Stephanie Doores

Factors affecting microwave heating of foods and microwave induced destruction of foodborne pathogens: a review

In the 30 year time period that microwave ovens have been used in households, several studies have examined variables that influence temperatures and bacterial destruction achieved in foods heated by microwave energy. Factors of primary importance include both physical and chemical (product mass, density, specific heat, ionic content, dielectric properties) parameters. The majority of evidence indicates that microwaves inactivate microbes by thermal effects alone. Concern about the safety of microwave-heated foods has arisen because of the abbreviated thermal treatment that microwave heating affords, the potential for temperature abuse of microwavable products, and the risk of pathogen survival in foodservice cook/chill foods. Foodborne pathogens such as Listeria monocytogenes , Staphylococcus aureus and Salmonella spp. have been the focus of most studies of microwavable food safety, primarily because of their ubiquitous nature and prevalence in causing illness. The practical implications of the findings of both basic and applied studies of bacterial destruction within microwave-heated products are examined.

The microbiology of apples and apple products.

The apple industry has reached an annual production level of 8.5 billion pounds. CA storage of 25% of this crop has enabled a fresh market on a year-round basis. To achieve high quality in raw fruit and processed apple products, careful attention must be paid to maintaining a microbiologically stable environment. The ecology of the microflora associated with the apple is a reflection of the orchard, handling, harvesting, and storage practices. Yeasts predominate on orchard fruit, molds may become a storage problem, and bacteria cause spoilage, off flavors, and loss of quality in juice products. Despite the microbial problems inherent in producing of quality product, the apple industry is faced with the occurrence of patulin. Patulin, a mycotoxin produced by Penicillium and Aspergillus species, has been associated with damaged fruit. Decreased temperatures, coupled with CA storage; can deter mold growth and patulin production. Laboratory detection methods for derivations of patulin are able to detect microgram quantities. Means to eliminate patulin formed in apple products include addition of ascorbate and SO2, extending fermentation, or charcoal filtering. However, degradation products of patulin have not been evaluated toxicologically.

Public Health Consequences of Macrolide Use in Food Animals: A Deterministic Risk Assessment †

The potential impact on human health from antibiotic-resistant bacteria selected by use of antibiotics in food animals has resulted in many reports and recommended actions. The U.S. Food and Drug Administration Center for Veterinary Medicine has issued Guidance Document 152, which advises veterinary drug sponsors of one potential process for conducting a qualitative risk assessment of drug use in food animals. Using this guideline, we developed a deterministic model to assess the risk from two macrolide antibiotics, tylosin and tilmicosin. The scope of modeling included all label claim uses of both macrolides in poultry, swine, and beef cattle. The Guidance Document was followed to define the hazard, which is illness (i) caused by foodborne bacteria with a resistance determinant, (ii) attributed to a specified animal-derived meat commodity, and (iii) treated with a human use drug of the same class. Risk was defined as the probability of this hazard combined with the consequence of treatment failure due to resistant Campylobacter spp. or Enterococcus faecium. A binomial event model was applied to estimate the annual risk for the U.S. general population. Parameters were derived from industry drug use surveys, scientific literature, medical guidelines, and government documents. This unique farm-to-patient risk assessment demonstrated that use of tylosin and tilmicosin in food animals presents a very low risk of human treatment failure, with an approximate annual probability of less than 1 in 10 million Campylobacter-derived and approximately 1 in 3 billion E. faecium-derived risk.

Antibacterial activity of the lactoperoxidase system against Listeria monocytogenes and Staphylococcus aureus in milk.

The lactoperoxidase system (LPS) was activated by addition of thiocyanate (SCN-) and hydrogen peroxide (H2O2) and utilizing the inherent milk lactoperoxidase (LP). For Listeria monocytogenes studies, initial concentrations of 2.4 mM SCN- and 0.6 mM H2O2 were added. The corresponding concentrations were 1.2 mM SCN- and 0.3 mM H2O2 for Staphylococcus aureus studies. The LPS increased the predicted time to reach half the maximum attainable CFU/ml by 326 h for L. monocytogenes at 10°C and by 6.3 h at 35°C. For S. aureus , the corresponding increases were 36 h at 10°C and 2.4 h at 37°C. During the initial period after activation of the LPS, bactericidal effects against L. monocytogenes at 35°C and S. aureus at 37°C were observed. After recovery from the effects of the LPS, growth rate of each pathogen was of similar magnitude as in the H2O2-treated and untreated milk, with the exception of L. monocytogenes at 10°C.

