A number of tests have been used traditionally to evaluate the quality of raw milk, including, but not limited to, somatic cell counts (SCC), different types of bacterial counts including the standard plate count (SPC) and tests for the presence of drug residues and added water.
The link between different raw milk quality parameters and the economics of dairy product processing and distribution is often not clear or well documented however, which poses a challenge for the development and implementation of financial incentive systems rewarding production of higher-quality raw milk.
This article will provide an overview of how key raw milk quality parameters used in producer incentive programs may be linked to product quality and potential financial benefits for processors. It will, hopefully, help in the continued development and improvement of producer quality programs to facilitate efficient and sustainable manufacture of dairy products.
Key raw milk quality parameters most frequently used for quality incentive programs include bulk tank SCCs (BTSCC) and total bacterial counts (the SPC or alternatives such as plate loop count or bactoscan). While regulatory limits under the U.S. Pasteurized Milk Ordinance (PMO) for Grade A producer milk are 750,000/ml BTSCC and 100,000/ml SPC, producers strive to meet more stringent standards often linked to quality incentives or premium payments offered by many cooperatives or other raw milk purchasers.
The rational for offering premiums may be to simply encourage and reward the production of high-quality raw milk as well as to procure raw milk that allows for more efficient processing and the production of higher-quality products with improved marketability (e.g., new customers; higher prices).
Milk quality and cheesemaking
While BTSCC levels are an important indicator of herd udder health, increased BTSCC levels can also have a negative effect on finished product quality and processing efficiency (specifically in cheese manufacturing).
Increased SCCs are associated with increased enzyme activity, primarily proteases and lipases. Proteolysis can result in decreased cheese yield, milk gelation and bitter flavors. Lipolysis is associated with rancid off-flavors. While different dairy products can be affected, the most well-documented value of low SCC raw milk relates to cheese manufacturing as there is broad consensus that with increased SCCs, cheese yield is reduced and sensory quality is compromised. For example, one study that compared cheese made from milks with <250,000 and >500,000 SCC found that the higher SCC raw milk resulted in increased rennet clotting time (25%), decreased moisture adjusted yield (9%) and inferior texture and flavor. A modelling study of cheese yield and quality concluded that increasing SCC from 100,000 to >400,000 could result in a reduced net revenue of 3.2%.
For dairy products other than cheese, there is limited published evidence for an association of SCC and product yield or finished product quality. There is some evidence that yogurt manufactured from high SCC milk may show decreased sensory quality; these effects may be more likely when raw milk with SCC >400,000 is used.
Milk quality affects milk flavor
While refrigerated storage and short shelf-life make SCC-associated defects less likely for pasteurized fluid milk products, one study noted increased lipolysis and proteolysis and detectable off-flavors at 14-21 days in pasteurized milks made from high (850,000) compared to low (45,000) SCC milk. Pasteurized milk produced from raw milk with 340,000 SCC also reached bitterness threshold values more quickly, after approximately 28 days at 43°F (6°C), compared to 54 days when raw milk with 25,000 SCC was used.
Some studies suggest that use of high SCC raw milk may increase the risk of proteolysis associated defects in UHT shelf-stable milk, although published data on SCC levels linked to these defects is limited. Overall, negative effects of high SCC on fluid milk quality may become more of a concern in extended shelf-life pasteurized milk. While use of lower SCC raw milk provides for a clear financial return in cheese manufacture, the direct economic benefits of using lower SCC raw milk in yogurt and milk manufacture remain less well defined.
Bacteria counts and product quality
Currently used methods to evaluate bacterial counts of producer raw milk enumerate a wide range of bacteria including those easily killed by pasteurization; bacteria that produce heat-stable enzymes; and bacterial spores that survive heat treatments and subsequently germinate into actively growing bacteria.
As the SPC may represent vastly different types of bacteria in different samples, it is challenging to define clear links between producer raw milk SPCs and finished product quality or processing efficiency. In addition, unlike for SCC, bacteria can grow and counts can increase during storage and transport, particularly if the bacteria present grow at refrigeration temperatures (“psychrotolerant” bacteria).
