Relation of the heat resistance of salmonellae to the water activity of the environment.
01 Mar 1970-Applied and Environmental Microbiology (American Society for Microbiology)-Vol. 19, Iss: 3, pp 429-433
TL;DR: All strains of Salmonella tested showed a greater increase in heat resistance than S. senftenberg 775W as the environment became drier, and Sucrose afforded the cells a greater degree of protection than did fructose, glycerol, and sorbitol.
Abstract: The effect of water activity (a(w)) on the heat resistance of eight strains of Salmonella was studied Heat resistance of the organisms increased as the a(w) of the heating menstruum was reduced Sucrose afforded the cells a greater degree of protection than did fructose, glycerol, and sorbitol A direct correlation between a(w) and heat resistance could not be established over the range of a(w) levels tested in this study There was variation among the strains of salmonellae in the magnitude of the increase in heat resistance as the a(w) level was reduced All strains of Salmonella tested showed a greater increase in heat resistance than S senftenberg 775W as the environment became drier Washed cells had D values 25 to 75% lower than unwashed cells Prior growth of the organisms in media with a reduced a(w) increased the heat resistance of the organisms when glycerol, but not when sucrose, was the controlling substance
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TL;DR: Sources and risk factors for contamination, survival, persistence, and heat resistance of Salmonella in low-moisture foods are reviewed and care must be taken when applying published D- and z-values to a specific food process.
382 citations
Cites background from "Relation of the heat resistance of ..."
..., solutes used to decrease the water activity) (50, 60), or the microscopic air-water distribution in foods (59), might be as or more important as the water activity itself....
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...glycerol, glucose) may contribute to a better recovery of the cells injured by heat or desiccation (47, 50, 70, 86, 88, 101, 133)....
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...(50) stated that survival of the organism during heating is a function of a medium composition rather than water activity of the surrounding environment....
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...(50) examined the effect of various sugar and sugar-alcohol solutions on heat resistance by using several serotypes of Salmonella....
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...It has been suggested that the high fat content of chocolate may protect Salmonella cells against the action of gastric acid in the stomach, which allows the cells to colonize the lower gastrointestinal tract and produce clinical symptoms, even when a very small number of the cells is present in the product (31, 34, 50)....
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TL;DR: The heat resistance of Salmonella is highly influenced by the strain tested, the type of experiment (log reduction versus end-point), culture conditions prior to the experiment, heating menstruum, and recovery conditions.
293 citations
Cites background from "Relation of the heat resistance of ..."
...75 using glycerol also decreased the heat resistance of Salmonella Typhimurium (58)....
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...These effectively lower aw, partially dehydrate cells, and increase heat resistance (10, 27, 28, 55, 58, 72, 141)....
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TL;DR: In this paper, the effects of heating menstruum on heat sensitivity were investigated using a factorially-designed experiment comparing 45 conditions of salt concentration, pH value and temperature, and the results showed a significant and consistent deviation from log-linear kinetics, particularly at low temperatures.
Abstract: Thermal inactivation of microorganisms has traditionally been described as log-linear in nature, that is the reduction in log numbers of survivors decreases in a linear manner with time. This is despite a plethora of data that shows consistent deviations from such kinetics for a wide range of organisms and conditions and that cannot be accounted for by experimental artifacts. Existing thermal death models fail to take such deviations into account and also fail to quantify the effects of heating menstruum on heat sensitivity. The thermal inactivation ofListeria monocytogenes ATCC 19115 has been investigated using a factorially-designed experiment comparing 45 conditions of salt concentration, pH value and temperature. Heating was carried out using a Submerged Coil heating apparatus that minimized experimental artifacts. Low pH values increased, whilst high salt concentrations decreased heat sensitivity. Results showed a significant and consistent deviation from log-linear kinetics, particularly at low temperatures. A number of distributions were tested for suitability to describe the variability of heat sensitivity within the population of heated cells (vitalistic approach). The use of the logistic function and log dose (log time) allowed the development of an accurate unifying predictive model across the whole range of heating conditions. It is proposed that this approach should be tested as a generalized modeling technique for death kinetics of vegetative bacteria.
