milk quality measures

Measures of Milk Quality Somatic Cell Count: The number of somatic cells in milk, referred to as the somatic cell count or SCC, is used throughout the world as an indicator of milk quality. The current regulatory limit for somatic cells in milk in the United States defined in the Grade "A" Pasteurized Milk Ordinance is 750,000 cells per milliliter (mL). For a variety of very good reasons, there is continuing pressure from animal health advocacy groups to reduce the regulatory limit for somatic cells in milk from the current 750,000 cells per mL to 400,000 or less.
Poor quality milk has a high number of somatic cells and is an inferior product with reduced processing properties resulting in dairy products with a reduced shelf-life. On the other hand, high quality milk has a very low number of somatic cells, a longer shelf-life, tastes better and is more nutritious. One characteristic feature of cows with mastitis is a significant elevation in the number of somatic cells in milk. Milk from uninfected mammary glands contains less than 100,000 somatic cells per mL. A milk SCC of more than 200,000 per mL suggests that an inflammatory response has been elicited, a mammary quarter is infected or is recovering from an infection, and that milk has reduced manufacturing properties. It is not uncommon for milk from cows with subclinical and/or clinical mastitis to contain several hundred thousand and even millions of somatic cells per milliliter of milk. Thus, an increase in the SCC of milk is a good indicator of mastitis or inflammation in the udder. Infection of the udder by mastitis pathogens alters milk composition and reduces milk yield. Most studies that evaluated the influence of mastitis on the composition of milk used SCC as the basis for determining the infection status of udders and for determining the degree of inflammation.
The bulk tank SCC (BTSCC) can be used to gauge the udder infection status of a dairy herd and also gives a good indication of the loss in milk production in a herd due to mastitis. As the BTSCC increases, the percentage of mammary quarters infected increases and the percentage production loss increases. Small increases in SCC can impact production. Most herd milk contains between 200,000 to 500,000 somatic cells per mL of milk. These herds are losing at least 8% in potential milk production. Thus, methods of mastitis control that reduce SCC will not only improve milk yield and composition but will also decrease economic losses due to mastitis.
A recent report published by the USDA Animal Improvement Program Laboratory summarized SCC data from all herds in the United States enrolled in the Dairy Herd Improvement (DHI) testing program for 2007. The good news is that the national SCC average for 2007 was 276,000 cells per mL of milk, which is 12,000 cells per mL lower than the 2006 figure. The bad news, however, was that 3.5% of U.S. herds had SCCs in excess of 750,000, and 24% of the national dairy herd had SCC of more than 400,000. In 2006, almost 4% of U.S. herds had SCC of more than 750,000 per mL, and 25% of the national dairy herd had in excess of 400,000 SCC per mL. The SCC of milk produced by dairy farms in the southern region of the United States over the last 10 years was about 35% higher than the U.S. average, with a yearly range of approximately 30% higher in 2000 to almost 41% higher than the U.S. average in 2003. These data demonstrate quite clearly that there is much room for improving milk quality in the United States, and this is particularly the case for milk produced on dairy farms in the Southeast.
Standard Plate Count (SPC): The SPC is an estimate of the total number of viable aerobic bacteria present in raw milk. This test is done by plating milk on a solid agar, incubating plates for 48 hours at 32 °C (90 °F), followed by counting bacteria that grow on plates. The SPC is used to monitor progress since consistent application of proper milking system cleaning practices, proper milking practices, udder hygiene and good mastitis prevention and control practices should allow dairy producers to produce milk with a low SPC, which is less than 5,000 colony forming units (cfu)of bacteria per milliliter. Federal regulations defined in the Pasteurized Milk Ordinance mandate that the milk SPC should not exceed 100,000 cfu/mL. However, most segments of the dairy industry feel that more stringent standards (SPC ≤10,000 cfu/mL) will result in higher quality milk. Though it is impossible to eliminate all sources of bacterial contamination of milk, milk from clean, healthy cows that has been properly collected generally has a SPC less than 1,000 cfu/mL. Consistent application of proper milking practices, udder hygiene and good mastitis prevention and control practices should allow dairy producers to produce milk with a SPC of ≤5,000 cfu/mL, while most farms can produce milk with counts of less than 10,000 cfu/mL. High bacteria counts (more than 10,000 cfu/mL) suggest that bacteria are entering milk from a variety of possible sources. The most frequent cause of high SPCs is poor cleaning of milking systems. Milk residues on equipment surfaces provide nutrients for growth and multiplication of bacteria that contaminate milk of subsequent milkings. Cows with mastitis (streptococcal and coliforms), soiled cows, unsanitary milking practices, failure to cool milk rapidly to <4.4°C (40°F), failure of the water heater, and extremely wet and humid weather can also contribute to high SPCs in raw milk (Figure 1). Some limitations of the SPC method include: 1) no indication of the bacterial types present, 2) no indication of the specific source of high counts, and 3) the SPC does not give a complete count of all bacteria as some bacteria only grow at lower temperatures