The proportions of milk stored within the gland cistern (cisternal milk) and within secretory alveolar tissue (alveolar milk) were determined at various intervals after milking in peak lactation cows and late lactation cows. The rate of milk secretion remained constant up to 12 h in both groups. Cisternal milk remained low (600 g or less) until after 4 h in both groups, then increased gradually to values at 12 h of 5·08 (peak lactation) and 2·60 kg (late lactation). Alveolar milk exceeded cisternal milk in both groups and at all time points up to 12 h, and was greatest in the peak lactation cows. A further measurement was made at 20 h in the late lactation group. Milk secretion rate remained constant between 12 and 20 h and most of this additional milk accumulated in the cistern. Alveolar fraction, defined as alveolar milk as a proportion of total milk, ranged from 0·43 to 0·94 at 8 h. There was a highly significant correlation between 8 h and 20 h alveolar milk fraction results, but not between 1 h and 8 h values, nor between alveolar milk at 0 h (effectively residual milk) and alveolar fraction at any time point. The milk volume results were used to construct a mathematical model describing milk flow between cisternal and alveolar compartments, and hence the pattern of milk accumulation. The results are discussed in relation to current hypotheses regarding local autocrine control of milk secretion by a feedback inhibitor in milk.

In a 2 × 2 × 3 factorial design grazing experiment we investigated the effect of fertilizer (none or 240 kg N/ha), amounts of clover grass available (low or high) and type and level of daily supplementary feed for each cow (3·5 kg barley, 3·5 kg concentrate mixture rich in protein and fat, or both, 7 kg) on the protein composition and renneting properties of their milk. The experiment was carried out in two successive grazing seasons (years) and included a total of 79 Danish Holstein cows. The effect on milk protein composition was determined in both years whereas the effect on renneting properties was determined only in the second year. Fertilization of the clover grass significantly decreased total milk protein concentration (− 1·4 g/kg; P < 0·01) and tended also to decrease the relative proportion of whey protein N. Fertilization had no effect on renneting properties. Increased availability of clover grass significantly increased milk protein concentration (1 g/kg; P < 0·05) and resulted in significantly poorer renneting properties, that is increased clotting time (P < 0·01) and decreased coagulum development. These effects seemed to be mediated through an effect on the pH of the milk (+ 0·05; P < 0·05) as the effect was markedly reduced when statistical correction was made for the actual pH. Use of the protein- and fat-rich concentrate mix (3·5 kg) significantly reduced the total protein content of the milk (P < 0·05) and increased the proportion of non-protein N (NPN) in total N compared with use of the other supplementary feeds (P < 0·05). We found no effect on renneting properties of the different supplementary feeds. Throughout the grazing season and independent of the main treatments, the NPN proportion of milk N increased at the expense of casein N. At the same time, renneting properties became poorer, especially with high clover grass availability.

Changes in fat concentration and triacylglycerol fatty acid (TGFA) composition were studied in milk from six periparturient cows 1 d before and 20 d after an experimentally induced Escherichia coli mastitis in the fore and rear homolateral quarters. Opposite fore and rear heterolateral quarters remained uninfected and were used as controls. Milk was collected from all individual quarters during the experiment. The fat concentration in milk from infected quarters did not change, but total fat production decreased owing to reduced milk production after the Esch. coli challenge. In milk from the heterolateral uninfected quarters fat concentration rose significantly 48 and 72 h after induction of mastitis, the rise being concomitant with a decrease in milk production. Throughout the experiment similar changes in TGFA composition were observed for both infected and uninfected quarters. There was an increase in all the even, odd-numbered, iso and anteiso short-chain TGFA from day +6 on after induction of mastitis. There was little change in the composition of 16:0 and 18:0 fatty acids, while the long-chain unsaturated fatty acids decreased. Using multivariate analysis, the results are presented visually. The observed changes in the TGFA can be ascribed to changes normally observed in cows' milk soon after parturition.

