Thermal treatment of aseptically processed tomato paste

The heat resistance characteristics of Bacillus coagulans (NRRL B-1103) spores suspended in buffer (pH 7.0, 4.5), tomato serum and tomato paste were studied. It was found that the heat resistance of spores was reduced significantly when buffer of pH 4.5, tomato serum or tomato paste was used as suspension medium instead of buffer pH 7.0. This effect was more apparent at higher temperatures. It was concluded that a thermal process of F_105°C = 3 min is capable of causing at least 3D destruction of spores of the most heat resistant strains of B. coagulans     

Heat resistance of Yersinia enterocolitica grown at different temperatures and heated in different media.

In the range of 4-20 degrees C, growth temperature did not influence the heat resistance at 54-66 degrees C for Yersinia enterocolitica at pH 7 in citrate phosphate buffer. However, when cells were grown at 37 degrees C. the D62 increased from 0.044 to 0.17 min. This increase was constant at all heating temperatures tested (z = 5.7-5.8). Growth temperature did not influence the proportion of heat-damaged cells after a heat treatment, as measured by their response to a 2% of sodium chloride added to the recovery medium. The sensitivity of heat treated cells to nisin or lysozyme depended on growth temperature: Whereas the number of cells grown at 4 degrees C surviving heat treatment was the same regardless of the presence of 100 IU/ml of nisin or 100 microg/ml of lysozyme in the recovery medium, that of cells grown at 37 degrees C was, in these media, lower. The pH of maximum heat resistance in citrate phosphate buffer was pH 7 for cells grown at 37 degrees C, but pH 5 for those grown at 4 degrees C. In both suspensions the magnitude of the effect of pH on heat resistance was constant at all heating temperatures. For cells grown at 4 degrees C the heat resistance at 54-66 degrees C, in skimmed milk or pH 7 buffer, was the same. For cells grown at 37 degrees C this also applied for heat treatment at 66 degrees C but at 56 degrees C the heat resistance in skimmed milk was higher.     

Heat resistance of Alicyclobacillus acidocaldarius in water, various buffers, and orange juice

The effect of the pH or the composition of the heating medium and of the sporulation temperature on the heat resistance of spores of a thermoacidophilic spore-forming microorganism isolated from a dairy beverage containing orange fruit concentrate was investigated. The species was identified as Alicyclobacillus acidocaldarius. The spores showed the same heat resistance in citrate-phosphate buffers of pH 4 and 7, in distilled water, and in orange juice at any of the temperatures tested (D120 degrees C = 0.1 min and z = 7 degrees C). A raise in 20 degrees C in the sporulation temperature (from 45 to 65 degrees C) increased the heat resistance eightfold (from D110 degrees C = 0.48 min when sporulated at 45 degrees C to 3.9 min when sporulated at 65 degrees C). The z-values remained constant for all sporulation temperatures. The spores of this strain of A. acidocaldarius were very heat resistant and could easily survive any heat treatment currently applied to pasteurize fruit juices.     

HEAT RESISTANCE IN DIFFERENT HEATING MEDIA OF LISTERIA MONOCYTOGENES ATCC 15313 GROWN AT DIFFERENT TEMPERATURES

This paper examines the influence of growth temperature on thermal resistance of Listeria monocytogenes ATCC 15313. Regardless of the incubation temperature, the heat resistance of Listeria monocytogenes increased during incubation until the stationary phase of growth was reached. The maximum heat resistance of cells grown at 4C, 20C or 37C was attained after 14 days, 36 h and 18 h of incubation, respectively. After longer incubation times the heat resistance of cells grown at 4C and 20C did not change but that of cells grown at 37C decreased. The maximum heat resistance was usually greater at higher incubation temperatures. However, the magnitude of these differences depended on the pH and composition of heating media. By raising the incubation temperature from 4C to 37C, the D₆₂ value in pH 7 citrate-phosphate buffer increased from 0.13 to 0.34 min. However, when skimmed milk was used as menstruum this difference was not observed. Cells grown at 37C attained maximum heat resistance at pH 7 but those grown at 4C, at pH 6. The magnitude of the effect of the incubation temperature on heat resistance was constant at all heating temperatures tested (z = 6 ± 0.8C). The higher heat resistance of cells grown at higher temperatures was not due to a greater capacity of heat damage repair     

