Comment

Milk is known to be one food commodity that is subjected to more legislative controls than any other food in the world. Today it is still a major agricultural produce in most countries and responsible for a significant contribution to gross domestic production (GDP).

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.

The emphasis now is to achieve the optimum scale of production for each business producing the best quality milk. In milk business the initial consideration is to optimise the yield and preserve its quality by cooling from the body temperature (37oC) to about below 5oC.

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.

Milk a wholesome food but perishable Milk is a prime source of nutrients for human being, and yet renders a great habitat for microbial growth and metabolism. Milk in its natural state is a highly perishable material because it is susceptible to rapid spoilage by the action of contaminating microorganisms and naturally occurring enzymes thus many processes have been developed over the years to enhance its utilization and safety (Singh & Bennett, 2002).

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 microorganisms. 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 thousand bacteria per millilitre of milk to few million. The Food Hygiene (England) Regulations 2006 states that plate count at 30oC 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 1 January 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 minimize 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 10 C.

Rapid cooling to below 4 C would slow down the growth of bacteria in the milk and almost completely stop all the activities that mostly responsible for the poor quality of ex-farm milk. It improves keeping quality of milk immensely and cooling to 4 C 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 4oC 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 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 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 intern increases the amount of milk per time while reducing the time between milking.

The conventional direct expansion cooling tanks will provide with slower cooling rates when they are overloaded with bulk milk (1-2 hrs 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 4oC 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 - 4 C 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. Consequently, conditions favouring lipolysis in a particular cow's milk would be attenuated by the conditions prevailing within the other cow milks present in the bulk mix at the point of cooling.

Some facts with respect to rapid cooling of milk 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 20oC using town water or bore-hole water. This milk could be further chilled to below 10oC 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 5oC, normally to 4-5oC 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. In some operations the cooled milk reaches 2-3oC at the exit point of the plate cooler but this is the final combined temperature of the milk after internally mixed (inside the plate pack) with very low temperature milk(may be below 0oC) with cool milk. By cooling milk to near freezing temperature it induce thermal shock to milk components such as protein which will become sensitive to heat treatment during processing to make products.

Wiking (2005) reported that 60-70% of FFA in pasteurised milk was due to milking and milk transfer to bulk tank where flow velocities develop friction against pipe walls leading to greater fat globule damage. The shear rate depends on the diameter of the pipe and the flow velocity. In addition, the presence of air, the temperature of the milk and fat content affect the stability of milk fat globules during mechanical treatment of milk. Air bubbles cause rupturing of the fat globules allowing globule contents to flow from damaged cell membranes. It is also evident that milk containing larger fat globules (>8µm) shows tendency to develop more FFA (Wiking, 2005). Mulder and Walstra (1974) indicated that the normal fat globules in milk range from about 1 µm to 10 µm and most globules are about 2-4µm. Wiking (2005) also found that in smaller fat globules, more shorter fatty acid chains (C12, C14, C16:1) dominates while in large fat globules C16 and C18 were the major fatty acids. One major reason for this variation was found to be due to feeding material used by the farmers. Therefore, feed material high in saturated fat supplement tends to have high fat milk, large globules and increase FFA which means it has the tendency to develop poor quality milk products arising from rancidity.

An important finding reported by Wiking (2005) is that the FFA was high when milk was pumped and agitated at 15-20oC and lowest at 5oC. This highlights that if the milk is slowly cooled in the direct expansion tank, it can cause grater damage to fat globules both by shear damage as well as frictional forces from gas bubbles. They also reported that pumping milk after cooling to 5oC reduces FFA which means installing a plate cooler as close as possible to udders.

The modern automatic milking systems allow more than two milking sessions due to shorter period per milking. The results showed that cows milked four milking per day had larger fat globules and more FFA than twice milked per day.

Optimum solutions for milk cooling 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 4-5oC 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 2-3oC from ice-bank system or a glycol unit.

This method would allow milk at 4-5oC 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 disadvantages as it encourages the oxidative rancidity. The milk transfer pump should be selected appropriately to match the duty so that mechanical damage arising from 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 pipe line where milk has to pass through the plate cooler to reach the pump. The milk temperature would be about 5oC when it reaches the pump and that enables to reduce the FFA in the cooled milk.

References Bachman, K. C. & Wilcox, C. J. (1990) Effect of Time of Onset of Rapid Cooling on Bovine Milk Fat Hydrolysis, J Dairy Sci. 73, 617-620 Food Hygiene (England and Wales) Regulations, 2006, A Practical Guide for Milk Producers; Milk Hygiene on the Dairy Farm, Food Standards Agency Mulder, H. & Walstra. P. (1974) The Milk Fat Globule, Commonwealth Agricultural Bureaux, Farnham Royal, Bucks, UK Singh, H, Bennett, R. J, (2002) Milk and Milk Processing in Food Microbiology Handbook ( Edi. R. K. Robinson) John Wiley, US Wiking, L. (2005) Milk Fat Globule Stability, Ph D Thesis, Swedish University of Agricultural Sciences, Uppsala, Sweden

Print

Email this

Comment

Add your comment

Name:
Email: *
Location:
**
	e.g. 123-123
Comment: Please note: All HTML tags will be ignored. Format Text:
By posting a comment, I confirm that I have read and agree to the terms of use. Comments are not moderated but we will react if anything that breaks the rules comes to our attention and we may delete inappropriate postings. Please treat other people with respect. You must not post anything that is abusive, indecent, unlawful or defamatory. Remember, you are personally liable for what you post on this site. If you wish to complain about a comment, contact us here.
* Your email address will not be displayed ** To avoid register now or login