283 citations
TL;DR: It can be seen that most factors reported to have an effect on the D-value are smaller than the variability of all published D-values, which shows the main effects that have to be included for a first impression on the performance of a heating process.
250 citations
TL;DR: This document is intended for use by the food industry, including food processors, food service operators, and food retailers; federal, state, and local food safety regulators; public health officials; food testing laboratories; and process authorities.
215 citations
Cites background from "Relation of the heat resistance of ..."
...For thermal inactivation studies, lower moisture or aw levels should be used because pathogens may have increased heat resistance under these conditions (10, 24, 25, 38, 102)....
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References
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TL;DR: In this paper, the authors considered the effects of the status of the water in the solution or substrate on the growth of molds, yeasts, and bacteria and to what extent these findings are applicable to all microorganisms.
Abstract: Publisher Summary This chapter is concerned with the quantitative data that are available at present, and with propounding the view that the water requirements of microorganisms are best considered in terms of the activity of the water in the immediate environment of the organisms. Whether or not the substrate is liquid or solid, all microorganisms grow only in aqueous solutions. For intermediate conditions, growth of some organisms may occur in very dilute solutions such as tap water or in highly concentrated solutions. Therefore, the chapter considers some of the simple properties of aqueous solutions insofar as these bear on the availability of water to microorganisms. It presents methods for study of water requirements. It considers the effects of the status of the water in the solution or substrate on the growth of molds, yeasts, and bacteria and to what extent these findings are applicable to all microorganisms. The rate of mold growth on foods has also been discussed. The chapter discusses factors affecting water requirements such as nutrition, temperature, oxygen, pH, and adaptation. In discussing some applications in food preservation, fresh foods, dried foods, concentrated foods, frozen foods, and canned foods have been considered.
684 citations
"Relation of the heat resistance of ..." refers methods in this paper
...The concentrations of sucrose and glycerol necessary to achieve the desired a, levels were derived from the data of Scott (8)....
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TL;DR: In this article, an apparatus is described for determining the equilibrium relative humidity (e.r.h.) of confectionery syrups and the results obtained are compared with those of other workers.
Abstract: Summary
An apparatus is described for determining the equilibrium relative humidity (e.r.h.) of confectionery syrups and the results obtained are compared with those of other workers. The identity of e.r.h. with the thermodynamic activity is pointed out and an equation for the activity coefficient as a function of concentration is derived. The agreement between e.r.h. values calculated by means of this equation and experimental results is demonstrated and some of the advantages of the use of equations of this type are discussed, including the ability to draw significant conclusions about the nature of the solvent-solute interaction.
294 citations
TL;DR: The heat resistance of the spores of six species of bacteria varied with water activity at which the spores were heated, although the magnitude of the changes differed greatly between species.
Abstract: SUMMARY: The heat resistance of the spores of six species of bacteria varied with water activity (aw) at which the spores were heated, although the magnitude of the changes differed greatly between species. At all aw values there was an approximately linear relation between the logarithm of the number of viable spores and the time of heating. The slopes of these straight lines were used to describe the observed death-rates as the time (D value) required to decrease the population by one log. unit. For all six species the greatest heat resistance was manifest at aw values of about 0.2–0.4, the maximum D values at 110° now varying from about 2 to 24 hr. At aw values less than 0.2 the heat resistance decreased; for spores rigorously dried over P2O5 (0.00 aw) the D values at 110° now varied between about 30 sec. and 30 min. When the spores were heated at aw values above 0.4 the resistance of 4 species decreased considerably, being lowest at 1.00 aw; with spores of Bacillus coagulans and B. stearothermo-philus the heat resistance decreased less at the high aw values; at 1.00 aw their D values were slightly greater than at 0.00 aw. At the high aw values the D values at 110° varied from less than 0.1 sec. for Clostridiuum botulinum type E to about 40 min. for B. coagulans and B. stearothermophilus. The Q10 for thermal death was about 10 at high aw values, decreasing to about 2 at aw values below 0.3. Under very moist conditions spores of B. stearothermophilus were about 50,000 times more heat resistant than were spores of C. botulinum type E; but at aw values less than about 0.5 this ratio fell to about 10. The convergence of this ratio resulted from more than a 100,000-fold increase in the resistance of the type E spores, and only a 20-fold increase in the resistance of the spores of B. stearothermophilus.
