Several media previously recommended for lactobacilli by other workers have been examined, but none was found to support good growth of all species. Experiments are described in which the optimum concentrations and combinations of various ingredients, particularly Tween 80, were determined. These resulted in the development of a practical medium which has proved satisfactory for the growth of all varieties of lactobacilli.

In the later stages of the reaction between rennin and casein drastic viscometric methods are undesirable, but in the first stages there is a fall in viscosity which may be satisfactorily measured in an Ostwald viscometer. Using fat-free milk, the viscosity at first falls linearly with time. At low rennet concentrations (Ce) this may be said to constitute a zero-order reaction (constant k0). At higher rennet concentrations and after longer times, the reaction passes to first order (constant k1). After very long times it doubtless becomes more complex.

The values of k0 are proportional, over a considerable range, to the milk concentration (Cm), those of k1 being independent of Cm.

For pure rennin k0 and k1 are proportional to Ce, but for commercial rennets they vary as a power (N) of Ce and the value of N appears to measure the rennin purity. The potentialities of the method for assessing rennet activities are also discussed.

If the reduced viscosity (specific viscosity÷Cm) of fat-free milks is plotted against Cm, good straight lines are obtained which may be extrapolated to zero concentration to give a reliable value of intrinsic viscosity. The intrinsic viscosity falls progressively during the protein breakdown process but the slope of the curves (‘second order term’) remains unchanged.

The composition and sensory characteristics of matured cheeses are controlled by a number of factors, among which the type of feed is important. The influence of feeding can be reflected by the presence in cheeses of terpenes and sesquiterpenes, compounds typically indicating their vegetable origin (Mariaca et al. 1997). Indeed, several investigators have already established that these compounds could characterize the forage even to a specific geographical location. Dumont & Adda (1978), Dumont et al. (1981), Guichard et al. (1987), Bosset et al. (1994) and Moio et al. (1996) were thus able to distinguish cheeses from lowland and upland regions. In most of these studies, the conditions under which milk was produced and processed were not completely controlled. Furthermore, all these studies analysed only the volatile fraction of cheeses and did not examine the volatile compounds in the forages used. Recently Bosset et al. (1994), in one part of the project described by Jeangros et al. (1997), showed that highland grass with a highly diversified botanical composition produces milk and cheeses with significantly different chemical compositions from those from lowland grass. To improve our understanding of the relationship between animal feed and cheese composition, we have investigated under controlled experimental conditions both the composition of the terpene and sesquiterpene fractions of four forages with different botanical diversities and the influence of those forages on the terpene fraction of cheeses.

Parmigiano Reggiano is a hard, cooked cheese produced in specific areas of Northern Italy. The raw material is obtained by mixing the partly skimmed evening milk and whole morning milk. The mixture is then heated to 22°C, and natural whey starter is added at 28–30 g/l after 1–2 min, bringing the pH of the mixture to 6·2–6·3. After coagulation, which takes ∼15 min and occurs at 32–33°C owing to the addition of calf rennet powder, the curd is broken up for 2–4 min, cut into fragments and cooked at a temperature raised gradually to 42–44°C and then more quickly to 55–56°C over 10–15 min. The curd is left undisturbed, covered by the whey, for 40–60 min, then removed and placed inside a circular wooden mould. The cheese is held at ∼20°C for 3 d during which it is turned at frequent intervals to facilitate complete whey drainage. The cheese, which now has its typical shape and size, is then salted by immersion in brine (260–280 g NaCl/l at 16–17°C) for 20–24 d. During this period the cheese absorbs 15–18 g NaCl/kg and its weight decreases by 4%. During the subsequent period of ripening (12–24 months) in store rooms at 16–18°C and 85% moisture, the cheese is frequently turned. At the end of ripening the cheese is cylindrical in shape, with a slightly convex side, 0·22–0·24 m high, 0·40–0·45 m diam. weighs 35–36 kg, has 320–330 g fat/kg dry matter and a minutely granulated internal structure with small holes formed by the activity of some heterofermentative lactic acid bacteria.

