Current Microbial Concerns in the Dairy Industry
By Joseph Odumeru, Ph.D.
Food safety is a pressing issue for governments, food processors, retailers and consumers worldwide and this sense of urgency is felt not only in the meat or seafood industries, but also in the dairy industry. The quality and safety of milk and dairy foods are of primary concern to the dairy industry and the obligation of dairy industry personnel. Food safety begins on the farm and continues through processing and transportation processes until the milk or milk product is consumed. There is an emphasis on on- farm control of foodborne pathogens because the farm environment is a reservoir for many microorganisms. In order to reduce the incidence of foodborne illness associated with milk and dairy products, a pathogen reduction program must begin on the farm and be applied throughout the food chain.
There are two categories of food safety concerns in the dairy industry. These include microbiological hazards and chemical hazards, primarily foodborne pathogens with regard to the former and chemical-related concerns such as antibiotics, pesticides and herbicides. The focus of this paper is on food safety concerns as they relate to microbiological hazards.
RECENT OUTBREAKS AND DRIVERS
Why is the food industry, including the dairy industry, concerned about foodborne pathogens? One of the worst nightmares for food processors or producers is to have their company show up in a news report as the source of a food- borne illness outbreak. Apart from the loss of consumer confidence and loss of sales, there also is a serious legal aspect about which food companies must be concerned. In addition, foodborne disease outbreaks tend to make the news on a more frequent basis than do chemical related food safety incidence. The health effects of chemical hazards are less dramatic than microbial-related effects, which may be a significant reason for this.
According to a report from the Council of Agricultural Science and Technology (CAST), an estimated 6.5 million to 33 million cases of foodborne diseases are reported each year in the U.S., at a cost of $5-8 billion annually and 900 fatalities. The incidence rate in Canada is estimated at 2.2 million cases per year at a cost of $ 1-3 billion annually. This cost estimate is a combination of the associated medical care and lost productivity. These are lots of cases and a lot of money, but it should be remembered that human pain and suffering cannot be measured in dollars. Hence, there is a concerted effort in the food industry to reduce the incidence of outbreaks related to these products and a number of programs have been put into place to accomplish this goal.
A large portion of reported cases of foodborne disease outbreaks is associated with contaminated milk and dairy products. Table 1 shows the incidence of the four most frequent foodborne disease- causing pathogens in Canada and the U.S. Tables 2 and 3 show some selected examples of outbreaks associated with both raw milk and pasteurized milk and dairy products. One of the largest outbreaks associated with raw milk, for example, occurred in Scotland in the late 1980s, and involved 1,268 cases of food- borne illness and two deaths. Over the years we have seen a dramatic decline in cases associated with contaminated raw milk as a result of the introduction of pasteurization. Outbreaks associated with raw milk are primarily reported in the U.S., which allows sales of raw milk. However, outbreaks of pasteurized milk and dairy products also have been reported; for example, one of the largest such outbreaks in the U.S. involved 18,000 cases of salmonellosis and seven deaths reported in 1985.
SOURCES OF FOODBORNE PATHOGENS IN DAIRY FOODS
There are two main sources of food- borne pathogens in dairy foods, contaminated raw milk and dairy products that have been contaminated during post-pasteurization processing or handling, by dairies, by those in the retail food chain or by the consumer.
Raw Milk. The largest number of dairy-related foodborne illness outbreaks can be attributed to contaminated raw milk. Of the known pathogenic bacteria in raw milk, Campylobacter, Listeria and Salmonella lead the pack, with verotoxigenic E. coli (VTEC), Staphylococcus aureus, and Bacillus cereus also implicated. Diseases such as tuberculosis, diphtheria, scarlet fever and brucellosis have declined dramatically to being non-existent as a result of mandatory regulations requiring pasteurization and other strategies such as the implementation of a brucellosis eradication program. Mycobacterium paratuberculosis is an emerging concern in the dairy industry, as well, particularly in light of the fact that while the other pathogens mentioned are effectively killed during pasteurization, M. paratuberculosis has been reported to survive this process. (It should be noted that there is currently a fierce debate as to whether this pathogen causes disease in humans, specifically Crohn’s Disease.)
