Survival of Listeria monocytogenes during the manufacture and ripening of Turkish white cheese

Listeria monocytogenes was enumerated during the manufacture and ripening of Turkish White cheese with particular reference to a) pasteurized milk, b) cheese milk after inoculation with L. monocytogenes (0 h), c) after curd formation (2 h), d) curd after pressing (6 h), e) curd after pH was reduced (17 h), f) curd after salting (32 h), and g) cheeses during ripening. Cheeses were also examined periodically for total solids, moisture and salt contents, pH values and aerobic plate count. An increase in the number of L. monocytogenes was observed during manufacture. Following salting and throughout the storage period, numbers of L. monocytogenes decreased at a rate depending on the salt concentration, starter activity and storage time. The initial microbial number had a significant (P < 0.01) effect on the survival of L. monocytogenes during the storage period.     

Fate of Listeria monocytogenes during the manufacture and ripening of Parmesan cheese

Parmesan cheese was made from a mixture of pasteurized whole and skim milk that was inoculated to contain ca. 10(4) to 10(5) cells of Listeria monocytogenes/ml. Curd was cooked at 51 degrees C (124 degrees F) for ca. 45 min. During cheese making, maximum numbers of L. monocytogenes appeared just before cooking; at this point, the increase over initial numbers was a .61 to 1.0 order of magnitude. During cooking of curd, the average decrease in numbers of L. monocytogenes was a .22 order of magnitude. During cheese ripening, numbers of L. monocytogenes decreased almost linearly and faster than reported for other hard cheeses. Listeria monocytogenes strain California died faster than did strain V7. Listeria monocytogenes were not detected in cheese after 2 to 16 wk of ripening, depending on the strain of the pathogen and the lot of cheese. Parmesan cheese made in this study was not a favorable medium for survival of L. monocytogenes.     

Survial of Listeria monocytogenes during the manufacture and ripening of Turkish White cheese

Listeria monocytogenes was enumerated during the manufacture and ripening of Turkish White cheese with particular reference to a) pasteurized milk, b) cheese milk after inoculation with L. monocytogenes (0 h( c) after curd formation (2 h( d) curd after pressing (6 h( e) curd after pH was reduced (17 h( f) curd after salting (32 h( and g) cheeses during ripening. Cheeses were also examined periodically for total solids, moisture and salt contents, pH values and aerobic plate count. An increase in the number ofL. monocytogenes was observed during manufacture. Following salting and throughout the storage period, numbers of L. monocytogenes decreased at a rate depending on the salt concentration, starter activity and storage time. The initial microbial number had a significant (P > 0.01) effect on the survival of L. monocytogenes during the storage period.     

Survival of Listeria monocytogenes during manufacture, ripening and storage of soft lactic cheese made from raw goat milk

Soft lactic cheeses were manufactured with raw goat milk inoculated with Listeria monocytogenes. The physico-chemical and microbiological characteristics of curds and cheeses were determined after each processing step as well as during ripening and refrigerated storage. The fate of Listeria monocytogenes was evaluated by enumeration on PALCAM agar and by a qualitative detection after a double selective enrichment procedure. The results showed that the physico-chemical and microbiological characteristics of lactic cheeses caused a decrease of Listeria monocytogenes counts. However, this decrease did not lead to the complete disappearance of the pathogen and Listeria monocytogenes was able to survive in soft lactic cheeses made with raw goat milk.     

Modelling the fate of Listeria monocytogenes during manufacture and ripening of smeared cheese made with pasteurised or raw milk

The dynamics of the physicochemical characteristics of foods help to determine the fate of pathogens throughout processing. The aim of this study was to assess the behaviour of Listeria monocytogenes during cheesesmaking and ripening and to model the growth observed under the dynamic conditions of the cheese. A laboratory scale cheese was made in 4 independent replicates from pasteurised or raw cow's milk, artificially contaminated with L. monocytogenes. No growth of L. monocytogenes occurred during raw milk cheese-making, whereas growth did occur in pasteurised milk. During ripening, growth occurred in raw milk cheese, but inactivation occurred in pasteurised milk cheese. The behaviour observed for L. monocytogenes was modelled using a logistic primary model coupled with a secondary cardinal model, taking into account the effect of physicochemical conditions (temperature, pH, water activity and lactate). A novel statistical approach was proposed to assess the optimal growth rate of a microorganism from experiments performed in dynamic conditions. This complex model had an acceptable quality of fit on the experimental data. The estimated optimum growth rates can be used to predict the fate of L. monocytogenes during cheese manufacture in raw or pasteurized milk in different physicochemical conditions. The data obtained contributes to a better understanding of the potential risk that L. monocytogenes presents to cheese producers (growth on the product, if it is contaminated) and consumers (the presence of high numbers) and constitutes a very useful set of data for the completion of chain-based modelling studies.     

