MILK GRADING.

Waikato Times, Volume 112, Issue 18777, 27 October 1932, Page 12

MILK GRADING.

THE CURD TEST. DECIDED NEED FOR PROGRESS. VARIOUS METHODS. No. i. (By G. M. Moir, Dairy Chemist Wallaceville Dairy Laboratory, in the New Zealand Journal of Agriculture.) Although all connected wtih our cheesemaking industry may not be agreed as to the desirability of milkgrading, yet in recent years many more have become converted to the need for progress in this matter. This development has been materially assisted by the success which has attended the introduction of crcamgrading. Moreover, in certain districts where special facilities have been available, local enthusiasm lias made it possble for milk-grading to be thoroughly tried out, with the result that the factories concerned have been enabled to make more easily and regularly cheese of highest quality. Thus a considerable body of opinion has been developed in favour of it, and no doubt as lime goes on the difficulties which stand In the way will be overcome and the principle become firmly established in the experience of all concerned.

All experienced dairy bacteriologists realise tliat no one test .is entirely satisfactory for determining the hygienic quality of milk. After carefully studying information obtained by diverse English, American, and Continental investigators of repute, Dr A. T. R. Mattick (head of the, Bacteriological Department, National Institute for Research in Dairying, Reading, writes: “One fact emerges from a consideration of the literature upon the methods at present in use, for determining tire bacteriological content of milk—viz., no one method is capable of giving exact information as to the numbers of living bacteria present." It follows, therefore, that 100

pe,r cent efficiency cannot be expected of any test. Again, it is quite possible that many tests which have been found exceedingly valuable for other purposes—e.g., for control of city milk-supplies —will not necessarily be the best for judging cheese-milk. • Milk for city people must, among other things, be specially free from souring organisms of the Streptococcus lactis type. These very common organisms, however, are of the same type which the ch&esemaker grows in his starter culture to add lo the milk to enable him to make the best cheese. Consequently, the presence in cheese-milk of a proportion of souring organisms may up to a point be desirable. However, it is not wise to encourage the milk-producer to permit these souring organisms to grow in his milk, because he may easily develop numbers which would spoil the milk for cheCisemaking, or at the same time he may easily introduce large numbers of objectionable types.

Sorting Out of Milks

Leaving aside the question of actual numbers, a lest of cheese-milk should, above all, sort out the milks rich in objectionable, types of bacteria, such ns those which give rise to gassy curds In the vat and subsequent unclean and off-flavours in the mature cheese. 'Fills Is the special viriue of the curd tost, which In fact flicks out milks heavily contaminated with organisms of the, conform type. Organisms of the “eoliform type" are so called because they are found In large numbers in the colon or lower intestine of animals. These germs are therefore, excreted In the faeces, and consequently cow-manure, dirt, and dust about the cow and Hie cowshed arc usually heavily Infected therewith. Very similar organisms are found associated with the dust of grain and fodder. Dairy bacteriologists have repeatedly emphasised the fact that tests which will reveal the collfonn types have an established value for controlling milk-supplies of all kinds. ‘ This was pointed out at the World's Dairy Congress, 1928, by the late Dr. B, Stenliouse Williams (Research Professor In Dairy Bacteriology al the National Institute for Research In Dairying, Reading, England). Mai tick cites experimental evidence, from which lie concludes: “The presence of colil'nrm. organisms in numbers In fresh milk is, therefore, evidence id" carelessness al some stag: i. i Hie handling. In many cases they ae-> of faecal origin. . . In the examinali*i.i nf farm samples, therefore. the enbi'orm P\sl is of great value." Tcsi-s fur rnlii'nm organisms in milk ~ic discussed in I lie. lal'sl edition lit 2 8 of "I airy Barieriolngy" |,y I ir. 11. \V. Hammer. Professor of Hairy Bacteriology al the si late College of fowa,

which Is one of [he principal dairyin States in America. lie points oi that although milk may have a con paratively low tolal count due to ad< quate cooling, yet at the, same (inexcessive numbers of coliform organ isms may be present due to contuiiin ation. Quite recently Canadian invesfiga tors. McGrady and Langevin, haw emphasised the value of the, coliform test as an adjunct to the plate count, which is agreed to be the most reliable method of estimating the numbers of bacteria in milk of the highest quality. They used coliform te;sts In duplicate, together wtih plate counts, to discover sources of contamination of highgrade pasteurised milk which in most cases had a plate count of less than 30,000 bacteria per cubic centimetre. They make the following observations- “ Both coli-aerogenes determinatio and plate counts were necessary fi the detection of recontamination, f the two methods the coli-nerogeiK yielded the sharper, and therefore tli more significant, as well as the mor consistent results in those ‘case where the organisms were found."

