For 17 phages active against Streptococcus cremoris, Str. lactis and Str. lactis subsp. diacetylactis, the killing efficiency of pasteurization (log No/N) at 72°C for 15 s in skim-milk showed large variations from > 6 to 0; the efficienty of killing during spray-drying ranged from 3.7 to 0.2 and phages survived well storage of milk powder at room temperature. Destruction in a heat exchanger was found to be greater than that calculated from biphasic curves obtained by heating phages in sealed ampoules. No relationship was established between lytic classification of phages and their heat resistance.

A fraction derived from the membrane of milk fat globules (MFGM) was isolated from fresh raw cream. The adsorption of MFGM at the interface of oil-in-water emulsions was studied as well as the stability of the emulsions under different conditions (pH, temperature treatment) and the surface potential. The emulsions prepared with MFGM isolates were stable at neutral pH and were destabilized at low pH. By analysis of the protein adsorbed at the interface by polyacrylamide gel electrophoresis, we also determined that the MFGM that covered the surface of the oil-in-water emulsion could not be displaced by surfactants such as Tweens and Triton X-100 and was not affected by the presence of other proteins such as caseins and β-lactoglobulin added to the emulsion. A strong molecular interaction between the adsorbed MFGM, composed of phospholipid and protein, may exist at the interface, rendering the newly formed membrane very little affected by the presence of other surfactants. This MFGM-stabilized emulsion was characterized by a behaviour very different from that of emulsions stabilized by other milk proteins.

Effects of systematic environmental factors and milk production and quality traits on milk coagulation properties (MCP), and on repeatability of those traits were estimated from 979 milk samples collected once a month over a period of 2 years from 83 Finnish Ayrshire cows. Estimation was based on a multitrait animal model and REML methodology. In addition, persistence of non-coagulation of milk in individual cows, and factors associated with it were established from a sub sample of 24 cows producing non-coagulating (NC) milk at least once. MCP were at their best during the first lactation, at the beginning and at the end of lactation, and during grazing seasons. Variation in MCP with systematic environmental factors was partly due to variation in composition and quality of milk, especially in pH and ln (somatic cell count, SCC). Coefficients of repeatability for milk coagulation time and curd firmness were 0·65 and 0·68. These estimates were of the same magnitude as those for protein content, but were higher than those for daily milk yield, fat content, pH, and SCC. Based on the repeatability estimates for the milk coagulation traits and effects of the environmental factors, cows should be sampled at least three times during a lactation to estimate reliably breeding values for the milk coagulation traits. A total of 10% of the milk samples did not coagulate in 30 min after addition of rennet. Cows that produced NC milk at least once (30% of the cows) could be classified into those that produced NC milk only a few times during a lactation and those that produced NC milk at almost every sampling. Based on logistic regression analyses, peak and mid-lactation, high milk yield, low protein and fat content and high pH increased the risk of non-coagulation of milk.

Cell-free extracts of both Lactobacillus bulgaricus and L. acidophilus demonstrated threonine aldolase activity, the end product of which was acetaldehyde, the major flavour compound of yoghurt. L. acidophilus also possessed an alcohol dehydrogenase activity capable of reducing acetaldehyde so that little yoghurt flavour was present in milks fermentation with this organism. Addition of threonine to fortified milk before fermentation with L. acidophilus increased acetaldehyde production and resulted in a well flavoured product similar to that of yoghurt made with L. bulgaricus. The contribution of these 2 enzymes to flavour production is discussed.

The formation of protein polymers in ultra-high-temperature (UHT)-treated milk during storage at various temperatures was examined by gel filtration. The extent of polymer formation was found to depend on both storage time and temperature. After some months of storage at the higher temperatures of 30 and 37°C, the extent of polymerization of the caseins and whey proteins due to reactions of the Maillard type was several times greater than the heat-induced changes resulting from the UHT processing itself. After storage for 6 months the following proportions of milk proteins were found to exist in the form of covalently bound polymers: 50% at 37°C, 40% at 30°C, 26% at 20°C and 21% at 4°C. In addition, further amounts of polymer were formed by disulphide bonding, the contribution of such polymers diminishing gradually in a temperature-dependent manner during storage due to continuing polymerization reactions. It appeared that αs1-casein may be preferentially involved in these polymerizations with β-casein reacting at a somewhat slower rate. Polymerization and associated reactions modifying molecular charge led to the expected alterations in electrophoretic mobility and a loss of definition in the bands due to the various protein components. Measurements of proteolytic breakdown indicated only very small increases in trichloroacetic acidsoluble fragments, suggesting that proteolysis was of minor significance.

