Oscillatory High Pressure Processing Applied to Mechanically Recovered Poultry Meat for Bacterial Inactivation

The effect of oscillatory high pressure processing on mesophile and psychrotroph populations of mechanically recovered poultry meat (MRPM) was evaluated. Vacuum‐packaged samples were subjected to cycles by alternating moderate pressure (60 MPa) and high pressure (450 MPa) at 20 °C, once or several times so that the total time under high pressure was 15 min. A continuous treatment at 450 MPa for 15 min at 20 °C was also performed. Oscillatory treatments did not generate significantly higher decreases in counts of both populations than continuous pressurization. Reductions from 3.2 to 3.8 log CFU g^–1 were found for mesophiles. Psychrotrophs proved more sensitive: one of the cyclic treatments induced a lethality of 5.2 log CFU g ^–1. Pressurization improves the microbiological quality of MRPM.     

Listeria innocua and aerobic mesophiles during chill storage of inoculated mechanically recovered poultry meat treated with high hydrostatic pressure

Mechanically recovered poultry meat (MRPM) was inoculated with Listeria innocua 910 CECT at a level of approximately 10⁸ CFU g⁻¹. Vacuum-packaged samples were treated by combinations of pressure (350, 400, 450 and 500 MPa), time (5, 10, 15 and 30 min) and temperature (2, 10 and 20°C) and later stored at 2°C for 2 months. Counts of L. innocua and aerobic mesophilic bacteria were determined 1, 4, 7, 15, 30 and 60 days after pressurisation. For mesophiles, in most treatments, pressurization at 2°C gave the significantly best results. High pressure caused a marked bactericidal effect on L. innocua: reductions higher than 7.5 log units were achieved in several cases. Some cells were just sublethally injured by pressure. Samples treated at 500 MPa for 30 min at 2°C had counts of only 2.3 log units after 60 days of chill storage. Noninoculated pressurised MRPM did not show Listeria growth throughout storage. These results suggest that high pressure processing can enhance the microbiological quality of MRPM.     

High pressure treatments on the inactivation of Salmonella Enteritidis and the characteristics of beef carpaccio

The effect of high pressure (HP) on Salmonella enterica subsp. enterica serovar Enteritidis in beef carpaccio stored under temperature abuse conditions (8°C) during 30days was investigated. After treatment, reductions of S. Enteritidis were 3.68 and 5.94 log cfu/g in samples pressurized at 450MPa for 5 and 10min, respectively, whereas the pathogen was only detected after enrichment of samples treated at 450MPa for 15min. During storage, counts of S. Enteritidis decreased 0.26 log cfu/g in non-pressurized carpaccio, 1.33 log cfu/g in carpaccio treated at 450MPa for 5min and were only detected after enrichment in carpaccio pressurized at 450MPa for 10 or 15min. Color (L*, a* and b*) varied with pressurization and storage, with higher changes in carpaccio treated at longer pressurization times. Shear resistance was slightly lower in treated samples just after pressurization, but increased at the end of the storage period. Maximum force was less affected by treatment.     

Inactivation of peroxidase and lipoxygenase in carrots, green beans, and green peas by combination of high hydrostatic pressure and mild heat treatment

The efficiency of high hydrostatic pressure (HHP) with the combination of mild heat treatment on peroxidase (POD) and lipoxygenase (LOX) inactivation in carrots, green beans, and green peas was investigated. In the first part of the study, the samples were pressurized under 250–450 MPa at 20–50 °C for 15–60 min. In the second part, two steps treatments were performed as water blanching at 40–70 °C for 15 and 30 min after pressurization at 250 MPa and 20 °C for 15–60 min. Carrot POD was decreased to 16% residual activity within the first 30 min at a treatment condition of 350 MPa and 20 °C and then it decreased to 9% at 60 min. When the carrots were water blanched at 50 °C for 30 min after HHP treatment of 250 MPa at 20 °C for 15 min, 13% residual POD activity was obtained. For green beans, the most effective results were obtained by two steps treatment and approximately 25% residual POD activity was obtained by water blanching at 50 °C for 15 min after pressurization at 250 MPa and 20 °C for 60 min. An effective inactivation of POD in green peas was not obtained. For carrots, LOX activity could not be measured due to very low LOX activity or the presence of strong antioxidants such as carotenoids. After pressurization at 250 MPa and 20 °C for 15 or 30 min, water blanching at 60 °C for 30 min provided 2–3% residual LOX activity in green beans. The treatment of 250 MPa for 30 min and then water blanching at 50 °C for 30 min provided 70% LOX inactivation in green peas.     

