Analyses of the neutral volatile components from 3 Cheddar cheese are presented. Two were made aseptically in an aseptic vat, with and without starter culture; the third was made with starter culture in an open vat. Gas–liquid chromatography and mass spectrometry detected the same volatiles in starterless cheese having little or no Cheddar flavour as in cheese made with starter and having a characteristic Cheddar flavour. Methyl disulphide and dimethyl sulphide were the only compounds consistently detected in higher concentrations in the cheese made with starter than in the cheese made without starter. However, using a total trapping technique, it was found that the combination of components recovered from the effluents of the chromatographic columns did not have the cheese-like aroma of the distillate vapours that were injected. Reasons for this are discussed.

A study was made of the formation of the extracellular proteolytic enzymes during the growth cycle of several species of Pseudomonas cultivated under different conditions of temperature and nutrition. Proteolytic activity was not proportional to growth. Expressed per unit of cell dry weight, the proteolytic activity showed a peak in the early logarithmic phase which was greater in cultures grown at 3 than at 28°C. Proteolytic enzyme was not formed in the absence of organic nitrogen. Of 16 organisms studied, Pseudomonas aeruginosa ATCC 10145 was the most prolific producer of proteolytic enzyme.

A laboratory method has been developed for studying the action of chemical disinfectants on milking equipment using a wet film and both metal and rubber surfaces. It was shown that milking machines may be soiled as effectively with fresh milk of low bacterial content as with aged milk of high bacterial content. When a single heat treatment preceded the soiling with milk and subsequent treatment with detergent and chemical disinfectant, a build-up of bacteria began on the third day. However, when the surfaces were heat treated daily 18 h before soiling, followed by cleaning and disinfection, no build-up occurred. These findings support the hypothesis that contamination of milk results from bacteria developing on the milking equipment and not merely from those added to it at each milking. The sites of bacterial contamination were shown to be the under surfaces of lid gaskets and the joints of the short milk tubes and the metal clawpiece nipples.

Total and differential cell counts were made on a total of 26272 quarter samples of foremilk taken at fortnightly intervals from cows in the Institute herd during the 12 years 1953–1965, and the samples were also examined bacteriologically.

The average cell count in each lactation up to the 7th was calculated.

The average total cell count of samples from which no staphylococci, streptococci or coliform organisms were isolated increased from the 1st to the 7th lactations from 0·19 to 0·60 million cells/ml, and from the 2nd lactation onwards the increase was due entirely to an increase in the number of polymorphs. With infected samples, however, changes in the average total cell count were caused by changes in the numbers both of polymorphs and of cells other than polymorphs.

The average total cell count of samples from which coagulase-negative staphylococci were isolated varied between 0·58 and 1·00 million/ml throughout the 7 lactations. For samples from which coagulase-positive staphylococci were isolated the cell count varied between 0·72 and 4·94 million/ml, the highest average total cell count occurring in the 5th lactation.

The average total cell count of samples from which streptococci were isolated varied between 1·48 and 4·62 million cells/ml, the highest value occurring in the 2nd lactation.

Lactic acid streptococci have an absolute requirement for magnesium and potassium and failed to grow in milk deionized with IRC 50 resin (sodium form) (Reiter unpublished; see also Fisons, 1958; Reiter, 1956). These findings were confirmed in preliminary experiments using the defined medium of Reiter & Oram (1962). Requirements for other metal ions have now been investigated. Two strains of Streptococcus lactis (ML3 and C10) and one of Streptococcus cremoris (HP) were used. They were maintained as milk cultures, and before use subcultured twice in the synthetic medium containing Ca, Mg, Fe, Zn, Mn, Co, Cu and Mo (cf. Reiter & Oram, 1962) and then finally in the synthetic medium containing only Mg and K (20 ppm.), and Fe (0·01 ppm.).

The proteolytic enzymes present in the culture supernatant of Pseudomonas aeruginosa ATCC 10145 were active in the pH range 5·5–9·0 with a maximum activity at pH 7·3. Heating for 15 sec at 72°C resulted in a 36% loss of proteolytic activity whereas heating for 30 min at 63°C resulted, in a 6% loss. Boiling for 2 min completely inactivated the proteolytic enzymes. At 2°C the proteolytic enzymes were stable for at least a month and casein was readily hydrolysed at this temperature.