The Malvern Particle Sizer, which applies Fraunhofer light diffraction theory, has recently become popular for measuring particle size distributions in dried milk and other powders. The size distribution is obtained by dispersing the powder in air or a suitable liquid in the laser light beam of the instrument. The suitability of the dry powder dispersion system (DPD system) of the Malvern Particle Sizer 2600c for characterizing agglomerated milk powders was examined. It was found that, although the instrument gave reproducible and reliable results for non-agglomerated powders (standard milk powders), milk powder agglomerates could be broken in the standard DPD system and erroneous size distributions might be obtained. However, the DPD system had potential for providing an objective test of the mechanical stability of milk powder agglomerates. In addition, the transient size distribution during dispersion of an instantized agglomerated milk powder in water was measured at intervals of about 20 s.

Each of the secondary lipid oxidation products pentanal, hexanal and heptanal was found to react with β–lactoglobulin (β–lg) in a two-phase model System (aqueous phosphate buffer–1-octanol) yielding fluorescent condensation products (emission maximum, 410 nm; excitation maximum, 350 nm). Protein polymers were detected by size-exclusion HPLC, and the rate of reaction paralleled the formation of fluorescent products, with the reactivity being pentanal > hexanal > heptanal. Simultaneously, the reaction also changed the intrinsic fluorescence of β–lg, and in particular pentanal reduced the intensity of tryptophan fluorescence (emission maximum, 332 nm; excitation maximum, 288 nm) by 30%. These findings are discussed with reference to the effect of peroxidizing lipids on the physical properties of whey proteins and the use of protein fluorescence (induced by the reaction with aldehydes) as marker for the oxidative status of milk and whey protein products.

Heat-induced gelation, an important functional property of β-lactoglobulin, was studied by measuring the rheological properties of both the A and B variants of the protein during and after heat treatment within a range of pH, temperature and concentration. Gel electrophoresis was used to determine the extent of denaturation and disulphide bond crosslinking of some samples. Both variants formed gel networks on heating at temperatures > 75 °C, and under most conditions the storage modulus (G′) of βlactoglobulin A gels was higher than the G′ of β-lactoglobulin B gels, in particular after cooling to 25 °C. A minimum protein concentration of 50 g/1 was required for gel formation at pH 7·0 in 0·1 M-NaCl by both variants at 80 °C. Increasing the protein concentration above 50 g/1 increased G′, the extent of increase being much greater for the A variant than the B variant. G′ of variant A gels was not much influenced by pH whereas G′ of variant B gels decreased slightly from pH 3 to pH 6 and increased between pH 6 and pH 9. When mixtures of the two variants were gelled G′ increased at the temperature of heating (80 °C) and after cooling (25 °C) as the relative quantity of variant A was increased. Comparisons of the loss of discrete protein bands from electrophoretic gels (native-PAGE, SDS-PAGE and SDS-PAGE of reduced samples) showed that heating β-lactoglobulin solutions of 100 g/1 at pH 7 in 0·1 M-NaCl and at 75, 80 and 85 °C caused a faster loss of both native and SDS-soluble β-lactoglobulin A than of β-lactoglobulin B. It was concluded that the loss of native β-lactoglobulin structure from these solutions during heating was faster than the formation of disulphidelinked aggregates, which was faster than gel formation for both β-lactoglobulin A and β-lactoglobulin B, and that each of these reactions was faster for β-lactoglobulin A than for β-lactoglobulin B. This contrasts with conclusions drawn from some previous studies and may arise from the differences in protein concentration between the present study (∼ 100 g/1) and the previous ones (< ∼ 10 g/1).

A simple and rapid method for determination of retinol in whole milk is described. Milk (400 mg) was diluted with a water–methanol–ethanol mixture (55:9:36 by vol.) and saponified with potassium hydroxide in a culture tube. Retinol was extracted into 10ml heptane-di-isopropyl ether (3:1) of which 100 μl was injected into the HPLC. Fluorescence detection ensured the specificity and sensitivity. The critical points in the analysis were the extraction of retinol from fatty samples and the instability of retinol towards photo-oxidation and air oxidation in alkaline solvents. There was an average recovery of 98·5 ± 2·4% and a CV of 3·6%.

Eight strains of Geotrichum candidum isolated from cheese were compared for aroma production. Twenty-four components, including alcohols, methyl ketones, fatty acids and esters, were identified by gas–liquid chromatography and gas chromatography–mass spectrometry of the volatile compounds produced by G. candidum cultures. Strains could be differentiated by their ability to produce dimethyldisulphide, phenol, phenylethanol and methyl ketones.