High pressure treatments on the inactivation of Salmonella Enteritidis and the characteristics of beef carpaccio

The effect of high pressure (HP) on Salmonella enterica subsp. enterica serovar Enteritidis in beef carpaccio stored under temperature abuse conditions (8 °C) during 30 days was investigated. After treatment, reductions of S. Enteritidis were 3.68 and 5.94 log cfu/g in samples pressurized at 450 MPa for 5 and 10 min, respectively, whereas the pathogen was only detected after enrichment of samples treated at 450 MPa for 15 min. During storage, counts of S. Enteritidis decreased 0.26 log cfu/g in non-pressurized carpaccio, 1.33 log cfu/g in carpaccio treated at 450 MPa for 5 min and were only detected after enrichment in carpaccio pressurized at 450 MPa for 10 or 15 min. Color (L*, a* and b*) varied with pressurization and storage, with higher changes in carpaccio treated at longer pressurization times. Shear resistance was slightly lower in treated samples just after pressurization, but increased at the end of the storage period. Maximum force was less affected by treatment.     

Inactivation of barotolerant Listeria monocytogenes in sausage by combination of high-pressure processing and food-grade additives

Food-grade additives were used to enhance the efficacy of high-pressure processing (HPP) against barotolerant Listeria monocytogenes. Three strains of L. monocytogenes (Scott A, OSY-8578, and OSY-328) were compared for their sensitivity to HPP, nisin, tert-butylhydroquinone (TBHQ), and their combination. Inactivation of these strains was evaluated in 0.2 M sodium phosphate buffer (pH 7.0) and commercially sterile sausage. A cell suspension of L. monocytogenes in buffer (10(9) CFU/ml) was treated with TBHQ at 100 ppm, nisin at 100 IU/ml, HPP at 400 MPa for 5 min, and combinations of these treatments. Populations of strains Scott A, OSY-8578, and OSY-328 decreased 3.9, 2.7, and 1.3 log with HPP alone and 6.4, 5.2, and 1.9 log with the HPP-TBHQ combination, respectively. Commercially sterile sausage was inoculated with the three L. monocytogenes strains (10(6) to 10(7) CFU/g) and treated with selected combinations of TBHQ (100 to 300 ppm), nisin (100 and 200 ppm), and HPP (600 MPa, 28 degrees C, 5 min). Samples were enriched to detect the viability of the pathogen after the treatments. Most of the samples treated with nisin, TBHQ, or their combination were positive for L. monocytogenes. HPP alone resulted in a modest decrease in the number of positive samples. L. monocytogenes was not detected in any of the inoculated commercial sausage samples after treatment with HPP-TBHQ or HPP-TBHQ-nisin combinations. These results suggest that addition of TBHQ or TBHQ plus nisin to sausage followed by in-package pressurization is a promising method for producing Listeria-free ready-to-eat products.     

Effects of High‐Pressure Processing on Inactivation of Salmonella Typhimurium,Eating Quality,and Microstructure of Raw Chicken Breast Fillets

Abstract: High‐pressure inactivation of Salmonella Typhimurium DMST 28913, eating quality, and microstructure of pressurized raw chicken breast meat was determined. The inoculated samples (approximately 7 log CFU/g initial load) were processed at 300 and 400 MPa, using pressurized medium of 25 to 35 °C during pressurization. Weibull model was well fitted to the survival curves with tailing. Least severe conditions with acceptable inactivation levels were 300 MPa, 35 °C, 1 min (approximately 2 log reduction) and 400 MPa, 30 °C, 1 min (approximately 4 log reduction). Based on these 2 conditions, the 400 MPa treatment yielded the raw chicken meat with higher L* value, greater cooking loss, and lower water holding capacity. Cooked chicken breast meat prepared from the pressurized samples had firmer texture than the control. Scanning electron microscopic images showed that higher pressure resulted in increasing extent of protein coagulation and the contraction of the muscle bundles. Practical Application: For raw chicken breast fillet, 300 MPa, 35 °C, 1 min was the condition that reasonably reduced the load of Salmonella Typhimurium. However, the pressurized samples had greater cooking loss. Marination with brine containing sodium chloride and phosphate prior to pressurization might help improve this eating quality.     