A pilot plant with a capacity of 2300 1./5 h day for the removal of cationic fission products from milk is described. The process involves the acidification of the milk with citric acid to pH 5.25 and its passage through an ion exchange resin charged with the ions of Ca, K, Na and Mg in the same proportions as those in which they occur in milk. The effluent milk is neutralized with potassium hydroxide. At the end of the day the plant and resin bed are washed and sterilized.

Two resin beds are provided and are used on alternate days, one being washed and regenerated while the other is in use. Regeneration is carried out with a solution which removes radioactive cations and restores the resin bed to its original ionic composition.

Bacteriological tests show that the method of cleaning both plant and resin bed is satisfactory.

Conclusions are drawn as to a suitable design for a larger scale plant.

AS previously described, the process consists essentially of the passage of milk at pH 5·2–5·25 through an ion exchange resin charged with a suitable mixture of the ions of Ca, K, Na and Mg.

When citric acid was used for reduction of the pH and 30 resin bed volumes (r.b.v.) were treated, the concentration of residual 85Sr was reduced to 2–4% whether it had been introduced in vivo or in vitro. There was no evidence that repeated use of the resin bed resulted in diminished radiochemical efficiency.

About the same efficiency of removal of 133Ba was obtained but this efficiency was achieved with 137Cs only if not more than 15 r.b.v. were treated. If 30 r.b.v. were treated the concentration of residual 137Cs was as much as 30%.

When hydrochloric acid was substituted for citric acid the residual concentration of 85Sr was 11%. Analyses of milks processed after adjustment of pH with either citric or hydrochloric acid showed the products to be satisfactory. The only important constituent lost was thiamine. Concentrations of heavy metals remained well below maximum values. Although the flavour of milk was detectably changed by the treatment it was still acceptable.

The results are discussed in relation to the problem of achieving maximum decontamination and to the design of a large scale plant.

The rate of increase of the complex rigidity modulus of renneted skimmilk was measured, both in a modified form of the torsiometer previously described and in the thrombelastograph, an instrument designed for the study of blood coagulation. Milk diluted with water was also tested.

The effect of holding the milk is to slow down the process of coagulation without affecting the ‘coagulation time’ (i.e. its starting time). In undiluted milk a fall in pH from 6.66 to 6.18 does not affect maximum firmness but a later softening of the curd is accentuated in diluted and undiluted milk. Again, the coagulation time is not affected but the rate of firming up is reduced. Potassium oxalate, used to reduce the calcium content, shows similar effects.

N-Ethyl maleimide, a reagent specific for sulphydryl groups, had no effect on undiluted milk; but with dilute milk, at a pH of about 6·0, there was a decrease in maximum firmness and there were also differences in the later softening of the curd.

The significance of these effects on bonds formed during postenzymic coagulation is discussed.

In order to determine how bacteria pass through the teat canal into the teat sinus, milk samples were taken by syringe, through the teat wall, from 12 quarters of 7 cows, before and after each milking for up to 16 days. Three of the teat canals were naturally infected before the start of the experiment and the remainder were artificially infected with Staphylococcus aureus by the Hadley–Wisconsin swab technique. In 3 of the quarters the inoculum was introduced into the teat sinus through the teat canal by the inoculation technique although the swabs were inserted only 3–5 mm into the canal. The other 9 of the 12 quarters were milked a total of 149 times during the experiment but only once was a colony-forming unit of the bacterium present in a teat canal infection isolated from the milk taken from the teat sinus after milking. One quarter developed an intramammary infection, the bacterium first being detected in the teat sinus prior to the third milking after inoculation of the teat canal. The invasion of the bacteria into the teat sinus therefore occurred between milkings. Staph. aureus persisted in teat canals between 5 and 32 or more milkings after inoculation. The experiment showed that bacteria in the teat canal are seldom refluxed into the residual milk in the teat sinus during correctly conducted milking with an efficient machine.

Intramammary infection present in quarters of dairy cows, either throughout the dry period or originating in the dry period, reduces the milk yield after calving. Relative to the milk yield of equivalent non-infected quarters of the same udder the reduction was found in this work to be about 35%. The relative milk yield of quarters found infected in late lactation was depressed by 48% but if the infection was eliminated during the dry period, the depression in milk yield was only 11% after parturition.

Folic acid activity was determined for National Dried Milk and for 5 proprietary dried milk foods for babies, for a proprietary liquid milk baby food and for 8 brands of tinned evaporated milk. For comparison, values were determined for mature breast milk, for raw bulk cow's milk, for bottled pasteurized cow's milk and for goat's milk.