Consistency of Coll Results. The same authors also say: “Parti cular attention is invited to the con sistency of the coli results as com pared with the plate counts; the lat ter may vary greatly, hut the coll figures are usually maintained from stage to stage. The striking significance of the coli results, where positive, Is also in marked contrast to the, uncertain significance of many of the differences in plate counts." It Is obvious fgpm these results that i>lk of apparently thoroughly satisfactory sanitary quality can be seriously con laminated with coliform organisms which are located at one or more particular spots in an otherwise 'clean city milk-bottling plant. Obviously, similar contamination may readily occur in a milking machine. Mention will be made later of the value of the curd test for detecting such contaminations in milking machines. Apart from the value of the conform organisms as Indicators of the sanitary conditions associated with the handling of milk, the presence of these organisms in . cheese-milk is particularly objectionable for other reasons. Evidence is available from three distinct sources. First, fundamental researches have shown that B. coli will produce 'chiefly acid and gas In a medium like milk, where in addition to protein there is a fermentable

carbohydrate such as lactose (milksugar). The organisms do not attack the protein hut live on the milksugar. If, however, the sugar is removed they begin to attack the protein, which they decompose, producing among other things foul-smelling, objectionable substances, such as Indole and skatole, which have characteristic faecal odours. Now this is exactly what is liable to occur In cheese, for after a week little milksugar is left, and after a fortnight the remainder has completely disappeared, so that the eoliform organisms must attack the protein (Casein). Seoond, that this actually occurs in cheese has been proved beyond doubt by experiments carried out by Dr. Whitehead, Bacteriologist at the Dairy Research Institute, Palmerston North. In these he added to pasteurised milk heavy doses of Conform organisms prior to using the milk for cheesomaking. On different days different eoliform cultures were used, and each day control cheese were made from the same pasteurised milk without the addition of these particular organisms. In both cases the usual amount of starter was added to each vat. In spite of the largo inoculation with gas-forming bacteria the experimental cheese showed no sign of gas-holes, but, what is much more Important, nil developed objectionable off-flavours, variously described as “unclean," “cowy," “like putrid meat." Moreover, these off-flavours were more pronounced when the experimental cheeses were more mature, but were entirely absent from the control cheese to which no eoliform organisms had been added. Third, the evidence obtained from the two foregoing lines of fundamental research in effect only confirms what lias long been realised by experienced and observant chcesemakers, who well know that gassy curds are liable lo make cheese which develops off-flavouis while maturing.

Wisconsin Curd Test. In the modern literature of dairy science comparatively few references are to he found to the curd test, because In the laboratory examination of milk a more refined eoliform test Is available. In Ills textbook on “Dairy Bacteriology,” Hammer emphasises the value of the Wisconsin curd test, which was Introduced at the end of last century. After describing the method of obtaining Ihe pat of curd lie says: "If it is solid tlirmiglioul. shows no sliminess al Ihe surface, and has an agreeable odour. Ihe milk was probably free from organisms Hint would be objectionable in eli'esemaking. while gas-boles in the eunl. sliminess, oil jee I ionable odours, etc., rdmw the preset of organisms Hum can lie expeeled to cause undesirable coiidil ions in el : in some instances III!' eui'll is so full of gas holes Ilia! it may he spongy. Milk with a very high

bacterial count may give a desirable type of curd and another lot with a low count may give a very undesirable curd, so that the test is of no value In comparing samples of milk from the standpoint of their keeping quality." In 1929 I visited the South German Dairy Research and Experimental Station at Welhenstephan. This institution is part of a large and oldestablished agricultural experiment station which Is surrounded by farm lands where for centuries agricultural pursuits, including buttermaking and

cheesemaking, have been carried on. The dairy bacteriologist. Dr. K. .1. Demeter, has had considerable experience in Germany and In America. When the subject of the grading of cheese-milk was discussed he introduced one of his assistants, who explained to me a curd test which they have developed for Judging the suitability of milk for cheesemaking. This test, called the “rennet curd test" (labgarprobe), they regard as the most Important for this purpose. A full account of this curd test and other-simple tests for milk-grading has recently been published by Zeller and Berwig In a German Journal dealing with dairy science. The authors have subjected the curd test, with various modifications, to a very exhaustive examination and comparison with other tests. For a period of three years milk used for cheesemaking has been regularly examined, so that hundreds of comparisons are available; for example, the results of 7564 comparisons between the curd test and fermentation test are tabulated. The curd test u-sed by Zeller and Berwig is carried out by adding to 50 c.c. of milk 2 c. ; c. or a fresh suitably diluted rennet solution. The tube containing the milk Is set for twelve hours In a water bath at a temperature of 102 deg. F. The curd is then removed from the whey, cut, and examined for form, texture and flavour. They point out that a similar test was Introduced about the year 1888 by Ilumann, who cut the curd to drain off ihe whey and used a kind of mould box in which to collect it to form a “little curd.” Owing to Its bothersome nature Bumann's apparatus was little used and for many years was quite forgotten. Essentially the same principle is used In the Wisconsin curd test, which also is tedious to operate in large numbers. However, by means of the apparatus known as the “New Zealand curd test,” elaborated by Messrs Singleton, Morgan and Byron at the Wallaceville Dairy Laboratory, a great deal of time and ■trouble can be saved.