Inactivation of natural microorganisms in litchi juice by high-pressure carbon dioxide combined with mild heat and nisin

The individual and combined effects of high-pressure carbon dioxide (HPCD), mild heat (MH) and nisin (200 ppm) on the inactivation of natural microorganisms, including aerobic bacteria (AB), yeasts and molds (Y&;M), in litchi juice were evaluated. The samples were treated at a pressure of 10 MPa and temperatures of 32, 42 or 52 °C for 5, 10, 15, 20, 25 or 30 min. Temperature played a prominent role in the inactivation of both AB and Y&;M when combined with HPCD, particularly for AB at 52 °C and Y&;M at temperatures ≥42 °C. Nisin increased the susceptibility of AB to the combined treatment of HPCD and MH (HPCD + MH). A reduction of 4.19 log cycles was achieved by HPCD + MH at 52 °C for 15 min, and complete inactivation of AB was obtained by combination of HPCD, MH and nisin (HPCD + MH + nisin). No significant effect of nisin was found on the inactivation of Y&;M.     

Effect of high hydrostatic pressure on Listeria innocua 910 CECT inoculated into Ewe's milk.

Ewe's milk standardized to 6% fat was inoculated with Listeria innocua 910 CECT at a concentration of 10(7)CFU/ml and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (200, 300, 350, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50 degrees C), and time (5, 10, and 15 min). To determine numbers of L. innocua, listeria selective agar base with listeria selective supplement and plate count agar was used. Low-temperature (2 degrees C) pressurizations produced higher L. innocua inactivation than treatments at room temperatures (25 degrees C). Pressures between 450 and 500 MPa for 10 to 15 min were needed to achieve reductions of 7 to 8 log units. The kinetics of destruction of L. innocua were first order with D-values of 3.12 min at 2 degrees C and 400 MPa and 4 min at 25 degrees C and 400 MPa. A baroprotective effect of ewe's milk (6% fat) on L. innocua was observed in comparison with other studies using different media and similar pressurization conditions.     

Factors influencing death and injury of foodborne pathogens by hydrostatic pressure-pasteurization

The key objective of hydrostatic pressure-pasteurization of food is the destruction of pathogenic bacterial cells at a high level (8log cycles) without adversely affecting the acceptance characteristics of a food. To achieve these goals, hydrostatic pressure-pasteurization may best be conducted at a moderate pressure, which alone will not kill the desired level of pathogens. However, along with hydrostatic pressure other parameters, namely pressurization-time, temperature and bactericidal compounds, such as bacteriocins, can be used to enhance bactericidal effects of pressurization.Our results with four pathogens indicated that viability loss at 25°C were minimal up to 207MPa, then increased at a rapid rate. However, even above 483MPa, not all species had 8log cycles viability loss. At and above 276MPa a large number of survivors were injured. At 207MPa and 25°C, pressurization for 30min did not greatly enhance viability loss and injury of the four pathogens. However, pressurization temperatures beyond 35°C greatly increased cell death and injury. By pressurizing the cells in the presence of a mixture of pediocin AcH and nisin viability loss of the pathogens was increased by an additional 1.3 to 5.1log cycles. This increase was due to the bactericidal effect of the two bacteriocins on sensitive and injured cells. These results indicated that at moderate pressure, a high level of destruction of pathogens can be induced within a short time by using moderate temperature in combination with bactericidal preservative(s).     

Destruction of Salmonella enteritidis inoculated in liquid whole egg by high hydrostatic pressure: comparative study in selective and non-selective media

The destruction of Salmonella enteritidis inoculated in liquid whole egg at approximately 10⁷−10⁸cfu ml^−lwas studied under combinations of pressure (350 and 450 MPa), temperature (50, 20, 2 and −15°C) and time (5, 10, 15 min and cycles of 5+5 and 5+5+5 min). One non-selective medium (tryptone soy agar) and two selective media (brilliant green agar and salmonella-shigella) were used to evaluate viability of S. enteritidis after pressurization. The inactivation rate increased with pressure and exposure time, being minimal at 350 MPa and −15°C for 5 min (over 1 log₁₀of reduction) and reaching total inactivation (8 log₁₀of reduction) in several treatments at 50°C. Treatments in cycles showed greater effectiveness than continuous treatments of the same total time. The effect of pressure was enhanced by elevated temperatures. The higher counts were obtained in the non-selective medium, indicating the presence of injured cells after pressure treatment. D -values obtained for two temperatures (2 and 20°C) and different times (0–60 min) under controlled pressure (400 MPa) showed that microbial inactivation followed a first-order kinetics with a decimal reduction time evaluated in tryptone soy agar medium of 9·5 min at 2°C and 8·8 min at 20°C.     

