Housed dairy heifers (62 cows) were fed concentrates exclusively through variable-time, out-of-parlour dispensers. They were observed to visit the dispensers on average 20·4 times/d but significantly less frequently (13·9 times/d) shortly after calving. The dispensers were occupied for 25·8% of the night (22.00–06.00 h) compared with 30·4% of the day (06.00–22.00 h). This difference was small but significant, whereas the differences between the proportion of night and day spent standing (32·5 v. 66·2%) or eating silage (4·8 v. 24·7%) were noticeably greater. In a herd of ∼ 80 cows in which the daily average number of visits to the dispenser was 20·5/cow there was a significant decrease in the number of visits/d (y) with increasing time after calving (x) where y = 23·4–0·030x (r = 0·88). Some cows repeatedly visited the dispensers more frequently than others (range 13·2–34·9±5·3 visits/d, giving a repeatability of 0·58). Cows in a grazing herd (∼ 60) visited the dispensers on average 14·6 times/d during an average 3·6 visits to a cubicle house to which they had continuous access. The number of visits to the dispensers (y) again decreased with increasing time after calving (x) where y = 22·7–0·072x (r = 0·45). The average time spent in the cubicle house (z) also decreased as the days after calving (x) increased (z = 8·4 – 0073x+0·000203x2, r =–0·66). All cows, however, stayed in the cubicle house during periods of heavy rainfall.

Quantitative cytological analysis demonstrated a marked and progressive increase in concentration of subepithelial plasma cells from milk-secreting parenchyma to the distal teat end mucosa in the normal, lactating bovine mammary gland. Parenchymal plasma cells exhibited typical ultrastructure with abundant, flattened stacks of rough endoplasmic reticulum, and hypertrophied Golgi components, while many cells in the teat end mucosa displayed an ergastoplasm distended with fine, flocculent material. Intraepithelial plasma cells were also observed in teat end mucosa. Results suggest that teat end tissues function by recruiting plasma cells to serve as protection against invading mastitis-causing organisms.

Oxytocin was infused for 60 min into the jugular vein of 8 lactating cows with full udders before milking. Under these circumstances milk loss occurred spontaneously in 3 cows and was evoked in 5 cows by reducing pressure around one teat (–30 cm water) using a plethysmographic chamber. Milk loss was measured continuously. The postsynaptic α-blocking agent prazosin (PRZS) injected either into the udder artery (2 or 5 mg) or into the jugular vein (200 mg) consistently induced a significant increase in milk leakage. It is suggested that PRZS effectively inhibits α-adrenoceptors in the teat sphincter muscles and thus promotes the action of β-adrenoceptors, in this way inducing relaxation of the teat sphincter.

N-acetyl-β-D-glucosaminidase (NAGase) levels and somatic cell counts (SCC) were determined monthly for 6 months in the bulk milk of 181 suppliers (1063 samples). A highly significant correlation (r = 0·74; P < 0·001) was found between supplier's bulk herd milk geometric mean NAGase activity and SCC. Monthly trends which grouped suppliers into various categories defined by different NAGase and SCC thresholds showed that a similar overall pattern was obtained with both SCC and NAGase. However, further analysis indicated that 18% of the bulk milk which had SCC < 500 × 103/ml had NAGase levels > 25 units. Also, 34% of samples with SCC > 500 × 103/ml had NAGase levels < 25 units. Overall, 24% of all samples did not have corresponding NAGase and SCC results. When the bulk milk of 2 commercial dairy herds was tested monthly over 12–18 months whilst the infection status of all quarters in the herds was regularly monitored, those herds with low incidence of mastitis (5% quarters infected) had significantly lower bulk milk SCC and NAGase levels than those with a high incidence of mastitis (22% of quarters infected). This suggests that NAGase measurement on bulk herd milk would be a simple means of monitoring infection status of dairy herds and of rapidly classifying a supplier's milk as being of low, medium or high SCC status. The possible combined use of SCC and NAGase levels in bulk milk monitoring schemes is discussed.

Glucose levels in 188 quarter fore milks from different cows were determined by an enzymic procedure. Mean glucose content was 0·22 mM (standard error ±0·009) and results ranged from 0·02–0·57 mM. Abnormal quarters had lower glucose levels (P < 0·01) than normal quarters but variability within each classification was large. Glucose content was negatively correlated with both somatic cell count (r = –0·49) and N-acetyl-²-D-glucosaminidase level (r = –0·61). Milk glucose was considered to be of limited value as a diagnostic test for mastitis.

