FURTHER OBSERVATIONS ON SLOW DEVELOPMENT OF ACIDITY IN CHEESE MANUFACTURE.

New Zealand Journal of Agriculture, Volume 47, Issue 6, 20 December 1933, Page 376

FURTHER OBSERVATIONS ON SLOW DEVELOPMENT OF ACIDITY IN CHEESE MANUFACTURE.

H. R. Whitehead,

Dairy Research Institute (N.Z.), Palmerston North

Trouble due to the occurrence of so-called “ non-acid ” milk is so common in cheese factories that the necessity for full investigation on the underlying cause or causes hardly needs emphasis.

In a previous it was shown that in one instance, the phenomenon of non-acid milk occurring in a cheese factory was due to the presence of a special type of bacterium in the milk brought to the factory by one supplier. Recently, a similar organism has been isolated from another sample of milk (from quite a different locality). In this case, although the main effect is the same, there is a difference in the mode of action of the organism. which is of additional interest in connection with the cheesemaking process.

Some milk samples were procured from a dairy factory during April, 1933,. originally for the purpose of a chemical investigation. As a matter of interest, - the portions of milk which remained after - the amounts needed for chemical analysis had been taken were pasteurized at 145 0 F. for thirty minutes and subjected to a vitality test, one starter culture being used for all the samples. Most of the milks gave normal acidity increases in the test, but in one sample there was practically no increase in acidity in the period between the two readings. A little of the raw sample still remained, so the milk was plated on yeast whey agar in an attempt to isolate any bacteria which were similar in reaction to the organism which had previously been shown to produce the non-acid substance in milk. There was a copious growth on the agar plate inoculated from the milk sample, and about a dozen small colonies similar in appearance to those produced by lactic streptococci were picked off into milk.

By the performance of vitality tests on milk samples in which these pure cultures had been grown, it was found that several of the strains of bacteria produced a typical non-acid reaction, provided that the milk in which they had been grown was subsequently pasteurized. The results of a typical experiment will serve to indicate the usual findings.

A sample of fresh Uncooled milk, direct from the Massey Agricultural College cow-shed, was divided into two portions. One portion was inoculated with a culture of one of the strains of. the bacterium (Di) isolated from the abnormal milk ; the other was retained as a control. Both portions of milk were allowed to stand at room temperature overnight. Next morning the control milk contained 960,000 bacteria per cubic centimetre by direct count ; it decolorized methylene blue in four hours at 37 0 C. ; and its acidity was 0-17 per cent, calculated as lactic acid. The inoculated milk contained 186 millions of bacteria per cubic centimetre ; it decolorized methylene blue in ten minutes ; and its acidity was 0-25 per cent. One-half of each batch of milk was pasteurized at 145 0 F. for thirty minutes, and then all four samples were subjected to a vitality test with the use of the same normal starter culture throughout. The results are given in the following table: —

It will be observed that the inoculated milk in the raw state gave an acidity increase only slightly lower than that given by the control (in some experiments, where a less extensive growth of the organism Dr occurred in the milk overnight, the acidity increase in the inoculated raw milk was actually higher than that in the control). On the other hand, it is evident that the inoculated milk inhibited acid production entirely after it had been pasteurized. This difference in result between the raw and pasteurized samples might be accounted for by the fact that the bacterium Di formed quite an appreciable amount of acid by itself when grown in milk, unlike the organism described in the previous article, which formed acid only very slowly. It is possible, however, that the non-acid substance is not fully formed in its potent state until the milk is heated.

Whatever may be the explanation, the foregoing results indicate quite clearly why no particular difficulty had been experienced with the original abnormal milk in the factory from which it was obtained —for the factory was making raw-milk cheese. Had the milk been pasteurized it is certain that slow vats would have been experienced, the extent of slowness ■ depending on the proportion of the milk in any particular vat. A further sample of milk was obtained from the same supplier about a month, later, and there was again no difficulty in isolating an organism which would render non-acid any milk in which it was grown. This is an indication of how persistently such organisms may be found in the milk of certain suppliers. Unfortunately, just at the time the last sample was obtained, the supplier in question ceased to bring his milk to the factory, as it was the end of the season. It was therefore impossible at the time to follow up the matter any further.

