Although widely adopted by the chemical and pharmaceutical industries in recent years, little published data is available regarding possible applications of high pressure homogenisation for dairy products. The objective of this work was to compare the effects of conventional (18 MPa, two-stage) and single or two-stage high pressure homogenisation (HPH) at 50–200 MPa on some properties of raw whole bovine milk (∼4% fat). Fat globule size decreased as HPH pressure increased and, under certain conditions of temperature and pressure, HPH yielded significantly smaller fat globules than conventional homogenisation. Fat globule size was also affected by milk inlet temperature. The pH of all homogenised milk samples decreased during 24 h refrigerated storage. Total bacterial counts of milk were decreased significantly (P<0·05) for milk samples HPH-treated at 150 or 200 MPa. Whiteness and rennet coagulation properties of milk were unaffected or enhanced, respectively, as homogenisation pressure was increased. Average casein micelle size decreased slightly when skim milk was homogenised at 200 MPa. Thus, HPH treatment has several, potentially significant, effects on milk properties.

Studies of the potential of high pressure homogenisation (HPH) for the combined pasteurisation/ homogenisation of raw bovine milk were undertaken. Raw milk was preheated to 45 8C and HPH-treated at 150, 200 or 250 MPa; milk outlet temperature at these pressures were 67, 76 . 8 and 83 . 6 8C, respectively, with a holding time of y20 s. Raw and commercially pasteurized and homogenized (CPH) milk samples were analysed as controls. Fat globules in HPH samples were approximately half the size of those in CPH samples, although differences were not significant (P >0 . 05). b-Lactoglobulin was denatured at pressures o150 MPa, although little denaturation of a-lactalbumin was observed. Numbers of psychrotrophic bacteria in raw milk were reduced by 2 . 73 log cycles by HPH at 150 MPa and were uncountable following HPH at 200 or 250 MPa. Mesophilic bacterial counts were reduced by 1 . 30, 1 . 83 and 3 . 06 log cycles by HPH at 150, 200 or 250 MPa, respectively. No viable Staphylococcus aureus nor coliform cells remained in any HPH milk samples. HPH did not affect the colour of milk and HPH samples did not cream during refrigerated storage. The activities of plasmin, alkaline phosphatase and lactoperoxidase in milk were all greatly reduced by HPH. Pseudomonas fluorescens, inoculated into milk (y10 6 cfu/ml), was reduced to undetectable levels by HPH at 200 MPa (milk inlet temperature, y10 8C); however, Ps. fluorescens proteinase was quite resistant to HPH under such conditions. Overall, owing to the significant increase in temperature and the possibility of varying the holding time, there may be potential applications for HPH as a novel liquid milk processing technique, combining many advantages of conventional homogenization and pasteurization of milk in a single process.

Three peptides produced by a Lactobacillus acidophilus DPC6026 fermentation of sodium caseinate and showing antibacterial activity against pathogenic strains Enterobacter sakazakii ATCC 12868 and Escherichia coli DPC5063 were characterized. These peptides were all generated from bovine ␣ s1 -casein and identified as IKHQGLPQE, VLNENLLR, and SDIPNPIGSENSEK. These peptides may have bioprotective applicability and potential use in milk-based formula, which has been linked to E. sakazakii infection in neonates.A number of bioactive peptides have been identified in milk proteins, such as casein and whey proteins, where they are present in an encrypted form, stored as propeptides or mature C-terminal peptides that are only released upon proteolysis (6, 9) The first antimicrobial peptides of casein origin were identified by Hill et al. (8), who isolated antibacterial glycopeptides, known as casecidins. Isracidin ␣ s1 -casein f(1-23) [␣ s1 -CN f(1-23), where f(1-23) refers to amino acids 1 to 23 of the peptide], a positively charged antimicrobial peptide with the primary amino acid structure R 1 PKHPIKHQGLPQEVLNENLLRF 23 (8), was shown to have a broad spectrum of activity against gram-positive bacteria (11). Given the highly proteolytic nature of lactic acid bacteria such as Lactococcus lactis (10,16) and Lactobacillus helveticus (13,20), it is not surprising that their use as microbial catalysts for the generation of bioactive peptides has been investigated (14). Characterization of peptides produced during casein degradation has been described for L. helveticus (5, 19) and to a lesser extent for Lactobacillus casei (2). Also, the cell wall-bound proteinase of Lactobacillus delbrueckii subsp. lactis ACA-DC 178 has been characterized, and its specificity for ␤-casein has been documented (17).The aim of this study was to investigate the potential of Lactobacillus acidophilus DPC6026 to generate antimicrobial peptides from bovine casein and to assess the activities of these peptides against pathogenic bacteria. Two peptide fragments were identified that exhibited antimicrobial activity similar to that of the characterized antimicrobial peptide isracidin against pathogenic strains such as Escherichia coli, Enterobacter sakazakii, Streptococcus mutans, and Listeria innocua, used in this study as a model for the known pathogenic strain Listeria monocytogenes.L. acidophilus DPC6026 was isolated from the porcine small intestine and stocked in the culture collection of Teagasc Dairy Products Research Centre, Cork, Ireland. This strain was propagated in MRS broth (Oxoid Ltd., Basingstoke, United Kingdom) anaerobically for 24 h at 37°C. L. innocua DPC3306, E. sakazakii ATCC 12868, E. sakazakii NCTC8155, purchased from the National Collection of Industrial and Marine Bacteria (Aberdeen, United Kingdom), and E. coli DPC6053 were employed as the test strains.L. acidophilus DPC6026 was used in fermentation on the basis of its proteolytic capabilities, demonstrated using smallscale (10 ml) casein fermentations followed by high-perform...

High pressure homogenisation (HPH) is a novel dairy processing tool, which has many effects on enzymes, microbes, fat globules and proteins in milk. The effects of HPH on milk are due to a combination of shear forces and frictional heating of the milk during processing; the relative importance of these different factors is unclear, and was the focus of this study. The effect of milk inlet temperature (in the range 10-50 degrees C) on residual plasmin, alkaline phosphatase, lactoperoxidase and lipase activities in raw whole bovine milk homogenised at 200 MPa was investigated. HPH caused significant heating of the milk; outlet temperature increased in a linear fashion (0.5887 degrees C/ degrees C, R2=0.9994) with increasing inlet temperature. As milk was held for 20 s at the final temperature before cooling, samples of the same milk were heated isothermally in glass capillary tubes for the same time/temperature combinations. Inactivation profiles of alkaline phosphatase in milk were similar for isothermal heating or HPH, indicating that loss of enzyme activity was due to heating alone. Loss of plasmin and lactoperoxidase activity in HPH milk, however, was greater than that in heated milk. Large differences in residual lipase activities in milks subjected to heating or HPH were observed due to the significant increase in lipase activity in homogenised milk. Denaturation of beta-lactoglobulin was more extensive following HPH than the equivalent heat treatment. Inactivation of plasmin was correlated with increasing fat/serum interfacial area but was not correlated with denaturation of beta-lactoglobulin. Thus, while some effects of HPH on milk are due to thermal effects alone, many are induced by the combination of forces and heating to which the milk is exposed during HPH.