Kinetics and Mechanism of Bacterial Inactivation by Ultrasound Waves and Sonoprotective Effect of Milk Components

Inactivation of Escherichia coli and Listeria monocytogenes were investigated in buffer and milk upon treatment with ultrasound waves (USW). In addition, sonoprotective effect of milk components and ultrasound-induced changes in bacterial cells were investigated using scanning electron microscopy (SEM). Bacterial cells were added to phosphate buffer, whole milk, skim milk, or simulated milk ultrafiltrate (SMUF). To determine the sonoprotective effect of milk components, lactose (5%), casein (3%), or β lactoglobulin (0.3%) was added to SMUF. Samples were sonicated with 24 kHz pulse USW while maintaining the system temperature between 30 to 35 °C. Aliquots were drawn at set times during sonication and bacteria were enumerated by surface plating appropriate dilutions on selective and nonselective media plates. Escherichia coli exhibited significantly higher D values in whole (2.43 min) and skim milk (2.41 min) than phosphate buffer (2.19 min). Listeria monocytogenes also showed higher D values in whole (9.31 min) and skim milk (8.61 min) compared to phosphate buffer (7.63 min). Data suggest that milk exerts a sonoprotective effect on these bacteria. Escherichia coli exhibited a log-linear inactivation kinetics followed by tailing whereas L. monocytogenes showed 1st-order kinetics throughout. Among the milk components tested, presence of lactose in SMUF resulted in significantly higher D values than SMUF for both organisms suggesting that lactose was exerting a protective effect on bacteria. SEM images showed that USW caused mechanical damage to the cell wall and cell membrane of bacteria leading to their inactivation.

Evaluation and Bacterial Populations Associated with Fresh Mushrooms (Agaricus Bisporus)

Abstract Doores, Stephanie, Kramer, Michael, and Beelman, Robert. 1986. Evaluation of bacterial populations associated with fresh mushrooms ( Agaricus bisporus ) Proc. Int. Symp. Tech Aspects Cult. Edible Fungi Total aerobic bacterial counts on mushrooms were determined for the first six flushes of the crop cycle and the casing material and water were sampled at selected intervals. Total bacterial numbers ranged from 2. 1 to 16 × 10 6 CFU/g; numbers were relatively constant through the fifth flush of the crop cycle and decreased slightly by flush 6. Counts from casing material were considerably higher, between 1. 7 and 2. 9 × 10 8 CFU/g and also were relatively constant over the cycle. Bacterial counts from tap water were lower and did not appear to contribute to overall microbial populations. The majority (54. 0%) of bacteria isolated from the individual flushes were identified as fluorescent pseudomonads with flavobacteria comprising the second largest group (10. 0%). The proportions of these two organisms remained relatively constant; however, the pseudomonads decreased between the fifth and sixth flush while the flavobacteria increased. Bacterial populations during postharvest storage at 13°C increased from an initial load of 10 7 CFU/g to almost 10 11 CFU/g. The same two groups predominated but the proportion of flavobacteria increased during later storage. A third group of pseudomonads having mucoidal colonies appeared during storage in a biphasic manner. Deterioration of mushroom quality as indicated by maturity and color measurement appeared concomitant with bacterial growth indicating that darkening is related to increased bacterial numbers. However, phytopathogenic strains of microorganisms responsible for blotch occurred in very low numbers during storage.

Viability loss of Salmonella species, Staphylococcus aureus, and Listeria monocytogenes in complex foods heated by microwave energy

The chemical composition of five foods (UHT milk, beef broth, pudding, cream sauce, and liquid whole egg) was examined to determine factors important in achieving uniform temperatures within foods heated in a 700 W microwave oven. Proximate analyses were performed on all food systems to relate their chemical composition to temperatures and to destruction of microwave-heated Salmonella species, Listeria monocytogenes Scott A and V7 and Staphylococcus aureus ATCC 25923. Microwave heating times were chosen such that the final mixed mean temperature achieved by systems was 60°C for Salmonella spp. and L. monocytogenes , and 65°C for S. aureus . The amount of destruction of Salmonella spp. varied from 3.17 log CFU/ml in UHT milk to 0.44 log CFU/ml in beef broth. L. monocytogenes strains incurred the greatest amount of destruction in pudding (2.39 log CFU/g), while the least amount of destruction was observed in cream sauce (1.63 log CFU/ml). There were no significant differences in the amount of destruction of S. aureus heated in the five foods. The pH and aw of these foods did not affect survival of thermally stressed Salmonella , L. monocytogenes , or S. aureus cells. Of the food components examined, sodium content was the primary influence on the uniformity of temperatures achieved within foods, and, in turn, on the survival of bacteria.

