Antioxidant properties of convective-air-dried Spirulina maxima: Evaluation of phycocyanin retention by a simple mathematical model of air-drying

- Spirulina maxima was air dried under various temperature and humidity conditions.
- Antioxidant properties of the dried algae were analyzed.
- The temperature profiles during drying were simulated by a mathematical model.
- Phycocyanin denaturation kinetics during drying was simulated.
- The major loss of phycocyanin occurred in the falling rate drying period.

Convective-air-dried Spirulina was investigated in terms of the resultant antioxidant properties, namely the phycocyanin content, total phenolic content, and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical scavenging activities. The loss of the antioxidant properties was apparently dependent on the drying conditions, especially on temperature. When the drying temperature was set lower than 50 °C, the antioxidant properties were retained to a larger extent, whereas lesser quantities could be retained at a drying temperature of 80 °C. A simple mathematical model was employed to simulate the temperature profiles during drying, and the resultant phycocyanin content was predicted using the temperature profiles and the experimentally obtained kinetic data of the degradation. These model equations were simple enough to be solved using commercial spreadsheet software, and the profiles of moisture content, mean temperature, and remaining phycocyanin content were predicted as a function of time. Based on the simulation results, an attempt was made to dry at two temperature steps, namely at 80 °C during the constant rate drying period, and at 50 °C during the falling rate drying period. This attempt, drying with two temperature steps, could successfully reduce the total drying time and retain greater amount of antioxidants in the resultant dried products.