Preservation of cell viability in fruit and vegetable tissues after freezing and thawing

Cryoprotection of horticultural products has been a matter of concern for the food industry in its search for efficient ways of improving the quality of frozen-thawed products. Research on the cryoprotection of plant tissues should make progress through a better understanding of the natural protective mechanisms of plant tissues during winter survival. The present study reports results on the use of vacuum impregnation (VI) alone or in combination with pulsed electric field (PEF) to obtain cryoprotectant solutions in the tissues. Spinach samples were treated with a combination of PEF and VI prior to freezing in liquid nitrogen and thawing at room temperature. VI was used to impregnate trehalose (40% w/w solution) as cryoprotectant and PEF was used to distribute the cryoprotectant in the extra and intracellular spaces of the tissue. Strawberries were treated with VI prior to freezing in liquid nitrogen ant thawing at room temperature. VI was used to impregnate a combination of trehalose (12% w/w) and cold acclimated winter wheat extract (AWWE, 0.02% w/w) containing antifreeze protein. Results were evaluated by assessing the maintenance of the texture of the tissue as well as by cell viability analysis using fluorescence microscopy. Results provide evidence that impregnating fruit and vegetables tissues using VI alone or in combination with PEF with the tested cryoprotectants improves drastically the cryoprotection of the treated tissues. Cryoprotection is proved through the maintenance of cell viability and texture after one freezing/thawing cycle. In the case of strawberries, cryoprotection was influenced by the heterogeneity of the tissues in the fruit and the viability of cells close to the surface (epidermal and probably also hypodermal) could not be preserved.