In a national survey of US dairy operations, 1685 dairy operations reported 47057 new dairy heifers (either births or acquisitions) and 4427 deaths (9·4%) of preweaned dairy heifer calves over a 3 month period. Stepwise logistic regression was used to identify management practices associated with high mortality among preweaned heifers in dairy operations where at least three dairy heifer calves were born alive or moved on to the operation. Analysis was done twice: once by separating all operations by size into high or low mortality; again using only operations with <2 and >10% mortality to eliminate dairy operations with intermediate levels of mortality from the comparisons. Results were similar. Dairy operations in the West were more likely to fall in the high mortality category than dairy operations in the rest of the country. In addition, the following dairy operation characteristics were associated with high death levels in both models: rolling herd average milk production <7710 kg, preweaned heifers placed in groups of seven or more, a male having primary responsibility for the care and feeding of preweaned heifers, calves not receiving hay or other roughages until >20 d old, calves fed on mastitic or antibiotic milk after colostrum and calves not given whole milk after colostrum.

The Dairy Herd Improvement Fund of Victoria recently identified a requirement for a simple and inexpensive in-line sampler to enable dairy farmers to collect representative milk samples for counting somatic cells. We found that the currently available simple in-line milk samplers, when connected to standard 35 ml collection vessels, terminate sampling early in a milking, and thus provide samples that are unrepresentative of the whole milking. We showed that cell count during a milking varies greatly, tending to be high for the first 1–2 l. Analyses of resulting samples will thus tend to overestimate cell counts if samplers are used in their traditional way. We found greater sampling rates in high-line compared with low-line milking systems, and consequently developed modified samplers suitable for both situations. Our samplers utilize low sampling rates (∼1–3%) and large collection vessels (450 ml). Compared with currently available simple in-line samplers, our type of sampler provided milk samples considerably more representative of the entire milking for the majority of cows. In conjunction with subsampling, they provided samples of appropriate size (12·5 ml minimum to 25 ml maximum) for testing fat, protein, lactose and cell count. Cell count results indicated that errors associated with the use of currently available simple in-line samplers could frequently be >200%. In contrast, we found that use of our samplers gave an estimate for cell count that was only slightly higher (mean 20%) than that from samples collected by an approved Tru-Test sampler.

Danish organic dairy production is characterized by a low input of antibiotics for udder treatment and a high input of other mastitis control procedures. A study was conducted in 14 organic dairy herds with the objectives of obtaining a comprehensive description of clinical mastitis cases and identifying characteristic patterns in these results. Clinical signs, inflammatory reactions and microbiological identifications were obtained from 367 cases of clinical mastitis occurring over 18 months. Cow characteristics and preincident values such as milk yield and somatic cell count were obtained for each cow. Signs of previous udder inflammation were present in two-thirds of the clinical mastitis cases. Severe local inflammatory reactions were found in 21% of the cases and some indication of generalized signs such as fever and reduced appetite were found in 35% of the cases. Logistic regression analyses were performed based on the results of an initial (exploratory) multiple correspondence analysis. Coliform mastitis (6% of the cases) was rarely preceded by pathogen isolation or inflammatory reactions in the same quarter. Coliform mastitis cases usually occurred in one quarter only. Escherichia coli infections were typically (truly) acute cases. Bacteriologically negative mastitis (20% of the cases) showed strong similarities with clinical coliform mastitis. Staphylococcus aureus cases (18% of the cases) occurred most frequently in late lactation or around drying-off. Prior isolation of Staph. aureus and slight decreases in milk yield were two factors that interacted but both were strongly and positively related to clinical Staph. aureus. Staph. aureus mastitis typically had a subclinical debut, and increasing degrees and duration of inflammation decreased shedding of this pathogen. Streptococcus dysgalactiae (9% of the cases) mastitis was typically persistent, virulent and manifest in periods of lower cow resistance. More patterns of subclinical and clinical Str. uberis mastitis (23% of the cases) seemed to be present.

The transepithelial potential difference (TEPD) and resistance (TER) in the alveoli, and the pressure in the mammary gland were measured in lactating mice at different times after they had finished suckling their young. Oxytocin administration caused a rise in TEPD, the amplitude of which decreased with time after cessation of the pups feeding. At 2·5 h after cessation, oxytocin administration caused a short-term decrease in TER, but at 5 h there was a two phase reaction: a fall in TER followed by a rise. At 20 h there was a one phase reaction, a rise in TER. TEPD in the mammary gland alveoli increased from its initial value of 18±1 mV to 25±1 mV at 2·5 h, and thereafter fell to zero by 15 h after suckling. The initial value of TER, 114±1 kΩ, increased to 131±4 kΩ at 2·5 h and then decreased by 5 h after suckling, but unlike TEPD it did not reach zero. These experiments show that extending the period after cessation of suckling does not cause a rise in intramammary pressure and that oxytocin injections cause a short-term rise in milk pressure. From these results, we propose a hypothesis of reducing ionic concentration gradients between intracellular fluid, cytoplasm of the secretory cell and milk at extended periods after cessation of suckling, starting at 2·5 h.