Survival of Listeria monocytogenes in commercial cheese brines.

The survival of Listeria monocytogenes was determined in commercial cheese brines collected from cheese factories in Wisconsin and northern Illinois. Survival of L. monocytogenes inoculated into commercial cheese brines ranged from < 7 d to over 259 d. Survival did not correlate with pH, salt content, nitrogen content, mineral content, or inherent microbial populations but was negatively associated with addition of sodium hypochlorite at the dairy plant. The L. monocytogenes generally survived longer in brines held at 4 degrees C than at 12 degrees C. Sodium hypochlorite or hydrogen peroxide inactivated L. monocytogenes when added to commercial brines in the lab at 10 to 100 ppm or 0.001% to 0.02%, respectively. Addition of 1% potassium sorbate or 1% sodium benzoate also decreased survival of L. monocytogenes. Laboratory filtration of commercial brines had a negative effect on survival in one of three brines tested. The L. monocytogenes did not grow during incubation in any of the commercial brine samples tested.     

Survival of Listeria monocytogenes after cleaning and sanitation of wooden shelves used for cheese ripening

The effect of cleaning and heat disinfection processes of wooden shelves used for cheese ripening on the survival of Listeria monocytogenes was examined. The cut boards were inoculated with a suspension containing 5.5?×?10⁷ colony forming units (CFU)/ml of L.??monocytogenes. Survival of L.??monocytogenes was investigated in the wood shavings. During the 24-hour incubation period in a humidity chamber, Listeria counts increased by 0.8 log units on average (n?=?9), compared to results 1?h after incubation, indicating the absence of antimicrobial properties of the wood in use. Additionally, after incubation for 24?h at room temperature, the boards were cleaned by soaking them for 15?min in a solution of hot alkaline detergent followed by brushing and rinsing with warm water. Some of the cleaned boards were subsequently heat treated at 80°C for 5?min and at 65°C for 15?min, respectively. The cleaning procedure alone was not sufficient to render L.??monocytogenes from the upper 2?mm wood layer inactive. In the case of both temperature-time combinations for heat disinfection, however, L.??monocytogenes was not detectable. The present study shows that the use of wooden shelves does not affect the hygienic safety of cheeses if such shelves are in good repair and are thoroughly cleaned and sanitized by heat treatment. Therefore, there is no reason to replace wood employed in cheese ripening processes with other materials.     

Behaviour of Listeria monocytogenes during the manufacture and ripening of Manchego and Chihuahua Mexican cheeses

The ability of Listeria monocytogenes to survive the Mexican Manchego and Chihuahua cheese-making processes and its persistence during the ripening stages of both cheeses was examined. Commercial pasteurized and homogenized whole milk was inoculated with Listeria monocytogenes (strain ATCC 19114) to a level between 2 x 10(6) and 9 x 10(6) CFU/ml. The milk was used to make Mexican Manchego and Chihuahua cheeses in a 25-l vat. Mexican Manchego cheese was ripened for 5 days and Chihuahua cheese for 6 weeks at 12 degrees C and 85% RH. Listeria present in the cheese was enumerated by diluting samples in sterile 0.1% peptone water and plating on Oxford agar. Duplicate samples were taken at each step of the manufacturing process. During the first week of ripening samples were taken daily from both cheeses. For Chihuahua cheese, samples were taken weekly after the first week of the ripening stage. During the manufacture of Mexican Manchego cheese, Listeria counts remained relatively constant at 10(6) CFU/ml, while with Chihuahua cheese there was a one log decrease in numbers (10(6) to 10(5) CFU/ml). After pressing both curds overnight, numbers of bacteria decreased in Mexican Manchego cheese to 8.2 x 10(5) but increased in Chihuahua cheese from 1.7 x 10(5) to 1.2 x 10(6) CFU/ml. During the ripening stage, counts of Listeria remained constant in both cheeses. However, since the Chihuahua cheese ripening stage is about 6 weeks, the number of bacteria decreased from 2 x 10(6) to 4 x 10(4) CFU/g. The results show that Listeria monocytogenes is able to survive the manufacture and ripening processes of both Mexican cheeses.     

