Testing Market Milk for Keeping Quality

New Zealand Journal of Agriculture, Volume 79, Issue 3, 15 September 1949, Page 241

Testing Market Milk for Keeping Quality

nrHE Milk Act, 1944, was passed with a view to ensuring the pro- ■ . . ’ ’ r . rz i & j r vision for human consumption in New Zealand of an adequate supply of milk of the highest quality. In this article S. J. Cowen, <» i 7 UM, t s n .. 7 x £ a • ix n i x Market Milk Instructor, Department of Agriculture, Palmerston North, describes how the methylene blue test is used by milk-treatment stations in helping town-milk producers to reach and maintain the high standard desired. • /

Z"\NE of the results of the Milk Act O was the formation of a Central Milk Council as a general guiding authority and this, in conjunction with the establishment of producer cooperative milk-supply associations, gave both direction and stability to the town-milk industry as a whole. This meant that town-milk producers could concentrate on the production of a high-quality milk without having to worry about the vagaries of what in the past had proved an erratic market, However the organisation of townrtowever, we organisation or town milk producer co-operatives brought with it the difficulties, inherent m a bulked milk supply Fortunately, the milk-treatment stations which were the receivers of this supply took steps to ensure the testing of the quality of each consignment, and officers of the Dairy Division of the Department of Agriculture have been untiring in their efforts to ensure that this testing for S?rV haS been maintalned reguiariy. M ~ , T ~ Necessity for Testing When urban areas were small and • i t 1 • -p ii -p the delivery of milk directly . from farm to consumer was economically practicable there was little need for a keeping-quality test; if the . milk did not keep, only one producer was affected . and' the consumer soon told him in what respect his milk was lacking. Where modern development in urban areas has necessitated the provision of central milk-treatment stations, as it has in New Zealand, this system of direct supply from producer to consumer has had to be superseded,

Under the new arrangement there is always the danger that one supply of milk of poor keeping quality may become mixed with the bulk supply of good milk in the milk-treatment station, and this could readily cause deterioration of the bulk supply. When the milk is delivered to the consumer it may give rise to a considerable number of complaints, many or all of which are . made public only too readily. This causes adverse criticism of a scheme which is the only means of ensuring a supply of milk to cn sumers throughout the year, and also creates the general impression that all the supply is of poor quality, whereas most o PP the producers concerned are supp i yin g fi rs t-class milk. pp ° ‘ , ~ Bulked milk is only as good as the poorest milk in the supply, so to safeguard the producers of good-quality muk a system of testing every supply of milk received is necessary .Normally sample oihhemnkJfceivlS from eaS producer and testing it for keeping quality, and the keeping-quality test which has been adopted almost universally in New Zealand and which is iiqpH eauallv as widely overseas is is usea equally as winery overseas is the methylene blue or reductase test. Before the introduction of the methylene blue test a direct method of testing keeping quality was used. It was usually carried out by holding a sample of milk at 60 degrees F. in a covered glass bottle; at. intervals the milk was . tasted,. and in addition a small portion was boiled in a test tube and the appearance and smell noted. As soon as any abnormality of taste or smell or evidence of clotting when

it boiled was noted, the life of the milk was considered to be at an end. Unfortunately, the result of this test could rarely be known before the milk had been delivered to the customers, and in this respect it failed to act as a corrective before the customer was inconvenienced. One advantage of this test was that it also picked out milk contaminated with non-bacterial flavours such as feed and absorbed taints; milk may have poor flavour but still be of good keeping quality. Obviously, it is essential that any system of testing for keeping quality be sufficiently rapid to ensure corrective action long before the customer receives the milk. This fact challenged scientists to seek a quicker method of obtaining such results, and as a result two scientists, Neisser and Wechsberg, developed the methylene blue test and introduced it in 1900. Description of Test Methylene blue is a dye which retains its colour as long as oxygen is present, but in the absence of oxygen changes to the colourless form of methylene white. As the bacteria usually found in milk consume oxygen, it will be seen that the more bacteria the milk contains the more quickly the dye is reduced in colour, and this speed of reduction provides the estimate of keeping quality. For laboratory purposes methylene blue is usually bought in powder form. The stock solution is prepared, by adding 0.1 gramme of dye to a flask containing 200 millilitres (ml.) of distilled water; after the dye has been dissolved a further 70ml. of distilled water is added, giving a stock solution of 0.1 gramme of dye in 270m1. of water. This solution may also be bought from dairy chemists, or the dye may be obtained in tablet form, packed in bottles

