Inactivation of Listeria monocytogenes in raw and hot smoked trout fillets by high hydrostatic pressure processing combined with liquid smoke and freezing. Innovative Food Science & Emerging Technologies, 64. 102427.

Understanding phase transition during high-pressure (HP) processing of foods is important both with respect to optimizing the process and improvement of product quality, but scientific information available in this area is very limited. In this study, the phase-transition behavior of water was evaluated using a HP differential scanning calorimetry (DSC). Tests were carried out under both isothermal pressure-scan (P-scan) and isobaric temperature-scan (T-scan) modes with distilled water prefrozen in the sample cell. P-scan was carried out at 0.3 MPa/min at two temperatures, -10 and -20C, and T-scan was carried out at 0.15C/min at two pressures, 0.1 and 115 MPa. The pressure-induced phase transition of water was accurately reproduced by the P-scan test. Ice melting latent heat during P-scan showed no significant difference ( P > 0.05) from the available reference data in literature. The relationship between P-scan tested (L m ) and reference latent heat was L m = 0.987 L (R 2 = 0.99, n = 6) suggesting a mean error less than 2%. T-scan mode was less appropriate and did not yield promising result. Measured values were less accurate than P-scan probably because of the influence of large heat capacity of sample cell. However, reliable and reproducible results obtained under P-scan mode suggested that the HP DSC can be used for the calorimetric determination of pressure-dependent water-phase transition in real food systems during HP freezing/thawing operations.

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A twin-screw extruder and a rotational rheometer were used to generate shear forces in concentrated gelatin inoculated with a heat-resistant isolate of a vegetative bacterial species,Microbacterium lacticum. Shear forces in the extruder were mainly controlled by varying the water feed rate. The water content of the extrudates changed between 19 and 45% (wet weight basis). Higher shear forces generated at low water contents and the calculated die wall shear stress correlated strongly with bacterial destruction. No surviving microorganisms could be detected at the highest wall shear stress of 409 kPa, giving log reduction of 5.3 (minimum detection level, 2 × 104 CFU/sample). The mean residence time of the microorganism in the extruder was 49 to 58 s, and the maximum temperature measured in the end of the die was 73°C. TheD 75°C of the microorganism in gelatin at 65% water content was 20 min. It is concluded that the physical forces generated in the reverse screw element and the extruder die rather than heat played a major part in cell destruction. In a rotational rheometer, after shearing of a mix of microorganisms with gelatin at 65% (wt/wt) moisture content for 4 min at a shear stress of 2.8 kPa and a temperature of 75°C, the number of surviving microorganisms in the sheared sample was 5.2 × 106 CFU/g of sample compared with 1.4 × 108 CFU/g of sample in the nonsheared control. The relative effectiveness of physical forces in the killing of bacteria and destruction of starch granules is discussed.

Synergistic action of high hydrostatic pressure (HHP) and freezing on inactivation of Escherichia coli K12 in phosphate buffered saline (PBS) was investigated by employing response surface methodology. Samples containing E. coli K12 were stored at 4, −24 and −80 °C overnight before they were pressurized. A maximum of 1.83 log reduction of CFU•ml -1 was obtained following a 9-min treatment at 400 MPa and 4±1 °C in samples stored at 4 °C whereas, 5.63 and 6.83 log reductions were obtained in samples frozen at −24 and −80 °C, respectively. Major disruption of E. coli cells observed by scanning electron microscopy and increased amounts of DNA and RNA measured in pressure treated frozen PBS samples indicated that the main mechanism of inactivation in frozen samples was due to cell rapture. The validity of enhanced microbial inactivation by freezing before HHP for a real food system was tested by using orange juice. Pressurization (250 MPa, 15 min) of frozen (−80 °C) orange juice resulted in 4.88, 4.15 and 4.61 log CFU•ml -1 reductions in number of E. coli for the samples having pH 3.2, 4.5 and 5.8, respectively. In the absence of freezing, the same treatment caused only up to 0.42 log reduction in samples having pH 4.5 and 5.8.

Inactivation of Escherichia coli strain ATCC 25922 in frozen and unfrozen milk by high pressure processing at low or subzero temperatures was investigated. Frozen ( − 21°C) and unfrozen (3°C) milk samples were exposed to a pressure of 300 MPa for 0.5-15 min at varying temperatures between − 5°C and 5°C. Reduction in the number of E. coli in unfrozen milk was enhanced by pressure treatment at subzero temperatures, whereas no survival was detected in frozen milk samples after a 10 min pressure treatment at − 3°C, corresponding to a minimum of 6.8 log cycle reduction in the number of E. coli. A substantial reduction of 3.8 log reduction in number of E. coli was observed after a 30 s pressure treatment at − 3°C. Results suggested that the mechanical forces could play a role in enhanced inactivation of E. coli in frozen milk samples.

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