Effects of spray drying conditions on physicochemical properties and antioxidant activities of raspberry powder

Abstract: In order to improve physicochemical properties and antioxidant properties of raspberry powder, maltodextrin was used as a drying carrier to investigate the effects of spray drying conditions on the physicochemical properties and antioxidant properties of raspberry powder. The process was performed using a laboratory-scale spray dryer. Before the mixture was fed into the drying chamber at room temperature (25 ℃) through a peristaltic pump, raspberry juice and maltodextrin were mixed together and the ratio of the amount of maltodextrin to solid content of raspberry juice was 1:1. The inlet air temperature was 210 ℃ and the outlet air temperature varied from 60 to 70 ℃. Spray drying was carried out with a drying air flow rate of 0.99 m3/min and an aspirator flow rate of 6.67 mL/min. The powders were obtained in the receiving flask. The results showed that the inlet air temperature (190-230 ℃), maltodextrin contents (70%-110%, calculated according to solids of raspberry juice), hot air flow (0.66-1.10 m3/min), and inlet flow rate (2.67-8 mL/min) had significant effects on the physicochemical properties and antioxidant properties of raspberry powder. The raspberry powder yield, bulk density, dissolution time, redness and yellowness values all showed an upward trend. The moisture content, brightness value, total phenolic content, the DPPH free radical scavenging rate, the reducing power, the superoxide free radical scavenging rate, the hydroxyl free radical scavenging rate and the ABTS scavenging rate all showed a downward trend. Moreover, when the addition of maltodextrin reached 110%, the yield, total phenolic content and antioxidant capacity significantly increased, while the inlet air temperature 220 ℃, hot air flow 0.99 m3/min and inlet flow rate 6.67 mL/min made the yield higher, and the total phenol content and antioxidant capacity were significantly lowered (P<0.05). Maltodextrin as a drying carrier raises the Tg (glass transition temperature) of raspberry powder, and it works together with inlet air temperature, hot air flow and inlet flow rate to regulate moisture contents. Since spray drying can quickly evaporate the water, the moisture decreases and the yield of the fruit powder increases, while the particles are not easily bonded, the space volume decreases and the bulk density increases. But because of the low moisture content, it is easy to plug internal moisture path and make it become hard, and the dissolution time increases. Moreover, browning reaction occurs during spray drying, resulting in changes in color. Inlet air temperature, the amount of maltodextrin, hot air flow and inlet flow rate have obvious effects on total phenolic content, DPPH scavenging capacity, reducing power, superoxide free radical scavenging rate, hydroxyl radical scavenging rate and ABTS scavenging rate (P<0.05), and the phenolic compounds have better effects on scavenging DPPH and superoxide anion free radicals because of lower IC50 value. Spray drying can cause the oxidation of polyphenols, decomposition reaction, especially at higher temperatures, so proanthocyanidins and other polyphenols are prone to decomposition, while polyphenols at higher concentrations easily cause interactions with protein, carbohydrate substances and minerals so as to induce discoloration of the raspberry powder.