Abstract
High hydrostatic pressure (HHP) is a novel food processing technology that is considered as an attractive alternative to conventional heat treatments for the preservation of foods, due to its lethal effects on pathogenic and spoilage microorganisms, while causing minor effects on food quality and sensorial attributes. This study is aimed at investigating how HHP treatments at varying intensities in the range 50–900 MPa affect the viability, membrane integrity, ultrastructure and molecular composition of Escherichia coli. Results of membrane integrity tests (measurement of cellular leakage and monitoring of propidium iodide uptake through fluorescence microscopy) and ultrastructural observations by transmission electron microscopy demonstrated that HHP gave rise to cellular enlargement, membrane damage or detachment, DNA and protein denaturation and loss of intracellular contents. Fourier-transform infrared (FTIR) spectroscopy analyses evidenced minor changes in molecular composition in response to high pressures, which were mostly observed on the spectral region w4 (1200–900 cm−1), mainly informative of carbohydrates and polysaccharides of the cell wall. These findings suggest that exposure of E. coli cells to HHP causes alterations in their physical integrity while producing minor modifications in biochemical cellular composition. The current study increases the knowledge on the mechanisms of E. coli inactivation by HHP and provides valuable information for the design of more effective food preservation regimes based on the integration of mild HHP in combination with other food preservation strategies into a multi-target hurdle technology approach.
Highlights
High hydrostatic pressure (HHP) is a food processing technology used to maintain the quality attributes of fresh foods, while extending their shelf life through the inactivation of pathogenic and spoilage microorganisms, as well as of endogenous enzymes
Stationary-phase cultures of Escherichia coli E218/02 and Escherichia coli C-600 were exposed to different pressure-time combinations (50 MPa, 24 h; 300 MPa, 5 min; 600 MPa, 5 min; and 900 MPa, 5 min)
Whereas HHP treatments at 50 MPa for 24 h did not give rise to significant reductions in the bacterial population, exposure to 300 MPa for 5 min resulted in the inactivation of 4.5 and 4.7 log cycles for E. coli E218/02 and E. coli C-600, respectively, and treatment at 600 MPa for 5 min gave rise to a 6.8 and 7.3 log reduction, respectively (Figure 1A)
Summary
High hydrostatic pressure (HHP) is a food processing technology used to maintain the quality attributes of fresh foods, while extending their shelf life through the inactivation of pathogenic and spoilage microorganisms, as well as of endogenous enzymes. Great research efforts have been made in recent years to evaluate the safety of HHP-treated foods by studying the inactivation kinetics by HHP of pathogenic microorganisms in food and food models [4]. The foodborne pathogens, Staphylococcus aureus, Listeria monocytogenes, Salmonella spp. and verocytotoxigenic Escherichia coli (VTEC), are among the bacterial species most extensively studied with this aim [1]. VTEC, characterized by the production of Shiga toxins, are important foodborne pathogens in the European Union, with 5,671 reported human cases in 2012, which yields a community incidence rate of 1.15 per 100,000 population [5].
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