S. J. Rowland

176. The Determination of the nitrogen distribution in milk

Details are given of a semi-micro Kjeldahl method for the determination of the total, casein, albumiij, globulin, proteose-peptone, and non-protein nitrogen of milk. The method is accurate, rapid, and particularly suitable for the determination of the smaller nitrogen fractions. The various nitrogen fractions are separated by the improved procedures described in the preceding paper.

175. The precipitation of the proteins in milk. I. Casein. II. Total proteins. III. Globulin. IV. Albumin and Proteose-peptone

Improved methods have been evolved for the separation of the total protein, casein, albumin, globulin, and proteose-peptone substances of milk. I. The use of acetic acid-sodium acetate solutions for the precipitation of casein is elucidated, and it is shown that the maximum precipitation of casein is rapidly effected from milk samples of varying casein content by the addition to 10 ml. of milk of about 80 ml. of water at 40° C. and 1·0 ml. of 10% acetic acid solution, followed, after 10 min., by 1·0 ml. of N sodium acetate solution. This maximum precipitation was found to be 1·0–1·4% greater than by Moirs method, and 2·4–3·8% greater than by the method of the Association of Oflicial Agricultural Chemists. A procedure is suggested for the determination of casein which avoids the tedious transfer and washing of the precipitate, and gives enhanced ease, accuracy and speed of working. II. The advantages of trichloroacetic acid for the precipitation of proteins in determinations of the total protein and the non-protein nitrogenous substances of milk are discussed. The trichloroacetic acid methods at present in use are shown not to give complete precipitation, and for this a rapid method employing, at room temperature, a final concentration of 12% acid in the milk-acid mixture is recommended. III. An accurate method for the precipitation of globulin uncontaminated with either albumin or casein is described. IV. Methods are given for the precipitation and separation of the albumin and proteose-peptone substances.

146. The heat denaturation of albumin and globulin in milk. II. Denaturation and degradation of protein at temperatures of 75–120° C

1. The denaturation of albumin and globulin took place rapidly in samples of milk heated at temperatures of 75°C. and above, and was complete in approximately 60, 30, 10–15, and 5–10 min. at 80, 90, 95, and 100°C. respectively. 2. There was no change in the non-protein nitrogen content of milk on heating at temperatures up to 100°C.; on continued heating at 95 and 100°C. extremely small amounts of proteose were produced by hydrolysis of protein. 3. In milk heated at 115 and 120°C. the denaturation of albumin and globulin was followed by appreciable hydrolysis of protein, which resulted in considerable increases in the proteose and in the non-protein nitrogen contents.

557. Studies of the secretion of milk of low fat content by cows on diets low in hay and high in concentrates: IV. The effect of variations in the intake of digestible nutrients

1. Digestibility trials were conducted, and the rate of passage of hay was measured, with five Shorthorn cows during a period of normal diet and during two experimental periods in which diets low in hay and high in concentrates were given. In the first experimental period the concentrates were cubes of the wartime type sold as National Cattle Food No. 1, and containing a variety of constituents; in the second they were a mixture offlakedmaize (50%), weatings (35%) and decorticated ground-nut cake (15%). For convenience these are referred to as concentrate ‘cubes’ and ‘mixture’ respectively. 2. During the initial and final control periods the cows consumed daily 17–21 lb. hay, and about 4·5 lb. concentrates per 10 lb. of milk produced. In the initial control period the concentrates were the concentrate cubes, and in the final they were the concentrate mixture. The hay was reduced to 4 lb. daily during the two experimental periods and the remainder of the standard requirements of the animals were met by concentrates. The concentrates were the cubes in the first experimental period and the mixture in the second experimental period. 3. Seducing the hay to 4 lb. did not affect the fat content of the milk when the other food in the diet was the concentrate cubes, but there was a striking mean fall of 1·04% fat when the cubes were replaced by the concentrate mixture. This represented a loss of over 30% in the yield of fat. 4. Digestibility trials, conducted in the initial control and first and second experimental periods, indicated that the fall in milk fat content was not the result of changes in the amounts of dry matter, crude protein, ether extract, crude fibre, cellulose, cellulosans or pentosans not in cellulose digested. The essential difference between the diet of low hay with the concentrate mixture and the other diets given in this experiment was that it provided a high intake of starch yet had little of the physical property of roughage. It is concluded that depression of milk fat content results from a combination of these two factors and probably originates from changes in the physical and biochemical processes of the reticulo-rumen. 5. The intake of starch equivalent, as calculated from the intake of digestible nutrients, was close to standard requirements in all periods of the experiments, but there was a surplus of digestible crude protein. 6. The mean solids-not-fat content of the milk rose 0·48% at the time of the fall in milk fat, and this was entirely due to an increase in milk protein. After the return to normal diets the recovery of solids-not-fat was slower, but no less complete, than the recovery in milk fat.

147. The soluble protein fraction of milk

1. The literature on the soluble protein fraction of milk has been reviewed to show the difficulties that are encountered in attempting to decide whether the fraction consists, as is usually supposed, of lactalbumin and lactoglobulin only, or whether it consists partly of secondary proteins of a proteose-peptone nature. 2. It has been shown that on boiling the casein-free filtrate of milk, maximum coagulation of albumin and globulin occurs at a pH. of 4–75–4–80. At this pS. prolonged boiling does not cause hydrolysis, and the amount of albumin and globulin N coagulated represents about 70 per cent of the total soluble protein N of milk. 3.By precipitation of the casein-free nitrate of milk with various con- centrations of trichloracetic acid, a partition of the soluble proteins has been effected which strongly suggests the presence of proteose-peptone substances to an extent in agreement with the results of heat coagulation. 4. The maximum amount of albumin and globulin rendered coagulable by heating milk itself has been determined by precipitating denatured albumin and globulin along with casein at pH. 4–7. This maximum represented an average of 76 per cent of the total soluble protein N in a series of normal milk samples, and reasons have been given for considering this maximum as the total of true albumin and globulin present. 5. The conclusion has been reached that the soluble protein fraction of normal fresh milk is composed of approximately 76 per cent albumin and globulin, and 24 per cent proteose-peptone substances.

