1. The mean and the standard error of the mean of milk yield and of butterfat percentage have been determined for each of the first five successive Milk Recording Society tests of both afternoon and morning milk.

2. The yield of butterfat and the coefficient of correlation of milk yield and butterfat percentage at each test have been calculated.

Figures showing the means and the standard deviation of butterfat percentage of milk samples, at the afternoon and morning milkings res-pectively, during the four quarters of the year are presented.

The probable sources of error due to the nature of the sample are discussed and the seasonal variability of mean and of standard deviation is considered.The likelihood of the mean of afternoon or of morning milk failing to reach certain specified minima during the respective seasons is calculated.

It is a well-established fact that during the summer months, particularly July and August, the chemical quality of milk tends to be poor. The samephenomenon has been reported in Germany (i) and in the United States of America(2, 3). The individual factor or factors responsible have not been thoroughly established, although the work of Ragsdale and Turner (4) andHays (5) shows clearly that the fat content of milk tends to be reduced as the environmental temperature increases. The extent of the summer depression in milk quality in this country is much greater during hot dry summers. Cranfield(6) and Golding et al. (7) found exceptionally poor-quality milk during the summers of 1929 and 1933 respectively. In both these years most cows were subjected to a higher temperature than usual and also to a variety of other unusual factors such as the changes in nutrition occasioned by dried-up pastures. The work reported here was planned in order to throw some light on the influence of temperature, and whether it may be considered solely responsible for low-quality summer milk or whether other factors such as nutritional ones may be partially responsible.

Litmus milk is probably the most useful medium to the dairy bacteriologist, since it indicates fairly accurately what will be the action upon milk of the micro-organism under investigation. In spite of the advantages of synthetic indicators, such as brom-cresjol-purple, litmus is still used because it gives a greater variety of reactions than a synthetic indicator. Thus the reducing power of a culture is adequately indicated by litmus, the normal E being about –0·09 volt at pH 5(1). Some lactic acid bacteria, e.g. Str. lactis, readily reduce litmus, while others do not or only very slightly, e.g. Sbm. plantarum. Plain litmus milk is, however, an inadequate medium for some bacteria, which either clot the medium only after several weeks growth, or never produce sufficient acid to bring about this change (2).

This paper contains the results obtained in the fourth and fifth, years of a study of the milk of a typical herd of healthy Shorthorn cows. The period of drought in the summer of 1933 is included and furnishes analyses which differ so much from the four previous years that the daily figures are given in Chart 1. In the earlier paper (i) weekly averages only could be published. The greatest seasonal variations which have been demonstrated by the daily analyses are:

(1) A fall of the percentage of fat in the morning's milk in the spring when the cows were on young grass. This was clearly shown in each of the five years.

(2) A fall in the percentage of solids-not-fat in the evening's and morning's milk during the period of drought in 1933, to an extent which had not previously been recorded.

The causes of these changes in the composition of milk will be further studied using the milk of individual cows.

The weekly analyses of morning's milk have also shown seasonal variations.

Relatively high values of the titratable acidity, formol titration, ash, calcium and phosphorus were observed from October 1931 to February 1932, followed by a general decrease in the summer months, while in the late autumn somewhat higher values were again found.

During the summer and early autumn of 1933 values which were lower than any previously obtained during this investigation were recorded for formol titration and for phosphorus, while the time of coagulation with rennet was longer.

Preliminary consideration of the data has revealed the possibility that the fall in non-fatty solids may be particularly associated with the phosphorus content.

The results of the present experiments may be summarised as follows:

(1) When minimal quantities of protein are fed in the maintenance rationof steers, a deficiency of lysine will lead to a marked increase in the urinary N output. A deficiency of cystine has no effect.

(2) The catabolism of body tissue is shown by the high creatine excretionand the S: N ratio of the excess S and N excreted. This is probably the resultof excessive deamination with the production of a mild acidosis.

(3) As regards urinary partition of N there is some indication that alysine-deficient diet causes a decreased percentage excretion of urea N and anincrease of ammonia N.

(4) The biological values have been calculated, the only marked difference occurring with wheat gluten, a protein very deficient in lysine

(5) An approximate value for the lysine requirements for maintenance has been calculated, i.e. 0·80 g. lysine N per 1000 lb. live weight.