The importance of milk proteins and the positive effect of administration of growth hormone (GH) on milk production, and the presence in some dairy cattle lines of greater GH concentrations prompted us to examine the presence of restriction fragment length polymorphism at the GH gene using the restriction enzyme TaqI and to investigate associations between this polymorphism in Simmental cows and bulls, as well as milk protein variants in Simmental cows, and milk production traits. Blood and milk were sampled from 279 Italian Simmental cows and semen was collected from 148 bulls of the same breed. Two fragment bands, denoted A and B, of 6200 and 5200 bp respectively, were examined and three patterns, AA, AB and BB, were found in both animal samples. All variants previously reported in other studies, for κ, β, and αs1-caseins, and β-lactoglobulin, were found in the cows' samples. For the cows' samples, a BLUP (Best Linear Unbiased Predictor) analysis of results was performed using a REML (Restricted Maximum Likelihood) program and known heritabilities, whereas for bulls we have performed a General Linear Model analysis. The effect of GH gene polymorphism, using TaqI restriction enzyme, on milk production traits was not significant, but bulls of BB pattern had a higher breeding value for milk yield than AA bulls (P<0·05). For the κ-casein genotypic effects, cows of AB genotype gave milk with 1·53±0·70 g/kg less fat than cows of AA genotype. In addition, breeding values for milk protein content were significantly higher in BB bulls, with 0·87±0·32 and 0·71±0·34 g/kg more milk protein than AA and AB bulls respectively. Thus, our results revealed a GH gene polymorphism and indicated significant effects of milk protein polymorphisms on milk production traits in the Italian Simmental breed.

The performances of falling film evaporators used in the New Zealand dairy industry for concentrating whole milk were evaluated by determining kg steam used/kg water evaporated, and the heat transfer coefficient of each pass in the evaporators. A specially written computer program was used to calculate the results. The heat transfer coefficients varied from 0·3 to 3·0 kW/m2K, and the steam consumption from 0·10 to 0·39 kg steam/kg evaporation, depending on the number of effects. The steam consumptions for whole and skim milk were similar. The momentum of the vapours passing down the tubes, the temperature difference across the tubes, the viscosity of the feed and the liquid loading were found to be the main factors controlling the heat transfer coefficient. A correlation between the heat transfer coefficient and these factors is presented, and other factors likely to have an influence on the performance are discussed. The correlation is compared with that obtained for skim milk.

Heavy coagulum formation in lactic casein manufacture results in high casein concentration, high starter bacteria concentration and low pH on the base of the coagulation vessel. No heavy coagulum was formed during slow coagulation of skim milk with glucono-δ-lactone, indicating that starter bacteria were necessary. Skim milk preheat treatments severe enough to cause denaturation of the more heat- stable whey proteins, α-lactalbumin and β-lactoglobulin, also caused a reduction in heavy coagulum formation, suggesting the heat inactivation of a co-factor involved in the process. These results suggest a mechanism for heavy coagulum formation similar to that for minor sludge formation in cottage cheese manufacture. Starter bacteria would clump together, probably assisted by a co-factor. These starter clumps would generate a region of low pH causing isoelectric precipitation of casein in and around the clumps. The clumps would then settle forming a layer on the base of the coagulation vessel which might become further compressed by gravity.

Two sandwich enzyme-linked immunosorbent assays (ELISA) have been developed for quantitation of bovine milk plasminogen and plasmin. The assays used two monoclonal antibodies, one specific for plasminogen and the other specific for plasminogen plus plasmin. Plasmin concentration was obtained by subtracting the first concentration from the second. The assays were sensitive (linear range, 5–75 ng/ml), repeatable (CV, 8 and 5% for plasmin and plasminogen titration respectively), specific (no cross reactivity with the major milk proteins) and directly applicable to skimmed milk with no particular pretreatment of the sample. However, ELISA did not permit complete titration of milk plasminogen and plasmin (only 60–90% could be quantified). This lack of accuracy was due to casein interfering with the plasmin–plasminogen titration by ELISA. Results obtained with ELISA or an enzymic technique on 20 milk samples collected from individual cows milked throughout pregnancy showed that the ELISA was particularly suitable for analysis of late lactation or mastitic milk where proteinase inhibitors interfered with enzymic quantitation.

