1. The effect of feeding levels of protein and energy appreciably higher than the Woodman standards on the yield and composition of the milk from twelve Ayrshire cows has been studied in two winter-feeding experiments.

2. In the first experiment the S.E. of the concentrate ration was held constant at 63½% and the D.C.P. content adjusted to 10, 15 and 17%. The 10 and 15% D.C.P. concentrates derived their protein from home-grown feedingstuffs, whereas the 17% D.C.P. concentrate was a bought-in commercial mixture. Feeding the three levels of D.C.P. led to no statistically significant differences in milk yield, milk composition or live-weight gain, although the commercial mixture (17% D.C.P.) resulted in milk with a slightly lower fat content.

3. The second experiment reversed the feeding conditions of the first, i.e. D.C.P. was held constant at 16% and three concentrates were fed which contained 59, 67 and 75% S.E. respectively. The two rations of higher S.E. content gave significantly more milk than the low S.E. ration and tended to give slight increases in live weight. There was a slight increase in the S.N.F. content of the milk when the S.E. of the concentrate was raised from 59 to 67% S.E. This was due almost entirely to an increase in the protein content of the milk. There was no further increase in S.N.F. with the 75% S.E. concentrate, and this latter concentrate produced milk with a slightly lower fat content.

4. Comparison of costs and returns showed that the cheap low protein concentrate, ration C, even though it gave the least milk, yielded the highest profit. It was shown, however, that at any reasonable fixed cost of starch equivalent the most profitable level of feeding above the Woodman maintenance standard was 3–3·3 lb. S.E./10 lb. of milk of 3·8–4·0% fat. Alternatively, the total ration should be 110–117% of the Woodman standard.

1. Two experiments were carried out to compare spring-dried grass and autumn-dried grass. In both experiments a 3×3 Latin square changeover design was used with periods of 4 weeks. In the first the control ration was a home-grown concentrate mixture (S.E. 58, D.C.P. 12·5%), in the second a high-quality mixture of imported feeds (S.E. 73·5, D.C.P. 16·6%). The concentrates or dried grass were fed in addition to a basal ration which supplied only the maintenance requirements of the cow.

2. In both experiments the milk yield from spring grass was 3–5% higher than that from autumn grass, but the differences were not statistically significant. In Exp. 1 the yield from the home-grown mixture was the same as from spring grass. The yield from the imported mixture in Exp. 2 was significantly higher than from both grass rations. There was a significant increase in milk yield when the cows went on to spring grazing in Exp. 1.

3. Expression of the feed input as a percentage of the Woodman standards showed that the level of feeding was similar in all three rations in the first experiment, and that the increased milk yield from the high-quality ration in the second experiment could be explained by the increased level of feeding which it gave. It was considered that the increased yield on spring grazing might have been caused simply by an increase in the amount of nutrients consumed.

In the investigation of an outbreak of acetone taint in bulk milk it was: (a) shown not to be due to the lactating animal; (b) reproduced by the inoculation into milk of a strain of Bact. cloacæ isolated from the tainted milk; (c) shown that seventy other strains of coliaerogenes organisms (including sixteen Bact. cloacæ) did not possess this ability to produce an acetone taint.

A search of the literature failed to produce a record of this taint being produced by factors other than acetonemia of the animal.

Note. Since writing the article another strain of Bact. cloacæ giving a positive Rothera's test has been isolated at Wye from a sample of milk which developed a typical sickly sweet taint on storage (Billing, E., personal communication, 5. xii. 55).

Batches of cream were artificially infected with M. tuberculosis, and subjected to varying heat treatments under conditions of H.T.S.T. and holder pasteurization. Guinea-pig and phosphatase tests were applied to the treated creams.

The minimum time-temperature conditions required for the inactivation of M. tuberculosis and phosphatase were found to be similar to those pertaining to milk.

The practical difficulties of controlling the effciency of cream pasteurization are discussed.

The use of a column to give countercurrent contact of steam and cream to strip taints from the cream is explained. An analysis based on the engineering concept of a transfer unit is presented, and generalized curves are given which can be used for any counterflow column using fresh steam. The generalized curves give the percentage reduction in taint for values of the stripping factor (mV/L) when the number of transfer units is known.

