Stability and microstructure of freeze-dried guava pulp (Psidium guajava L.) with added sucrose and pectin.

Freeze-dried guava pulp powders, formulated with the addition of sucrose (0-20 g/100 g pulp) and pectin (0-1.0 g/100 g pulp), were obtained, and their stability was evaluated with respect to the water adsorption isotherms, thermal analysis and microstructure. The GAB (Guggenheim-Anderson-de Boer), Peleg and BET (Brunauer-Emmett-Teller) models were used to evaluate the water adsorption. The microstructure was examined using optical microscopy with polarized light and scanning electron microscopy (SEM). The GAB and BET parameters showed that the moisture content of the monolayer (Xm) increases with increasing pectin concentration in the adsorption isotherms. Optical microscopy micrographs showed that the pulp consisting of sucrose showed crystalline structures present in a higher amount and size, and this behavior is enhanced with increasing relative moisture. SEM showed that the increase in sucrose and pectin concentrations produced powders with lower porosity, providing greater stability to the product. The glass transition temperature increased slightly with increasing pectin concentration and decreased with increasing moisture content in the guava pulp powder. The kinetic curves, ratio of the increase of the water content against the storage time, of the guava pulp treated with 20 g sucrose per 100 g pulp and higher pectin concentration (0.5 or 1.0 g pectin per 100 g pulp) showed reduced adsorption and yielded freeze-dried pulps that were more stable.

Effects of added sucrose and pectin on the rheological behavior and freezing kinetics of passion fruit pulp Studied by response surface methodology.

The effects of pectin and sucrose addition on the rheological parameters and freezing kinetic of passion fruit pulp were studied. The effect of the air-blast freezing of passion fruit at -20 °C on the rheological parameters before and after freezing was evaluated. The rheological analyses were carried out using a viscometer at 25 °C, and the readings were converted into rheological measurements using the Mitschka method and fitted to the Ostwald-de Waele model. The freezing kinetics was analyzed using a controlled-temperature cold-stage instrument, which was coupled to a microscope integrated with a video capture system. The concentrations of sucrose and pectin were established using a Composite Rotational Design. The pulps combined with the additives exhibited a pseudoplastic behavior, and the values for the index flow (n) and consistency index (k) before and after freezing differed and were dependent on the additive concentrations. The rates of increase for the frozen areas were evaluated in a microfreezer and were significantly influenced by the additive concentrations. These concentrations were those that presented a lower index flow and a higher consistency index. The results are discussed in terms of the solubility and interaction of the pectin added to the fruit pulp with low pH, the presence of sucrose and low temperature.