Sensitivity of spores of Bacillus subtilis and Clostridium sporogenes PA 3679 to combinations of high hydrostatic pressure and other processing parameters

The combined effects of pressure, temperature, pH and the presence of nisin or sucrose laurate on the survival of spores of Bacillus subtilis 168 and Clostridium sporogenes PA 3679 were investigated. Spore populations of PA 3679 were reduced by 2.5-log₁₀ when exposed to 404 megapascals (MPa) at 25°C, pH 4.0 for 30 min, but the same treatment at pH 7.0 resulted in a <0.5-log₁₀ reduction in spore counts. Pressurization of B. subtilis spores at 70°C, pH 6.0 or 7.0 for 15 min at 404 MPa resulted in a 5-log₁₀ reduction as compared to a <0.5-log₁₀ reduction for the same pressurization treatment at 25°C. For the inactivation of spores of B. subtilis and PA 3679, the addition of nisin to the plating medium appeared to be synergistic in some instances when combined with pressurization at elevated temperatures and reduced pH. B. subtilis 168 was resistant to 0.1% sucrose laurate, but when combined at ≤6.0 pH with a 15-min treatment of 404 MPa at 45°C, a dramatic synergistic effect eliminated spore suspensions of 1×10⁶/ml.     

Inactivation ofEscherichia coliinoculated in liquid whole egg by high hydrostatic pressure

The resistance ofEscherichia coliin liquid whole egg was studied at several pressures (300, 350, 400 and 450MPa), temperatures (50, 20, 2 and –15°C) and times (5, 5+5, 10, 5+5+5, 15min). The highest reduction was obtained at 50°C (about 7log₈units). At 20 and –15°CE. coliwas more resistant to pressure than at 50 and 2°C. The intermittent treatments were more effective than continuous treatments at lower pressures (350MPa). The destruction increases upon increasing the pressure and the time treatment. Survivor curves were studied at 400MPa for two temperatures (20 and 2°C) and different times (0–60min), obtaining a decimal reduction time of 14.1min at 20°C and 9.5min at 2°X.     

The effect of certain antibiotics on bolti fish (Tilapia nilotica) preservation

The shelf life of bolti fish (Tilapia nilotica) caught in the river Nile, has been successfully prolonged for about 9 days by dipping in tetracycline (TC) or nisin solution followed by refrigeration, although the initial microbial contamination in the fresh fish was very high. The caught fish were gutted and treated with 10 and 20 ppm TC solutions for 10 and 15 min by dipping, and with 500 and 1000 R.U. nisin/g fish for 20 and 30 min. The treated and control fish samples, were stored and refrigerated at (4 +/- 1) degree C for 12 days. Total bacterial counts, the most probable number of coliform bacteria and lactic acid bacteria in the fish treated with TC or nisin, were lower than those of the control, especially at 20 ppm TC, 15 min dipping, and 1000 R.U. nisin/g, 30 min. Further more TC was more effective against yeasts and moulds. This result suggest that antibiotics would help in transporting chilled fish from the highdam lake in Aswan to Cairo (about 900 km). According to the present results it may be recommended to use the antibiotic TC, with the concentration of 20 ppm for 15 min dipping, for prolonging the shelflife of bolti fish. The presence and increase of coliform bacteria in fish, draw the attention to the necessity of hygienic measures when dealing with such fish until to the consumption.     

