The effects of temperature, air speed and IR radiation on drying kinetics and some quality factors of saffron

Saffron is a dry, red color of the Crocus Sativus L. It is the most expensive spice in the world. Drying is an important step in the process of saffron production. Drying causes physical, chemical and biochemical changes to achieve the desired properties of saffron. In this study, the kinetics of drying saffron stigma using a combined hot air-infrared dryer at three temperature levels of 40, 50 and 60°C and two hot air flow speeds of 0.3 and 0.6 m/s as well as an ordinary hot-air dryer using the same conditions were studied. The effect of the studied parameters on the amount of crocin, picocrocin and safranal and the microbial load of the final product were also investigated. The results showed that, the Midley model was found to be the best model in fitting data of both drying methods. Infrared irradiation and increased temperature and air velocity caused a dramatic reduction in the drying time of saffron. The infiltration coefficient ranged from 1.8003×10-08 to 2.38346×10-08 m2/s for the hot air-infra-red method and 5.07×10- 10 to 2.7892×10-09 m2/s for hot air drying. The amount of activation energy in the combination method of hot air-infrared  varied from 10.43 kJ/mol to 11.6 kJ/mol, and was obtained at 34.1 kJ/mol and 62.4 kJ/mol in hot-air drying. In terms of maintaining the color strength of saffron, the highest crocin was obtained when the combination of hot air-infrared at 50 ° C and 0.3 m / s was used with an average of   1%E440nm 278.5, which was the highest among all other treatments. According to the national standard of Iran, microbial tests were carried out including identification of E. coli, identification of enterococci, identification of sulfite reducing clostridia and counting mold and yeast on different treatments. No significant differences were observed between the treatments.