As part of an EU-funded research project a representative survey of consumer attitudes concerning high pressure processing (HPP) of foods was carried out. 3000 adults aged 14 years and over, in France, Germany and the UK were interviewed in face-to-face computer assisted personal interviews (CAPI) and asked to indicate their opinion by evaluating 35 positive and 25 negative statements about the new technique, to compare the new technique with the conventional techniques, and they were asked if they would buy products preserved using High Pressure Pasteurization. The concept used for the data analysis was that of a market segmentation model using sociodemographical, geographical and psychographical attributes. The average acceptability rate is discussed with respect to the MAYA threshold value (Most Advanced Yet Acceptable), a pragmatic market research threshold value.

Heat (85 degrees C for 20 min) and pressure (600 MPa for 15 min) treatments were applied to skim milk fortified by addition of whey protein concentrate. Both treatments caused > 90 % denaturation of beta-lactoglobulin. During heat treatment this denaturation took place in the presence of intact casein micelles; during pressure treatment it occurred while the micelles were in a highly dissociated state. As a result micelle structure and the distribution of beta-lactoglobulin were different in the two milks. Electron microscopy and immunolabelling techniques were used to examine the milks after processing and during their transition to yogurt gels. The disruption of micelles by high pressure caused a significant change in the appearance of the milk which was quantified by measurement of the colour values L*, a* and b*. Heat treatment also affected these characteristics. Casein micelles are dynamic structures, influenced by changes to their environment. This was clearly demonstrated by the transition from the clusters of small irregularly shaped micelle fragments present in cold pressure-treated milk to round, separate and compact micelles formed on warming the milk to 43 degrees C. The effect of this transition was observed as significant changes in the colour indicators. During yogurt gel formation, further changes in micelle structure, occurring in both pressure and heat-treated samples, resulted in a convergence of colour values. However, the microstructure of the gels and their rheological properties were very different. Pressure-treated milk yogurt had a much higher storage modulus but yielded more readily to large deformation than the heated milk yogurt. These changes in micelle structure during processing and yogurt preparation are discussed in terms of a recently published micelle model.

High isostatic pressures up to 600 MPa were applied to samples of skim milk before addition of rennet and preparation of cheese curds. Electron microscopy revealed the structure of rennet gels produced from pressure-treated milks. These contained dense networks of fine strands, which were continuous over much bigger distances than in gels produced from untreated milk, where the strands were coarser with large interstitial spaces. Alterations in gel network structure gave rise to differences in rheology with much higher values for the storage moduli in the pressure-treated milk gels. The rate of gel formation and the water retention within the gel matrix were also affected by the processing of the milk. Casein micelles were disrupted by pressure and disruption appeared to be complete at treatments of 400 MPa and above. Whey proteins, particularly β-lactoglobulin, were progressively denatured as increasing pressure was applied, and the denatured β-lactoglobulin was incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly by rennet, but the presence of denatured β-lactoglobulin interfered with the secondary aggregation phase and reduced the overall rate of coagulation. Syneresis from the curds was significantly reduced following treatment of the milk at 600 MPa, probably owing to the effects of a finer gel network and increased inclusion of whey protein. Levels of syneresis were more similar to control samples when the milk was treated at 400 MPa or less.

The effects of high pressure (HP) on plasmin activity, β-lactoglobulin denaturation and proteolysis during subsequent storage of HP treated milk, were studied. Fresh raw milk samples were exposed to a range of pressures from 50 to 800MPa, for times of 1, 10 or 30min, at 20°C. Residual plasmin activity and whey protein denaturation were measured immediately post HP-treatment. Indices of proteolysis were measured during post-HP storage. Treatment at pressures >300MPa resulted in extensive β-lactoglobulin denaturation. Plasmin activity decreased in milk treated at pressures ⩾400MPa; the loss of activity was not well correlated with β-lactoglobulin denaturation. Compared to raw milk, treatment at 50MPa had little effect on proteolysis during storage of treated milk measured as increases in pH 4.6-soluble N and liberation of proteose peptones, but at pressures of 300–400MPa, proteolysis was increased relative to raw milk. After pressurisation >500MPa, proteolysis during storage of milk was less than that observed in raw milk. Overall, HP influenced proteolysis in milk in a way which is different from that produced by heat, in terms of subsequent susceptibility of casein to proteolysis during storage or incubation. In particular, HP treatment at pressures of 300–500MPa can increase proteolysis in milk, possibly through changes in micelle structure facilitating increased availability of substrate bonds to plasmin, which has implications for products prepared from milk thus treated.

