Influência da dinâmica de partículas inertes no rendimento da secagem de polpa de acerola utilizando secador rotatório não-convencional

Abstract

Acerola (Malphigia emarginata D.C.) is a tropical fruit originating in the Antilles and Brazil is the largest producer in the world. For owning high levels of vitamin C and other bioactive compounds, acerola has become popular, being known for its antioxidant capacity. Due to the high perishability of this fruit, drying is a simple and low-cost alternative for preservation, improving storage and transportation, in addition to reducing weight, volume and inhibiting microbial activity. Therefore, the rotary dryer with inert filling (SRRI) presents itself as a promising alternative for the production of powdered dehydrated products from food pastes. In this equipment, a current of heated air comes into contact with the pulp and the inert particles inside the drum which, during its rotation, promote particle-particle and particle-drum wall collisions. Through the phenomena of heat and mass transfer, the formation of powder occurs, which is collected by a cyclone. In view of the above, this work aimed to investigate the drying of acerola pulp using a rotary dryer with inert filling (SRRI) experimentally in order to obtain a powdered dehydrated product, in addition to evaluate how inert particles of different densities influence the drying performance. The numerical study was carried out through simulations using the Lagrangian numerical approach (Discrete Element Method – DEM), whose contact law allowed obtaining information about how particle-particle and particle-drum wall interactions occur. For each type of inert particle (stainless steel, ceramic and polypropylene) of two different sizes (25.40 mm in diameter and 12.70 mm in diameter), 5 simulations were performed varying the fraction of minor inerts (FI), totaling 15 simulations, in order to evaluate the normal collision force, the collision velocity and the number of collisions. The filling degree parameters were fixed at 35% and rotation speed at 70 rpm. It was possible to analyze that, with the increase in the revenue of smaller aggregates, the number off collisions between particles per second also increased, which is favorable to the drying process. In the condition with the lowest percentage of FI, the collision force for stainless steel spheres was 424.77% greater than ceramic spheres and 1209.27% greater than polypropylene spheres. The properties of the inert spheres also had influence on the particle velocity. The speed of the stainless steel spheres was 10.79% higher when compared with the ceramic ones and 66.40% higher when compared with the polypropylene ones in the case in which the highest values of speed were obtained. Regarding the experimental study, the characterization of the acerola pulp was first carried out, with analysis of moisture, water activity, pH, density, ash content and composition analysis in FRX. Three PCC's were carried out (one for each type of inert particle) in order to evaluate the effect of maltodextrin concentration, minor inert fraction and temperature on the drying yield. Each PCC had 16 experiments, totaling 48 experiments. For all analyzed conditions, the maximum moisture of the product obtained was 3.2% and the highest water activity was 0.193, guaranteeing the inhibition of microbial activity. With regard to drying performance, the best results were obtained using inert stainless steel particles, followed by ceramic and polypropylene particles. The highest values of maltodextrin concentration, inert fraction and temperature generated a higher drying yield. The optimization performed resulted in a concentration of 21.72% of maltodextrin, 75.03% of minor inert fraction and 95.74°C, with a desirability of 1.0 (highest possible value). For the experiments carried out under these conditions, yields of 82,73% were obtained for stainless steel, 67,15% for ceramics and 53,91% for polypropylene. Therefore, it is concluded that the SRRI showed good efficiency for the dehydration of acerola pulp and is promising for further studies and for industrial application.