In ruminants, protective immunoglobulins are transferred to the newborn via colostrum to mediate maternal immunity. There is a high selectivity in the transport of immunoglobulins from the maternal plasma across the mammary barrier into the colostrum, and only IgG1 is transferred in large amounts. We have recently analysed the expression of the neonatal Fc receptor (FcRn) in sheep mammary gland around parturition. Re-analysing this issue in bovine confirmed our previous data indicating that FcRn is homogeneously localized in the mammary gland acinar cells before parturition, however a remarkable difference was observed in the pattern after calving, where only the apical side of the cells was strongly stained. The presence of the FcRn in the acinar epithelial cells of the mammary gland and the obvious change in distribution before and after parturition indicate that FcRn plays an important role in the IgG transport during colostrum formation in ruminants.

Accurately assessing the economic impacts of diseases and other factors that affect milk production requires that the demand for milk be taken into account. Because demand for milk in the USA is relatively inelastic (i.e., consumers generally purchase a somewhat fixed amount over a given time frame, regardless of fluctuations in price), consumers tend to reap much of the benefit of enhanced production. An examination of the economic impacts of Johne's disease indicated that reduced milk production, associated with the determination of dairy operations as Johne's-positive, reduced consumer surplus by $770 million ± $690 million, and resulted in a total loss of $200 million ± $160 million to the US economy in 1996. Most of the economic surplus lost by consumers was transferred to producers, whose economic surplus increased by $570 million ± $550 million as a result of the reduced milk production associated with Johne's disease. Uncertainty analysis showed that the estimated reduction in milk production on Johne's-positive dairy operations accounted for most of the uncertainty in the economic-impact estimates. If Johne's disease had not been present on US dairy operations, then an additional 580 million ±460 million kg of milk would have been produced, but the price would have fallen by 1·1±1·0 cents/kg, and the total value of the milk would have decreased by $580 million ± $560 million.

The contamination of pasteurized milk by Bacillus cereus during the filling process was studied in two dairy plants. Samples of pasteurized milk were taken at four different sites along the production line. The samples were stored at 7 °C for 7 d, or at 10 °C for 5 d, before plate counting and random selection of B. cereus isolates. Isolates of B. cereus were typed by the polymerase chain reaction (PCR)-based method randomly amplified polymorphic DNA (RAPD). Samples taken at three different sites between the pasteurizer and the filling machine were all holding similar low concentrations of B. cereus, while an increase of the B. cereus count was seen in the consumer packages. More B. cereus of different RAPD types was growing in the consumer packages than in samples taken just before the filling machine. Several RAPD types found in the consumer packages were not detected in the samples taken just before the filling machine.

Thirty-nine multiparous Holstein cows were used to measure the effect of propylene glycol treatment around parturition on milk yield, reproductive efficiency and some hormone and metabolite concentrations. Cows were assigned randomly to control (n = 19) or propylene glycol treated (n = 20) groups. Propylene glycol (300g) was administered directly mixed with the diet from day 10 prior to the expected calving date until parturition (day 0) and orally after dilution in 1 l water on days 3, 6, 9 and 12. Blood samples were collected on days −20, −5, 0, 3, 10, 25 and 50 while milk samples were taken weekly until 13 weeks post partum. Body condition scores, recorded on days −20, 15 and 50, were not affected by propylene glycol administration. Propylene glycol did not significantly affect milk yield or composition but linear somatic cell score measured from the first 13 weeks post partum was reduced by propylene glycol administration (P < 0·01). Moreover, propylene glycol reduced milk urea (−25 mg/l, P < 0·05), especially during the first 9 weeks post partum. Plasma insulin concentrations were similar in both groups during the experiment while insulin-like growth factor I (P < 0·05) and insulin-like growth factor-binding protein 3 (P < 0·001) levels were higher on days 10, 25 and 50 post partum in the propylene glycol group. Propylene glycol administration decreased plasma non-esterified fatty acid concentrations (P < 0·05 to P < 0·01) but increased total cholesterol levels (P < 0·01) after parturition while 3-hydroxy-butyrate levels were unaffected by the treatment. Changes in the hormone and metabolic concentrations after propylene glycol administration in the last few days of gestation and the first week of lactation seem to indicate that energy balance in the treated group was probably more positive than in the control group. There was also evidence that propylene glycol administration prevented fatty liver syndrome and hastened the resumption of oestrous cycles (P < 0·001).

