Cooling milk rapidly is essential for inhibiting bacterial growth and maintaining good milk quality in vat bulk milk.
Milk cooling standards
The quicker milk is cooled after milking, the better the quality. As outlined by Food Standards Australia New Zealand (FSANZ), milk must be chilled to 5°C or less within three and a half hours from the start of the milking process. Milk should be kept at, or below, 5°C until collected from the farm.
Milk that is collected within that three-and-a-half-hour period, which is still measuring above 5°C (for example if the tanker arrives before chilling is complete), will be subject to a risk assessment by the milk processor. Milk that fails this risk assessment cannot be used for human consumption.
Most food safety programs require dairy farmers to monitor and record milk cooling performance and ensure bulk tank thermometers are calibrated regularly.
Plate heat exchangers (PHE), or plate coolers, are the most efficient way to reduce milk temperature quickly after milking.
A PHE consists of very thin stainless steel plates where water or coolant (for example, glycol mixture) flows along one side of each plate and milk flows along the other. This allows heat to be exchanged from the milk to the water, or coolant, via the plate. Some pre-cooling systems have a first water plate cooler and then a secondary plate cooler using chilled coolant. The capacity of a PHE is adjusted by adding or removing plates.
Some practical tips for peak pre-cooling performance include:
- Regularly check the effectiveness of your PHE. This can be done by comparing the difference between the incoming cooling water or coolant temperature and outgoing milk temperature. The difference should be no greater than 2°C. For example, if water entering the PHE is 10°C, outgoing milk should be no greater than 12°C
- Temperatures can be measured by applying a PVC strip thermometer to the water or coolant inflow and milk outflow pipes. The ratio of flow rates between the water and milk is critical. A ratio of between 2:1 and 3:1 of water to milk flow rate is required.
- If the gap between the end-plates is too small, the flow through the plate cooler will be impeded and performance can be compromised. Allow 2-3 mm/plate (refer to manufacturer’s specifications for further details)
- If a plate cooler is not working efficiently, it should be checked by a refrigeration technician.
- Maintain PHE cleanliness by always running the plant wash with a filter sock in place. Blockages caused by build-up of soiling material within the plates affect heat exchanger capacity and reduce performance.
Vats and chilling systems
It is estimated that milk cooling may account for as much as 30 per cent of the total energy costs of operating a dairy. But energy demand and dairy operating costs can be reduced by adopting changes that result in heat recovery from the cooling system. Cooling milk in the vat is influenced by various factors, including:
- Vat design.
- Type of refrigeration unit.
- Volume of milk to be cooled.
- Schedule of pick-ups.
- Prevailing environmental conditions.
Key facts to consider when deciding on a refrigeration system are:Direct expansion
The most common system and best choice if access to power and reliability of supply is not a problem. Plate coolers significantly reduce size and operating costs of refrigeration units.Scroll compressor refrigeration units
Scroll compressors are 10 to 15 per cent more energy efficient compared with traditional ‘open drive’ units. They are now available for large capacity vats (>30,000 litres).Open drive compressor refrigeration units
Traditional design, known for their reliability but less energy efficient that comparable scroll compressors.Thermal storage systems
Less energy efficient than direct expansion but may be the only option if grid power is limited. Initial capital costs are higher than direct expansion systems. Economics become more favourable for larger herds producing more than 15,000L/day (individual circumstances greatly influence the economics).On demand thermal storage systems (chiller units)
Require a smaller quantity of cooling medium (usually glycol), but chiller unit needs to be larger to cope with demand. More costly to operate than other systems during milking (often at peak tariff rates).