Inhibition of milk ejection in cows by oxytocin receptor blockade (Atosiban) and α-adrenergic receptor stimulation (phenylephrine) prior to prestimulation was compared with inhibition of milk ejection in unfamiliar surroundings. In addition, Atosiban and phenylephrine were administered after a 1 min prestimulation or 1 min after the start of milking. Oxytocin concentrations increased during milking in all treatments. The spontaneously removed milk fraction (before oxytocin was injected) was similar for Atosiban and phenylephrine treatments and in unfamiliar surroundings, but lower than in controls. Peak flow rates were similar in all treatments, but reduced as compared with controls when phenylephrine and Atosiban were administered before prestimulation. Peripheral (Atosiban, phenylephrine) and central (unfamiliar surroundings) inhibition of milk ejection reduced the amount of available milk similarly. Drug treatments resulted in similar peak flow rates; however, teats were contracted after phenylephrine administration but not after Atosiban. The inhibition induced by Atosiban could be abolished by oxytocin injection, but not that induced by phenylephrine, which was antagonized by α-adrenergic receptor blockade. These results indicate that inhibition of milk ejection through activation of α-adrenergic receptors is based on blockade of milk flow into the cistern, but not through the teats.

Cortisol response to stress appears to differ between lactating and non- lactating animals. Lactating (14 d post partum) and non-lactating sheep were fitted with probes so that drugs and hormones could be infused directly into the posterior pituitary and paraventricular nucleus of the hypothalamus. The animals were also fitted with instruments to allow monitoring of heart rate, body temperature and blood cortisol levels. Their reactions to a source of acute stress (a barking dog) were then followed, with or without drug and hormone manipulation. Results in both lactating and non-lactating animals indicated shortcomings in the use of cortisol as a stress indicator. Infusing prolactin and oxytocin into either the posterior pituitary or the paraventricular nucleus of the hypothalamus suppressed cortisol responsiveness to stress in both lactating and non-lactating animals (the latter to a greater extent). In the absence of drugs, lactating animals had a slightly higher basal level of cortisol and a lower cortisol response to stress than their non-lactating counterparts. Despite suppression of cortisol responses, with or without drugs, other indicators of stress still changed with the presence of a barking dog, suggesting the complexity of control involved in stress responses.

Four mid-lactation Holstein dairy cows (mean milk yield on day of experiments 26·1 kg/d) were used in a series of experiments to establish the contribution of non-insulin-mediated glucose uptake to total glucose uptake at basal insulin concentrations. A secondary objective was to determine whether somatostatin affects the action of infused insulin. In part I of the experiment a primed continuous infusion of [6,6-2H]glucose (45·2 μg/kg per min) was begun at time 0 and continued for 5 h. After 3 h of [6,6-2H]glucose infusion (basal period) a primed continuous infusion of insulin (0·001 i.u./kg per min) was administered for 2 h. Coincident with the insulin infusion, normal glucose was also infused in order to maintain the plasma glucose concentration at euglycaemia. Part II of the experiment was the same as part I except that somatostatin was infused for 2 h (0·333 μg/kg per min) instead of insulin. In part III of the experiment both insulin and somatostatin were infused for the final 2 h. Plasma insulin levels were increased by insulin infusion (to 0·1476 and 0·1290 i.u./l for parts I and III respectively) and were reduced by somatostatin infusion in part II (to 0·006 i.u./l) relative to the basal periods (mean 0·021 i.u./l). Glucose uptake during somatostatin infusion (2·50 mg/kg per min; part II) was 92·0% of that observed in the respective basal period (2·72 mg/kg per min). Circulating insulin levels were much lower than the dose of insulin that causes a half maximal effect on glucose uptake (0·06–0·10 i.u./l for ruminants); consequently insulin-mediated glucose uptake was probably absent in part II. Secondly, glucose uptake following insulin only infusion (4·05 mg/kg per min) was significantly lower than that observed when insulin plus somatostatin was infused (4·69 mg/kg per min), indicating that somatostatin either directly or indirectly enhanced the action of insulin on glucose uptake.