Isoelectric focusing and 2-dimensional electrophoresis were used to study the development of the pH 4·6-insoluble fraction during Camembert cheese ripening. Modifications of this fraction were due mainly to the action of 5 proteinases: rennet (chymosin + bovine pepsin), plasmin and Penicillium caseicolum aspartyl-and metalloproteinases. Rennet was inactive on β-casein, but acted very early on αs1-casein. However, rennet and P. caseicolum aspartyl-proteinase had a very similar action on the latter substrate, which prevented clear definition of the respective actions of these proteinases on αs1-casein after 7 d of ripening. Plasmin action on β-casein was important from 21 and 35 d of ripening at the surface and in the centre of the cheese respectively, suggesting an important influence of pH changes during maturation. The respective activities of the metallo-and aspartyl-proteinases of P. caseicolum were characterized and followed using β-casein degradation products as markers. The metallo-proteinase activity was detectable immediately after the development of the Penicillium (7 d), while that of the aspartyl-proteinase was observed 3 d later. Thereafter, the amount of β-casein degradation peptides resulting from the metalloproteinase decreased while that resulting from the aspartyl-proteinase increased, suggesting a more important role of the latter enzyme.

High performance gel chromatography of the casein micelles disaggregated by 6 m-urea was carried out on a TSK-GEL G4000SW column using 6 M-urea synthetic milk serum as the effluent. The eluate was divided into two fractions. Fast eluted fraction 1 decreased from 67·5 to 57·3% on reduction of casein micelles and was not observed in reduced colloidal phosphate-free casein micelles. Fraction 1 from reduced casein micelles contained 1·7 times as much Ca and Pi bound to casein as did whole casein micelles, while fraction 2 essentially contained only bound Ca. These facts indicated that fraction 1 of reduced casein micelles consisted of the casein aggregates cross-linked by colloidal Ca phosphate (CCP). Polyacrylamide-gel electrophoresis showed that fraction 1 of reduced casein micelles contained more αs1- and αs2-caseins and less β-casein than whole micellar casein. No κ-casein was detected in fraction 1 of reduced casein micelles. It is suggested that the ester phosphate groups of casein are the sites for interaction with CCP.

The triglyceride compositions of 76 Australian milk fats obtained from 3 factories during a whole year were determined by gas-liquid chromatography. It was shown that all the data fitted the equation

with a S.D. of of 0·7088 where C40, C42 and C44 are the weight percentages of triglycerides with carbon numbers 40, 42 and 44. The triglyceride compositions of 10 samples of milk fat from countries other than Australia were also shown to fit the equation. If the equation is assumed to define pure milk fat, it is shown that as little as 5% of any non-milk fats can be detected with > 99% confidence. The amount of non-milk fat in mixtures of milk and non-milk fats can be quantified to better than ±2% with > 95% confidence. Also, the identity of the non-milk fat can usually be deduced by calculation of its triglyceride composition. Analyses of known mixtures and of chocolate and a table spread comprising butter and vegetable oil showed an agreement between observed and expected results well within the predicted confidence limits.

To obtain information about the possible release of β-casomorphins from β-casein under in vitro conditions, cows' milk was incubated with 13 strains of Gram-negative or Gram-positive bacterial species isolated from bovine milk. After incubation periods of 1–24 d, milk samples were assayed for β-casomorphin-4, -5, -6 or -7 immunoreactive materials. In general, no β-casomorphin immunoreactive material was found in samples incubated with non-caseolytic strains, e.g. Pseudomonas putida or Streptococcus faecalis subsp. faecalis; however, high levels were found in several samples incubated with caseolytic strains, e.g. Ps. aeruginosa or Bacillus cereus. When such materials were present, the amounts of β-casomorphin-4, -6 or -7 greatly exceeded β-casomorphin-5 immunoreactivity. It was concluded that while some of these materials probably represented authentic β-casomorphins, others were more likely to be β-casomorphin precursors.

