Effect of nisin in combination with EDTA, sodium lactate, and potassium sorbate for reducing Salmonella on whole and fresh-cut cantaloupet

Nisin (50 microg/ml), EDTA (0.02 M, disodium salt), sodium lactate (NaL, 2%), and potassium sorbate (KS, 0.02%) were tested individually and in various combinations as sanitizer treatments for reducing Salmonella on whole and fresh-cut cantaloupe. Whole cantaloupe and fresh-cut pieces were inoculated with a five-strain cocktail of Salmonella to give 4.76 +/- 0.23 log CFU/cm2 and 3.42 +/- 0.13 log CFU/g, respectively. Inoculated whole melons and fresh-cut pieces were stored at 5 degrees C for 7 days. Washing treatments were applied to inoculated whole melons at days 0, 3, and 7 of storage, and surviving bacterial populations were determined. The effect of the washing treatments on transfer of Salmonella to fresh-cut pieces prepared immediately after treatment was also determined. Directly inoculated fresh-cut pieces were treated at day 0, and surviving bacteria were enumerated at days 0, 3, and 7 of storage. The combination treatments of nisin-EDTA, nisin-NaL, nisin-KS, NaL-KS, and nisin-NaL-KS all resulted in reductions of approximately 3 log CFU/cm2 at day 0 for whole melons. When tested alone, all compounds, along with water washes, were ineffective. After 3 and 7 days of storage, the five combination washing treatments were less effective, resulting in reductions of approximately 2 log CFU/cm2. None of the combination treatments completely eliminated transfer of pathogen survivors to fresh-cut pieces. The combination treatments nisin-NaL, nisin-KS, NaL-KS, and nisin-NaL-KS, but not nisin-EDTA, gave significant (P < 0.05) reductions of Salmonella directly inoculated onto fresh-cut pieces. Washing with nisin-NaL-KS was significantly (P < 0.05) more effective than the other three combination treatments, resulting in a reduction of 1.4 CFU/g. Inhibition by the four effective treatments carried over from day 0 through day 7 of storage, with no increase in the population of Salmonella on the stored fresh-cut pieces. Sensory evaluations indicated that treatment of fresh-cut pieces with nisin-NaL and NaL-KS, but not nisin-KS or nisin-NaL-KS, were acceptable in terms of appearance, odor, and overall acceptability. After the required regulatory approval, treatment of whole cantaloupe with nisin in combination with EDTA, NaL, KS, or NaL and KS and of fresh-cut pieces with nisin-NaL or NaL-KS could help ensure the microbiological safety of fresh-cut cantaloupe.     

Use of hydrogen peroxide in combination with nisin, sodium lactate and citric acid for reducing transfer of bacterial pathogens from whole melon surfaces to fresh-cut pieces

Hydrogen peroxide (2.5%) alone or hydrogen peroxide (1%) in combination with nisin (25 microg/ml), sodium lactate (1%), and citric acid (0.5%) (HPLNC) were investigated as potential sanitizers for reducing Escherichia coli O157:H7 or Listeria monocytogenes populations on whole cantaloupe and honeydew melons. Whole cantaloupes inoculated with E. coli O157:H7 and L. monocytogenes at 5.27 and 4.07 log10 CFU/cm2, respectively, and whole honeydew melons inoculated with E. coli O157:H7 and L. monocytogenes at 3.45 and 3.05 log10 CFU/cm2, respectively, were stored at 5 degrees C for 7 days. Antimicrobial washing treatments were applied to inoculated whole melons on days 0 or 7 of storage and surviving bacterial populations and the numbers transferred to fresh-cut pieces were determined. At days 0 and 7 treatment with HPLNC significantly (p<0.05) reduced the numbers of both pathogens, by 3 to 4 log CFU/cm2 on both types of whole melon. Treatment with HPLNC was significantly (p<0.05) more effective than treatment with 2.5% hydrogen peroxide. While fresh-cut pieces prepared from stored whole melons were negative for the pathogens by both direct plating and by enrichment, fresh-cut pieces from cantaloupe melons treated with 2.5% hydrogen peroxide were positive for both pathogens and pieces from honeydew melons were positive for E. coli 0157:H7. The native microflora on fresh-cut melons were also substantially reduced by HPLNC treatment of whole melons. The results suggest that HPLNC could be used to decontaminate whole melon surfaces and so improve the microbial safety and quality of fresh-cut melons.     

