A. Angersbach

Preservation of liquid foods by high intensity pulsed electric fields—basic concepts for process design

Abstract In excess of a critical transmembrane potential ΔϕM of −1 V produced by high intensity pulsed electric fields a rapid electrical breakdown and local conformational changes of cell membranes occur which result in a drastic increase in permeability and an equilibration of the electrochemical and electrical potential differences of the cell plasma and the extracellular medium. As irreverible membrane permeabilization impairs most vital physiological control systems, high intensity pulsed electric fields may be applied as a highly effective process for the microbial decontamination of liquid foods. The efficiency of the treatment is largely influenced by the inherent properties of the foods and of the spoiling microorganisms. In addition a number of technical limitations have to be considered. In this review an approach is presented which reduces the diversity of parameters that affect microbial inactivation during pulsed power treatment. In particular, the required total specific energy input is discussed.

Impact of high-intensity electric field pulses on plant membrane permeabilization

The application of high-intensity electric field pulses in food processing provides exciting opportunities to food companies seeking mild, safe processing technologies. The technique offers potential both as a preservation technology and as an adjunct to other processes, such as drying. In respect of such uses of high-intensity pulsed electric fields, the controlled permeabilization of plant membranes is of particular value. This review examines recent work in this area and considers attempts to adapt and optimize the technology.

Electrophysiological Model of Intact and Processed Plant Tissues: Cell Disintegration Criteria

Frequency versus conductivity relationships of food cell system, based on impedance measurements as characterized by polarization effects of the Maxwell−Wagner type at intact membrane interfaces, are presented. The electrical properties of a biological membrane (represented as a resistor and capacitor) are responsible for the dependence of the total conductivity of the cell system on the alternating current frequency. Based on an equivalent circuit model of a single plant cell, the electrical conductivity spectrum of the cell system in intact plant tissue (potato, carrot, banana, and apple) was determined in a frequency range between 3 kHz and 50 MHz. The electrical properties of a cell system with different ratios of intact/ruptured cells could also be predicted on the basis of a description of a cell system consisting of elementary layers with regularly distributed intact and ruptured cells as well as of extracellular compartments. This simple determination of the degree of cell permeabilization (cell disintegration index, po) is based upon electric conductivity changes in the cell sample. For accurate calculations of po, the sample conductivities before and after treatment, obtained at low‐ (fl) and high‐frequency (fh) ranges of the so‐called β‐dispersion, were used. In this study with plant cell systems, characteristic conductivities used were measured at frequencies fl = 3 kHz and fh = 12.5 MHz. The disintegration index was used to analyze the degree of cell disruption after different treatments (such as mechanical disruption, heating, freeze−thaw cycles, application of electric field pulses, and enzymatic treatment) of the plant tissues.

Use of pulsed electric field pre-treatment to improve dehydration characteristics of plant based foods

Abstract Conventional dehydration of fruits and vegetables affects their physical and biochemical status leading to shrinkage, change of colour, texture and taste. Alteration of the physical properties of foods with minimal influence on the quality could be a means of reducing drying time, minimising quality degradation and saving energy. Increasing consumer markets for minimally processed fruits and vegetables have prompted researchers to study combined methods as preservation techniques. Pulsed electric field is one of the more promising non- thermal processing method inducing membrane permeabilisation within a very short time (μs to ms range) leaving the product matrix largely unchanged while positively affecting mass transfer in subsequent processing of foods. Rapid and accurate on-line determination of the state of cell membrane systems is important in optimising various processes (i.e. minimizing cell damage in minimal processes, monitoring disruption for mass transfer purposes and inducing biosynthetic stress/wound reactions/responses). This paper reviews recent work on the use of pulsed electric fields as an upstream process in dehydration and rehydration of plant based foods. An effective and simple method for quantifying extent of membrane permeabilization is also discussed and suggestions for future work are highlighted.

Processing concepts based on high intensity electric field pulses

Emerging non-thermal food processing techniques are receiving considerable attention. This is because of their potential for quality and safety improvement of our food supply, for the ability to enhance conventional processing operations or to create alternative ones, as well as modify existing or non-conventional raw materials, food constituents or processed foods. High intensity electric field pulse technology is one of the most advanced emerging non-thermal processing methods and is currently undergoing intensive scientific and developmental evaluation. The objectives of this review are to summarize and identify the key advantages of this emerging technology and to convert it into optimum use. Those attempts are a combination of research on a kinetic basis and on a cellular level and the subsequent transfer of the knowledge gained into pilot scale.

Osmotic dehydration of bell peppers: influence of high intensity electric field pulses and elevated temperature treatment

Abstract Osmotic dehydration of bell peppers using sucrose and sodium chloride as osmotic agents as influenced by moderate thermal treatment (25–55 °C) and high intensity electric field pulses at varying field strengths (E=0.5–2.5 kV/cm) was studied. Two product quality indicators (vitamin C and carotenoids) were evaluated. Increasing temperature resulted in water loss from 32% to 48% and increasing field strength resulted in water loss from 36% to 50% of initial moisture content. Both conditions enhanced solid gain during osmotic dehydration of bell pepper. Air drying reduced vitamin C to approximately 5% of initial concentration while increasing temperature (25–55 °C) during osmotic dehydration decreased residual vitamin C concentration after osmotic dehydration from 20% to 4% and high intensity electric field (2.5–0.5 kV/cm) decreased it from 13% to 7% of initial value. Carotenoids reduced from 80% to 55% as a result of temperature increase and from 74% to 62% of initial fresh content as a result of high intensity electric field pre-treatment. Results obtained at field strength 2.5 kV/cm were comparable and in some cases better than those at elevated temperature of 55 °C suggesting high intensity electric field as an attractive alternative to conventional thermal processing.

