Ο δυόσμος ως πρότυπο φυτό για τη βελτιστοποίηση της ξήρανσης βιοδραστικών ουσιών μέσω εκχυλισμάτων

Abstract

Dried mint is a popular product that is commonly used as an ingredient in the food industry, in domestic cooking or as a decoction. The effects that drying has on the quality parameters of mint, can be limited or controlled by the study and optimization of the process parameters; this was the main aim of the present study. The quality parameters evaluated included colour, microbial safety and stability, and concentration in health-promoting compounds. Initially, the effect of the application of different drying technologies was studied; results showed that freeze-drying leads to a product containing the highest amounts of phenolic compounds and possesses the highest antioxidant capacity. The industrial application of freeze-drying is extremely difficult though, owing to its high cost. Industrially, the use of convection (hot-air) drying is most common, due to the capability it provides to handle large quantities of fresh material in a reasonable amount of time and under controlled conditions. Therefore, a pilot scale convection oven was designed and manufactured, and a bi-factorial experiment was performed. Four levels of temperature (40, 50, 60 και 70°C) were combined with four levels of air velocity (1.5, 2.0, 2.5 και 3.0 ms-1), leading to a total of 16 treatments. Temperature had a main effect on all parameters studied, and in some cases interaction of the two factors was also statistically significant. The optimum color, based on lightness and greenness, was achieved by drying at 40°C and 3.0 ms-1, probably due to the lowest temperature and the shortest duration of the process. In contrast, the maximal recovery of phenolic compounds and antioxidant capacity was attributed to the material dried by 50°C at 1.5-2.0 ms-1. All combinations achieved high microbial quality of the end-product. Findings indicate that the optimal drying conditions depend on the intended use. Mathematical modeling was utilized in order to describe the changes in moisture content during drying. Effective moisture diffusivity (Deff) was calculated and ranged from 1.956x10-12 to 2.192x10-11 m2s-1, supporting its limited use as a constant. Therefore, a linear model was developed by exploring the relationship between water flux and the driving force of drying, which is the gradient between water activity in the material and in the air. The model provided promising results, since the calculated constant was consistent. Finally, the use of dried mint as a source of phenolic ingredients for the use in functional foods or the cosmetics industry was studied. Ultrasonic assisted extraction using 70% ethanol produced the highest results. The method was further studied with the use of an ultrasonic probe and was subsequently mathematically modeled. In order to describe the enhanced retrieval of phenolic compounds and hydroxycinnamic acid derivatives, and antioxidant capacity, the term sonochemical potential was coined. The gradient in sonochemical potential is the driving force for the heightened flux of the studied parameters.