Despite the fact that consumption of contaminated raw milk poses a potential hazard to the consumer and that many foodborne illness outbreaks have been traced to this source, it remains legal to sell raw milk in some U.S. states. However, these states are under pressure to ban the sale of raw milk. In the province of Ontario, Canada, it is illegal to sell raw milk. Over the years, some milk producers have lobbied to remove this ban. In order to confirm the risk of drinking raw milk, the University of Guelph conducted a survey of 1,720 bulk tank raw samples in Ontario for the presence of the four most common pathogens—Listeria, Campylobacter, VTEC and Salmonella. The study, published in 1997 in the Journal of Food Protection, showed the presence of Listeria monocytogenes with the highest prevalence (2.73%) in the raw milk samples tested, followed by Salmonella (0.17%), Campylobacter (0.47%) and VTEC (0.87%). Of the samples tested, 4.13% contained at least one of these four pathogens, and 0.12% contained two or more of these organisms. We also looked at the effect of pooling raw milk, which is typically done with bulk tank milk going into dairies, and the chances of finding any of these four pathogens in the pooled raw milk. For example, if one was to randomly pool milk from 10 various farms with a prevalence rate of Listeria monocytogenes at 2.73% per farm (or at 4.13% prevalence rate for the presence of at least one of the four pathogens), the chances of contamination with Listeria monocytogenes per farm would increase to 24.18% and to a 34.41% prevalence rate for the presence of contamination by any of the four pathogens in milk pooled from the 10 farms. Clearly, the chance of finding pathogens in a tanker truck increases with the number of pooled milk obtained from various farms.
Post-Pasteurization Contamination. It is clear that contamination of dairy product can occur post-pasteurization. This may be as a result of cross-contamination of finished product with raw product, inadequate sanitation procedures in the plant environment, or inadequately sanitized equipment. Since the introduction of pathogens can occur at any stage along the post-pasteurization processing, distribution, storage and handling chain, a system of good laboratory methods for detecting and tracing sources of microbial contamination, as well as control measures for preventing such contamination are essential elements of the dairy industry food safety program.
Control measures for preventing contamination begin at the farm level and are implemented during animal production with adequate environmental sanitation, separating infected or sick animals from young calves and other healthy animals, as well as the implementation of Good Manufacturing Practices (GMPs) during milking and storage. Milking must be performed using procedures that will prevent contamination of milk by microorganisms from the udder and teats. Cold storage at 4-5°C (40°F) and strategies for ensuring that all equipment that comes into contact with milk is properly sanitized are other important control measures implemented at the dairy processing and after-processing stages of production. For example, separation of raw milk from finished product and adequate environmental sanitation in both dairies and retail stores will reduce the incidence of post-pasteurization contamination.
While the key control measures— maintenance of high sanitation standards, GMPs and the prevention of cross-contamination of finished dairy product by raw milk—help immensely in the control of microbiological hazards, the implementation of a Hazard Analysis & Critical Control Points (HACCP) plan provides better control of food safety hazards. While HACCP is not mandatory for the milk and dairy product industry, it has been voluntarily implemented in many dairy processing plants across North America. This science-based approach in which potential sources of food safety hazards—chemical, biological and physical—in the operation are identified, critical control points (CCPs) established, monitoring procedures instituted and verification that control measures are working effectively can be implemented at both the farm and processing levels.
One of the key benefits of HACCP is an increased ability to identify problems before they occur, which results in the opportunity for the producer or processor to establish control measures critical to food safety aims. On the farm, for example, biological CCPs, primarily pathogens, can he identified at critical junctures during animal production, milking and storage. Control measures for identified hazards might include the development and implementation of an on-farm sanitation program that ensures the cleanliness of equipment and tools used for milking, and the cleanliness of the water used on the farm so that it meets the standards for potable water.