Synergistic effect of high hydrostatic pressure pretreatment and osmotic stress on mass transfer during osmotic dehydration

During osmotic removal of water from foods, the osmotic dehydration front moves from the surface of the food in contact with the surrounding osmotic solution to the centre, which results in disintegration of cells due to osmotic stress. When the food is pretreated with high hydrostatic pressure (HHP), it also results in cell permeabilisation. The cell permeabilisation index (Zp, as measured by an electrophysical measurement based on electrical impedance analysis) after high pressure treatment increases with time. Osmotic dehydration of HHP-treated foods is faster than that of untreated foods. The state of the cell membrane during osmotic dehydration of high-pressure-pretreated samples can change from being partially to totally permeable, which leads to significant changes in the tissue architecture resulting in increased mass transfer rates during osmotic dehydration as compared to untreated samples.

Comparative evaluation of the effects of pulsed electric field and freezing on cell membrane permeabilisation and mass transfer during dehydration of red bell peppers

Abstract The extent of cell membrane permeabilisation due to high intensity electric field pulses (HELP) varying number of pulses (1–50) using electric field of 2 kV/cm, 400 μs pulse duration and freezing on mass transfer and vitamin C content during osmotic (50° Brix sucrose at 40 °C) and convective air (60 °C, 1 m/s for 5 h) dehydration of red bell peppers was studied. Total pore area due to HELP increased with number of pulses while freezing resulted in total pore area of almost 6 times as greater as the highest value from the HELP process. Higher water loss was observed for all HELP treated than for prefrozen samples while slow freezing provided samples with the highest solids uptake. The correlation coefficient (R2) of linear regression between water loss and solids gain estimated from either total solids or soluble solids measurement ranged from 0.954 to 0.998 suggesting the possibility of using the soluble solids method in evaluating mass transfer kinetics during osmotic dehydration process. Drying rate during convective air-drying was more enhanced by HELP than by freezing. Electrical conductivity of the osmotic solution increased with the degree of permeabilisation to a given medium value after which no further increase in the release of the intracellular ions was observed. Minimal vitamin C depletion was observed immediately after HELP treatment. The order of magnitude of vitamin C retention was untreated>frozen>HELP pretreated samples with 1 pulse>5 pulses>50 pulses>10 pulses>20 pulses after osmotic dehydration. The reduction in vitamin C content of HELP treated samples after convective drying ranged from approximately 11 to 24% while freezing resulted in approximately 24% decrease compared to the untreated samples.

Effects of high hydrostatic pressure or high intensity electrical field pulse pre-treatment on dehydration characteristics of red paprika

The effects of various pre-treatments (hot water blanching, skin treatments, high pressure and high intensity electric field pulse treatment) on the dehydration characteristics of red paprika (Capsicum annuum L.) were evaluated and compared with untreated samples. Hot water blanching (100°C, 3 min) prior to dehydration (fluidised bed dryer at 60°C, 6 h and 1 m/s) resulted in the permeabilisation of 88% of the cell membranes in paprika, which in turn resulted in a higher mass and heat transfer. Skin treatments (such as lye peeling and acid treatment), as practised conventionally, increased dehydration rates but affected only the skin permeability. The application of high hydrostatic pressure (HHP, 400 MPa for 10 min at 25°C) or high intensity electric field pulses (HELP, 2.4 kV/cm, pulse width 300 μs, 10 pulses, pulse frequency 1 Hz) pre-treatments resulted in cell disintegration indexes of 0.58 and 0.61, respectively. Cell permeabilisation of these physical treatments resulted in higher drying rates, as well as higher mass and heat transfer coefficients, as compared to conventional pre-treatments.

Evaluation of process-induced dimensional changes in the membrane structure of biological cells using impedance measurement

The impact of high intensity electric field pulses, high hydrostatic pressure, and freezing‐thawing on local structural changes of the membrane was determined for potato, sugar beet tissue, and yeast suspensions. On the basis of the electrophysical model of cell systems in biological tissues and suspensions, a method was derived for determining the extent of local damage of cell membranes. The method was characterized by an accurate and rapid on‐line determination of frequency‐dependent electrical conductivity properties from which information on microscopic events on cellular level may be deduced. Evaluation was based on the measurement of the relative change in the sampleapos;s impedance at characteristically low ( fl) and high ( fh) frequencies within the β‐dispersion range. For plant and animal cells the characteristic frequencies were fl ≈ 5 kHz and fh > 5 MHz and for yeast cells in the range fl ≈ 50 kHz and fh > 25 MHz. The observed phenomena were complex. The identification of the underlying mechanisms required consideration of the time‐dependent nature of the processing effects and stress reactions of the biological systems, which ranged from seconds to several hours. A very low but significantly detectable membrane damage (0.004% of the total area) was found after high hydrostatic pressure treatment of potato tissue at 200 MPa. The membrane rupture in plant tissue cells was higher after freezing and subsequent thawing (0.9% of total area for potato cells and 0.05–0.07% for sugar beet cells determined immediately after thawing), which increased substantially during the next 2 h.