The term “starters” as used in dairying is understood to refer to cultures of lactic acid bacteria which induce “souring” when inoculated into milk, cream or other product. Souring will usually occur without the use of starters, but the results will be erratic. With the help of starters, however, the reaction can be controlled so that it proceeds in a desired direction. The term generally suggests those starters which are used in the making of butter, but in its widest sense it must include all cultures which are used in the dairy for the production of any type of souring.

The most important fact brought out by a review of the present position of our knowledge of mineral metabolism and the mineral requirements in the dairy cow is, that, in spite of the amount of work which has been done on the subject, our information is exceedingly scanty. We know that deficiency of calcium or phosphorus may affect rate of growth, milk yield, breeding capacity, and in all probability susceptibility to certain diseases. We know also that even on what are counted good rations, depletion of the tissues, especially of calcium, in heavy milking cows during lactation is common. In some districts deficiency of iodine causes pathological conditions, and it is common knowledge based on practical observations that even on pasture there is liable to be a deficiency of chlorine. With regard to the possibility of deficiencies of other mineral elements and the effect of such deficiencies on the health and milk production of cows, we have not yet even begun to accumulate knowledge. It is thus impossible to make a final pronouncement with regard to any of the main problems. We need many more data on the metabolism of minerals in the lactating animal and on feeding experiments of long duration with rations of different mineral content. Such information as we have, however, suggests that the subject is one of great potential practical importance, and that if the necessary information were available it might be applied to the more economical production of milk, both by increasing milk yield and by reducing mortality from disease.

With regard to the information we do possess, much of it is not immediately applicable in practice because the data have been accumulated with diets and under conditions which have no parallel in practice. The following conclusions sum up in a general way the practical bearing of our present knowledge.

Four organisms commonly found in contaminated milk (B. coli, B. subtilis, a staphylococcus, and B. faecalis alkaligenes) were grown in milk in association with lactic streptococci, and the acidity produced in a given time was determined. B. subtilis appeared to stimulate the production of acid. B. coli caused a decrease in acid production in some experiments, and an increase in others. The staphylococcus had a slight stimulative effect. B. faecalis alkaligenes had only a slight effect which was barely beyond experimental error. There was no indication that any of the organisms are responsible for the cessation of acidity production which sometimes occurs in milk during cheese manufacture. However, the results were not conclusive in this connection, since the experimental conditions did not simulate those obtaining in a cheese vat.

A Study of the chemical changes involved in the ripening of Cheddar cheese becomes largely a study of protein degradation. The result of the breakdown of the casein through the combined effect of rennin, milk enzymes and proteolytic bacteria is to furnish water-soluble decomposition products which increase in amount as the ripening progresses. In consequence a method used by many workers in the past to follow the course of ripening was to determine at intervals the percentage of soluble nitrogen present in the cheese. It must be remembered, however, that the decomposition products which make up this soluble nitrogen factor comprise different compounds such as caseoses, peptones, polypeptides, amino acids and ammonia, which are of decreasing molecular complexity in the order named. All these compounds are soluble in water to varying extents and, therefore, a determination of the soluble nitrogen merely gives an estimate of the sum total of the nitrogen which they represent, and gives no information as to the way in which it is distributed amongst the different types.

The preceding investigation left a little doubt as to the effect produced by pasteurisation of clean milk upon the flavour of the mature cheese. For although the cheese made from milk, flash-pasteurised at 165° F., appeared to develop a desirable flavour more rapidly than the raw control throughout the greater part of the ripening period, yet at the end this was spoilt by a very slight bitterness.

1. Milk of varied hygienic quality has been “flash”-pasteurised at different temperatures and portions have been coagulated with rennin at intervals after pasteurisation.

2. Analyses performed upon the whey separated from the coagula of the raw and pasteurised milk have shown that the mineral content of the whey depends largely upon the acidity.

3. The increase in the coagulation time with rennin caused by pasteurisation is associated with a decrease in the whey nitrogen, so that the one is approximately a linear function of the other.