It has been shown that alterations in the hydrogen-ion concentration of milk result in differences in the numbers of bacteria which survive the process of heating at 145° F. for 30 min. As the pH decreases the number of bacteria surviving also decreases. The possible bearings of these findings on various processes are discussed.

The writer seeks to establish the interrelationship and variability of the three tests used for gauging the hygienic quality of milk, plate count, coliform content and keeping quality. Keeping quality is denned, and after discussing previous work and source of material, and noting the technique followed, the method of classification is described. Plate count and coliform content are treated logarithmically. Treatment of keeping-quality results presents especial difficulties, and the test is described in detail to disclose these, and reasons given for tabulating by half-days. The relationship between plate count and keeping quality is linear, but non-linearity occurs in those relationships which concern coliform content. This is because the first group “absent from 1 ml.” is not truly a definite class, but contains samples of superior quality. It is shown that an increase of one “stage” of coliform contamination reduces the average keeping quality as much as an increase of 0–54 in the logarithm of the plate count (approximately seven times). Variability of each term is discussed. The standard deviation of keeping quality is of the order of 1 half-day, that of logarithm of plate count is 0·77 and that of logarithm of coliform content in excess of unity. These variabilities are too great to permit of reasonable forecast of one term from the other two.

1. The feeding of butterfat in the form of butter or cream had no significant influence on the yield or composition of the milk.

2. At the beginning and end of the fat-feeding periods there was atendency for some of the cows to go off feed.

(1) 419 samples were counted in each of ten stations (laboratories) in order to assess the accuracy of the plate-count method under the conditions existing in practice.

(2) When, in each count, that plate which would ordinarily be used in practice is selected for the final count, it is found that the range which would include 95 per cent, of counts in the same sample is as wide as from one-ninth of the mean to nine times the mean when inter-station differences are taken into account. For 80 per cent, of counts in the same sample the corresponding range would be from 1/(4·1) of the mean to 4·1 times the mean.

(3) Systematic differences between stations under these conditions account for between 3 and 4 per cent, of the total experimental variation.

(4) When thirty-one complete samples in which all counts were made at a dilution of 1/10 were statistically analysed separately it was found that the range which would include 95 per cent, of counts in the same sample was from 1/(3·7) of the mean to 3·7 times the mean. For 80 per cent, of counts in the same sample the corresponding range would be from 1/(2·3) of the mean to 2·3 times the mean. There were no systematic station differences.

(5) When all plates are counted at the same dilution the error is therefore much reduced and this result suggests that the systematic station differences exhibited by the 419 samples as well as some of the random errors occurring in them is due to the combining of plate counts made at different dilutions.

In effect this means that owing to the different conditions encountered during transit comparable figures for any sample could not be expected.

The isolation from cow's milk of an organism allied to Actinomyces farcinicus merits recording on account of the rarity in this country of such organisms and on account of failure to find in the literature a description of the organism.

Figures are given for the yield, specific gravity and composition of samples of milk collected at varying intervals throughout lactation from eight Cheviotewes kept under experimental conditions and of odd samples taken from ewesunder natural grazing conditions in different parts of Scotland.

The figures recorded do not show any marked differences in compositionas a result of wide differences in the calcium content of the rations.

The various methods tested in this work have revealed the criteria of utility, U, shown in the following table for the detection of added water as measured by for the “difference” modes, and for the ratio modes by , where M3/M4 is the ratio of the maximum and minimum seasonal values of M. All the figures are for 10 per cent, added water. Utility values equal to and less than unity in the table are useless.

1. With the cows and machines tested there was a great range in the duration of milking, the coefficient of correlation between yield and duration of milking being 0·429.

2. With some individual cows the correlation between yield and duration was high, and lines representing the regression of the duration of milking relative to the amount of milk drawn have been constructed.

3. Average composite curves representing the rate of milking at various stages during the complete operation have also been set out for varying yield groups. The main conclusions to be drawn from these curves are:

(a) With some cows the rate of flow was noticeably checked about –1 min. after milking commenced. This check was of temporary duration.

(b) Maximum rate of flow in all yield groups was reached about 2 min. after milking commenced.

(c) A very much more rapid rate of flow was recorded with high yields than with low yields.

(d) The decrease in rate of flow near the end of milking was rapid, especially with high yields.

