A technique for monitoring the movement of the teatcup liner during milking, using video recordings, has been investigated. The liner was videoed through a transparent shell and the recordings were subsequently processed frame by frame by an image analyser. The width of the liner and its variation with pressure difference across the liner walls were calculated at several locations along the liner length, and analysis of these results allowed the pressures at which the liner buckled and the walls touched to be determined and some estimate of the degree and duration of liner collapse to be made.

The transport of α-aminoisobutyric acid (AIB) by lactating rat mammary tissue has been examined. AIB uptake by mammary tissue was via both Na+-dependent and Na+-independent pathways. AIB uptake via the Na+-dependent pathway was inhibited by (methylamino)isobutyric acid (MeAIB) whereas AIB uptake via the Na+-independent pathway was blocked by 2-aminobicyclo[2, 2, 1]-heptane-2-carboxylic acid (BCH). A small fraction of AIB influx persisted in the presence of both MeAIB and BCH. The Na+-independent moiety of AIB uptake was strongly inhibited by phenylalanine, tryptophan, leucine, isoleucine and methionine. AIB efflux from mammary tissue slices was found to be both Na+-dependent and Na+-independent. Trans-stimulation of AIB efflux by other amino acids was not observed; in contrast, external phenylalanine, tryptophan and leucine inhibited AIB efflux. The results are largely consistent with the presence of systems A and L in lactating rat mammary tissue. However, the Na+-independent fraction of AIB transport may represent transport via a tissue specific form of system L.

The whey proteins α-lactalbumin and βMactoglobulin have been investigated as potential markers of mammary development in sows by measuring their concentrations in plasma. The whey proteins were isolated from porcine milk by gel filtration, ion-exchange and hydrophobic interaction chromatography, characterized by several criteria and used to raise antibodies. Specific radioimmuno-assays were set up for porcine α-lactalbumin and β-lactoglobulin and validated for use in porcine blood and milk. Plasma levels of the whey proteins were measured in sows that were pregnant, suckling litters post partum, weaned abruptly at birth or were pregnant but mastectomized. Both whey proteins showed similar patterns in plasma post partum, falling from a maximum 1 d after parturition to values < 0·02% those in milk by day 4–5 post partum in suckling sows and showing a transient peak associated with early involution before declining to very low concentrations in non-suckling sows. α-Lactalbumin was first detected in the last week prepartum, rising markedly in the 3 d before parturition, correlated with rising prolactin (r = 0·986) and falling progesterone (r = –0·998). β-Lactoglobulin rose much earlier from 5 weeks prepartum, at the time when lobulo-alveolar mammary development is occurring, and correlated (r = 0·929) with oestradiol-17β. In mastectomized sows, concentrations of whey proteins in plasma were reduced by 90% or more when compared with intact animals, though following a similar pattern. This study shows that whey protein concentrations in plasma vary with physiological state and reflect aspects of the development of the mammary gland. The very different profiles for α-lactalbumin and β lactoglobulin prepartum indicate that they are differently controlled.

Mammary tissue from pigs on days 60, 80, 90, 100 and 100+ (days 106–111) of pregnancy has been cultured in vitro as explants. The total accumulation in tissue and culture medium of the whey proteins α-lactalbumin and β-lactoglobulin has been measured using specific radioimmunoassays. The control, uncultured tissue showed progressive morphological development from sparse, non-secretory epithelial tissue on day 60 to full lobulo-alveolar development with some accumulated secretion from day 100. In uncultured explants β-lactoglobulin could be detected consistently from day 90 (13 ± 12 ng/μg DNA, n = 4) and α-lactalbumin from day 100 (1·3 ± 0·5 ng/μg DNA, n = 11). At all stages of pregnancy, both whey proteins increased markedly during the period of culture (up to 7 d). Stimulation of α-lactalbumin appeared to be primarily under prolactin control. Prolactin increased α-lactalbumin accumulation to a similar extent alone, or in the presence of insulin and/or corticosterone. The response to prolactin was dose-dependent over the range 0·4–20 nM (10–500 ng/ml). Porcine prolactin was more potent than ovine prolactin. There was no effect of porcine growth hormone and no synergism detected between prolactin and tri-iodothyronine. By contrast, no specific hormonal requirements were established for accumulation of β-lactoglobulin, which appeared to increase in vitro if tissue remained viable in various combinations of insulin, corticosterone and prolactin. It was not stimulated by growth hormone. There was some indication of a prolactin-sensitive component in longer term cultures after day 4.