4. The chemical changes produced by “flash”-pasteurisation of milk appear to be increased both by the use of higher temperatures, and also by the treatment of milk of low hygienic quality, as compared with clean milk.

1. Representative strains of organisms of the colon group have a deleterious influence on the flavour of Cheddar cheese when they are added to the milk immediately before the start of the process of cheese manufacture.

2. They do not produce gas holes in the cheese, even when present in large numbers, provided that an active culture of lactic streptococci is used as “starter.” It seems most probable that their metabolism is modified by association with the lactic streptococci, so that, although the colon bacilli grow in the cheese, they produce little or no gas.

3. Under the conditions of these experiments, they have no influence on the texture of the cheese.

4. They do not appreciably affect the production of acid by the “starter” in the milk or curd.

5. Under the experimental conditions described, they do not cause decolorisation of annatto in a coloured cheese. It seems unlikely, therefore, that they are responsible for mottling in cheese.

Weekly tests of the acidity of the evening milk of each cow in the herd of the Agricultural Research Institute of Northern Ireland, Hillsborough, Co. Down, were made over a period of almost two years. A graph showing the influence of the stage of lactation on the acidity of the milk is given.

In the course of some experiments on the manufacture of cheese, in which it was necessary to add cultures of various organisms to milk in the cheese vat immediately before the addition of rennet, a curious anomaly in the direct microscopic count (Breed method) was observed. The organisms to be used were grown for 24 hours on a meat-extract peptone agar. The growth was then washed off with sterile saline, and the resulting suspension was added to the milk in the vat. After the milk had been stirred thoroughly, samples were taken and a direct count, according to the method of Breed, was carried out in order to determine approximately the number of organisms which had been added. With several of the types of organisms used (staphylococci, streptococci and Gram-positive bacilli) this procedure gave a reasonable result but, in the case of Gram-negative bacilli of the coliform group, it was rarely possible to detect under the microscope any definite organisms, except those present originally in the milk and observed in the control sample. This observation proved consistent over a long series of experiments.

Consideration has been given to the various methods available for the estimation of salt in cheese. The following three methods have been found to give concordant results, and to permit accurate estimation of known added quantities of chloride.

(1) Careful ashing with small amounts of potassium nitrate, after addition of some dilute caustic soda, and subsequent titration of chloride according to the method of Volhard.

(2) Solution of the cheese in caustic soda, precipitation of the proteins by nitric acid, and titration (accordmg to Volhard) of chloride in an aliquot portion of the filtrate (van der Burg's method, with some modifications introduced by the authors).

(3) Titration by the Volhard method after destruction of the proteins by hot nitric acid in the presence of excess silver nitrate.

Methods (2) and (3) have been found suitable for routine analysis of cheese.

It would be difficult to over-estimate the importance of disease of the cow's udder, and the financial loss from this cause is felt by all who are engaged in the industry of milk production. The loss is aggravated by several factors, and of these reference must be made in the first place to the wide distribution of mastitis.

1. The most important factor in the growth of the red spot organism in milk is the presence and growth of certain other bacteria. The reason for this is discussed.

2. The bacteria most active in metabiotic stimulation of pigment production are those inducing low oxidation-reduction potentials in milk.

3. The influence of factors such as variations in the method of manufacture and the composition of the milk and curd on the incidence of the fault have been investigated under practical cheese-making conditions.

4. There is no correlation between the hydrogen-ion concentration and other properties of the ripe cheese and the extent of the red spot.

5. Measures for the control of red spot have been suggested.

The experiments have shown that although butter of exceptionally fine flavour can be produced by the use of pure culture starters of S. paracitrovorus under laboratory control, the uncertainties under practical conditions, due to its weak growth, are too marked to warrant its general use. Both S. citrovorus and S. paracitrovorus are unable to compete successfully with the inevitable contaminants encountered in practice. The value of vegetable media such as grass, silage and bean agar for growing streptococci such as S. paracitrovorus has been confirmed, but as observed by Orla-Jensen et al. the subsequent growth in milk media lacks the vigour which might be expected.