Combining nonthermal technologies to control foodborne microorganisms

High pressure homogenization versus heat treatment: effect on survival, growth, and metabolism of dairy Leuconostoc strains

The effect of high pressure homogenization (HPH) with respect to a traditional heat treatment on the inactivation, growth at 8°C after treatments, and volatile profile of adventitious Leuconostoc strains isolated from Cremoso Argentino spoiled cheeses and ingredients used for their manufacture was evaluated. Most Leuconostoc strains revealed elevated resistance to HPH (eight passes, 100 MPa), especially when resuspended in skim milk. Heat treatment was more efficient than HPH in inactivating Leuconostoc cells at the three initial levels tested. The levels of alcohols and sulfur compounds increased during incubation at 8°C in HPH-treated samples, while the highest amounts of aldehydes and ketones characterized were in heated samples. Leuconostoc cells resuspended in skim milk and subjected to one single-pass HPH treatment using an industrial-scale machine showed remarkable reductions in viable cell counts only when 300 and 400 MPa were applied. However, the cell counts of treated samples rose rapidly after only 5 days of storage at 8°C. The Leuconostoc strains tested in this work were highly resistant to the inactivation treatments applied. Neither HPH nor heat treatment assured their total destruction, even though they were more sensitive to the thermal treatment. To enhance the inhibitory effect on Leuconostoc cells, HPH should be combined with a mild heat treatment, which in addition to efficient microbial inactivation, could allow maximal retention of the physicochemical properties of the product.

Electrode activation sequencing employing conductivity changes in irreversible electroporation tissue ablation

Irreversible electroporation (IRE) uses high-voltage pulses applied to tissue, which cause dielectric breakdown of cell membranes resulting in cell death. IRE is a promising technique for ablation of nonresectable tumors because it can be configured to spare critical structures such as blood vessels. A consequence of pulse application is an increase in tissue electrical conductivity due to current pathways being opened in cell membranes. We propose a novel IRE method introducing electrode switching and pulse sequencing in which tissue conductivity is first increased using preparatory pulses in order to form high-conductivity zones, which then helps provide higher electric field intensity within the targeted tissue as subsequent pulses are applied, and hence, enhances the efficiency and selectivity of the IRE treatment. We demonstrate the potential of this method using computational models on simple geometries.

A comparative study on the structure of Saccharomyces cerevisiae under nonthermal technologies: high hydrostatic pressure, pulsed electric fields and thermo-sonication

Nonthermal technologies are becoming more popular in food processing; however, little detailed research has been conducted on the study of the lethal effect of these technologies on certain microorganisms. Saccharomyces cerevisiae is a yeast related to spoilage of fruit products such as juices; novel technologies have been explored to inactivate this yeast. Three nonthermal technologies, high hydrostatic pressure (HHP), pulsed electric fields (PEF) and thermo-sonication (TS), were used to evaluate and to compare the structural damage of yeast cells after processing. Processing conditions were chosen based on previous experiments to ensure the death of cells; HHP was conducted at 600 MPa for 7 min (room temperature, 21 °C); for PEF, 30.76 kV/cm at 40 °C and 21 pulses (2 μs each), and finally for TS the conditions were 120 μm, 60 °C and 30 min in continuous and pulsed modes; all treatments were applied in apple juice. Cells were prepared for electron microscopy using an innovative and short microwave assisted dehydration technique. Scanning electron microscopy showed the degree of damage to the cells after processing and illustrated the important and particular characteristics of each technology. Cells treated with high hydrostatic pressure showed a total disruption of the cell membrane, perforation, and release of the cell wall; scars were also observed on the surface of the pressurized cells. PEF treated cells showed less superficial damage, with the main changes being the deformation of the cells, apparent fusion of cells, the formation of pores, and the breakdown of the cell wall in some cells. Finally, the thermo-sonicated cells showed a similar degree of cellular damage to their structure regardless of whether the TS was applied continuously or pulsed. The main characteristics of cellular death for this technology were the erosion and disruption of the cellular membrane, formation of orifices on the surface, lysis of cells causing the release of intracellular contents, roughness of the cell membrane, and displacement of cell debris to the surface of other cells. This study confirms some theories about cell inactivation and presents new and detailed results about nonthermal technologies, but also shows that after using the above mentioned conditions, recovery of cells, specifically those that are pressurized and thermo-sonicated, it is not possible to do it following the high extent of damage observed in the entire population. Furthermore, a faster methodology that was used in sample preparation for electron microscopy provided high quality resolution images, allowing closer study of the detail of structural lethal effects on treated cells.

