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.

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.

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.

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.

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.

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.