All dairy operators want to offer a quality product. Farmers providing raw, organic milk face additional challenges for keeping bacteria populations low.
Nicole Martin, Ph.D., from Cornell University’s Food Safety Laboratory, recently presented “Bacterial Populations in Organic Raw Milk Impacting Finished Product Quality” at the NOFA-NY Winter Conference.
Martin serves as assistant research professor in Dairy Foods Microbiology and as associate director of the Milk Quality Improvement Program at Cornell.
“We characterize bacteria using a number of different parameters,” Martin said, including cell morphology, gram reaction, pH range, oxygen requirements, growth temperature and ability to tolerate various environments.
Different types of bacteria can survive at different temperatures. “We use that to indicate the implications of these organisms in raw milk and finished products,” Martin said.
Other environmental factors can influence bacterial ability to survive, such as high salt or high acid concentrations and high temperature exposure.
Unlike fungal spores, “endospores are formed under stressful environmental conditions,” Martin said. “There might be a lack of nutrients or a lack of moisture. There might be some sort of chemicals that might eventually kill that organism.”
Typically, heat (pasteurization), drying (powders), radiation and sanitizers kill bacteria. But spores that survive these processes can cause problems for the quality of the resulting dairy products if their environmental conditions return to favorable. The spores begin to multiply at that point.
“These spores go on to cause spoilage or even safety issues in the finished products,” Martin said.
Unlike the idiom “What happens in Vegas stays in Vegas,” how farms operate follows their products.
“What happens on the farm does not stay on the farm,” Martin said. “Dairy farms are not Las Vegas. Pasteurization is not a magic bullet. Pasteurization does not correct things that went wrong at the farm. High quality raw milk is essential to manufacturing high quality processed dairy products.”
But not all farm factors affect the finished product’s quality in the same way. Martin wants raw milk testing to focus on the factors that affect the quality of the finished product. Choosing the correct tests is critical.
Psychrophilic/psychrotolerant enzyme producers and gram-positive spore formers are two examples of raw milk microbial drivers of finished product quality.
The enzyme producers grow at low temperatures. Factors that contribute to growth include high initial contamination, improper cooling and long hold times. Heat-stable proteases and lipases produced by psychrophilic and psychrotolerant bacteria in raw milk lead to reduced yield in cheese (estimated 3% – 4%) and defects in flavor, odor and body defects in cheese, yogurt and milk.
“One of the defects that these bacteria produce is called sweet curdling in fluid milk,” Martin said. “We have some psychrotolerant spore formers that like to grow in high temperatures.” These form a protelytic enzyme.
“It’s called ‘sweet curdling’ because there’s no acid development,” she said, “so this can be a pretty severe defect when we have these psychrotolerant spores that grow in high temperature in a short time. And then we have what are called anaerobic-forming bacteria like Closterium, which can produce defects in some styles of cheese, especially in semi-hard and hard aged cheese like gouda, Swiss and some Italian-style cheeses.”
These organisms like to grow in the absence of oxygen. In wheels of cheese, they can cause long cracks and large holes, deforming the cheese, and result in off flavors and odors. Dried products can also carry concentrated spores, which cause problems when reconstituted.
Martin said that raw milk quality as defined by the quality of the finished product relies upon the milk’s total bacteria count, preliminary incubation count, coliform count, laboratory pasteurization count and spore count.
“We have to have below a certain level of total bacteria in raw milk for it to be compliant with the pasteurized milk ordinance, but we have other tests that are done on raw milk pretty frequently,” Martin said.
Farm sources and factors affecting total bacteria count include mastitic cows, milking time hygiene, equipment and water and raw milk handling.
“There’s no regulatory limit here and there are really only guidelines that suggest there’s a limit for those anerobic spores, and that is less than one spore per milliliter of milk – so a very, very, very low limit on those organisms – and that would be right for manufacturers who are making a product that is susceptible to the type of spoilage that these organisms cause,” Martin said.
The low levels can be very challenging to detect compared with a limit of 100,000 colony-forming units per milliliter.
To better control bacteria in organic raw milk, Martin said producers must start with process control as their foundation, then build on that by monitoring their quality parameters and troubleshooting when issues arise. It all begins with hygiene.
“Incomplete and inconsistent removal of dirt from teats allows bacteria from the environment, like soil, bedding and manure, to enter the bulk tank,” Martin said. Inadequately cleaned towels, soiled gloves or other contact points may transfer soil on to teats prior to milking, leading to bacterial contamination too.
Bedding types and practices influence spores in the bulk tank. “Bedding management has been shown to be a critical factor for determining bacterial populations and numbers on the teat end,” Martin said. “What is the animal spending 12 to 14 hours lying in? Spore levels in unused bedding have been shown to both directly and indirectly impact spore levels in bulk tank raw milk. Organic materials have significantly higher spore levels than inorganic bedding.”
Producers using organic bedding are still able to produce low-spore raw milk, however. Increased frequency of adding clean bedding to stalls was associated with reduced levels of spores in the bulk tank.
Equipment also plays a role, as milk must be chilled quickly and held at the appropriate temperature to limit growth of psychrophilic bacteria (ideally at less than 40º F). Equipment should also be cleaned adequately. One area that may be overlooked is rubber parts. If not replaced on time, they can dry out and harbor bacteria in the cracks.
Martin added that udder hygiene also matters. “Training milking personnel on focused teat end cleaning resulted in greater than 40% reduction in bulk tank raw milk spore levels,” she noted.
Farms at risk for dirty udders include those with overstocking, loose consistency of manure, infrequent removal of bedding and grooming of stalls, poor cleanliness of cow walkways, frequent access to outside areas and rushed animals, which results in manure splattering.
Martin also said that teat end condition is impacted by a number of factors, including overmilking, poor milking preparation, teat skin condition, genetics, parity and stage of lactation. Forestripping is associated with lower bulk tank spore levels.
by Deborah Jeanne Sergeant