Search

The effects of whey protein isolate (WPI) films and coatings incorporating lysozyme (LZ) on the inhibition of Listeria monocytogenes both in and on microbial media, as well as on cold-smoked salmon, were studied. The antimicrobial effects of LZ were examined using various growth media by turbidity and plate counting tests. Disc-covering and disc-surface-spreading tests were also used to evaluate the effects of WPI films incorporating LZ. Smoked salmon was used as a model food to test the antimicrobial effects of WPI coatings incorporating LZ, both initially and during storage at 4 and 10 degrees C for 35 days. Tensile properties (elastic modulus, tensile strength, and percentage of elongation), oxygen permeability, and color (Hunter L, a, and b) of WPI films with and without LZ were also compared. LZ inhibited L. monocytogenes in broth and on agar media. The number of cells surviving after LZ treatments depended on the type of media. WPI films incorporating 204 mg of LZ per g of film (dry basis) inhibited the growth of a preparation of 4.4 log CFU/cm2 L. monocytogenes. WPI coatings prepared with 25 mg of LZ per g of coating solution initially inactivated more than 2.4, 4.5, and 3.0 log CFU/g of L. monocytogenes, total aerobes, and yeasts and molds in smoked salmon samples, respectively. The WPI coatings incorporating LZ efficiently retarded the growth of L. monocytogenes at both 4 and 10 degrees C. The anti-L. monocytogenes effect of LZ-WPI coating was more noticeable when the coating was applied before inoculation than when the coating was applied after inoculation. Significantly higher elastic modulus values and lower percentage of elongation and oxygen permeability values were measured with the WPI films incorporating LZ than with the plain WPI films.

Propylene oxide (PPO) is commonly used to reduce microbial populations in U.S. bulk raw almonds, but the process has not been validated for reduction of foodborne pathogens. The reduction of Salmonella Enteritidis phage type (PT) 30 inoculated onto almonds was evaluated after exposure to a standard commercial PPO treatment. Almonds were inoculated with Salmonella Enteritidis PT 30 to approximately 8.0 log CFU/g after drying. Inoculated almonds were placed in bags designed for gaseous sterilization and positioned in the center of 900-kg bins or 22.7-kg boxes of warmed almonds. Almonds were further warmed to an initial temperature of 23 to 34 degrees C, treated with PPO (0.5 kg/m3 for 4 h), and held for 0 or 2 days at 38 to 43 degrees C followed by storage for 2 to 5 days at 15 to 18 degrees C. Salmonella Enteritidis PT 30 was recovered by vigorously shaking 100 g of almonds in 100 ml of Butterfield's phosphate buffer, plating onto tryptic soy or bismuth sulfite agar, and incubating at 35 degrees C for 24 or 48 h, respectively. Populations of Salmonella Enteritidis were consistently reduced by > 5.0 log CFU/g (5.2 to > 8.6 log CFU/ g) when initial counts were compared with counts obtained 5 days after PPO treatment. Reductions of 1.2 to 4.4 log CFU/g occurred during post-PPO storage. Reductions were not significantly improved (P < 0.05) when almonds were held at 38 to 43 degrees C after PPO treatment. PPO residues were > 400 ppm immediately after removal from the PPO chamber and declined to < 300 ppm during post-PPO storage. PPO is an effective treatment for reducing populations of Salmonella Enteritidis PT 30 on bulk almonds.

Cut or intact surfaces of fresh strawberries were spot inoculated with a five-strain cocktail of nalidixic-acid resistant Listeria monocytogenes (10(6) (low inoculum) and 10(8) (high inoculum) CFU per three-berry sample). Inoculated strawberries were dried for 1 h at 24 degrees C and were stored in loosely closed containers at 4 or 24 degrees C. An initial population reduction of approximately 0.6 and 1.2 log cycles, high and low inoculum, respectively, was observed on intact but not cut berries after the 1-h drying period. A decrease of 1.4 and 3.3 log cycles per intact sample was observed over 48 h for the high and low inoculum, respectively, when stored at 24 degrees C. When held at 4 degrees C, a reduction of approximately 3 log cycles per intact-berry sample was observed for both inocula over the 7-day storage period. Populations on cut surfaces remained constant at both temperatures and both inoculum densities throughout the storage period. Sliced, inoculated strawberries (6.7 log CFU/25-g sample) with or without 20% sucrose were frozen at -20+/-2 degrees C. After 28 days of frozen storage, populations of L. monocytogenes determined on tryptose phosphate agar supplemented with nalidixic acid (TPAN) had declined by 0 to 1.2 log cycles, with and without 20% sucrose, respectively. Counts on modified Oxford agar supplemented with nalidixic acid were significantly (P< or =0.05) lower (0.5 to 1.8 log CFU/g) than on TPAN indicating that some cell injury had occurred. Results of this study indicate that L. monocytogenes is capable of survival but not growth on the surface of fresh intact or cut strawberries throughout the expected shelf life of the fresh fruit and can survive on frozen strawberries for periods of at least 4 weeks. On whole strawberries held at 24 degrees C, significantly faster declines (P< or =0.05) of L. monocytogenes were observed when lower rather than higher inoculum levels were applied.