Prolidase activity from cytoplasm of Lactococcus lactis subsp. cremoris (Streptococcus cremoris) AM2 was partially purified. The enzyme had Mr 42000 and optimum activity between pH 7·35 and 8·25 in citrate, phosphate and borate buffers while in a universal buffer system an optimum pH between 8·3 and 9·0 was observed. The activity was strongly inhibited by the chelating agents E.DTA, 1,10-phenanthroline and 8-hydroxyquinoline. Inhibition was also noted with dithio-threitol, N-ethylmaleimide and bacitracin. The enzyme was active on all amino-acylproline substrates tested except Gly-Pro and Gip-Pro and also showed activity against Pro-Pro. While most prolyl amino acids tested were not hydrolysed, hydrolysis was noted with Pro-Ala and Pro-Val. Km values of 20 mM and 10 mM were obtained with Phe-Pro and Met-Pro respectively; however, substrate inhibition was observed with Ile-Pro and Leu-Pro.

The effects of decreasing pH and micellar calcium concentrations of reconstituted skim milk and caseinate solution were studied by 1H and 17O NMR spectroscopy. The proton transverse relaxation rate 1/T2 of skim milk decreased as the pH decreased, reaching a minimum at pH 5·3. However, as the pH fell sodium caseinate solution showed a continuous increase in 1/T2, with no minimum. Analysis of proton relaxation as a function of the interpulse time in the CPMG (Carr-Purcell-Meiboom-Gill) sequence demonstrated that both the proton exchange mechanism and ‘bound’ water contributed to proton relaxation in skim milk. The study of 17O relaxation rate as a function of pH confirmed the change in protein hydration upon acidification. Increasing the amount of EDTA showed that the proton transverse relaxation rate of skim milk decreased until a plateau was reached when the micellar calcium was totally solubilized. With excess EDTA the relaxation rates of skim milk and caseinate solution were identical. A strong correlation was also found between the pH dependent relaxation rate and the solubilization of micellar phosphorus as detected by 31P NMR. Together, these results suggested that aggregation of caseins by calcium and colloidal calcium phosphate is mainly responsible for the excess hydration in skim milk micelles.

An objective test has been developed to help in assessing the value, as a method for determining the heat stability of milk, of the subjective test described by Davies & White (1966a). The objective test showed that coagulation time, as determined by the subjective test, is a reasonably accurate measure of the induction period that precedes the onset of initial rapid coagulation, but that coagulation proceeds in 2 different ways depending primarily on whether the coagulation of the milk is good (initial clots large) or poor (initial clots small). The latter finding confirms the view that in studying heat stability these 2 categories of milk should be examined separately.

The increase in levels of free fatty acids (FFA) in morning and afternoon milk after 48 h cold storage at 5 °C was determined at two stages of lactation in milk from 96 cows. Lipolysis was positively correlated to day of lactation (r = 0·6, P < 0·001), while the correlations between lipolysis and milk yield and fat content respectively were due to these factors being related to day of lactation. In afternoon milk the level of FFA increased almost three times above that in morning milk. Lipolysis was closely related to the lipoprotein lipase (LPL) activity associated with the cream fraction (r = 0·8, P < 0·001). Afternoon milk had higher LPL activity in the cream fraction than morning milk. Lipase activity in the cream increased with increasing stage of lactation. Cooling of milk increased LPL activity associated with the fat phase. This effect was greater in afternoon milk from late lactation than in morning milk from early lactation. When heparin was added to milk the LPL activity associated with the fat increased substantially; again, this effect was greater in afternoon milk from late lactation than in morning milk from early lactation. Lipolysis was higher in heparinized milk than in normal milk, and there was a close correlation between lipolysis and LPL activity associated with the fat (r = 0·82, P < 0·001). Characteristics of the milk fat globule influencing the attraction of LPL were found to be fundamentally important to lipolysis.