Radioreceptor binding studies were designed to localize and determine the number of α and β-adrenergic receptors in the mammary gland of lactating cows. 3H-prazosin, 3H-rauwolscine and 3H-dihydroalprenolol were used for the regional characterization of α1, α2- and β-adrenergic receptors by competitive inhibition of binding of 3H-ligands with unlabelled adrenergic agonists and antagonists. The α1-, α2- and β2-adrenergic receptor subtypes could thus be demonstrated in the regions of the teats, large mammary ducts and parenchyma. Tissues of the teat wall, of the large mammary ducts above the gland cistern and of the mammary parenchyma were prepared to determine the density of α1, α2- and β-receptors by saturation binding assays using 3H-prazosin, 3H-rauwolscine and 3H-dihydroalprenolol respectively. Binding to high affinity sites was reversible within minutes and saturable. Equilibrium was reached within minutes. The number of α1-and α2-adrenergic receptors decreased from the teat to the mammary ducts to the parenchyma. Most of the α1- and α2-adrenergic receptors were found in the teat wall, whereas in the parenchyma α-adrenergic receptors were absent or barely detectable. The density of β-adrenergic receptors was similar in the teat wall and the large mammary ducts, but much lower in the parenchyma. Thus, α1, α2- and β-adrenergic receptors were found mainly in the milk purging system and hardly at all in mammary parenchyma. Inhibition of milk removal by α-adrenergic stimulation is possibly due to constriction of teat wall and to constriction of the mammary ducts, whereas enhanced milk flow after β-adrenergic stimulation is possibly due to relaxation not only of the teat sphincter and teat wall, but probably also of the large mammary ducts.

Samples of lactic coagulum were obtained from various depths in a lactic casein silo. The casein content at the base of the silo was in the range 38–62 g/kg compared with 25–30 g/kg in the bulk of the silo. This layer of higher casein concentration, called heavy coagulum, was up to l m deep and appeared long before the bulk milk had fully coagulated. It had a lower pH than the bulk, e.g. pH 4·35 when the bulk pH was 5·2. The heavy coagulum layer was removed from the silo last during pump-out. A laboratory coagulation tube was designed to permit more intensive study of heavy coagulum formation. Starter bacteria levels in the heavy coagulum were 10–160 times those in the bulk. Heavy coagulum had a visibly different curd structure (a much more granular gel), produced differently shaped extrusion curves, and had a higher gel strength than coagulum from the bulk. The heavy coagulum phenomenon had many similarities to minor sludge formation in Cottage cheese manufacture.

The percolation model of Steventon et al. (1991) successfully detected changes in the mechanism of whey protein concentrate gelation with pH and temperature by comparing simulated with experimental gelation times. The results demonstrated that at 71 °C and pH 5·2 the formation of an aggregate from two denatured protein molecules (initiation) was the rate-determining step in the gelation process, while at pH 7·0 the addition of denatured protein molecules to an aggregate (propagation) was rate-determining. At pH 5·9, the gelation process was also initiation limited, with the rate being slower than for pH 5·2 solutions, probably owing to electrostatic effects. Analysis of the temperature dependence of the gelation time, and percolation analysis both showed that there was a change in the rate-controlling reaction at 73 °C for gelation at pH 5·2 and 7·0. In the case of pH 7·0 gelation, this change in rate-controlling reaction was not due to a change from denaturation- to aggregation-controlled gelation, but was probably due to a change in the relative rates of interactions between protein molecules. Gelation at pH 5·2 was aggregation-controlled at temperatures below 73 °C, and denaturation-controlled at higher temperatures; there appeared to be another change in rate-limiting reaction at 80–85 °C without a change in mechanism (i.e. it remained denaturation-limited). The activation energies of the rate-limiting reactions determined from analysis of the temperature dependence of gelation time and from percolation analysis were in agreement. This is evidence that the changes in the rate-controlling reaction of whey protein concentrate gelation with temperature and pH were real.

The arsenic content of cows' milk consumed in Spain was determined. A procedure using dry ashing hydride generation atomic absorption spectrometry was developed for the purpose. Experimental conditions for the destruction of organic matter were established and an interference study was carried out. The method developed has a detection limit of 0·1 ng/g, a relative SD of 5%, and a recovery (mean ± CSI) of 100 ± 4%. The accuracy of the method was checked by analysis of a certified sample of IAEA milk powder (mean ± CSI: certified, 4·85 ± 0·32 ng/g; found, 5·04 ± 0·81 ng/g). The proposed procedure was used to analyse cows' milk samples. The extremely low arsenic levels found do not present toxicological problems.

A sandwich ELISA (enzyme-linked immunosorbent assay) of the two-site type has been successfully developed for the detection of cows' milk in goats' or ewes' milk. The assay uses two monoclonal antibodies (MAb) raised in mice against cows' β-lactoglobulin (β-lg). These MAb recognize different epitopes of the β-lg, which are sufficiently distinct to allow simultaneous binding of the corresponding antibodies. One of the MAb recognizes a species-specific epitope of the bovine β-lg and was adsorbed to a plastic microtitration plate (capture antibody). The second MAb was labelled with peroxidase and used to detect the captured cows' β-lg. Factors affecting assay performance were investigated. The optimized assay is highly specific, reproducible (intra- and inter-assay CV were 8 and 13% respectively) and sensitive: as little as 5 ng β-lg/ml or 1 part cows' milk per 100000 parts goats' or ewes' milk can be detected. The technique is robust, cheap, rapid, reliable and suitable for high sample throughput, semi-automation and screening surveys. The MAb used guarantee the high specificity of the assay and indefinite reagent supply of constant quality once approved by collaborative national or international trials.