Synergistic inhibition of Listeria monocytogenes on cold-smoked rainbow trout by nisin and sodium lactate

The inhibition of Listeria monocytogenes and mesophilic aerobic bacteria in cold-smoked rainbow trout by nisin, sodium lactate or their combination was studied. Nisin (4000-6000 IU/ml), sodium lactate (60%) or their combination (1:1) were injected into rainbow trout at an industrial scale before the smoking process, or injected into the finished smoked product. Both types of fish samples were smoked, sliced and vacuum-packed according to normal practice in the plant. Packages were opened and L. monocytogenes was inoculated (10(3)-10(4) log colony forming units (cfu)/g) onto the fish samples, which were then vacuum packed again. Samples were stored at 8 degrees C for 17 days or at 3 degrees C for 29 days. Listeria and mesophilic aerobic bacteria counts were measured once a week. The effects of treatments on sensory characteristics and storage stability were also analyzed. Both nisin and lactate inhibited the growth of L. monocytogenes in smoked fish, but the combination of the two compounds was even more effective. The combination of nisin and sodium lactate injected into smoked fish decreased the count of L. monocytogenes from 3.26 to 1.8 log cfu/g over 16 days of storage at 8 degrees C. The level of L. monocytogenes remained almost constant (4.66-4.92 log cfu/g) for 29 days at 3 degrees C in the samples injected before smoking and which contained both nisin and sodium lactate. The treatments did not affect the sensory characteristics of cold-smoked rainbow trout. Based on a triangle test, the sensory quality of all test samples remained unchanged for 23 days of storage at 3 degrees C, whereas the control fish prepared without additives or additional salt remained unchanged only for 16 days.     

Effects and interactions of sodium lactate, sodium diacetate, and pediocin on the thermal inactivation of starved Listeria monocytogenes on bologna

The effects and interactions of temperature (56.3-60 °C), sodium lactate (SL; 0-4.8%), sodium diacetate (SDA; 0-2.5%), and pediocin (0-10,000 AU) on starved Listeria monocytogenes (10⁷ CFU/g) on bologna were investigated. Bologna slices containing SL and SDA in the formulation were dipped in pediocin, surface inoculated, and treated at various temperatures using combinations of parameters determined by central composite design. D-values were calculated. The observed D-values ranged from 2.8 min at 60 °C to 24.61 min at 56.3 °C. Injury ranged from 9.1 to 76% under various conditions. The observed D-values were analyzed using second order response surface regression for temperature, SL, SDA, and pediocin, and a predictive model was developed. Predicted D-values were calculated and ranged from 3.7 to 19 min for various combinations of parameters. Temperature alone reduced the predicted D-values from 33.96 min at 56.3 °C to 11.51 min at 60 °C. Addition of SL showed a protective effect. Other combination treatments either reduced or increased D-values depending on temperature. The combination of SL and SDA was effective at lower temperatures, however, higher levels of SDA at higher temperatures made the organism more heat resistant. Pediocin (up to 5000 AU) with increasing temperature and SDA reduced D-values. Depending on temperature and concentration, the interactions between various additives can affect thermal inactivation of L. monocytogenes on bologna. Starvation rendered L. monocytogenes more susceptible to heat and additives.     

