Investigating the Effect of Roasting Temperature and Storage Time of Hazelnut Seed on the Color and Quality of its Dough

Introduction  All the different species of hazelnut trees produce edible nuts. Hazelnuts are one of the most nutritious nuts, with a protein content of about 12%. They are also a good source of energy, with a fat content of about 60%, and an excellent source of carbohydrates, half of which are dietary fiber. Hazelnuts also contain minerals (Ca, Mg, P, K), vitamins (E and B), and antioxidants. Hazelnut kernels may be eaten raw or roasted. Roasting alters and significantly enhances the color, texture, and appearance of nuts. These changes increase the overall palatability of the nuts and are mainly related to non-enzymatic browning. Non-enzymatic browning, however, causes a decrease in nutritional value due to decrease in protein digestibility and loss of essential amino acids. The most widely used nut roasting method is the convective heat transfer process, which is performed in a hot air oven working either in continuous mode or in batch systems. Therefore, modifying this process for improving product quality is very important. Color is among the most important quality attributes of dehydrated foods for consumers. Empirically, color is also an effective quality indicator because the brown pigments increase as the browning and caramelization reactions progress. Therefore, this study aimed to investigate the effect of high roasting temperature and storage time on color changes and oxidation of hazelnut paste oil.   Materials and Methods  After getting hazelnut kernels, the Roasting process was performed with hot air at 130 and 170 ℃ for 20 minutes. Hazelnut paste is produced as practiced in the industry. Quality analysis was roasted hazelnut paste color changes and fat oxidation. L *, a *, b * color index, (∆E), acidity, and peroxide value were measured in three intervals (0, 10, 30 days’). All experiments were performed in three replications. Average Data was compared using one-way ANOVA and done by SPSS version 21.   Results and Discussion  The main purpose of roasting is to improve the flavor, the color, and the crispy and crunchy texture of the product. The roasting process is very important to determine the characteristic roasted flavor and color of the product: in particular, temperature modulation is an important independent variable significantly affecting the quality features of hazelnut. In this study, ANOVA analysis results showed a significant difference between the treatments. Both roasting temperature and storage time affect the acidity and peroxide value. The increased temperature and storage increased the acidity and peroxide index, which indicated fat oxidation at higher temperatures. For ten days, the sample's peroxide value increased. After that, these parameters decreased because the first autoxidation products changing to the second product. Due to the release of free fatty acids after the basic hydrolysis of triglycerides, the acidity of the samples increased. The Browning color of roasted products was desirable. Discoloration during the roasting process of hazelnut kernels is mainly related to non-enzymatic browning. The brown pigments increase with the development of browning reactions. Results showed that the color index includes the L*, a*, b*, and ∆E values of the samples only affected by roasting temperature. With increasing the temperature, the L* value decreased but other values increased. Color indicators significantly changed and are not desirable for the consumers. The rate of color changes increased, including browning, indicating a non-enzymatic browning reaction in the samples during roasting.   Conclusion According to the results, hazelnuts roasted at 130℃ introduced as the best treatment. It is recommended that the storage time does not exceed ten days.