SLOW DEVELOPMENT OF ACIDITY IN CHEESEMAKING.

New Zealand Journal of Agriculture, Volume XXXVII, Issue 5, 20 November 1928, Page 305

SLOW DEVELOPMENT OF ACIDITY IN CHEESEMAKING.

BACILLUS SUBTILIS SHOWN TO BE A FREQUENT CAUSE.

G. F. V. Morgan,

N.D.A., N.D.D., Bacteriologist, Dairy Division, and J. Curle, Dairy Instructor.

At the present time the question of slow-working curds in the process of cheesemaking is one of wide interest to the dairy industry, the spasmodic or periodical occurrence of this nuisance being dreaded by dairy companies, factory managers, and workers alike. The chief concern from the companies’ point of view is that the slow curd nearly always turns out to be one of very poor quality, having a tendency to inferior flavours instead of ripening normally. For the factory staff the trouble means working for long hours on raw material that experience has taught can only result in a very poor finished‘product. The problem of slow curds and failing starters has long been present in New Zealand, and some of the possible causes of the trouble have been investigated both in field and laboratory. and a considerable amount of data collected.

In a typical case which occurred recently in the Wellington District the points primarily investigated were (i) whether the trouble originated from the milk as received at the factory possibly becoming contaminated at the farm with alkali-producing organisms of a putrefactive type, or of a type that would retard the development of a lactic ferment ; (2) whether the trouble was due to the starter itself or to contamination of the milk for cheesemaking, after pasteurization, by the plants or vats. One point that seemed clear from the outset was that the cause was bacteriological, and not due to. a chemical fault in the composition of the milk as was sometimes supposed.

In this outbreak the work of investigation was carried out at a small factory, having only six milk-suppliers, where the trouble had already commenced. Each supply of raw milk was sampled as it arrived at the factory, the samples being taken in thoroughly sterilized bottles. Other samples were then taken from the milk in each vat after it had been pasteurized and cooled and had remained long enough in the vats to be thoroughly mixed by the stirrers. Finally a sample was taken of the starter then in use at the factory, which was showing signs of failing. ' ' '

The results of the bacteriological analysis of these samples pointed to the fact that certain spore-forming aerobes of the subtilis mesentericus group were at the source of the trouble. In view of the fact that both starter and cheese milk were pasteurized, and that organisms of the Streptococcus lactis type used as cheese starters are in themselves very virile and not likely to become attenuated or checked by the ordinary alkali-producing bacteria, this supposition, seemed most likely to be justified.

The results above mentioned were as follows: The farmers’ milk showed a normal bacterial flora, the growth of acid-producing organisms of the Streptococcus lactis kind and acid-producing “ weeds ” appeared

typical of normal cheese milks, and shake cultures in litmus lactose plates, showed no sign of a preponderance of alkali-producing organisms. The same, however, was not the case with the samples of milk from the vats. These samples showed a heavy contamination by alkaliproducing rods of the subtilis type, and on plating out in litmus lactose shake cultures a considerable amount of alkali appeared in the neighbourhood of these colonies, this being an indication of what was going on in the milk. The most surprising results, however, were obtained from the plate culture of the starter, which showed no trace of acidity development, only very occasional pin-point colonies of the Streptococcus lactis type, but a very large number of colonies of subtilis. (Two days later this starter was dead.)

> The results of these experiments showed quite clearly that the fault did not lie with the farmers’ milk, but that the milk became contaminated, after pasteurization, by the plant itself. (It has already been shown that B. subtilis is practically the only inhabitant of factory plants in parts of the system between the pasteurizer and the coolers.) It was also obvious that a starter which had become fairly heavily contaminated with spore-formers of this type was unreliable and likely to fail at any time.

After these data had been collected at the factory, further work was carried out at the laboratory (Wallaceville) on the effects of this type of organism in milk, also its power to develop in and finally overcome pure culture starters. It was shown that contamination of active starters by subtilis in very small quantities has little effect on . their action. - However, if subtilis once obtained a firm footing it would increase rapidly in numbers, though. its full effect might not be felt for four or five days after inoculation, when the starter appears to go quite dead. The results of daily subcultures of contaminated starters in the laboratory were checked by plating out when the subcultures were made, and showed a daily increase of subtilis colonies until the time when the starter was so weak that it took two days to coagulate at 70° F. Smears were also made at each stage. In connection with this daily increase in subtilis a curious fact was noted. The increase in subtilis seemed constant until the starter appeared dead, then suddenly decreased in number very rapidly till it seemed to have almost disappeared, and the Streptococcus lactis seemed to revive again and work normally for a time, though isolated rods still appeared. Whether this sudden incidence and almost total disappearance work in regular cycles is still being investigated ; factory experience rather points to its probability.

