Gamma irradiation of fine-emulsion sausage containing sodium diacetate

Listeria monocytogenes, a psychrotrophic foodborne pathogen, is a frequent postprocess contaminant of ready-to-eat (RTE) meat products, including frankfurters and bologna. Ionizing radiation can eliminate L. monocytogenes from RTE meats. Sodium diacetate (SDA) incorporated into fine-emulsion sausages inhibits the growth of L. monocytogenes. Irradiation of L. monocytogenes suspended in SDA solutions resulted in synergistic reductions of the microorganism. L. monocytogenes populations were reduced by > 9 log10 units at a radiation dose of 1.5 kGy when suspended in 0.125% SDA solution. In contrast, the D10-values (the ionizing radiation doses required to reduce the population by 90%) were 0.58, 0.59, 0.57, and 0.53 kGy for L. monocytogenes populations suspended in emulsions containing 0, 0.125, 0.25, and 0.5% SDA, respectively. The D10-values for L. monocytogenes surface inoculated onto frankfurters dipped in 0, 0.125, 0.25, and 0.5% SDA solutions were 0.58, 0.53, 0.54, and 0.52 kGy, respectively. Postirradiation growth of L. monocytogenes suspended in beef bologna emulsion at 9 degrees C was dependent on SDA concentration and ionizing radiation dose. Very small, but statistically significant, changes in bologna redness, lipid oxidation, and shear force were observed for the beef bologna emulsion with the highest SDA concentration (0.5%) and irradiation dose (3.0 kGy). SDA can inhibit the proliferation of L. monocytogenes surviving the irradiation process with minimal impact on fine-emulsion sausage color, lipid oxidation, and firmness when used within regulatory limits.     

EFFECT OF IONIZING RADIATION ON BEEF BOLOGNA CONTAINING SOY PROTEIN CONCENTRATE

Soy protein concentrate (SPC), an extender, is a common additive in ready-to-eat (RTE) meat products. SPC contains antioxidants that could potentially interfere with the ability of ionizing radiation to eliminate Listeria monocytogenes from RTE meat products. When L. monocytogenes was inoculated into cooked beef bologna emulsion containing 0, 1.75, or 3.5% SPC the gamma radiation D₁₀ values, at radiation doses of 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 kGy, were 0.66, 0.68, and 0.71kGy, respectively. Soluble antioxidant power, as determined by the Ferric Reducing Antioxidant Power (FRAP) assay was 1958, 3572, and 5494 mol in bologna emulsion containing 0, 1.75 and 3.5% SPC, respectively. Soluble antioxidant power was not affected by ionizing radiation. SPC did not prevent ionizing radiation induced lipid oxidation as determined by Thiobarbituric Acid Reactive Substance (TBARS) assay. Hunter color analysis of both unirradiated and irradiated bologna slices containing SPC indicated decreased a value as a result of irradiation, while the addition of SPC helped maintain b-value and L-value. The inclusion of SPC did not represent a barrier to ionizing radiation pasteurization of fine emulsion sausages for the parameters examined.     

Reduction and survival of Listeria monocytogenes in ready-to-eat meats after irradiation

A five-strain Listeria monocytogenes culture was inoculated onto six different types of ready-to-eat (RTE) meats (frankfurters, ham, roast beef, bologna, smoked turkey with lactate, and smoked turkey without lactate). The meats were vacuum packed and stored at 4 degrees C for 24 h prior to irradiation. Populations of L. monocytogenes were recovered by surface plating on nonselective and selective media. The margins of safety studied include 3-log (3D) and 5-log (5D) reduction of pathogenic bacteria to achieve an optimal level of reduction while retaining organoleptic qualities of the meats. A 3-log reduction of L. monocytogenes was obtained at 1.5 kGy when nonselective plating medium was used. The dosages for 3-log reduction were 1.5 kGy for bologna, roast beef, and both types of turkey and 2.0 kGy for frankfurters and ham on the basis of use of selective medium. The D10-values ranged from 0.42 to 0.44 kGy. A 5-log reduction of L. monocytogenes was obtained at 2.5 kGy with nonselective medium. With selective medium, the dosages were 2.5 kGy for bologna, roast beef, and both types of turkey and 3.0 kGy for frankfurters and ham. Survival of L. monocytogenes in the same RTE meat types after irradiation was also studied. Meats were inoculated with 5 log L. monocytogenes per g and irradiated at doses of 2.0 and 4.0 kGy. Recovery of the surviving organisms was observed during storage at temperatures of 4 and 10 degrees C for 12 weeks. Preliminary results showed no growth in meats irradiated at 4.0 kGy. Survivors were observed for irradiated meats at 2.0 kGy stored at 10 degrees C after the second week. No growth was observed in samples irradiated at 2.0 kGy stored at 4 degrees C until the fifth week.     

