ESTUDIO DE LA TRANSFERENCIA DE CALOR EN EL TERMOPROCESO DE PURÉ DE GUAYABA (Psidium guajava L.) EN INTERCAMBIADORES DE SUPERFICIE RASPADA ANALIZANDO EL EFECTO SOBRE SUS COMPONENTES BIOQUÍMICOS

Abstract

Was studied heat transfer, residence times and effect on the biochemical properties of the product in a scraped surface heat exchanger of three parts: heating, pre-cooling and cooling, was used guava puree as work fluid. Experimental variables were: feed stream, heating temperature and rotational speed of scrapers. The fluid inlet temperature pre-cooling and cooling was kept constant at 20±1 and -10±0.5 °C respectively in all experiments. The scrapers speed of was the same for the bodies of the three stages. To evaluate the effect of experimental variables, we used a 23 factorial design with three replications at the central point, considering as response variables: the internal coefficient of heat transfer in each of the bodies, percentage retention of ascorbic acid, phenols total antioxidant activity, total change in color and degree of inactivation of the enzyme pectin methyl esterase. Internal coefficients experimental heat transfer compared with those predicted by the equation of Skelland for each operating condition used. The indicated results that the heat transfer coefficients are obtained when higher working power to flow and rotation speed of the upper scraper (400 mL/min and 500 rpm). By comparing the heat transfer coefficients obtained with experimental and theoretical equation Skelland, it was observed that the latter over-estimate the coefficients in approximately 20% compared to the experimental but strongly depend on the viscosity value which is considered in the calculation, which must include both the average temperature variation of the working fluid as the effect of rotating scrapers on viscosity. The thermal treatments to guava puree generally managed to retain about 80% of vitamin C, total phenolics and antioxidant activity, with good color retention, however the only conditions of operation as a whole inactivated the enzyme pectin methyl esterase, were those in which the puree reached on average 90 °C to leave the heating stage, showing the highest residence time (Tm = 6.24 min), which cause the greater change in color and less retention of biochemical components analyzed.