With the SPC, bacterial defects in processed dairy products are generally linked to numbers considerably above the PMO limit of 300,000/ml for commingled raw milk (i.e., >1,000,000/ml), at the time of manufacturing. Specific concerns with high raw milk SPCs include:
- an increased risk of some cells surviving pasteurization with subsequent outgrowth and spoilage;
- the production of heat-resistant enzymes at sufficient levels to cause defects; and
- the milk is already damaged (spoiled) when processed.
In general, high bacteria counts in raw milk are associated with levels of microbial enzymes sufficient to cause defects associated with fermentation by-products (e.g., lactic acid); breakdown of proteins (bitter peptides, coagulation) and fat (rancidity); and other enzymatic pathways (e.g., fruity off-flavor).
Defects in fluid milk
Fluid milk products are likely more susceptible than manufactured products to defects associated with high SPCs in raw milk. In one report, bitter flavor was detected within seven days in pasteurized milk made from raw milks with 16 and 27 million SPC, whereas product made from raw milk with 11 million SPC was still considered good at 14 days.
In another report, a malty defect in pasteurized milk caused by high numbers of Lactococcus lactis in the raw milk, resulted in consumer complaints and product withdrawal. While heat-stable microbial enzymes have been suggested as a concern for many dairy products, including pasteurized milk, defects have been documented best in shelf-stable UHT products, as these enzymes are most active at ambient temperatures. Most research indicates that bacteria counts need to exceed 1-10 million/ml to cause defects. Some bacteria have been reported to produce sufficient enzyme to cause damage at 250,000/ml or less; these reports appear to be the exception and further study in this regard is warranted.
For manufactured products made from raw milk with counts >1-10 million, documented defects include increased levels of proteolysis and/or lipolysis in yogurt, cottage cheese and hard cheeses; decreased cheese yields; and reduced heat-solubility and increased foaming capacity in milk powders. Overall, with current product shelf-life expectations, product defects appear to be linked to SPC values considerably above legal limits. As shelf-life expectations and products are changing, alternative tests that quantify specific spoilage microorganisms may provide for better assessment of product concerns and milk value.
Alternative microbial methods
Among microbial tests other than the SPC, methods that detect and enumerate bacterial spores in raw milk appear to have the most direct link to economically relevant quality criteria for finished products as spores in raw milk can survive pasteurization and other heat treatments and potentially cause defects, even if present at low levels.
For example, sporeforming butyric acid bacteria (BAB) (e.g., Clostridium tyrobutyricum) cause late gas-blowing defects during cheese aging, making the affected cheese unsaleable, potentially resulting in significant economic losses for cheese makers. Testing raw milk to secure a supply that is low in BAB and thus less likely to cause late gas-blowing provides a direct benefit to the processor.
In Netherlands, programs that pay premiums for milk with low levels of BAB spores are in place. Other sporeformers that can influence finished product quality include psychrotolerant spore formers (e.g., Paenibacillus), which cause spoilage in pasteurized milk. As many end users set stringent requirements for levels of meso- and thermophilic spores in dairy powders, raw milk testing and premium programs for different sporeformer types may be valuable as low spore count raw milk may produce end products that demand a higher price.
While a variety of test methods can be used to evaluate raw milk quality, not all have a direct and quantifiable link to the value of the milk for subsequent processing. Based on available data, SCC and SPC affect the value of raw milk for processing for some products, but not all.
For example, while low SCC milk may allow for improved cheese yields, providing a direct financial return for a processor, there is currently limited evidence that low SCC raw milk has a defined economic benefit for yogurt manufacture.
Continued use of incentives to provide high-quality raw milk is important for the dairy industry as a whole as consumer demands for products made from high-quality and sustainable agricultural commodities are likely to increase.
As expectations rise for increased quality and longer shelf-lives, additional work investigating the value of SCC and SPC levels to the different processing sectors might be important, but further study and development of testing and premium programs that target specific microbial groups that have direct impact on quality will likely be more significant in the future.