Factors affecting the inactivation of the natural microbiota of milk processed by pulsed electric fields and cross-flow microfiltration

Prior to processing milk and cream were standardised and homogenised. Skim milk was cross-flow microfiltered (CFMF) prior to treatment with pulsed electric fields (PEF) or high temperature short time (HTST) pasteurization. The effect of temperature of the skim milk and product composition on the efficacy of PEF treatment was determined. The electrical conductivity of the product was related to fat and solids content and increased 5% for every g/kg increase of solids and decreased by nearly 0·7% for every g/kg increase of fat. From the three microbial groups analyzed (mesophilic, coliform, and psychrotroph) in milks differences (P<0·05) in the inactivation of mesophilic microorganisms were observed between the counts following PEF treatment, while HTST pasteurization resulted in higher reductions in all different counts than those obtained after PEF. Increasing the skim milk temperature prior to PEF treatment to about 34°C showed equivalent reductions in microbial counts to skim milk treated at 6°C in half the time. The reductions achieved by a combination of CFMF and PEF treatments were comparable to those achieved when CFMF was combined with HTST pasteurization. A higher reduction in coliform counts was observed in homogenised products subjected to PEF than in products that were only standardised for fat content.

Inactivation of polyphenol oxidase from watermelon juice by high pressure carbon dioxide treatment

The inactivation of polyphenol oxidase from watermelon juice with high pressure carbon dioxide (HPCD) treatment was investigated. The maximum reduction of polyphenol oxidase (PPO) activity inactivated by HPCD treatment was 95.8% at 30 MPa and 50 °C for 30 min, which was far higher than 50.9% of control treatment at 50 °C for 30 min. The inactivation of PPO was adequately described by a two-fraction model, which indicated that a labile and stable fraction might present in PPO from watermelon juice. The kinetic rate constants kL and kS of labile and stable fractions were 1.976 and 0.041 min(-1) by HPCD treatment of 30 MPa and 50 °C. And the labile fraction was easier to be inactivated by kinetic analysis. HPCD treatment with the combined effects of pressure, temperature, pH reduction, and time was stronger to inactivate PPO from watermelon juice than control treatment at the same temperature.

LacI(Ts)-regulated expression as an in situ intracellular biomolecular thermometer

In response to needs for in situ thermometry, a temperature-sensitive vector was adapted to report changes in the intracellular heat content of Escherichia coli in near-real time. This model system utilized vectors expressing increasing quantities of β-galactosidase in response to stepwise temperature increases through a biologically relevant range (22 to 45°C). As judged by calibrated fluorometric and colorimetric reporters, both whole E. coli cells and lysates expressed significant repeatable changes in β-galactosidase activity that were sensitive to temperature changes of less than 1°C (35 to 45°C). This model system suggests that changes in cellular heat content can be detected independently of the medium in which cells are maintained, a feature of particular importance where the medium is heterogeneous or nonaqueous, or otherwise has a low heat transfer capacity. We report here that the intracellular temperature can be reliably obtained in near-real time using reliable fluorescent reporting systems from cellular scales, with a 20°C range of detection and at least 0.7°C sensitivity between 35 and 45°C.

