Association of Yersinia enterocolitica with the manufacture of cheese and occurrence in pasteurized milk.

Raw milk in southern Ontario frequently contains Yersinia enterocolitica. The potential for transmission of this organism by cheese manufactured from unpasteurized milk was evaluated by examination of milk and cheese curd samples from cheese manufacturing plants and finished cheddar and Italian cheeses. The incidence of Y. enterocolitica was lower in cheese curd samples (9.2%) than in raw milk (18.2%). Most of the curd samples showed a positive phosphatase test, indicating production from raw milk. One curd sample yielded Y. enterocolitica after 4 weeks of storage at 4 degrees C but was negative after 8 weeks. All samples of cheddar and Italian cheeses, most of which showed a positive phosphatase test, were negative for Y. enterocolitica. One out of 265 samples (0.4%) of pasteurized fluid dairy products contained Y. enterocolitica.     

Microbiological quality of Pecorino Siciliano "primosale" cheese on retail sale in the street markets of Palermo, Italy

Pecorino Siciliano (PS) "primosale" is a traditional Sicilian fresh soft cheese made from sheep's milk. Short-ripening time and production from unpasteurized or raw milk can facilitate bacterial contamination of PS "primosale". The microbiological quality of "primosale" on retail sale in the street markets of Palermo, Italy was studied by detecting the common food pathogens Listeria monocytogenes and Staphylococcus aureus and indicator microorganisms, such as Escherichia coli, Enterobacteriaceae and Staphylococcaceae. In our study, 4% and 44% of the samples, respectively, did not comply with the acceptability levels fixed by European regulations for S. aureus and E. coli. A high contamination of bacteria belonging to Enterobacteriaceae and Staphylococcaceae was found in 42% and 50% of the cheeses analyzed, respectively. Such results indicate poor husbandry and poor hygiene practices during milk collection or preservation or during cheese production processes and handling. In addition, the retail sale conditions may have played a role in cheese contamination since a correlation was found between poor microbiological quality and some selling parameters. This study emphasizes the need to improve production hygiene throughout the PS food chain in line with the traditional cheese-making procedures. Labelling of PS with clear information on whether the cheese was prepared from raw milk also requires improvement.     

Fate of Escherichia coli O157:H7 during the manufacture of Mozzarella cheese

AIMS: The fate of Escherichia coli O157:H7 was investigated during the manufacture of Mozzarella cheese. METHODS AND RESULTS: The Mozzarella cheese was made from unpasteurized milk which was inoculated to contain ca 10(5) cfu ml(-1)E. coli O157:H7. Two different heating temperatures (70 and 80 degrees C), commonly used during curd stretching, were investigated to determine their effects on the viability of E. coli O157:H7 in Mozzarella cheese. Stretching at 80 degrees C for 5 min resulted in the loss of culturability of E. coli O157:H7 strains, whereas stretching at 70 degrees C reduced the number of culturable E. coli O157:H7 by a factor of 10. CONCLUSIONS: The results show that stretching curd at 80 degrees C for 5 min is effective in controlling E. coli O157:H7 during the production of Mozzarella cheese. Brining and storage at 4 degrees C for 12 h was less effective than the stretching. Significance and Impact of the Study: Mozzarella cheese should be free of E. coli O157:H7 only if temperatures higher than or equal to 80 degrees C are used during milk processing.     

Real-time visual/near-infrared analysis of milk-clotting parameters for industrial applications

The economical profitability of the dairy industry is based on the quality of the bulk milk collected in the farms, therefore it was based on the herd level rather than on the individual animals at real time. Udder infection and stage of lactation are directly related to the quality of milk produced on the herd level. However, improvement of milk quality requires testing each animal's milk separately and continuously. Recently, it was postulated that online equipment can estimate milk quality according to its clotting parameters, and thus result in better economical return for cheese making. This study further investigated the potential application of the AfiLab™ equipment to provide real-time analysis of milk-clotting parameters for cheese manufacture and cheese yield on quarter (1018) and individual cow (277) levels. Days in milk, lactose, log SCC and udder infection were found to have a significant effect on curd firmness and cheese properties and yield. The results clearly indicate that: (a) the parameter Afi-CF determined with the AfiLab™ is suitable for assessing milk quality for its clotting parameters, a value which is not provided by merely measuring fat and protein content on the gland and the cow levels; (b) bacterial type is the single major cause of reduced milk quality, with variations depending on the bacterial species; and (c) early and late lactation also had negative effects on milk-clotting parameters. Cheese made from the various milk samples that were determined by the Afilab™ to be of higher quality for cheese making resulted in higher yield and better texture, which were related mainly to the bacterial species and stage of lactation.     