Fate of Listeria monocytogenes inoculated onto the surface of model Turkey frankfurter pieces treated with zein coatings containing nisin, sodium diacetate, and sodium lactate at 4 degrees C

The antimicrobial effects of zein coatings containing nisin, sodium lactate, and sodium diacetate against Listeria monocytogenes on turkey frankfurters at 4 degrees C were determined. Our objectives were to determine whether zein, nisin, lactate, and diacetate alone or in combination could control the growth of L. monocytogenes on full-fat turkey frankfurters at 4 degrees C and to determine whether lactate or diacetate had any synergistic effect on the activity of nisin. Turkey frankfurter pieces surface inoculated with L. monocytogenes strain V7 were treated with zein-ethanol-glycerol (ZEG), zein-propylene-glycol (ZPR), ethanol-glycerol (EG), propylene glycol (PR), nisin (N), sodium lactate (L), or sodium diacetate (D) alone or in combination. Over 28 days, treatment with N or D alone reduced L. monocytogenes counts on frankfurters by 6.6 or 6.3 log CFU/g, respectively. N-D treatment reduced L. monocytogenes by 6 log CFU/g. The zein solvents EG and PR reduced L. monocytogenes by about 5.6 and 5.2 log CFU/g, respectively, similar to the results obtained with ZEG and ZPR, which suggests that zein powder per se had no antimicrobial activity. After 28 days, ZEG-N-D, ZEG-N-D-L, ZPR-N-D, and ZPR-N-D-L yielded no detectable CFU. L alone was ineffective. No synergies were observed. N and D when used singly and the combinations of N-D, ZEG-N-D, ZEG-N-D-L, ZPR-N-D, ZPR-N-D-L, EG, and PR were effective as inhibitors of the growth of recontaminating L. monocytogenes cells on full-fat turkey frankfurters.     

山梨酸钾、异抗坏血酸钠及包装方式对鲜切梨的影响

目的在于寻找一种操作简单、费用低廉且有效的保鲜贮藏方法。通过正交实验,观察鲜切梨表面微生物的生长和表面颜色的变化情况,筛选出最佳的处理方案为:0.1%山梨酸钾、2.0%异抗坏血酸钠处理,保鲜盒包装,5℃贮藏。然后测定此条件下鲜切雪花梨的生理生化变化,结果表明,鲜切梨内层各项指标要优于外层。0.1%山梨酸钾、2.0%异抗坏血酸钠处理,保鲜盒包装可以有效抑制PPO和POD活性,提高CAT活性,抑制膜脂过氧化程度的加重,改善鲜切梨的感官及内在品质。     

Evaluation of sodium lactate as a replacement for conventional chemical preservatives in comminuted sausages inoculated with Listeria monocytogenes

Sodium lactate (SL) as a potential replacer for potassium sorbate (PS) or sodium benzoate (SB) in comminuted sausages was evaluated. Sausages manufactured with 3.3% SL were compared with a control and 0.05 or 0.1% of PS and SB with regard to its influence on changes of chemical composition, physico-chemical and textural properties, and the growth of inoculated Listeria monocytogenes (LM) stored at 4?°C for up to 8 weeks. The sausages contained 62-64% moisture, 15-17% fat and 12-14% protein with pH range of 6.10-6.15 and water activity (a(w)) range of 0.936-0.941. Sausages containing 3.3% SL alone had lower (P<0.05) thiobarbituric acid reacting substances (TBARS) values than the control and those of PS (0.05-0.1%). Lightness values of sausages varied (P<0.05) among preservatives and storage times, while yellowness values tended to increase with storage time. Textural attributes (springiness and hardness) were reduced after 2 and 6 weeks storage, respectively. Sodium lactate at an incorporation level of 3.3% to sausage formulation had an antilisterial effect similar to those of 0.05-1.0% of PS or SB and delayed the lag phase for the growth of Listeria monocytogenes at least 2 weeks, compared with the control.     

Nisin和柠檬酸对鲜切皇冠梨储藏期间单增李斯特菌生长的影响

分别采用0.05mg/m L乳酸链球菌素(Nisin)、0.3%柠檬酸复配溶液和自来水清洗接种单增李斯特菌(LM)的鲜切皇冠梨,将清洗后的鲜切皇冠梨分别于4、12、24℃的条件下储藏,以未清洗的样品为对照,测定储藏过程中LM的生长情况。结果显示Nisin和柠檬酸清洗能显著降低(p<0.05)鲜切皇冠梨中LM的初始带菌量。在4℃储藏过程中,Nisin和柠檬酸清洗的鲜切皇冠梨中LM数量没有明显变化(p>0.05),自来水清洗和未清洗的鲜切样品中LM数量略有升高。在12℃和24℃储藏期间,三种处理方式的鲜切皇冠梨LM数量均显著上升(p<0.05)。Nisin和柠檬酸复配清洗处理结合低温储藏能有效控制鲜切皇冠梨中LM的生长。      

