Shelf life of fresh-cut fruits and salad leaves treated with Pulsed Electric Field (PEF)

Abstract

Application of Pulsed Electric Field (PEF) treatment on five fresh commodities was analysed for the changes in the metabolic and physiological effects on their shelf life. Treatment parameters were optimised in order to achieve uniform electroporation on the samples, which was analysed by Propidium Iodide staining for the leaf samples. The viability of electroporation of greenhouse (GH) rocket, commercial rocket and spinach leaves was evaluated by FDA vital staining. Bulk evaluation of electroporation of mango and kiwi fruits along with the leaf samples was done by impedance measurement, monitoring tissue leakage caused by permeabilization. The evaluation of metabolic responses of the samples was carried out by isothermal calorimeter where increase in metabolic heat production (thermal power) of all the treated samples was recorded. The increase in thermal power is the result of an increase in the internal metabolic activity of tissues, provoked by PEF induced stress. A decrease in metabolic heat rate was observed with increasing intensity of PEF parameters, due to the possible consequences of loss in cell viability.
After the optimisation of PEF parameters, the samples were treated, packed and stored at 4 °C. Measurements for the colour change was recorded using spectrophotometer and composition of O2 and CO2 was monitored using a gas analyser. PEF treatment showed effective results in preserving the colour of greenhouse rocket leaves. No effects of treatment were observed in commercial rocket and spinach leaves and the treatment resulted in deterioration of mango and kiwi fruits. Composition of oxygen and carbon dioxide did not show differences among samples.

Popular Abstract

With growing concerns of consumers regarding processed foods, there has been an increase in minimal processing of food products by the industries. Minimal processing does not involve processing techniques such as heat treatment, the addition of additives and preservatives because they reduce the nutritional profile of fresh fruits and vegetables. However, cutting the fresh produce exposes their inner constituents to the outer environment which reduces their shelf life. Spoilage of fresh-cut products can be caused as a result of oxidation when oxygen in the air reacts with the produce, presence of microorganisms that may cause degradation and chemical reactions within the product. Also, fresh green vegetables have fairly short shelf life under refrigerated conditions, therefore minimal processing could be challenging to aim for longer storage life. Extending shelf life is not only aimed at reducing food wastage, but it also increases the profitability of farmers, industries and supermarkets.

Pulsed Electric Field (PEF) Technology is a processing technique which uses high voltage electric pulses that creates pores in the cell membrane of living cells. This process of creation of pores in the membrane by the application of electrical field is termed as electroporation. Depending on the strength of electric field application, PEF is categorised in either reversible or irreversible form. The irreversible electroporation is most widely used where the application of very high electric field strength creates permanent damage to the cell membrane. This method is employed to deactivate spoilage-causing microorganisms and to better extract of juices and pigments from the cell interior. Reversible electroporation, on the other hand, is achieved by using a lower strength of electric pulses which provides the cells to repair their pores and continue with normal functionality. It was theorised that if reversible electroporation alters the chemical, physical and enzymatic reactions inside the cells of fresh products, it might influence their shelf life after the cells recover from the electroporation. Every plant cell respires and produces heat due to chemical reactions taking place, as a result of metabolic activity, This project explores the possibilities of reversible PEF treatment on the change in the metabolism of fresh fruits and green vegetables and its effects on their storage shelf life.

Mango, kiwi fruit, baby spinach leaves, greenhouse rocket and commercial rocket leaves were five different fresh commodities chosen for this project. In order to differentiate between the electrical treatments which could generate reversible and irreversible electroporation, the PEF treatment parameters were first tested by treating the products with different strengths of the electric field. In order to identify the uniformity of treatment on the surface of the leaves, they were treated with fluorescent chemicals that were examined under the microscope where the bright nucleus of electroporated cells can be identified. The effect of electroporation inside the surface was observed by resistance measurement. Depending on the strength of treatment, the leakage of cell’s inner constituents in the solution presents resistance to the flow of current which is measured over a range of frequencies, where the lower value of resistance shows more leakage or degradation of cells and vice versa. Changes in the metabolic heat were recorded by a calorimeter, an instrument which measures the heat produced by a product with a function of time at a constant temperature. Treatment with PEF generates stress response in plant tissues which increases its metabolic heat, and that could be measured in the calorimeter. For the fruits, however, due to different cell structures, their PEF parameter was established using both resistance and calorimeter measurements. A decrease in resistance and metabolic heat was noted with an increase in the strength of the applied electric field. This drop in the measurement was the result of the degradation of tissues (irreversibility of the treatment) because higher electric field strength deteriorates more cells.

After obtaining the optimum parameters, the samples were treated and packed in sealed plastic packages and stored at refrigerated temperature. The gas composition (carbon dioxide and oxygen content) inside the packages and colour of the samples was measured in order to understand the effects of PEF treatment both in terms of respiration rate as well as the colour change of samples during their storage life. The treatment showed effective results on greenhouse-grown spinach leaves in preserving the green colour. On the other hand, the shelf life of commercial rocket and spinach leaves showed no difference, and the treatment was found to be detrimental for the fruit samples. No difference was observed between Carbon dioxide and Oxygen levels inside the packages for any commodity which could be the result of normal respiration rate after the recovery of cells in treated samples.

• author: Soni, Chandan
• supervisor: Federico Gómez Galindo
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLTM01 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Agriculture and Food Sciences; Technology and Engineering
• keywords: PEF, pulsed electric field, reversible electroporation, calorimeter, metabolic rate, shelf life, fresh commodities, food engineering, livsmedelsteknik
• language: English
• id: 8997057
• date added to LUP: 2019-10-29 11:21:36
• date last changed: 2019-10-29 11:21:36