Reducing the colloidal calcium phosphate (CCP) content of milk by 40% or increasing it by 20% did not significantly affect the heat-induced pH-dependent dissociation of micellar κ-casein. However, changes in soluble Ca and phosphate affected the dissociation of κ-casein markedly; decreasing the phosphate concentration or increasing the Ca concentration reduced the formation of non-sedimentable N (NSN) and non-sedimentable 12% TCA-insoluble N-acetyl-neuraminic acid (NANA). Dialysis of milk against water for short periods (∼ 5 h) reduced the formation of both NSN and non-sedimentable 12% TCA-insoluble NANA, as did NaCl at concentrations above 0·05 M. Modification of protein amino groups by succinylation promoted the release of κ-casein while amidation of carboxyl groups had the opposite effect. It appears that the pH-dependent dissociation of κ-casein produced on heating milk above 90°C is controlled by electrostatic interactions. The effects of soluble ions such as Ca2+ or Na+ appear to be due to shielding of such negatively-charged groups as seryl phosphate and carboxyl on the protein, thus reducing the release of κ-casein.

The aim of the study was to describe the characteristics of cleaning procedures for milking equipment applied in intensive dairy farms in Lombardy (Italy) and to study their relationships with bacterial count of bulk milk and hygienic condition of milking machine components. A group of 22 dairy farms was visited twice (winter and summer) in order to collect bulk tank milk and post-rinse water samples and swabs from liners and milk receiver. Samples were analysed to determine: standard plate count (SPC), laboratory pasteurization count (LPC), psychrotrophic bacteria count (PBC), coliform count (CC) and Escherichia coli. Cleaning procedures were monitored using electronic milk flow meters with specific software for the measurement of the duration of each cleaning phase, circulating solution temperature and electrical conductivity, turbulence and water filling percentage of pipelines. The results showed that farms classified as high and low milk total bacteria count significantly differed both in terms of liners and receiver bacterial contamination and in terms of water temperature reached during the detergent phase of cleaning milking equipment. Significant positive correlations were found among total bacteria count in milk and bacterial contamination of the liners. Maximum water temperature reached during the cleaning cycle of milking equipment was very low (34·4±8·9°C on average); most of the observations (88·6%) corresponded to water temperatures <45°C. Cleaning temperature was related to psychrotrophic bacteria count of milk and post-rinse water and coliform count in liners. Routine check and regulation of water temperature during the washing phase of the milking machine can be a simple and effective way to control one of the main risk factors for bacteriological quality of bulk tank milk.

On the basis of complete permeability by high molecular weight reagents of casein micelles in milk and a uniform distribution of the 3 different casein subunits, a model of the micelle structure is proposed. It is composed of an average repeating unit of 1 κ-, 2 αs1;- and β-casein subunits assembled in a 3-dimensional network or branched polymer made of 130–130000 monomers, in which the trimers of κ-casein occupy the nodes and the copolymers of αs1;- and β-caseins make up the branches. All the associations between subunits are through non-covalent bonds. The chemical composition varies with the number of αs1;- and β;-casein subunits in the branches. This proposed structure is strongly supported by evidence from electron microscopy and a scale model has been made. It leads to an understanding of the role of κ-casein in micelle formation and opens new perspectives in explaining some properties of the caseins. It offers an interesting example of a new type of quaternary structure of protein subunits.

The effect of the proportion of clover in the diet (200, 500 or 800 g/kg total dry matter (DM)) on milk production of cows housed indoors and fed on a mixture of perennial rye-grass and white clover was measured in mid (Expt I) and late (Expt II) lactation. Higher clover contents increased the nutritive value of the diets, resulting in increased energy and protein intakes. DM intakes of cows offered 500 or 800 g clover/kg DM diets ad lib. (Expt I and Expt II, Period 1) were not significantly different but were 11–17% greater (P<0·05) than intakes of cows fed on 200 g clover/kg total DM diets. Cows offered restricted allowances (Expt II, Period 2) had similar intakes irrespective of diet. In Expt I cows fed on 500 or 800 g clover/kg DM diets ad lib. produced 30 or 33% respectively more milk (P<0·05) than cows fed on 200 g clover/kg total DM diets. During Expt II, Period 1, cows fed on 500 or 800 g clover/kg DM diets ad lib. produced 18 or 16% more milk (P<0·05) respectively than cows given 200 g clover/kg total DM diets. In both these experiments the increased milk yields were due to increased intake and the higher nutritive value of the high clover diets. There was no difference in the feed conversion efficiencies of cows if maintenance energy requirements were taken into account. However, cows on restricted allowances (Expt II, Period 2) showed no significant difference in milk yield, indicating that the effect of increased nutritive value was very slight. There were no consistent effects on milk fat, protein or lactose concentrations. Concentrations of blood and milk urea increased as the clover content of the diet increased (Expt 1 only), and this was associated with increased milk non-protein N and a decreased ratio of casein N[ratio ]total N. Both trials indicated an optimum clover content in the diet for milk production of 600–700 g/kg total DM.