Control of Listeria monocytogenes in ready-to-eat meats containing sodium levulinate, sodium lactate, or a combination of sodium lactate and sodium diacetate

ABSTRACT:  This study investigated the use of sodium levulinate to prevent outgrowth of Listeria monocytogenes in refrigerated ready-to-eat (RTE) meat products. Turkey breast roll and bologna were formulated to contain 1%, 2%, or 3% (w/w) sodium levulinate, 2% sodium lactate, a 2% combination of sodium lactate and sodium diacetate (1.875% sodium lactate and 0.125% sodium diacetate), or no antimicrobial (control). Samples of the RTE products were sliced, inoculated with 10² to 10³ CFU/cm² of a 5-strain cocktail of L. monocytogenes , vacuum packaged, and stored at refrigeration temperature for 0 to 12 wk. Counts reached 10⁸ CFU/cm² on control turkey roll product after 8 wk, and over 10⁷ CFU/cm² on control bologna after 12 wk. Addition of 2% or more sodium levulinate to turkey roll and 1% or more sodium levulinate to bologna completely prevented growth of L. monocytogenes during 12 wk of refrigerated storage. A consumer taste panel with pathogen-free samples found no differences in the overall liking among the preparations of turkey roll or among preparations of bologna. These results show that sodium levulinate is at least as effective at inhibiting outgrowth of L. monocytogenes in RTE meat products as the current industry standards of lactate or lactate and diacetate, and levulinate addition does not alter the overall liking of the RTE meat products.     

Antioxidant power,lipid oxidation,color, and viability of Listeria monocytogenes in beef bologna treated with gamma radiation and containing various levels of glucose

Ionizing radiation can be used to pasteurize ready-to-eat (RTE) meat products. Thermal processing of RTE meats that contain dextrose results in the production of antioxidants that may interfere with ionizing radiation pasteurization of RTE meat products. Beef bologna was manufactured with dextrose concentrations of 0, 2, 4, 6, and 8%. Antioxidant activity, as measured by the Ferric Reducing Antioxidant Power assay, increased with dextrose concentration but was unaffected by ionizing radiation. Lipid oxidation increased significantly in irradiated bologna (4 kGy) that contained dextrose. Hunter color analysis indicated that the addition of dextrose reduced the ionizing radiation-induced loss of redness (a-value) but promoted the loss of brightness (L-value). The radiation resistance, D10-value, of Listeria monocytogenes that was surface-inoculated onto bologna slices was not affected by dextrose concentration. L. monocytogenes strains isolated from RTE meats after listeriosis outbreaks were utilized. Increased antioxidant activity generated by thermal processing of dextrose in fine emulsion sausages does not present a barrier to radiation pasteurization of RTE meats. However, a high dextrose concentration in combination with gamma irradiation increases lipid oxidation significantly.     

Electron beam and gamma irradiation effectively reduce Listeria monocytogenes populations on chopped romaine lettuce

Fresh, chopped romaine lettuce contaminated with a seven-strain cocktail of Listeria monocytogenes (in a solution containing approximately 10(8) organisms per ml) that had attained a level of contamination of between 7 and 8 log CFU/g was packaged in 15-g samples. The lettuce was irradiated with a Co60 source at 1.15 or 0.51 kGy and then stored at 4 degrees C. In addition, samples contaminated with isolated strains 16397, 0733, and 1992 were subjected to either electron beam irradiation at doses ranging from 0.3 to 1.2 kGy or gamma irradiation at 0.56 kGy without subsequent refrigerated storage. All postirradiation and control samples were diluted with Butterfield's phosphate buffer and plated in duplicate on modified Oxford media. Samples that received electron beam or gamma irradiation without subsequent refrigerated storage were also plated in duplicate on modified Oxford media plates coated with two 7-ml layers of basal yeast extract agar. Electron beam irradiation yielded D10-values (the dose required to eliminate 90% of the microbial population) of 0.16, 0.17, and 0.19 kGy for strains 16397, 0733, and 1992, respectively. The corresponding log reductions obtained for these same three strains at 0.56 kGy of gamma irradiation were 2.91, 2.62, and 2.66 log, respectively. Gamma irradiation at 1.15 and 0.51 kGy with subsequent refrigerated storage (4 degrees C) reduced populations by > 5 and > 2 log, respectively, compared with controls. Neither the irradiated samples nor the control samples showed increases in population during the storage periods. Our results indicate that low-dose irradiation can effectively reduce or eliminate L. monocytogenes on chopped romaine lettuce, improving the safety of ready-to-eat salads.     

