PhD: Non-thermal preservation of fruits and vegetables

Elena Velickova
Department for Food Technology and Biotechnology, Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University
January, 2013


The non-thermal preservation methods could be easily combined with the traditional preservation methods to produce high quality food products. The new mild processing techniques used in this study include: osmotic dehydration, vacuum impregnation, pulse electric field and edible coatings.

Five raw materials with different tissue structure such as potato, carrot, Jerusalem artichoke, apple and strawberry were used for the osmotic treatment. The changes in tissue’s density and porosity under different temperatures and concentrations of the osmotic solutions were evaluated. The effect of different parameters on the osmotic dehydration of the different plant tissues in terms of water loss and solid gain, were investigated. The water removal rate was in the range from 0.076 to 0.624 gwater/gѕм·h, while the solute uptake’s rate was from 0.040 to 1.043 g/gѕм·h. The water diffusion coefficient was in the range of 0.8 to 16·10-11 m2/s. Osmotic treatment deactivates the enzymes and it preserves well the aroma and the color, the mechanical and nutritive properties of the fruits and vegetables. The dried apples with well preserved optical and nutritive properties, having the common characteristics of crispy products were transformed in a new product, apple chips.

The effect of cryoprotectants on the freezing tolerance of strawberry tissues were evaluated by means of vacuum infusion and pulse electric field. The cryoprotectants used in this study were trehalose and antifreeze protein. They were vacuum infused in the strawberries’ apoplast. The pulse electric field enabled direct impregnation of the plant cells with cryoprotective solutes. The tissue absorbed 9% and 14% of antifreeze protein and trehalose. When the combination of the both solutions was used the tissue absorbed 19%. The combined effect of both cryoprotectants significantly improved the freezing tolerance of the treated strawberries and maintained 90% viability of the plant cells after thawing.

The minimal processing of fruits was carried out with edible films. Edible coatings act as moisture and gas barriers and improve the quality and shelf-life of minimally processed fruits and vegetables. For that purpose 10 chitosan-based edible films were prepared. Chitosan films were modified by crosslinking, application of a beeswax coating on both interfaces of the film and preparation of composite films. All edible films were characterized in terms of their optical, mechanical, barrier, sorption and thermal properties. The incorporation of the beeswax into the edible films decreased the water vapour permeability value from 4,15·10-11 to 1.5·10-11 mol·m/m2·s·Pa. The edible coatings as a preservation method were tested on strawberries. The coatings modified the respiration rates of the strawberries and slowed down their metabolism extending their shelf-life at 20 ºC. The three-layer coatings enabled the best protection of the overall quality of the strawberries, but the chitosan and composite coatings were more preferable for the consumers.

The emerging technologies in combination with the already existing methods besides providing microbial stability, fresh look and extended shelf life also gave better preservation of the sensorial and nutritional properties of the fruits and the vegetables.