This study was conducted to examine the effects of mastitis and stage of lactation on plasminogen activator (PA) activity in milk somatic cells. An assay System, which measures the plasmin-mediated hydrolysis of the chromogenic substrate D-valyl-leucyl-lysine p−nitroanilide, was used to assess PA activity present within milk somatic cells. Milk cell associated PA activity was increased (P < 0·05) by 50% in the presence of fibrin fragments. This suggests that milk somatic cells contain tissue PA which, unlike urokinase PA, is preferentially activated in the presence of fibrin fragments. An increase of the milk somatic cell count from < 5 × 104 to > 106 cells/ml resulted in an 8-fold increase in PA activity per cell. Elevated levels of PA activity were associated with milk somatic cells isolated from mastitic quarters obtained from cows in early (<4 months in lactation) or late lactation (>8 months in lactation). We conclude that PA activity is increased during severe mastitic inflammation. Although the physiological function of this enzyme is as yet unclear, we propose that it may be involved in the conversion of plasminogen to plasmin, contributing to the higher levels of plasmin occurring in milk isolated from mastitic quarters.

Colostrum and milk feeding are key factors for the newborn ruminant survival, affecting the future performance of the animal. Nowadays, there is an increasing interest in the potential of feeding newborn ruminants (mainly goat kids and lambs) with colostrum and milk from other more productive ruminant species (mainly cows). Although some studies regarding differences between colostrum and milk from these three species have been performed, herein we conduct for the first time a comparison using a proteomics 2-Dimensional Electrophoresis gel-based approach between these three ruminant species. In this study colostrum and milk samples from six Holstein cows, six Canarian sheep and six Majorera goats were used to determine the chemical composition, immunoglobulin G (IgG) and M (IgM) concentrations and proteomics profiles. Results showed that in general sheep colostrum and milk contained higher fat, protein and lactose percentages compared to bovine and goat samples. Additionally, no differences in the IgG or IgM concentrations were found among any of the three studied species, with the exception of sheep colostrum that showed the highest IgM concentration. With reference to the proteomics-based approach, some high abundant proteins such as serum albumin precursor, beta-caseins or different immunoglobulins components were found in colostrum, milk or even both. Nevertheless, differences in other proteins with immune function such as serotransferrin or lactoperoxidase were detected. This study shows that despite the similar immunoglobulin concentrations in colostrum and milk from the three studied species, differences in several immune components can be detected when these samples are studied using a proteomics approach. Finally, this study also provides a base for future investigation in colostrum and milk proteomics and metabolomics.

Bacillus cereus is a common contaminant in raw milk. The spores survive pasteurization and psychrotrophic strains of B. cereus often limit the keeping quality of pasteurized milk stored at > 6 °C (Griffiths, 1992). High numbers of B. cereus in pasteurized milk are most frequent when the cows are grazing (Slaghuis et al. 1997), mainly owing to increased levels of spores in raw milk resulting from teat contamination by soil (Christiansson et al. 1999). However, high numbers can also be found in pasteurized milk while the cows are housed indoors, and this is probably caused by additional contamination at the dairy plant (te Giffel et al. 1996; Larsen & Jørgensen, 1997; Lin et al. 1998). There is little information available about the sites of recontamination in the dairy. The use of typing techniques capable of discrimination below the species level, such as fatty acid profiles and random amplification of polymorphic DNA–polymerase chain reaction (RAPD–PCR), could be helpful in demonstrating contamination routes (Lin et al. 1998; Nilsson et al. 1998).

Spores of B. cereus are very hydrophobic and readily adhere to surfaces of steel, glass and rubber (Rönner et al. 1990), and short cleaning-in-place programmes do not always eliminate all the spores (Rönner & Husmark, 1992). Spores adhering to surfaces are more difficult to eliminate by disinfectants than spores in solution (te Giffel et al. 1995). Many B. cereus spores germinate rapidly in milk upon heat activation and, if allowed to propagate undisturbed on surfaces, may form biofilms that are extremely difficult to eliminate (Mosteller & Bishop, 1993; Wirtanen et al. 1996; Kumar & Anand, 1998).