Changes in milk lipid composition were measured during the course of mastitis infections induced in cows by infusion of either Escherichia coli endotoxin or Streptococcus agalactiae into 2 quarters of the udder; untreated quarters were used as controls. Experiments were divided into 3 distinct phases: a pre infusion period during which several samples were collected before infusion; a post infusion period following infusion and corresponding to the occurrence of elevated cell counts in milk from infused quarters; a recovery period followed after a short pause and represented the return of the cell count in milk from infused quarters to the level in the corresponding controls.

Milk total fatty acid composition was unaffected by the infusion. Free fatty acid (FFA) composition did, however, undergo some alteration. There was a significant increase in long-chain saturated acids in milk from infused quarters relative to the corresponding controls during the post infusion period. FFA concentration (mequiv./100 g fat) also increased significantly during this period although the net increase was only slight. Phospholipid and cholesterol concentrations were significantly higher post infusion in milk of infused quarters.

The results suggest that changes in concentration and composition of those milk constituents synthesized and secreted by the mammary epithelium occur after secretion in the alveolar lumen and milk ducts.

Calcium and phosphate concentrations in milk ultrafiltrate prepared at 37 °C over the pH range 4.0–8.0 have been measured and used to calculate the apparent solubility products for various calcium phosphates in milk. Micellar calcium phosphate had a constant Ca/Pi = 1·58·10·03 at all pH values and a near-invariant ion activity product for CaHP04. Apparent solubility products have also been obtained for brushite, hydroxyapatite and amorphous tricalcium phosphate in milk salt solution over a similar pH range. No evidence was found for a hydroxyapatite or amorphous tricalcium phosphate phase in milk.

κ-casein from porcine milk was isolated and purified by affinity chromatography on thiopropyl Sepharose followed by hydroxyapatite chromatography. The amino acid composition of this protein is similar to that of bovine κ-casein. The sugar composition was established and sow κ-casein was found to be highly glycosylated. It contains 12 N-acetylneuraminic acid, 10 N-acetylgalactosamine, 2 N-acetylglucosamine and 22 galactose residues.

Whole milks concentrated 1·5–4-fold and acidified and citrated milks concentrated 2·8-fold by ultrafiltration at 50 °C were analysed for chemical changes relevant to further processing, storage or nutrition. Fat and protein were entirely retained in the concentrate. The retention of water-soluble vitamins, Ca, Mg, phosphate and trace minerals depended on the proportion bound to the protein. Ascorbic acid was rapidly destroyed during concentration. Because of the differential retention of nitrogenous components, protein comprised a progressively higher proportion of the total N as the milk became more concentrated. No denaturation of whey protein or disruption of casein micelles was detected during concentration of whole milk, but some solubilization of the casein occurred after citration. Reduction of fat globule size occurred early in the concentration process, damage to the fat globule membrane was indicated and the milk became more susceptible to lipolysis. Apart from a tendency for preacidified or precitrated concentrates to gel, no change in the susceptibility of the milks to heat damage was detected.

Milk fat globule membrane (MFGM) material was isolated from commercial pasteurized milks and pasteurized creams using a procedure specifically adapted for these products (McPherson et al. 1984a). Pasteurized cream membranes contained higher lipid levels while pasteurized milk membrane material had lower lipid contents than fresh raw MFGM. Electrophoretic analysis showed that membrane material from both commercial products contained, in addition to native MFGM polypeptides, significant amounts of β-lactoglobulin with lower levels of caseins and other skim-milk components. The incorporation of β-lactoglobulin was more pronounced in pasteurized cream membranes. Examination of the lipoprotein complexes present in these membrane preparations by linear sucrose density gradient centrifugation showed that pasteurized milk membrane material had a similar profile to fresh raw MFGM except that preferential binding of skim-milk components was found in some of the lipoprotein fractions. Pasteurized cream membranes showed considerably different density gradient patterns with only 2 lipoprotein fractions present, of which the major one was a low density complex.