Radiation sensitivity and postirradiation growth of foodborne pathogens on a ready-to-eat frankfurter on a roll product in the presence of modified atmosphere and antimicrobials

Intervention technologies including ionizing radiation, antimicrobials, and modified atmospheres (MA) can be used to inhibit the growth of or inactivate foodborne pathogens on complex ready-to-eat foods such as sandwiches. However, the effect of these technologies when used in combination (the hurdle concept) on the survival of foodborne pathogens is unknown. The ability of ionizing radiation to inactivate Escherichia coli O157:H7, Salmonella, Listeria monocytogenes, and Staphylococcus aureus inoculated onto a frankfurter on a roll product containing the antimicrobials sodium diacetate and potassium lactate in the presence of an MA (100% N2, 50% N2 plus 50% CO2, or 100% CO2) was investigated. The radiation resistances (D10-values) of the foodborne pathogens were 0.43 to 0.47 kGy for E. coli O157:H7, 0.61 to 0.71 kGy for Salmonella, 0.53 to 0.57 for L. monocytogenes, and 0.56 to 0.60 for S. aureus. The MA had no effect on the radiation resistance of the pathogens. During a 2-week storage period under mild temperature abuse (10 degrees C), none of the pathogens were able to proliferate on the frankfurter on a roll product, regardless of the MA used. However, application of sublethal doses of ionizing radiation resulted in increased mortality of the gram-positive pathogens L. monocytogenes and S. aureus during the storage period regardless of the MA. Although the pathogens were unable to proliferate on the frankfurter on a roll product during the storage period, application of a postpackaging intervention step was needed to actually inactivate the foodborne pathogens. Ionizing radiation used in combination with sodium diacetate and potassium lactate resulted in additional mortality of L. monocytogenes and S. aureus, independent of the MA, during the 2-week storage period.     

Irradiation inactivation of Listeria monocytogenes in low-fat ground pork at freezing and refrigeration temperatures

Gamma radiation effectively controls Listeria monocytogenes in uncooked and in ready-to-eat foods. This study was conducted to determine if gamma radiation could be used to control L. monocytogenes in ground pork. Ground pork was contaminated with L. monocytogenes, kept at refrigeration (4 degrees C), chilling (0 degrees C), and freezing (-18 degrees C) temperatures overnight, exposed to gamma radiation and stored at 4 degrees C for 7 days, and at 0 and -18 degrees C for 60 days. Following irradiation, the meat was assayed for L. monocytogenes viable counts and lipid oxidation. A triangle test was performed to determine if sausage made from the irradiated and nonirradiated ground pork differed in sensory quality. It was observed that a 5-log reduction of L. monocytogenes viable counts would require a 3.0-kGy radiation dose. The results of a 60-day storage study of ground pork inoculated with 10(5) to 10(6) CFU of L. monocytogenes per gram indicated that counts for nonirradiated meat remained fairly constant at refrigeration, chilling, and freezing temperatures. However, irradiation of ground pork at 3.0 kGy could inactivate L. monocytogenes totally in ground pork subsequently held at all the temperatures used in this study. Lipid oxidation measurements, as determined by the thiobarbituric acid-reactive substance assay, ranged from 0.16 nmol/g for nonirradiated ground pork and 0.20 nmol/g for meat irradiated at 3.0 kGy. Sensory panelists could distinguish between irradiated and nonirradiated sausage but were divided on whether irradiation adversely affected the sausage quality. Our results suggest that gamma radiation could be useful to control L. monocytogenes in ground pork and improve the safety of ground pork products.     

