Exploring the linear viscoelastic properties structure relationship in processed fruit tissues

Abstract

Texture is a quality attribute that is critical in determining the acceptability of raw and processed fruits, so it is of primary concern in product development and/or preservation techniques design. Texture is a sensory attribute that can only be sensed by people. Perceived texture results from an array of sensory inputs, arising before and during consumption (Jack et al., 1995). As a rule, one texture property is based on various physical properties. However, product texture is closely related with product rheology, and this is one of the most obvious reasons for studying fruit rheology. Mechanical properties of biologic tissues depend on contributions from the different levels of structure: the molecular level (i.e., the chemicals and interactions between the constituting polymers), the cellular level (i.e., the architecture of the tissue cells and their interaction) and the organ level (i.e., the arrangement of cells into tissues and their chemical and physical interactions) (Ilker and Szczesniak, 1990; Waldron et al., 1997; Jackman and Stanley, 1995a; Alzamora et al., 2000). Fruits are composite materials and consist of various structural elements with different mechanical properties. The edible portion of most plant foods is predominantly composed of parenchymatous tissue. The parenchyma cells, approximately 50–500µm across and polyhedral or spherical in shape, show, from out to inner, the middle lamella that glue adjacent cells; the primary cell wall with the plasmodesmata; the plasma membrane; a thin layer of parietal cytoplasm containing different organelles (mitochondrias, spherosomes, plastids, chloroplasts, endoplasmic reticulum, nucleus and so on); and, bound by the tonoplast membrane, one or more vacuoles that contain a watery solution of organic acids, salts, pigments, and flavors that are responsible for the osmotic potential of the cell. Cells and intercellular spaces are arranged into tissues, and these last into the final organ (Brett and Waldron, 1996). This chapter is part of a comprehensive study on the relationship between structure, rheology and texture of raw and minimally processed fruit. With this aim, this chapter is intended to explore the correlation between the linear viscoelastic (oscillatory shear and creep) properties and the microstructure/ultrastructure of selected fruits (melon, apple), as affected by osmotic dehydration and/or calcium incorporation.