1. Data are presented on variation in age at eruption of permanent incisor teeth in one-egg (MZ) and two-egg (DZ) twins and in pairs of half sisters (HZ). The 1st pair of incisors erupted when animals were on average 100 weeks old and the other three pairs came at roughly 26-week intervals thereafter. Variation in age at eruption increased from 1st to 4th incisors, but the coefficient of variation remained constant at about 8%. The mean interval between eruption and complete emergence was about 5 weeks for the first three pairs of teeth and nearly 10 weeks for the 4th pair.

2. Variation in age at eruption within MZ pairs was no greater than that between left and right sides of the same animal. DZ displayed greater within-pair variances than MZ twins, and HZ pairs greater than DZ.

3. Correlations of age at eruption of teeth with body weight and with gain in weight were negative but non-significant. Age at eruption of 1st and of 3rd teeth was positively correlated with age at 1st and 2nd calving, but eruption of 2nd and of 4th teeth was not.

4. For 2nd, 3rd and 4th incisors, the intra-pair variance of the HZ group was in excess of that expected. This can be interpreted to mean that the estimates of environmental and genetic variation derivable from MZ and DZ pairs are not adequate to account for all the variation within HZ pairs.

Following the earlier papers in this series ((1)–(4)) further work has been carried out on the fats and greases which attack milking rubbers. The sources from which they come have been investigated and an estimate made of the composition of the fatty material obtained from used milk liners.

The importance of fat absorption as a cause of deterioration in the rubber teat-cup liners of milking machines has already been demonstrated (l, 2), and it is well known that the composition of the rubber affects its resistance to fat. It was considered desirable, therefore, to see what changes in composition could usefully be made, and how these would affect fat absorption under normal milking conditions. To this end, and with the cooperation of a large number of farms and Milk Production Officers, the four experiments reported here were carried out.

A method of cleaning milking equipment is described which involves the immersion of all items coming into contact with milk in a caustic soda solution for the whole period between milkings. It is expected that this method of maintaining the equipment in satisfactory bacteriological condition would need less attention to detail than existing techniques. The procedure outlined may only be applied to apparatus which can be packed into a basket no larger than can be readily handled. It therefore applies only to direct-to-can milking equipment, and suitable milking and immersion cleaning apparatus are described.

The process has been tried for a year on 8 commercial farms. The results are promising in that milk of good quality has been consistently produced, but the bacteriological state of the milking clusters was often below present standards. Deposition of calcium soap and inorganic calcium salts was marked on farms where the total hardness of the water was of the order of 200 p.p.m. Work is continuing.

Two farms, using direct-to-can milking, were used to compare utensil cleaning with hot detergent solution followed by hypochlorite on one farm and steam on the other, with immersion of the complete milking equipment in 5% caustic soda between milkings. Control and caustic treatments were used alternately for periods of 8 weeks for a total of 8 months.

Bacteriological tests made at weekly intervals on fresh milk and milk aged for 24 hr. at 22° C. showed little difference between the control methods, which involved brushing the milk contact surfaces, and the caustic soda method where there is no brushing.

The majority of micro-organisms in the bulk milk came from the udder and the utensils contributed little to the milk. It appeared that where utensils are satisfactorily cleaned, milking technique will be the factor which mainly affects keeping quality.

A study of the predominant flora revealed that milk from equipment cleaned by immersion in caustic soda had a simpler flora (mainly micrococci) than the control treatments.

A study has been made of the cultural and biochemical characteristics of heat-resistant thermophilic and mesophilic spore-forming bacteria isolated from raw and sterilized milk. The tests hitherto used in classifying this group of bacteria have been examined and some modifications suggested; as a result a simplified method of distinguishing different resistant spore-forming organisms in milk is proposed. The types of micro-organisms were few. The thermophiles were restricted to B. calidolactis and of the mesophiles B. subtilis predominated in sterilized milk and B. licheniformis in raw milk.