The use of the bacteriocin, nisin, as a preservative in ricotta-type cheeses to control the food-borne pathogen Listeria monocytogenes

The efficacy of nisin to control the food-borne pathogen Listeria monocytogenes in ricotta-type cheeses over long storage (70 d) at 6–8°C was determined. Cheeses were prepared from unpasteurized milk by direct acidification with acetic acid (final pH 5·9) and/or calcium chloride addition during heat treatment. Nisin was added in the commercial form of Nisaplin^® pre-production to the milk. Each batch of cheese was inoculated with 10²–10³ cfu g⁻¹ of a five-strain cocktail of L. monocytogenes before storage. Shelf-life analysis demonstrated that incorporation of nisin at a level of 2·5 mg l⁻¹ could effectively inhibit the growth of L. monocytogenes for a period of 8 weeks or more (dependent on cheese type). Cheese made without the addition of nisin contained unsafe levels of the organism within 1–2 weeks of incubation. Measurement of initial and residual nisin indicated a high level of retention over the 10-week incubation period at 6–8°C, with only 10–32% nisin loss.     

Large outbreak of Salmonella enterica serotype paratyphi B infection caused by a goats' milk cheese,France, 1993: a case finding and epidemiological study.

OBJECTIVE--To assess the magnitude of a nationwide outbreak of infection with Salmonella enterica serotype paratyphi B and identify the vehicle and source of infection. DESIGN--A case finding study of S paratyphi B infection between 15 August and 30 November 1993; a pair matched case-control study; an environmental investigation at a processing plant that produced a raw goats'' milk cheese incriminated in the outbreak; phage typing and genotyping of food and human S paratyphi B isolates. SETTING--France, 15 August to 30 November 1993. SUBJECTS--273 patients with S paratyphi B infection; 59 pairs of cases and controls matched for age, sex, and city of residence. MAIN OUTCOME MEASURES--Numbers of cases and incidence rates by region of residence and age; matched odds ratios for dairy food preferences. RESULTS--Among the 273 cases there was one death; 203 (78%) strains belonged to phage type 1 var 3. The incidence of infection was greatest in the region where goats'' milk cheese is commonly produced. Comparison of cases and controls showed a 12-fold greater risk of illness (95% confidence interval 1.6 to 92.3) from eating brand A unpasteurised goats'' milk cheese. S paratyphi B isolates of phage type 1 var 3 were recovered from cheese A, goats'' milk at the plant processing cheese A, and goats'' milk supplied to the plant by a single farm. Genotypic IS 200 typing of food and human 1 var 3 phage type isolates showed a common IS 200 pattern. CONCLUSION--This outbreak emphasises the potential health hazards of widely distributed unpasteurised milk products in France and the need for their close bacterial monitoring.     

Enhancement of cheese flavors with microbial esterases

The characteristic flavor of hard Italian cheeses is associated with the presence of fatty acids, particularly butyric acid, liberated from milk fat during the ripening process. To ensure proper development and control of flavor, animal pregastric esterases or lipases are routinely added to the milk before coagulation of the curd. Such esterases are also used to generate flavor in enzyme modified cheese and other dairy products. Esterases from microbial sources have been investigated as agents to enhance flavor in cheese. We have found that an esterase from Mucor miehei exhibits the type of lipolytic activity needed for this application. Romano and fontina cheeses of excellent quality have been prepared by the use of this esterase. It has also been used successfully in the preparation of enzyme modified cheese, and, in turn, processed American cheese.     

Prevalence and survival of Campylobacter jejuni in unpasteurized milk.

Campylobacter jejuni was isolated from 1 to 108 (0.9%) milk samples obtained from the bulk tanks of nine grade A dairy farms and from 50 of 78 (64%) cows producing grade A milk. Survival of eight Campylobacter strains in unpasteurized milk (4 degrees C) varied greatly: the most tolerant strain showed a less than 2-log10 decrease in viable cells after 14 days, and the most sensitive strain showed a greater than 6-log10 decrease after 7 days. One strain was still recoverable 21 days after the inoculation of milk. Inactivation of the different strains corresponded with an increase in milk aerobic plate count and a decrease in milk pH; however, no absolute correlation could be made between the rates of change of these parameters and the rates of campylobacter inactivation. When held at 4 degrees C, C. jejuni was most stable in brucella broth, died most rapidly in unpasteurized milk, and was inactivated at an intermediate rate in sterile milk. Our results indicate the presence and possible persistence of C. jejuni in raw grade A milk and reaffirm the need for pasteurization of milk.     

