This series of four papers has outlined a method for predicting the bactericidal efficiency of an ultra-high-temperature sterilizing plant. The method has been illustrated with reference to the performance of an A.P.V. 200 gal./hr. plate-type sterilizing plant, treating at different temperatures milk heavily inoculated with suspensions of B. subtilis and B. stearothermophilus spores.

The method involves the determination of the temperature and flow time distributions in the plant, and of the thermal death characteristics of the organism to be considered. The investigations of plant performance are considered in Part I. The determination of the sporicidal effect of the plant from this information is described in Part II. The effect is expressed in terms of the slope of the thermal death line for the organism considered, at the operation temperature of the plant. By expressing the result in this form it becomes general and applicable to any type of organism, at any operating temperature close to that for which the plant is designed.

The thermal death time of any organism at the operating temperature may be obtained by laboratory experiment, by extrapolation if necessary. In Parts III and IV, laboratory data for B. subtilis spores and B. Stearothermophilus spores are used to calculate the performance of the plant, and the calculated results are compared with the results of direct plant experiment. The agreement is satisfactory.

The interpretation of the results with spores of B. stearothermophilus is complicated by their very marked inhibition by u.h.t.-treated milk. Only about one spore per million germinates and grows in such milk. A false impression of the number of truly surviving organisms after the heat treatment may therefore be obtained. It is not known to what extent this effect occurs with spores of other strains of B. stearothermophilus or with other organisms.

It is not suggested that the theoretical method of estimation of performance will give reliable information on the spoilage level to be expected from a plant under practical dairy conditions. The uncertainties as to the number and types of incident organisms are too great. Comparisons can be made between different plants, however, by comparing the results of the theoretical analyses for the plants. No bacteriological data are then required. This may be a less difficult procedure for manufacturers than a direct bacteriological experiment using heavily inoculated milk. The analysis also enables the contribution to the overall lethal effect of the different part of a sterilizing plant to be assessed.

Improved growth of Streptococcus cremoris 924 was obtained in the presence of CO2.

When fast and slow acid-producing variants of Str. cremoris 924 were mixed and the mixture subcultured daily the resulting cultures were all active acid producers, irrespective of the initial ratio of fast to slow cells. A balance is established between the two types of cell; probably this will vary with the cultural conditions and vary between strains of Str. cremoris and Str. lactis for any particular conditions of maintenance.

Providing the strains are correctly selected, single-strain starter cultures can be maintained by daily subculture in litmus milk, apparently indefinitely. Activity varies with the present method within wide limits and this variation seems to be a function of the culture.

Freeze drying is a satisfactory method of maintaining active single-strain starters.

The author wishes to thank Miss Shelagh Gibbs for technical assistance, Miss H. R. Chapman for making the cheese, and Messrs. Cow and Gate, Ltd., for supplying the dried milk powder for these experiments.

1. Results are reported of an experiment with fattening pigs, made under commercial conditions, to obtain further information on the comparative value of dried skim-milk and white fish meal as protein supplements in a basal meal fed in conjunction with unrestricted amounts of whey.

2. All the pigs received, together with the unrestricted supply of whey, 3 lb. of a basal meal, reduced to 2 lb./pig/day by 13 weeks of age, fed dry once daily. Four different basal meals, comprising the four experimental treatments, were used, containing respectively 10, 7 and 5% white fish meal and 10% dried skim-milk as the protein supplement.

3. 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 8 weeks of age to bacon weight. Comprehensive carcass measurements were made on all the pigs.

4. None of the treatment differences for any of the variables studied was significant at the 5% level. There was, however, a trend for rate of growth to be reduced, efficiency of food conversion to decline and dressing percentage to improve with reduction of protein level in the diet. There was also an indication that the rate of growth and efficiency of food utilization of the pigs given the basal meal containing 10% white fish meal were rather better than for the pigs receiving the basal meal with 10% dried skim-milk as the protein supplement, although the mean differences were relatively small.

5. The results are discussed in relation to those obtained in earlier experiments completed under similar conditions at the Cow and Gate farm.

6. On the basis of the experimental evidence obtained so far, it was concluded that either 10% white fish meal or 15% dried skim-milk should be considered the minimum amounts of these protein supplements that should in practice be included in the basal meal fed with unrestricted whey in the manner described.

7. Possible effects of season on whey consumption are discussed in relation to some preliminary findings on the effect of whey acidity on the performance of fattening pigs.

8. Results of another preliminary experiment are mentioned in which a reduction in the daily allowance of the basal meal to 1 lb. or less had a very adverse effect on the performance of the pigs. It was concluded that a daily allowance of 2 lb./pig of meal given in conjunction with an unrestricted supply of whey, should be considered the absolute minimum in order to obtain a reasonable rate of growth.