Persistence of serological and biological activities of staphylococcal enterotoxin A in canned mushrooms

Outbreaks in 1989 of staphylococcal food poisoning linked to the consumption of imported canned mushrooms indicated that staphylococcal enterotoxins (SE) may survive a commercial retort process. To examine this possibility, fresh mushrooms were blanched in boiling water for 5 min and cooled 5 min in sterile water inoculated with enterotoxigenic type A Staphylococcus aureus strain 743, to yield approximately 1.3 ×103 staphylococci per g. Inoculated mushrooms were incubated 20 h at 30°C to simulate time-temperature abuse prior to canning. Mushrooms were sealed in 211 × 212 cans and thermally processed in a still retort to F values of 7, 12, and 18 min at 121 and 127°C. Pre- and post-thermal process staphylococcal enterotoxin A (SEA) serological activity was estimated from a standard curve with purified SEA using a commercial enzyme-linked immunosorbent assay (ELISA) kit. SEA was chromatographically separated from 4-can composite extracts of each F value and temperature. A feline emetic assay was used to determine the biological activity. The dose, administered on a body-weight basis, was equivalent to approximately 0.5 servings of mushrooms and brine for humans. The presence of SEA in the samples was confirmed by Western blotting using anti-SEA immunoglobulin G (IgG). The pre-thermal-process concentration of SEA was about 58 ng/g of mushrooms. Serological and biological activities were detected after all sterilizing values tested at 121 and 127°C. The inactivation of serological activity occurred in two phases, with a rapid initial rate and a distinctly slower rate at higher F values. Attenuation of biological activity, noted by a reduction in the number of emetic episodes and an increase in time to an emetic response, was observed with increasing F values of the processes.

Survival of Salmonella species heated by microwave energy in a liquid menstruum containing food components

Common food constituents were examined to determine the protective influence they may exert on a mixture of Salmonella species heated by microwave energy. A model system was developed, wherein combinations of sucrose, sodium chloride, caseinate and corn oil, all at 1.0% (w/v) concentrations, were added to 0.3 mM phosphate buffer (pH 6.8), producing a total volume of 100 ml. Salmonella -inoculated solutions were heated for 47 sec in a 700 watt microwave oven. Temperatures at localized areas were monitored by fluorescent fiberoptic (fluoroptic) thermometry as the solutions heated, and by mercury thermometer after heating. The mean final temperature achieved for the various combinations of solutes was not significantly different and varied only by 4°C as measured by a mercury thermometer. However, solutions containing NaCl afforded the Salmonella spp. up to 170 times the protection of the phosphate buffer control. This protective effect occurred although the mean final temperature of NaCl-containing solutions would be as high as those solutions lacking salt. Fluoroptic thermometry temperature profiles revealed that surface temperatures were higher when NaCl was present in solution, due to decreased depth of penetration of the microwaves. This re-sulted in decreased temperatures at greater depths. Of the solutes evaluated, only NaCl appears to confer a significant protective effect.

Commercially Available Rapid Methods for Detection of Selected Food-borne Pathogens

ABSTRACT Generally, the enumeration and isolation of food-borne pathogens is performed using culture-dependent methods. These methods are sensitive, inexpensive, and provide both qualitative and quantitative assessment of the microorganisms present in a sample, but these are time-consuming. For this reason, researchers are developing new techniques that allow detection of food pathogens in shorter period of time. This review identifies commercially available methods for rapid detection and quantification of Listeria monocytogenes, Salmonella spp., Staphylococcus aureus, and Shiga toxin-producing Escherichia coli in food samples. Three categories are discussed: immunologically based methods, nucleic acid-based assays, and biosensors. This review describes the basic mechanism and capabilities of each method, discusses the difficulties of choosing the most convenient method, and provides an overview of the future challenges for the technology for rapid detection of microorganisms.