This paper describes how we demonstrated the involvement of a pasteurizer in the contamination of pasteurized milk by B. cereus in a commercial dairy plant using a combination of classic microbiological analyses and typing of strains by RAPD–PCR.

Polymorphism of caprine milk proteins was studied by capillary electrophoresis. Identification of casein (CN) fractions was effected by using isolated fractions from cation-exchange fast protein liquid chromatography. Genetic polymorphisms in caprine αs2-CN, αs1-CN, β-CN and κ-CN have been determined. κ-CN A and B, β-CN A and null, αs2-CN A, B and C, αs1-CN A, B, C and null, and other forms with intermediate and low αs1-CN content have been identified. The capillary electrophoresis method made it possible to analyse whole caprine milk using simple sample preparation and was rapid, automated and suitable for phenotyping studies. This method may also permit the quantitative study of different protein fractions.

The ability of Listeria monocytogenes to grow at low temperatures and increase to hazardous levels during storage of dairy products is of great concern to the dairy industry. List. monocytogenes was detected in 3·6% of 774 cows' milk samples from 114 farms in Central Spain (Gaya et al. 1998). Ewes' and goats' milks produced in Spain had contamination levels of 2·2 and 2·6% respectively (Rodríguez et al. 1994; Gaya et al. 1996). List. monocytogenes may survive during the manufacture and ripening of most cheese varieties, with higher risk in soft surface-ripened cheeses owing to the rise in pH during ripening (Maisnier-Patin et al. 1992; Giraffa & Carminati, 1997).

The efficacy with which actively growing bacteriocin-producing cultures inhibit List. monocytogenes in some cheese varieties has been demonstrated. Nisin-producing starters inhibited List. monocytogenes V7 in Camembert cheese (Maisnier-Patin et al. 1992) and List. innocua BL86/26 in raw ewes' milk Manchego cheese (Rodríguez et al. 1998). A lacticin 3147-producing starter culture (McAuliffe et al. 1999) reduced List. monocytogenes Scott A levels in cottage cheese. Inoculation of milk with enterocin AS-48-producing Enterococcus faecalis INIA 4 inhibited List. monocytogenes strains Ohio and Scott A during the manufacture and ripening of Manchego cheese (Nuñez et al. 1997).

Semi-hard cheese varieties from raw or pasteurized cows' milk mixed with different proportions of ewes' and/or goats' milk represent ∼ 50% of Spanish cheese production. The objective of the present work was to evaluate the antilisterial activity of three bacteriocin-producing lactic acid bacteria strains isolated from milk (Rodríguez et al. 2000) during the manufacture and ripening of a semi-hard cheese manufactured from raw milk.

The effects of increased somatic cell count, whether caused by infection or by experimental infusion of bacterial endotoxin, on the distribution in milk of caseins between the micellar and soluble forms were investigated. The relationship of somatic cell count to some cheese-making parameters was also studied. With quite modestly elevated cell counts (2–3 × 106/ml) increases of up to 37% in total casein in the soluble phase were observed, most of which was contributed by β-casein, while κ- and αs1-caseins increased only slightly. With storage at 4°C, the concentrations of all the caseins, Ca and phosphate in the soluble phase increased substantially during the first 48 h, but this was followed by a slight decline on further storage. Rennet clotting time, losses of fat in whey, curd moisture, and losses in curd yield and rigidity were all greater the higher the somatic count. Clear differences were detectable in these parameters between milks of very low cell count (e.g. 5 × 104 cells/ml) and milks with counts more typical of those found in bulk supplies (e.g. about 5 × 105 cells/ml). If these findings can be reproduced in commercial practice even a modest reduction in bulk milk somatic cell counts might be expected to bring definite benefits.

The ‘mushroom compound’ previously reported as a gas chromatographic fraction of oxidized dairy products has been identified as oct-l-en-3-ol. The alcohol was found to have a flavour threshold value of 1 part in 109 in water, 1 part in 108 in skim-milk, and 1 part in 107 in butterfat. A mechanism for the formation of oct-1- en-3-ol is discussed.