Evaluation of Listeria innocua as a suitable indicator for replacing Listeria monocytogenes during ripening of Camembert cheese

The suitability of Listeria innocua for use as an indicator for replacing Listeria monocytogenes during the cheese-making and ripening of Camembert cheese was evaluated. Pasteurized whole milk inoculated with either L. innocua or L. monocytogenes was used to make Camembert cheese, which were ripened in three stages. All cheese was ripened in three stages: room temperature (∼20 °C) and relative humidity of 60% for 36 h; 12 °C and relative humidity of 93% for 2 weeks; and 7 °C and relative humidity of 85% for 3 weeks. Results showed that population values of L. innocua and L. monocytogenes on day 1 were 7.16 and 6.11 log₁₀ CFU/g, respectively, which declined to 6.54 and 5.45 log₁₀ CFU/g, respectively, during subsequent 20 days. Thereafter, L. innocua and L. monocytogenes populations increased to 7.38 and 6.06 log₁₀ CFU/g on day 35 of ripening, respectively. During ripening, surface and interior of cheeses were analysed for populations of L. innocua and L. monocytogenes , respectively. The data were collected on day 1, 5, 10, 15, 20, 25, 30, and 35 of ripening. Generally, the growth of L. innocua and L. monocytogenes is faster in surface than in centre. Top centre, bottom centre and bottom surface locations had similar population values during ripening. There were no significant differences ( P  > 0.05) between batch and section of cheese. The ripening time and locations had significant effect ( P  < 0.05) on the survival and growth of L. innocua and L. monocytogenes . The trends of survival and growth of L. innocua and L. monocytogenes were similar. These results indicated that L. innocua can be considered as an indicator for L. monocytogenes during ripening of Camembert cheese.     

Glycolysis and related reactions during cheese manufacture and ripening

Fermentation of lactose to lactic acid by lactic acid bacteria is an essential primary reaction in the manufacture of all cheese varieties. The reduced pH of cheese curd, which reaches 4.5 to 5.2, depending on the variety, affects at least the following characteristics of curd and cheese: syneresis (and hence cheese composition), retention of calcium (which affects cheese texture), retention and activity of coagulant (which influences the extent and type of proteolysis during ripening), the growth of contaminating bacteria. Most (98%) of the lactose in milk is removed in the whey during cheesemaking, either as lactose or lactic acid. The residual lactose in cheese curd is metabolized during the early stages of ripening. During ripening lactic acid is also altered, mainly through the action of nonstarter bacteria. The principal changes are (1) conversion of L-lactate to D-lactate such that a racemic mixture exists in most cheeses at the end of ripening; (2) in Swiss-type cheeses, L-lactate is metabolized to propionate, acetate, and CO2, which are responsible for eye formation and contribute to typical flavor; (3) in surface mold, and probably in surface bacterially ripened cheese, lactate is metabolized to CO2 and H2O, which contributes to the increase in pH characteristic of such cheeses and that is responsible for textural changes, (4) in Cheddar and Dutch-type cheeses, some lactate may be oxidized to acetate by Pediococci. Cheese contains a low level of citrate, metabolism of which by Streptococcus diacetylactis leads to the production of diacetyl, which contributes to the flavor and is responsible for the limited eye formation characteristic of such cheeses.     

