Mastitis is an infection of the udder which negatively impacts milk quality. Pooling milk dilutes the effect of single infected cows and herds but in most jurisdictions the cumulative effect of mastitis, especially subclinical mastitis, is significant. Olson as cited in Eck and Gillis (2000) estimates a cheese yield loss of 1% if 10% of the milk is from cows with subclinical mastitis. Further, as noted below, the quality effects of mastitic milk are probably of more economic importance than the yield effects.
Causative organisms include human pathogens such as E. coli and Staphylococcus aureus. Nonbacterial infections such as prototheca infection also cause high SCC. Based on new automated procedures for bacteria counting, Ontario producer milk data suggests that prototheca is a common mastitic agent and frequently contributes to high SCC and bacterial counts.
Typical Ranges of Somatic Cells
Somatic cells include any type of 'body' cell in the milk, such as skin cells (epithelial) from the cows' udders and leucocytes of several types. Leucocytes are white blood cells which are part of the cow's immune response to infection in the udder, so they are used as an index of mastitis or udder infection. Several observations are relevant:
- The milk of healthy cows should contain less than 100,000 somatic cells per ml of milk. Higher counts indicate subclinical mastitis (infection in the udder).
- Clinical mastitis is associated with counts greater than 1,000,0000/ml
- Producer milks in Ontario average about 200,000 cells/ml but counts less than 100,000 can readily be achieved with good herd management
Critical Ranges With Respect to Milk Quality
There is evidence that counts as low as 100,000 cells/ml affect cheese yield (Barbano et al, 1991, J. Dairy Sci. 74:369) and the quality of other dairy products such as ultra-high temperature milk. SCC in the range of 250,000 - 500,000 are associated with altered milk composition and decreased cheese yield. When counts exceed 1,000,000 cells/ml, altered milk composition and reduced cheese yield, are obvious.
Gross composition effects of udder infection are not significant for SCC less than about 250,000/ml. Above that level the following trends are observed:
- Little change in fat content
- Increased mineral content, particularly more cloride.
- Decreased lactose content which balances the osmotic effects of increased mineral content.
- Protein effects:
- Less casein
- More whey proteins, especially immunoglobulins
- More nonprotein nitrogen
- Increased pH (up to 7.5 whereas 6.7 is normal)
High SCC are normally associated with shedding of pathogenic (to humans) bacteria in the milk including E. coli, S. aureus and others. Basically, whatever organism is causing the udder infection, including the algae, prototheca will be present in the milk. Further, growth factors present in high SCC milk encourage growth of both E. coli and S. aureus (Amer. J. Vet. Res. 45:2504). The growth rates of some lactic cultures are also affected; Streptococcus thermophilis grows faster and Lactobacillus acidophilus is inhibited.
Significance To Cheese Milk
Cheese yield is affected in two ways:
- Mastitic milk contains more plasmin, a heat stable milk protease which degrades protein and causes more protein to be lost in the whey.
- Reduced casein directly affects cheese yield.
- Poor curd formation (longer flocculation time, slower rate of curd firming, and reduced maximum firmness) contributes to yield loss as fines.
Perhaps more important than yield are the effects of subclinical mastitis on cheese quality (J. Dairy Res. 53:645). Modest levels of SCC cause several quality problems:
- Decreased curd strength due to high whey proteins, low caseins, high pH and altered calcium-phosphate-caseinate balance. As noted above, these changes affect cheese yield, but they also impact quality.
- Higher moisture cheese due to impaired curd syneresis.
- Soft, less elastic, sticky and grainy cheese texture.
- Increased flavour intensity, usually with off flavours
Significance to Fluid Milk
Very high counts (>2 million) will cause milk to taste salty and result in many quality problems. Lower counts, even as low as 300,000 can increase development of bitter flavour due to increased levels of plasmin. This is a particular problem with ultra-high-temperature processed milk because the enzyme is heat stable and the storage time is long enough to permit significant protein degradation.