Ionizing radiation sensitivity of Listeria monocytogenes ATCC 49594 and Listeria innocua ATCC 51742 inoculated on endive (Cichorium endiva)

Ionizing radiation inactivates the pathogenic bacteria that can contaminate leafy green vegetables. Leaf pieces and leaf homogenate of endive (Cichorium endiva) were inoculated with the pathogen Listeria monocytogenes (ATCC 49594) or Listeria innocua (ATCC 51742), a nonpathogenic surrogate bacterium. The radiation sensitivity of the two strains was similar, although L. innocua was more sensitive to the type of suspending leaf preparation. During refrigerated storage after irradiation, the population of L. monocytogenes on inoculated endive was briefly suppressed by 0.42 kilogray (kGy), a dose calibrated to achieve a 99% reduction. However, the pathogen regrew after 5 days until it exceeded the bacterial levels on the control after 19 days in storage. Treatment with 0.84 kGy, equivalent to a 99.99% reduction, suppressed L. monocytogenes throughout refrigerated storage. Doses up to 1.0 kGy had no significant effect on the color of endive leaf material, regardless of whether taken from the leaf edge or the leaf midrib. The texture of leaf edge material was unaffected by doses up to 1.0 kGy, whereas the maximum dose tolerated by leaf midrib material was 0.8 kGy. These results show that endive leaves may be treated with doses sufficient to achieve at least a 99.99% reduction of L. monocytogenes with little or no impact on the product's texture or color.     

Effect of combining nisin and/or lysozyme with in-package pasteurization for control of Listeria monocytogenes in ready-to-eat turkey bologna during refrigerated storage

This study investigated the efficacy of in-package pasteurization combined with pre-surface application of nisin and/or lysozyme to reduce and prevent the subsequent recovery and growth of Listeria monocytogenes during refrigerated storage on the surface of low-fat turkey bologna. Sterile bologna samples were treated with solutions of nisin (2 mg/ml=5000 AU/ml), lysozyme (10 mg/ml=80 AU/ml) and a mixture of nisin and lysozyme (2 mg nisin+10mg lysozyme/ml) before in-package pasteurization at 65 degrees C for 32s. In-package pasteurization resulted in an immediate 3.5-4.2 log CFU/cm(2) reduction in L. monocytogenes population for all treatments. All pasteurized treatments also resulted in a significant reduction of L. monocytogenes by 12 weeks compared to un-pasteurized bologna. In-package pasteurization in combination with nisin or nisin-lysozyme treatments was effective in reducing the population below detectable levels by 2-3 weeks of storage. Results from this study could have a significant impact for the industry since a reduction in bacterial population was achieved by a relatively short pasteurization time and antimicrobials reduced populations further during refrigerated storage.     

Control of Listeria monocytogenes on frankfurters and cooked pork chops by irradiation combined with modified atmosphere packaging

This study was conducted to investigate the efficacy of controlling Listeria monocytogenes on frankfurters and cooked pork chops with irradiation and modified atmosphere packaging (MAP) containing a high concentration of CO(2). Frankfurters and cooked pork chops were inoculated with a five-strain cocktail of L. monocytogenes and packaged in vacuum or high-CO(2) MAP. Irradiation was applied to each product at 0, 0.5, 1.0, or 1.5 kGy. No significant packaging effect was found for the radiation sensitivity of L. monocytogenes. Radiation D(10)-values for L. monocytogenes were 0.66 ± 0.03 and 0.70 ± 0.05 kGy on frankfurters and 0.60 ± 0.02 and 0.57 ± 0.02 kGy on cooked pork chops in vacuum and high-CO(2) MAP, respectively. High-CO(2) MAP was more effective than vacuum packaging for controlling the growth of survivors during refrigerated storage. These results indicate that irradiation and high-CO(2) MAP can be used to improve control of L. monocytogenes in ready-to-eat meats.     

Assessment of the potential for Listeria monocytogenes survival and growth during alfalfa sprout production and use of ionizing radiation as a potential intervention treatment