A simple and rapid competitive enzyme-linked immunosorbent assay method has been developed to detect the residual activity of chymosin in various types of cheese without extraction. In a cheese of the Saint-Paulin type, concentrations of chymosin detected at the end of cheesemaking in curd and whey represented 22·5 and 73·5% respectively of the total amount added to milk, so most of the chymosin was detected using this method. In a cooked pressed cheese of the Comté type, only 14% of the chymosin added to milk was detected in curd and all the chymosin in whey was inactivated. It was thus possible to demonstrate the presence of active residual chymosin in cooked pressed cheese. When measurements were carried out in 12 different cheeses, chymosin concentrations ranged between 384 ng/g in a Camembert cheese and 86 ng/g for an 8-month old Comté cheese. Chymosin appeared to be highly stable during cheese ripening.

The nature and composition of the unsaturated (monoenoic) oxo fatty acids in Cheddar cheese have been re-evaluated. Following isolation by column chromatography and silver ion HPLC, the fatty acids were identified in the form of their picolinyl ester derivatives by gas chromatography–mass spectrometry. For confirmation, they were analysed similarly following deuteration of the double bonds with deuterium and Wilkinson's catalyst. Fifteen fatty acids were identified fully, only five of which corresponded to those reported in an earlier study. Isomers with double bonds in positions 5, 7, 9 and 11–15 were identified, with oxo groups in positions 8, 9, 10, 13, 16 and 17, and with chain-lengths varying from C14 to C18, but with C18 predominating.

Skim milks were pre-acidified to pH 6·4 and concentrated by ultra-filtration to give retentates with protein levels of 210 g/1. Retentates were blended with skim milk and cream to give standardized milks with protein levels ranging from 30 to 82 g/1. These were used for the manufacture of Cheddar cheese in conventional equipment. Increasing milk protein level resulted in reduced gelation times, increased curd firming rates and a decrease in the set-to-cut time when cutting at equal firmness values (i.e. elastic modulus, G′, ∼ 16 Pa). As the curd firming rates increased with milk protein level, it became increasingly difficult to cut the curd cleanly, without tearing, before the end of the cutting cycle. Reflecting the tearing of curd, and consequent curd particle shattering, fat losses in the running wheys were greater than those predicted on the basis of volume reduction (due to ultrafiltration) for milks with protein levels > 50 g/1. Reduction of setting temperatures, in the range 31–27 °C, and the level of added rennet brought the set-to-cut times and curd firming rates of concentrated milks closer to those of the control milk. While increasing milk protein level in the range 30–70 g/1 had little effect on cheese composition, it resulted in slower proteolysis and maturation.

Physicochemical, organoleptic and microbiological characteristics of semi-hard ewes' milk cheeses, frozen immediately after manufacture under two different conditions at −35 and −80°C and stored at −20°C for 4 months, were studied during subsequent ripening. Frozen storage after either freezing condition did not result in marked alterations in overall composition or rheological and sensory properties of the cheeses after 45 d ripening. Water activity decreased over the ripening period and this decrease was more pronounced in the frozen batches. The water-holding capacities of the protein in the frozen cheese batches were higher than for the control batch. The level of proteolysis, estimated from the level of amino groups, was higher in the previously frozen cheese. Freezing also reduced the viability of the microbial flora in the cheese. The differences in these characteristics for the two freezing conditions were not significant.

The performance of khoa, a local heat-concentrated milk product, as a milk solids source in ice cream was tested by replacing 250, 500, 750 and 1000 g/kg of the condensed whole milk in control ice cream. Proximate composition was little affected by the inclusion of khoa at any level tested. At levels of inclusion > 250 g/kg the viscosity of both fresh and aged experimental mixes was significantly (P < 0·05) increased compared with controls. Some mixes containing khoa appeared slightly off-white, but there were no overall differences from control. Control and experimental mixes were identical in whipping capacity and overrun. In organoleptic evaluation, the colour score of the ice creams declined significantly (P < 0·05) when 750 or 1000 g/kg of the condensed milk was replaced with khoa. Ice creams containing khoa were equivalent or superior to controls in flavour. In objective tests, the experimental ice creams showed significantly (P < 0·05) faster meltdown compared with control, but no differences were found in subjective assessments. We conclude that khoa can be used successfully as a milk solids source for ice cream without any serious adverse effects on quality, the only problem being the dull colour.