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THE EFFECT OF HIGH SCC LEVELS ON MILK AND CHEESE QUALITY

Discipline: mastitis; Key words: somatic cell counts, pasteurized fluid milk quality, shelf-life study, cheese, total solids, protein. 

Although most of us are familiar with Somatic Cell Count (SCC) as indicator of mastitis and udder health, one tends to forget the details of how the relationship works, what are the implications and also, what are the limitations of the method. To fill in the details for the reader, I went back in history and consult research that has been done in the early years after the development of SCC as a standard indicator. For that purpose I have consulted publications by the Agriculture and Horticulture Board of the UK, a paper by Dr Y. Ma and colleagues of Cornell University, which was published in the Journal of Dairy Science, Volume 83 of 2000 (pages 264 to 274), with the title: Effects of Somatic Cell Count on Quality and Shelf-Life of Pasteurized Fluid Milkand a paper by Drs I. Politis and K. F. NG-Kwai-Hang of McGill University, Quebec, also published in the Journal of Dairy Science, Volume 83 of 1988 (pages 1711 to 1719), with the title: Effects of Somatic Cell Count and Milk Composition on Cheese Composition and Cheese Making Efficiency.   

The principles of SCC use are: Somatic cells originate because of infection, the majority of them are leukocytes (white blood cells) - which become present in increasing numbers in milk usually as an immune response to a mastitis-causing pathogen - and a small number of epithelial cells, which are milk-producing cells shed from inside of the udder when an infection occurs. Somatic Cell Count is quantified as the number of cells per ml of milk. In general terms:

  • An individual cow SCC of 100 000 or less indicates an 'uninfected' cow, where there are no significant production losses due to subclinical mastitis.

  • A threshold SCC of 200 000 would determine whether a cow is infected with mastitis. Cows with a result of greater than 200 000 are highly likely to be infected in at least one quarter.

  • Cows infected with significant pathogens have an SCC of 300 000 or greater.

Dairy farmers are financially rewarded for low herd SCCs and penalised for high ones, because cell counts reflect the quality of the milk produced and how mastitis can affect its constituent parts, having implications for its keeping abilities, its taste and how well it can be made into other dairy products such as yoghurt or cheese (see below). Thus, the principle is that SCC is also a main indicator of milk quality.

For the cow though, a lower SCC indicates better animal health, as somatic cells originate only from inside the animal's udder. SCC monitoring is important because as the number of somatic cells increases, milk yield is likely to fall, primarily due to the damage to milk-producing tissue in the udder caused by mastitis pathogens and the toxins they produce, particularly when epithelial cells are lost. A particularly low SCC is sometimes regarded as a sign of poor immune response, but it may be simply due to a low level of infection. The immune response is a reflection of how quickly the immune system reacts to the disease challenge, rather than how many white blood cells are present before infection occurs.

Cell counts tend to reflect a response to contagious mastitis pathogens: the Bactoscan count, on the other hand, indicates the level of bacterial contamination from external sources, such as insufficient cleaning of the milking equipment or poor udder and teat preparation prior to milking, and can indicate a high level of environmental pathogens.

Effect on milk and dairy product quality: The results of the studies on pasteurized milk and cheese referenced above are significant.

In the milk study, milk was collected from eight Holstein cows four times before and four times after udder infection with Streptococcus agalactiae. Post-infection milk had significantly higher SCC of 849 000 cells per ml than pre-infection milk (45 000 cells perml). High SCC raw milk had more lipolysis and proteolysis than low SCC raw milk. Pasteurized milk were stored at 5°C and analyzed for lipolysis, proteolysis, microbial quality and sensory attributes at 1, 7, 14 and 21 days. During refrigerated storage, the average rates of free fatty acid increase by lipolysis and casein hydrolysis in high SCC milk were, respectively, three and two times faster than those in low SCC milk. Low SCC milk maintained high organoleptic quality for the entire 21-day shelf-life period. However, for high SCC milk, between 14 and 21 days, sensory defects were detected, which resulted in low overall quality ratings. The sensory defects mainly included rancidity and bitterness and were consistent with higher levels of lipolysis and proteolysis. Hence, mastitis adversely affected the quality of pasteurized fluid milk. 

In the cheese study, monthly milk samples with varying SCC were collected over one year from 42 Holstein cows. Milk was analyzed for fat, protein, lactose, casein and SCC and was used for laboratory scale cheese making. Cheese was assayed for fat, protein, total solids, and salt. Losses of milk components in the whey were also determined. The results showed that the levels of SCC in milk were negatively related to fat, protein, total solids and fat in the cheese. An increase of SCC from 100 000 to above 1 000 000 per ml resulted in a cheese containing approximately 6.8, 3.6, 4.9 and 1.5% less fat, protein, total solids and fat, respectively and 4.4% more moisture in the non-fat substances. Contrary, the levels of SCC in the milk were positively related to protein losses in the whey. Overall, the protein losses increased approximately 6.8% for the first million increase in SCC per ml. Thus, the results show that the composition and quality of cheese are negatively affected by high SCC.