Human milk and raw and pasteurized cow's milk all had much the same folate activity, equivalent to about 54 µg folic acid/1. Values for goat's milk were much lower, around 6 µg/1.

Values for the reconstituted baby milks ranged from 9 to 65 µg/1, though only 2 of the products had markedly lower values than breast milk. The question is discussed whether the folic acid requirement in infants can be met by formulas based on cow's milk without having recourse to folic acid supplementation.

The milk produced by old cows frequently contains a lower concentration of solids-not-fat (S.N.F.) than that of young cows, mainly because of a low concentration of lactose. This can result from mastitis or possibly from other factors associated with ageing. The composition of the milk of 2 groups of old cows has been compared with that of substantially disease-free first-lactation cows. A comparison has also been made between the milk from the 4 quarters of each cow.

The first part of the work was done with 8 old cows and 4 first-lactation cows, and the milk was examined bacteriologically, cytologically and chemically at frequent intervals throughout the lactation. There was strong evidence that most of the difference in chemical composition of the milk of the old cows compared with that of the young cows was the result of mastitis, mainly subclinical. The milk from the old cows had a higher incidence of infection, a lower concentration of lactose and casein, higher concentrations of serum albumins, globulins and proteose-peptones and contained more total and polymorph cells. There was also some indication of another effect, not directly associated with disease in the current lactation, that led to a decrease in the lactose concentration of the milk of the old cows after about 80–90 days in lactation.

In the second part of the work, the milk from 23 old and 8 first-lactation cows in 8 commercial herds was examined twice, once after 58 and again after 163 days in lactation. Milk of abnormal composition as a result of disease was produced by 30% of the quarters of the old cows at the first examination and by 62% of the quarters at the second examination.

Some indication is given of the probable loss of milk and milk constituents that can be attributed to disease.

(1) A tentative scheme is proposed for the quantitative estimation of the effects of various factors on one major milk constituent, lactose, from analyses of the milk of individual cows within herds. The potential lactose content in the milk of individual cows is predicted from the observed potassium-to-lactose ratio in the milk, and the difference between the predicted potential and the actual lactose content is partitioned into fractions that are attributed to effects due to age, inter-quarter difference and changes with stage of lactation. The scheme was applied to 2 commercial herds, one (herd A) producing milk of low and the other (herd B) milk of normal SNF content.

(2) Lactation mean values for fat, SNF and lactose contents of the milk of individual cows showed a wide range of the same order in each of the 2 herds. Of the mean difference in SNF percentage of 0·30 between the herds, 0·14, or 47%, was due to a difference in lactose content.

(3) The predicted potential milk lactose content of the cows in the 2 herds ranged from 5·04 to 5·66 (g/100 g milk water). Herd mean values were 5·394 for herd A and 5·244 for herd B.

(4) Herd mean values for the effect on the lactose content (g/100 g milk water) of age, of changes with stage of lactation and of inter-quarter difference were −0·127, −0·080, and 0·073 respectively in herd A, and −0·197, −0·058 and −0·078, respectively, in herd B.

(5) The relative importance of the various factors in accounting for differences between the herds in the lactose content of their bulk milk were: predicted potential lactose content 61%, effect of age 28%, effect of changes with stage of lactation 9% and effect of inter-quarter difference 2%.

(6) The errors attached to the estimation of potential milk lactose content and the effects of age, of changes with stage of lactation and of inter-quarter difference are discussed.

An investigation was made of the death-rate of Bacillus subtilis spores in suspension in hydrogen peroxide concentrations of 10, 15 and 20% at 25, 50, 60, 70 and 80°C. Average logarithmic survival curves were plotted against normal time and death-rate constants and Q10 values were calculated from the graphically determined decimal reduction times. This treatment of the experimental findings was found to be warranted in the temperature range 60–80°C. The first-order death-rate constant was ∽ 0·1 s−1 for 10% hydrogen peroxide solution at 60°C and the Q10 was about 1·6. Increases in the concentration from 10 to 15% and from 15 to 20% each gave an increase of about 50% in the rate constant. The application of these data to commercial practice is discussed.

Samples were tested for fat percentage from every consignment of herd bulk milk from 10 herds over a period of 12 months. The mean monthly within-herd variance was 0·043. Also, triplicate samples were taken from 8 herd bulk supplies for 8 days and tested in duplicate and an analysis of variance was applied to the within-herd fat percentage in order to find the contribution sampling, testing and biological variances. It is shown that when dealing with tanker-collected milk where composite samples are not used the testing accuracy is secondary to frequency of sampling. By combining the variances in the formulae given, the accuracy of different systems may be obtained and compared using either single tested or composite samples.