Numerical method for prediction of final microbial load in inoculated cylindrical cans of tomato concentrate heated in a pilot plant

A finite difference numerical method to predict temperatures and destruction of a thermally labile species in cylindrical cans of conduction heating food was applied to compute final microbial load in an inoculated 18°Brix canned tomato concentrate at pH = 4.5 processed under conditions simulating industrial practice. Predictions of product temperature and final microbial load in three can sizes compared well with experimental heat penetration data and estimation of surviving species of Bacillus coagulans ATC 8038 spores inoculated into the cans. The application of the model as an aid to establishing process schedules is illustrated for a can with a required IS¹⁴₁₀₀= 11.44 min. The model predicts that a 29.5 min heating time at 105.6°C would provide an IS¹⁴₁₀₀= 11.77 min.     

Heat resistance of juice spoilage microorganisms

The heat resistance of various yeasts (Saccharomyces cerevisiae, Rhodotorula mucilaginosa, Torulaspora delbrueckii, and Zygosaccharomyces rouxii), molds (Penicillium citrinum, Penicillium roquefortii, and Aspergillus niger), and lactic acid bacteria (Lactobacillus fermentum and Lactobacillus plantarum) obtained from spoiled acid or acidified food products was determined in 0.1 M citrate buffer at pH values of 3.0, 3.5, and 4.0. S. cerevisiae was the most heat resistant of the microorganisms in citrate buffer, and its heat resistance was further evaluated in apple, grapefruit, calcium-fortified apple, and tomato juices as well as in a juice base with high fructose corn syrup. Decimal reduction times (D-values) and changes in temperature required to change the D-value (z-values) for S. cerevisiae were higher in the juices than in citrate buffer at all pH values tested. The D57 degrees C(135 degrees F)-values varied from 9.4 min in the juice product with pH 2.8 to 32 min in a calcium-added apple juice with pH 3.9. The S. cerevisiae strain used in this study can be used in thermal-death-time experiments in acidic products to calculate process conditions and in challenge tests to validate the calculated temperatures and hold times during processing.     

Thermal inactivation of the wine spoilage yeasts Dekkera/Brettanomyces

The heat resistance of three strains of Dekkera/Brettanomyces (Dekkera anomala PYCC 5,153, Dekkera bruxellensis PYCC 4,801 and Dekkera/Brettanomyces 093) was evaluated at different temperatures between 32.5 and 55 degrees C. Thermal inactivation tests were performed in tartrate buffer solution (pH 4.0) and in wines. In the studies employing buffer as the heating menstruum, measurable thermal inactivation began only at temperatures of 50 degrees C. When heating was performed in wine, significant inactivation begins at 35 degrees C. Subsequent thermal inactivation tests were performed in buffer at various levels of pH, ethanol concentration, and various phenolic acids. Results from experiments in buffer with added ethanol suggest that the greater heat sensitivity shown in wines can be largely attributed to ethanol, although potentiation of this effect might be due to the phenolic content, particularly from ferulic acid. In the range of pH values tested (2.5-4.5), this factor had no influence in the heat inactivation kinetics. Relevant data, in the form of D and Z values calculated in the various environments, potentially useful for the establishment of regimes of thermal control of Dekkera/Brettanomyces yeasts in wine and contaminated equipment is presented.     

Thermal Inactivation of Clostridium perfringens Enterotoxin in Buffer and in Chicken Gravy

The time-temperature relationships for the inactivation of purified enterotoxin prepared from Clostridium perfringens were determined by Vero cell assay in two different heating menstrua. Enterotoxin diluted to an initial concentration of 12.5 μg/g in chicken gravy and in 0.1M phosphate buffer both at pH 6.1 was heated at temperatures of 59–65°C. Average inactivation times in gravy ranged from 72.8–1.5 min and in buffer from 149.4–2.4 min.     