Washing conditions that included a soak or brush scrub were evaluated for removal of Salmonella from the surface of smooth (honeydew) or complex (cantaloupe) melon rinds. Melon rinds were spot-inoculated onto a 2.5 cm2 area of rind (squares) with approximately 6.0 log(10) CFU/square of an avirulent nalidixic acid-resistant strain of Salmonella typhimurium. Melons were washed by immersion in 1500 ml of water or 200 ppm total chlorine and allowed to soak or were scrubbed over the entire melon surface with a sterile vegetable brush for 60 s. Inoculated sites, uninoculated sites ("next to" sites) that were adjacent to inoculated sites, and sites on the side of the melon opposite (remote sites) the inoculated site were excised and pummeled in a stomacher for 2 min prior to plating onto tryptic soy or bismuth sulfite agar supplemented with 50 microg/ml nalidixic acid. S. typhimurium was reduced on the rind of cantaloupe by 1.8 log CFU/melon after soaking for 60 s in 200 ppm total chlorine, which was significantly better than the 0.7 log CFU/melon achieved with soaking in water. For both water and 200 ppm total chlorine, scrubbing with a vegetable brush was shown to be significantly (0.9 log CFU/cantaloupe) more effective than soaking alone. When honeydew melons were soaked or scrubbed in water, reductions of 2.8 log CFU/melon or >4.6 log CFU/melon (four of five samples), respectively, were observed. However, when water treatments were used, the presence of Salmonella-positive "next to" and remote sites indicated that bacteria were spread from inoculated site on the rind to uninoculated sites either through the rinse water (40-70 CFU/ml of Salmonella) or scrub brush (400-500 CFU/brush). Transfer to other sites occurred more often with cantaloupe than honeydew melons. This transfer was eliminated when 200 ppm total chlorine was used. When 200 ppm total chlorine was used, Salmonella could not be detected in the water or on the scrub brush. For optimal microbial removal in food service and home settings, melons should be scrubbed with a clean brush under running water. However, to ensure the benefits of brushing, instructions for cleaning and sanitizing brushes must also be emphasized. For food service settings where concentration and pH can be adequately measured, the use of chlorinated water may provide additional benefit.

The effects of method for applying inoculum and of drying time after inoculation on survival and recovery of foodborne pathogens on iceberg lettuce and parsley were studied. Five-strain mixtures of Escherichia coli O157:H7, Salmonella, or Listeria monocytogenes were applied to lettuce and parsley by dip, spot, or spray inoculation methods. Inocula were dried for 2 h at 22 degrees C or for 2 h at 22 degrees C and then 22 h at 4 degrees C before being treated with water (control) or chlorine (200 microg/ml). Significantly higher populations (CFU per lettuce or parsley sample) of E. coli O157:H7 and Salmonella (alpha = 0.05) were recovered from dip-inoculated produce than from spot- or spray-inoculated produce. This difference was attributed to larger numbers of cells adhering to lettuce and parsley subjected to dip inoculation. Populations of E. coli O157:H7 and Salmonella recovered from lettuce inoculated by spot and spray methods were not significantly different, but populations recovered from spot-inoculated parsley were significantly higher than those recovered from spray-inoculated parsley, even though the number of cells applied was the same. Significantly different numbers of L. monocytogenes were recovered from inoculated lettuce (dip > spray > spot); populations recovered from dip-inoculated parsley were significantly higher than those recovered from spot- or spray-inoculated parsley, which were not significantly different from each other. Populations of pathogens recovered from lettuce and parsley after drying inoculum for 2 h at 22 degrees C were significantly higher than or equal to populations recovered after drying for 2 h at 22 degrees C and then for 22 h at 4 degrees C. Significant differences (water > chlorine) were observed in populations of all pathogens recovered from treated lettuce and parsley, regardless of inoculation method and drying time. It is recommended that spot inoculation with a drying time of 2 h at 22 degrees C followed by 22 h at 4 degrees C be used to determine the efficacy of chlorine and other sanitizers in killing foodborne pathogens on lettuce and parsley.