Three groups of 12 lactating Comisana ewes were housed in separate rooms of the same building that contained straw-bedded pens with total areas of 12, 18 and 24 m2. The ewes were assigned to groups with different stocking densities: high (HSD, 1 m2/animal), medium (MSD, 1.5 m2/animal) or low (LSD, 2 m2/animal). The LSD treatment resulted in lower air concentrations of total microorganisms and coliform bacteria than the other two. Ewes in the LSD group gave greater yields of milk, total protein and fat than those in the other two groups. LSD treatment increased milk casein content and improved clotting time and clot firmness. LSD also resulted in ewes giving milk with lower somatic cell counts and smaller concentrations of mesophiles, psychrotrophs and faecal coliforms than MSD and HSD. Subclinical mastitis occurred in three ewes of the MSD group and four ewes of the HSD group, while no cases were recorded in the LSD group. These results indicate that stocking density is a critical factor in dairy sheep housing and suggest that a space allocation <2 m2/animal may adversely affect the performance and health of the lactating ewe.

Whey protein complexed and became co-sedimentable with casein micelles after heating milk at ≥ 90°C for 10 min at pH ≤ 6·9 while at higher pH values (7·3) whey proteins and κ-casein-rich protein dissociated from the micelles on heating. κ-Casein-deficient micelles were more sensitive to heat, Ca2+ or ethanol than whey protein-coated or native micelles and were readily coagulable by rennet. Isolated κ-casein added to skim milk before preheating (90°C for 10 min) did not associate with the micelles at pH ≥ 6·9. Sodium dodecyl sulphate increased the level of both non-sedimentable N (NSN) and N-acetylneuraminic acid (NANA) and shifted the NSN-pH and NANA-pH curves to more acidic values while cetyltrimethylammonium bromide had the opposite effect. It is suggested that the pH-dependent dissociation in micellar κ-casein, which appears to be reversible, depends on the surface charge on the micelles; at a certain negative charge, disruption of hydrophobic and electrostatic forces could result in the dissociation of κ-casein from the casein micelles.

When κ-casein (κ-CN) was added to milk, and the milk was subsequently pH adjusted (pH 6·5−6·9) and heated (90°C/15 min), the serum contained considerably higher levels of denatured whey proteins than the milks without added κ-CN. When milk at pH 6·5−6·9 was heated at 90°C for different times, κ-CN was found in the serum in the early stages of heating and before significant levels of whey proteins were denatured. κ-CN reached its maximum level in the serum before the whey proteins were fully denatured. When milk at pH 6·5−6·9 was heated at 20−90°C for 15 min, κ-CN dissociated from the casein micelles at all temperatures, with the level in the serum increasing with the temperature and the pH at heating. κ-CN dissociated from the micelles at temperatures below those at which significant levels of the whey proteins were denatured. When taken together, the results from these experiments strongly indicate that the dissociation of κ-CN from the micelles can precede the interaction of the denatured whey proteins with κ-CN, and that there is a preferential interaction of the denatured whey proteins with serum-phase κ-CN.

The present study describes the purification of a post-proline dipeptidyl aminopeptidase from the cytoplasm of Streptococcus cremoris AM2. On the basis of its elution from a calibrated Sephadex G200 column, the enzyme had a molecular weight of 117000 and exhibited a broad pH optimum activity between 6·0 and 9·0. The activity was most comprehensively inhibited by phenylmethylsulphonylfluoride and more modestly inhibited by 1,10-phenanthroline and 8-hydroxyquinoline but not by EDTA. A range of peptides containing either proline or alanine as the penultimate amino acid residue could act as substrates. The presence of proline on the carboxy side of the scissile bond prevented hydrolysis. However the enzyme could release Pro-Pro from Pro-Pro-Gly-Phe-Ser-Pro. The significance of this substrate specificity is considered in the context of removal of either single proline residues or prolylproline sequences from oligopeptides during cheese ripening.