The physicochemical characteristics and the renneting properties of milks from Lacaune ewes are reported. The ewes belonged to divergent genetic lines termed ‘low’ or ‘high’ and were selected on a combination of fat and protein yield. The 179 second to fifth parity ewes were sampled three times during lactation at about 50, 135 and 175 d in milk. Physicochemical and renneting variables were principally influenced by days in milk and somatic cell count, whereas β-lactoglobulin genotype, parity and divergent lines had relatively few effects. Fat and protein content increased throughout lactation, whereas rennet gel firming rate decreased. Salt distribution and micellar characteristics were also significantly affected by days in milk. Milks with high somatic cell count were characterized by high values of pH and soluble nitrogen content and rennet coagulation properties were significantly affected at >3×105−5×105 cells/ml milk. Divergent lines had a strong effect on milk yield, but no effect on milk composition consistent with the selection criteria. Gel firming rate was slightly higher in milks from high yield ewes. Protein content, Ca concentration, pH and micelle characteristics were the main factors related to variations in the renneting properties of the milks.

The aim of this work was to determine by ion-exchange liquid chromatography and isotope ratio mass spectrometry the specific 15N enrichment of amino acids in casein and plasma proteins in cows receiving three successive daily oral doses (300, 150 and 150 g) of (15NH4)2SO4 (10 atom per cent isotopic enrichment) and to examine the 15N enrichments obtained with regard to nitrogen transport and metabolism in the lactating cow. To investigate the 15N distribution in amino acids in casein and in plasma proteins, samples of 15N-labelled casein and plasma proteins were extracted either from a pool of several milkings (36–96 h after starting to administer the tracer) or from pooled venous blood (removed on the fourth day after the start of administration) from the four lactating cows. 15N enrichments of the proteins studied, expressed as atoms percent excess, were 0·2509 for casein and 0·0577 for plasma protein. Chromatographic fractionation of the amino acid mixture (protein hydrolysates) resulted in nine groups containing between one and four amino acids: Asp, Ser and Thr; Glu; Pro; Gly, Ala, Val and Met; Ileu and Leu; Tyr; Phe; His and Lys; and Arg. High 15N incorporation was demonstrated in all individual or groups of amino acids studied. In both proteins, Glu appeared to be the most enriched amino acid, Phe and Arg the least enriched. Most aliphatic molecules with a single amino group were highly enriched. The much lower (3·5–7·7-fold) enrichments in plasma protein compared with casein suggest considerable intracellular dilution at the site of liver protein synthesis. Finally, the amino acid separation methods are discussed and suggestions for improving them considered.

The peptide Pro130–Thr–Ser–Thr–Pro–Thr–Ile–Glu–Ala–Val–Glu140–Ser–Thr–Val–Ala–Thr–Leu–Glu–Ala–Ser–Pro150–Glu–Val–Ile, which corresponds to residues 130–153 of κ-casein B, was synthesized and the conformation of the peptide in solution investigated by circular dichroism (CD) spectroscopy, structure prediction algorithms and 1H-nuclear magnetic resonance spectroscopy. In a solution containing the structure-enhancing solvent trifluoroethanol the CD spectrum was typical of a peptide in the α-helical conformation and nuclear magnetic resonance showed that the amino acids between Ile136and Ser149 (κ-casein numbering) were predominantly in the α-helical conformation, but that Pro130 to Thr135 and Pro150 to Ile153 were not. In addition, Thr133–Pro134 and Ser149–Pro150 were primarily in the trans conformation, the residues from Thr131 to Thr135 were in unordered structures and the residues from Glu151 to Ile153 were in an extended conformation. Residues Glu137 to Glu140 and Thr145 to Ala148 also displayed some 310-helix character. When the peptide was dissolved in 10 mM-cetyltrimethylammonium chloride solution at pH 6, the CD spectra indicated that the proportion of helical structure was comparable to that of the peptide in trifluoroethanol solution (400 ml/l), whereas when the peptide was dissolved in buffer alone or in 10 mM-SDS solution, the CD spectra were consistent with a low helical content. Acidification of these solutions to pH 2·85 resulted in a slight increase in the helical content of the peptide in buffer and more markedly in buffer containing SDS. When the peptide was in 5 mM-CaCl2 solution at neutral pH, the CD spectrum indicated that some ordered structure was present. Taken together these results indicate that the ionizable residues Glu137, Glu140, Glu147 and Glu151 could be important in determining the stability of the putative helix. The structure predictions found that the sequence from Glu137 to Pro150 would be more likely to be in a helical than any other conformation in the intact bovine protein, but that pig, sheep and goat κ-caseins did not give a prediction of a strongly helical region in this part of the molecule.