Effect of high hydrostatic pressure and high-pressure homogenisation on <Emphasis Type="Italic">Lactobacillus plantarum</Emphasis> inactivation kinetics and quality parameters of mandarin juice

The inactivation kinetics of Lactobacillus plantarum in a mandarin juice treated by thermal treatment (45–90 °C), high-pressure homogenisation (HPH) (30–120 MPa at 15 and 30 °C) and high-pressure processing (HPP) (150–450 MPa at 15, 30 and 45 °C) were fitted to different Weibullian equations. A synergic effect between pressure and temperature was observed in HPH and HPP treatments achieving 2.38 log cycles after 120 MPa at 30 °C for 10 s (final T of 45 °C) and 6.12 log cycles after 400 MPa at 45 °C for 1 min (final T of 60 °C), respectively. A combined treatment of 100 MPa at 15 °C for 10 s and 300 MPa at 15–30 °C for 1 min in HPH and HPP, respectively, was needed to the first logarithm microbial population decline. Weibull model accurately predicted microorganism inactivation kinetics after HPH and HPP processing when displaying single shoulder or tail in the survivor curves, whereas when a more complex trend was observed after thermal treatment, the double-Weibull equation was found more appropriate to explain such behaviour. Equivalent treatments that achieved the same degree of microbial inactivation (77 °C–10 s in thermal processing, 120 MPa–10 s at 30 °C in HPH processing and 375 MPa–1 min at 30 °C in HPP) were selected to study the effects on quality parameters. The application of dynamic pressure led to a decrease in sedimentable pulp, transmittance and juice redness, thus stabilising the opaqueness and cloudiness of mandarin juice. Pectin methyl esterase (PME) was found to be highly baroresistant to static and dynamic pressure. Carotenoid content remained unaffected by any treatment. This study shows the potential of high-pressure homogenisation as an alternative for fruit-juice pasteurisation.     

Inactivation of spores of Bacillus cereus in cheese by high hydrostatic pressure with the addition of nisin or lysozyme

The objective of this work was to study high hydrostatic pressure (HHP) inactivation of spores of Bacillus cereus ATCC 9139 inoculated in model cheeses made of raw milk, together with the effects of the addition of nisin or lysozyme. The concentration of spores in model cheeses was approximately 6-log10 cfu/g of cheese. Cheeses were vacuum packed and stored at 8 degrees C. All samples except controls were submitted to a germination cycle of 60 MPa at 30 degrees C for 210 min, to a vegetative cells destruction cycle of 300 or 400 MPa at 30 degrees C for 15 min, or to both treatments. Bacillus cereus counts were measured 24 h and 15 d after HHP treatment. The combination of both cycles improved the efficiency of the whole treatment. When the second pressure-cycle was of 400 MPa, the highest inactivation (2.4 +/- 0.1 log10 cfu/g) was obtained with the presence of nisin (1.56 mg/L of milk), whereas lysozyme (22.4 mg/L of milk) did not increase sensitivity of the spores to HHP. For nisin (0.05 and 1.56 mg/L of milk), no significant differences were found between counts at 24 h and 15 d after treatment. Considering that mesophilic spore counts usually range from 2.6 to 3.0 log10 cfu/ml in raw milk, HHP at mild temperatures with the addition of nisin may be useful for improving safety and preservation of soft curd cheeses made from raw milk.     

Inactivation of Bacillus cereus spores in milk by mild pressure and heat treatments

The objective of this work was to study the germination and subsequent inactivation of Bacillus cereus spores in milk by mild hydrostatic pressure treatment. In an introductory experiment with strain LMG6910 treated at 40 degrees C for 30 min at 0, 100, 300 and 600 MPa, germination levels were 1.5 to 3 logs higher in milk than in 100 mM potassium phosphate buffer (pH 6.7). The effects of pressure and germination-inducing components present in the milk on spore germination were synergistic. More detailed experiments were conducted in milk at a range of pressures between 100 and 600 MPa at temperatures between 30 and 60 degrees C to identify treatments that allow a 6 log inactivation of B. cereus spores. The mildest treatment resulting in a 6 log germination was 30 min at 200 MPa/40 degrees C. Lower treatment pressures or temperatures resulted in considerably less germination, and higher pressures and temperatures further increased germination, but a small fraction of spores always remained ungerminated. Further, not all germinated spores were inactivated by the pressure treatment, even under the most severe conditions (600 MPa/60 degrees C). Two possible approaches to achieve a 6 log spore inactivation were identified, and validated in three additional B. cereus strains. The first is a single step treatment at 500 MPa/60 degrees C for 30 min, the second is a two-step treatment consisting of pressure treatment for 30 min at 200 MPa/45 degrees C to induce spore germination, followed by mild heat treatment at 60 degrees C for 10 min to kill the germinated spores. Reduction of the pressurization time to 15 min still allows a 5 log inactivation. These results illustrate the potential of high-pressure treatment to inactivate bacterial spores in minimally processed foods.     

