Behavior of Yersinia enterocolitica and Salmonella typhimurium in Crottin goat's cheese

In order to evaluate the behavior of Yersinia enterocolitica and Salmonella typhimurium in Crottin goat's cheese, inoculated products stored at 5, 15 and 25 degrees C were analysed together with chemical and microbiological characteristics of the cheese. In general, low counts of microorganisms were detected. None of the samples showed the presence of Escherichia coli, Salmonella spp. or Y. enterocolitica. In the inoculation tests, Y. enterocolitica and S. typhimurium were inhibited during storage; nevertheless, these bacteria survived for extensive periods. The counts at the end of the experiments at 5 and 15 degrees C were high, indicating that contamination with high bacterial numbers represents a potential health hazard. The primary mathematical models used to analyse the behavior of Y. enterocolitica and S. typhimurium were the Vitalistic, Gompertz's empirical and Churchill's model. The mean square error was calculated for the three models in order to evaluate the goodness-of-fit of each one. For Y. enterocolitica, the Vitalistic model was the best at the three temperatures. For S. typhimurium, there was no significant difference between the three models at 5 and 15 degrees C; the Churchill model was clearly the best at 25 degrees C. These results confirm that, in order to predict the risk of transmission of pathogenic microorganisms in foods using mathematical models, it is essential to analyse their behavior in specific foods.     

Antagonistic action of Lactobacillus delbrueckii ssp. lactis RM2-5 toward spoilage organisms in cottage cheese

Cells of Lactobacillus delbrueckii ssp. lactis RM2-5 were added to cottage cheese stored at 7 degrees C in different amounts to determine if they would inhibit the growth of Pseudomonas fluorescens, also inoculated into the cheese samples. In addition, experiments were conducted in which no spoilage organisms were added to determine the effect of the lactobacilli on the natural background flora in the cottage cheese. For most experiments, as the numbers of lactobacilli increased, the numbers of spoilage organisms were lower than in the control on any given day of storage. In cheese inoculated with P. fluorescens, the numbers of spoilage organisms in the control had increased 5 log cycles by d 7, whereas the treatment containing the highest level of L. delbrueckii ssp. lactis RM2-5 (1.0 x 10(9) cfu/g) had not, and did not, increase over the course of the 21-d study. In the experiments where no spoilage organisms were added, lactobacilli significantly retarded the growth of gram-negative bacteria in the cheese. However, in these experiments, mold growth on the samples became a limiting factor during extended storage. The results of these experiments indicate that lactobacilli could be effective at helping control gram-negative spoilage bacteria in cottage cheese, thus potentially extending its shelf life.     

COMPARISON OF ANTIMICROBIAL ACTIONS OF MONOLAURIN AND SORBIC ACID

The antimicrobial effects of monolaurin and sorbic acid were compared in laboratory media, cottage cheese and a pork homogenate. Sorbic acid (250–1000 ppm) was more effective than monolaurin (250–1000 ppm) in inhibiting the growth of Staphylococcus aureus S-6 and Salmonella enteritidis 13076 in pH 5.2 trypticase soy broth. Monolaurin was more effective than sorbic acid against the growth of S. typhimurium 13311 under similar conditions. No synergistic effects were observed when combinations of monolaurin and sorbic acid were tested. In the cottage cheese study, 800 ppm sorbic acid effectively inhibited the growth of coliforms and yeasts and molds, but only slightly inhibited the growth of psychrotrophs. Monolaurin at 800 ppm was ineffective in controlling the growth of any of the microorganisms tested. The use of 3000 ppm monolaurin in a pork homogenate had no effect on the outgrowth and gas formation by Clostridium sporogenes PA3679, whereas 3000 ppm sorbic acid delayed outgrowth and gas formation. Combinations of monolalurin and sorbic acid indicated that sorbic acid was the active anticlostridial agent.     

Influence of cooling temperatures on aflatoxin formation in milk products (author's transl)

In the literature several contradictionary results have been published on the aflatoxin formation at temperatures below 10 degrees C. Therefore experiments with pastes made from milk and cheese powder artificially contaminated with Aspergillus parasiticus, were performed at temperatures of 1 degree C, 5 degrees C, and 10 degrees C for 28 days at a relative humidity of 90--95%. Even at 1 degree C, the aflatoxins B1, B2, G1, G2, and M1 could be determined quantitatively. The lactose content did not have a significant influence on the aflatoxin values. Even storage of cheese (camembert and cottage cheese) in a 10% salt solution did not inhibit aflatoxin formation at 20 degrees C.     