Autolysis of and proteolysis by various Lactococcus lactis subsp. cremoris strains were monitored in cheese ‘juice’ extracted by hydraulic pressure up to 63 d ripening. Viability was lowest for strain AM2 (non-bitter), intermediate for strain HP (bitter) and highest for the defined mixed strains G11/C25 (non-bitter). Autolysis monitored by the levels of the intracellular marker enzymes lactate dehydrogenase (EC 1.1.1.27), glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and post-proline dipeptidyl aminopeptidase proceeded in the order AM2 > G11/C25 > HP. Differences in autolysis between strains did not appear to be due to differences in stabilities of the marker enzymes, populations of non-starter lactic acid bacteria or levels of the marker enzymes in the strains. Proteolysis, as measured by gel permeation FPLC and free amino acid analysis of the cheese juice was highest for AM2, intermediate for G11/C25 and lowest for HP. The results of this study provided some evidence that different Lactococcus strains used for cheesemaking had different autolytic patterns during ripening, the effects of which on ripening and flavour development have not yet been clearly demonstrated.

Untreated whole milk, skim milk, sweetened milks, sweetened and unsweetened creams and ice-cream mixes were preincubated with a culture of Enterococcus faecium, then subjected to heat treatment in a pilot-scale plate heat exchanger using a hold of 15 s at 76°C. The liquids were examined for γ-glutamyltranspeptidase (GGTP) activity and total streptococcal count before and after heat treatment and the results plotted against water activity (aw). There was a good correlation between reduction in GGTP activity, destruction of streptococci and aw, demonstrating the potential of the enzymic assay for assessing the severity of HTST heat treatments above the minimum for pasteurization.

Two different investigations were conducted on the chemiluminescent activity of bovine milk polymorphonuclear neutrophil leucocytes (PMN) activated by different stimuli: zymosan, derived from the wall of Saccharomyces cerevisiae (Experiment A), and Streptococcus uberis and Escherichia coli (Experiment B). In Experiment A, a quarter with a phagocytic activity of PMN < 20 mV/1000 PMN following stimulation with zymosan was found to be 23 times more likely to be a clinical mastitis case than a quarter with phagocytic activity above this threshold value. In Experiment B, calculation of the odds ratio showed similar results following stimulation with Str. uberis or Esch. coli. These results provide evidence that immunocompromisation of mammary gland defences could predispose to clinical mastitis. They also support the need to challenge phagocytic cells with appropriate stimuli, and the Esch. coli test seems to be the most sensitive.

The effect of brining time and smoking on the free fatty acid content of Idiazabal cheese during ripening was examined. The main free fatty acids considered underwent at least some increase during the first stage of ripening before day 90 and tended to level off around a constant value towards the end of the ripening period. There were significant differences in free fatty acid levels during ripening among cheeses with different brining times and between smoked and unsmoked cheeses. Brining time and smoking exerted marked effects on lipolytic activity during cheese ripening, depending upon the free fatty acid involved and ripening time. In general, brining and smoking led to increases in free fatty acid levels at the end of the ripening period; the different behaviour of butyric acid may be due to a specific lipolytic activity.

Several methods used for sample preparation and for the determination of volatile flavour compounds in dairy products were tested. Steam distillation was an advantageous method for isolating volatile substances. The extraction yields for acetaldehyde, ethanol and diacetyl were >90%, and acetoin was partly separated from diacetyl. After steam distillation of the sample, gas-liquid chromatography was found to be a rapid method for the determination of acetaldehyde, ethanol and diacetyl in a wide range of concentrations corresponding to those found in dairy products. However, the lower limits for reproducible measurements were high (250, 1250 and 65 μM respectively). From the same sample, colorimetry was shown to be an accurate method for the detection of low levels of diacetyl and acetoin (12 and 57 μM respectively). Diacetyl interfered in the colorimetrie determination of acetoin. However, the interference was < 10% when the diacetyl: acetoin molar ratio was < 0·033. Dynamic head-space gas chromatography proved to be a sensitive method for acetaldehyde and diacetyl determination when a ‘purge and trap’ injector and a capillary column were used. The ranges suitable for determination lay between 25 and 100 μM for acetaldehyde and between 2·9 and 11·4 μM for diacetyl. Acetoin was analysed by HPLC on a cation-exchange column and detected by refractometry for concentrations ranging from 250 to 4000 μM.