184 citations
TL;DR: Of approximately 300 cultures of Salmonella, representing 75 different serotypes, none was found to be as heat-resistant as S. senftenberg 775W; however, S. blockley 2004 was 5 times more heat- resistant and S.senftenberg775W was 30 times moreHeat-resistant than S. typhimurium Tm-1, the reference strain in this study.
Abstract: Of approximately 300 cultures of Salmonella, representing 75 different serotypes, none was found to be as heat-resistant as S. senftenberg 775W. However, S. blockley 2004 was 5 times more heat-resistant and S. senftenberg 775W was 30 times more heat-resistant than S. typhimurium Tm-1, the reference strain in this study. All other strains of Salmonella tested, including 19 strains of S. senftenberg and 7 strains of S. blockley, had decimal reduction times at 57 C of about 1 min, equivalent to that of the reference organism, Tm-1. As observed in other bacterial species, strain 775W is more heat-sensitive in the log phase than in the stationary phase of growth. Cells from cultures grown at 44 C were more heat-resistant than those grown at either 35 or 15 C; the medium of growth, whether minimal or complex, made no appreciable difference in heat resistance. Cells from cultures limited by a carbon source were killed at a much slower rate than those limited by a nitrogen source and exhibited a 1-hr lag at 55 C before a significant rate of kill was attained. For any given set of growth conditions, strain 775W was always more heat-resistant than another strain of S. senftenberg, 197B, which has normal heat resistance.
167 citations
"Relation of the heat resistance of ..." refers background in this paper
...Recently, Ng and co-workers (5) studied the heat resistance of several serotypes of Salmonella....
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TL;DR: The results of experiments performed to determine the heat resistance of Salmonella in miLk and dark chocolate are presented and viable salmonellae were enumerated by the threetube most probable number technique.
Abstract: The problem of Salmonella contamination in dried foods has underscored our lack of knowledge concerning the heat resistance of enteric microorganisms in dry and semidry environments. Confectionery manufacturing is one of the food industries concerned with dry-heat destruction of Salmonella. The discovery of these organisms in several chocolate-containing products in the recent past has raised the question of whether they can be killed by heating the molten chocolate prior to casting. The results of experiments performed to determine the heat resistance of Salmonella in miLk and dark chocolate are presented in this paper. S. typhimurium and S. senftenberg 775W were inoculated into milk chocolate in the following manner. The cells resulting from 24-hr growth in 600 ml of Trypticase Soy Broth (BBL) incubated statically at 37 C were harvested centrifugally, resuspended in 20 ml of sterile skimmed milk, and lyophilized. Commercially formulated milk chocolate was dispensed in Sorvall Omnimixer cups in 100-g quantities and warmed to the test temperature in an oil bath. The lyophilized cells were added to the molten chocolate and blended for 1 min at top speed on the Omnimixer. The cups were returned to the oil bath and immersed so that the level of the chocolate was /4 (1.9 cm) inch below the surface of the oil. Agitation of the inoculated chocolate throughout the remainder of the experiment was accomplished by a mechanical stirrer. Periodically, 1-ml samples were taken by using wide-bore pipettes and a vacuum-pressure pump. Dilutions were made in sterile 30% sucrose, and viable salmonellae were enumerated by the threetube most probable number technique. Nutrient broth was used as the recovery medium, and tubes showing turbidity after 48 hr at 37 C were streaked on MacConkey Agar (Difco) and incubated at 37 C. Isolates were confirmed biochemically and serologically. The D values obtained in replicate trials at 70, 80, and 90 C are presented in Table 1. The averages of the D values are plotted against
98 citations
"Relation of the heat resistance of ..." refers background in this paper
...Goepfert and Biggie (3) and Riemann (7) demonstrated that S....
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