We thank the Director of Messrs C. & G. Prideaux, Ltd., Bacon Curers, Motcombe, Shaftesbury, Dorset, for permission to take carcass measurements of the experimental pigs, and Messrs K. Lawrence and P. Prideaux who most conscientiously took these measurements. The help of Mr J. Rowell of the Agricultural Research Council Statistics Group, Cambridge, in making the statistical analyses is also very gratefully acknowledged. We should also like to thank Mrs J. Sedgwick for making the whey acidity determinations.

The viscosity of concentrated skim-milk changed markedly with time upon the addition of urea, calcium-complexing agents or alkali, the viscosity-time curve passing through a distinct maximum. Also, the characteristic opacity of the milk disappeared in the presence of these reagents.

These phenomena are explained in terms of a postulated expansion of the casein micelles followed by their disintegration into ‘molecular’ units.

Electrostatic repulsions within the casein micelles are considered to be the cause of expansion.

Calcium, either in the ionic atmosphere of the casein micelle or as calcium bridges within the micelle, and hydrogen bonds are regarded as two of the major factors responsible for maintaining the stable structure of the casein micelle.

The authors wish to thank Mr E. F. Woods, Division of Protein Chemistry, C.S.I.R.O., Melbourne, and various staff members of the Dairy Research Section, C.S.I.R.O., Melbourne, for helpful discussion during the preparation of the manuscript. The technical assistance of Miss Audrey K. Morris is also gratefully acknowledged.

Flavour concentrates from mature New Zealand Cheddar cheese were prepared by steam distillation methods. Using 2:4-dinitrophenylhydrazine reagent the presence of carbonyl compounds in the distillate was established. By means of paper chromatography, and other methods, the following compounds were identified in the distillate: acetoin, diacetyl, acetaldehyde, acetone, 2-butanone, 2-pentanone, 2-heptanone, 2-nonanone and 2-undecanone, and their approximate concentrations in New Zealand Cheddar cheese were determined. The possible role of these compounds in the formation of Cheddar cheese flavour is discussed.

The authors gratefully acknowledge the helpful advice and criticism of Dr H. R. Whitehead and Dr W. A. McGillivray, and the technical assistance of Mr P. L. McLaughlin.

An essentially exponential relationship between maximum viscosity and concentration of casein was observed for concentrated skim-milk undergoing treatment with calcium hydroxide. Positive temperature coefficients of viscosity were found for samples treated with alkali, while negative coefficients were found with sodium hexametaphosphate. The time of appearance of the maximum viscosity was independent of temperature in concentrated skim-milk treated with sodium or calcium hydroxide; however, when sodium hexametaphosphate was used the maximum appeared earlier at higher temperatures. The viscosity of all systems studied decreased with increasing rate of shear, the deviation from Newtonian being greatest when the viscosity was at its maximum. The effects are explained in terms of a swelling and disintegration of the casein micelles as was postulated in the previous paper.

The authors wish to thank Miss Audrey K. Morris, Dairy Research Section, C.S.I.R.O., Melbourne, for valuable technical assistance, and to express their appreciation for the helpful advice of other members of this Section during the preparation of the manuscript.

The volatile sulphur compounds present in New Zealand Cheddar cheese have been studied by means of gas entrainment and steam-distillation techniques. Hydrogen sulphide was the only sulphur compound which could be detected, and after removal of this compound from the gas stream or steam distillate only a mixture of carbonyl compounds remained. Subsequent removal of these compounds completely destroyed the original cheesy aroma of the gas stream or distillate.

The parts played by hydrogen sulphide and carbonyl compounds in Cheddar cheese flavour are discussed.

The author wishes to record his gratitude to Dr H. R. Whitehead and Dr W. A. McGillivray for their helpful advice and criticism and to Mr P. L. McLaughlin for technical assistance.

1. The partition of riboflavin between the free and the protein-bound forms was measured in the milk of six Ayrshire cows, three of which were treated with l-thyroxine.

2. As lactation progressed there was a slow decline in the total riboflavin content of the milk for which the initial values were 0·86 to 1·09 µg./ml. milk. The rate of decline ranged from 0·01 to 0·003 µg./ml. milk per day.

3. Thyroxine had its expected effects in stimulating milk yield and heart rate, and in preventing the rise of phosphatase in the milk as lactation progressed, but it did not affect either the concentration or the partition of riboflavin.

4. Reasons are advanced for the marked contrast between the effect of thyroxine in increasing the phosphorylation of thiamine and the absence of any such effect on riboflavin.

We thank Miss S. McLauchlan and Miss M. Lightbody for technical assistance, and Glaxo Laboratories Ltd., Greenford, Middlesex, for a gift of L-thyroxine.

Slices and blocks of cheese were coloured with aqueous methylene blue solution and vacuum-packed or wrapped in the films to be compared. The extent of bleaching of the dye after 14 days gave a good indication of the extent to which oxygen was excluded by the film. Similar results were obtained with vacuum-packed slices and with wrapped rindless cheeses.

With polythene-coated and wax-coated MSAT cellophane films there was little or no reduction of methylene blue on the cheese surface. Similar films, including a lamination of foil or MXXT cellophane, allowed complete or almost complete bleaching.