Optimization of olive leaves' thin layer, intermittent near-infrared-drying

Abstract

The study aimed to examine the effect of intermittent near-infrared radiation on the quality of olive leaves (OLs) -spread as a thin layer- upon drying using response surface methodology. Temperature and time were selected as independent variables and color, total phenol, flavonoid content, antioxidant activity (DPPH●, CUPRAC assays), the concentration of oleuropein, verbascoside, luteolin-7-O-glucoside and triterpenic acids (oleanolic and maslinic) were used as dependent variables. A statistical model was found for greenness (a*), total phenol content, and antioxidant activity (CUPRAC assay), which were favored upon heating for short time at high temperature. The optimal conditions (116 °C, 3 min) were compared with freeze-drying and oven-drying (120 °C, for 3 and, 12 min). Infrared-drying was faster and superior regarding OLs greenness and oleuropein level compared to oven-drying. Freeze-drying was generally inferior and of equal performance to the two thermal techniques only regarding total flavonoid and triterpenic acid levels. Industrial relevanceThe interest in the exploitation of OLs as a source of bioactive compounds for Pharmaceutical, Food, and Feed applications is continuously growing. Drying is a key step to producing material of high quality. Since conventional approaches can be long-lasting or energy-consuming due to the slow heat transfer there is a need to seek out innovative techniques that can be fast, therefore, economical, and eco-friendly, but also efficient. Thus, in the present work, the efficiency of intermittent near-infrared radiation in producing dried OLs of high quality is discussed and optimum conditions are provided including evidence for its superiority compared to freeze- and oven-drying.