B.I.O. Ade-Omowaye

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.

Nutritional improvement of plant foods by non-thermal processing.

As a result of the increasing consumer demand for minimally-processed fresh-like food products with high sensory and nutritional qualities, there is a growing interest in non-thermal processes for food processing and preservation. Key advanced technologies such as high-pressure processing, pulsed electric fields, dense gases and ultrasound are being applied to develop gentle but targeted processes to further improve the quality and safety of processed foods. These technologies also offer the potential for improving existing processes as well as for developing new process options. Furthermore, by adding new process dimensions (such as hydrostatic pressure, electric fields, ultrasonics, supercritical CO2) to the conventional process variables of temperature and time, they facilitate enlargement of the availability of unit operations. These operations might be applied effectively in unique combination processes, or as subsequent processing tools in more-targeted and subsequently less-intensive processes for food preservation and modification than the currently-applied processes.

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.

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.