Effects of aging on functional properties of caprine milk made into Cheddar- and Colby-like cheeses

The effects of cheese milk obtained at three times during lactation (weeks 4–5, 12–15, and 21–23) and cheese storage (up to 16 or 24 weeks) on meltability, sliceability, and color changes upon heating (232 °C for 5 min, high baking temperature, HT, or 130 °C for 75 min, low baking temperature, LT) of caprine milk cheeses were evaluated. The cheeses were manufactured from milk from Alpine goats and based on the procedures of Cheddar and Colby cheese manufacture. In Cheddar-like cheese, the sliceability (force required to slice sample) was at its highest when the cheese was made with milk from weeks 12–15 into lactation. Color change was variable although it tended to be lowest in cheese made at weeks 4–5 into lactation. In Colby-like cheeses, meltability was at its highest and sliceability was very poor (after 8 weeks of aging) when made with milk obtained later in lactation. Color changes were variable at the two different baking temperatures. As expected during aging, the meltability of the cheeses increased and the force required to slice the cheeses decreased with the significant changes occurring within the first 16 weeks for Cheddar-like and the first 8 weeks for Colby-like cheeses. The color changes upon heating were variable for aged Cheddar-like cheeses and did not change significantly for aged Colby-like cheeses. Color changes were highly correlated with proteolysis occurring during storage. Cheese milk obtained at different times of lactation and aging of the cheese impact the functional properties of caprine milk cheeses and will affect their optimal utilization.     

The microstructure and distribution of micro-organisms within mature Serra cheese

The distribution of micro-organisms in mature Serra, a traditional Portuguese cheese made from unpasteurised ewes' milk without added starter culture, was examined by light microscopy and electron microscopy. Four populations of micro-organisms were recognized according to their position within the cheese: (i) those present as apparently axenic colonies within the curd matrix; (ii) bacteria growing along curd junctions; (iii) yeasts and bacteria present in the smear on the surface of the cheese and (iv) bacteria found in cracks which penetrated the outer part of the cheese from the rind. Two types of crystals were observed, together with contaminants of vegetable origin and somatic cells originating from the milk.     

Improvement in Salmonella detection in milk and dairy products: comparison between the ISO method and the Oxoid SPRINT Salmonella test

The Oxoid SPRINT Salmonella test was compared with the ISO method (ISO 6579: 1993) for the detection of Salmonella in milk and dairy products. Samples were artificially contaminated, in some cases with sublethally injured salmonellas. Experiments with raw milk, soft cheese made from heat-treated milk (mould-ripened and with smear) and soft cheese with smear made from raw milk showed no significant differences between the SPRINT and ISO methods. With dried milk products and mould-ripened soft cheese made from raw milk the reference method gave significantly more positive results. The addition of ferrioxamine E to pre-enrichment (ISO) or pre-enrichment/enrichment broth (SPRINT test) did not improve Salmonella detection.     

Evaluation of amino acid-decarboxylative microbiota throughout the ripening of an Italian PDO cheese produced using different manufacturing practices

Aim:  To investigate the presence of biogenic amines (BAs) in Montasio cheese produced by using different cheese manufacturing practices.
Methods and Results:  Three batches of Montasio cheese were made in the following way: batch A using raw milk and natural milk culture, batch B with thermized milk and natural milk culture and batch C with thermized milk and natural milk culture added of a commercial starter culture. During 120 days of ripening analyses were performed for microbial counts and BA content; indeed, the potential to produce BAs was screened in lactic acid bacteria and Enterobacteriaceae isolates. At the end of ripening, the total BA contents of cheeses from batches A, B and C were 166·3, 207·3 and 29·8 mg kg⁻¹, respectively. Amino acid decarboxylase activity was widespread among isolates.
Conclusions:  The BA content of Montasio cheese from the three batches was below the threshold proposed as potentially toxic. The highest BA content was found in cheese produced using thermized milk and natural milk culture; therefore, the thermal treatment of milk was not enough by itself to reduce the counts of decarboxylase-positive bacteria in cheese. The use of selected starters guaranteed a low BA content in Montasio cheese.
Significance and Impact of the Study:  The study of the effects of some technological processes on the incidence of decarboxylative microbiota in 'protected denomination of origin' cheeses could provide useful information on the hygienic risk related to their production.     

