Inactivation ofEscherichia coliin milk by high pressure processing at low and subzero temperatures

Abstract

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.

Inactivation of Listeria monocytogenes in raw and hot smoked trout fillets by high hydrostatic pressure processing combined with liquid smoke and freezing

“…Subsequently, we introduced freezing prior to HHP treatment. Freezing combined with HHP showed to exert antimicrobial effects as shown previously (Bulut, 2014a;2014b;Su et al, 2014b). The liquid smoke enhanced the antimicrobial effects of HHP and freezing, but this additional effect was not major.…”

“…Also, sterile BHI broth containing 3.80 % (w/v) NaCl was prepared. Subsequently, all samples were heat-sealed after removing all air as described by Bulut (2014b) and finally placed individually into a second plastic bag made of polyamide/polyethylene and vacuum packed for 30 s using a Fresco 300 Packaging Machine (SousVidetools.com).…”

“…Phase transitions of ice crystals during the pressurization of frozen food have been previously suggested to take place during the HHP treatment that results in cellular damage and increased microbial inactivation (Luscher et al, 2004;Shen et al, 2005;Su et al, 2014a). Several studies have proposed that the application of HHP on frozen products at low or subzero temperatures and atmospheric conditions can enhance the microbial inactivation and thus allow the usage of lower pressure treatments (Knorr et al, 1998;Fernández et al, 2007;Vaudagna et al, 2012;Bulut, 2014a;Bulut, 2014b;Albertos, 2016;Parlapani et al, 2019). The effectiveness of the combined HHP and freezing treatment depends on the characteristics of the product (pH, aw, composition, etc.…”

Inactivation of Escherichia coli K12 in phosphate buffer saline and orange juice by high hydrostatic pressure processing combined with freezing

“…Similar observations were reported previously by Su, et al (2014) who showed more than 6 log reduction in CFU•ml -1 for E. coli ATCC 25922 in frozen nutrient broth after 1 min at 350 MPa and room temperature. In previous work (Bulut, 2014b) we reported a 3.8 log reduction in CFU•ml -1 of E. coli ATCC 25922 in frozen milk after 1 min exposure to 300 MPa at…”

“…The authors suggested that solid-solid phase transitions were mainly responsible for the observed drop in cell viability. Bulut (2014b) showed that a 5 min pressure treatment at 300 MPa and 0 °C resulted in 1.8 log cycles of CFU•ml -1 reduction in numbers of E. coli strain ATCC 25922 in milk. Freezing the samples at -21°C overnight prior to pressurization under the same conditions increased the log reduction to 5.2 log CFU•ml -1 .…”