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A special feature for South West Farmer by Miss Upuli Wijewardene BSc, Pg D (HACCP) and Dr H M Premlal Ranjith PhD, FIFST, MBIAC, CSci, Diotte Consulting & Technology Limited, Nantwich.

Milk is known to be one food commodity that is subjected to more legislative controls than any other food in the world.

Today the UK milk production amounts to 14,130 million litres and in 2005 the total UK milk and dairy products market was estimated to be worth £8.16 billion at retail selling prices.

In milk business the initial consideration is to optimise the yield and preserve its quality by cooling from the body temperature (37C) to about below 5C. Although the main advantage is the control of microbial population in milk, there are other factors such as damage to its components such as milk fat globules as well as aeration from agitation could become a problem due to the methods used for cooling the milk especially when using a highly automated milking system.

Bovine milk is generally collected from lactating animals and can be easily contaminated from the milking and subsequent handling procedures in the farm if the hygiene guide lines are not followed. The abundant constituents such as carbohydrates, proteins, fats together with the neutral pH of milk encourage the growth of a variety of micro-organisms. The initial bacterial count or the microbial quality of milk can vary noticeably due to milk handling practices. The microbial population of milk can vary from few a thousand bacteria per millilitre of milk to a few million. The Food Hygiene (England) Regulations 2006 states that plate count at 30C per ml of raw cow's milk should be < 100 000.

According to the practical guide for milk producers to the Food Hygiene Regulations for England and Wales, 2006 prepared by the Dairy Hygiene Inspectorate and new EU food hygiene legislation, which came into effect on January 1, 2006, it is important that milk must be cooled immediately after milking to not more than 8C in the case of daily collection, or not more than 6C if collection is not daily to minimise bacteria multiplication.

Temperature is the most important factor affecting bacterial growth, reproduction and decomposition of food. Bacteria can only develop within certain temperature limits, and these limits vary from one species to another. In general, growth of bacteria in milk and milk products is considerably reduced by cooling to below 10C.

Rapid cooling to below 4C would slow down the growth of bacteria in the milk and almost completely stop all the activities that are mostly responsible for the poor quality of ex-farm milk. It improves the keeping quality of milk immensely and cooling to 4C in conjunction with milking methods, makes it possible to deliver milk every other day or at two or three day intervals, provided that the farm milk container/tank is well insulated.

The unsanitary production and processing practices on the farms produces milk with very high initial bacterial count and at the blood temperature it is a favourable environment to increase in numbers. This increase would reach significant proportions if control measures are not in place to immediately cool to preferably around 4C within a short period. Avoiding post milking contamination and following good hygiene practices, and efficient cooling of milk immediately after milking, are good control measures to ensure high quality milk.

Milk cooling methods Direct expansion cooling system cools milk while in the bulk holding tank due to a refrigerated base plate at the bottom of the tank and efficient agitation in the tank. In this system the heat of the milk goes through the stainless steel wall of the tank to the refrigerant and with the evaporation of refrigerant, it removes the heat from the milk. Direct expansion tanks do not have a cold buffer, so that energy must always be available.

In present dairy farming, milk flow is comparatively high and prolongs milking periods which induces potential cooling problems to the farmers. This resulted from growing number of farms opting for keeping an economic quantity of cows to increase the volume desirable to operate the unit. The modern technologies are responsible for speeding the milking process and this in turn increases the amount of milk per time while reducing the time between milking.

The conventional direct expansion cooling tanks will provide slower cooling rates when they are overloaded with bulk milk (one to two hours to cool based on the volume). Therefore, the bacterial count will be higher. The long cooling times combined with prolonged agitation may also cause buttering of milk and increases the percentage entrained air and dissolved oxygen.

The cooling of milk immediately after milking using an instant cooling system will be more effective solution for these problems where it cools the milk down to about 4C in a matter of seconds before milk reaches the storage tank. This is an in-line system where milk goes from the milking units to the balance tank and then pumped to the plate cooler at a constant rate.

When the milk leaves the plate cooler, the temperature of milk has been reduced to about 2 - 4C and the milk is pumped continuously to the insulated storage tank, where it can be kept, with occasional agitation, until collection.

According to the study of Bachman and Wilcox (1990), immediate, rapid cooling of raw milk samples increased the fat hydrolysis that occurred during cold storage. However, they concluded that use of plate coolers and refrigerated receiver jars to cool raw milk on commercial dairy operations rapidly and efficiently are unlikely to increase the incidence of hydrolytic rancid flavour detected in bulk milk shipments, because under farm conditions, milk from several cows is bulked before cooling.

Most farms are taking advantage of plate cooling systems for in-line milk cooling before sending into the bulk storage tank. Milk may be cooled to about 20C using town water or borehole water. This milk could be further chilled to below 10C in approximately 20-30 minutes in the bulk milk storage tank (direct expansion type). Alternatively an additional plate heat exchanger is used with chilled water to cool milk immediately to below 5C, normally to 4-5C before entering the bulk tank. It is important not to employ oversize pumps to transfer milk, as this tends to inflict mechanical damage to fat globules which will rapidly increase the free fat in the raw milk. As indicated before, free fat is quickly turned to free fatty acids (FFA) due to lipolysis which is responsible for the rancid flavour in milk.

If a glycol chilling unit is used, then all precautions must be taken to ensure that milk is kept above freezing temperature within the plates through out the cooling period.

Considering the information available from the literature it may confuse some readers to decide as to the best solution for cooling milk in the farm of various daily output. It is unquestionable that cooling raw milk immediately after milking to 4-5C is advantages to minimise the proliferation of micro-organisms. For rapid cooling of milk in modern farms, plate type heat exchangers are used where one section is supplied with town or bore-hole water and chilled section use water at 2-3C from ice-bank system or a glycol unit.

This method would allow milk at 4-5C to be stored in the bulk tank with very minimum agitation. Therefore, it should reduce fat globule damage both due to shear and frictional forces from air bubbles. Continuous agitation also increases the dissolved oxygen content in milk which is disadvantaged as it encourages the oxidative rancidity.

The milk transfer pump should be selected appropriately to match the duty so that mechanical damage arising from an over size pump is minimised. A chilling plate heat exchanger placed near the milking unit may not be practical for large farms but a centrifugal pump can be installed to create suction in the pipeline where milk has to pass through the plate cooler to reach the pump. The milk temperature would be about 5C when it reaches the pump and that enables to reduce the FFA in the cooled milk.

For a full report on this subject, log on to www.southwestfarmer.co.uk

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