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

Regulation of the Ice Ⅰ to Ice III high pressure phase transition meta-stability in milk and its bactericidal effects

This study was focused on a novel approach of creating perturbations under high pressure (HP) meta-stable Ice Ⅰ to Ice Ⅲ phase transition and its bactericidal effects. Experiments were carried out under subzero high pressure processing conditions using Escherichia coli suspended in milk, and the microbial inactivation before and after the meta-stable state regulation was compared. The phase transition position of unperturbed milk was 302 MPa/-37.5 °C. The volume change resulting from the phase transition was employed as the perturbation mechanism. Glucose (5 %, 20 %) and sodium chloride solutions (5 %, 20 %) were used as regulatory sources. Glucose solutions accelerated the phase change of the milk better than the sodium chloride solution and resulted in an optimum phase transition position of milk at 243 MPa/-30.6 °C. The induced perturbations accelerated meta-stable transformation and enhanced the microbial destruction. At 330 MPa/3s, compared to the unfrozen samples, the lethality of E. coli in the frozen-regulated samples significantly increased by 1.79 log. The relationship between the E. coli inactivation within the phase change pressure range and the pressure was not continuous, but a segmented one, both before and after meta-stable state regulation. A higher level of E. coli destruction was accomplished by a 5 min pressure-holding of frozen samples at 220 MPa and 280 MPa as compared to the one-pulse and two-pulses treatments without holding time. The maximum lethality of 6.73 log was achieved at 280 MPa/5 min in the frozen-regulated application.

Recent Progress in the Synergistic Bactericidal Effect of High Pressure and Temperature Processing in Fruits and Vegetables and Related Kinetics

BACKGROUND

Traditional thermal processing is a widely used method to ensure food safety. However, thermal processing leads to a significant decline in food quality, especially in the case of fruits and vegetables. To overcome this drawback, researchers are extensively exploring alternative non-thermal High-Pressure Processing (HPP) technology to ensure microbial safety and retaining the sensory and nutritional quality of food. However, HPP is unable to inactivate the spores of some pathogenic bacteria; thus, HPP in conjunction with moderate- and low-temperature is employed for inactivating the spores of harmful microorganisms. Scope and approach: In this paper, the inactivation effect of high-pressure and high-pressure thermal processing (HPTP) on harmful microorganisms in different food systems, along with the bactericidal kinetics model followed by HPP in certain food samples, have been reviewed. In addition, the effects of different factors such as microorganism species and growth stage, process parameters and pressurization mode, and food composition on microbial inactivation under the combined high-pressure and moderate/low-temperature treatment were discussed.

KEY FINDINGS AND CONCLUSIONS

The establishment of a reliable bactericidal kinetic model and accurate prediction of microbial inactivation will be helpful for industrial design, development, and optimization of safe HPP and HPTP treatment conditions.

Frozen-Phase High-Pressure Destruction Kinetics of Escherichia coli as Influenced by Application Mode, Substrate, and Enrichment Medium

The synergistic effect of frozen-phase high pressure (HP) on the inactivation of E. coli ATCC 25922 cultures in suspension medium, Chinese bayberry juice (pH 3.0), and carrot juice (pH 6.5) was evaluated. The survivor count of E. coli remained at 3.36 log CFU/mL on a nonselective brain heart infusion (BHIA) medium, while no survivor was detected on a selective violet red bile agar (VRBA) medium after a 5 min hold pressure at 250 MPa in a frozen culture suspension. BHIA was suitable for safe testing of the injured E coli cells after HP treatment in frozen state. Frozen Chinese bayberry juice showed higher sensitivity to HP treatment for its matrix property with high sterilizing efficiency at 170 MPa. Two pulses exhibited a significant inactivation effect in frozen samples compared with one pulse, especially for the Chinese bayberry juice with different pressure levels. The destruction kinetics of HP pulse mode followed the first-order rate kinetics with a Zp value of 267 MPa in frozen carrot juice. Our results evaluated the influenced factors of frozen HP destruction effects, including the medium, substrate, and application mode. The frozen HP destruction kinetics of pulses afford us better understanding of the technology application in the food industry.

