1. An experiment was conducted to compare the relative value for winter milk production of artificially dried cocksfoot and a mixture of timothy and meadow-fescue each cut in the spring and autumn.

2. A double balanced 4×4 Latin square design with eight Ayrshire cows and four periods each of 3 weeks' duration was used.

3. The dried grasses were fed in equal amounts (17 lb./cow/day) and equal quantities of each grass were consumed in terms of dry matter. Approximately half the total dry matter intake was grass.

4. No differences were detected between the cocksfoot and the timothy/meadow-fescue mixtures in their value for milk production.

5. With both cocksfoot and timothy/meadow-fescue the spring-grown grass gave slightly higher milk yields than the autumn-grown grasses but the differences were not statistically significant.

6. Feeding the spring-grown dried grasses led to a slightly higher solids-not-fat content in the milk than when the autumn-grown grasses were fed.

7. The feeding of the autumn grown timothy/meadow-fescue grass mixture was associated with a significant loss of live weight in the cows. It was suggested that this loss of weight was due primarily to changes in the amount of the contents of the rumen and intestines.

Machine milking has been studied in three cows by cineradiography at 50 frames/sec. The findings of Pier et al.(5) have been confirmed.

The effects of variation in the pulsation rate, ratio and liner tension were investigated and the results compared with those obtained by other methods. The proportion of each milking cycle during which milk flows has been determined.

1. Seven sets of monozygous twins were kept under the same open-grazing conditions for one year. Milks were analysed for fat, solids-not-fat, lactose and total protein contents, and butterfats were isolated at fortnightly intervals for estimation of iodine value, refractive index, softening point, saponification value, Reichert value, carotene content and vitamin A content. The results are presented both graphically and statistically.

2. Consistent differences in body weights of the cows of a twin set did not render the twin set unsuitable for use in experimental work on causes of changes in milk yield, fat yield, milk composition or butterfat characteristics.

3. The advantages of monozygous twins over unrelated cows was shown: (a) by the close general correspondence of the within-twin-set results as compared with the between-twin-set results throughout the period of the trial; (b) by the similarity in the forms of the curves for the different sets of twins; (c) by the marked similarity of the reaction of the cows of a twin set to change in environmental conditions—this applied particularly for butterfat characteristics such as iodine value and refractive index; (d) by the ‘twin efficiency values’ for butterfat properties computed from the whole-lactation weighted means, viz. iodine value, 7·8; saponification value, 3·4; Reichert value, 5·5; carotene content, 7·8; vitamin A content, 1·7; and vitamin A potency, 2·5.

4. There were wide differences in the degree of approach to ‘identicality’ with different sets of twins. The cows in two of the sets showed a consistently close within-twin-set relationship for all characteristics studied.

1. Results are reported of an experiment carried out under commercial conditions to obtain information on the comparative value of dried skim-milk and white fish meal as protein supplements for fattening pigs.

2. There were four treatments. Control pigs (i) received a meal mixture containing 10% white fish meal, reduced to 7% from 130 lb. live weight. In the other experimental groups, the fish meal was replaced by dried skim-milk, (ii) on a weight for weight basis, (iv) on a protein basis and (iii) intermediate between (ii) and (iv). All pigs were fed the meal dry and ad lib. up to a maximum of 6½ lb./head daily. Unrestricted water was available.

There were five pens of nine group-fed pigs on each treatment, involving a total of 180 pigs. The pigs were on experiment from approximately 10 weeks of age to bacon weight. Comprehensive carcass measurements were made on all the pigs.

3. The results showed that a significantly better rate of growth was obtained when the white fish meal was replaced by dried skim-milk on a weight for weight basis. Increasing the percentage of skim-milk in the diet up to 15% resulted in a slightly better performance of the pigs, but no further improvement was obtained by raising the level of skim-milk to 20%. These results were discussed in relation to an apparent set-back that the control pigs suffered at the start of the experimental period.

4. The inclusion of dried skim-milk in the diet had an adverse effect on the carcass grading, particularly in so far as it resulted in a thicker layer of back fat over the loin as compared with that in the control pigs.