Comparison between a PCR-ELISA test and the vero cell assay for detecting Shiga toxin-producing Escherichia coli in dairy products and characterization of virulence traits of the isolated strains

AIMS: This paper provides information on a PCR-ELISA method for detecting Shiga toxin-producing Escherichia coli (STEC), and on their prevalence in dairy products. METHODS AND RESULTS: The sensitivity and specificity of the test was evaluated using pure cultures, spiked and naturally-contaminated samples. A comparative study with vero cytotoxicity testing was conducted, and STEC isolated from naturally-contaminated samples were characterized. The PCR-ELISA test was highly specific and sensitive, and detected 14% more positive samples than the vero cell assay. The prevalence of STEC in raw milk and unpasteurized cheese was 21.5% and 30.5%, respectively, while samples from the 'dairy environment' and from pasteurized cheese were less contaminated. The 34 strains of STEC isolated from natural samples showed that some of them carried virulence genes. CONCLUSION: No conclusion can be drawn at the moment concerning the potential risk to consumers. SIGNIFICANCE AND IMPACT OF THE STUDY: These data show the necessity of valuable screening methods to appreciate the virulence of STEC.     

Q fever through consumption of unpasteurised milk and milk products – a risk profile and exposure assessment

Q fever is a zoonotic disease caused by the bacterium Coxiella burnetii which is endemic in cattle, sheep and goats in much of the world, including the United Kingdom (UK). There is some epidemiological evidence that a small proportion of cases in the developed world may arise from consumption of unpasteurised milk with less evidence for milk products such as cheese. Long maturation at low pH may give some inactivation in hard cheese, and viable C. burnetii are rarely detected in unpasteurised cheese compared to unpasteurised milk. Simulations presented here predict that the probability of exposure per person to one or more C. burnetii through the daily cumulative consumption of raw milk in the UK is 0·4203. For those positive exposures, the average level of exposure predicted is high at 1266 guinea pig intraperitoneal infectious dose 50% units (GP_IP_ID₅₀) per person per day. However, in the absence of human dose–response data, the case is made that the GP_IP_ID₅₀ unit represents a very low risk through the oral route. The available evidence suggests that the risks from C. burnetii through consumption of unpasteurised milk and milk products (including cheese) are not negligible but they are lower in comparison to transmission via inhalation of aerosols from parturient products and livestock contact.     

Quantitative isolation efficiency of O26, O103, O111, O145 and O157 STEC serotypes from artificially contaminated food and cattle faeces samples using a new isolation protocol

Aims:  A range of new differential and confirmation plating media for some non-O157 Shiga toxin producing Escherichia coli (STEC) serotypes (O26, O103, O111, O145) and both sorbitol-positive and -negative O157 were evaluated using artificially contaminated samples.
Methods and Results:  Dairy products (raw milk, cheese made from pasteurized milk and raw milk), meat (ground beef, fermented meat) and cattle faeces were artificially contaminated using clinical STEC strains. Isolation efficiency was 100%, 82·3%, 88·5%, 65·9%, 64·3% and 15·8%, respectively, for an inoculum size of ≤100 CFU 25 g⁻¹. The consecutive use of differential and confirmation media limited the incidence of false positive isolates from 0% for raw milk samples, cheese made from pasteurized milk and for fermented meat to 2·1% for cheese made from raw milk, and to 8·9% for ground beef.
Conclusions:  Data presented in this paper indicated that the efficiency of the applied isolation method was dependent on sample-to-sample variation but not on the inoculum size.
Significance and Impact of Study:  Data in this paper indicated that isolation of low levels of non-O157 and sorbitol-positive O157 STEC from food samples is possible.     

Biotechnological Methods to Accelerate Cheddar Cheese Ripening

ABSTRACT

Cheese is one of the dairy products that can result from the enzymatic coagulation of milk. The basic steps of the transformation of milk into cheese are coagulation, draining, and ripening. Ripening is the complex process required for the development of a cheese's flavor, texture and aroma. Proteolysis, lipolysis and glycolysis are the three main biochemical reactions that are responsible for the basic changes during the maturation period. As ripening is a relatively expensive process for the cheese industry, reducing maturation time without destroying the quality of the ripened cheese has economic and technological benefits. Elevated ripening temperatures, addition of enzymes, addition of cheese slurry, attenuated starters, adjunct cultures, genetically engineered starters and recombinant enzymes and microencapsulation of ripening enzymes are traditional and modern methods used to accelerate cheese ripening. In this context, an up to date review of Cheddar cheese ripening is presented.