Survival of Salmonella typhimurium and Escherichia coli O157:H7 in cheese brines

Survival of Salmonella typhimurium and Escherichia coli O157:H7 was studied in model brines and brine from three cheese plants. Three strain mixtures of S. typhimurium and E. coli O157:H7 (10(6) CFU/ml) were inoculated separately into 23% model brine with or without added pasteurized whey (2%) and as a combined inoculum into the commercial brines. The model brines were incubated at 8 and 15 degrees C for 28 days, and the commercial brines at 4 and 13 degrees C for 35 days. Populations of both pathogens in the model brine + whey decreased slowly over 28 days (1.0-2.0 log CFU/ml) with greater survival at 8 degrees C than at 15 degrees C. Corresponding decreases in model brine without whey were 1.9-3.0 log CFU/ml, with greater survival at 8 degrees C than at 15 degrees C. Both S. typhimurium and E. coli O157:H7 survived significantly better (P < 0.05) at 4 degrees C than at 13 degrees C in two of the commercial brines. The survival of each pathogen in the commercial brines at 13 degrees C was significantly influenced by brine pH. Both pathogen populations decreased most rapidly in commercial brines during the first week of storage (2.5-4.0 and 2.3-2.8 log CFU/ml for S. typhimurium and E. coli O157:H7, respectively) with significant recovery (ca. 0.5 log CFU/ml increase) often occurring in the second week of storage. Counts changed little thereafter. Overall, E. coli O157:H7 survived better than S. typhimurium, with differences of 0.1-1.2 log CFU/ml between the two pathogens. Results of this study show that cheese brine could support the survival of contaminating S. typhimurium and E. coli O157:H7 for several weeks under typical brining conditions.     

Comparison between the growth of Yersinia enterocolitica, listeria monocytogenes, escherichia coli O157:H7 and salmonella spp. in ground beef packed by three commercially used packaging techniques

Growth of the pathogens Yersinia enterocolitica, Listeria monocytogenes, Escherichia coli O157:H7 and strains of Salmonella were compared in ground beef packed in modified atmospheres of 60% CO2/40% N2/0.4% CO (high CO2/low CO mixture), 70% O2/30% CO2 (high O2 mixture) and in chub packs (stuffed in plastic casings). The ground beef was inoculated with rifampicin-resistant or nalidixic acid/streptomycin-resistant strains of the pathogens (final concentration 10(2) - 10(3) bacteria/g) and stored at 4 and 10 degrees C for up to 14 days. At 4 degrees C the shelf life, based on colour stability and background flora development, was prolonged for the high CO2/low CO mixture compared to the two other packaging methods, but at 10 degrees C the shelf life was < 8 days for all the packaging methods. Growth of Y. enterocolitica was nearly totally inhibited both at 4 and 10 degrees C in the high CO2/low CO mixture, while the bacterial numbers in the samples packed in the high O2 mixture increased from about 5 x 10(2) bacteria/g at day 0 to about 10(4) at day 5 at 4 degrees C and to 10(5) at 10 degrees C. Growth in the chub packs was even higher. L. monocytogenes showed very little growth at 4 degrees C in all treatments. At 10 degrees C there was slow growth from about 5 x 10(3) bacteria/g to about 10(4) at day 5 in the high CO2/low CO mixture, while the numbers in the high O2 mixture and the chub packs were about 10 times higher. Growth of E. coli O157:H7 at 10 degrees C in the ground beef was nearly totally inhibited in both the high CO2/low CO mixture and the high O2 mixture. Growth in the chub packs was higher, as the number of bacteria increased 3 log in 5 days. The Salmonella strains (S. typhimurium, S. dublin, S. enteritidis and S. enterica 61:k:1,5,(7)) in the ground beef stored at 10 degrees C for 5 and 7 days grew to a higher number in the high CO2/low CO mixture than in the high O2 mixture. This study shows that the growth of Y. enterocolitica and L. mononcytogenes in ground beef stored in the high CO2 /low CO mixture was not increased as a result of prolonging the shelf life. However, the observed growth of strains of Salmonella at 10 degrees C in this mixture and in chub packs does emphasise the importance of temperature control during storage.     