Effectiveness of potassium lactate and sodium diacetate in combination with irradiation to control Listeria monocytogenes on frankfurters

ABSTRACT:  The use of antimicrobial ingredients in combination with irradiation is an effective antilisterial intervention strategy for ready-to-eat meat products. Microbial safety was evaluated for frankfurters formulated with 0% or 3% added potassium lactate/sodium diacetate solution and inoculated with Listeria monocytogenes before or after treatment with irradiation (0, 1.8, or 2.6 kGy). Frankfurters were stored aerobically or vacuum packaged and L. mo nocytogenes counts and APCs were determined while refrigerated. The incorporation of lactate/diacetate with or without irradiation had a strong listeriostatic effect for aerobically stored frankfurters. Outgrowth was suppressed and counts were not different from initial counts (5.2 log CFU/frank compared with 5.0 log CFU/frank); however, those without the additive increased steadily (5.4 to 9.3 log CFU/frank). Irradiation treatments alone had higher L. monocytogenes counts after 3 wk. For vacuum-packaged frankfurters, both the addition of lactate/diacetate and irradiation were effective at controlling growth after 8 wk. Large and incremental reductions in total counts were seen for irradiation treatments. Initial counts were reduced by 3 log CFU with the application of 1.8 kGy while 2.6 kGy decreased counts over 5 log CFU. These reductions were maintained throughout storage for lactate/diacetate-treated frankfurters. By 8 wk, L. monocytogenes counts on 1.8 and 2.6 kGy irradiated frankfurters without lactate/diacetate increased to 7.43 and 6.13 log CFU, respectively. Overall, lactate/diacetate retarded the outgrowth of L. monocytogenes on frankfurters throughout aerobic storage and the combination of irradiation and 3% lactate/diacetate reduced and retarded growth of L. monocytogenes , especially during the last 2 wk of vacuum-packaged storage.     

Synergic antimicrobial activity of lysozyme, nisin, and EDTA against Listeria monocytogenes in ostrich meat patties

Synergic antimicrobial activity of lysozyme (250 ppm), nisin (250 ppm), and disodium ethylenediamine tetraacetic acid (EDTA) (20 mM) against Listeria monocytogenes and meat-borne spoilage bacteria in ostrich patties packaged in air and vacuum was studied. The antimicrobial treatment decreased the L. monocytogenes population in ostrich patties below the official limit of the European Union (<2 log CFU/g). The total viable counts for the treated samples (air and vacuum) showed a reduction of 1 log cycle until to 2 d of storage; after this period the cell load increased. Moreover, the reduction of 2 log cycle for the lactic acid bacteria was observed. Enterobacteriaceae and Pseudomonas spp. were not affected by the antimicrobial treatment in both packaging atmospheres. Sensory evaluation did not differ between treated and untreated samples as regard to the color. The ostrich patties packaged in vacuum had a desirable odor during the storage time and were not affected by antimicrobial treatment. The off-odors for the patties packaged in air developed faster in the control while the odor scores for the treated samples remained above the rejection point up to the end of storage. PRACTICAL APPLICATION: Great interest is developing in food bio-preservation, because of the ever-increasing needs to protect consumers' health and to valorize the naturalness and safety of food products.     