Alfalfa seeds (Australian, nondormant, nonscarified) were treated with 20,000 ppm active chlorine, sprouted in canning jars for 5 days, and packaged and stored at 5 degrees C for up to 9 days. Seeds or sprouts were inoculated with a three-strain cocktail of Listeria monocytogenes at one of three points during the process-day 0 (before 24-h aqueous seed soak), day 1 (after 24-h aqueous seed soak), or day 5 (after sprouting, before prepackaging 10 ppm chlorine rinse)--or control (no inoculum), and the ability of the inoculum to survive and grow was evaluated. Total bacterial numbers on uninoculated seeds increased dramatically during the first 24-h the seeds were soaked, from 3.5 to ca. 8.0 log CFU/g, and remained at this level during refrigerated storage. When the seeds were inoculated with a cocktail of L. monocytogenes (log 5 CFU/10 ml) on day 0 or 1, the population of the pathogen increased dramatically, to within 1 to 2 logs of the total, and remained high during refrigerated storage. When sprouted seeds were inoculated with L. monocytogenes later in the process (day 5), the inoculum survived but did not grow more than ca. 1 log CFU/g, regardless of whether the inoculation level in each jar was low (10(3)) or high (10(5)). Irradiation of sprouts with beta radiation at 3.3 or 5.3 kGy, but not 1.5 kGy, was effective at eliminating L. monocytogenes from inoculated sprouts (6 log CFU/g) without causing noticeable changes in appearance or odor. In summary, L. monocytogenes can grow on sprouts during production, can survive on refrigerated sprouts, and may be eliminated on sprouts with beta radiation.     

Impact of nitrite on detection of Listeria monocytogenes in selected ready-to-eat (RTE) meat and seafood products

ABSTRACT:  The impact of sodium nitrite (NaNO₂) on detection and recovery of Listeria monocytogenes from select ready-to-eat (RTE) foods including smoked salmon, smoked ham, beef frankfurters, and beef bologna was assessed. Nitrite-containing (NC; 100 to 200 ppm NaNO₂) or nitrite-free (NF) foods were inoculated with a 5-strain cocktail of L. monocytogenes by immersion into Butterfield's buffer solution containing 5.4 to 7.4 × 10³ L. monocytogenes per milliliter. Inoculated products were vacuum-packaged and stored at 5 °C. A weekly comparative analysis was performed for presence of L. monocytogenes using 5 detection methods on products held at 5 °C for up to 8 wk. L. monocytogenes initially present at <100 CFU/g during the first 2 wk of storage increased throughout the study, attaining final populations of approximately 1 × 10⁴ to 1 × 10⁵ CFU/g. Lactic acid bacteria predominated throughout the study in all products. Exposure to NaNO₂ (100 to 200 ppm) resulted in 83% to 99% injury to the L. monocytogenes strains tested. The genetic-based BAX^® System (DuPont™ Qualicon, Wilmington, Del., U.S.A.) and modified USDA/FSIS methods detected 98% to 100% of Listeria -positive food samples and were consistently superior to and significantly different ( P < 0.05) from conventional cultural methods in recovering Listeria from NC samples. Data show that nitrite-induced injury adversely affects detection and recovery of L. monocytogenes from NC food, confirming earlier findings that nitrite-induced injury masks L. monocytogenes detection in NC RTE food products. Nitrite-injured Listeria can subsequently repair upon nitrite depletion and grow to high levels over extended refrigerated storage.     

Shelf life evaluation for ready-to-eat sliced uncured turkey breast and cured ham under probable storage conditions based on Listeria monocytogenes and psychrotroph growth

The growth variability of three Listeria monocytogenes ribotypes in ready-to-eat (RTE) sliced uncured turkey breast and cured ham was studied under storage conditions that RTE foods are likely to encounter. Three product treatments studied were: (1) a control; (2) a formulation subjected to high pressure processing to reduce initial microbial load (HPP); (3) a formulation containing 2.0% potassium lactate and 0.2% sodium diacetate (PL/SD). After separate inoculation with individual L. monocytogenes ribotypes and packaging each treatment under air and vacuum, the packages were stored at 4, 8, or 12 degrees C and the counts of L. monocytogenes and psychrotrophic bacteria (PPC) were determined for several weeks. The Baranyi model was used to estimate lag times and growth rates. Significant effect of strain difference was noted in both sliced products (P<0.05). In the absence of antimicrobials (HPP and control), the growth rate (GR) of L. monocytogenes strains increment from 4 to 8 degrees C and from 8 to 12 degrees C was approximately 10 and 2 fold, respectively. The addition of PL/SD was effective in restricting the growth of L. monocytogenes and PPC at 4 degrees C, but at 8 and 12 degrees C significant growth was observed (more than 100-fold increase) (P<0.05). In PL/SD samples, vacuum packaging slowed down the onset and the rate of growth of L. monocytogenes at 12 degrees C in sliced ham and at 8 and 12 degrees C in sliced turkey breast. Generally, the time to increase by 2-logs was greater in control samples than as observed in HPP-treated samples. When antimicrobials were present, the current results showed that L. monocytogenes was able to grow more than 100-fold within the typical quality-based shelf life of 60 to 90 days at 8 and 12 degrees C. The findings of this study should be useful in setting the duration of a safety-based shelf life for RTE sliced meat and poultry foods.     