Microbiological changes in cottage cheese varieties during storage at +7°C

Studies have been made of microbiological changes in cottage cheese varieties during storage at 7°C. Commercially prepared products obtained directly from three manufacturers were at an initial pH in the range 4·6 to 5·1 and differed significantly in their content of sorbic acid/sorbate and of viable lactic streptococci. In many of the varieties that did not contain sorbic acid/sorbate multiplication occurred of Pseudomonas spp., Enterobacteriaceae and yeasts; spoilage occurred in some batches due to formation of a surface film of Pseudomonas fluorescens or of surface colonies of Sporobolomyces roseus and Trichosporon sp. at times equal to, or slightly greater than the recommended storage life. Where high numbers of viable lactic streptococci were present in varieties immediately after manufacture, no marked decrease in pH or increase in titratable acidity was observed during storage at 7°C. In varieties that contained sorbic acid at a concentration higher than approximately 500 mg kg^?1 little increase in numbers of bacteria or yeasts was observed during storage at 7°C. Decrease in quality due to separation of whey occurred in some products from each manufacturer, but no clear correlation was observed between separation of whey and pH or the number of lactic streptococci present.Bacteria of public health significance, Escherichia coli type I, Staphylococcus aureus and Vibrio spp. were not detected in these products.Fifty-nine samples produced by six manufacturers, purchased from retail outlets, were examined on the sell-by date; the results confirmed the conclusions drawn from studies during storage.     

The effect of temperature on the growth and persistence of Salmonella in fermented liquid pig feed

Two studies were conducted to investigate the effect of temperature on the fate of Salmonella enterica subsp. enterica serovar typhimurium DT104:30 in fermented liquid pig feed. These were (1) by co-inoculation of feed with S. typhimurium DT104:30 and Pediococcus pentosaceus, as the fermenting organism, and (2) by fermenting feed for 48, 72 or 96 h prior to inoculation with S. typhimurium DT104:30. In co-inoculated feed incubated at 20 degrees C, S. typhimurium DT104:30 persisted for at least 72 h. In contrast, in feed incubated at 30 degrees C, no S. typhimurium DT104:30 were detectable 48 h after inoculation. In prefermented feed, S. typhimurium DT104:30 died four to five times faster in feed maintained at 30 degrees C (D(value) 34-45 min) compared with feed maintained at 20 degrees C (D(value) 137-250 min). This was not entirely due to differences in lactic acid concentration as feed fermented for 72 or 96 h at 20 degrees C and feed fermented for 48 h at 30 degrees C contained similar concentrations of lactic acid (160-170 mM). Low numbers of S. typhimurium DT104:30 were still detectable in fermented feed 24 h after inoculation at 20 degrees C. In contrast, none were detectable 6-7 h after inoculation at 30 degrees C. The results of these studies indicate that it would be advisable for pig producers to control the temperature of liquid feed tanks to reduce the risk of Salmonella contamination.     

Modified Atmosphere Packaging to Maintain Direct-Set Cottage Cheese Quality

Direct-set cottage cheese packaged in barrier containers was flushed with 100% CO₂ 75% CO₂:25% N₂, 100% N_2, or air, and stored at 4°C for 28 days. Quality was assessed by sensory, microbiological, and chemical tests. No change was observed in headspace gas composition during storage. Psychrotrophic and lactic acid bacteria counts increased for air-treated samples, but counts for cottage cheese packaged under modified atmospheres remained unchanged. Product discoloration was not observed. Acidity increased over storage life, but lactic acid did not contribute towards increased acidity. Sensory characteristics of cottage cheese packaged under modified atmospheres remained satisfactory after 28 days, with 100% CO₂ best.     

Fate of unirradiated Salmonella in irradiated mechanically deboned chicken meat.

