Two experiments with dairy cows and one with suckler cows and their calves were conducted to examine the use of secretion rate measurements for estimating total milk production. In the first experiment both 4- and 6- h intervals between measurements gave similar estimates of total 7-d milk yield. The second experiment compared estimated and measured milk composition as well as yield. Milk and solids-not-fat yields were underestimated with dairy cows as a result of an extended milking interval before measurement. However, fat yield was overestimated, indicating that all residual milk was not removed at the first oxytocinaided milking. It was concluded that for the beef cow, previous interval effects would be eliminated by the frequency of calf suckling, but that residual milk effects might cause a 3–6% and a 16% overestimation of milk and fat yields respectively.

In the third experiment, the milk yield of suckler cows was estimated from measurements of secretion rate and from changes in calf weight; good agreement was obtained provided there were at least 3 consecutive controlled sucklings.

Quarter-milk cell counts and milk yields from 1000 cows in the province of Noord-Brabant were obtained over a 12-month period. The data from 933 animals, mainly heifers, were used to examine statistically the relationship between cell counts, as a criterion for sub-clinical forms of mastitis, and milk yield. Losses in milk were assessed by quarter, by pairs of quarters and by cow. The conclusions were that the losses in milk due to sub-clinical forms of mastitis, as indicated by cell count, can be substantial. The existence of a compensating increase in milk yield from a healthy gland, in response to the loss of milk from a neighbouring quarter with raised cell count, could not be demonstrated.

A new spectrofluorimetric assay procedure for bovine milk N-acetyl-β-D-glucosaminidase (NAGase) is described for use as a routine screening test for the detection of abnormal uddersecretions. This procedure uses 4-methylumbel-liferyl-N-acetyl-β-D-glucosaminide as substrate. On the basis of the greater sample throughput, increased product sensitivity detection of NAGase and the absence of turbidity problems, it is considered to be superior to a previously reported spectrophotometric procedure (Kitchen, 1976). The correlation coefficient between the somatic cell count and the fluorimetric procedure using 243 quarter fore-milk samples was 0·86. Distribution studies on bovine milk and mammary gland homogenates indicated that this enzyme activity was located predominantly in the soluble whey protein fraction and the post-microsomal supernatant. Mammary gland secretory cells contained high levels of NAGase and appeared to be the major source of the enzyme in milk whilst NAGase from other sources (white blood cells, blood serum) contributed only a minor proportion (5–15%) of the total activity in milk. The implications of these findings on the value of the NAGase test as a means of mastitis diagnosis are discussed.

Pellets produced by the centrifugation of bovine milk and of pre- and post-partum colostrum consistently contained small cell fragments (1–4 μm long) morphologically characterized by their possession of abundant microvilli along one margin. They were more common in strippings than in fore-milk and during the first 4–6 weeks of lactation than at later stages. Their possession of cytoplasmic fat droplets and putative casein micelles suggests that they originate from secretory cells of alveoli or small ducts. The significance of their numerous microvilli is discussed.

The variation in kinematic viscosity (ν) with solids concentration and temperature has been determined for native skim-milk concentrates and reconstituted New Zealand skim-milk. Below 40% (w/v) total solids, samples reconstituted from a single batch of skim-milk powder model the behaviour of native concentrates obtained from an evaporation plant at all times of the year. Above 40% (w/v) agethickening and seasonal variations in the viscosity of native concentrates become significant. Age-thickening can be reduced by cooling the concentrate. These results are discussed in relation to milk powder manufacture.

A progressive change takes place in the heat stability of skim-milk during concentration. At the maximum in the coagulation time (CT)–pH profile of milk concentrated to over 20% of total solids (TS) the total N depletion curve changed from single- to 2-stage and CT became insensitive to the addition of urea. Furthermore, addition of β-lactoglobulin to skim-milk concentrates destabilized the heated milk whilst the opposite effect was observed in the presence of sulphydryl-group blocking agents. As a result of these observations, it has been suggested that the mechanism of coagulation in concentrated milk is similar to that which occurs within the minimum of the CT–pH profile of skim-milk at normal levels of TS.