Development of a laboratory scale clean-in-place system to test the effectiveness of "natural" antimicrobials against dairy biofilms

A laboratory scale system, partially reproducing dairy plant conditions, was developed to quantify the effectiveness of chlorine and alternative sanitizers in reducing the number of viable bacteria attached to stainless steel surfaces. Stainless steel tubes fouled in a continuous flow reactor were exposed to a standard clean-in-place regime (water rinse, 1% sodium hydroxide at 70 degrees C for 10 min, water rinse, 0.8% nitric acid at 70 degrees C for 10 min, water rinse) followed by exposure to either chlorine (200 ppm) or combinations of nisin (500 ppm), lauricidin (100 ppm), and the lactoperoxidase system (LPS) (200 ppm) for 10 min or 2, 4, 8, 18, or 24 h. There was significant variation in the effectiveness of the alkaline-acid wash steps in reducing cell numbers (log reduction between 0 and 2). Following a 10-min treatment, none of the sanitizers significantly reduced the number of attached cells. Two hours of exposure to chlorine, nisin + the LPS, or lauricidin + the LPS achieved 2.8, 2.2, and 1.6 log reductions, respectively. Exposure times > 2 h did not further decrease the number of viable bacteria attached to the stainless steel. The effectiveness of combinations of nisin, lauricidin, and the LPS was similar to that of chlorine (P > 0.05), and these sanitizers could be used to decontaminate the surfaces of small-volume or critical hard-to-clean milk processing equipment.     

The use of nisin as a preservative in fish sausage stored at ambient (28 ± 2 °C) and refrigerated (6 ± 2 °C) temperatures

Summary The efficacy of nisin at three different concentrations, 12.5, 25 and 50 ppm, on the keeping quality of fish sausage in synthetic casing at ambient (28 ± 2 °C) and refrigerated (6 ± 2 °C) temperatures was assessed. Gel strength, expressible water content, total volatile base nitrogen, total plate count and aerobic spore counts were affected by the storage temperatures and nisin concentrations used. Fish sausage treated with 50 ppm of nisin was acceptable after storage at ambient temperature for 20–22 days compared with the control, which were acceptable only for 2 days. The keeping quality of the sausages, at refrigerated temperature, varied from 30 days in the control to 150 days in 50‐ppm nisin‐treated samples. The residual nisin decreased slowly in samples stored at refrigerated temperature, whereas, in fish sausages stored at ambient temperature, the decrease was rapid. Nisin at 50‐ppm level showed a significant effect (P ≤ 0.05) on the gel strength and overall acceptability scores both at ambient and refrigerated temperature.     

Destruction of Listeria monocytogenes in sturgeon (Acipenser transmontanus) caviar by a combination of nisin with chemical antimicrobials or moderate heat

The objective of this study was to investigate the effect of nisin in combination with heat or antimicrobial chemical treatments (such as lactic acid, chlorous acid, and sodium hypochlorite) on the inhibition of Listeria monocytogenes and total mesophiles in sturgeon (Acipenser transmontanus) caviar. The effects of nisin (250, 500, 750, and 1,000 IU/ml), lactic acid (1, 2, and 3%), chlorous acid (134 and 268 ppm), sodium hypochlorite (150 and 300 ppm), and heat at 60 degrees C for 3 min were evaluated for a five-strain mixture of L. monocytogenes and total mesophiles in sturgeon caviar containing 3.5% salt. Selected combinations of these antimicrobial treatments were also tested. Injured and viable L. monocytogenes cells were recovered using an overlay method. Treating caviar with > or =500 IU/ml nisin initially reduced L. monocytogenes by 2 to 2.5 log units. Chlorous acid (268 ppm) reduced L. monocytogenes from 7.7 log units to undetectable (<0.48 log units) after 4 days of storage at 4 degrees C. However, there were no synergistic effects observed for combinations of nisin (500 or 750 IU/ml) plus either lactic acid or chlorous acid. Lactic acid caused a slight reduction (approximately 1 log unit) in the microbial load during a 6-day period at 4 degrees C. Sodium hypochlorite was ineffective at the levels tested. Mild heating (60 degrees C for 3 min) with nisin synergistically reduced viable counts of L. monocytogenes and total mesophiles. No L. monocytogenes cells (<0.48 log units) were recovered from caviar treated with heat and nisin (750 IU/ml) after a storage period of 28 days at 4 degrees C.