material (polypropylene) also were evaluated after mild, moderate, and extreme treatments. A decreasing trend was noted for elastic modulus; however, no significant differences were found between untreated samples and those given mild and moderate treatments (P > 0.05). Overall, these results indicate that pulsed UV light can inactivate P. roqueforti and L. monocytogenes on the surface of hard cheeses.

Antifungal activity of 10-oxo-trans-8-decenoic acid and 1-octen-3-ol against Penicillium expansum in potato dextrose agar medium.

In mushrooms, 10-oxo-trans-8-decenoic acid (ODA) and 1-octen-3-ol are secondary metabolites produced naturally by the enzymatic breakdown of linoleic acid. Both compounds were determined to inhibit the mycelial growth of Penicillium expansum PP497A, a common food spoilage organism, when added to potato dextrose agar medium. ODA and 1-octen-3-ol were inhibitory at concentrations of > 1.25 mM (230 microg/g for ODA and 160 microg/g for 1-octen-3-ol). At pH 5.6, 1-octen-3-ol was more inhibitory than ODA. However, at pH 3.5, both compounds (especially ODA) were more inhibitory than they were at pH 5.6. This finding indicates that the undissociated carboxyl of ODA was important for inhibition. At a concentration of 2.5 mM and a pH of 3.5, ODA and 1-octen-3-ol inhibited growth by 43.1 and 41.9%, respectively. An additive effect was observed when both compounds were added at a combined concentration of > or = 1.25 mM; when both were added at a combined concentration of 2.5 mM, mycelial growth was inhibited by 48.8 and 72.8% at pHs of 5.6 and 3.5, respectively. Although the antifungal activity levels for these two compounds were lower than those observed for equal molar concentrations of sorbate, a common antifungal compound, these findings indicate that further investigation of the potential of ODA and 1-octen-3-ol for use as natural food preservatives is warranted.

Antimycotic and Antiaflatoxigenic Effect of Lactic Acid Bacteria: A Review †.

Lactic acid bacteria are extensively used in the fermentation of a wide variety of food products and are known for their preservative and therapeutic effects. Many lactic acid bacteria species have been reported to inactivate bacterial pathogens, and numerous antibacterial substances have been isolated. However, the antimycotic and antimycotoxigenic potential of lactic acid bacteria has still not been fully investigated. Fermented foods such as cheese can be contaminated by molds and mycotoxins. Mold causes spoilage and renders the product unusable for consumption, and the presence of mycotoxins presents a potential health hazard. A limited number of reports have shown that lactic acid bacteria affect mold growth and aflatoxin production. Although numerous lactic acid bacteria such as Lactobacillus spp. were found to inhibit aflatoxin biosynthesis, other lactic bacteria such as Lactococcus lactis were found to stimulate aflatoxin production. The morphology of lactic acid bacteria cells has also been found to be affected by the presence of fungal mycelia and aflatoxin. Lactococcus lactis cells became larger and formed long chains in the presence of Aspergillus flavus and aflatoxins. Numerous investigations reported that low pH, depletion of nutrients, and microbial competition do not explain the reason for aflatoxin inhibition. Some investigators suggested that the inhibition of aflatoxin is due to lactic acid and/or lactic acid bacteria metabolites. These metabolites have been reported to be heat-stable low-molecular-weight compounds.

Inhibition of Growth and Aflatoxin Production of Aspergillus flavus by Lactobacillus Species †.

A mixture of Lactobacillus species from a commercial silage inoculum reduced mold growth and inhibited aflatoxin production by Aspergillus flavus subsp. parasiticus . Actively growing Lactobacillus spp. cells totally inhibited germination of mold spores. Culture supernatant broth from the mixture of strains inhibited mold growth but did not destroy mold spore viability. Some mold spores were observed microscopically to have germinated and produced short nonbranching germ tubes; then growth ceased. While the pH of the culture broth and supernatant were about 4.0, acidification of nonfermented broth to pH 4.0 with HCl and lactic acid did not cause a similar inhibition of spore germination. The mixture of Lactobacillus species growing in a dialysis sack inhibited aflatoxin production by the A. flavus culture growing outside of the sack in broth, whereas mold growth was not affected. The pH values outside of the dialysis sack in the control and the treatments were similar (6 to 7) throughout the incubation period. When a dialysis sack with a molecular weight cutoff (MWCO) of 1,000 was used, there was little inhibition of aflatoxin B1 production, but with MWCOs of 6,000 to 8,000 and 12,000 to 14,000 aflatoxin production was greatly inhibited. In mixed culture experiments, levels of aflatoxin B1 and G1 were depressed compared to the control (monoculture). Mold growth in this case was also reduced compared to the monoculture system. Purified isolates of Lactobacillus from the commercial mixture had a slight effect on mold growth and aflatoxin production, but supernatant liquid of one isolate was quite inhibitory to production of aflatoxins B1 and G1, without affecting mold growth.

Detection of Molds in Foods and Feeds: Potential Rapid and Selective Methods †.