The chemical method of Tolle, Zeidler & Heeschen, (1966) for dispersing the fat in milk to allow electronic counting of the cells in suspension has been examined. The optimum conditions prescribed for obtaining the best estimates of cell counts were largely confirmed though inaccurate counts occasionally occurred. The reproducibility of counts made on the same milk samples was acceptable. For milk samples from individual quarters, cell counts determined after the chemical treatment and those after centrifugal separation of the fat could be related by a linear regression line of unit slope through the origin. The surface active liquid Lissapol NXP (I.C.I. Ltd.) appeared to be a suitable substitute for the Witopal CO (Chemischen Werken in Witten/Ruhr), an alkylphenol polyglycol ether, used by Tolle et al. (1966).

The flavour quality and stability of 3 different UHT creams are described. The rate of oxidation was measured by the thiobarbituric acid (TBA) and peroxide methods as well as by organoleptic assessment. Lipid oxidation developed in some UHT creams on prolonged storage, particularly in creams manufactured during the winter months and stored at 18°C, while little or no oxidation occurred at 4 °C. Creams manufactured during the summer months were most resistant to oxidation. Creams with no detectable oxidized flavour had TBA values ≤ 0·08; values ≥ 0·16 and peroxide ≥ 2·0 were associated with pronounced oxidized flavour. These relationships between the chemical and organoleptic assessments were highly significant (P < 0·01). Low peroxide levels (< 2·0) were not necessarily indicative of good flavour quality.

(1) In a previous paper (Walsh, Rook & Dodd, 1968) a scheme was devised in which the potential milk lactose content for individual cows was predicted from the observed potassium-to-lactose ratio in the milk, and the difference between the predicted potential and the actual milk lactose contents was partitioned into effects due to age, changes with stage of lactation and inter-quarter difference. The scheme was based on repeated analyses for potassium and lactose of milk from the separate quarters of the udder of each animal at intervals throughout a single lactation, and was applied to 2 commercial herds. A simplified scheme, in which analyses are confined to milk secreted in mid-lactation and in which the measurement of the effect of changes with stage of lactation is omitted, has now been applied to a further 6 commercial herds. The results are reported here, together with those for the mid-lactation period for the 2 herds studied previously.

(2) The potassium and lactose contents of the milk of uninfected quarters of the heifers in 4 of the 6 herds and of the second-lactation animals in the 5th herd, which had no heifers, were found to conform with the relationship established previously (Walsh & Rook, 1964) on which the prediction of potential lactose content is based. The values for heifers in the 6th herd did not conform, and the results for this herd are therefore not reported.

(3) The range of herd mean values for SNF content was 8·17–8·62% and for lactose content 4·33–4·72%. The overall range for all herds for the predicted potential lactose content of the milk of individual cows was 5·06–5·71 (g/100g milk water). Herd mean values for predicted potential lactose content were, however, generally similar, ranging from 5·240 to 5·486 (g/100g milk water).

(4) The range of values for individual cows and the herd mean values for the effects of age and inter-quarter difference showed important differences from herd to herd. Herd mean values for the effect of age varied from −0·028 to −0·266 (g/100g milk water), and there was a similar range, of −0·074 to −0·251 (g/100g milk water), for the effect of inter-quarter difference. In herds where the effect of age was high, the effect of inter-quarter difference also was high.

(5) The effects of inter-quarter difference and of udder infections were similar. The loss in milk lactose content was about twice as high in quarters infected with streptococci as in quarters infected with staphylococci.

(6) Estimates of herd values for potential lactose content and for the combined effects of age and inter-quarter difference based on analyses of herd bulk milk agreed with corresponding estimates based on analyses of the milk of individual animals within the herd.

(7) The importance of variation in milk lactose content as a source of variation in SNF content and the relative importance of predicted potential lactose content and the effects of age and inter-quarter difference as a source of variation in milk lactose content are discussed.

The teats of milked and unmilked quarters of 6 cows were deliberately contaminated twice daily by immersion in a suspension of streptococci and staphylococci. Only the unmilked quarters became infected, 7 with Staphylococcus aureus and 3 with Streptococcus uberis. Neither previous infection with the same strain or a different strain of pathogen nor existing infection with Corynebacterium bovis appeared to influence the results.