Evolution of microbial populations during traditional Feta cheese manufacture and ripening

In three different dairies (A, B and C) located in Peloponess region (Southern Greece), traditional Feta cheese trials took place February to March using mixtures of sheep's and goat's milk. Only small variations in the evolution of microbial groups were observed during the whole ripening period. The main groups, such as thermophilic cocci, mesophilic lactococci, thermophilic lactobacilli, nonstarter lactic acid bacteria (NSLAB), presumptive Leuconostoc, enterococci and micrococci, reached their highest levels during the first 16 days, and then declined approximately 1-2 log units until the end of ripening. The remaining groups investigated, comprising yeasts, coliforms and Escherichia coli, were highest at day 4. The yeasts remained constant, while coliforms and E. coli decreased sharply and were not detectable after 120 days of ripening. A number of 146 isolates (dairy A) taken from all stages of the manufacturing and ripening process were purified and studied. Lactobacillus plantarum (58/146) and isolates of related species Lactobacillus pentosus and Lactobacillus paraplantarum (16/146) were the most common microorganisms found during cheese ripening. Streptococcus thermophilus (23/146) and Lactobacillus delbrueckii subsp. bulgaricus (20/146) were detected in high levels up to 20 days, and then gradually reduced. Enterococcus faecium (29/146) was found in all manufacturing and ripening stages.     

Behaviour of Listeria monocytogenes in Lighvan cheese following artificial contamination during making,ripening and storage in different conditions

This study investigated the behaviour and fate of Listeria monocytogenes at different ripening temperatures and NaCl concentrations in traditional Lighvan cheese. L. monocytogenes was added to raw sheep's milk. After producing the cheese, they were stored in 8%, 12% and 15% NaCl at 4, 9 and 14 °C. Sampling was performed for 150 days. Different temperature and NaCl concentrations had a significant effect on the survival of L. monocytogenes (P < 0.001). The lowest growth and survival rates of L. monocytogenes were in 15% NaCl at 14 °C and 12% NaCl at 14 °C, respectively. Also, the highest growth and survival rates of the bacterium were in 8% NaCl at 4 °C.     

Listeria monocytogenes survival during production and storage of water buffalo Mozzarella cheese

The ability of Listeria monocytogenes to survive during the manufacture of water buffalo Mozzarella and to grow during its shelf life was evaluated. A wild‐type and a reference strain were used to contaminate raw milk. The viable count of the reference strain ATCC 9525 dropped after the stretching process, and in the cheese, it fell to below 100 cfu/g. When the wild‐type strain was used, however, stretching did not appear to have any effect on the pathogen. The artificially contaminated cheeses were stored, for eleven days, at 4, 20 and 30 °C. Pathogen populations increased at 20 (≈2.60 log cfu/g) and 30 °C (≈1.95 log cfu/g).     

Fate of Yersinia enterocolitica during manufacture, ripening and storage of Lighvan cheese

This study aimed to evaluate the behavior of virulent Yersinia enterocolitica (YE) during the manufacture, ripening and storage of Lighvan cheese with particular reference to strains of YE, initial inoculation level, and storage time. Three strains of YE with low (1 log cfu/ml) and high (3 log cfu/ml) inoculation levels were inoculated to raw whole ewe's milk which was then used for manufacturing of Lighvan cheese. Throughout the manufacturing, ripening and storage periods the number of YE was counted on selective media. Enumerated colonies were then confirmed by duplex PCR using ail and virF genes. Moreover, some microbial and physiochemical characteristics of the cheese samples were examined. According to the results, initial inoculation level and storage time had statistically significant (P<0.01) effects on persistency of YE, while strain type exhibited no statistically significant (P>0.01) impact on survival of the pathogen. Results showed a rapid increase in the number of YE during manufacturing, however, in the ripening and storage periods the number of YE was decreased and eventually it was eliminated in all cheese batches after 4 months of storage.     

The isolation and identification of yeasts obtained during the manufacture and ripening of Cheddar cheese

Sources of yeast, which may contaminate the curd during the manufacture of Cheddar cheese, were examined in a single cheese factory. A total of 77 yeast species present in the factory environment, manufacturing and ripening of Cheddar cheese were identified according to cellular long-chain fatty acid analysis and verified with conventional identification techniques. Product line samples were taken at critical control points in the manufacturing process and analysed after incubation at 25°C for 96 h. The progression of yeast species during cheese-making and ripening was monitored after renneting and at subsequent 48-h intervals. Dominant species wereDebaryomyces hanseniiandCryptococcus albidus, whileYarrowia lipolytica, Rhodotorula minuta, Torulaspora delbrueckii, Rhodotorula glutinisandKluyveromyces marxianuswere present at low numbers. The results obtained showed that yeasts were present in all cheese samples examined, at quantities ranging from 9×10²to 1·4×10⁷cfu g⁻¹.     