Escherichia coli O157[ratio ]H7 in spiked samples of raw milk and ice-cream was enriched in tryptic soy broth for 4 h, captured by immunomagnetic separation, subjected to amplification by polymerase chain reaction of parts of the verotoxin genes (SLT-I and SLT-II), and detected by agarose gel electrophoresis. Using this method, as few as 1 cfu Esch. coli O157[ratio ]H7/g food could be detected in <10 h.

Growth of Leuconostoc mesenteroides in milk was studied with respect to the proteinase and peptidase activities of the strains and their nutritional requirements. Ln. mesenteroides grew poorly in milk since none of the 14 strains studied exceeded 5×108 cfu/ml at the end of growth. Few strains displayed proteinase activity, and this did not contribute much to growth. The pattern of peptidase activities varied with the strain. Nitrogen starvation and a high requirement for Mn2+ were involved in the cause of growth deficiencies. Addition of amino acids, 50 mg Mg2+/l and 0·08–0·49 mg Mn2+/l stimulated growth of most leuconostoc strains up to 5×108 cfu/ml. Addition of 5 g glucose/l to milk containing amino acids, Mg2+ and Mn2+ or yeast extract stimulated the growth of seven and eight strains respectively up to 109 cfu/ml. No growth advantage was found in a N2 atmosphere. However, the addition of small amounts of Mn2+ to milk suppressed the inhibitory effect of aeration on the growth of Ln. mesenteroides UD23, suggesting a protective role of Mn2+ against O2 toxicity.

Acid aspartyl proteinase of Penicillium roqueforti was purified from culture filtrates using FPLC ion-exchange chromatography on Mono Q and size exclusion chromatography on Superdex 75. The purity obtained allowed us to produce, using rabbits, a specific antiserum which was then used to develop a sandwich ELISA. The test was sensitive (detection limit, 0·25 ng/ml), reproducible (CV, 3·1–6·9%) and closely correlated with enzyme activity measurement (r=0·995, P<0·001). This ELISA should be a valuable tool for monitoring acid aspartyl proteinase level in culture filtrates and for determining the enzyme activity[ratio ]enzyme protein ratio of acid aspartyl proteinase obtained after genetic recombinations or site-directed mutagenesis.

Microstructural and textural characteristics of rennet curd made from rehydrated non-fat dried milk standardized to 40 g fat/l were compared with those of non-fat curd made with capsule-forming non-ropy lactic culture. Microstructure examination using confocal scanning laser microscopy indicated that the presence of a capsule-forming culture produced an open casein network with large pores. Distribution of fat globules was observed within the undisturbed casein structure after staining with a fluorescent lipophilic dye which was added to the milk before coagulation. Bacterial capsules and milk fat reduced curd tension and firmness of rennet curd to similar levels.

Some of the textural changes that occur early in cheese maturation may be related to the redistribution of water within the cheese matrix. To examine this, a model cheese curd system was devised and explored. Initially, cheese curd was prepared using starter and chymosin and the curd pH was controlled by varying the draining and salting pH values. The quantity of serum that could be centrifuged from the resultant curd was less for lower pH curd and decreased in volume with time. The curd pH decreased with time. In the protocol finally adopted, milk was acidified with lactic acid and coagulated with Rennilase 46L. After cheddaring, salting and light pressing, the samples of this curd were finely diced and mixed with glucono-δ-lactone to give curd samples with comparable moisture contents, similar casein proteolysis rates but different pH values. The quantity of serum that could be centrifuged from these samples was greater for pH 5·6 curd than for pH 5·2 curd and decreased faster for the lower pH curd. Neither the curd moisture nor the pH changed significantly during curd storage and the casein proteolysis was low. These results for the model curd system are consistent with known water absorption characteristics of casein curd under ‘equilibrium’ conditions and the effects of pH and mineral salts on this absorption. It was concluded that, during the early stages of cheese ripening, there may be a redistribution of moisture within the cheese, related to the basic properties of the protein matrix and the transient effects of curd salting, rather than as a direct consequence of glycolytic and proteolytic changes.

