Influence of ultrasonic pretreatment on the drying rate of lentil sprouts in hot-air and infrared dryers

Introduction: Lentil is one of the five major pulses produced in the world, and the annual production is around 4.5 million tonnes (Chelladurai and Erkinbaev 2020). Lentils have a relatively higher protein, carbohydrate, and energy content than other legumes. Lentil seeds are composed of about two-thirds carbohydrates and 24–30% protein. In addition, lentils are also a good source of certain amino acids, such as lysine and arginine (Lee et al. 2007). In many of the developing countries, lentil is considered as a stable source of protein due to its higher dietary protein and complex carbohydrate content (Chelladurai and Erkinbaev 2020). Sprouting process involves changes in the nutritional, biochemical, and sensory characteristics which improve the quality of legumes (El-Adawy et al. 2003). Ultrasound pre-treatment as non-thermal food processing technology could be a better pre-treatment technique for food processing, due to its benefits which comprise energy saving, preservation of original freshness and nutritional contents, keeping bioactive compounds, the decline in processing duration, and cost. Ultrasound pre-treatment accelerates the mass transfer in dehydration and drying of fruit and vegetable slices mostly due to the breakdown of cells and the creation of microchannels (Awad et al. 2012; Ghorbani et al. 2013). One of the best methods for the preservation of agricultural product is drying, which consists in removing water from the manufactured goods. One of the best ways to decreasing in drying time is to provide heat by infrared radiation. Infrared heating has many advantages including high heat transfer rate, uniform heating, low processing time, high efficiency, low energy consumption, low energy costs, and improving final product structure, porosity and quality (Salehi 2020). We found no report on the effects of ultrasound pretreatment on the hot-air and infrared drying kinetics of sprouted lentils in the literature. Hence, the purpose of this study was to estimate the impacts of ultrasound pretreatment and drying approaches on the drying time, mass transfer kinetic, effective moisture diffusivity (Deff), and rehydration of sprouted lentils. In addition, the moisture ratio changes of sprouted lentils during drying were modeled.
Material and methods: Lentil seeds were washed and soaked in tap water for 24 hr at room temperature (25±1°C). Soaked seeds were kept inside a polyethylene container covered with a clean kitchen towel and allowed to germinate for 48 hr in the dark at room temperature (25±1°C). In this research, the effect of ultrasound time and dryer type (hot-air and infrared) on the drying time, effective moisture diffusivity coefficient and rehydration of lentil sprouts were investigated and drying kinetics were modeled. To apply the sonication treatments on the germinated lentils, a Backer vCLEAN1-L6 ultrasonic bath (Iran) was employed with a frequency of 40 kHz and a power of 150 watts. The tank of the device was filled with 6L of distilled water and, then, after the temperature of the water reached to 25°C, the germinated lentils were placed directly in the bath. To apply ultrasound pre-treatment, the sprouts were placed inside the ultrasonic bath machine for 0, 3, 6, and 9 minutes, and after leaving the machine and removing extra moisture, the samples in thin layers were placed in the hot-air (with a temperature of 70°C) and infrared (power of 250 W) dryers. The dehydration kinetics of sprouted lentils has been explained using 4 simplified drying equations (Henderson and Pabis, Newton, Page, and Wang and Singh). Fick's second law of diffusion using spherical coordinates was used to calculate the moisture diffusivity of germinated lentils at various hot-air and infrared drying conditions. The rehydration tests were conducted with a water bath (R.J42, Pars Azma Co. , Iran). Dried sprouted lentils were weighed and immersed for 30 min in distillated water in a 250 ml glass beaker at 50°C.
Results and discussion: The results showed that sonication treatment, causes an increase in moisture removal rate from the sprouts, an increase in the effective moisture diffusivity coefficient, and as a result, reduces the drying time. By increasing sonication time from zero to 9 min, the average drying time of sprouts in the hot-air and infrared dryers decreased from 156.7 min to 103.3 min, and from 32.7 min to 24.3 min, respectively. The average drying time of the samples in the hot-air dryer was 134.2 min and in the infrared dryer was 28.8 min. Also, the average effective moisture diffusivity coefficient calculated for the samples placed in the hot-air dryer was equal to 3.76×10-10 m2/s and for the infrared dryer it was equal to 1.6×10-9 m2/s. The time of ultrasound and drying treatment had significant effects on the rehydration of samples, and the value of this parameter was higher for samples dried in the hot-air dryer.
Conclusion: Kinetic modeling of lentil sprouts weight changes during drying was carried out by models in the sources, followed the Page model was selected as the best model to predict moisture ratio changes under the selected experimental conditions. Generally, 9 minutes pre-treatment by ultrasound and then using an infrared dryer is the best condition for drying lentil sprouts.