Mechanically deboned chicken meat was irradiated at 0, 1.25 and 2.50 kGy (Cesium 137) and inoculated with Salmonella dublin ATCC 15480, Salmonella enteritidis ATCC 9186 or Salmonella typhimurium ATCC 14028. Samples were then stored at 5 degrees C and 10 degrees C and were subjected to microbiological analysis directly after irradiation and inoculation (time 0), and after 24, 72, 120, 168 and 216 h of storage. Samples stored at 20 degrees C were examined at time 0 and after 6, 12 and 24 h of storage. Irradiation at 1.25 and 2.50 kGy caused an average reduction in bacterial levels of 2.23 and 3.44 logs, respectively. S. dublin, S. enteritidis and S. typhimurium showed very small, insignificant changes in numbers, during storage of meat for 9 days at 5 degrees C. The final populations of S. dublin and S. typhimurium in samples irradiated before inoculation and stored at 10 degrees C or 20 degrees C were greater than the equivalent populations in samples which had not been irradiated before inoculation. Reduction of indigenous microflora in mechanically deboned chicken meat by irradiation may create better conditions for the growth of salmonellae and may thus increase the risk of salmonellosis when accidental contamination and temperature abuse occur after a radiation treatment. Therefore, irradiated mechanically deboned chicken meat should be properly refrigerated and protected against contamination.     

Comparative survival of Salmonella typhimurium DT 104, Listeria monocytogenes, and Escherichia coli O157:H7 in preservative-free apple cider and simulated gastric fluid

This study compared the survival of three-strain mixtures (ca. 10(7) CFU ml(-1) each) of Salmonella typhimurium DT104, Listeria monocytogenes, and Escherichia coli O157:H7 in pasteurized and unpasteurized preservative-free apple cider (pH 3.3-3.5) during storage at 4 and 10 degrees C for up to 21 days. S. typhimurium DT104 populations decreased by <4.5 log10 CFU ml(-1) during 14 days storage at 4 and 10 degrees C in pasteurized cider, and by > or =5.5 log10 CFU ml(-1) during 14 days in unpasteurized cider stored at these temperatures. However, after 7 days at 4 degrees C, the S. typhimurium DT104 populations had decreased by only about 2.5 log10 CFU ml(-1) in both pasteurized and unpasteurized cider. Listeria monocytogenes populations decreased below the plating detection limit (10 CFU ml(-1)) within 2 days under all conditions tested. Survival of E. coli O157:H7 was similar to that of S. typhimurium DT104 in pasteurized cider at both 4 and 10 degrees C over the 21-days storage period, but E. coli O157:H7 survived better (ca. 5.0 log10 CFU ml(-1) decrease) than S. typhimurium DT104 (> 7.0 log10 CFU ml(-1) decrease) after 14 days at 4 degrees C in unpasteurized cider. In related experiments, when incubated in simulated gastric fluid (pH 1.5) at 37 degrees C, S. typhimurium DT104 and L. monocytogenes were eliminated (5.5-6.0 log10 CFU ml(-1) decrease) within 5 and 30 min, respectively, whereas E. coli O157:H7 concentrations decreased only 1.60-2.80 log10 CFU ml(-1) within 2 h.     

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

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

Behavior of Yersinia enterocolitica strains inoculated in model cheese treated with high hydrostatic pressure

The effects of high hydrostatic pressure treatment and the ability for survival, repair, and growth of three human pathogenic serotypes (O:1, O:3, O:8) of Yersinia enterocolitica were investigated in washed-curd model cheese made with pasteurized bovine milk. Samples were treated at 300, 400, and 500 MPa for 10 min at 20 degrees C and analyzed at 0, 1, 7, and 15 days to assess the viability of the Yersinia population. A long-term study (up to 60 days of ripening after high hydrostatic pressure treatment) was also undertaken. Treatments at 400 and 500 MPa caused maximum lethality, and only the treatment at 300 MPa showed significant differences (P < 0.05) between serotypes; the most baroresistant was O:3. Ability to repair and grow was not observed after 15 days of storage at 8 degrees C. Yersinia counts in untreated cheese samples also decreased below the detection limit at day 45 in the long-term study. These results suggest that the cheese environment did not allow recovery of injured cells or growth. A primary contributing factor to this effect seemed to be the low pH resulting from the production of lactic acid during cheese ripening.     