Aminopeptidase P was purified 65·3-fold from the cytoplasm of Lactococcus lactis subsp. cremoris AM2 with a 5·8% yield. The purified enzyme was found to consist of one polypeptide chain with a relative molecular mass of 41600. Metal chelating agents were found to be inhibitory and Mn2+ and Co2+ stimulated activity 7-fold and 6-fold respectively. The purified enzyme removed the N-terminal amino acid from peptides only where proline (and in one case alanine) was present in the penultimate position. No hydrolysis was observed either with dipeptides even when proline was present in the C-terminal position or when either N-terminal proline or pyroglutamate was present preceding a proline residue in the penultimate position of longer peptides. On the basis of this substrate specificity either aminopeptidase P or post-proline dipeptidyl aminopeptidase are necessary along with a broad specificity aminopeptidase to effect complete hydrolysis of casein-derived peptides containing a single internally placed proline residue. However, both aminopeptidase P and post-proline dipeptidyl aminopeptidase would be required together with a broad specificity aminopeptidase in order to completely hydrolyse casein-derived peptides that contain two internally placed consecutive proline residues. As bitter casein-derived peptides are likely to contain either single prolines or pairs of prolines, aminopeptidase P appears to be an important enzyme for debittering.

In all, 4379 isolates from 35 products, including 24 artisanal cheeses, were surveyed with a view to identifying strains that could be used as starters in commercial dairy fermentations. Of the isolates, 38% were classified as Lactococcus, 17% as Enterococcus, 14% as Streptococcus thermophilus, 12% as mesophilic Lactobacillus, 10% as Leuconostoc and 9% as thermophilic Lactobacillus. Acid production by the isolates varied considerably. Of the 1582 isolates of Lactococcus and 482 isolates of mesophilic Lactobacillus tested, only 8 and 2% respectively produced sufficient acid to lower the pH of milk to <5·3 in 6 h at 30°C. In contrast, 53, 32 and 13% of Str. thermophilus, thermophilic Lactobacillus and Enterococcus isolates respectively reduced the pH to 5·3. These isolates were found only in some French, Italian and Greek cheeses. Bacteriocins were produced by 11% of the 2257 isolates tested and 26 of them produced broad-spectrum bacteriocins which inhibited at least eight of the ten target strains used, which included lactic acid bacteria, clostridia and Listeria innocua. The most proteolytic of the 2469 isolates tested were Str. thermophilus from Fontina cheese followed by Enterococcus from Fiore Sardo and Toma cheese and thermophilic Lactobacillus from all sources. Exopolysaccharides were produced by 5·3% of the 2224 isolates tested.

Confocal laser scanning microscopy was used to examine components of whole milk powder. Powder was dispersed in a glycerol suspension containing a fat-soluble fluorescent dye which was excited by laser light to produce images of the fat at the surface of and inside the whole milk powder particle. Lactose crystals, produced in the milk prior to spray drying, were seen to be in the diamond-like, α-hydrate form. The surface coverage and the stability of a wetting agent sprayed on to the surface of whole milk powder particles were examined. The wetting agent was shown to be located as a thin surface layer that partly migrated into creases and folds on the particle surface during storage at 37°C.

Several peptides were isolated from the diafiltration retentate, prepared using 10 kDa membranes, of the water-soluble extract from a commercial mature Cheddar cheese and identified by amino acid sequencing and mass spectrometry. Most of the peptides were from the N-terminal half of β-casein, but peptides from αs1- and αs2-caseins were also identified; the extract also contained α-lactalbumin. Identified peptides showed the important role played by lactococcal cell envelope proteinases in the degradation of primary proteolytic products from αs1- and β- caseins, produced by chymosin and plasmin respectively. Plasmin seemed to be involved in the hydrolysis of αs2-casein. Several phosphopeptides were identified and the action of phosphatase on these peptides was evident.

Two methods were compared for the determination of free fatty acids (FFA) from acetic to long-chain acids in samples with a large excess of triacylglycerols (TG) (1[ratio ]200, w/w), such as cheese and other dairy products. In method 1, after fat extraction, FFA were separated from TG by aminopropyl-bonded phase chromatography, injecting the fraction containing FFA directly into the gas chromatograph. In method 2, extracted fat was treated with tetramethylammonium hydroxide, the methyl ester derivatives being formed in the injector. Cheese samples and standard mixtures of FFA and TG in different proportions were analysed by both methods. The cheese sample contained 2·4 times more FFA when analysed by method 2 as compared with the result obtained with method 1. The composition of the standard mixtures analysed by method 1 closely reflected that of the original mixture and gave 90–100% recovery of FFA, regardless of their chain length and the ratio of FFA[ratio ]TG (1[ratio ]1 or 1[ratio ]200, w/w). The composition of samples with a FFA[ratio ]TG ratio of 1[ratio ]200 (w/v) was severely distorted (as compared with the original composition of the sample) when analysed by method 2. Varying recoveries of FFA were also obtained, the largest differences being found for the shorter-chain components. We conclude that the FFA fraction should be separated from the TG fraction before derivatization and chromatographic analysis, particularly for samples in which the FFA represent a minor fraction of the TG.

