PROBIEN   20416
INSTITUTO DE INVESTIGACION Y DESARROLLO EN INGENIERIA DE PROCESOS, BIOTECNOLOGIA Y ENERGIAS ALTERNATIVAS
Unidad Ejecutora - UE
capítulos de libros
Título:
Exploring the linear viscoelastic properties-structure relationship in processed fruit tissues
Autor/es:
ALZAMORA, STELLA M.; VIOLLAZ, PASCUAL; MARTÍNEZ, VANESA; NIETO, ANDREA; SALVATORI, DANIELA
Libro:
Food Engineering: Integrating Approaches
Editorial:
Springer
Referencias:
Año: 2008; p. 143 - 167
Resumen:
Texture is a quality attribute that is critical in determining the acceptability of raw and processed fruits, and so it is of primary concern in product development and /or preservation techniques design.  Texture is a sensory attribute that only can 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 the 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 arrangements 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, @ 50–500 mm 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) The three major structural aspects that contribute to textural properties of plant-based foods are turgor, the force exerted on the cell membrane by intracellular fluid, cell wall rigidity, and cell-cell adhesion, determined by the integrity of the middle lamella and the plasmodesmata. In addition, the relative percentage of the different tissues, size and shape of the cells, ratio of cytoplasm to vacuoles, volume of intercellular spaces (which may contain either fluids or interstitial air), type of solutes present, and presence of starch and its state are also important (Ilker and Szczesniak , 1990; Alzamora et al, 2000). The plant cell wall is a strong fibrillar network that gives each cell its stable shape. Various alternative models of cell wall structure have been proposed based on indirect evidence and biochemical hunches (Cosgrove 1997, 1998).  In general, the models postulate that three intertwined networks, stuck together perhaps by various non – covalent adhesions and entanglements,  are responsible for bearing tensile stresses within the wall: 1) a stretch resistant, load – bearing cellulose /hemicelluloses network, where the hemicelluloses are thought to form a hydrogen – bonded surface coat over the microfibrils and may bridge between microfibrils; 2) a compression – resistant pectic polysaccharide network, where the junction zones are originated by calcium cross – linking and by ester linkages with dihydroxy – cinnamic acids, and 3) a third network consisting of structural proteins covalently linked by oxidative phenolic cross – bridges and other linkages (Carpita and Gibeaut, 1993). The cellulose microfibrils (with crystalline and amorphous regions) are remarkably stable and usually undergo negligible breakdown under stress, playing a major rol in determining the strength and structural bias of the cell wall; the disposition of microfibrils in space also determines many of the mechanical properties of the wall. Pectins make up a hydrophilic phase with gel - like properties situated in the space between microfibrils, preventing aggregation and collapse of the cellulose network and allowing for easy slippage between the celluloses – hemicelluloses. The wall matrix is approximately 75% water by mass resembling a very dense aqueous gel. The macro- micro- and ultrastructure of tissues are affected by processing, occurring several modifications which influence the mechanical behavior and the perceived texture. Thus, establishing and understanding a relation between texture and instrumental mechanical parameters also requires determining what are the most essential structural elements and to which fundamental rheologic parameters they are primarily linked. Knowledge resulting from investigations on plant cell structure during processing and its influence on viscoelastic behavior (determined by small deformation tests) and failure properties (determined by large deformations methods) can be utilized for the development of tailor-made technologies or the optimization of existing techniques for the production of processed fruits with specific mechanical properties (Kunzek et al, 1999).                 At the macroscopic level, fruits are viscoelastic systems that exhibit a combination of elastic and viscous behavior under mechanical loading, which means that force, distance and time- in the form of rate, extent and duration of load - determine the value of measurements (Pitt, 1992). Determining the structural characteristics of polymers has been for long the main focus of linear viscoelastic tests (Ferry, 1980). It is assumed that these techniques are very sensitive means of studying microstructure. However, the use of linear viscoelastic data for characterizing  food materials has just began in the last fifteen years and today research efforts are focused in correlating transient or dynamic methods to sensory characteristics and structure of foods (mainly dairy products and gels). Until now, these tests have not been very much used to elucidate the structure – function correlation in vegetable tissues. 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 a) 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.