Measurements of residual native protein remaining after heat treatment of cheese whey have been made using quantitative polyacrylamide gel electrophoresis. The results have been expressed in terms of kinetic constants. The effect of concentration was investigated up to 3 times the normal total solids content, dialysis treatments were used to study the effect of non-protein constituents, and the effect of pH was studied at pH 4, 6 and 9. The results indicated that increased total solids (TS) concentration slowed the denaturation of β-lactoglobulin A and B (β-lg A, β-lg B) but hastened the denaturation of α-lactalbumin (α-la). However, increased lactose concentration slowed the denaturation of both α-la and β-lg, perhaps by preventing formation of heat-induced complexes. Increased Ca concentration, up to 0.4 mg/ml, tended to slow the denaturation of both proteins, but further increase in Ca up to 0.9 mg/;ml produced little effect. The rate of denaturation of both α-la and β-lg was slower at pH 4 than at pH 6 or 9, and was probably slowest at the isoelectric point. However, not all the changes associated with pH could be explained in terms of net molecular electrostatic charge. The genetic variants of ′β-lg showed different heat stabilities – below 90 °C β-lgA was more stable than β-lgB, but above 90 °C the situation was reversed at all TS concentrations, and pH 6. However, at pH 4 and 9, °-lgA was less stable than °-lgB over the entire temperature range at normal concentration.

Mixed cultures of different EPS-producing Streptococcus thermophilus strains in combination with a Lactobacillus delbrueckii subsp. bulgaricus strain with negligible EPS-production were used for yoghurt production. The yoghurt texture was characterised with respect to sensory, rheological and microstructural properties and the EPS-concentrations were determined. The cultures resulted in yoghurts with highly different texture properties, and positive interactions between certain Streptococcus thermophilus strains were observed. The underlying properties of yoghurt texture are multidimensional, but a number of microstructural characteristics were apparent in the yoghurts with the highest mouth thickness, creaminess and viscosity. A strong protein network, not too dense and with medium size pores containing EPS, seems associated with these properties. The presence of capsular polysaccharides (CPS) also appeared to be beneficial as did a combination of EPS types, which were distributed differently in the protein network (in serum pores, respectively in association with protein). Obviously, a certain concentration of EPS must be present to provide for these effects on yoghurt texture, but other factors than concentration per se seem more important.

A novel 2-stage gravity separation scheme was developed for fractionation of raw, whole bovine milk into fractions enriched in small (SFG) or large (LFG) fat globules. The volume mean diameter of fat globules in SFG, LFG or control (CTRL) milk was 3·45, 4·68 and 3·58 μm, respectively. The maximum in storage modulus (index of firmness) decreased with increasing fat globule size for rennet-induced gels formed from SFG, LFG or CTRL milks. Miniature (20 g) Cheddar cheeses were manufactured using each of the 3 milks. There were no significant (P>0·05) differences in the pH, moisture and fat in dry matter levels between cheeses made using any of the 3 milks, however, the fat content of the cheese made using SFG milk was ~1% lower than that of cheese made using LFG or CTRL milk in each of the 2 trials. Image analysis of confocal scanning laser micrographs of the cheeses illustrated that the star volume of fat globules in the cheeses decreased significantly (P[les ]0·05) as the size of fat globules in the milks used for cheesemaking was reduced. This indicates that it is possible to manipulate the size distribution of fat globules in Cheddar cheese by adjusting the fat globule size distribution of the milk used for cheesemaking. The concentration of free fatty acids (FFA) increased in all cheeses during ripening. At 120 d of ripening, the concentration of FFA varied significantly (P[les ]0·05 and P[les ]0·001 for trials 1 and 2, respectively) with fat globule size, with cheeses made in trial 2 from LFG, SFG or CTRL milks having total FFA levels of 3391, 2820 and 2612 mg/kg cheese, respectively.