The effect of pH on the thermal resistance of Clostridium sporogenes (PA 3679) in asparagus purée acidified with citric acid and glucono-delta-lactone.

The influence of type of acid, pH and temperature on heat resistance of Clostridium sporogenes (PA 3679) spores were investigated in white asparagus pureé acidified with citric acid and glucono-delta-lactone (GDL). The pH values studied were: 4.5, 4.8, 5.1 and 5.4 at temperatures of 110, 115, 118 and 121 degrees C. The addition of citric acid and GDL to reduce pH significantly diminished heat resistance of the spores. The two acids investigated differed in their effect on heat resistance at the various pH-levels. The most pronounced effect was observed at the lower heat treatment temperatures investigated. The z values ranged from 10.24 to 13.09 degrees C in the asparagus purée with acids added and with significant differences between the two acids and the various pH levels.     

Evaluation of the thermal resistance of different Salmonella serotypes in pork meat containing curing additives

The preliminary heat resistance evaluation of 94 Salmonella strains was carried out in culture medium (Trypticase soy broth, TSB). The heat resistance of three S. typhimurium strains (ATCC 14028, 133 and 1116), a strain each of S. derby B4373, S. potsdam 1133, S. menston 179. S. eppendorf 166, and S. kingston I124 was determined also in pork meat containing curing additives. As expected, the eight Salmonella strains showed greater heat resistance in pork meat than in TSB. At the lowest temperature (58 degrees C), the heat resistance increased 1.5-4 times, and it was most pronounced for the strains being most heat sensitive in TSB. S. potsdam 133 was the most resistant strain in pork meat, with D-values at 58 degrees C, 60 degrees C and 63 degrees C of 4.80, 1.57 and 0.30 min, respectively. The most sensitive strain turned out to be S. kingston 1124, with D-values of 2.79. 0.92 and 0.24 min, at the same temperatures. According to collected data, the heating processes, as applied to cured pork meat, providing an internal temperature of 60 degrees C for 9-10 min or of 63 degrees C for 3-4 min can be expected to provide a > or = 7 D kill of Salmonella belonging to the serotypes studied.     

Heat resistance of Bacillus cereus, Salmonella typhimurium and Lactobacillus delbrueckii in relation to pH and ethanol

The aim of the present work was to investigate the effect of ethanol alone and in combination with low pH on the heat resistance of specific bacteria. The bacteria chosen are representative of heat resistant and heat sensitive pathogens (Bacillus cereus and Salmonella typhimurium) and of relatively heat resistant spoilage microorganisms (Lactobacillus delbrueckii). The chosen bacteria were treated at different temperatures ranging between 70 and 97 degrees C for B. cereus, 48 and 54 degrees C for S. typhimurium and 44 and 60 degrees C for L. delbrueckii, in media of different pH (3, 5 and 7) and ethanol content (0, 2.5, 5, 7.5 and 10%). Both factors proved to be very effective in reducing the heat resistance of the bacteria examined in this work. At pH 7, an increase in ethanol content up to 10% caused D values to decrease by up to 100-fold (S. typhimurium). A drop from pH 7 to pH 3 also caused up to a 100-fold reduction in the D values (S. typhimurium). For B. cereus the regression analysis of the log10 of the D value in relation to temperature, pH and ethanol content was used to produce a second order polynomial equation. The z values increased at decreasing pH and cells were more sensitive to ethanol at lower pH. For S. typhimurium the polynomial equation produced to describe the relationship between log10 of the D values and temperature and ethanol content was also a second order equation indicating that the relationship between z values and ethanol was non-linear. For L. delbrueckii, z values were independent of the ethanol content of the heating medium. Acid tolerance at 25 and 37 degrees C (L. delbrueckii and S. typhimurium) and acid adaptation (S. typhimurium) were also tested. L. delbrueckii was more ethanol and pH tolerant than S. typhimurium at heat treatment temperatures whilst S. typhimurium was more acid tolerant than L. delbrueckii at incubation temperatures (25 and 37 degrees C). Acid adaptation increased the tolerance of S. typhimurium to low pH at 25 degrees C but failed to improve its thermal resistance at 48 degrees C. In all bacteria used, the effects of pH and ethanol were more evident at lower treatment temperatures and therefore their significance becomes greater in view of reduced thermal processing and/or changes in product formulation.     