It is further shown that the number of transfer units for any particular design of column depends on the value of the stripping factor. The total resistance to the transfer of taint (HTU)L depends on a liquid-side resistance (HTU)l and a steam-side resistance (HTU)g such that (HTU)L=(HTU)l+L/mV(HTU)g.

Results are given for an experimental column in which the liquid flows as (a) a spray, (b) a falling film or wetted wall. A strong deodorizing effect occurs at the bottom of the column and the resistances to transfer are

Resistances for a full-size plant 3 ft. diameter processing at 10,000 lb./hr. are assessed from the above figures as:

Restriction of the process of taint transfer is controlled largely by the liquid-side resistance. Because the minimum obtainable taint level depends on the liquid-side resistance (no matter how much steam is used) a low liquid-side resistance or a tall column must be used for satisfactory deodorization. A minimum of 4 transfer units is suggested, as this will allow a taint to be reduced to nearly 1·8% of the original value.

A design method is given based on hypothetical taint conditions, the generalized curves, and the characteristics of any proposed counterflow column.

The extent of removal of the reference substances diacetyl and acetoin from water and cream in the Vacreator under varied conditions of operation has been investigated.

1. It was not affected by change in diameter or length of the uptake-pipe, or, with water, by changing the position of the equilibrium-valve to the top of the uptake-pipe or by introduction of the steam at the bottom of the pasteurizer bend instead of at the normal steam inlet.

2. It was affected only slightly by inclusion of baffles in the uptake-pipe, to provide a more intimate contact of steam and cream.

3. With diacetyl in water it was not affected by introduction of the major proportion of the steam via the infuser connexion, and with diacetyl in cream it was not affected until the proportion of steam introduced via the infuser was greater than 50% of the total amount of steam used. With acetoin in both water and cream there was a progressive slight reduction in extent of reference substance removal with increase in proportion of steam introduced via the infuser.

4. With diacetyl and acetoin in water it was not affected by increase in rate of flow of steam and cream at constant intensity of treatment.

5. With diacetyl it was not affected by temperature of steam distillation over the range 138–197°F., but with acetoin there was a progressive reduction in extent of removal with decrease in distillation temperature.

6. Steam is most effectively used if it is applied in separate stages instead of in one stage, i.e., a multiple Vacreator uses steam more efficiently than a single-unit machine. This effect is more marked with diacetyl, which has a high equilibrium coefficient, than with acetoin, which has a low coefficient.

7. The equilibrium coefficient computed from steam treatment of diacetyl solutions in the Vacreator is of the same order as that determined by the laboratory continuous equilibrium vaporization apparatus.

8. The efficiency of cream-deodorization equipment may be lowered by condensation of vapour back into the liquid, e.g. by use of excess cooling water in the Vacreator jackets.

9. There are indications that removal of diacetyl from cream is somewhat slower than from water.

The thanks of the author are due to Mr J. K. Scott for helpful discussion, to Mr E. R. Elley who did the analytical work, and to Mr B. Le Heron for assistance in the operation of the Vacreator and collection of samples.

1. During 1950–53 the milk of 814 Ayrshire cows was sampled six times during the lactation at intervals of approximately 5 weeks, starting towards the end of the first month of lactation. The weight of milk at successive evening and morning milkings was recorded and samples from each mixed in proportion to the yield. The milk was analysed for total solids, fat, S.N.F. by difference, crude protein (N×6·38), casein and lactose.

2. The weighted lactation average was calculated for each constituent of the milk of each cow and used in genetic studies (see Part II), and in assessing the effect of age on milk composition. The analyses of the individual samples were used to determine the separate effects of stage of lactation and of season on milk composition.

3. Advancing lactation caused the following changes in milk composition:

(a) Milk yield was highest 45 days after calving and then fell regularly to the end of lactation.

(b) Total solids, S.N.F., fat, crude protein and casein contents fell rapidly for 45 days, with fat and total solids continuing to fall for a further 30 days. The concentrations of all these constituents then increased continuously for the remainder of the lactation, rising more rapidly after about 200 days.