Demonstration of Escherichia coli Inactivation in Sterile Physiological Saline under High Pressure (HP) Phase Transition Conditions and Analysis of Probable Contribution of HP Metastable Positions Using Model Solutions and Apple Juice

It was demonstrated that the inactivation of high pressure (HP) treatment on Escherichia coli survival in sterile physiological saline (SPS) was influenced by the treatment conditions: unfrozen, frozen-thawed and fully frozen (phase transition). In order to probe the enhanced phase transition microbial destruction, vibration effects of phase transition position were created and discussed. Test samples were placed in HP chamber for treatment (150/240/330 MPa, no holding time) at room temperature and a special cooling device was used to maintain the phase transition conditions. Results showed that the phase transition from ice I to ice III of frozen SPS could be realized based on the cooling of a 20% sodium chloride solution. HP treatment under fully frozen conditions produced the best lethal effect compared to unfrozen and freeze-thaw samples. Vibration tests were carried out by using model solutions and apple juice to explore the behavior of phase transition. A synchronous and advance phase transition of internal apple juice was realized, respectively, by using pure water and 5% sodium chloride solution as external vibration sources, and the advance phase transitions of external pure water were realized by using 5% sodium chloride solution and 5% glucose solution as internal vibration sources.

Synergetic Inactivation Mechanism of Protocatechuic Acid and High Hydrostatic Pressure against Escherichia coli O157:H7

With the wide application of high hydrostatic pressure (HHP) technology in the food industry, safety issues regarding food products, resulting in potential food safety hazards, have arisen. To address such problems, this study explored the synergetic bactericidal effects and mechanisms of protocatechuic acid (PCA) and HHP against Escherichia coli O157:H7. At greater than 200 MPa, PCA (1.25 mg/mL for 60 min) plus HHP treatments had significant synergetic bactericidal effects that positively correlated with pressure. After a combined treatment at 500 MPa for 5 min, an approximate 9.0 log CFU/mL colony decline occurred, whereas the individual HHP and PCA treatments caused 4.48 and 1.06 log CFU/mL colony decreases, respectively. Mechanistically, membrane integrity and morphology were damaged, and the permeability increased when E. coli O157: H7 was exposed to the synergetic stress of PCA plus HHP. Inside cells, the synergetic treatment additionally targeted the activities of enzymes such as superoxide dismutase, catalase and ATPase, which were inhibited significantly (p ≤ 0.05) when exposed to high pressure. Moreover, an analysis of circular dichroism spectra indicated that the synergetic treatment caused a change in DNA structure, which was expressed as the redshift of the characteristic absorption peak. Thus, the synergetic treatment of PCA plus HHP may be used as a decontamination method owing to the good bactericidal effects on multiple targets.

Inactivation of Escherichia coli O157:H7 in apple juice via induced electric field (IEF) and its bactericidal mechanism

Non-conventional heating technology based on electric fields can be utilized to process liquid foods. In this study, the induced electric field (IEF) was investigated to clarify its inactivation mechanism on E.coli. Staining results show that inactivation of E.coli by IEF can be attributed to the reversible destruction of the cell membrane, followed by the denaturation of intracellular enzymes, and finally the irreversible rupture of the cell membrane. The increased levels of extracellular proteins and nucleic acids were also observed. IEF treatment at 400 Hz and 800 V (or 53 V/cm) results in a reduction of 4.5 log CFU·mL^-1 in the number of E.coli. Storage life analysis shows that IEF treatment can improve the stability of apple juice and the content of bioactive components. Thus, IEF is a potential technique for liquid food processing.

Non-thermal Technologies for Food Processing

Food is subjected to various thermal treatments during processes to enhance its shelf-life. But these thermal treatments may result in deterioration of the nutritional and sensory qualities of food. With the change in the lifestyle of people around the globe, their food needs have changed as well. Today's consumer demand is for clean and safe food without compromising the nutritional and sensory qualities of food. This directed the attention of food professionals toward the development of non-thermal technologies that are green, safe, and environment-friendly. In non-thermal processing, food is processed at near room temperature, so there is no damage to food because heat-sensitive nutritious materials are intact in the food, contrary to thermal processing of food. These non-thermal technologies can be utilized for treating all kinds of food like fruits, vegetables, pulses, spices, meat, fish, etc. Non-thermal technologies have emerged largely in the last few decades in food sector.

Non-Thermal Methods for Ensuring the Microbiological Quality and Safety of Seafood

A literature search and systematic review were conducted to present and discuss the most recent research studies for the past twenty years on the application of non-thermal methods for ensuring the microbiological safety and quality of fish and seafood. This review presents the principles and reveals the potential benefits of high hydrostatic pressure processing (HHP), ultrasounds (US), non-thermal atmospheric plasma (NTAP), pulsed electric fields (PEF), and electrolyzed water (EW) as alternative methods to conventional heat treatments. Some of these methods have already been adopted by the seafood industry, while others show promising results in inactivating microbial contaminants or spoilage bacteria from solid or liquid seafood products without affecting the biochemical or sensory quality. The main applications and mechanisms of action for each emerging technology are being discussed. Each of these technologies has a specific mode of microbial inactivation and a specific range of use. Thus, their knowledge is important to design a practical application plan focusing on producing safer, qualitative seafood products with added value following today’s consumers’ needs.