Effect of UHPH on indigenous microbiota of apple juice: a preliminary study of microbial shelf-life.

The effect of ultra high pressure homogenisation (UHPH) at 100, 200 and 300 MPa on apple juice at two different inlet temperatures (4 and 20 degrees C) was studied. Raw and conventional heat treated (PA) apple juice was compared with UHPH treated juice after treatment and during 60 days of storage at 4 degrees C. The microbial quality was studied by enumerating aerobic mesophilic counts (AM), psychrotrophs (PS), moulds and yeasts (MY), lactobacilli (LB), enterobacteriaceae (EB), and faecal coliforms (FC). PA samples were below detection level (< or = -2 log cfu/mL) for all groups, the same as UHPH treated juices at 200 MPa and above but for around 1 log cfu/mL for AM. Those AM counts did not change during 60 days storage at 4 degrees C and proved to be spores. Juices treated at 100 MPa shown little reductions in microbial counts, and surviving microorganisms significantly increased their numbers during storage. This research work showed that non-thermal methods such a UHPH technology may give new opportunities to develop "fresh like" apple juice with a shelf-life equivalent to PA in terms of microbiological characteristics without affecting the product quality.

High pressure inactivation of microorganisms inoculated into ovine milk of different fat contents.

High hydrostatic pressure inactivation of Escherichia coli, Pseudomonas fluorescens, Listeria innocua, Staphylococcus aureus, and Lactobacillus helveticus were studied. These microorganisms were inoculated at a concentration between 10(7) and 10(8) cfu/ml in Ringer solution and in ovine milk adjusted to 0, 6, and 50% fat content to evaluate the baroprotective effect of fat content on inactivation of microorganisms. Treatments of pressurization consisted of combinations of pressure (100 to 500 MPa) and temperature (4, 25, and 50 degrees C) for 15 min. Gram-negative microorganisms were more sensitive than were Gram-positive ones (more destruction P. fluorescens > E. coli > or = List. innocua > Lb. helveticus > S. aureus). Pressurizations at low temperature (4 degrees C) produced greater inactivation on P. fluorescens, List. innocua, and Lb. helveticus than at room temperature (25 degrees C), whereas for E. coli and S. aureus the results were opposite. Ovine milk per se (0% fat) showed a baroprotective effect on all microorganisms, but percentage of fat (6 and 50%) did not show a progressive baroprotective effect in all pressurization conditions or for all microorganisms.

Sensitivity of Staphylococcus aureus and Lactobacillus helveticus in ovine milk subjected to high hydrostatic pressure.

Ovine milk, standardized to 6% fat, was inoculated with Staphylococcus aureus CECT 534 and Lactobacillus helveticus CECT 414 at a concentration of 10(7) cfu/ml and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (200, 300, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50 degrees C), and time (5, 10, and 15 min). Staphylococcus aureus was highly resistant to pressure; only pressurizations at 50 degrees C of 500 MPa for 15 min achieved reductions of > or = 7.3 log units. For L. helveticus, the number of surviving cells was reduced considerably at pressures of 400 MPa or more (up to 4.5 log units at 50 degrees C for 15 min), and pressure was more effective at low (2 and 10 degrees C) and moderately high (50 degrees C) temperatures than at room temperature (25 degrees C). Both species showed first-order kinetics of destruction in the range 0 to 60 min. The D values for S. aureus were 20 min (2 degrees C at 450 MPa) and 16.7 min (25 degrees C at 450 MPa), and D values for L. helveticus were 7.1 min (2 degrees C at 450 MPa) and 9.1 min (25 degrees C at 450 MPa). Lactobacillus helveticus showed higher rates of survival of pressure than those reported in previous studies for other Lactobacillus spp.

Inactivation of Listeria innocua inoculated in liquid whole egg by high hydrostatic pressure.

The resistance of Listeria innocua, as a model microorganism for Listeria monocytogenes, to high hydrostatic pressure in liquid whole egg was studied at several pressures (300, 350, 400, and 450 MPa),temperatures (- 15, 2, and 20 degrees C), and times (5, 10, and 15 min). Listeria innocus was added to liquid whole egg at approximately 10(6) CFU/ml. Listeria innocua was not totally inactivated in any of the treatments. In general, reduction was better at 2 degrees than at room temperature, but the greatest inactivation was obtained at 450 MPa at 20 degrees C for 15 min (over 5 log of reduction), The results indicate that microbial inactivation was increased with prolonged exposure to pressure. D values for Listeria innocua were obtained at 400 MPa for two temperatures (2 and 20 degrees C), and different times (0 to 20 min). The microbial inactivation followed apparent first-order kinetics, exhibiting a decimal reduction time of 7.35 min at 2 degrees C and 8.23 min at 20 degrees C.

Effect of high hydrostatic pressure on Escherichia coli and Pseudomonas fluorescens strains in ovine milk.

Ovine milk that had been standardized to 6% fat was inoculated with Escherichia coli 405 CECT and Pseudomonas fluorescens 378 CECT at a rate of 10(6) and 10(7) cfu/ml, respectively, and treated with high hydrostatic pressure. Treatments consisted of combinations of pressure (300, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50 degrees C), and time (5, 10, and 15 min). Inactivation (> 6 log cfu/ml) of both strains was observed at 50 degrees C for all pressures and treatment times. A similar level of inactivation occurred at > or = 450 MPa and 25 degrees C for E. coli and at > or = 400 MPa and 10 degrees C for P. fluorescens. Destruction was lowest at 10 degrees C for E. coli and at 25 degrees C for P. fluorescens. The test strain of E. coli was more baroresistance than was the P. fluorescens strain.

Effect of high hydrostatic pressure on Listeria innocua 910 CECT inoculated into Ewe's milk.

Ewe's milk standardized to 6% fat was inoculated with Listeria innocua 910 CECT at a concentration of 10(7)CFU/ml and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (200, 300, 350, 400, 450, and 500 MPa), temperature (2, 10, 25, and 50 degrees C), and time (5, 10, and 15 min). To determine numbers of L. innocua, listeria selective agar base with listeria selective supplement and plate count agar was used. Low-temperature (2 degrees C) pressurizations produced higher L. innocua inactivation than treatments at room temperatures (25 degrees C). Pressures between 450 and 500 MPa for 10 to 15 min were needed to achieve reductions of 7 to 8 log units. The kinetics of destruction of L. innocua were first order with D-values of 3.12 min at 2 degrees C and 400 MPa and 4 min at 25 degrees C and 400 MPa. A baroprotective effect of ewe's milk (6% fat) on L. innocua was observed in comparison with other studies using different media and similar pressurization conditions.