The influence of Lactobacillus plantarum culture inoculation on the fate of Staphylococcus aureus and Salmonella typhimurium in Montasio cheese.

The growth and survival of Staphylococcus aureus and Salmonella typhimurium were investigated during the manufacturing and ripening of raw milk Montasio cheese. Initial inoculated populations in the cheese milk were about 10(5) cfu/ml for S. aureus and 10(6) cfu/ml for S. typhimurium. Samples of curds and cheeses were taken during manufacturing and storage and analysed for pH and microbial populations. S. aureus increased slightly in number during the early period of ripening and attained a population of about 10(6) cfu/ml during the remaining period of storage. S. typhimurium decreased during cheesemaking and storage but persisted through 90 days. The addition of Lactobacillus plantarum culture (0.2% v/v) produced a marked reduction in populations of the test strains in 10 days of storage. Enterotoxin A was not detected in Montasio cheese even with a S. aureus population of 1.1 X 10(7) cfu/ml. L. plantarum strains were also tested by the spot method and the associative growth approach for their antagonistic activity against S. aureus and S. typhimurium. The compound excreted by L. plantarum was active only toward S. aureus. Furthermore, its activity was destroyed by protease treatment. These results indicated that while the growth of S. typhimurium is reduced by the acid production, S. aureus inhibition can be ascribed to bacteriocin production.     

Influence of probiotic strains added to cottage cheese on generation of potentially antioxidant peptides,anti-listerial activity,and survival of probiotic microorganisms in simulated gastrointestinal conditions

Our objective was to evaluate the viability of probiotic microorganisms added to cottage cheese under simulated gastrointestinal conditions, the release of potentially-antioxidant peptides, and their antimicrobial effect on Listeria monocytogenes. Cottage cheeses were prepared in triplicate, incorporating Lactobacillus casei, Lactobacillus rhamnosus GG, the commercial mix YO-MIX™ 205, or a control without probiotic addition. The probiotic population remained at >10⁶ cfu g⁻¹ during 28 days of storage at 8 °C. Cheeses made with added probiotics showed an increased metabolic activity with higher levels of lactic and acetic acids. Higher numbers of potentially bioactive peptides were observed in cheeses added with probiotics. L. monocytogenes population was reduced by about one log cycle after 20 days of storage, in cheeses with probiotics added. Our results indicate that cottage cheese is a good vehicle for probiotic bacteria.     

Prevalence and behaviour of Bacillus cereus in a REPFED of Italian origin

This paper reports on the prevalence and behaviour of Bacillus cereus in gnocchi, a REPFED of Italian origin. A survey of gnocchi under varying storage conditions, revealed that, although B. cereus was found in 33% of the samples, the contamination level was lower than 10(2) CFU/g for the unstored and the refrigerated (8 degrees C) samples. Counts increased with increasing storage temperatures and prolonged storage times in samples prepared without sorbic acid. The effect of different formulations (sorbic, citric and lactic acid) and storage conditions (8, 12 and 20 degrees C) have been evaluated in a challenge testing with spores of B. cereus. Results indicate that the use of sorbic acid in association with citric or lactic acid to pH 5.0 is effective in inhibiting growth of B. cereus and the anticipated shelf life of the product is safe even if temperature abuse occurs. If sorbic acid is omitted, lactic acid can inhibit B. cereus growth during storage at 8 degrees C. On the contrary, when temperature abuse occurs (12 and 20 degrees C), lactic or citric acid are not able to prevent growth of B. cereus.     

Stability of aflatoxin M in milk.

This three-part study was designed to determine aflatoxin M recovery from pasteurized and/or stored cow's milk. (a) Aflatoxin M was added to samples of raw Holstein milk at a concentration of 2.0 mug/liter. Half of each sample then was pasteurized at 63 C for 30 min, and both raw and pasteurized portions were stored at 4 C up to 17 days. (b) Samples of raw milk, pasteurized (77 C, 16 s) skim milk, dry cottage cheese curd, and cottage cheese whey were taken from a commercial operation in an area in which natural contamination had been encountered. (c) Milk from a cow dosed with aflatoxin B1 was stored frozen (-18 C) in bulk and in assay-size sample containers for 120 days. Aflatoxin M was recovered completely after either storage or pasteurization in (a) and (b). In (c), a recovery deficiency was detectable after 68 days of storage, which increased to 45% of the original value by 120 days. These observations differ from those of others in that loss of aflatoxin M was significant after pasteurization or storage of raw milk, totaling 87% loss after 120 days of frozen storage. Aflatoxin M partitioning between curd and whey in the preparation of cottage cheese agrees with more recent studies, but differs from previous reports. Three possible explanations for the differences are offered.     