Physicochemical properties, proteolysis and storage stability of samples of raw milk, UHT milk, UHT milk with added KIO3 and UHT milk in which the original level of milk plasmin activity had been restored by the addition of exogenous plasmin were compared. UHT treatment of milk resulted in association of denatured β-lactoglobulin with micelles (as demonstrated by immunogold localisation combined with electron microscopy), and greatly reduced plasmin activity, while this milk was extremely stable over storage. Preserved raw milk was unstable over storage, and showed extensive proteolysis, including clear evidence of non-plasmin proteolytic enzymes. The microscopic appearance of micelles in raw milk changed considerably over storage, with the formation of chains of small submicellar particles, and, after 84d of storage, cross-linked aggregates of micellar particles were recovered from the milk. UHT milk with added KIO3 behaved somewhat like raw milk during storage, showing extensive plasminogen activation, rapid proteolysis and formation of sediments at a similar time, and of similar appearance, to those seen in raw milk. The addition of plasmin to UHT milk after heating reduced the stability of the milk, increased proteolysis, and lead to the early formation of sediments. The results of this study suggest strongly that plasmin activity is a major influence on the storage stability of UHT milk.

There is increasing interest in the use of high pressure treatment as a minimal process for improving the quality and stability of foods. We have applied this technology to skim milk and studied the effect on the structure and functional properties of milk proteins; in particular the size and composition of the micelles and their gelation properties. Ln cheese making, the addition of rennet causes casein micelles to coagulate and form a gel. The rheology of the gels is affected by the properties of the milk proteins.

SummaryEffects of long-term deep-freeze storage on the stability of fat in raw sheep's milk were assessed by measuring free fatty acid (FFA) concentrations, lipoprotein lipase activity and the amount of solvent-extractable fat. Portions of milk from three herds were frozen and stored at – 12, – 20 and – 27 °C. There was a gradual increase in FFA during 6 months storage, the rate of lipolysis being affected by storage temperature and differences between herds. After 6 months storage the residual lipase activity was only 2% (– 12 °C), 11% (– 20 °C) and 24% (– 27 °C) of the initial activity in fresh milk. The potential for lipolysis in the stored samples, after they were removed from storage and allowed to thaw to 4 °C, gradually fell as storage time was prolonged. This may be due to the loss of lipase activity.

The properties of high fat foam are influenced by a number of factors. When the same milk is put through different separators, the stability of the resulting whipped creams can be very different. Cream produced by one separator was found to have acceptable whipping properties, but with cream from the other, the foam collapsed and the fat globules showed extensive clustering, clumping, and coalescence. This chapter examines whipped cream, which appears to have been severely damaged during separation and shows the effects of this damage on the whipped cream properties. It concludes that whipping cream, which contains large fat aggregates, resulting from disruption to the fat phase during separation, has poor whipping properties. Adsorbed at the air bubble surface is a large number of large, needle-like fat crystals. What has still to be confirmed is whether these crystals are directly responsible for the reduced foam formation in these creams.

The role of milk serum proteins in the development of foam structure during the whipping of 38% fat cream was investigated. Proteins were removed from the aqueous phase by washing the cream with simulated milk ultrafiltrate. The natural protein complement was replaced with either casein or whey protein preparations. When creams with different protein compositions were whipped to a defined end point the structural properties of the foams were similar but removal of protein caused a 50% reduction in whipping time. Other creams were whipped for fixed times of between 30 and 180 seconds to investigate the development and formation of the foam structure. This was greatly influenced by the protein composition. The presence of whey protein resulted in an initial lag phase in structure development, which was not so apparent when the added protein was casein. The lag phase was attributed to differences in structure of globular and disordered proteins and their behaviour at interfaces.

SummaryCows were infused with Escherichia coli endotoxin to provide a pool of milks with high somatic cell counts (SCC). This was mixed with bulk tank milk obtained from either of two Institute herds, producing a range of milks containing 0, 10, 20, 30 and 100% high SCC (HCC) milk. Creams containing 38% fat were produced from these milks on nine separate occasions; five times from winter milks and four times from summer milks. The SCC of the milks, the free fatty acid concentration of the unwhipped creams and the whipping time, stiffness and overrun of the whipped cream were measured. Whipping time and stiffness increased and overrun (volume increase) decreased as the proportion of HCC milk in the mixture increased. However, these changes were statistically significant only when comparing the 100% HCC milk with the other treatments containing lower levels of HCC milk. Creams produced in the summer took less time to whip and were less stiff than winter creams but there was no change in overrun. The source of bulk milk supply affected the stiffness and overrun of the whipped creams. Free fatty acid concentration appeared to have no direct effect on whipping characteristics.

SUMMARYThe effect of aeration and agitation during milking on free fatty acid (FFA) levels in raw milk was investigated by comparing (a) a dual vacuum system with a ‘round the barn’ installation and (b) a claw piece requiring high air bleed with conventional claw pieces. Reduced milk transfer pressure resulted in a reduction in lipolysis of 54%, while high air bleed led to a 21% increase. FFA values were related to milk yield, stage of lactation and to a lesser extent the incidence of mastitis. These relationships were largely independent of the type of milking equipment. Fat content and somatic cell count did not affect the level of lipolysis. Increases in FFA due to activation by added blood serum were strongly related to the initial FFA concentrations.

