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A processing perspective: SCC and spore-forming bacteria

Michael Gould Published on 18 April 2014

A large portion of any dairy farmer’s milk check can come from quality bonuses. These quality bonuses ordinarily pay additional money for shipments of milk that test low in somatic cell count (SCC) as well as bacteria, which is measured by total plate count.

These are on top of the usual payments for butterfat and protein. All of these quality components have a direct effect on the finished dairy product that is created with this milk.

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Elevated SCC is a function of a cow’s immune response system. Leukocytes, or white blood cells that are measured by SCC testing, act as an immune response, targeting pathogens in the teat canal that cause mastitis. However, they have a very different effect in dairy products.

Leukocytes break down during milk processing and throughout the shelf life of a dairy product in the store or in a consumer’s refrigerator. This breakdown releases heat-resistant enzymes from inside the leukocyte that can damage proteins and milk fat in the product.

Pasteurization and heat treatment cannot destroy these enzymes and therefore cannot be used to prevent the deterioration of milk.

If somatic cell count were kept below 25,000 cells per mL of milk, high-heat pasteurization of a 2 percent gallon of milk could be expected to last 55 days without experiencing high levels of bitterness associated with leukocyte breakdown.

This is nearly a doubling in shelf life compared to the 28-day shelf life of high-somatic cell count 2 percent milk (SCC 340,000) with the same low bacterial counts.

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One of the main consequences of damage caused by high SCC is the production of highly flavorful (and not in a good way) free fatty acids and peptide compounds associated with the decomposition of protein that make off flavors in milk products.

Rancid flavors (which are caused by the degradation of fat compounds into free fatty acids) and bitterness (which are contributed by proteins degrading into peptides) are accelerated over the shelf life of a product made with high-SCC milk.

Damaged protein in milk used in cheese-making can also reduce yields in cheese processing. Increased somatic cells counts lead to a reduction in the number of pounds of cheese that can be produced from each hundredweight of milk. Above 400,000 SCC can lead to losses between 2 to 5 percent in finished cheese yield and reduce whey quality.

Bacterial spoilage is the most prevalent cause of dairy product degradation. The detrimental effects of the poor control of bacterial spoilage organisms can range from the production of off flavors to the growth of harmful pathogens to protein and fat degradation.

Much of traditional dairy processing has focused on slowing this degradation. Pasteurization of fluid milk, culturing of yogurt products, drying of dairy powders and cheese-making processes are all designed to increase the shelf life of milk.

Advances in cooling, cleaning and aseptic product-filling technologies have slowed the rate of bacterial degradation leading to ever-improving product quality and increases in product shelf life. Further reduction in on-farm bacteria can only serve to increase finished product quality. Great-quality dairy products can only be made from great-quality milk.

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One of the difficulties in modern dairy processing is controlling spore-forming bacteria found in milk. Under standard cold storage conditions, these bacteria stay in their spore phase, where they are protected from heat, acid and drying.

Spores can be activated by the heat in pasteurization systems. Once spores are activated by the heat of pasteurization, they can begin their growth and reproduction phases at cold storage temperatures.

When spore-forming bacteria are in their growth phase, they behave like other spoilage bacteria by damaging the quality of proteins, fats and producing off flavors. This spoilage from spore-forming bacteria is one of the leading causes of off flavors in the gallon of milk hidden in the back of a refrigerator.

Spore-forming bacteria are particularly difficult to deal with in dairy powders, shelf-stable fluid milk and high-heat-treated products. Many of these products are export market-bound and can face challenging storage conditions that can increase milk quality degradation. As soon as dairy powders are heated and reconstituted with water, they can begin their own growth and reproduction stages.

As for shelf-stable and high-heat- treated products with an extended shelf life, having spores in these products can mean spoilage before the specified shelf life has been reached. This, in turn, may cause consumers to throw away products and dissuade customers from buying these products in the future.

A decrease in the number of spore-forming bacteria present in raw milk can be effective in ensuring that milk reaches its expiration date without defects and could potentially increase the shelf life of dairy products.

On-farm control of both somatic cell count and the bacterial load of raw milk is a constant battle to be fought. Ensuring that cows are frequently evaluated for somatic cell count and keeping high-SCC milk out of the bulk tank can help to keep SCC counts shipped to the processor in check.

Keeping bacteria out of milk will always be about following proper procedures. This includes milking clean, dry teats in the parlor, keeping the milking equipment free of manure and debris, following good cleaning procedures and properly cooling raw milk in the bulk tank.

Developing procedures to keep spore-forming bacteria out of raw milk is an area of ongoing research. It has proven difficult to determine the exact origin of spore-forming bacteria on farms or in processing.

Current research is focused on finding bacterial DNA at different stages of milk production and processing. Being able to determine which bacteria come into contact with the milk at each stage will enable scientists to find better ways to eliminate specific spore-forming bacteria in milk and therefore improve final product quality.

From a processing point of view, somatic cell count and bacterial spore levels can have an effect on shelf life as well as product quality. Any processor knows that the end dairy product – be it fluid milk, cheese, yogurt, butter or dairy powders – is only going to be as good as the quality of raw milk they receive from producers.

By lowering the somatic cell count and bacterial load, including spores, of each producer’s milk, processors are not only more flexible with the products that they can make with the incoming raw milk, but they also can maximize final product flavor and texture, and even increase product shelf life.

The strength of the dairy market hinges on creating products that meet the needs of the farmer, processor and consumer. Producing milk that has low somatic cell count and reduced spoilage organisms stands to benefit everyone involved. PD

Michael Gould is a dairy product development scientist. He is a Cornell University graduate with experience as a dairy herdsman and in yogurt, cheese and fluid milk manufacturing.

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