A study was undertaken to evaluate methods for applying inoculum and to examine the effect of inoculum drying time on survival and recovery of foodborne pathogens inoculated onto the surface of raw, ripe tomatoes. Five-strain mixtures of Escherichia coli O157:H7, Salmonella, or Listeria monocytogenes were applied to tomatoes by dip, spot, or spray inoculation methods. Inocula were dried for 1 or 24 h at 22 degrees C before tomatoes were treated with water (control) or chlorine (200 micrograms/ml). Significantly (alpha = 0.05) larger populations (CFU per tomato) of E. coli O157:H7 and Salmonella were recovered from dipinoculated tomatoes than from spot- or spray-inoculated tomatoes. This difference was attributed to larger numbers of cells adhering to tomatoes subjected to dip inoculation. Populations of E. coli O157:H7 and Salmonella recovered from spot- and spray-inoculated tomatoes containing the same initial number of cells were not significantly different. Significantly different L. monocytogenes population sizes were recovered from inoculated tomatoes (dip > spot > spray). Populations of pathogens recovered from tomatoes were significantly larger when inocula were dried for 1 h compared with 24 h. Significant differences (water > chlorine) were observed in the sizes of populations for all pathogens recovered from tomatoes treated with chlorine, regardless of inoculation method or drying time. Results indicate that inoculation method, drying time, and treatment affect survival and/or recovery of foodborne pathogens inoculated onto the surface of tomatoes. We recommend that spot inoculation with a drying time of 24 h at 22 degrees C be used with standard methods to determine the efficacy of chlorine and other sanitizers for killing foodborne pathogens on tomatoes.

The efficacy levels of practices used by consumers to wash smooth-surface fruits and vegetables were compared. Golden Delicious apples were spot inoculated near the blossom end with 50 microl of a cocktail of six serotypes of Salmonella enterica (with a total inoculum level of approximately 10(9) CFU per apple). The inoculum was dried for 1.5 h, and apples were either treated immediately or held for 24 h prior to treatment. Treatments included wetting with approximately 5 ml of water, vinegar (5% acidity), or a 200-ppm chlorine solution, rubbing for 5 or 30 s, rinsing with 200 to 600 ml of 24 or 43 degrees C water, and drying with a sterile paper towel. Residual populations of Salmonella were determined by rubbing the treated apple for 30 s in 20 ml of Dey-Engley neutralizing broth and plating on tryptic soy agar and bismuth sulfite agar. Rubbing treatments carried out for 5 and 30 s both resulted in a significant reduction in Salmonella populations (1 log10 CFU per apple) relative to populations on samples held for 30 s. A 5-s rub followed by a 200-ml flowing-water rinse reduced populations by 3 log10 CFU per apple. No further decrease in population was obtained by rinsing with 400 or 600 ml of water. Increasing the rinse water temperature to 43 degrees C did not significantly improve microbial removal. Drying the apple with a sterile paper towel resulted in an additional decrease of approximately 0.4 log10 CFU per apple. A reduction of 3.2 log10 CFU was achieved with a combination of wetting with water, rubbing for 5 s, rinsing with 200 ml of water, and drying with a paper towel for apples inoculated just prior to or 24 h before treatment. Reductions obtained for apples treated with 5% vinegar and with a 200-ppm chlorine solution were significantly larger (2.1 to 3.2 log10 CFU per apple, respectively) than those achieved with water.

In 1999, consumption of alfalfa sprouts contaminated with Salmonella Mbandaka led to a multistate outbreak of salmonellosis. In this study, the implicated alfalfa seed lot (no. 8,119) was confirmed to be contaminated with Salmonella Mbandaka at a detection frequency of approximately 72% per replicated 100 g of seed. The sensitivity of detection was improved by a combination of nonselective and selective enrichment of 5.0 ml of germination effluent, followed by immunomagnetic separation. Detection of low levels of viable cells with nonselective enrichment, employed to enhance the recovery of stressed or injured cells, was facilitated by the application of Salmonella-specific polymerase chain reaction (PCR). With PCR assays, Salmonella Mbandaka was detectable on seed stored at 5 degrees C for at least 11 months, but at an increasingly diminishing frequency. Using conventional techniques, viable populations were detected in the seed germination effluent from seeds stored for up to 8 months. Seed treatments with buffered (to pH 7) and unbuffered solutions of calcium hypochlorite, providing approximately 2,000 and 20,000 ppm of free chlorine, for 10 min were equally effective in eliminating viable populations of Salmonella Mbandaka. However, aqueous heat treatments at up to 85 degrees C for 1 min did not eliminate the naturally occurring contaminant from the seed. Reductions of > 15% in germination were observed following heat treatments of 65 degrees C for > or = 6 min or 70 degrees C for > or = 4 min. On the basis of these results, aqueous heat treatments alone do not appear to be a viable alternative to hyperchlorination as an effective method to eliminate Salmonella from alfalfa seed.

Listeria monocytogenes ATCC 19115, Scott A, and UAL500 were evaluated for sensitivity to nisin (0 to 50 μg/ml) using a direct plating method. Nisin (10 μg/ml) decreased an initial population of L. monocytogenes (10 9 CFU per ml) by 6- to 7-log cycles. Sensitivity to nisin was enhanced by addition of 2% NaCl or by reduction of the medium pH from 6.5 to 5.5 with either hydrochloric or lactic acid. Mutants resistant to 50 μg/ml nisin were detected at frequencies of 10 -6 to 10 -8 . Nisin-resistant L. monocytogenes mutants should be expected to arise when nisin is used as an antimicrobial in food systems.