An optimum or a high heat treatment of milk before yogurt preparation was applied according to an experimental design with the deliberate aim of making stirred yogurts with varying properties. Images of sections of yogurt samples were obtained using a transmission electron microscope. These were digitized and segmented via a threshold determination to separate the protein structure from the background. Four spatial statistical estimates were determined in order to evaluate their applicability for characterization of yogurt microstructure. The volume content was estimated and compared with the protein content obtained through chemical analysis. Heat treatment had no significant effect on volume content, as expected. The surface area of the protein aggregates was a useful measure to describe the difference between casein networks obtained by an optimum and a high heat treatment. The star volume gave a measure of the average local size of pores in the yogurt gel. The optimum heat treatment resulted in yogurt with small values for the star volume, indicating a less coarse structure of the casein network compared with yogurt prepared from high heat milk. The covariance function was able to differentiate between yogurts made using two types of heat treatment.

Lowering the lactose content of milk in vivo has two potential benefits: to develop milk products for lactose intolerant people (Delmont, 1983) and to produce a more concentrated milk, reducing milk storage volume and milking frequency without affecting the overall fat and protein yields. Lactose synthesis in the lactating mammary gland results from the interaction of α-lactalbumin (α-la) with a Golgian UDP-galactosyltransferase (EC 2.4.1.22), and it has been suggested that lactose and α-la contents are directly related (Fitzgerald et al. 1970). Evidence that α-la is the only protein responsible for the in vivo induction of lactose synthesis came recently from knock-out experiments of the relevant gene (Stinnakre et al. 1994; Stacey et al. 1995), which disrupt lactation, as lactose-deprived milk of α-la-deficient mice is highly viscous. However, although the lactose, fat and protein contents of the milk were found to be directly related to the quantity of α-la present (Stinnakre et al. 1994), only in mice homozygous for the α-la null allele was there a significant alteration of milk concentration (Stacey et al. 1995). Stacey et al. (1995) also demonstrated that human α-la could substitute for its mouse counterpart via homologous recombination.

In the present study, we report the generation of transgenic mice producing only bovine α-la by back-crossing animals from four independent bovine α-la transgenic lines with α-la-deficient mice. Our work confirms that foreign α-la can substitute for the mouse protein and suggests a negative correlation between the α-la content and protein and fat concentrations in milk.

Milk proteins have largely been considered as providing essential amino acids, but oligopeptides derived from milk proteins have been shown to possess biological functions. In vitro, opioid activity was first reported in bovine β-casein hydrolysate by Brantl et al. (1979). Precursors of biologically active peptides have been demonstrated in vivo after digestion of milk (Scanff et al. 1992). Peptides that inhibit platelet aggregation (Fiat et al. 1989), stimulate the immune system (Migliore-Samour et al. 1989), inhibit angiotensin I converting enzyme (Maruyama & Suzuki, 1982) and are involved in intestinal Ca solubilization and absorption (Sato et al. 1986) have also been isolated from bovine casein hydrolysates.

Bioactive peptides from whey proteins and their physiological effects have received less attention than those from casein. Peptides with opiate-like activity include the lactorphins, residues 50–53 in bovine and human α-lactalbumin and 102–105 in bovine β-lactoglobulin (β-lg) (Chiba & Yoshikawa, 1986; Yoshikawa et al. 1986; Antila et al. 1991). Yamauchi (1992) has reported that a peptide derived from β-lg induced contraction of guinea pig ileum longitudinal muscle in the absence of electric stimulation and agonist. The fragment, containing residues 146–149 of β-lg (His–Ile–Arg–Leu), was called β-lactotensin.

The purpose of this study was to determine whether β-lactotensin was released from bovine β-lg by in vitro proteolysis using different proteolytic enzymes. The pharmacological activity of this tetrapeptide was characterized in guinea pig ileum in vitro using a synthetic fragment.

Success in making a cheese acceptable in both flavour and texture depends partly on curd properties which determine the retention of fat and moisture and, thus, cheese yield and composition (Green & Grandison, 1993). When considering farmhouse and/or Appelation d'Origine Contrôlée cheese where milk treatments are not allowed, curd properties depend only on milk composition and clotting characteristics, which are subject to wide fluctuations over the year owing to the cow's stage of lactation and nutritional factors (O'Keeffe, 1984; Macheboeuf et al. 1993). These variations often result in seasonal differences in the chemical or sensory properties of the cheese (Kefford et al. 1995) which are sometimes difficult to understand (Grandison et al. 1985). Only a few studies have assessed the cheesemaking quality of milk by actually making ripened cheese; most studies report milk coagulation properties and curd firmness.

The object of this work was to study the influence of the chemical composition and clotting characteristics of the milk on the chemical and sensory properties of ripened cheeses when milk characteristics vary widely.