Survival and acid resistance of Listeria innocua in Feta cheese and yogurt, in the presence or absence of fungi

The objective of the study was to assess the survival of Listeria innocua, alone or coinoculated with fungal isolates, during storage of Feta cheese (pH 4.43 to 4.56) and yogurt (pH 4.01 to 4.27) at 3 to 15 degrees C. The acid resistance of the bacterium during subsequent exposure to pH 2.5 for 3 h was also evaluated in samples stored at 3 and 10 degrees C. In Feta cheese, L. innocua survived better than it did in yogurt at all temperatures. At 5, 10, and 15 degrees C, the pH of cheese increased due to fungal growth, and this enhanced the survival of L. innocua more than during storage at 3 degrees C. Moreover, during storage of Feta cheese, L. innocua was capable of surviving the subsequent exposure for 3 h in broth of pH 2.5, in contrast to cultures not inoculated in the product (control cultures; 24 h at 30 degrees C in broth). In yogurt, L. innocua reduced more than 5 log within 15 days of storage at 5, 10, and 15 degrees C, whereas extended survival was observed at 3 degrees C until day 22, with total reduction of approximately 4.5 log. In contrast to what was observed in Feta cheese, surviving populations of L. innocua in yogurt were eliminated after subsequent exposure for 3 h to pH 2.5. The findings indicate that growth of fungi on the surface of Feta cheese and yogurt may compromise the safety of these products by enhancing survival of the bacterium. Particularly, when fungi increase the pH of Feta cheese, L. innocua demonstrates better survival and prolonged storage may raise concerns for the development of acid-resistant Listeria populations.     

Growth of pathogenic Yersinia enterocolitica strains in minced meat with and without protective gas with consideration of the competitive background flora

The growth of two pathogenic Yersinia enterocolitica strains (O:3, O:9 at 1 degree C, 4 degrees C, 10 degrees C and 15 degrees C) in minced meat produced under sterile conditions and in minced meat with a normal background flora was investigated. In addition, the influence of treatment with a protective gas (20% CO2, 80% O2) was tested. In minced meat produced under sterile conditions and incubated under normal atmospheric conditions, Y. enterocolitica increased in numbers by only 1 log at 1 degree C and 3.5 logs at 4 degrees C within 14 days. At 10 and 15 degrees C, there was about a 5-log increase in cell numbers within 5 days. There was a marked inhibition of growth by the background flora on Y. enterocolitica. This effect was so marked that only a slight additional inhibition on the growth of Y. enterocolitica at 1, 4 and 10 degrees C was observed under the CO2:O2 modified atmosphere. This is in contrast to the situation where minced meat with a low total bacterial count was used, and the growth of Y. enterocolitica was adversely affected by the CO2:O2 environment. The significance of raw minced meat in the development of human Y. enterocolitica infections is also discussed.     

The effect of pH on the initiation of growth of cottage cheese spoilage bacteria

The influence of pH on strains of Pseudomonas spp. and Enterobacter agglomerans that cause spoilage of cottage cheese varieties during storage at 7 degrees C has been investigated. In a culture medium adjusted to the required pH with HCl, 57 of 64 strains of Pseudomonas spp. grew at pH 4.8 when incubated at 7 degrees C but a very low proportion of strains grew at pH 4.7 or pH 4.6 and none at pH 4.5. At 20 degrees C some of the pseudomonads grew at pH 4.4. Three out of nine strains of E. agglomerans grew at pH 3.8 when incubated at 7 degrees C and at pH 3.6 when incubated at 20 degrees C. In cultures of E. agglomerans at controlled pH and 20 degrees C, after a lag phase the doubling time at pH 4.1 was 2 h, and at pH 7.0 was 1.4 h.