Effects of high pressure homogenisation of raw bovine milk on alkaline phosphatase and microbial inactivation. A comparison with continuous short-time thermal treatments

Abstract

Raw whole milk of high microbial quality (<or=4 x 104 cfu/ml) was processed using a approximately 15 l/h homogeniser with a high pressure (HP) valve immediately followed by cooling heat exchangers. The effects of homogenisation between 100 and 300 MPa (HP valve) with an initial milk temperature Tin=4 degrees C or 24 degrees C was investigated on the inactivation of: (i) endogenous alkaline phosphatase (ALP); (ii) endogenous milk flora and (iii) two Gram positive (Listeria innocua and Micrococcus luteus) and one Gram negative (Pseudomonas fluorescens) strains inoculated into milk. Temperatures T1 and T2 measured before and immediately after the HP valve, and fat globule size distributions were also determined. ALP activity slightly decreased after homogenisation above 250 MPa when Tin=4 degrees C (corresponding T2>58 degrees C), but markedly decreased above 200 MPa when Tin=24 degrees C (T2>60 degrees C). In contrast to inactivation induced by continuous short-time thermal treatments, ALP inactivation induced by HP homogenisation was clearly due to mechanical forces (shear, cavitation and/or impact) in the HP valve and not to the short (<<1 s) residence time at temperature T2 in the same valve. Inactivation of the three exogenous microorganisms led to similar conclusions. Homogenisation at 250 MPa or 300 MPa (Tin=24 degrees C) induced a 2-3 log cycle reduction of the total endogenous milk flora and a 1.5-1.8 log cycle reduction of inoculated List. innocua. Higher reduction ratios (2-4 log cycles) were obtained for the two other microorganisms. The highest levels of ALP inactivation corresponded to the highest extents of microbial reduction. Running the milk twice or three times through the homogeniser (recycling), keeping temperature T1 approximately 29 degrees C and pressure=200 MPa, increased homogenisation efficiency.

Opportunities for Ultra-High-Pressure Homogenisation (UHPH) for the Food Industry

“…However, it was observed a shift in the distribution of the population with respect to raw milk towards heat resistant and halotolerant bacteria. Similar results were obtained by Pereda et al [45] and Picart et al [50]. However, complete sterilisation of whole milk was achieved by UHPH at 300 MPa with inlet temperatures of 75 and 85°C [1].…”

“…Plasmin activity decreases proportionally with its original activity as inlet temperature or UHPH pressure increases [14,30,31], but complete inactivation has not yet been observed. Concerning the enzyme alkaline phosphatase, Picart et al [50] observed an increase in the activity upon UHPH treatment at 200 MPa with inlet temperatures below 24°C which might help to release enzyme bound to the fat globule membrane or unfold the enzyme making active sites more accessible to the substrate. Complete inactivation of the enzyme was observed in whole milk with inlet temperatures C30°C [14,31,45].…”

“…Such a lack of effect has been attributed to the flash time of the treatment, which has been estimated in some cases as lower than 1 s [50] with a velocity of 100 m/s [24]. Indeed, in the case of milk, UHPH samples, regardless of the increase of temperature experienced in the UHPH valve, suffered less Maillard reaction, less whey protein denaturation and did not present lactose isomerisation compared to a commercial pasteurised milk with a concomitant preservation of essential amino acids and as a consequence with a better nutritional value desired by consumers [49].…”

Structural and Rheological Properties of Frozen Concentrated Orange Juice (FCOJ) by Multi-Pass High-Pressure Homogenisation (MP-HPH)

“…Nevertheless, it must be highlighted that MP-HPH is not a practical processing (in an industrial perspective); thus, it would be more interesting if a small number of passes were effective. Therefore, similarly to Picart et al [33] , only three passes were evaluated.…”

Characterization of Whey Protein Oil-In-Water Emulsions with Different Oil Concentrations Stabilized by Ultra-High Pressure Homogenization

“…These technologies generate intense disruptive forces that break up the oil and water phases and lead to the formation of small oil droplets [8]. Ultra-high-pressure homogenization (UHPH) is a non-thermal processing technique, which works at significantly higher pressures (≥200 MPa) than high-pressure homogenization (150-200 Mpa) or microfluidization MPa) [9,10].…”