Thermal resistance parameters for pathogens in white grape juice concentrate

The heat resistance of Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes that were in stationary phase, had been exposed to high osmotic pressure, or were acid adapted was evaluated in white grape juice concentrate (58 degrees Brix, pH 3.3). The most heat-resistant cells of all three pathogens were those exposed to high osmotic pressure or in stationary phase. Unlike in single-strength juices, in concentrate the acid-adapted cells for all three pathogens were less heat resistant than were cells in the other physiological states. E. coli O157:H7 had the highest heat resistance for all temperatures tested (e.g., D62 degrees C = 1.8 +/- 0.3 min, with a z-value of 9.9 +/- 0.6 degrees C). L. monocytogenes exposed to high osmotic pressure had the highest z-value (12.3 +/- 1.2 degrees C), although its D-values for all temperatures tested were lower (e.g., D62 degrees C = 0.93 +/- 0.1 min) than those for E. coli O157:H7. Salmonella was the most sensitive of the pathogens under all conditions. Based on the results obtained in this study, one example of a heat treatment that will inactivate 5 log units of all three pathogens in white grape juice concentrate was calculated as 1.5 min at 71.1 degrees C (z = 10.3 degrees C). Validation studies confirmed the predicted D71 degrees C for E. coli O157:H7 exposed to high osmotic pressure.     

Thermal Inactivation of Clostridium botulinum Toxin Types F and G in Buffer and in Beef and Mushroom Patties

The time-temperature relationships for the inactivation of crude toxins prepared from Clostridium botulinum type F strain Wall 8 and type G strain G89 were determined in three different heating menstrua. Toxins dilated to 7,700–32,000 mouse LD₅₀ units/0.5 ml in beef and mushroom patties (pH 6.05), in 0.1M phosphate buffer (pH 6.05), and in 00.1M acetate buffer (pH 5.0) were heated at temperatures from 65.6–76.7°C. In the beef patties, inactivation times ranged from an average of 108.1–1.6 min for type F toxin, and from an average of 158.8–1.8 min for type G toxin. Less time was required in phosphate buffer.     

Characterization of a wild strain of Alicyclobacillus acidoterrestris: heat resistance and implications for tomato juice

This article reports the characterization of a wild strain of Alicyclobacillus acidoterrestris and describes the implications of the heat resistance of this microorganism in tomato juice. The strain (labeled as A. acidoterrestrisγ4) showed pH and temperature ranges for growth typical of the species (3.0 to 6.0 for the pH and 35 to 60 °C for the temperature); heat resistance in tomato juice was as follows: D(T) values of 40.65, 9.47, and 1.5 min (at 85, 90, and 95 °C, respectively) and z-value of 7 °C. A treatment at 70 °C for 15 min was found to be optimal for spore activation, whereas Malt Extract Agar, acidified to pH 4.5, showed good results for spore recovery. Concerning the implications of heat resistance of A. acidoterrestris on tomato juice, high temperatures required for spore inactivation determined a general decrease of the antioxidant activity (increase of the redox potential and reduction of the chain-breaking activity), but not the formation of brown compounds (namely, hydroxymethylfurfural), thus suggesting an effect on the secondary antioxidants (carotenoids and ascorbic acid) rather than on lycopene. PRACTICAL APPLICATION: Alicyclobacillus acidoterrestris is an emerging spore-forming microorganism, capable of causing spoilage in tomato juice. Due to their high thermal resistance, spores could be used as targets for the optimization of heat processing; this article reports on the assessment of thermal resistance of a wild strain of A. acidoterrestris, then focusing on the effect of the thermal treatment necessary to inactivate spores on the quality of tomato juice.     

