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Biochemical post-harvest transformations and quality of foods and raw materials

Abstract

Alimentary raw materials and foods are normally organs or living tissue which once harvested continue with their physiological processes. Their quality and conservation depend on these transformations which in turn depend on the genetic patrimony of the plant (organ or tissue) and how this expresses itself in the face of a "stress" represented by the harvesting, storage and future processing. The functional properties (organoleptic, nutritional, conservation and processibility) derive from the biochemical transformations in diverse components such as simple and complex carbohydrates, pigments, vitamins, etc. Thus the knowledge of these transformations and their mechanisms and the possibility of their control are important in the maintenance of quality and the generation of new products, raw materials or technological processes. The thematic objective of this project involves the study of molecular bases associated with the sweetening and the texture which are important attributes of plant quality. The research involves two main themes of conceptual and methodological convergence. The first concerns the transformations suffered by the carbohydrates (disappearance of starch, synthesis of sucrose) in fruits after harvest. Here this concerns studying how these transformations occur during ripening and their control at the level of enzymes of some metabolic routes. The second theme has to do with texture with the hypothesis that it is associated largely with the structural transformations of the cell wall mediated by specific hydrolases. These changes would be the cause of softening in ripening fruits and, in beans, of hardening and increased processing time owing to storage. The starch-sucrose transformation and the synthesis of sucrose are important components of the sensory and nutritional quality and of processibility in fruits. In previous works we studied the variations in the composition of carbohydrates in the ripening of the banana, observing that in the climacteric peak there is an increase in amylasic activity, at the same time as the starch disappears, synthesizing into sucrose. Also confirmed was the incorporation of G14-1_P in sucrose formed in the period. Immediately we observed that the sucrose phosphate synthase (SPS), has its activity increased but, the sucrose synthase (SS) was decreased, a result which was confirmed after partial purification of these enzymes. At the same time, in fruits which do not accumulate starch, the role of these enzymes is not clear. Giving continuity, the objectives of this investigation cover the study of the transformations of the carbohydrates (starch and sucrose), during the ripening of fruits, the tracking of the activity of the amylases and synthetases, their purification and partial characterization, the study of the kinetic properties and their spatial and post-translational expression, to establish the control mechanism involved in the process during this period. The methodology will involve the ripening of fruits harvested green, with the enzymatic activity and the expression of enzymes being tracked through Western, Northern and Southern blottings and other usual techniques of molecular biology. Slices of banana will be infiltrated with specific metabolic inhibitors to study the importance of these enzymes in the development of sugars in the tissues. Very fast changes in texture occur in the ripening of climacteric fruits and in the processing of plant products in general, but the mechanisms of control are still not well understood. In this case, the mechanisms involved will be studied in papaya, combining the study of the chemical composition, structural organization and enzymology of the cell wall and middle lamella, in fruits subjected to normal ripening processes or controlled by irradiation. The methodology will involve instrumental trials alongside biochemical trials, involving the measurement of the activity, partial purification and characterization of enzymes (polygalacturonase, pectin-methylesterase, cellulases and B-galactosidase) and study of the existence of control at the level of proteic synthesis. This knowledge is important for the development of new varieties through technologies such as recombinant DNA, as was already successfully achieved with the partial unbinding of polygalacturonase from the tomato. Another problem linked to texture is that of the bean (Phaseolus vulgaris) stored in conditions of high humidity and temperature, prevalent in several regions of the country, developing a defect known as post-harvest hardening. Rehydration is made more difficult and cooking time is increased at a sensory, nutritional and economic cost. In previous works, we studied the changes that occur in the amyloplast, their consequences in the generation of non-available, resistant starch and we observed alterations in the cell wall. By optical and electronic microscope it was observed that the problem is linked, at least in part, to the non separation of the aged bean cells by the action of cooking. It was also observed, on the other hand, that thermal reactions lead to a reduction in the digestibility of albumin during the processing of the bean. Given this, the objectives of this research will be to study the chemical composition of the cell wall (pectines, hemicellulose, cellulose), composition in phenolic substances and try to identify crossed bonds involving phenols and pectines in beans stored in normal and adverse conditions. For this purpose the instrumental methodology will be used involving mass and infrared spectrometry and high resolution liquid chromatography. It is hoped that based on the study of these changes we will be able to identify some of the causes and establish technologies for the solution of the problem through the participation of improvers, genetic engineering or specific technology. (AU)

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