Both of the organisms found to produce the non-acid phenomenon in milk, have proved on examination in the laboratory to be lactic streptococci, indistinguishable in most respects from the bacteria which constitute the normal population of starter cultures. They differ solely in their power of producing during their growth in milk some substance which acts as a powerful inhibitory agent on the subsequent growth of normal lactic streptococci. It has also been found, in experiments carried out up to the present, that the inhibitory substance formed by these two peculiar organisms acts only on the lactic acid bacteria ; it has no restraining effect on B. Coli or on B. Subtilis.

The importance of these abnormal streptococci in dairy work hardly needs emphasis, for it is evident that anything which hampers the normal development of acid in the process of cheese manufacture is likely to have an adverse influence on the quality of the cheese. Factory-managers

who experience trouble with non-acid milk are usually able to narrow down the probable source to the milk of three or four suppliers. Vitality tests carried out on these few milks should then show exactly which milk or milks are responsible. But it is essential, in view of the discovery of this second type of organism described in the present article, that the milks should be tested in the pasteurized as well as in the raw state. This certainly involves more trouble, especially where facilities for such work in the factory are meagre ; but it should be possible to devise some rough method for the pasteurization of the small batches of milk at 145 0 F. for thirty minutes ;■ even a .pasteurization carried out by raising the milk to 150° F. and then cooling immediately will serve. Having found which milk supply is at fault, every effort should be made to induce the farmer to adopt cleanly methods of milking and to cool the milk efficiently. Even if the bacteria responsible for the trouble cannot be eliminated entirely from the milk, efficient cooling will check their growth overnight and reduce their effect to a minimum. If the milk will not decolorize methylene blue in four or five hours, it cannot give rise to the type of non-acid trouble described in the present article, for there will not be a large enough number of the abnormal streptococci present. It would also , appear to be extremely important that the factorymanager should make sure that the milk he is using for the preparation of his starter is not infected with the organisms which produce the non-acid reaction, for it is obvious that even after an efficient pasteurization such milk will not permit a normal growth of the starter bacteria in it. It is desirable therefore that he should occasionally perform a vitality test (using an active starter) on the milk which he selects for the preparation of his bulk and mother-culture starters. The trouble involved is small compared with the difficulties in manufacture which may be avoided.

Spread ability of Butter .-—Referring to work on this problem by the Dairy Research Institute at Palmerston North in 1932-33, the Research Committee reports: "Experiments indicate that butter made from cream which is cooled relatively slowly in the last stages of the process is more spreadable than that made from cream cooled very rapidly after a temperature of about 55° F. is reached. Efforts, however, made to improve spreadability showed that great care needs to be taken to avoid greasiness of body. Further experimental work is necessary before the technique, can be applied in factory practice. It has also to be pointed out that any modification in present methods of manufacture should only apply to butter . which , would be marketed in- Great Britain during the winter months.” . Chemical Control of Noxious Weeds. — The Director of the Fields Division reports that several new specifics were tried experimentally during the past year, but generally they proved useless for the purpose. In addition, several further tests were made with chlorates, and these went to show that sodium chlorate is still the most efficacious and cheapest for the treatment of ragwort. Occysol was further tested; and gave very good results at a strength of 5 per cent. Calcium chlorate at the same strength gave about an 80-per-cent. kill.

Reference. . ( l ) Whitehead and Riddet : N.Z. Journal of Agriculture, April, 1933, Vol. 46, p. 225.

Acidities as Percentage of Lactic Acid. —- ' Acidities as Percentage of Lactic Acid. Inoculated, ■ Raw. ■ Control, . Raw. Inoculated, Pasteurized. Control, Pasteurized. First acidity reading 0-42 0-38 0-13 0-38 Second acidity reading . . 0-63 0 • 72 o-14 0-72 Increase . . ■ . . O-2I 0-34 ■ 0 • 01 - . • o-34

Table 1.