SUMMARYThe effect of separating conditions on lipase activity and free fatty acid levels in preheated milk, cream and skim milk was measured on nine occasions during a 12-week period covering the seasonal change from winter feeding to summertime grazing. This change consisted of four periods each representing a different type of forage intake, namely: silage, kale, daytime grazing and 24 h grazing. Milk was separated at 30, 40, 50 and 60°C with preheating times of 10, 25 and 55 s. Results were expressed both as absolute values and in terms of changes relative to the original unheated milk. Lipase activity and free fatty acid concentration were significantly reduced as separation temperature increased but were not influenced by holding time. The loss of activity in cream was progressive so that at 60°C only 40% of the original activity remained. Up to 50°C little change occurred in preheated milk or skim milk activity, while at 60°C 83 and 76% respectively of the original activity remained. The amount of activity calculated to be associated with the fat fraction of the cream also decreased with temperature. Activity varied significantly with date; maximum values were observed during the first 3 weeks of summertime grazing. Relative activity values indicated that the susceptibility of milk lipase to heat inactivation also varied with date. Lipolysis was also significantly affected by date. Cream free fatty acid levels were lower during the period of daytime grazing and were significantly higher than those in preheated milk. The correlation between lipase activity and free fatty acid levels was generally poor, accounting for between 0 and 34% of the variance. Possible reasons for the effect of separating temperature on lipolysis in cream are discussed.

Lipolysis caused by milk lipase increases during raw milk separation but the extent of free fatty acid (FFA) development depends on separation temperature. FFA levels in pasteurized double cream decrease with increasing separation temperature between 40 and 52°C. This decrease is associated with a corresponding decrease in lipase activity. In normal commercial production, FFA levels in pasteurized cream vary not only from day to day but between batches within days. Taste panel assessment suggests that as FFA values increase there is an increase in the overall defect detected, but that rancidity itself becomes apparent only at values above 1.7 mEq/100g fat.

Changes in milk lipid composition were measured during the course of mastitis infections induced in cows by infusion of either Escherichia coli endotoxin or Streptococcus agalactiae into 2 quarters of the udder; untreated quarters were used as controls. Experiments were divided into 3 distinct phases: a pre infusion period during which several samples were collected before infusion; a post infusion period following infusion and corresponding to the occurrence of elevated cell counts in milk from infused quarters; a recovery period followed after a short pause and represented the return of the cell count in milk from infused quarters to the level in the corresponding controls. Milk total fatty acid composition was unaffected by the infusion. Free fatty acid (FFA) composition did, however, undergo some alteration. There was a significant increase in long-chain saturated acids in milk from infused quarters relative to the corresponding controls during the post infusion period. FFA concentration (mequiv./100 g fat) also increased significantly during this period although the net increase was only slight. Phospholipid and cholesterol concentrations were significantly higher post infusion in milk of infused quarters. The results suggest that changes in concentration and composition of those milk constituents synthesized and secreted by the mammary epithelium occur after secretion in the alveolar lumen and milk ducts.

SummaryThe role of blood serum lipoproteins in stimulating milk lipolysis was investigated by adding blood serum and heparin to the milk of normal cows and cows in which elevated somatic cell counts were induced by intramammary infusion ofEscherichia coliendotoxin andStaphylococcus aureus. There was considerable variation between individual milks in the extent to which lipolysis was stimulated. In some milks there was almost no response. For values obtained from 29 cows during the first 6 months of lactation a relationship was observed between free fatty acid (FFA) levels in the untreated and serum-stimulated samples (r= 0·776). FFA values were higher after 18 h at 4 °C in milks from infused quarters than in those from control quarters, but this difference was not entirely due to higher values from the infused quarters at 0 h. Both blood serum and heparin stimulated lipolysis in high cell count milks and control milks. The response to heparin was greater than that to blood serum, but the response to both was highly correlated with FFA levels in the untreated milks. There were no differences in the ability of skim-milks prepared from infused and control quarters to promote lipolysis in recombined milks containing sterilized homogenized milk fat globules as a source of substrate. Incubation of blood serum with 2 μg/ml trypsin for 1 h at 37 °C reduced the ability of the serum to promote lipolysis. The importance of lipase activation by blood serum lipoproteins in relation to milk lipolysis is discussed.

SummaryA quantitative method for rapid routine analysis of individual free fatty acids (FFA) in milk was developed. Lipid was extracted from milk in ether and FFA were recovered by shaking the extract with anion exchange resin Amberlyst 26. The resin-bound FFA were methylated directly and the individual acids quantified, using internal standards, by gas-liquid chromatography. The properties of the resin were measured. The validity of the method was established by extraction of FFA mixtures and milk. Individual acids were, on average, found to be within 6% of the actual concentration present in the mixture. An average coefficient of variation of 4·3% was achieved for the major individual fatty acids on repeated extraction of a single milk sample.