with instructions for preparing the solution. The stock solution must be stored in a dark cupboard. To prepare the test a few millilitres of stock solution is poured into a sterile test tube and the stock solution returned to the cupboard. One millilitre of stock solution from the test tube is added to 9ml. of distilled water, and Iml. of this solution is added to 10ml. of milk, giving a final concentration of 1 part of dye in 300,000. Excess dye should not be returned to the stock solution. The glass test tube into which the 10ml. of milk and Iml. of dye solution are placed is 6in. long and fin. in diameter, with provision on the surface of the tube for marking the number of the sample. The tube also carries a small mark indicating the 10ml. capacity, which is convenient for measuring the milk directly into the tubes. For plugging the tubes rubber corks are normally used. The other equipment required for the test includes a supply of Iml. and 10ml. pipettes and graduated cylinders and a constant-temperature bath, preferably provided with thermostatic control to ensure the maintenance of a steady temperature of 98 degrees F. during the test. These baths can be heated by electricity or by gas or spirit lamps, but in general practice baths heated by electricity are found to be most convenient. Inside the bath are racks to hold tubes. The bath is covered with a lid, as it is desirable that the test be carried out in the dark; some old-model baths had glass fronts, but because of the effect of light on the samples it was found necessary to paint the glass black. It is essential that glassware and corks to be used for a methylene blue test be sterile. The glassware can be sterilised either by being heated in a dry-heat oven at 340 degrees F. for 2 hours or by being placed in boiling

water and the water kept boiling for 10 to 15 minutes. The rubber corks used for closing the tubes when they contain the milk and dye are sterilised in boiling water. After the samples to be tested are ready the procedure is as follows: Test tubes, racks, a basket of . corks, tongs for handling the corks from the basket, pipettes, and methylene blue solution are all placed in a convenient position. Each sample is mixed, and either 10ml. of milk is pipetted into each test tube or milk is poured into the tube up to the 10ml. mark. Then Iml. of the prepared methylene blue solution is pipetted into each test tube or run into it from a special burette, and the tubes are corked and inverted to mix the dye thoroughly through the milk. . Each tube is numbered when the sample is added and then placed in position in the rack, which is numbered correspondingly; that ensures the maintenance of the identity of the sample from the producer. The rack of test samples is placed in the water bath and the water level maintained about the level of the sample of milk in the test tubes. The water temperature is controlled at 98 degrees F. within a tolerance of 1 degree, as this is the temperature considered most suitable for the reproduction of the bacteria usually found in milk. The test tubes are inverted every half-hour until the colour of the contents changes to white. The period between the start of the test and the time when the colour finally changes to white is expressed as the result of the —for example, if a test started at 9 a.m. and the sample was reduced to . white at 3.30 p.m., the result of the test would be 6-j hours. A good-quality milk, from healthy cows and well cooled and handled under clean conditions, will contain comparatively-few bacteria and may not bring about a change of colour in 10 hours; milk of the same age but

produced, cooled, and handled under unclean conditions will contain large numbers of bacteria and may change the colour to white in 30 minutes.

Milk that has been poorly cooled

invariably causes a change of colour in a shorter time than if the same milk had been properly cooled at the time of production.

By regular testing and recording of results any variation in keeping quality is quickly noted. This information is passed on by the treatment station to the producer concerned and to the local Stock Inspector of the Livestock Division, Department of Agriculture, who assists the producer in locating and clearing up his trouble.

Producers' Responsibilities

The result of the methylene blue test is influenced by the number of bacteria present in the milk, which varies with cleanliness, temperature, and time. Therefore, to produce milk with good keeping quality every person handling milk must declare war on all bacteria to prevent their gaining admission to the supply. No amount of testing can absolve the producer from his responsibility in maintaining a high quality in the milk produced. If clean milk is to be produced, a definite routine must be observed:—

First, all cows milked must be healthy and clean. The cows should be properly prepared for milking by the clipping of all surplus hairs from the udder .-and thorough washing of the udder with clean water. The taking of a few streams of milk from each teat into a receptacle enables the attendant to examine the milk and in healthy cows removes a great source of bacteria by cleaning out the teat duct. Milk produced in such conditions will have a low bacterial count. Second, all equipment with which milk comes in contact must be thoroughly clean, and that can be achieved only with a plentiful supply of hot water and the correct technique of cleaning the plant. This is explained in bulletins which can be procured free from the nearest office of the Department of Agriculture, and Stock Inspectors of the Department will readily give advice and assistance. Third, milk must not be exposed to the atmosphere. Particles of dust floating in the air may fall into cans or on coolers, thus admitting unknown numbers and types of bacteria. Yards and surroundings should be kept clean and bails and yards thoroughly wet before the cows come in.

