Frontiers in Heat and Mass Transfer (FHMT), 14, 23 (2020) DOI: 10.5098/hmt.14.23 Global Digital Central ISSN: 2151-8629 1 CONVECTIVE HOT AIR DRYING KINETICS OF RED BEETROOT IN THIN LAYERS Abhishek Dasore a* , Tarun Polavarapu b , Ramakrishna Konijeti c , Naveen Puppala d , a Assistant Professor, School of Mechanical Engineering, RGM College of Engineering and Technology, Nandyal, Andhra Pradesh, 518501, India b Assistant Professor, Department of Mechanical Engineering, SRK Institute of Technology, Vijayawada, Andhra Pradesh, 521108, India c Professor, Department of Mechanical Engineering, Koneru Lakshmaiah Education Foundation, Guntur, Andhra Pradesh, 522502, India d Professor, Agricultural Science Centre at Clovis, College of Agricultural, Consumer and Environmental Sciences, NMSU, NM, 88101, USA ABSTRACT The effect of air temperature on drying kinetics of red beetroot slices was investigated experimentally in a cabinet tray dryer. Drying was carried out at 70, 75, 80, and 85 ° with an air velocity 2 m/s and relative humidity 30 %. The drying data thus obtained were analyzed to get effective diffusivity values by applying the Fick’s diffusion model. Effective diffusivity increased with increasing temperature. An Arrhenius relation with an activation energy value of 35.59 kJ/mol expressed the effect of temperature on the diffusivity. Also, within the given operating range, the average heat transfer coefficient at the air-product interface is estimated. Experimental data were fitted to ten mathematical models available in the literature. The Midilli et al. and Wang & Singh models are given better prediction than the other models and satisfactorily described drying characteristics of red beetroot slices. Keywords: Air temperature, Drying kinetics, Mathematical modeling, Red beetroot, Transport properties 1. INTRODUCTION In developing countries like India, post-harvest losses are more than 30% due to poor holding capacity, disorganised transport facilities and limited market access. If this high volume of losses is reduced, then it can address the food insecurity situation, a major threat being faced by many developing countries (Alizadeh and Allameh, 2013). Drying is one of the most preferable methods for reducing these post-harvest losses of agricultural products. A considerable amount of moisture content from the products can be removed by drying in order to minimise microbial spoilage and to maintain its desired levels of nutrients. Red beetroot (Beta vulgaris L.), which is used as a drying material in present work, is a root vegetable containing several essential nutrients and is a great source of fibre, manganese, potassium, iron, vitamin C and folate. Besides being used as a food, dried red beetroots find applications in food colorant and medicinal fields (Stintzing and Carle, 2004; Kaur and Kapoor, 2002). Traditionally, red beetroots are dried in open sun. But now-a-days a high volume of the total production of red beetroots is mechanically dried by forced convection. As it requires smaller investment and yields high- quality end products (Gokhale and Lele, 2011). Generally, drying processes are modelled mathematically in order to optimise the existing drying systems or to include a novel process design (Xia and Sun, 2002, Abhishek et al., 2018). Sometimes, drying data obtained experimentally is applied to existing drying equations to choose a sufficiently accurate drying model which is capable of predicting the moisture removal rates and elucidating the performance of drying process of each specific product under the general conditions employed in normal commercial relevant facilities (Sacilik and Elicin, 2006; Erbay and Icier, 2010). The model parameters like transfer coefficients, drying constants of modelling are directly related to the drying conditions i.e., * Corresponding author. Email: dasoreabhishek@gmail.com temperature and velocity of the drying medium inside the mechanical dryer. Although many experimental and mathematical investigations have been carried out in analysing drying characteristics of various fruits and vegetables (Kohli et al., 2018; Sadaka and Atungulu, 2018; Waheed and Komolafe, 2019; Doymaz and Karasu, 2018; Togrul and Pehlivan, 2004; Lopez et al., 2000; Goyal et al., 2006; Midilli and Kucuk, 2003; Akpinar and Bicer, 2005; Abhishek et al., 2019; Doymaz, 2004; Babalis et al., 2006; Lin and Cze, 2018; Nistor et al., 2017; Komolafe et al., 2019), no study has analysed the drying behaviour of red beetroot and estimated its drying parameters such as moisture effective diffusivity, activation energy and heat transfer coefficient during drying. Hence, the present work aims at determining the effect of the air temperature on drying kinetics of the red beetroot. Also, compared ten thin layer drying equations that best describe its drying kinetics. Further, estimated the transport properties like effective moisture diffusivity, activation energy, heat transfer coefficient. 2. MATERIALS AND METHODS 2.1 Sample preparation Fresh good quality red beetroots are obtained from local market in southern state of India. Before drying, red beetroots are washed and hand peeled and cut into rectangular shaped slices having dimensions (60 ± 0.2) × (60 ± 0.2) × 5 . Pre-treatment like blanching, soaking and salting is avoided since it causes loss of water-soluble betalain pigments. 2.2 Drying equipment and measuring instruments A laboratory-scale hot air induced draft convective tray dryer is used to dry the red beetroot slices. The dryer comprises of a drying chamber, heating system, variable speed induced draft fan, rectangular air duct, Frontiers in Heat and Mass Transfer Available at www.ThermalFluidsCentral.org