Most laboratories still rely on traditional microbiological methods to detect molds in foods and feed. These methods are modified bacteriological methods. Plate count techniques are time consuming and do not detect dead fungi, which are a sign of past contamination. Development of rapid methods to detect molds in foods is still in its embryonic stage. Recently mycologists have begun to develop media that are differential and selective for particular mold species. The use of these media is of great value for the detection of specific groups of fungi such as toxigenic fungi. Other potential rapid methods include chemical and biochemical assays for, e.g., chitin and ergosterol, and immunological and electrical impedance methods.

Aspergillus flavus and Aspergillus parasiticus : Aflatoxigenic Fungi of Concern in Foods and Feeds † : A Review.

Aspergillus flavus and the closely related subspecies parasiticus have long been recognized as major contaminants of organic and nonorganic items. A. flavus , a common soil fungus, can infest a wide range of agricultural products. Some A. flavus varieties produce aflatoxins, which are carcinogenic toxins that induce liver cancer in laboratory animals. A. flavus var. flavus , A. flavus subsp. parasiticus , and A. nomius share the ability to produce aflatoxins. Identification of the A. flavus species group is mainly based on the color and macroscopic and microscopic characteristics of the fungus. A. flavus growth and aflatoxin biosynthesis depend on substrate, moisture, temperature, pH, aeration, and competing microflora. The growth of A. flavus and aflatoxin production are sometimes unavoidable. Aflatoxins are considered natural contaminants; the ideal control approach is prevention of mold growth and aflatoxin production. The detection of members of the A. flavus species group in foods and feed is generally carried out by using plate techniques such as surface spread or direct plating. Research on alternative fungal detection methods is still in its infancy. Few immunoassay techniques have been investigated in this regard. Aflatoxins are generally analyzed by chemical methods, although immunochemical methods which use antibodies are becoming common analytical tools for aflatoxins.

Growth and Production of Enterotoxins A and D by Staphylococcus aureus in Salad Bar Ingredients and Clam Chowder.

Growth and production of enterotoxins A and D (SEA, SED) by two strains of Staphylococcus aureus were determined in salad bar ingredients and clam chowder. Salad bar ingredients included lettuce, canned black olives, canned green olives, tomato, green pepper, blue cheese salad dressing, blue cheese crumbles, celery, and croutons. Total S. aureus were determined by plate count on Baird-Parker agar. Enterotoxins were quantified by using an ELISA technique. S. aureus did not survive in salad dressing, with pH 4.3. With the exception of olives and blue cheese, S. aureus survived on all ingredients for more than 12 h. After 24 h, the total number of cells decreased on most of the ingredients. S. aureus grew well on green pepper during the first 24 h, reaching 105 CFU/g, but no enterotoxins were detected. S. aureus also increased in moist and dry plain croutons, but there was no detectable production of enterotoxins. S. aureus growth was excellent in clam chowder with cell counts exceeding 108 CFU/g after 12 h at 42°C. Production of SEA and SED began shortly after 3 h. Maximal levels of SEA and SED were 0.29 and 1.6 ng/g, respectively, after 12 h. In brain heart infusion broth, the production of SEA and SED reached 21.9 and 36.3 ng/ml, respectively, after 24 h at 37°C.

Effects of Oleuropein, Tyrosol, and Caffeic Acid on the Growth of Mold Isolated from Olives.

The effects of oleuropein, tyrosol, and caffeic acid on the growth of mold species isolated from Moroccan olives were studied. Oleuropein at 0.2%, 0.4%, and 0.6% slightly stimulated fungal growth. Tyrosol at 0.2%, 0.4%, and 0.6% inhibited the growth of all mold species tested. Caffeic acid was less inhibitory than tyrosol.

Effects of Potassium Sorbate and Natamycin on Growth and Penicillic Acid Production by Aspergillus ochraceus 1.

Potassium sorbate at 500, 1000 and 1500 µg/ml delayed initiation of growth and sporulation by Aspergillus ochraceus 0L24 in yeast extract-sucrose (YES) broth at 15°C, 25°C and 35°C. At 25°C, sporulation and growth were more rapid. Potassium sorbate at 500 µg/ml resulted in an increase in mycelial weight, but at 1000 and 1500 µg/ml the mycelial mass was decreased. Potassium sorbate also reduced or prevented production of penicillic acid, especially at 15 and 35°C. Natamycin at 1, 10 and 20 µg/ml delayed initiation of growth and sporulation in YES broth. At 20 µg of natamycin/ml, mycelial growth was inhibited by 80 to 100% and penicillic acid production was completely inhibited. Growth and penicillic acid production on olive paste by A. ochraceus in the presence of potassium sorbate and natamycin showed that sorbate at 1500, 3000, and 6000 µg/g delayed growth and sporulation. Also, the extent of growth was greatly reduced by 3000 and 6000 µg of potassium sorbate/g. Penicillic acid production was reduced over the control at all the potassium sorbate levels. At 6000 µg of sorbate/g, no penicillic acid was detected after 21 d of incubation. Natamycin at 85, 175, and 350 µg/g delayed growth and sporulation by A. ochraceus on olive paste. Increasing levels of natamycin resulted in decreased growth. Production of penicillic acid was also decreased by natamycin, 350 µg of natamycin/g decreased penicillic acid production by 96%.