Providing the best quality milk to customers is the dairy farmer’s goal. However, there’s a constant battle with bacteria, and it isn’t easy to win.
Dr. Nicole Martin, assistant research professor in Dairy Foods Microbiology and associate director of the Milk Quality Improvement Program at Cornell University, discusses the impact of spores on organic raw milk.
Martin explained that some bacteria produce resistant spores that survive pasteurization and cause spoilage. Spores are naturally found in soil, plant material and water, so it’s relatively easy for them to get into raw milk and survive pasteurization or the cheesemaking process.
“In fluid milk, there are a number of different types of spore-forming bacteria that can cause spoilage over shelf life,” said Martin. “Because spore-formers go back to a metabolically vegetative form, it takes some time at cool temperatures. For the first 10 days, there’s no growth, but by 17 to 21 days of shelf life, they reach spoilage levels.”
Bacteria grow and start producing enzymes that result in odor and flavor defects in milk or cheese, eventually causing product spoilage. These organisms are the biological barrier to shelf-life extension.
Processors strive to extend shelf life for several reasons, including the ability to distribute milk to more outlets, reach new markets and create more flexible scheduling. Shelf life also influences customers’ product experience, especially for those who are hesitant to purchase milk or milk products due to potential spoilage before the product is consumed.
One main spore-forming group are psychrotolerant spores, which are cold tolerant and can grow at low temperatures. These spores are responsible for fluid milk spoilage. They grow at refrigeration temperatures and produce enzymes that break down milk protein, resulting in “sweet curdling.”
Another group are anaerobic butyric acid-producing bacteria (BAB), which grow in the absence of oxygen. The most common is Clostridium, which causes a defect in certain styles of cheese known as “late blowing.”
“It’s called late blowing because it causes gas production 60 to 90 days into the aging process,” said Martin. “This organism also produces butyric acid, which has an unpleasant smell and taste.” These issues cause economic losses for cheesemakers whose cheeses are susceptible to this type of spoilage. Dietary silage contains high levels of butyric acid, which can lead to high levels in raw milk.
Several groups of spore-forming bacteria are relevant in dairy powders. Because dairy powders are a big opportunity for dairy exports, mesophilic and thermophilic spores are used as quality indicators for exported milk powders.
Mesophilic bacteria grow at mid-range temperature (ambient to body temperature); thermophilic bacteria grow at much higher temperatures. Powdered milk can host spores from both raw milk and the processing facility.
“Spores are found in the laboratory pasteurization count test,” said Martin. “It’s a common test applied to raw milk premium programs, especially for some organic producers who have to have a laboratory pasteurization count test below a certain level that can affect their premium and contract.”
Although spores are ubiquitous in the natural environment, dairy farmers can manage spore levels. “We can reduce spores through simple, practical implementation of interventions at the farm level,” said Martin. “We know at the farm level nothing happens in isolation. A lot of the factors that affect milk quality are interrelated.”
The condition of udders and teats is significant, as well as the milking system, housing and bedding management. Environmental influences and mastitis management are also factors.
Cows’ environment, whether inside or outside, is a factor in bacterial spores in milk. Photo by Sally Colby
Which practices should be prioritized to reduce spores in bulk tank milk, either to control Lab Pasteurized Counts (LPC) or to improve the quality of the finished product? What are the spore levels in raw milk, farm practices and management, and do meteorological conditions drive spore levels in organic bulk tank raw milk?
Because environmental and management factors are related to spoilage, identifying the cause of spores in raw milk is complicated. Martin described a national study that included milk samples from farms in New York, Pennsylvania and Vermont. The initial survey included questions about farm location, how long has the farm been certified organic, udder hygiene scoring, percent dry matter intake from pasture, stocking density, employees (family, hired, part- or full-time), milker training, glove usage, udder stimulation, pre- and post-dipping, housing, bedding, milking system, milking frequency and milking system location on the farm.
Over 600 raw milk samples were tested for psychrotolerant spores, mesophilic and thermophilic spores and anaerobic BAB.
Certain factors influenced higher spore counts, including herds that were certified organic for fewer than nine years and clipping/flaming udders.
These results led to a study to determine whether removing udder hair reduced spores in organic bulk tank raw milk and milk powder. The study included four farms that had not been removing udder hair. Researchers collected bulk tank milk for a week and made milk powder from the initial samples, then removed udder hair by singeing. Milk from samples after singeing was also made into powder.
“There was reduction in both thermophilic and mesophilic spores,” said Martin. “We saw no difference in psychrotolerant spores, and virtually no difference in anaerobic spores.”
Although there weren’t enough samples for a valid study, the major drop in both thermophilic and mesophilic spores suggest that removing udder hair lowers spore counts.
Pasteurization is the most effective way to kill spores. The high-temperature, short time pasteurization (HTST) process is typically run at a minimum of 72º C for 15 seconds. “Spores react differently to different temperatures,” said Martin. “When HTST temperatures are below 76º Celsius for 20 seconds or so, spores don’t germinate well. They need a high heat treatment to shock them into growing. We see them growing much faster when the heat treatment is above 76º.”
Martin said spores originating from natural environments that end up in raw milk on the farm impact finished product quality and raw milk testing. However, there’s no single approach to reducing spores in organic bulk tank raw milk. Dairy farmers must be aware of the influence of management, weather conditions, stocking density, worker training and other factors, and work within their own farm’s conditions to find ways to reduce spores in bulk tank milk.
by Sally Colby
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