FREEZE DRYING OF STRAWBERRY. EXPERIMENTAL STUDY AND MATHEMATICAL MODELLING OF THE SUBLIMATION PERIOD

REALE, V. ADRIAN; TORREZ IRIGOYEN, MARTÍN R.

Buenos Aires

Congreso; 11th WORLD CONGRESS OF CHEMICAL ENGINEERING; 2023

Freeze-drying is a dehydration technique that involves freezing the sample and then sublimating the ice in a primary drying or sublimation period (SP), ending with a desorption stage or secondary drying (SS) of the adsorbed water that was not previously frozen. Both steps are carried out under high vacuum. The technique, when carried out at low temperatures (T below 50 ºC), usually retains a high proportion of the micronutrients present in the fresh fruit, particularly bioactive compounds (vitamins and antioxidants). The purpose of this work was to develop theoretical/practical knowledge of this technique to add value to fruit and vegetable production in the highly productive peri-urban belt of La Plata (CPP), but with a low level of industrialization. The products thus obtained can be exported or destined for the domestic consumer market. A Rificor L-A-B4-C, manufactured in Argentina, was used, equipped with a vacuum chamber with four thermostatised shelves and trays of 0.30 m in diameter. During the process, the absolute pressure of the drying chamber was monitored by means of a Pirani type sensor, as well as the tray temperature (TT). The only programmable value is TT, which provides the heat of sublimation and desorption heat, which was set at values of 30, 40 and 50 ºC. We worked with strawberry slices from the CPP, 0.01 m thick, which were frozen at -40 ºC and then freeze-dried at the set Tb. The experiments were carried out in duplicate, taking a frozen slice, placing it inside the equipment, and weighing the slice every 15 minutes with a digital scale installed inside the freeze-dryer drying chamber. The moisture content of the samples during the process was determined by the official method AOAC 934.06. The transition from SP to SS was obtained theoretically from the calculation of the moisture content at the end of the sublimation period. To mathematically model the SP, a mathematical model was developed based on balance equations of mass transfer and applying regression techniques to the model by using experimental data. The adjusted dried layer permeability (b) values for SP were found to be between 5.4×10-10 and 7.5×10-10 kg water (m Pa s)-1, being comparable with those found in the literature for freeze-dried products. The moisture content vs. time curves predicted by the model fitted the experimental data satisfactorily. In turn, primary drying times of 12, 9 and 8 h at Tb of 30, 40 and 50 °C, respectively, were calculated, showing a decrease in drying time as Tb increases. A positive linear correlation between b with Tb was found. These parameters obtained by fitting would be useful in the design of freeze-drying equipment and processes, oriented at obtaining value-added fruit products of high nutritional quality.