Inactivation of Listeria monocytogenes on Beef Bologna and Cheddar Cheese Using Polyvinyl-idene Chloride Films Containing Sorbic Acid

ABSTRACT: Polyvinylidene chloride (PVDC, Saran^R F-310) films containing sorbic acid (0%, 1.5%, and 3.0% w/v) were prepared with use of a solvent-casting method and were then placed between slices of commercially produced beef bologna that were previously surface-inoculated with L. monocytogenes at 10³ or 10⁵ CFU/g. In addition, cubes of commercial Cheddar cheese were surface-inoculated to contain 10³ or 10⁵ Listeria monocytogenes colony-forming units (CFU) /g and then wrapped with the sorbic acid-containing films. Films containing 1.5% and 3.0% (w/ v) sorbic acid prevented growth of L. monocytogenes on bologna slices with populations as much as 7.1 logs lower after 28 d of storage at 4 °C compared with the sorbic acid-free controls. In contrast, numbers of Listeria remained relatively stable on Cheddar cheese with populations decreasing < 1.3 logs after 35 d of storage. With use of the sorbic acid-containing films, common spoilage organisms were also inhibited on both products. After 28 d of contact with bologna and Cheddar cheese, these films retained 7% and 60% of their original sorbic acid content, respectively, with the control film retaining 85% of its original sorbic acid content. Given these findings, sorbic acid-containing films may be useful in enhancing the safety and shelf-life of ready-to-eat delicatessen products.     

Effects of high-pressure processing on the safety, quality, and shelf life of ready-to-eat meats

Ready-to-eat (RTE) meats (low-fat pastrami, Strassburg beef, export sausage, and Cajun beef) were pressure treated at 600 MPa, 20 degrees C, for 180 s to evaluate the feasibility of using high-pressure processing (HPP) for the safe shelf-life extension of these products. After processing, samples were stored at 4 degrees C for 98 days during which time microbiological enumeration and enrichments were performed. Additionally, sensory analyses were undertaken to determine consumer acceptability and purchase intent over the duration of storage. Counts of aerobic and anaerobic mesophiles, lactic acid bacteria, Listeria spp., staphylococci, Brochothrix thermosphacta, coliforms, and yeasts and molds revealed that there were undetectable or low levels for all types of microorganisms throughout storage. Comparison of consumer hedonic ratings for unprocessed and processed meats revealed no difference in consumer acceptability, and no deterioration in the sensory quality was evident for any of the products tested during the study. Additionally, inoculated pack studies were conducted to determine if HPP could be used as a postlethality treatment to reduce or eliminate Listeria monocytogenes and thus assess the potential use of HPP in a hazard analysis critical control point plan for production of RTE meats. Inoculated samples (initial level of 10(4) CFU/g) were pressure treated (600 MPa, 20 degrees C, for 180 s) and stored at 4 degrees C, and survival of L. monocytogenes was monitored for 91 days. L. monocytogenes was not detected by plating methods until day 91, but selective enrichments showed sporadic recovery in three of the four products examined. The results show that HPP at 600 MPa, 20 degrees C, for 180 s can extend the refrigerated shelf life of RTE meats and reduce L. monocytogenes numbers by more than 4 log CFU/g in inoculated product.     

Irradiation and modified atmosphere packaging for the control of Listeria monocytogenes on turkey meat.

When radiation-sterilized ground turkey meat was inoculated with Listeria monocytogenes, packaged under mixtures of nitrogen and carbon dioxide, and irradiated with gamma-radiation doses of 0 to 3.0 kGy, there was a statistically significant (P < 0.05), but probably not a biologically significant, lower (0.39 log) predicted bacterial survival in the presence of 100% carbon dioxide than in the presence of 100% nitrogen. Possibly because all atmospheres contained oxygen and because a response surface design was used, gamma-radiation resistance was not significantly (P < 0.05) different in air than in modified atmosphere packaging (MAP) mixtures containing 5% O2 or containing 20, 40, 60, and 80% CO2 and balance N2. The antilisterial effects of MAP mixtures containing 17.2, 40.5, and 64% CO2 and balance N2 were compared to those associated with air and vacuum packaging on turkey inoculated with approximately 5 x 10(3) CFU/g. Samples were irradiated to doses of 0, 0.5, 1.0, 1.5, 2.0, and 2.5 kGy and were stored at 7 degrees C for up to 28 days. Irradiation treatments were significantly more lethal in the presence of air packaging than in either vacuum packaging or MAP, and in those samples that received >1.0 kGy, there was a concentration-dependent CO2 inhibition of L. monocytogenes multiplication and/or recovery.     