Survival of Escherichia coli, strain W, during the manufacture of cottage cheese.

Cottage cheese was manufactured in 10-liter experimental vats by the direct-acid-set method from milk that was inoculated with a heat resistant strain of Escherichia coli. Growth or survival of Strain W (ATCC 9637) E. coli was determined at various stages of the cheese making operation after the cheese-skim milk was inoculated to give counts of 2.5 X 10(4) or 4.0 X 10(5) cells/ml. Numbers of coliform organisms remained constant at the inoculated concentration in the cheese milk up to a cooking temperature of 43 C. At 43 C, when curd was separated from the whey, the curd (not washed) had coliform counts that were two log cycles greater than the whey. These trends were in milks with both cell counts. Washing of the curd with acid and 10 ppm chlorine reduced the number of coliform cells in the curd at all cooking temperatures as compared with unwashed curd. Acid wash of the curd at pH 5 did not reduce the coliform counts below those of unwashed curd. Cooking temperatures of 54 C were necessary to destroy (less than 1 cell/ml) E. coli Strain W, in either the unwashed or acid-chlorine washed curd. Holding curd with an initial average log count of 6.26 coliform cells/ml at constant temperatures of 38, 43, 49, and 54 C confirmed that 54 C for 50 min was necessary to reduce the average count to less than 1 cell/ml in isolated curd. Coliforms in whey were reduced to that concentration after 10 min at 54 C.     

Evolution of chemico-physical characteristics during manufacture and ripening of Castelmagno PDO cheese in wintertime

Biochemical, volatile and textural profiles during manufacture and ripening were determined in samples of Castelmagno PDO cheese obtained from three different batches in the main artisan cheese plant of Castelmagno PDO production area. At the end of manufacture, samples were characterised by a pH of 6.57% and 52.4% moisture content. The HPLC analysis of organic acids and sugars showed the exhaustion of lactose content, while Urea-PAGE indicated extensive primary proteolysis of both β-casein and α_s1-casein. During ripening, cheeses were characterised by high degradation of β-casein and α_s1-casein, due to bacterial action. RP-HPLC profiles showed a high production of peptides eluted between 20 and 30 min. In total, 92 volatile compounds were identified in cheese headspace. Texture profiles showed an increase in hardness, gumminess, chewiness and adhesiveness values, as well as a decrease in cohesiveness during ripening.     

High pressure treatment: applications in cheese manufacture and ripening

High pressure (HP) treatment has emerged as a food processing technology primarily due to increasing interest in novel methods for preservation of foods. Applying HP to food products modifies interactions between individual components, influences rates of enzymatic reactions and can inactivate microorganisms. This paper reviews studies of HP induced changes in milk relevant to cheesemaking, including the effects of HP on rennet coagulation time, rate of curd formation and cheese yield. Published studies on the effects of direct HP treatment of cheese and specifically the effects of HP on cheese ripening characteristics, functionality and microbiology, are also reviewed.     

Survival of Listeria monocytogenes in egg washwater

The viability of Listeria monocytogenes strains Scott A, 78-34, and 81-861 in artificial egg washwater at different temperatures and pH values was determined. After a 4-h incubation, less than a 1-log decrease in viability of strains Scott A and 78-34 was found at 33 degrees C with alkaline detergent (pH 8.0-10.5); however, up to a 3-log decrease in viable numbers was found in neutral pH controls lacking detergent. At 42 degrees C, survival was generally poorer; complete loss of viability (greater than 4-log decrease in viable numbers) was found within 2 h at neutral pH. Strain 81-861 was more sensitive to the test conditions than the other two strains. Viability of all strains was markedly lower in synthetic washwater at the lower pH values (pH 7-9) containing whole egg than washwater in which whole egg was omitted. The presence of whole egg appeared to have no effect on survival at pH 10.5. A limited survey of two egg wash facilities in Southeastern Ontario revealed Listeria innocua in environmental samples from both plants, and in washwater from one plant. These results suggest that Listeria spp. can survive normal commercial washwater conditions, and can be found in commercial egg wash plants.