11

H.—34

an involved piece of work, and before embarking on such an investigation it seemed desirable to investigate in the College factory whether the ordinary factory milk-supplies from different localities, made into cheese under the same conditions, did actually give different results. It was found that cheese made from milk from all districts investigated developed slit openness. With regard to mechanical openness, on the other hand, the results were not conclusive. More difficulty was experienced in making cheese free from mechanical holes from the milk from certain districts. Definite results were upset by the difficulty of procuring milk of the same bacterial flora and numbers from day to day. There were indications in some experiments that the development of mechanical holes has a relation to the sanitary quality of the milk ; but this does not preclude the possibility of chemical peculiarities also exerting an influence. Definite conclusions on the points can only be formed after further experiment. (b) Bacterial content of the milk : As pointed out in the last annual report, no outstanding difference was revealed by Mr. H. R. Whitehead between factories little affected and badly affected with openness, in the bacteriological analyses of their raw milk and pasteurized milk. The milk-supplies in general were found to be unsatisfactory to enable the cheesemaker to exercise reasonable control over the process of manufacture. This state of affairs calls for national farm inspection, milk-grading as soon as it can be introduced, the proper care of milk on the farm, the careful cleaning and sterilization of milking-machines and milk-cans, and the cessation of the pernicious practice of carting whey back to farms where there are not adequate facilities for the sterilization of the cans used for this purpose. In order to establish the effect on cheese-texture of certain gas-producing germs, commonly found in milk, Mr. H. R. Whitehead carried out a series of experiments in which these organisms were added to the milk just previous to the manufacture of cheese. The detailed experiments will be published at an early date. The principal finding from the point of view of the problem in hand was that so long as an active starter was used they did not exercise any influence upon texture, but they produced very offensive flavours in the mature product. The effect which these organisms have upon sustained action on the milk previous to pasteurization remains to be investigated. The necessity of precluding all bacterial contamination has led to studies concerning methods of pure-milk production under New Zealand conditions. This is being studied closely by Mr. H. R. Whitehead, and is reported upon under the heading " Pure-milk Production." In the course of experimental work with very pure milk, it has become apparent that such milk requires a much longer " ripening " period than ordinary milk supplied to dairy factories and that the curd will not carry so much acid at salting. Further careful study is required in regard to the handling of very pure milk in its relation to what is popularly known as " desirable cheddar-cheese flavour." (c) Slow milk and mechanical openness : It sometimes happens that the cheesemaking process is slowed up in one way or another either while the curd is in the whey or subsequent to drying. This abnormality has been attributed to contaminated milk and to impurity or lack of vitality of the starter, but definite evidence of any active cause in the milk itself has not yet, in any instance, been adduced. No one has been able to reproduce the non-acid condition in a cheese-vat by addition of supposedly causative germs to normal milk. There is, however, some evidence that certain germs growing for some time in a starter culture will reduce its vitality. Even pure starter cultures are known to lose their vitality for some unknown reason. In any case, an apparent non-acid condition does occur with milk of good sanitary quality as well as with milk of poor sanitary quality, so that it seems almost certain that the starter is responsible in some instances. Moreover, our experiments show that abnormal slowing-up on the cheesemaking process is conducive to mechanical holes in the cheese even though manufacture is prolonged to give the curd every chance of developing as much acid as possible before salting. Sometimes there may be an apparent slowing-up in acid-production between milling and salting, and even an apparent reduction in the amount of acid may occur. This has been experienced in our experimental makes, especially in slow vats; It is often associated with high fat content of any whey pressed out, and, since fat has no acidity, the acidity of the normal volume of whey used in testing is reduced. At this stage of manufacture the hot-iron test of a portion of curd taken from the centre of the mass is a better test. The feel of the curd to an experienced maker is even more reliable. The Effect of Details in Manufacture on Slit and Mechanical Openness. —The art of cheddar-cheese manufacture lies in keeping acid-development, and moisture-expulsion working in harmony, so that at the time of salting, the curd is well broken down, resilient to the feel, and free of excesß moisture. This is effected in practice by adjusting from time to time, as the curd requires, the amounts of starter and rennet, ripening-period, cooking-temperature, and acidity at various stages in the process of manufacture. When salt is added to the curd, moisture is expelled and bacterial activity restrained, the effect upon each depending upon the amount of salt used. Acid-development is dependent on bacterial activity, which in turn depends upon the vitality of the acid-producing germs supplied largely by the starter, and upon the moisture content of the curd. This action continues after the salt is added, and it exercises a great influence on the quality of the cheese. Moisture is partly expelled by heat, and partly by acid-development. For these reasons, if a slow-working curd and a fast-working curd are salted at the same acidity and the same rate of salting, the texture, body, and flavour of the resulting cheeses are quite different. The chemical condition of the curd at the time of salting would appear to be the principal factor influencing the cheese later on. This is determined by the conditions referred to above. If the cheesemaker had an exact knowledge of the chemical state of his milk and curd at three critical stages—namely, setting, running the whey, and salting of the curd —he would be in a better position to control his manufacture. Unfortunately, this information is not yet available ; indeed, there is not even a simple moisture test for curd that can be performed in a few minutes. Thus the cheesemaker has very largely to rely upon judgment, though he has some assistance from the acidity of the curd and the hot-iron test, if he is accustomed to the use of the latter. Experiments are in