The material surrounding the fat globules in commercial pasteurized homogenized milk was isolated by centrifugation of the freeze – thaw extract of the fat globules through a 52·5% (w/v) sucrose solution in simulated milk ultrafiltrate. The purified membrane material contained higher neutral lipid levels than fresh raw milk fat globule membrane (MFGM) material. Caseins and whey proteins, mainly β-lactoglobulin (β-lg) were found to be the major protein components of homogenized milk membrane material. Native MFGM polypeptides comprised only a small proportion of the total protein in these membranes. The total amount of membrane material isolated from homogenized milk was substantially higher than that from fresh raw milk. Analysis of homogenized milk membrane material by linear sucrose density gradient centrifugation showed a different pattern to that found with other milks, indicating the presence of new lipoprotein complexes. Commercial ultra heat treated (UHT) milks were also examined, but it was difficult to obtain isolated MFGM material free from core fat. However, fat globule fractions were isolated from these products by high speed centrifugation and these were analysed intact. Membrane material in these milks contained caseins and whey proteins (mainly β-1g). Natural MFGM polypeptides were present at extremely low levels. Phospholipid levels in UHT milk membranes were greatly reduced compared with membrane material isolated from other milks.

The viscosity of heat treated creams increased with time (t). The rate (k) was dependent on homogenization pressure, fat content and heat treatment temperatures. Viscosity and its rate of increase could be related by the expressions:

log η = log η0 + kt

(where η0 is the viscosity at t = 0),

log η0 = a1 + b1φ/d

(where φ is the fat volume fraction, d is the globule diameter) and

k = a2 + b2φ/d.

Different values of a1b1a2 and b2 were found for homogenized and unhomogenized creams.

Bovine, caprine, ovine and human milks have been clarified by addition of Triton X-100 and EDTA followed by a short incubation. Generally, addition of 4·2% Triton X–100 and 5·4% EDTA was sufficient and with skim-milk samples no dilution was necessary. With whole milks some dilution and greater amounts of Triton X-100 or longer incubation times or higher temperatures were required. These factors were interrelated to some extent and the optimum conditions will be determined by convenience and any particular requirements of the experiment. The attainment of optically transparent milk samples greatly facilitates colorimetric measurements; as examples we have shown that total protein may be estimated by the biuret reaction, NH2 groups measured with 2,4,6-trinitrobenzene sulphonic acid and SH groups by reaction with 5, 5′-dithiobis(2-nitrobenzoic acid). Acid phosphatase activity in bovine milk was followed colorimetrically by reaction with p-nitrophenyl phosphate to demonstrate the suitability of the clarification procedure in the measurement of enzymic reactions.

Of the various factors influencing the volume transferred by the loop in the automated plate loop technique for enumerating bacteria in milk, the speed of the loop as it emerged from the milk sample was the most important. Increasing this speed from 3 to 6 cm/s resulted in a 23% increase in volume transferred by a standard loop of internal diam. 1·45 mm. The depth of dip was also important, but changes in temperature of the sample from 5 to 20 °C caused no significant change in volume transferred. The results indicate that if the transfer of 1 µl milk is required at speeds commonly used in the automated plate loop technique, the internal diam. of the loop should be reduced from the present standard 1·45 to 1·3 mm.

An apparatus for the objective determination of the heat coagulation time of milk powder solutions is described. In the apparatus a chromium plated iron piston is moved by means of a ring magnet, driven by a control mechanism, through a heated cylinder containing a small sample of milk. When the milk coagulates, the increased impedance between the inner wall of the cylinder and the metal piston overcomes the magnetic strength of the ring magnet and the magnetic coil moves away from the piston core, resulting in a momentary change in coil current, which after amplification is used to stop a digital clock. The method was tested with 10 and 20 % milk powder solutions at various pH values, at temperatures of 120 and 140 °C. The results are comparable with those obtained using a traditional method involving visual determination of coagulation (Davies & White, 1966).

Commercial and pure chicken pepsin preparations were compared with rennet as cheesemaking coagulants. From the milk clotting and caseinolytic activities and enzymic stabilities, predictions were made of the relative activities of the chicken pepsins and rennet during cheese ripening. By this means mixtures of chicken and pig pepsins for cheesemaking trials were formulated. These and mixtures of chicken pepsins with rennet were approximately additive in coagulating activity provided they were used immediately. The amounts of chicken pepsins used in laboratory scale Cheddar cheesemaking trials were minimized by acidification and calcification of the milk. The nature of the coagulant did not affect the cheesemaking conditions, yields or cheese composition. Cheeses made with chicken pepsins alone showed faster proteolysis, more intense flavour, off-flavours and bitterness and were softer than those made with rennet. Cheeses made with chicken pepsin/rennet mixtures were intermediate and those made with chicken pepsin/pig pepsin mixtures were similar to those made with rennet. It was concluded that neither of the chicken pepsin preparations was suitable for Cheddar cheesemaking and that the predictive tests had led to correct assessments.