APPLICATION OF RAPID MICROBIOLOGY
Rapid and automated methods for the screening and detection of pathogens are useful application in a dairy industry HACCP program. Recently, there has been a move in the dairy industry toward the use of rapid methods to quickly screen raw milk for bacteria levels as a general indicator of adequate sanitation and milk quality. For example, rapid methods that generate bacterial counts within less than one hour of testing is routinely used in the Laboratory Services Division at the University of Guelph, Canada. In addition to microscopy, there are automated instruments available that can generate these counts in the “real-time” mode required by HACCP The Bactoscan 8000S system (Foss Electric, Denmark) is one example of a direct method used in the Guelph lab for bacterial count. With this system, technicians are able to process 80 samples per hour, and results are available within five minutes of testing. The system has a built-in centrifuge to separate bacteria from other milk components prior to staining with acridine orange and a built-in microscope used to count fluorescent- stained bacterial cells. Further tests can be performed on milk samples with high bacterial counts to assess whether the high count is due to environmental contaminants or due to the presence of pathogens such as Staphylococcus aureus. A new version of the system recently acquired by the Laboratory Services Division is the Bactoscan FC, which uses flow cytometry technology for the estimation of bacterial counts of raw milk. This system can process 100 to 140 samples per hour.
Other rapid methods using parameters that are not dependent on viable counts include measurements of ATP concentration and electrical impedance and conductance. Methods involving measurements of radioactive CO2, heat generation, lipopolysacharide or enzyme activities are used infrequently. ATP measurement to assess environmental hygiene or microbial load in milk involves the detection of the bioluminescent reaction catalyzed by the enzyme luciferace in the presence of luciferin, magnesium ions and oxygen. The light emission is measured in relative light units using a luminometer. Although ATP bioluminescence is commonly used for hygiene monitoring, it can be used for assessing the microbial load of raw milk. Automation of this technology is required for routine testing of large number of raw milk samples.
The electrical impedance and conductance technology involves measurement of the changes in the electrical impedance and conductance of the growth medium. These changes are initiated by production of charged molecules from uncharged or weakly charged molecules as bacteria utilize nutrients during growth. Specialized instruments, such as the Bactometer and Malthus systems. are used for the measurement of the changes in the electrical impedance and conductance respectively. These instruments have been used for the estimation of bacterial counts of raw and pasteurized milk and assessment of post-pasteurization contamination. There are reports of the application of the conductance method for the detection of Salmonella in milk. These two systems provide an automated assessment of milk quality with computerized data handling and storage.
Rapid methods for pathogen detection in milk and dairy products also include the use of immunoassays, nucleic acid hybridization, polymerase chain reaction (PCR), lux gene technology and enzyme- linked fluorescent immunoassay (ELFA). Pathogen detection, unlike the measurement of total microbial load, requires preenrichment of the dairy samples in order to increase the sensitivity of detection. Hence, tests in most cases require more than 18 hours to obtain results.
In today’s dairy industry, potentially harmful pathogenic contamination can occur at any stage of milk production: on the farm, in the processing plant, in the retail store, and even in the consumer’s home. Therefore, steps must be taken to control microbial contamination at various points along the food chain. As indicated, HACCP implementation is an essential component of the control measure arsenal available to ensure food safety in the dairy industry. Emerging rapid and automated methods are being developed and improved upon to help the dairy industry to effectively address emerging microbial challenges and concerns. The implementation of HACCP from farm to retail distribution of milk and dairy products is a must for food safety assurance in the dairy industry.
Joseph Odumeru, Ph.D., is a senior research scientist and food microbiology laboratory supervisor at the Laboratory Services Division, and an adjunct professor, Department of Food Science at the University of Guelph, Ontario, Canada. His work experience includes 20 years of diagnostic microbiology and research work in industrial, medical and food microbiology. Odumeru’s research interests include the development of rapid methods for the detection, enumeration and identification of microorganisms in food and environmental samples. His current research projects include the investigation of milk as a potential source of human exposure to Mycobacterium paratuberculosis by PCR and culture methods, antibiotic resistance profiles of Campylobacter and Salmonella isolates from beef, hog, poultry and ready-to-eat processed meats and use of automated systems for environmental monitoring of Listeria in food processing plants.