Changes in heat resistance resulting from pH and nutritional shifts of acid-adapted and non-acid-adapted Listeria monocytogenes Scott A

Stationary-phase Listeria monocytogenes cells that were either pH dependent acid adapted or not acid adapted were heat challenged at 60 degrees C in a two-level full factorial design for three variables. The three variables and the levels consisted of tryptic soy broth (TSB) and sterile cell-free culture supernatant (sterile TSB), the presence and absence of 1% added glucose, and pH 4.8 and pH 7. Non-acid-adapted cells were most heat resistant when challenged in TSB (mean decimal reduction times at 60 degrees C: D60 = 1.16 min). In the absence of added glucose, non-acid-adapted cells had similar D60-values for inactivations at pH 4.8 and pH 7; however, the presence of glucose caused non-acid-adapted cells challenged at pH 4.8 to be more heat sensitive (D60 = 0.65 min) than those inactivated at pH 7 (D60 = 1.03 min), indicating an interaction between glucose and pH. Overall, the significantly decreased heat resistance of the acid-adapted cells was due to the presence of glucose (D60 = 0.78 min without glucose, D60 = 0.59 min with glucose). Acid-adapted cells heat challenged in TSB had similar D60-values for inactivations at pH 4.8 and pH 7; however, acid-adapted cells in sterile TSB challenged at pH 4.8 (D60 = 0.52 min) had significantly lower heat resistance than did cells challenged at pH 7 (D60 = 0.76 min), indicating an interaction between the medium and pH. The L. monocytogenes survivor data were modeled to extract information on the frequency distribution of heat resistance within heat-challenged populations, and the frequency distribution characteristics of mean, mode, and variance were compared among treatment conditions. Significant differences in the frequency distribution data were compared with the D60-values. These data indicated that the presence and level of cross-protection is highly dependent on the physiological state of the cells and nutrient availability at the time of heat challenge. Such conditions should be considered to ensure that stressed pathogens in foods are destroyed or inactivated.     

Combined effects of heat, nisin and acidification on the inactivation of Clostridium sporogenes spores in carrot-alginate particles: from kinetics to process validation

Combined effects of mild temperatures, acidification and nisin on the thermal resistance of Clostridium sporogenes ATCC 11437 spores were assessed. Inoculated carrot-alginate particles were used as a solid-food model for the validation of the spore inactivation during the flow of a solid-liquid food system through the holding tube of an aseptic processing unit. Inactivation kinetics was studied in a water bath with the spores inoculated into carrot-alginate particles and in Sorensen's phosphate buffer. For temperatures of 70-90 degrees C, D-values in the buffer were 24.9-5.7 min, much lower than those evaluated for the particles (115.1-22.2 min). Statistical analyses showed significant synergistic effects of temperature and pH on spore inactivation for both media. Acidification reduced the heat resistance of the spores by reducing the D-values. Nisin was not significantly effective at the lower concentrations (up to 750 IU/g). The combination of 90 degrees C, pH: 4.5 and 500IU/g nisin resulted in a ten-fold decrease of the D-value for spores inoculated in the particles (from 111.1 to 10.6 min). Microbial validation tests were conducted using a pilot-scale aseptic processing unit with a mixture of carrot cubes (10%) and carrier liquid of 2%-carboxymethylcellulose solution (90%). Spore-inoculated carrot-alginate particles (initial counts of 106 CFU/g, obtained after come-up-time pre-heat) with pH 3.5 and 2000 IU/g nisin were processed at 90 degrees C in the aseptic processing unit. Microbial analysis showed no spore survivors in the particles after passing through the holding tube (5.2-6.0 min of residence time). The proposed combination of these hurdles significantly enhanced the spore inactivation rate (D(90)=1.17 min) as compared to that for thermal treatment only (D(90)=19.6 min).     