(c) The changes in lactose content were opposite from those of fat and protein and smaller in magnitude. The changes bore a marked resemblance to those for yield. The value rose to a maximum after 45 days, fell slowly until about 165 days after calving and then more quickly. The lactose content of milk from cows in their first lactation fell much more slowly with advancing lactation than that in the milk of older cows.

4. Seasonal effects were of smaller magnitude than those arising from advancing lactation and caused the following changes in milk composition:

(a) Yield rose steadily from January to the May-June period, and then fell to a minimum during October-November.

(b) Fat content was at a maximum in October, falling steadily to a minimum in June.

(c) Crude protein and casein contents rose to a peak in May-June and again in September. Lowest values occurred from January to March.

(d) Lactose content was at a steady high level from January to June, falling to a lower level by August at which it remained for the rest of the year. Again there was some similarity in the pattern of change in lactose content and in yield. The range of variation in the values for lactose was less than those for protein and fat, and the variations were mainly opposite in sign.

(e) Total solids and S.N.F. were at a minimum in March and April, but whereas S.N.F. reached their highest level in May-June, total solids were highest in October.

5. Increasing age of the cow resulted generally in increasing yields of poorer quality milk. The difference in composition between milk from the first and the grouped ninth and later lactations was fat 0·19%, S.N.F. 0·34%, crude protein 0·08%, casein 0·21% and lactose 0·25%.

6. On the evidence of cell counts, a number of the samples came from cows with some inflammation of the udder. The average casein numbers, however, did not indicate any serious incidence of mastitis, and it was concluded that although lactose contents may have been lowered slightly, the effect of disease on the average composition was small.

7. Although the figures are not closely comparable, it appears that there has been little change in the composition of the milk of Ayrshire cows in Scotland since 1921–22.

8. The number of individual cow samples deficient in fat was 2% and in S.N.F. 16% of the total. In 560 samples of farm bulk milk none were deficient in fat and only 1·6% deficient in S.N.F. May, June and July were the months with most fat-deficient samples, and the period from January to April provided the largest number of samples deficient in S.N.F., closely followed by the period July to October.

9. Possible causes of the seasonal changes in milk composition are discussed. These changes are thought to be caused mainly by feeding although this does not give a complete explanation of all variations.

1. A statistical analysis has been made of lactation records from 500 dam-daughter pairs of the Ayrshire breed. Measurements were taken of milk yield, fat, S.N.F., crude protein, casein and lactose.

2. A correction was made for the age of the cow. This contributed a substantial fraction of the total variation for milk yield, S.N.F. and lactose contents. Month of calving had a trivial effect on the lactation average for the characters when compared with the individual variation between cows.

3. After correction for age, the variation between lactations was highest for fat content (standard deviation 0·35%) and slightly less for S.N.F. (0·26%).

4. The heritability was estimated by the dam-daughter correlation within herds. Values in the neighbourhood of 0·5 were obtained for S.N.F., crude protein, casein and ‘ash’ (determined by difference), about 0·35 for fat and lactose, 0·25 for milk yield and 0·13 for non-casein nitrogen.

5. The phenotypic correlations between yield and lactose were positive for both dam and daughter groups. The correlations between yield and the other constituents were more negative in the daughter group than in the dam group. The correlation between yield and S.N.F. was, in fact, positive in the latter. The correlation between fat and S.N.F. averaged 0·36, being derived mainly from the correlation between fat and casein (0·43), though that with lactose was also positive (0·13).

6. The variation in S.N.F. was mostly determined by that in casein (r=0·85). There was a positive correlation between lactose and casein (0·24), and a negative one between lactose and non-casein nitrogen (–0·41). Lactose was negatively correlated with ‘ash’ (–0·31). ‘Ash’ was positively correlated with non-casein nitrogen (0·23)

7. There were positive and significant genetic correlations between casein and both fat and lactose, and that between the last two was almost significant.

8. No significant genetic correlations were found between yield and the chemical constituents, although figures of practical importance may exist which did not reach significance in an experiment of this size.

9. The practical significance of these results is discussed. It is concluded that selection for yield alone is not likely to cause any great decline in S.N.F. content of the milk, and that this could probably be held in check by paying some attention to the fat content of the milk.