5. There was also some indication that the carcasses from the animals given dried skim-milk had slightly thicker bellies and a smaller breadth of eye muscle than the carcasses of the control animals that received fish meal.

6. The results have been compared with those of an earlier trial, which was in most details the same as that reported here, the main difference being that in the earlier experiment individual feeding and one breed of pig were used.

Bacteriophage active against a lactic streptococcus was first isolated in this laboratory in February 1935. In May of the same year some batches of phage for Streptococcus cremoris strain HP were prepared by lysing the bacteria in skim-milk culture and preparing a Seitz-filtered whey as described by Whitehead & Cox(1). At that time it was not the practice to adjust the reaction of phage filtrates and the pH of the batches prepared in 1935 was not recorded. It was probably between 5·2 and 6·1 (Hunter & Whitehead (2)). Samples of four different preparations of the phage were filled into sterile tubes and sealed off in a blow-pipe flame. Two of the samples, sealed on 24 May 1935, had titres of 107 (as determined by spotting of standard loops of dilutions on a bacterial mat) and the other two, sealed on 16 July 1935 had titres of 106. The sealed tubes were stored in the refrigerator at 4–7° C.

A method has been described for the routine estimation of total phospholipids and of lecithin, cephalin and sphingomyelin in butter. Butter serum was treated with ether and light petroleum in a modified Röse-Gottlieb procedure to extract the phospholipids which then were estimated in the extract.

Chloroform-methanol extracted from the butter serum slightly more phosphorus than did the ether-light petroleum mixture. Ethanol-ether, benzene-ethanol and benzene-isopropanol, when used to extract a protein-phospholipid precipitate formed by the addition of trichloroacetic acid to butter serum, also removed slightly more phosphorus than did the ether-light petroleum mixture. The higher phosphorus contents of these extracts was due partly to a slightly more efficient extraction of phospholipids and partly to extraction of non-phospholipid phosphorus compounds.

Samples of milk, skim-milk and raw cream from one factory and of pasteurized cream, buttermilk and butter from two factories were taken twice monthly for varying periods for estimation of total phospholipids and (in butter only) of lecithin, cephalin and sphingomyelin.

Seasonal variations in the phospholipid contents of milk, skim-milk and raw cream over 13 months probably were due to the greater proportion of small fat globules in autumn (late lactation) milk compared with spring (early lactation) milk.

There were regularly recurring seasonal variations in total phospholipid content of the butters over 3 years. Maximum results were found during autumn and minimum results in winter. Lecithin, cephalin and sphingomyelin contents followed the same trends as the total phospholipid contents.

Seasonal variations in phospholipid contents of buttermilks and butters were due mainly to variations in the amount of phospholipid per unit of fat in the cream. Differences in butter-making technique had little effect on the proportion of phospholipids from the cream retained in the butter.

Average results for total phospholipid content of the products examined were: milk, 0·038% (0·031–0·050%); skim-milk, 0·018% (0·014–0·023%); buttermilk, 0·156% (0·103–0·191%); butter, 0·133% (0·099–0·181%). The average result for percentage of total phospholipid in the fat of the raw creams examined was 0·44% (0·38–0·51%); and in the fat of the pasteurized creams 0·41% (0·35–0·49%). Average weighted results for total phospholipid and for lecithin, cephalin and sphingomyelin contents of butter were 0·139, 0·041, 0·051 and 0·047% respectively.

The gross energy content of the milk of the cow, sow, ewe, goat and of human milk has been determined by bomb calorimeter, and compared with results obtained using conversion factors and chemical analysis.

Correction for non-protein nitrogen content led to values agreeing within 0·5–1·0% for the milk of all species. For human colostrum other factors may be involved, and a closer estimate of gross energy value of human milk was obtained by treating all nitrogenous constituents as being protein.

An extraction-titration method is described for the determination of free fatty acids in small samples of fresh milk.

Recoveries of fatty acids added to milk ranged from 63% for butyric acid to 97% for stearic acid. For a composite sample approximating milk fat in fatty acid composition the recovery was 83%.

Interference from citric and lactic acids was not significant for reasonably fresh samples.