578. The composition of the milk of the blue whale

The detailed composition of samples of milk from three blue whales (Balaenoptera sibbaldi) is reported in terms of total solids, fat, solids-not-fat, total protein, casein, soluble proteins, non-protein nitrogen, lactose, chloride, ash, calcium, phosphorus, sodium, potassium, magnesium, vitamin A, thiamine, biotin, riboflavin, nicotinic acid, vitamin B6, pantothenic acid and vitamin B12.The composition of the milk of the whale is compared with that of other species.

187. The effect of subclinical mastitis on the solids-not-fat content of milk

The solids-not-fat content (expressed as a percentage of fat-free milk) has been determined for 247 samples of milk from the individual quarters of sixty-two cows. The samples were also examined bacteriologically for the presence of mastitis streptococci and 121 were found to be infected, 114 uninfected, and twelve “doubtful”. The solids-not-fat content of the uninfected samples varied from 8·24 to 10·28 and of the infected samples from 4·26 to 9·92. For the uninfected samples of the Shorthorn, Friesian, Ayrshire and Guernsey breeds it averaged 9·36, 9·11, 9·82 and 9·83 respectively, but in the infected samples only 8·44, 8·38, 8·28 and 9·49%. Of the 247 samples, 36 % were below 8·80 in solids-not-fat content. Of 121 infected samples, 63% were below while of the 114 uninfected samples only 9% were below this standard. Hence 88% of the samples below 8·80 were infected samples, thus showing that subclinical mastitis accounts for a very high percentage of samples low in solids-not-fat content. In these samples the average value for solids-not-fat was 7·93%. The few uninfected samples that were low in solids-not-fat averaged 8·56%.

186. The casein number. A chemical method for diagnosis of mastitis

The accuracy of the casern number of milk as the basis for a chemical method for the detection of mastitis has been investigated. The number was determined for 247 samples of milk from the individual quarters of sixty-two cows, and the samples were examined bacteriologically for mastitis streptococci. Of the 247 quarters 49% were found to be infected—mostly subclinically—involving 74 % of the cows. The total nitrogen content of the samples varied from 362· to 687·4 mg. per 100 ml., and the casein nitrogen from 153·0 to 554·0. The casein number varied from 42·3 to 83·4. The casein number averaged 79·2, 80·4, 79·5 and 80·3 in the samples from the uninfected quarters of the Shorthorn, Friesian, Ayrshire and Guernsey cows respectively; and 71·7, 74·1 and 70·3 in the samples from the infected quarters of the Shorthorn, Friesian and Ayrshire breeds respectively. For the diagnosis of mastitis from the casein number, a figure of 78·0 and less was taken as indicating an infected quarter, and on comparing this chemical method with the bacteriological method, out of a total of 243 quarters, the chemical result differed from the bacteriological for 21 quarters—representing 8·6 % of the total. Reasons are given for considering that this estimate of the error is excessive and that in practice the casein number would provide a valuable and rehable method for the diagnosis of mastitis.

The yield and composition of kale and cabbage with and without nitrogenous top-dressing

In 4 consecutive years, 1948 to 1952, combined varietal and manurial trials were carried out to obtain information on the yields of marrow-stem kale, thousand-head kale and cattle cabbage in the presence and absence of sulphate of ammonia applied as a top-dressing at the rate of 6 cwt./acre. Yield and composition of all crops were apparently affected by differences in weather conditions. In most years, and particularly for cabbage, the yield declined with successive harvests. Increased green crop, dry matter and crude protein yields resulted from the application of nitrogenous fertilizer in 3 years, but in the other year, a very dry season, the response to the top-dressing was negligible. Marrow-stem kale both with and without the additional fertilizer, yielded more, in terms of fresh crop, dry matter and crude protein, than thousandhead kale with the same manurial treatment. Cabbage yields often approached those of marrowstem kale at the first harvest, but were nearer to those of thousand-head by the last harvest of each year.

575. Colour changes in heated and unheated milk: III. The effect of variation in milk composition on the whitening and browning of separated milk on heating

The effect of natural variations in the pH, and in the casein, soluble-protein and lactose contents on the whitening and browning of separated milk has been studied by holding samples of known composition, obtained from individual cows, at a temperature of 110°C. for various times and determining their reflectances after treatment. The rate of browning, measured as a rate of loss of reflectance at a wave-length of 4260 A., was found to depend on the pH of the milk. Apparent variations of the rate of browning with protein content are considered to have been caused by associated variations of pH with protein content in the milk from the different cows. The rate of browning varied over a range of 2:1 for the samples examined. The whitening of milk, measured as the maximum rise of reflectance occurring on heating, was found to be much more variable, and over the same range of samples varied by 5·5:1. The soluble-protein content and pH affect the amount of whitening, but there appears to be some other factor of importance which was not covered by the chemical analyses that were made. A considerable range of reflectances is shown by different milk samples after a fixed heat treatment. These differences are determined more by variations in the amount of whitening than by variations in the rate of browning.