Drying kinetics and quality analysis of black turmeric (Curcuma caesia) drying in a mixed mode forced convection solar dryer integrated with thermal energy storage

Highlights

• A mixed mode forced convection solar dryer with thermal storage has been developed.

• Drying kinetics of black turmeric has been investigated.

• Over all drying efficiency of the system was 12%.

• Developed solar dryer saves 60% of drying time compared to open sun drying.

• Quality analyses of samples dried in solar dryer and open sun drying are presented.

Abstract

Performances of a mixed mode forced convection solar dryer integrated with paraffin wax based thermal energy storage have been studied for drying the sliced black turmeric (curcuma caesia). Thin layer drying kinetics of sliced black turmeric dried in a solar dryer has been compared with the open sun drying. Two 200 g samples of black turmeric were chosen and one was placed in the solar dryer and another was placed in the open sun. The samples were dried from initial moisture content of 73.4% (w.b) to 8.5% (w.b) in 18.5 hr in the solar dryer and the sample took 46.5 hr in open sun drying. Ten thin layer drying kinetic models were fitted with experimental data and Two term model and Page model were found to be best suited for predicting the drying kinetics of sliced black turmeric dried in the solar dryer and in open sun, respectively. The overall solar air heater efficiency and the overall solar dryer efficiency were found to be 25.6% and 12.0%, respectively. Quality analyses of fresh, solar dried and open sun-dried samples were also carried out in terms of colour, anti-oxidant activity, and flavonoids.

Introduction

Curcuma longa L (Turmeric) and Curcuma Caesia (Black Turmeric) are from the family of a perennial herb. Turmeric is known as ‘Indian saffron’ and is an important commercial spice crop grown in India. It is used in different forms such as a condiment, flavouring, colouring agent and also a principal ingredient in Indian cookery as curry powder. Both turmeric and black turmeric have anti-cancer and anti-viral properties and hence, they are also used in the drug industries. The increase in demand for natural products as food additives makes the turmeric an ideal product for preparing the food items. Black turmeric is a rear perennial herb available in North East, Odisha and some places in South India. It contains a good percentage of curcumin which possesses many curative properties among all the curcuma or turmeric species. It is an anti-inflammatory and antibiotic compound and helps in improving the antioxidant capacity of the body [1]. Black turmeric requires a warm and humid climate and grown in diverse tropical conditions from sea level to 1500 m above MSL and temperature in the range of 20–35 °C with an average rainfall of 1500 mm per annum.

Drying is an energy-intensive process. It reduces the moisture content of the product to a level below which the deterioration does not occur. Open sun drying (OSD) is still widely used in many tropical and subtropical countries, as it is the cheapest method. For drying the turmeric, the common practice in India is to dry it under open sun. However, the quality of the dried products in the OSD gets affected due to rodents, sudden rain, improper heating, etc. This can be overcome by introducing the solar drying technology. A natural convection solar dryer with biomass back up heater was developed for drying of ginger, turmeric, and guduchi in the climatic conditions of Delhi [2]. It was observed that the hybrid mode solar dryer took 36 hr to dry 18 kg of sliced turmeric of size 5-7 mm from initial moisture content of 319.7% (d.b.) to 8.8% (d.b.), while in natural convection solar drying mode, it took about 96 hr and in open sun drying mode, the reported drying was 275 hr. Forson et al. [3] designed a mixed mode natural convection solar dryer and tested its performance for drying of cassava. At full load condition, the reported dryer efficiency was 2.3%. Double pass forced convection solar dryer was designed and developed for drying the chilli [4]. The initial moisture content of 90.2% (w.b) was reduced to 10% (w.b) in 32 hr, while in open sun drying, the product took about 93 hr. The colour test was done for both the samples and it was found that the original colour was preserved in the sample dried in the solar dryer. Thin layer drying kinetics of mint and thymus were studied by using a V- corrugated solar air heater coupled with a drying chamber and it was found that Midilli Kucuk model was best suited for mint and Page and Modified Page models were suited for thymus drying [5]. A laboratory-type mixed mode natural convection solar dryer with backup heater (1.8 kW electric resistance heater) was developed for drying pineapple [6] and the experiments were performed under four different drying conditions viz., (i) solar drying mode, (ii) solar drying in daytime and backup heating during night time, (iii) solar and backup heating during daytime (hybrid) and (iv) only using electrical heating. Among these, hybrid mode of drying was found more suitable and the moisture content of 912% (d.b) was reduced to 155% (d.b) in 7 hr in hybrid solar drying mode and took about 23 hr in solar drying mode.