Relationship between the apparent heat resistance of Bacillus cereus spores and the pH and NaCl concentration of the recovery medium

Conventional heat resistance data, D values, were previously established by other workers at optimal condition for spores outgrowth. However, in canned food conditions of outgrowth are generally suboptimal in term of pH, salt concentration, water activity. The combined effects of pH and NaCl level of the recovery medium for the D value and z(pH) value were studied. Spores of Bacillus cereus were heated at 95 degrees C in phosphate-citrate buffer media at pH 7. Cells were recovered at 25 degrees C in Nutrient Agar with pH ranging from 5 to 7 and 1% to 4% (w/w) NaCl concentration. For each condition D' values (decimal reduction time associated with the recovery media characteristics) were determined. The results show a major influence of the recovery pH on the D' values. This effect is characterised by the z'(pH) values, distance of recovery medium pH from optimum recovery pH* medium (6.7) which leads to a tenfold reduction time of D value. The increase of the salt concentration leads to a slight decrease of D' value. However z'(pH) values are not significantly affected by the salt concentration. A simple three parameter model describing the effects of pH and NaCl concentration of the recovery medium upon the heat resistance of spores is proposed. The interaction between pH and salt concentration is sufficiently low to be neglected by the model.     

Thermal resistance of spores from virulent strains of Bacillus anthracis and potential surrogates

The objective of this study was to determine the thermal resistance of spores of Bacillus anthracis and potential surrogates. The heat resistance of spores suspended in buffer (pH 7.0 or 4.5), milk, or orange juice was determined at 70, 80, and 90 degrees C. D-values for B. anthracis strains Sterne, Vollum, and Pasteur ranged from < 1 min at 90 degrees C to approximately 200 min at 70 degrees C and were lower under acidic than under neutral conditions. The D-values for B. anthracis spores fell within the range obtained for spores from eight strains of Bacillus cereus, Bacillus thuringiensis, Bacillus mycoides, and Bacillus subtilis. However, there were significant differences (P < 0.001) among the D-values of the strains. The z-values in pH 7.0 buffer and milk averaged approximately 10.5 degrees C and were not significantly different among strains (P < 0.05). The z-values in pH 4.5 buffer and orange juice averaged 12.9 and 13.9 degrees C, respectively, significantly (P < 0.05) higher than those obtained in milk or in pH 7.0 buffer. The significance of this difference was driven by large differences among a few strains. The z-values for B. anthracis strain Pasteur were twice as high in the acid media than in the neutral media. This study confirms that B. anthracis spores are not unusually heat resistant and that spores from validated Bacillus species are appropriate surrogates for thermal resistance studies.     

Predicting thermal inactivation in media of different pH of Salmonella grown at different temperatures

The influence of the growth temperature and the pH of the heating medium on the heat resistance at different temperatures of Salmonella typhimurium ATCC 13311 was studied and described mathematically. The shift of the growth temperature from 10 to 37 degrees C increased heat resistance of S. typhimurium fourfold. The pH of the heating medium at which heat resistance was maximum was pH 6 for cells grown at 37 degrees C, but changed with growth temperature. The alkalinization of the heating medium from pH 6 to pH 7.7 decreased the heat resistance of cells grown at 37 degrees C by a factor of 3. Neither the growth temperature nor the pH modified the z values significantly (4.9 degrees C). The decimal reduction times at different treatment temperatures, in buffers of different pH of cells of S. typhimurium grown at different temperatures, were accurately described by a mathematical equation (correlation coefficient of 0.97). This equation was also tested for Salmonella senftenberg 775W (ATCC 43845) and Salmonella enteritidis ATCC 13076, strains in which the correlation coefficients between the observed and the theoretically calculated values were 0.91 and 0.98, respectively.     

Heat resistance of Listeria monocytogenes in vegetables: evaluation of blanching processes

The heat resistance of a Listeria monocytogenes composite (serotypes 1/2a, 1/2b, and 4b) was determined in fresh broccoli florets, sweet green peppers, onions, mushrooms, and peas using an end-point procedure in polyester pouches. The heat resistance of L. monocytogenes was higher in peas (D(60 degrees C) = 1.0 min) and mushrooms (D(60 degrees C) = 0.7 min) than in other vegetables tested (D(60 degrees C) in onions = 0.2 min) and was highest when cells were subjected to starvation before the thermal death time experiments (D(60 degrees C) of starved L. monocytogenes in mushrooms = 1.6 min). The results showed that blanching can be used as an antilisterial treatment (inactivation of 5 logs of L. monocytogenes) when the cold spot of vegetables is treated for at least 10 s at 75 degrees C or instantaneously (<1 s) at temperatures above 82 degrees C.