The method gives a higher recovery of the lower molecular weight fatty acids than is possible by earlier methods.

For titrating very dilute solutions of fatty acids α-naphtholphthalein was shown to be a more accurate indicator than phenolphthalein; it also provides a better defined endpoint.

A method for the determination of the fat content of milk from measurements of the turbidities of diluted milk suspensions in the near infra-red region has been shown to provide greater accuracy than the alkali-addition method in the visible region. Variations in results due to breeds of cows and lactation times have been studied, and excluding early lactation samples, the maximum deviation within a given breed was found to correspond to ±0·2% in the fat concentration.

1. This paper is Part I of a series dealing with the relation between the chemical composition of milk and the stability of the caseinate complex to ethanol (Part II), rennet (Part III) and heat (Part IV).

2. A description is given of the milk samples, which were taken from herd bulk milk, from individual cows in different stages of lactation and also from cows with subclinical mastitis.

3. The methods used to measure the stability of the caseinate complex to the three coagulating agents are described, together with the methods used to make a detailed chemical analysis of the milk samples.

4. The chemical composition of the samples is given and also the relation between composition and stage of lactation, and the interrelations of the concentrations of certain milk constituents.

The authors thank Miss M. H. King and Miss R. E. M. Stevenson for technical assistance, Mr N. H. Strachan and Mr R. C. Voss (West of Scotland Agricultural College) for the sodium and potassium determinations and Dr P. S. Blackburn for the differential cell counts and bacteriological examination of the milks.

1. The variation in the stability of milk protein to ethanol and the relationship between milk composition and ethanol stability were examined.

2. Samples of herd bulk milk were very similar in stability to ethanol; the range of aqueous ethanol solutions required to coagulate the caseinate complex in an equal volume of milk was only 80–84% (v/v) ethanol. Samples from individual cows showed a wide variation in stability; coagulation was caused by ethanol solutions ranging in strength from 66 to 90% (v/v) ethanol.

3. Colostrum was very unstable to ethanol but stability rapidly increased during the post-colostrum period to higher levels in mid-lactation. Late lactation and subclinical mastitis milk showed no definite bias towards stability or instability.

4. The strength of ethanol required to coagulate the caseinate complex in an equal volume of milk was inversely related to the concentration of ionized calcium in the milk. The correlation coefficient was –0·76 (significant at 0·001 level) indicating that approximately 60% of the variation in stability was accounted for by the variation in the concentration of ionized calcium.

5. The relationships between the concentrations of other milk constituents and stability to ethanol could be attributed to the interrelations of the concentrations of these constituents and the concentration of ionized calcium.

1. The variation in the time required for the coagulation of milk by rennet and the relationship between milk composition and renneting time were examined.

2. The renneting times of herd bulk milks varied over the relatively narrow range of 3·2–5·4 min., whereas the renneting times of milks from individual cows varied from 1·4 to 12·9 min. and two did not coagulate.

3. In general, milk quick to coagulate gave a firm curd (early lactation milk) and milk slow to coagulate gave a soft curd (late lactation and subclinical mastitis milk).

4. The renneting time of milk was related to the stage of lactation of the cow. In early lactation, renneting times were short and as lactation advanced there was a progressive increase in renneting time; the increase became more pronounced towards the end of lactation.

5. The property of milk most closely related to renneting time was acidity. As pH increased from 6·4 to 7·2, there was a curvilinear increase in renneting time from 1·5 to 13 min. Titratable acidity was related to renneting time in the opposite way to pH.

6. Although milk containing a relatively high concentration of ionized calcium coagulated quickly with rennet, the renneting time of milks containing average, or less than average, amounts of ionized calcium was not related to their content of this constituent.

7. Weak relationships between the concentrations of other milk constituents and renneting time could be attributed to the interrelations of the concentrations of these constituents and pH.

1. The variation in the stability of milk protein to heat and the relationship between milk composition and heat stability were examined.

2. The coagulation times of the majority of the milk samples decreased by a factor of about 3 with an increase in temperature of 10°C. over the range 130–150°C. Because of the general proportionality of the coagulation times at 130, 140 and 150°C., the coagulation time at 130°C. only were used as a measure of the stability of the samples to heat.

