| dc.description.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. | |
