Cooling of Milk and Cream and Chilling of Town Milk

New Zealand Journal of Agriculture, Volume 95, Issue 6, 16 December 1957, Page 574

Cooling of Milk and Cream and Chilling of Town Milk

By

M. J. McFETRIDGE,

Supervisor of Farm Dairy Instruction, Department of Agriculture, Hamilton SUFFICIENT cooling of milk or cream is essential for the production of a good product, *” irrespective of whether it is for manufacturing or town supply milk. Such cooling is a necessary preliminary .in securing best results from subsequent chilling of town milk. The principal advantage of efficient cooling is that development of bacteria: is retarded or stopped at low temperatures. Consequently milk and cream produced under good conditions and efficiently cooled will maintain its quality for a much greater period than will uncooled milk or cream, which will begin to deteriorate immediately after production. Cooling will also assist in reducing the incidence of churned cream.

TO obtain efficient cooling of milk or cream it is necessary to have a good supply of cold water at the dairy. The source of supply should be as close as possible to the dairy and there should be sufficient water available to give a ratio of 3 parts of cooling water to 1 part ,of milk being cooled. An efficient cooler operated within its rated capacity .will cool milk to within 3 degrees F. of the temperature of the water at the inlet of the cooler. Several makes of cooler now available will easily do this, but often good equipment fails because of faulty installation. Points which should be watched by the farmer are:— m The water supply to the cooler LU must be both adequate and cold enough. SThe quality of the cooling water LTJ should be considered. If it has a high mineral content, a coating will form on the inside of the tubes, acting as an insulator and making the cooler ineffective. Cooler tubes should .be cleaned internally from time to time if the water has a high mineral content. S Restrictions at the cooler outlet I —' caused by faulty or makeshift piping are quite common. Care should be taken to see that both outlet and inlet piping are of correct size and have no restrictions. If the outflow water is used for udder washing, it should be taken off through a bypass to prevent any back pressure, which would restrict the flow of water through the cooler tubes.

oSome means of controlling the rate of milk flow over the cooler is essential if maximum efficiency is to be obtained. The rate of milk flow over the cooler should not be greater than the manufacturer’s rating and the most satisfactory way of regulating the flow is by a control vat. Vats are available in sizes to suit individual requirements, and the size of the milking machine, breed of cow, and type of milking shed should all be taken into account before a vat is installed.

Variations in Milk Flow The following are typical figures of the variations in milk flow during one milking where no control vat was installed. The figures were obtained from dairies taken at random during a special investigation made by officers of the Dairy Division. Size of Rate of flow of milk machine over cooler and breed No. of Max. gals. Min. gals, of cows milkers per hour per hour Jersey: 3 single .. 2 64 30 3 double .. 1 122 27 3 double ..1 53 21 4 double ..2 60 32 8 single .. 2 144 76 8 single .. 3 211 25 Friesian: 4 single .. 2 63 27 4 single .. 2 40 23 4 double ..2 109 ■ 49 6 single .. 2 108 43 6 double .. 3 192 44 6 double ..2 200 30 These figures show that proper control of milk flow is essential. The installation of a control vat at one of the above dairies gave an improvement of 6 degrees F. in the over-all temperature of the milk produced. z Heavy milking breeds usually have a faster milking rate than light breeds. Elevated-bail milking sheds usually give a faster rate of milking than do orthodox sheds, and in herringbone sheds there are wide variations in the quantity . of milk being discharged from the releaser. Increase in the number of teat-cups, for instance by doubling up the plant, will also increase the volume of milk delivered from the releaser and inevitably will extend the time (if cooling is to be done effectively) to well beyond the actual milking time. Chilling of Town Supply Milk Milk chilling equipment is being installed in many town supply dairies and this is being encouraged by the Dairy Division of the Department of Agriculture as being an important factor in the provision of a better product to consumers of milk. Farmers

contemplating installing chilling plant should not overlook the possibility of increases in the numbers of their herds becoming possible by better management and improved pastures enabling increased carrying capacity, or by doubling up of their machines enabling more cows to be handled. They should plan the installation of chilling plant adequate for their foreseeable production; otherwise they might be faced with repetition of the expense of installation or with reduced efficiency because of inadequate plant. Factors affecting the efficiency are: — n Proper control of milk flow over L--I the cooler to ensure that it does not exceed the rated capacity of the unit. RH Efficient primary cooling by havLr ing an adequate supply of cold water to cool the milk to as low a temperature as possible on the watercooling section. The chilling installation should be in a well-ventilated position to obtain maximum efficiency from the compressor. Coolers should be installed in the dairy or on the milk stand and should be easily accessible for cleaning.

Cool rooms or cabinets should be close to the water cooler with easy access for placing the can in the room and for loading out. Two types of chilling unit, known as ice bank and direct expansion, are in general use. Either is suitable and will be satisfactory if it is installed and operated correctly. Important factors are: — ryi Efficient primary cooling by the ' — water cooler (as discussed earlier in this article). This is important, as the more effective this cooling is the more efficiently will the chilling unit function. Rfl The quantity of milk to be chilled ' — l during the peak months of the producing season. Allowance should be made , for maximum quantities, as an under-estimation would result in the unit being incapable of chilling the milk to 40 degrees F. at the time when it is .most needed, particularly if an ice bank system is installed. Purpose of Chilling The purpose of chilling milk is to maintain the quality of the milk as it leaves the cow. Chilling should never be regarded as a means of offsetting bacterial growth resulting from an unsatisfactory standard of cleanliness. Practically all types of bacteria are most productive at the temperature of uncooled milk, or at the temperature it is received from the cows. Other bacteria still thrive at low temperatures. Chilling must therefore be regarded as but one of the safeguards of quality.