The possible hypocholesterolaemic properties of milk and fermented milk products have been investigated in groups of albino rats given a basal diet, basal diet plus cholesterol, and basal diet plus cholesterol together with whole milk or standard or bifidus yogurt. The yogurts were fortified with skim milk powder, condensed whey or lactose-hydrolysed condensed whey. After 30 d, triacylglycerols, total cholesterol, HDL-cholesterol and LDL-cholesterol were measured in serum. Whole milk and ordinary yogurt had no hypocholesterolaemic effect, but standard yogurt containing lactose-hydrolysed condensed whey and all bifidus yogurts lowered serum cholesterol. In general, yogurts changed HDL-cholesterol little, but tended to raise triacylglycerols. There was marked lowering of LDL-cholesterol in rats given either type of yogurt fortified with whey proteins. This study has demonstrated in a rat model that bifidus yogurts and yogurts fortified with whey proteins can reduce total and LDL-cholesterol, and suggests that if they have the same effect in human subjects they have potential value in cholesterol-lowering diets.

The whey proteins of the cow, goat and sheep have previously been fractionated on an analytical scale by reversed-phase HPLC (De Frutos et al. 1992), anion-exchange FPLC (Andrews et al. 1985; Manji et al. 1985; Laezza et al. 1991) and gel permeation FPLC (Andrews et al. 1985; Hill & Kakuda, 1990). Anion-exchange and gel permeation FPLC can readily be scaled up for laboratory preparation of whey protein fractions. There is some indication, however, that anion-exchange FPLC does not give complete separation of β-lactoglobulin and α-lactalbumin from the other minor whey protein fractions (Girardet et al. 1989).

In previous work it has been shown that gel permeation FPLC gives a satisfactory fractionation of the whey proteins of the cow (Law et al. 1993), goat (Law & Brown, 1994) and sheep (Law, 1995). In this paper we describe a scaled-up method of gel permeation that can be used for fairly rapid preparation or purification of four main whey protein fractions from the milks of these species.

Lipolysis has many effects in dairy technology. Undesirable consequences include deleterious effects on the organoleptic properties of milk and some cheeses; however, it is desirable in some other cheeses.

The usual methods for detecting lipolysis in milk measure the amounts of free fatty acids liberated from triacylglycerols (International Dairy Federation, 1991). Lipase activity is detected by incubation of samples with substrates such as tributyrin (Castberg et al. 1975) or triolein (Nilsson-Ehle & Schotz, 1976; Shirai et al. 1982). A few reports describe methods using synthetic chromogenic substrates, e.g. p-nitrophenyl butyrate (Shirai & Jackson, 1982), β-naphthyl caprylate (McKellar, 1986; McKellar & Cholette, 1986) or 4-methylumbelliferyl oleate (Stead, 1983). Tsakalidou et al. (1992) have developed the detection of esterase activities on electrophoresis gels.

The camel population of Saudi Arabia is ∼607000 out of a world population of 17 million. All are dromedaries (Camelus dromedarius) of four major ecotypes (Elamin & Wilcox, 1992), giving daily milk yields of 3·5–35·0 kg/animal (Knoess, 1977). Camel milk is low in lactose compared with cows' milk (Elamin & Wilcox, 1992). However, levels of potassium, magnesium, iron, copper, manganese, sodium and zinc are higher than in cows' milk (Sawaya et al. 1984; Abu-Lehia, 1987). Farah (1993) found low levels of potassium and phosphorus in Egyptian dromedaries. There is little difference between the milk of dromedaries and that of bactrian camels (Kheraskov, 1961).

In the Gulf region and some southern Mediterranean countries, substantial amounts of camel milk are consumed by the local people. The investigation reported here was undertaken to shed more light on the nutritional contribution made by camel milk.

Lameness is a major welfare concern in dairy cattle. Estimates of the annual incidence of lameness range from 4 to 30%, and even in well managed herds as many as 15% of animals can be affected (Esselmont, 1990). In addition to the cost in animal suffering, lameness is accompanied by loss of production on a scale comparable, in temperate countries, with that caused by mastitis. Lost production, veterinary charges and milk discard costs coupled with reduced fertility or premature culling in turn make lameness a major economic factor in dairy farming. In the UK alone, the estimated cost in lost production is £44–£90 million per annum, equivalent to £10–20 per cow (Booth, 1989; Esselmont, 1990).