Encapsulation of nisin and lysozyme in liposomes enhances efficacy against Listeria monocytogenes

The efficacy and stability against Listeria monocytogenes of nisin and lysozyme encapsulated in phospholipid liposomes was evaluated. Antimicrobial-containing liposomes were prepared by hydrating dried lipids with buffer containing nisin, nisin plus the fluorescence probe calcein, or calcein and lysozyme. Mixtures were then centrifuged and sonicated, and encapsulated liposomes were collected using size-exclusion chromatography. Antimicrobial concentration in liposomes was determined by bicinchoninic acid assay prior to determination of antimicrobial activity against strains of L. monocytogenes. When nisin was encapsulated in liposomes, protein concentrations of 0.39, 0.27, and 0.23 mg/ml for phosphatidylcholine (PC), PC-cholesterol (7:3), and PC-phosphatidylglycerol (PG)-cholesterol (5:2:3), respectively, were obtained. Encapsulation of nisin with calcein yielded protein concentrations of 0.35, 0.39, and 0.28 mg/ml for PC, PC-cholesterol, and PC-PG-cholesterol, respectively. Encapsulation of calcein with lysozyme resulted in protein concentrations of 0.43, 0.26, and 0.19 mg/ml for PC, PC-cholesterol, and PC-PG-cholesterol, respectively. Encapsulated nisin in 100% PC and PC-cholesterol liposomes inhibited bacterial growth by >2 log CFU/ml compared with free nisin. Growth inhibition with liposomal lysozyme was strain dependent, with greater inhibition observed for strains 310 and Scott A with PC-cholesterol and PC-PG-cholesterol liposomes. Inhibition of L. monocytogenes indicated the potential of liposomes to serve as delivery vehicles for antimicrobials in foods while improving stability of antimicrobials.     

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.     

Protein variations in Listeria monocytogenes exposed to sodium lactate, sodium diacetate, and their combination

Most studies of the effect of adverse conditions on survival of Listeria monocytogenes have focused on stress caused by acid or sodium chloride. However, no information is available on resistance of this pathogen to stress caused by salts of organic acids. Sodium lactate and sodium diacetate are generally recognized as safe substances and are approved as ingredients for use in foods. We evaluated antilisterial properties of each of these salts and the enhanced inhibition effected by their combination in ready-to-eat meat products at pH 6.3. Changes in proteins found in this pathogen were studied in the presence of the salts in a chemically defined medium at the same pH using a proteomic approach. The total numbers of protein spots obtained from two-dimensional electrophoresis were 198, 150, and 131 for sodium diacetate, sodium lactate, and the control, respectively. Sodium diacetate treatment produced the highest number of unmatched proteins (124 versus 53 in lactate), the greatest increase in expression (20 versus 5 in lactate), and the highest number of novel proteins (90 versus 45 in lactate). The number of repressed proteins was highest in the combination treatment (41 versus -30 in the single salt treatment). Six proteins that increased or decreased by > or = 10-fold were further investigated; oxidoreductase and lipoprotein were upregulated, and DNA-binding protein, alpha amylase, and two SecA proteins were downregulated or completely suppressed by the salt treatment. Identification of all protein spots is essential for comparison with proteins induced or suppressed under other stress conditions.     

Behavior of Listeria monocytogenes inoculated on cantaloupe surfaces and efficacy of washing treatments to reduce transfer from rind to fresh-cut pieces

Attachment and survival of Listeria monocytogenes on external surfaces (rind) of inoculated cantaloupe, resistance of the surviving bacteria to chlorine or hydrogen peroxide treatments, transfer of the pathogen from unsanitized and sanitized rinds to fresh-cut tissues during cutting and growth, and survival of L. monocytogenes on fresh-cut pieces of cantaloupe were investigated. Surface treatment with 70% ethanol to reduce the native microflora on treated melon, followed by immersion in a four-strain cocktail of L monocytogenes (10(8) CFU/ml) for 10 min, deposited 4.2 log10 CFU/cm2 and 3.5 log10 CFU/cm2 of L monocytogenes on treated and untreated cantaloupe rinds, respectively. L. monocytogenes survived on the treated or untreated cantaloupe rinds for up to 15 days during storage at 4 and 20 degrees C, but populations declined by approximately 1 to 2 log10 CFU/cm2. Fresh-cut pieces prepared from inoculated whole cantaloupes stored at 4 degrees C for 24 h after inoculation were positive for L. monocytogenes. Washing inoculated whole cantaloupes in solutions containing 1,000 ppm of chlorine or 5% hydrogen peroxide for 2 min at 1 to 15 days of storage at 4 degrees C after inoculation resulted in a 2.0- to 3.5-log reduction in L. monocytogenes on the melon surface. Fresh-cut pieces prepared from the sanitized melons were negative for L. monocytogenes. After direct inoculation onto fresh-cut pieces, L. monocytogenes survived, but did not grow, during 15 days of storage at 4 degrees C. Growth was evident by 4 h of storage at 8 and 20 degrees C. It is concluded that sanitizing with chlorine or hydrogen peroxide has the potential to reduce or eliminate the transfer of L. monocytogenes on melon surfaces to fresh-cut pieces during cutting.     

