H.—34.

At another commercial factory experiments were made throughout the season to determine whether it is possible to protect the starter from air-borne phage under somewhat crude conditions and without much expenditure. During the course of this work the mechanism of the development of air-borne phage was made clear. It has previously been shown that bacteriophage commonly develops in the whey in cheese-vats, even when the process is proceeding normally. The reason for this is not known, but it may be accepted as a normal occurrence in cheesemaking practice. The separator which is used to remove the last traces of fat from the cheese whey revolves at approximately six thousand revolutions per minute, and it was found this season that the separation process resulted in the emission of a finely atomized whey (usually containing phage). This proved to be the major source of the air-borne phage in the factory building. The concentration of phage in the air was so strong on occasions that the mere removal of a cotton-wool plug from a flask of sterilized milk for ten seconds was enough to cause immediate failure of a starter in that milk. This finding emphasized the necessity for removing the starter mother culture from the factory environment. By complete closure of the starter cans and inoculation through a small orifice plugged with cotton-wool it did prove possible to maintain the bulk starters with only very occasional failures in the factory buildings. The whole, trend of the results, however, showed that, while various makeshift devices gave considerable relief from starter failures, nothing short of a special room completely cut off from the main factory building gave security. Several dairy companies are now contemplating the building of special starter-rooms. Experience over the next season or two will show how near complete success with starter cultures the average factory can come. Single-strain starter cultures were used throughout the work mentioned above, but there is ample evidence to support the view that cultures containing several different strains of streptococci such as have commonly been prepared from commercial powder will be more steady in activity under commercial conditions if they are similarly protected from air-borne contamination. (b) Technique of Cheese-manufacture with Active Starters (W. Riddet and H. R. Whitehead). —- The single-strain starters R t and HP were used throughout the season for the manufacture of cheese in the experimental factory. The paramount influence of the acidity at the time of removal of the whey from the curd on the character of the cheese was confirmed. Other factors such as the rate of acid development throughout the process, the acidity when the curd is milled, and the acidity at salting have a very subsidiary influence. The optimum acidity at the drying stage naturally varies with the composition of the cheese milk. It ranges between 0-20 per cent, and 0-28 per cent, lactic acid (for whey draining from the curd immediately after drying). Drying the curd at too low an acidity (say, 0-20 per cent, instead of 0-23 per cent.) tends to give a cheese with a weak " rubbery " body, while drying at too high an acidity (say, 0-26 per cent, instead of 0-23 per cent.) tends to give a cheese with a firm, but sticky body and a sharp acid taste. The minor influence of the rate of acid development on cheese quality makes it possible within limits to reduce the time the curd is cooked in the whey where an active starter is used. Similarly, the times between drying and milling and milling and salting can often be reduced without any significant influence on cheese characteristics becoming apparent. There are slight differences in the effect of the various single-strain starters which have to be recognized and allowed for in the cheesemaking process. When a cheesemaker becomes accustomed to the use of a particular single-strain starter, its regularity of behaviour gives him an excellent chance to adjust his making procedure so as to get the best possible result. (c) The Sources of Lactobacilli important in Cheese-ripening (I. R. Sherwood). —In earlier reports the important influence of lactobacilli upon the ripening of Cheddar cheese has been discussed. During the past year an investigation of the sources of these organisms has been carried out. The Milk employed in Cheese-manufacture : In New Zealand cheesemaking practice the milk is usually flash-pasteurized. This heat treatment destroys most of the organisms in the milk, but it has been shown in the present investigation that an appreciable proportion of the lactobacillal population survives and passes into the cheese-curd. Milking-machines and Utensils used in the Production of the Milk : Owing to the extensive use of milking-machines in New Zealand, a special study was made of the bacterial flora of selected machines. By the use of a selective cultivation procedure it was shown that all types of lactobacilli of common occurrence in cheese also occurred in milking-machines. In individual machines one or two types usually predominated. Experiments carried out on the Institute machines showed that while metal parts and rubber parts in good condition were fairly easily sterilized, yet worn rubbers displaying cracks and scratches could not be sterilized satisfactorily by ordinary methods. From other utensils used in milk production —cans, buckets, &o. —lactobacilli were frequently, but not invariably, isolated. Sources from which Lactobacilli may gain Access to Milking-machines and Utensils : Earlier workers have shown that lactobacilli occur in ensilage and dung. Other sources are — (1) Dust from the Cowyards and Environs : Examination of samples of dust from cowyards • -and adjoining roadways revealed the presence of lactobacilli (Sbm. casei, Sbm. plantarum, and betabacteria). These organisms were found to gain access readily to milking-machine parts in the sheds, especially the teat-cups. Although in practice the extent of contamination which occurs in this way is usually relatively small, yet once the organisms gain a foothold in the machines, rapid multiplication may occur under suitable conditions. (2) Grasses : Lactobacilli were isolated from grasses by the use of a laboratory modification of the ensilage-making process. Perennial rye was found to harbour three varieties of Sbm. plantarum (varieties 2, 3, and 4, described in the last report). The same varieties of Sbm. plantarum were isolated from brown-top and a few other grasses. From red and white clovers and from cocksfoot the only lactobacilli isolated were all strains of Sbm. flantarum, variety 1. There would thus appear to be some specificity in the occurrence of lactobacilli on grasses, but seasonal and climatic factors have not yet been adequately investigated.

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