Relative contributions of turgor, cell wall and middle lamella to the mechanical performance of vegetable tissues determined through controlled osmotic stress.

ROJAS A. M.; MARANGONI ALEJANDRO G.; GERSCHENSON L. N.; LATORRE M. E.; DE ESCALADA PLÁ M. F.

Food Engineering

Nova Science Publishers, Inc.

Lugar: Hauppauge, New York.; Año: 2011; p. 259 - 310

Epidemiologic evidence of a protective role exerted by fruits and vegetables in relation to cancer prevention is substantial. The strength of this scientific information guides US national policymaking in diet and health issues and facilitates community and local programs that address national dietary goals to increase fruit and vegetable consumption. The degree of acceptability of fresh fruits and vegetables is a combination of attributes, properties or appearances that give each commodity value in terms of human food. Consumers judge quality of fresh fruits and vegetables on the basis of their appearance and firmness at the time of the initial purchase. Their texture is strongly linked to turgor as well as to the relative resistance of the cell wall and middle lamella during mastication. Turgor is the hydrostatic pressure exerted by the intracellular liquid on the cellular membranes, whose selective permeability gives rise to osmosis. Since plasmalemma has little mechanical resistance, the pressure exerted on and supported by the cell wall accounts for the elasticity of the cells and tissue. The pectin layer between neighbouring cells which constitutes the middle lamella is the polymeric material that produces the adhesion of adjacent cells. The ratio of the mechanical resistances of the cell wall and middle lamella was suggested to determine the sensory perception of juiciness or dry, chalky granular texture or mealiness during mastication. Vegetable tissue softening during ripening, processing and/or storage is attributed to changes in the polysaccharide components of the cell wall as well as to the turgor loss derived either from senescence or from denaturalization of the cell membranes. These two events lead to changes in cell wall thickness, size and shape of cells and volume of intercellular spaces. It is then important to establish the prevailing cellular components in relation to textural properties of fruit or vegetable products as a way to optimize preservation treatments, minimizing changes in the mechanical response of plant tissues. The present chapter deals with the engineering studies on tissue failure as well as on the ?at rest? (structural) response of edible fruit and vegetable tissues after being submitted to artificially induced turgor stress in order to determine the relative magnitude of the contributions of turgor pressure, cell wall, and middle lamella to the rheological properties of the vegetable tissues. A proposed model and methodology was applied with that objective within kiwifruit as a model system and the results obtained from testing different fruit and vegetable as well as their relevance to the structure/texture relationship is discussed.