In a study of coliform organisms in cream attention has been directed towards the occurrence of specific types within the group, and to their effect upon flavour and quality.

Coliform organisms were isolated from eighty-eight (or 72 per cent.) of 112 samples of cream examined. By preliminary enrichment methods 128 strains were isolated, of which ninety-eight (or 76·5 per cent.) were typical coli strains, fourteen (or 11 per cent.) aerogenes, and sixteen (or 12·5 per cent.) intermediate.

Twenty-nine coliform organisms were tested for their effect upon cream flavour and quality. The aerogenes and intermediate types were found to be similar in their effect and to produce a more rapid and serious deterioration in quality than the coli strains of faecal type.

1. Six types of surface discolorations are described and a list is given of the fungi causing these discolorations. The fungi listed are shown to be capable of reproducing the discolorations under experimental conditions.

2. The surface moulding of butter is shown to depend largely on the weather conditions at the time of removal from cold store (temperature and humidity), and the foodstuffs available from the wrapping materials.

3. The butter, boxwood and parchment are capable, in the presence of free water, of supplying substances which stimulate the germination of fungal spores.

4. The water-soluble extract of parchment stimulates fungal germination and growth. Its value as a nutrient is masked when parchment is in contact with butter. Even a well-washed parchment is capable of supporting fungal growth.

5. The butter and the boxwood are even more important as sources of soluble food material than parchment of high “water-soluble” content.

6. Glycerine is a source of food for fungi and a glycerine-softened parchment is more liable to mould attack than a glycerine-free paper.

7. It has been shown that the softening of parchment with salt does not prevent mould attack. Two strains of Cladosporium herbarum and Penicillium sp. attacked salt-softened parchment more easily than untreated parchments.

The titratable acidity of fresh milk has been studied by several investigators, but the effects of the various buffers present have not been precisely defined. Early statements as to the cause of the titratable acidity were expressions of opinion unsupported by experimental evidence; the theories of indicators and buffer action were not considered. The acids and bases present were calculated by arbitrary rules to various combinations of salts, and the acidity was stated to be due to the presence of acid salts in these hypothetical mixtures.

1. The ratio of the weight of fat in the milk required to make 1 lb. of butter has been taken to indicate the size of the fat globule in the milk, and so the type of “ cream line ” produced.

2. The Dairy Show records have been analysed from this standpoint, and variations in the above ratio studied together with variations in milk yield, fat yield and fat percentage. The effects considered are those between breeds and within breeds, the latter including milk yield, period of lactation and ageof cow.

3. With a constant milk yield the fat percentage does vary between breeds, but not to the same extent as it does when no account is taken of the differences in average yield of the different breeds; the same applies tothe fat: butter ratio, i.e. size of fat globule. The effect of equal increases of milk yield on the butter:fat ratio is much greater between breeds than it is within any one breed.

4. It is suggested that the size of the fat globule is determined by two factors: (a) the rate of butterfat formation by the cell, which varies with breed and stage of lactation, and (b) the rate of milk secretion which affects the size of the globules by the rate at which they are washed out from the cell.

1. A description is given of internal and subsurface discolorations in unsalted and lightly salted butter.

2. Six strains of the causative organism Cladosporium herbarum are described.

3. These strains are shown to be capable of reproducing the defect under experimental conditions.

4. Four of the strains tested proved capable of slight growth at 20–24° F. (−6·6 to −4·4° C.). after 3½ months.

5. None of the strains grew at normal cold store temperatures, 15–18° F. (−9·4 to −7·7° C).

6. Salt retards the development of internal spotting. Under favourable conditions it was shown that discolorations developed in 6, 10, 16 and 25 days in butter containing 0, 5, 10 and 15 per cent, of salt in the butter serum.

1. The colours of the butter for individual cows at the London Dairy Show have been measured on a colour scale of yellow shades on cellulose strips, and the results analysed statistically.

2. The mean values and variability curves of the ranges of butter colour in the different breeds of dairy cattle are given. Circumstances which may affect these values are indicated.

3. The two most important factors affecting butter colour are the genetic character of the cow and the method of feeding.

4. A slight increase in the shade of colour was found to occur under most of the various conditions which give rise to increased milk yield. It is suggested that this may be due to the greater intake of colouring matter (associated with the greater food intake) in proportion to butterfat produced, since increase in yield is usually associated with decrease in fat percentage.