The addition of simple aldehydes brought about large increases in the heat stability of both skim-milk and concentrated skim-milk over a comparatively wide milk–pH range. The coagulation time–pH minima of type A milks were completely removed by aldehyde treatment. Some sugars, which react readily as aldehydes on heating, were also shown to stabilize concentrated milk to prolonged heat treatment at 120 °C.

Micelles of milk heated at 95 °C for 10 min or autoclaved at 121·7 °C for 15 min were observed by electron microscopy to bear filamentous appendages, whereas micelles from raw milk had relatively smooth contours with no appendages. Experiments suggested that the appendages were composed of denatured β-lactoglobulin and showed that their development was sensitive to sulphydryl blocking agents. Firmness and syneresis of yoghurt were associated with the extent to which micelles coalesced during fermentation; the appendages of micelles in heated milk appeared to inhibit coalescence giving rise to a firmer curd with a lower tendency to syneresis.

The reaction of αs1- and β-casein with ferrous ions and O2 has been studied. It has been found that phosphoseryl residues present in these proteins rapidly catalyse the oxidation of iron from the ferrous to the ferric state with the formation of stable ferric-phosphoprotein complexes and the consumption of the theoretical amount of O2. When, however, the phosphoseryl residues occurred in groups, the formation of ferric complexes was accompanied by uptakes of O2 in quantities significantly in excess of that required for the oxidation of ferrous to ferric iron.

The significance of these findings is discussed in relation to hypotheses which have been advanced concerning the possible biological function of phosphoprotein.

The effect of reductive methylation on the properties of αs-, β- and κ-caseins was studied using preparations of these proteins in which 20 or 60% of the lysyl residues had been reductively methylated. This modification appeared to affect the physicochemical properties of the protein to an insignificant extent and in particular, electrophoretic mobility, sensitivity to precipitation by Ca2+ and solubility of Ca salts of individual proteins were only slightly altered. The use of caseins which had been subjected to methylation with radioactive formaldehyde is suggested as a potentially valuable method for the study of protein interactions.

The recovery of proteins from whey by complex formation with carboxymethyl cellulose (CMC) has been studied. After precipitation of the CMC–whey protein complex at pH 3·2, the complex was separated, washed and redissolved at pH 7. The CMC was separated from the protein by precipitation as its Ca salt. By this method protein concentrations in the supernatant liquor of up to 6 times that of the original whey were achieved. Between 10 and 70% of the protein was precipitated with the Ca CMC, depending on the pH and the excess of Ca present. The effective protein binding capacity of the recovered CMC decreased on re-use because of firm binding of both protein and Ca to the CMC.

The precipitation of proteins from whey by modified carboxymethyl cellulose (CMC) was examined. Carboxymethyl cellulose containing acetyl, methoxyl or carboxymethyl secondary amides with iso-propyl, n-propyl, iso-butyl, n-butyl or cyclohexyl side chains precipitated more protein than did CMC of similar carboxyl content. This effect was greatest in CMC containing secondary amide groups. The increase in protein precipitation gained by introduction of secondary amide groups into CMC was no greater however than the increase obtained by introduction of a similar number of carboxymethyl groups into the polymer. Carboxyethyl cellulose did not form insoluble complexes with whey proteins under the conditions examined.

Peptide and amino acid transport in Streptococcus lactis was both energy-and temperature-dependent. Separate systems were present for the transport of amino acids, dipeptides and oligopeptides as well as for individual amino acids. Both tri- and tetra-peptides were readily transported while an apparent size restriction at the point of uptake resulted in a relatively low rate of uptake of the pentapeptide. The effects of pH, temperature and salt concentration on the uptake of amino acids and peptides were studied. Exchange reactions played a significant role in the uptake of both peptides and amino acids.

Activation of the antibacterial lactoperoxidase system in milk, i.e. increasing the thiocyanate concentration to 0·25 mM and adding an equimolar amount of H2O2, results in a substantial reduction of the bacterial flora and prevents the multiplication of psychrotrophic bacteria for up to 5 d. This treatment has no effect on the physico-chemical properties of milk and does not lead to the accumulation of resistant bacteria. The practical application of the lactoperoxidase system in prolonging the storage period of raw milk at low temperatures is discussed.