Introduction: Roasting is an essential process that improves the taste, color, texture and appearance of the product. The shelf-life is also extended as a result of roasting. The temperature and the duration of roasting are the most important factors that influence favorable traits. IR is a novel technique for roasting that, compared to conventional heating, has positive advantages such as shorten heating time, significant energy saving and uniform heating. This study aimed to investigate the effect of IR, compared to conventional, roasting on some physicochemical and quality properties of soybeans. Moreover, the conditions of roasting soybeans via the two mentioned methods were optimized using Response Surface Methodology (RSM).
Material and methods: Soybean seeds were collected from a commercial farm in Gorgan (North of Iran) and were dried in an oven at 40 °C for 48 hours until the moisture content became lower than 5 % w/w. For each treatment, 25 g of raw soybeans were spread in glass petri dishes and were then roasted under the conditions selected for each experiment. In conventional roasting an electrical oven with the temperature range of 180 to 260 °C and time duration of 5 to15 min was used. IR roasting was performed using an IR-warm air apparatus with a constant power of 1300 W at the air temperature of 180-240 °C for 5-15 min. After the temperature equilibrium was reached, the samples were packed in polyethylene bags and were kept at 4 °C until further analysis. For each roasting method, a central composite design consisted of two variables of time and temperature (each in three levels) and a total of 13 experiments were applied. Response surface analysis was performed using Design-Expert software. The moisture content of samples was determined by drying the samples in a drying oven at 105 °C until a constant weight was reached. The total phenol content was measured quantitatively by the Folin-Ciocalteu colorimetric method based on the reaction of reagents with the active hydroxyl groups of phenolic compounds. The radical scavenging activity of the samples was determined by the DPPH radical. The force needed to break the roasted seeds was evaluated using a texture analyzer equipped with a load cell of 25 Kg. The color of samples was evaluated in a special box under controlled conditions (in terms of light intensity and camera position) using a digital camera and the color parameters (L*, a* and b*) and the color change (ΔE) were determined.
Results and Discussion: According to the results, the second- and first-order models were suggested for the study of time and temperature effects on moisture reduction that were both significant (p<0.05). In two methods, total phenolic content and antioxidant activity models were significantly (p<0.05) second-order. With increasing time and temperature, these above values increased. Hardness and color differences of oven roasting were both first-order but only color differences were first-order for infrared roasting. Effects of two parameters were significant in all models. Optimum conditions for soybeans roasting sing oven and infrared were 223°C – 13 min and 231°C – 11 min, respectively. In optimum condition, experimental data for the moisture content, total phenolic content, antioxidant activity, hardness and color differences were: (1.10, 4.53, 42.75, 9.03, and 4.93) , (1.58, 4.93, 47.85, 6.20, and 4.79) respectively. Based on above results, infrared can be introduced as a replacement of conventional oven method for the roasting of soybeans.