Intermittent availability of solar energy, seasonal fluctuations, and sudden rain are the major drawbacks in the solar drying process. To overcome these drawbacks, the solar dryer can be integrated with auxiliary heat sources like biomass, LPG burners, and electrical heaters for supplying the required heat to sustain the continuous drying process. Integration of thermal energy storage devices with solar dryer is an optimistic solution for continuous solar drying. Both sensible heat storage [[7], [8], [9], [10], [11], [12], [13], [14]] (rock bed, pebbles, sand and gravels) and latent heat storage [[15], [16], [17], [18], [19]] (mostly phase change materials) have been extensively used in solar dryers for drying of different agricultural products. Solar dryers integrated with the latent heat storage (LHS) have many advantages such as high heat storage capacity, dissipation of heat at near constant temperature, low volume and mass, etc. Shalaby and Bek [15] designed and tested an indirect-type forced convection solar dryer composed of two solar air heaters, a drying chamber integrated with a LHS unit and a blower for drying two medicinal plants (O. Basilicum and T. Neriifolia). Two vertical cylindrical containers filled with paraffin wax (melting point temperature 49 °C) consist of thirty-two embedded copper tubes each (for circulating the air) were used as thermal storage. Hybrid solar dryer with evacuated tube heat pipe collector with PCM based thermal energy storage was developed for drying the garlic cloves [16]. The Midilli Kucuk model was found suitable to describe the drying behaviour of garlic cloves. The investigators reported that the exergy efficiency varied in the range of 5–55% during the first 3 hr of the drying without the recirculation of exhaust air and with recirculation of the exhaust air; the exergy efficiency varied in between 67% and 88%. Mushrooms were dried in a hybrid solar dryer consisting of two solar collectors (one acted as solar heater and other one as an accumulator), a dryer and a 5 kW electrical heater [17]. One hundred copper tubes attached with fins and filled with 14 kg of paraffin wax of the melting temperature in the range of 58–60 °C were positioned above the absorber plate of the solar heater. The incorporation of the accumulator saved 40–70% electrical energy. The thermal efficiency of the solar air heater and the accumulator were in the range of 22–67% and 10–21%, respectively. Jain and Tiwari [18] developed a passive solar dryer integrated with a PCM based energy storage unit and tested the solar dryer by drying mint. Forty - eight cylindrical tubes of length 0.75 m and 0.05 m in diameter filled with 48 kg of paraffin wax (thermal storage unit) were positioned below the drying trays in the zigzag arrangement. In this drying system, the thermal storage maintained the drying air temperature around 6 °C higher than the ambient temperature for 5–6 hr after the sunset. The dryer was tested by drying the pretreated mint leaves, and it took 24 hr to reduce the moisture content of 12 kg of mint leaves from 4.8 kg of water/kg of dry matter to 0.11 kg of water/kg of dry matter. Rabha and Muthukumar [19] designed and developed a forced convection solar dryer with thermal energy storage for drying red chilli. The dryer consisted of two air heaters connected in series, a shell and tube heat exchanger filled with paraffin wax and a parallel flow tunnel dryer. The chilli was dried from the initial moisture content 73.5% (w.b) to the final moisture content of 9.6% (w.b) in 40 hr and the authors also estimated the exergy efficiency, specific energy consumption and overall efficiency of the dryer. Performance of the forced convection mixed mode solar dryer with thermal energy storage (granulated paraffin wax) was studied for drying of apricot [20].

It is evident from the literature that the drying air velocity, temperature, relative humidity, and size of the sample affect the drying kinetics of the products. Midilli et al. [21] studied the drying kinetics of shelled and unshelled pistachios in a solar dryer and under open sun drying conditions. They found that logarithmic model was best suited for the solar dried product. Sacilik et al. [22] dried organic tomatoes in a direct-type passive solar tunnel dryer and also under open sun drying to study the thin layer drying kinetics of the product. It was observed that the colour quality of the solar dryer dried product was better than the sun drying one. Borah et al. [23] studied the drying kinetics of sliced turmeric (thickness 10-14 mm and 50 mm length, quantity 12 kg) and lump turmeric (12 kg) in a solar conduction dryer. Four drying kinetic models were fitted to the experimental data and it was found that the Page model fits well with the experimental data. Rabha et al. [24] designed and developed a forced convection solar dryer without thermal energy storage under the climatic conditions of Guwahati (North Eastern part of India) for drying the ghost chili pepper. The overall thermal efficiency of the drying chamber was found as 4.1%. They also observed that the Midilli-Kucuk model was best suited for representing the drying process of the ghost chilli dried in a solar dryer and the Page model was suitable for the open sun-dried products. The time required for reducing the moisture content level from 589.6% (d.b.) to the desired moisture content of 12% (d.b.) was 123 hr in the solar dryer and in open sun drying, the sample took 193 hr including night hours.

Several studies on the development and testing of natural and forced convection solar dryers, mixed mode solar dryers and solar dryers with thermal energy storage systems have been reported in the literature. Most of the reported studies on solar drying were focused on investigating the drying kinetics of various agricultural products such as red chilli, mint, turmeric, pepper, apricot, tomato, grapes, etc. and very few studies were reported on the quality analysis of the dried sample. Further, it is observed that there is no study reported on the drying kinetics of high-value agriculture products such black turmeric and cardamom. Therefore, the present work is focused on the development of a mixed mode forced convection solar dryer integrated with thermal energy storage for investigating the drying kinetics of black turmeric. The available drying kinetics models are also used for investigating the thin layer drying kinetics of sliced black turmeric dried in mixed mode solar dryer and under open sun. Quality analyses of fresh, solar dried and open sun-dried black turmeric are also presented.