1. Details have been given of a modified Stiven test for determining the efficiency of pasteurization in milk.

2. A ‘long’ form and a ‘short’ form of the test have been described.

3. The ‘long’ and ‘short’ tests have been compared with the Aschaffenburg & Mullen and Kay & Graham tests.

4. The work has led to the following conclusions:

(a) The ‘long’ Stiven test seems best to distinguish between exposure for 24 min. and exposure for 30 min. to 62·5° C, though compared with the other three tests this ‘long’ Stiven test as assessed in the present work is possibly too stringent.

(b) There was little difference between the results of the ‘short’ Stiven test, the Aschaffenburg & Mullen and the Kay & Graham tests when used for the routine examination of specimens.

(c) The ease of performance and the stability of the reagents make the Stiven test convenient to use, and it should be helpful to those who have difficulty in matching blues and yellows.

(d) It should be of value as a confirmatory procedure.

The theory of the scattering of light in colloidal solutions is considered. As a result it is suggested that the reflectance changes in milk which involve a rise or fall in the general level of reflectance are caused by an increase or decrease in the mean size of the colloidal particles. Whether the particles responsible are the casein micelles or the soluble protein depends on the phenomenon to be explained.

The change in reflectance on the browning of milk, which is dependent on wavelength, is considered to have a more complex cause, involving the production of absorbing molecules.

The fat globules in milk and cream by taking up the fluorescent dye phosphine from aqueous solution are rendered fluorescent, and show under the blue light fluorescence microscope a bright yellowish green coloration against the black background. Even very small fat globules (in homogenized milk, cream, skim milk, or buttermilk) and very thin fat layers on the milk-air boundary or on glass can be distinguished owing to the high contrast.

By using glycerol containing two fluorochromes, phosphine for the fat and basic fuchsin for the proteinaceous matter, a very sharp differentiation of these two components in milk-powder particles is attained. Differences in the state of dispersion of fat caused by manufacturing technique and storage conditions, and affecting the properties of powders, are clearly made visible.

1. The problem of unsatisfactory curd formation in cottage-cheese manufacture has been studied in relation to milk composition.

2. Addition of calcium chloride to the milk did not produce significant improvement.

3. There was a distinct breed difference in the curd formation; Jersey milk produced satisfactory curd whilst Friesian milk invariably produced unsatisfactory curd.

4. Dilution of Jersey milk to the same level of precipitable casein as Friesian milk resulted in unsatisfactory curd.

5. The presence of 10 % Friesian milk in Jersey milk rendered the latter unsatisfactory.

6. The Walker formol titration for determination of casein gave significantly high values with Friesian milk. This is attributed to the different distribution of nitrogen in Friesian milk.

1. Steam distillates containing the flavouring substance(s) from Cheddar cheese were examined chemically. After the volatile fatty acids had been removed, the only substances identified in the flavour concentrates were ethanol, butyraldehyde, ethyl acetate and ethyl butyrate. None of these compounds appeared to be connected with the cheese flavour.

2. The essential flavour compound(s) could not be identified but some of its properties were elucidated.

3. The findings of other workers that the lower volatile fatty acids, diacetyl and acetyl methyl carbinol contribute to Cheddar-cheese flavour could not be confirmed. Flavour tests with numerous other compounds failed to show that they contributed to the typical flavour of Cheddar cheese.

The author gratefully acknowledges the helpful criticism and advice of Dr H. R. White-head throughout the investigation.

Quantitative data have been obtained concerning the amounts of individual amino-acids present in the aqueous phase of Cheddar cheese during the ripening process. They differed from the amounts found in casein by the larger amounts of basic amino-acids, smaller quantities of proline and the presence of ornithine.

Tasting tests showed that amino-acids contribute an important background flavour to Cheddar cheese.

An unidentified substance or group of substances, representing 7–9% of the total ninhydrin-positive material in the serum of ripe cheese, has been observed.

The author gratefully acknowledges the interest and advice of Dr N. J. Berridge, and the technical assistance of Mrs K. Kelly. Thanks are also due to Miss H. R. Chapman for making the cheese.