عنوان مقاله [English]
Introduction: Due to the nutritional status and health effects of raspberry as a fruit rich in nutrients such as anthocyanins and phenolic compounds, the resulting powder can while have these compounds, increasing the variety and consumption of raspberry products (Si et al., 2016). However, because of its high moisture content and vulnerable texture, raspberry fruit corrupts very quickly and is difficult to keep fresh, even in cold storage conditions. Therefore, maintaining the quality of raspberry fruit after harvest and also increasing its storage time is one of the goals of raspberry growers around the world (Palonen and Weber, 2019). The drying process of fruits and vegetables prevents chemical and biochemical spoilage of the products and increases their shelf life by reducing the moisture content. However, some undesirable physical and chemical changes such as loss of color, texture and nutritional value may also occur during drying, which reduce consumer acceptance (Dehghannya et al., 2017; Kayran and Doymaz, 2017). Foam mat drying is an alternative method of drum dryer, spray dryer and freezer dryer for the production of food powder. Food products become a stable foam by adding foaming agents and stirring in the presence or absence of foam stabilizers and are dried using different heat streams. Therefore, with the proper selection of foaming agents and drying conditions, a high quality powder can be obtained (Qadri et al., 2010; Javed et al., 2018). Despite the impact of air velocity on drying kinetics and other process parameters and consequently on the qualitative and functional characteristics of the dried product, few studies have been done on the effect of air velocity on foam mat drying of agricultural and food products. Therefore, according to the potentials of raspberry fruit and foam mat drying method, the aim of the present study was to evaluate the effect of drying air velocity (2 to 4 m/s) on the effective moisture diffusion coefficient, drying kinetics and also on the physicochemical properties of raspberry powder produced by the foam mat drying method.
Material and methods: Black raspberry fruit after being prepared from the local market in the north of the Iran (Gilan province) was stored at a temperature of 4C. The raspberry fruit was first completely crushed by a food processor and passed through mesh number 30. Ovalbumin and methylcellulose with a concentration of 0.5% were used as foaming agent and foam stabilizing agent, respectively and added to the raspberry pulp. The resulting mixture was mixed with an electric stirrer at maximum speed for 10 minutes and the resulting foam was spread in a thickness of 7 mm on an aluminum plate. Drying was performed with a convective hot air dryer at a temperature of 70C and an air velocity of 2 to 4 m/s. Drying kinetics and effective moisture diffusion coefficient, as well as physicochemical properties of raspberry powder such as bulk and tap density, powder flowability, absolute density, water solubility index (WSI) and water absorption index (WAI), powder mass porosity, moisture content and color were evaluated according to the methods described by Dehghannya et al (Dehghannya et al., 2018, 2019).
Results and discussion: In the present study, the drying kinetics of raspberry by the foam mat drying method and the physicochemical properties of the resulting powder under the influence of air velocity were investigated. The results showed that the effective moisture diffusion coefficient (Deff) was in the range of 1.391 10-8 to 1.723 10-8 m2/s and the increase of drying air velocity from 2 to 3 m/s and 3 to 4 m/s had a significant effect on it (p < 0.05). However, increasing the drying air velocity had no significant effect (p> 0.05) on moisture content, moisture ratio and drying rate of the product during drying. Changes in Carr index and Hausner ratio at different drying velocities were statistically significant (p <0.05) and increasing the air velocity from 2 to 3 m/s reduced their values, which indicated the improvement of the powder flowability at air velocity of 3 m/s. Furthermore, the mean values of MC, WSI, WAI, Chroma index and brightness of raspberry powder significantly increased with increasing drying air velocity (p <0.05). However, the bulk and tap density and absolute density and the overall color difference of raspberry powder decreased with increasing drying air velocity (p> 0.05).
Conclusion: The results of this study showed that the hot air velocity used in the drying process can be effective on the effective moisture diffusion coefficient and thus on the quality characteristics of the produced powder. So that, its effect on effective moisture diffusion coefficient, bulk and tap density, flowability, absolute density, WSI, WAI, moisture content and brightness of powder were significant. It should be noted, that the effect of air velocity in different values and in accordance with other process conditions, on different parameters is variable. Raspberry powder produced by foam mat drying method, due to its high anthocyanin content, can be used as a natural additive in dairy products, desserts, jams, jellies and other food products and the results of this study can increase the quality of the powder and improve its nutritional and functional properties by optimizing the drying process.