Variation in the temperature of measurement, pH, and concentration of calcium, citrate and phosphate ions have been consistently found to affect the reflectance of separated milk in such a way that the entire curve of reflectance versus wave-length in the visible spectrum is raised or lowered. At temperatures between 10 and 50°C. the reflectance rises reversibly with temperature. The change is not instantaneous, and if the temperature is changed abruptly the reflectance change to the new value is exponential, with a time constant of about 40 min.

Reflectance is independent of pH in the range 5·8–6·6, but for greater values it drops sharply.

Added calcium ions increase the reflectance of separated milk, and added citrate and phosphate ions decrease it, independently of the simultaneous changes in pH.

It is suggested that the reflectance changes which have been found to occur are all caused by changes in the sizes of the casein particles in milk which are brought about by the factors considered. The results given by other authors on viscosity and bound water content, and on direct electron-micrographic observation of casein, are cited to support this contention.

It is further suggested that the change in casein particle size with temperature may be due to a change in the calcium-ion distribution between solid and liquid phases.

The viscosity and reflectance results for separated milk heated to 70°C. and above are apparently anomalous, but they can be explained on the hypothesis that denatured soluble proteins aggregate into particles having a high ration of length to breadth.

The results of examining the fat globule size in milk samples from thirty-one cows which were in-patients in a Veterinary Hospital, have been recorded. Eleven of these cows did not develop a body temperature over 102·4° F., and samples collected on the second day after admission and 5 days later were examined and compared. Although the rise in yield was significant, the differences in fat percentage and fat globule size were not. The other twenty cows developed raised body temperatures, and the association between body temperature and milk composition was considered in the following temperature ranges: 104° F. or over, from 102·5–103·9° F., and 102·4° F. or below. The results showed that a rise in body temperature was accompanied by a fall in yield and also by an increase in the fat percentage and in the average fat globule diameter.

The carotene and vitamin A contents of the butterfat from the milk yielded by the cow in early lactation may be higher than normal for a period of up to 10 or 12 days after parturition.

Seasonal changes in carotene content of butterfat from small herds of cows were governed almost entirely by the availability of carotene in the pasture and not by lactational influences.

Seasonal changes in vitamin A content of the herd butterfats were relatively small during most or all of the lactational period. In certain years an increase in the vitamin A content during the autumn months was probably due to seasonal conditions and not to a lactational effect.

A lysogenic culture, prepared in the laboratory from a strain of Streptococcus lactis, was used as a cheese starter in commercial factories. It was attacked in turn by two other unrelated phage races. The lysogenic condition, which involved slight morphological and physiological changes, persisted in the subsequent forms resistant to one or both the new phage races. Acquired resistance to any one of the three phages did not protect the culture from the other two phages.

In nature such interactions between phage races and lactic acid bacteria must be constantly taking place, giving rise to similarly related strains.

Two of the three phage races produced spreading haloes around their plaques due to a lysin released during phage action. The lysin may also interfere with the survival of secondary growth after attack by these phage races. Production of this type of lysin is thus a property of the phage race and not of the bacterial strain.

A reaction closely resembling the nascent phage phenonomenon was studied. When lysing the homologous strains of Str. lactis, two phage races were found to be capable of simultaneously inducing lysis of a heterologous strain and of other phage-resistant homologous strains. Seitz filtered potent lysates of the homologous strains also caused lysis. Subjecting the lysates to a temperature of 60°C. for 5 min. destroyed the lytic activity on heterologous strains but did not inactivate the phage.

These two phage races produced haloes around their plaques while a third phage, unable to induce lysis of heterologous cultures, showed no haloes. Addition of I3-to the medium suppressed the appearance of haloes, providing further evidence that they are due to the production of bacterial lysins.

Evidence was obtained to show that several other possible mechanisms were not responsible for the observed lysis, and it was concluded that a heat-labile lysin or lysins capable of destroying the heterologous strain was produced during lysis of the homologous strain. The relationship of this to the nascent phage and other similar phenomena observed elsewhere is discussed.

The problem of bacteriophage in cheesemaking has been reviewed recently by Whitehead(1) and Mattick(2). Results since reported by other workers and ourselves call for some modification of and addition to the existing concepts of the problem.