Microbial growth and the effects of mild acidification and preservatives in refrigerated sweet potato puree

Refrigerated sweet potato puree is a convenient form of sweet potato that can be used as an ingredient in formulated foods. The microbiology of refrigerated sweet potato puree during storage for up to 5 weeks was evaluated. Because the puree was made by comminuting steam-cooked sweet potatoes before refrigeration, no naturally occurring vegetative bacterial cells were detected during a 4-week period of refrigerated storage at 4 degrees C. However, if postprocessing microbial contamination of the puree were to occur, contaminating microorganisms such as Listeria monocytogenes could grow during refrigerated storage. The effects of acidification or the addition of potassium sorbate and sodium benzoate on a population of L. monocytogenes inoculated into refrigerated (4 degrees C) sweet potato puree were determined. Inoculation of the refrigerated puree with L. monocytogenes at 10(6) CFU/ml resulted in a 3-log increase after 3 weeks storage of nonsupplemented puree. Supplementation of the sweet potato puree with 0.06% (wt/vol) sorbic acid or benzoic acid plus mild acidification of the sweet potato puree with citric acid to pH 4.2 prevented growth of L. monocytogenes during storage at 4 degrees C.     

Control of Listeria monocytogenes on cold-smoked salmon using chitosan-based antimicrobial coatings and films

The relatively high incidence of Listeria monocytogenes in ready-to-eat (RTE) products such as cold-smoked salmon is of serious concern. The objective of this study was to evaluate the efficacy of chitosan-based edible coatings and films incorporating 3 generally recognized as safe (GRAS) antimicrobials, sodium lactate (SL), sodium diacetate (SD), and potassium sorbate (PS), against L. monocytogenes on cold-smoked salmon. Salmon samples were surface-inoculated with a 5-strain cocktail of Listeria monocytogenes to a final concentration of 4.4 log CFU/cm(2) and then either coated with chitosan solutions or wrapped with chitosan films with or without the 3 antimicrobials. The samples were then vacuum packaged and stored at 4 °C for 30 d. The chitosan coatings with or without the antimicrobials consistently showed higher efficacy against L. monocytogenes than chitosan films having the same compositions. The most effective film treatments, chitosan films containing 1.2% SL/0.25% SD or 2.4% SL, achieved ≥ 1.3 log reductions of L. monocytogenes during the 30 d of refrigerated storage, while the most effective coating treatments, chitosan coatings containing 1.2% SL/0.25% SD or 0.15% PS/0.125% SD, achieved ≥ 2.8 log reductions. Practical Application: This study shows that chitosan-based edible coatings and films hold promise and can potentially assist fishery industries in their efforts to control L. monocytogenes.     

In-package Pasteurization Combined with Biocide-impregnated Films to Inhibit Listeria monocytogenes and Salmonella Typhimurium in Turkey Bologna

ABSTRACT: The inhibitory effects of in-package pasteurization (3–5D, decimal reduction times) combined with a nisin (7%, w/w) containing wheat gluten film were tested over an 8-wk storage period against Listeria monocytogenes and Salmonella Typhimurium populations inoculated on refrigerated bologna. Bologna slices subjected to the in-package pasteurization process reducedL. monocytogenes populations 3.8- to 7.0-log colony-forming units (CFU)/g, and the remaining population fluctuated between 1.2- and 38-log CFU/g over the 2-mo storage period. S . Typhimurium was reduced 5.7- to 7.3-log CFU/g, and the remaining population progressively declined from 100 to <10 CFU/g over 2 mo of storage. The wheat gluten film containing nisin was effective in reducing the population of L. monocytogenes (2.75-log reduction with pasteurization; 1-log reduction without pasteurization), but was not effective against S . Typhimurium (<1-log reduction). Combining both treatments significantly reduced the L. monocytogenes populations and prevented outgrowth over the 2-mo storage period but provided no added inhibitory effect against S . Typhimurium compared with only pasteurization.