Inhibition of Listeria innocua in hummus by a combination of nisin and citric acid

The effect of nisin or citric acid or combinations of these two inhibitors on the inactivation of a cocktail of three Listeria innocua strains was investigated in a model brain heart infusion (BHI) broth and hummus (chickpea dip). In BHI broth, citric acid had a limited ability to inhibit L. innocua growth. Nisin initially reduced L. innocua concentrations by about 3 log cycles; however, L. innocua reached concentrations similar to those of the control after 5 days at 22 degrees C. In combination, the effects of 500 IU/ml nisin and 0.2% citric acid were synergistic and resulted in complete elimination of L. innocua in the BHI broth. The inhibition of L. innocua by nisin (500 or 1,000 IU/g), citric acid (0.1, 0.2, or 0.3%), or their combinations also was evaluated in hummus. Citric acid alone did not affect L. innocua growth or the aerobic bacterial plate count. A combination of 1,000 IU/g nisin and 0.3% citric acid was somewhat effective (approximately 1.5-log reduction) in controlling the concentration of L. innocua and the aerobic plate count for up to 6 days. This combination also may be useful, in addition to proper hygienic practices, for minimizing the growth of the pathogen Listeria monocytogenes in hummus.     

Survival and growth of Listeria monocytogenes in broth as a function of temperature, pH, and potassium lactate and sodium diacetate concentrations

The objective of this study was to determine the antimicrobial effect of a combination of potassium lactate and sodium diacetate (0, 1.8, 3, and 4.5%; PURASAL P Opti.Form 4, 60% solution) on the survival and growth of Listeria monocytogenes Scott A in pH-adjusted broth (5.5, 6.0, 6.5, and 7.0) stored at 4, 10, 17, 24, 30, and 37 degrees C. Appropriate dilutions of broth were enumerated by spiral plating on tryptose agar and counted with an automated colony counter. Growth data were iteratively fit, using nonlinear regression analysis to a three-phase linear model, using GraphPad PRISM. At pH 5.5, the combination of lactate-diacetate fully inhibited (P < 0.001) the growth of L. monocytogenes at all four levels and six temperatures. At pH 6.0, addition of 1.8% lactate-diacetate reduced (P < 0.001) the specific growth rate of L. monocytogenes and increased lag time; however, 3 and 4.5% completely inhibited the growth at the six temperatures studied. Efficacy of the lactate-diacetate mixture was decreased as pH increased and incubation temperature increased. Thus, at pH 6.5, at least 3% was required to retard (P < 0.001) the growth of L. monocytogenes in broth. There was a limited effect of the lactate-diacetate level on the specific growth rate of the pathogen at pH 7.0. However, 1.8 and 3% significantly lengthened the lag time at 4 and 10 degrees C. These results suggest that 1.8% of lactate-diacetate mixture can be used as a substantial hurdle to the growth of L. monocytogenes when refrigerated temperatures are maintained for products with pH less than 6.5.     