3. The coagulation times of herd bulk milks ranged from 17·2 to 59·0 min. at 130°C., whereas the range for samples from individual cows was 0·6–86·2 min.

4. Samples of colostrum were very unstable to heat, and milk from cows in late lactation tended to have the longest coagulation times, but otherwise there was little relation between the heat stability of milk and the stage of lactation of the cow.

5. Although colostrum samples were comparatively rich in ionized calcium, their marked instability to heat appeared to be caused solely by their high content of lactalbumin plus lactoglobulin.

6. The stability to heat of the calcium caseinate-calcium phosphate complex in all samples, other than colostrum, could not be closely related either to the concentration and composition of the complex or to the composition and salt-balance of the aqueous phase.

7. When the calcium phosphate content of the caseinate complex was relatively low, the heat stability of the complex tended to be inversely related to the concentration of ionized calcium in the milk, but in general coagulation time was not related to the concentration of ionized calcium.

A method for measuring the increasing rigidity moduli and internal viscosities of milk during rennet setting is described. Curves are given showing the degree of reproducibility on replicate samples of dried and liquid milks, and of the effects of changes in calcium levels in the case of the former.

A tentative interpretation of the shape of the main part of the setting curve is proposed. The upper part of the curve generally becomes erratic with the onset of syneresis, but some dried milks do not show this behaviour.

Some curious phenomena concerning the production of syneretic liquid (whey) are described and methods for obviating the disturbing effects of syneresis are discussed.

The rennet-catalysed coagulation of milk casein has been examined calorimetrically. The heat of coagulation and syneresis was found to be less than 0·1 cal.'40 ml. of milk, the limiting sensitivity of the apparatus.

Heats of reaction approaching 0·5 cal. were observed when the milk was acted upon by the enzymes trypsin and papain. It is doubtful whether, at this stage, the results can be correlated with the several phases of the reaction.

A viscometric study of solutions of alkaline (sodium and potassium) caseinate was made to determine the effects of renneting with and without the addition of calcium ions. There was no real viscometric effect of these treatments at high dilutions. At a higher temperature, the viscosity increment of the solutions decreased. It was suggested that particle aggregation occurs more readily at higher temperatures, leading to a greater particle symmetry as well as causing some dehydration.

Fluorescence microscopy was applied to the examination of casein in milk, curd and cheese, to check or support the picture obtained by electron microscopy. The best fluorochroming effect was shown by thioflavine T, acridine orange, coriphosphine and basic fuchsin.

In milk, larger casein particles (0·1–0·8 μ in diameter) were rendered visible as minute nearly spherical corpuscles in lively Brownian movement. The sediment obtained on centrifuging the whole milk at about 900 g contained predominantly large particles (average diameter 0·;8 μ). On acid or rennet coagulation the casein particles aggregated to flocs, which finally formed a three-dimensional framework.

From curd or cheese freezing-microtome sections (10 μ thick) were examined. The curds which did not possess the fibrous (‘chicken-breast’) texture, characteristic for wellcheddared curd, under the fluorescence microscope displayed a granular structure, whereas curds with a pronounced fibrous texture showed a network of thin micro-fibres, particularly in sections cut along the direction of macro-fibres.

The sections (10 μ or less thick) of curd and cheese on the surface of water slowly spread out. The spreading was accompanied either by conversion of the form of casein from granular into fibrous, or by stretching of the micro-fibres in the already existing network.

The grain boundaries in curd and cheese (Cheddar, Emmental, Edam) were almost free of fat. No grain boundaries existed in process cheese.

It has been shown that the degree of homogenization can be tested directly by measuring the transmission of milk samples of thickness ca. 0·1 mm., using wavelengths in the near infra-red region.

The purpose of this paper is to show how the performance of a continuous heat-treatment plant may be predicted from the temperature distribution and the flow time distributions obtained as described in the first paper of this series (1), using thermal death-time characteristics appropriate to the micro-organisms of greatest importance. The calculations are carried out using the data obtained for the plant described in the previous paper, and they provide an estimate of the bactericidal effectiveness to be expected from that plant.