One other point of interest observed in the laboratory was that-in contaminated samples that were not subcultured daily there was a very marked increase in the amount of subtilis present, as compared with those that were regularly subcultured, also that starters incubated at a high temperature developed subtilis much more readily than those incubated at about 6o° to 70° F.

During the time the laboratory experiments were being carried out a number of alleged slow milks from factories in various localities were tested. With one exception these milks were contaminated with organisms of the subtilis. type to a fairly marked degree.

THE FACTORY EXPERIMENT

It was then decided to try, under factory conditions, an active starter contaminated with subtilis, with an active pure starter as a control.

Organisms of the B. subtilis type found in the starters giving slow acid development when used in the cheesemaking were employed in a starter sufficient to work part of a vat of milk. In order to test this starter a vat was filled with pasteurized milk in the ordinary way, and after being well agitated was divided between two vats. One vat had a good normal working starter added ; the other the starter to which had been added the contaminating culture obtained from the laboratory.

The first vat worked normally, acidity developing as looked for in good cheesemaking practice, while the other vat behaved in a manner similar to that obtained when working with what is commonly known as a slow or almost dead starter. At the time of setting, both vats had the same acidity by the alkaline and Marshall tests, and the same acidity in the whey when the curd was cut, while acidity developed to the same extent in each vat during the first two hours after setting. This is usually found to occur with slow starters. Slow starters develop acidity normally up to a certain point (about o-i6 on the alkaline test), and then, as in this case, acid development becomes slow and at times almost stops.

In this experiment the curd in the first vat, containing a good starter, was ready for the salt in six and a half hours from setting, while the curd in the vat with the contaminated starter was ten and a half hours reaching the same stage of maturity. The only difference noticed was that whereas in using the ordinary factory slow starter acid development is slow all the time, in this case there was a rapid development of acidity while the curd was draining, thus tending to show that although the culture. used was highly contaminated there was also present a very active lactic acid germ.

The contaminated starter received from the laboratory was subcultured at the factory on the day following its receipt, and tested 0-76 per cent, acidity. On the day it was used it tested 0-70 per cent., while the starter used in the control vat was 0-90 per cent, when used.

The following table gives a comparison between the times and acidities of the cheese made with the contaminated starter, a typical slow starter, and the control vat with a normal active starter :—

Samples taken at various stages from the experimental vat showed that B. subtilis was present in considerable numbers in the starter on the day it was used, though Streptococcus lactic was also present and apparently healthy. A sample from the vat after pasteurization and the addition of starter showed that subtilis was beginning to get ahead. Samples taken at various acidities during the cooking stage showed a steady increase in subtilis ; and a final sample taken from the “ white whey ” when cheddaring showed a very great development of subtilis, but also showed an increase in virility of the Streptococcus lactis.

CONCLUSION

In conclusion, it may be stated that apart from a considerable amount of subtilis appearing in samples of slow milks and slow starters, the conditions found in cheesemaking are suitable for the development and propagation of this organism. In the first place, pasteurized milk is a good medium for its growth ; and, in the second place, its spores can remain virile in the pasteurization plant itself, close to the pasteurizer. Experiments have shown that subtilis can remain alive and grow beside an active culture of Streptococcus lactis. Its biological characteristics show that it is capable of producing marked alkalinity in milk followed by an alkaline peptonization of casein, with the production of ammonia. This accounts for the slow curd developing weak spongy characteristics when maturing. •

Time. Control Cheese (Active Starter). Experimental Cheese (Starter and . B. subtilis). Ordinary Slow Cheese (Slow Starter). Acidity Percentage. Acidity Percentage. Acidity Percentage. Setting . . 0-19 0-19 0-19 . Cutting . . 0-13 0-13 0-13 11 hours 0-14 0-14 ... 0-14 3 hours . . 0-18 0-17 0-16 5 hours . . o-86 0-27 Commencement of slowness. 6-J- hours 1-00 Considerable variation in times and acidities. 7j- hours o-73 io hours 1-00