Effect of high temperature on drying behavior and food properties of hazelnuts

Hazelnut is a highly important agricultural product for our country and is one of the indispensable components of the food industry due to its diverse uses in various sectors such as cosmetics and personal care, pharmaceuticals and supplements, handicrafts, and decoration. Hazelnut production is especially widespread in the Black Sea region of our country and is an important agricultural export product. To preserve the quality and durability of hazelnuts, they must be stored after drying. Hazelnuts are mostly dried under the sun, while a smaller proportion is dried using mechanical systems. The moisture content of freshly harvested hazelnuts can be up to 38%, and this value must be reduced below the equilibrium moisture content of 6% through drying methods. The drying time and temperature significantly affect the food properties of hazelnuts. This study investigates the effects of mechanical drying temperature on the drying behavior and food properties of hazelnuts. In the experiments, two hazelnut varieties Tombul and Çakıldak were used. Fresh hazelnuts harvested from the branch were husked and dried simultaneously in ovens at five different temperatures (45°C, 50°C, 55°C, 60°C, 65°C) and five different durations (1 h, 2 h, 3 h, 4 h, and continuous). For comparison purposes, hazelnuts were also dried in the shade and under sunlight. Moisture, oil content, free fatty acidity, peroxide, protein, total phenolic compound and flavonoid values were determined on dried samples. While the initial moisture of fresh Çakıldak and Tombul hazelnuts was 31% and 30.14%, respectively, the internal hazelnut moisture was reduced to 5.41% and 4.43% on average as a result of drying. Considering the oil analysis results, the average oil ratios of Çakıldak and Tombul hazelnuts were found to be 53.72% and 57.22%, respectively. For 45 ⁰C, 50 ⁰C continuous, 55 ⁰C continuous, 60 ⁰C continuous and 65 ⁰C drying conditions, the % peroxide value was determined as 0.48, 0.9, 0.44, 0.65, 0.9 meq/kg for Tombul hazelnuts, respectively, while for Çakıldak hazelnuts, it was determined as 1.3, 1.4, 2, 1.6, 1.4 meq/kg, respectively. While the average free fatty acid (FFA) value of Tombul hazelnuts was 0.12%, the average free fatty acid (FFA) values of Çakıldak hazelnuts were measured as 0.11%. The average protein% value for Çakıldak and Tombul hazelnuts is 19.42 and 17.45, respectively. For Çakıldak hazelnut, the average total phenolic compound values were measured as 10.46 g GAE kg-1 dw, 11.48 g GAE kg-1 dw, 10.42 g GAE kg-1 dw and 10.58 g GAE kg-1 dw under drying conditions of 45 ℃, 50 ℃, 55 ℃, 60 ℃ and 65 ℃, respectively, while for Tombul hazelnut, the average total phenolic compound values were measured as 9.04 g GAE kg-1 dw, 10.48 g GAE kg-1 dw, 11.58 g GAE kg-1 dw and 10.52 g GAE kg-1 dw, respectively. When the average flavonoid values for Çakıldak hazelnut were compared according to 45 ℃, 50 ℃, 55 ℃, 60 ℃ and 65 ℃ drying conditions were measured as 0.8 g QE kg-1 dw, 0.72 g QE kg-1 dw, 0.7 g QE kg-1 dw, 0.72 g QE kg-1 dw and 0.8 g QE kg-1 dw, respectively, while the average flavonoid values for Tombul hazelnut were measured as 1.6 g QE kg-1 dw, 1.18 g QE kg-1 dw, 0.88 g QE kg-1 dw, 1.06 g QE kg-1 dw and 1.14 g QE kg-1 dw, respectively. For Çakıldak hazelnuts, the time required for the hazelnuts to drop to the desired moisture value in continuous drying at 45 ℃ is 3030 minutes, while for Tombul hazelnuts it is 3080 minutes. The increase in drying temperature caused the drying time to shorten in both varieties.