Fourth, . milk should be reduced quickly to 60 degrees F. or' cooler to preserve the natural flavour and improve the keeping quality. This requires a good supply of cold water. Bacterial growth is reduced considerably by cooling to 60 degrees, though bacteria cannot be killed by cooling, but merely remain dormant until the temperature rises. The low temperature should be maintained by keeping the cans in a cool, shady position. It is also in the producers’ interest to insist that all milk-collection lorries be fitted • with good covers so that their milk is delivered to the depot in good condition; the test sample is taken at the depot, and poor carrying conditions have a great influence on test records.

Assistance to Producers

The methylene blue test can be applied extensively in an advisory capacity, and any resentment of such testing is not in the, best interests of a clean milk supply, as the results can be of great help to producers striving to attain perfection.. The testing of every delivery of milk received at the depot under the same time and temperature conditions enables producers to be advised of any deterioration in keeping quality before the supply falls below the legal minimum test result of 4 hours set by Department of Health Regulations.

This requirement means that the milk must give a result of not less than 4 hours at the time of delivery to the consumer, and not just at the time of delivery to the treatment station, as some producers erroneously believe.

For this purpose it is necessary that all samples taken at the depot be held for a set period so that all are subjected to the same lapse of time between the normal milking hours and the start of the test. For example, samples taken from loads received early in the morning should be held until samples are taken from the last load delivered for that shift, thus placing all suppliers on the same basis.

It is advisable that samples collected during the morning milk delivery be held until 4 p.m. and then put in the cool-storage room at 38 to 40 degrees F. until the following morning. Samples taken during the evening delivery should be held at a temperature not exceeding 65 degrees and tests started by 8.30 the following morning. By this procedure all milk received from producers is subjected to the same conditions. Milk failing to comply with the 4-hour standard, because of unhygienic conditions or poor cooling, is identified and producers are advised so that they can take steps to remedy the defect on their own initiative or with the help of the local Stock Inspector.

Overseas and New Zealand

Standards

Though the general quality of the milk in New Zealand is high, the methylene blue standards which are legally obligatory are markedly lower than those enforced overseas. In New Zealand the milk must not reduce the dye in 4 hours or less, and a milk of

good keeping quality can give a result as high as 10 hours quite easily. In the United States of America methylene blue standards have been defined for different categories of milk, class 1 being of the highest quality and class 4 of definitely-low quality. The classes are defined as follows: Class 1, not decolorised in 8 hours; class 2, decolorised in less than 8 hours but not less than 6 hours; class 3, decolorised in less than 6 hours but not less than 2 hours; class 4, decolorised in less than 2 hours. Classes 3 and 4 are not of market-milk standard, which is interesting in view of the lowness of standards in New Zealand, though the general quality of milk in the Dominion certainly is not lower than that normally attained in America. In England the standard required for good-quality raw milk is as follows: Samples are held at atmospheric shade temperature for 24 hours after production and when tested must retain their blue colour for not less than 4J hours in summer and not less than 5J hours in winter. When evening milk is

sampled it is held until 5 p.m. on the day after production and, if it is not convenient to start the test then, the samples are placed in cold storage at 40 degrees F. until the following morning. The regular system of ageing the samples practised in England makes for a stricter test than the one used in New Zealand, but the required standard is not less than 4| hours, compared with the New Zealand standard of not less than 4 hours for un-aged samples. No matter how high the standard were set in New Zealand a large proportion of producers would comply continually, and that is to the credit of the main body of town-milk producers generally. No town-milk producer should be satisfied if the milk-treat-ment station informs him that his milk just passed the legal standards —that is, gave a result of 4| or 5 hours. By overseas standards that is a poor result, and any producer not attaining a standard of at least 6 hours should realise that there is something wrong in his method of production or in his method of cooling. Cooling by itself will not remove trouble caused by lack of cleanliness anywhere in the line of production, and the methylene blue test properly used quickly shows up an unwise town-milk producer who may be depending on mechanical refrigeration to cover other defects in production.

The methylene blue test is invaluable to the producer, to the milk-treatment station, and to the consumer. It is in the interests of every town-milk producer to ensure that his milk is being regularly checked by this test at the treating station to which he sends his milk. Failure on the part of the milk-treatment stations to use the test is really a breach of moral responsibility to the producer on the one hand and to the consumer on the other.