Modeling the growth of Listeria monocytogenes in cured ready-to-eat processed meat products by manipulation of sodium chloride, sodium diacetate, potassium lactate, and product moisture content

A central composite second-order response surface design was employed to determine the influences of added sodium chloride (0.8 to 3.6%), sodium diacetate (0 to 0.2%), potassium lactate syrup (0.25 to 9.25%), and finished-product moisture (45.5 to 83.5%) on the predicted growth rate of Listeria monocytogenes in cured ready-to-eat (RTE) meat products. Increased amounts of both sodium diacetate (P < 0.11) and potassium lactate (P < 0.001) resulted in significant reductions in the growth rate constants of L monocytogenes. Increased finished-product moisture (P < 0.11) significantly increased growth rate constants. The nfluence of sodium chloride was not statistically significant. The second-order statistical factor for lactate was significant (P < 0.01), but all two-way interactions were not. In general, predicted growth rates exceeded actual growth rates obtained from inoculation studies of four cured RTE meat products (wieners, smoked-cooked ham, light bologna, and cotto salami). The final model will be useful to food technologists in determining formulations that will result in finished cured RTE meat products in which L. monocytogenes is not likely to grow.     

Survival of Listeria monocytogenes Scott A on vacuum-packaged raw beef treated with polylactic acid, lactic acid, and nisin

Low-molecular-weight polylactic acid (LMW-PLA) and lactic acid (LA) were used to inhibit growth of Listeria monocytogenes Scott A on vacuum-packaged beef. Nisin was also used simultaneously as an additional hurdle to the growth of this pathogen. Inoculated beef cubes were immersed in a solution of 2% LMW-PLA, 2% LA, 400 IU/ml of nisin, or combinations of each acid and nisin for 5 min and drip-dried for 15 min. The cubes were then vacuum-packaged and stored at 4 degrees C for up to 42 days. Surface pH values of beef cubes treated with 2% LMW-PLA, the combination of 400 IU/ml of nisin and 2% LMW-PLA (2% NPLA), or 400 IU/ml of nisin alone were significantly reduced from 5.59 to 5.18, 5.01, and 5.19, respectively, whereas those decontaminated with 2% LA or 400 IU/ml of nisin and 2% LA (2% NLA) were significantly decreased from 5.59 to 4.92 and 4.83, respectively, at day 0 (P < or = 0.05). The 2% LMW-PLA, 2% LA, 2% NPLA, 2% NLA, and 400 IU/ml of nisin showed immediate bactericidal effects on L. monocytogenes Scott A (1.22-, 1.56-, 1.57-, 1.94-, and 1.64-log10 reduction, respectively) compared with the initial number of 5.33 log10 CFU/cm2 of the untreated control at day 0 (P < or = 0.05). These treatments, combined with vacuum-packaging and refrigeration temperature, succeeded to inhibit growth of L. monocytogenes during storage up to 42 days. At the end of 42 days, the numbers of L. monocytogenes Scott A remaining viable on these samples were 1.21, 0.36, 2.21, 0.84, and 0.89 log10 CFU/cm2, respectively.     

Modeling time to inactivation of Listeria monocytogenes in response to high pressure, sodium chloride, and sodium lactate

A mathematical model was developed to predict time to inactivation (TTI) by high pressure processing of Listeria monocytogenes in a broth system (pH 6.3) as a function of pressure (450 to 700 MPa), inoculum level (2 to 6 log CFU/ml), sodium chloride (1 or 2%), and sodium lactate (0 or 2.5%) from a 4°C initial temperature. Ten L. monocytogenes isolates from various sources, including processed meats, were evaluated for pressure resistance. The five most resistant strains were used as a cocktail to determine TTI and for model validation. Complete inactivation of L. monocytogenes in all treatments was demonstrated with an enrichment method. The TTI increased with increasing inoculum level and decreasing pressure magnitude, from 1.5 min at 700 MPa and 2 log CFU/ml, to 15 min at 450 MPa and 6 log CFU/ml. Neither NaCl nor sodium lactate significantly influenced TTI. The model was validated with ready-to-eat, uncured, Australian retail poultry products, and with product specially made at a U.S. Department of Agriculture, Food Safety and Inspection Service (FSIS)-inspected pilot plant in the United States. Data from the 210 individual product samples used for validation indicate that the model gives "fail-safe" predictions (58% with response as expected, 39% with no survivors where survivors expected, and only 3% with survivors where none were expected). This model can help manufacturers of refrigerated ready-to-eat meats establish effective processing criteria for the use of high pressure processing as a postlethality treatment for L. monocytogenes in accordance with FSIS regulations.