Estimated reading time: 4 minutes
Quality defects in milk and dairy products can result from elevated somatic cell counts (SCCs) in milk from animals infected with foot-and-mouth disease (FMD).
Jompie Burger, general manager of the Dairy Standard Agency (DSA), discussed the risks and contamination concerns associated with FMD-related SCCs in milk during a recent DSA webinar.
FMD virus resistance
The FMD virus is environmentally resilient but can be inactivated outside a pH range of six to nine. Lowering the pH of milk is therefore beneficial, although excessive lowering can compromise product quality. Fermented products such as amasi and yoghurt typically reach a pH of ~4,6 at which the virus struggles to survive; it is also unstable under alkaline conditions. Heat treatment at 72°C for 15 seconds can reduce viral load.
The problem with the FMD virus is that it is shed into milk before clinical disease signs appear. In infected animals, the virus can replicate in the udder, while teat lesions can increase the risk of bacterial infections, leading to clinical and subclinical mastitis. Secondary infections are often caused by environmental pathogens such as Staphylococcus and Streptococcus species.
Burger highlighted the following key points regarding the impact of SCC on pasteurised milk:
- High SCC affect casein fractions in both raw and pasteurised milk, especially in low-fat products. In high-fat products, heat treatment may not fully eliminate viruses, as they can be protected within fat globules.
- The main effect of high SCC in pasteurised milk is enzymatic breakdown (hydrolysis) of casein, causing sensory defects such as bitterness.
- Milk may develop rancid off-flavours, often described as soapy, bitter, with a lingering aftertaste.
- This bitterness is linked to increased levels of short-chain fatty acids.
- Rancidity is a major quality defect in market milk, but these flavours do not necessarily indicate the presence of FMD; they result from casein breakdown due to high SCCs, which may have other causes.
- A decrease in casein nitrogen (CN) during cold storage, especially in high-SCC milk, indicates proteolytic activity even after pasteurisation.
FMD and SCC levels
In FMD-infected animals, SCCs rise sharply during the incubation period and lesion development. High SCCs can quickly escalate, rendering the milk unsuitable for further processing.
Elevated SCC in milk also negatively affects cheese production – it reduces fat and protein recovery in cheese due to poorer rennet coagulation, weaker cheese-making properties, and increased proteolysis and lipolysis. High SCC can also cause slow, weak coagulation due to changes in milk proteins, mineral imbalances, and elevated pH. As SCC increases, the protein content of cheese decreases significantly.
In typical cheese-making, 10â„“ of milk will produce 1kg of cheese; with high-SCC milk, the yield will decrease substantially. Research shows that SCC as low as 100 000 cells/mâ„“ can affect the organoleptic properties of cheese. The ideal in cheese production is SCC levels below 200 000 cells/mâ„“.
High SCC milk can pose various food safety risks, especially when secondary infections are present in the cow’s udder. While studies show that bovine leukocytes are not harmful to humans, high-SCC milk can harbour pathogens and toxins.
Key protocols
Once FMD is detected, strict protocols must be established to ensure proper milk collection, transport, and treatment. Once a farm is deemed FMD positive, no animals or animal products, including milk, are allowed to leave the farm without a strict, individualised assessment by veterinary authorities.
Key protocols for FMD-infected milk management include:
- Milk collected from quarantined areas may be transported only with a Red Cross permit.
- Milk tankers must undergo rigorous cleaning and disinfection before and after transport, following strict ‘come clean, go clean’ protocols.
- Only one designated entry point is used for vehicles collecting milk, with mandatory disinfection stations operated by trained personnel.
- Milk from affected areas cannot be sold for conventional use; it must undergo specialised processing to destroy the virus. For export purposes, the standard protocol is double pasteurisation or ultra-high temperature (UHT) (148°C for three seconds) to ensure the FMD virus is completely inactivated. Procedures to ensure inactivation include heating milk to 100°C for 20 minutes, or a pH shift below six or above nine.
- Processing plants must ensure that milk from quarantined farms is kept entirely separate from uninfected milk, which often requires processing only once all other products are completed, followed by a full plant disinfection.
- Normal bleach and iodine do not kill the FMD virus. Use only approved disinfectants that include 2% sodium hydroxide, 4% sodium carbonate, and citric acid.
- Contaminated materials, including milk that cannot be transported, should be destroyed on-site under veterinary supervision.
– Christal-Lize Muller, Plaas Media
For more information, contact Jompie Burger at 012 665 4250 or jompie@dairystandard.co.za