Inactivation kinetics and virulence potential of Salmonella Typhimurium and Listeria monocytogenes treated by combined high pressure and nisin

The aim of this study was to characterize the physiological and molecular changes of Salmonella Typhimurium and Listeria monocytogenes in deionized water (DIW) and nisin solutions (100 IU/g) during high pressure processing (HPP). Strains of Salmonella Typhimurium and L. monocytogenes in DIW or nisin solutions were subjected to 200, 300, and 400 MPa for 20 min. The Weibull model adequately described the HPP inactivation of Salmonella Typhimurium and L. monocytogenes. Salmonella Typhimurium and L. monocytogenes populations were reduced to less than 1 CFU/ml in DIW and nisin solutions under 400 MPa. The highest b value was 5.75 for Salmonella Typhimurium in nisin solution under 400 MPa. L. monocytogenes was more sensitive to pressure change when suspended in DIW than when suspended in nisin. The pressure sensitivity of both Salmonella Typhimurium and L. monocytogenes was higher in DIW solution (141 to 243 MPa) than in nisin solution (608 to 872 MPa). No recovery of HPP-injured cells in DIW and nisin solutions treated at 400 MPa was observed after 7 days of refrigerated storage. The heterogeneity of HPP-treated cells was revealed in flow cytometry dot plots. The transcripts of stn, invA, prfA, and inlA were relatively down-regulated in HPP-treated nisin solution. The combination of high pressure and nisin could noticeably suppress the expression of virulence-associated genes. These results provide useful information for understanding the physiological and molecular characteristics of foodborne pathogens under high-pressure stress.

Preservation technologies for fresh meat - a review

Fresh meat is a highly perishable product due to its biological composition. Many interrelated factors influence the shelf life and freshness of meat such as holding temperature, atmospheric oxygen (O(2)), endogenous enzymes, moisture, light and most importantly, micro-organisms. With the increased demand for high quality, convenience, safety, fresh appearance and an extended shelf life in fresh meat products, alternative non-thermal preservation technologies such as high hydrostatic pressure, superchilling, natural biopreservatives and active packaging have been proposed and investigated. Whilst some of these technologies are efficient at inactivating the micro-organisms most commonly related to food-borne diseases, they are not effective against spores. To increase their efficacy against vegetative cells, a combination of several preservation technologies under the so-called hurdle concept has also been investigated. The objective of this review is to describe current methods and developing technologies for preserving fresh meat. The benefits of some new technologies and their industrial limitations is presented and discussed.

Overview of current meat hygiene and safety risks and summary of recent studies on biofilms, and control of Escherichia coli O157:H7 in nonintact, and Listeria monocytogenes in ready-to-eat, meat products

As meat consumption is increasing around the world, so do concerns and challenges to meat hygiene and safety. These concerns are mostly of a biological nature and include bacterial pathogens, such as Escherichia coli O157:H7, Salmonella and Campylobacter in raw meat and poultry, and Listeria monocytogenes in ready-to-eat processed products, while viral pathogens are of major concern at foodservice. A major goal of scientists, industry, public health and regulatory authorities is to control pathogenic microorganisms and improve meat product hygiene and safety within a country and internationally. This paper is not a comprehensive or critical review of the scientific literature on the broad area of meat hygiene and safety, but it provides an overview of major current meat hygiene and safety issues, and then a summary of studies on biofilm formation by pathogens, control of E. coli O157:H7 in nonintact meat products, and control of L. monocytogenes in ready-to-eat meat products, conducted at the Center for Meat Safety & Quality and Food Safety Cluster of Colorado State University in recent years.

Pressure effects on the structure and stability of the hyperthermophilic trehalose/maltose-binding protein from Thermococcus litoralis

In this work, we investigated the effect of pressure on the structure and stability of the recombinant D-trehalose/D-maltose-binding protein isolated from the hyperthermophilic archaeon Thermococcus litoralis (TMBP). The spectroscopic results obtained both in the absence and in the presence of maltose or trehalose revealed that the TMBP-Mal complex exhibits a larger structural stability under high pressure values than TMBP-Tre complex. In addition, the results also pointed out that pressure induces reversible denaturation transitions of the protein structure. By combining the fluorescence results obtained with 8-anilino-1-naphtalene sulfonate as extrinsic probe and the intrinsic indolic fluorescence of TMBP, it is evident that the protein structural changes above 400 MPa that involve the exposure to the solvent of a large portion of the hydrophobic protein domains are preceded by a partially unfolded protein structural state. The spectroscopic results have been interpreted and discussed by taking into account the X-ray structure of the protein and, in particular, the interactions of maltose and trehalose within the three-dimensional structure of TMBP.