Study of activation energy and moisture diffusivity of various dipping solutions of ivy gourd using solar dryer

The study is aimed to enhance the shelf life of ivy gourd through the solar drying method in open, forced, and natural convection mode. Ivy gourd is treated as the primary agent to prepare medicines and the stems, leaves and flowers are used to cure diseases related to diabetics, ulcer and skin. The normal shelf life is 2–3 days and it can be increased up to 6 months with an effective drying process. The experiment is intended to find the best drying process among the open, natural, and forced convection mode with an initial dipping method with ascorbic acid, lemon juice, sugar solution, honey solutions individually, and a control sample (without dipping). A 3 kg sample of ivy gourd is dipped in 10 g/L of each of the solutions and it is used for the three drying processes individually. The obtained results are indicating that the forced convection method for ascorbic acid is best among the other drying method, with the highest moisture diffusivity is 7.88 × 10−8 m2/s and the lowest activation energy of 21.12 kJ/mol. The lemon juice sample is found to have better sensory appeal in terms of colour (darkness) and shrinkage followed by honey, ascorbic acid, and control sample, whereas the honey-dipped sample offers a better taste followed by lemon juice-dipped samples, control, and ascorbic acid-dipped samples, respectively. The influence of dipping solution and drying mechanisms on the functionalities of drying are discussed with suitable illustrations.


Introduction
The developing countries are experiencing food scarcity due to the inefficacy to preserve food supplies compared to low production (Elavarasan and Natarajan 2021a, b;Kumar et al. 2022). The medicinal values that existed in vegetables are partially used in the current situation due to inefficient and uneconomical preservation processes. Ivy gourd is one of the tropical vegetables which remain under-treated though it contains many nutraceutical properties. Solar drying is considered one of the prominent food preservation techniques for many years, which can be supportive to maintain Responsible Editor: Philippe Garrigues * Sendhil Kumar Natarajan sendhil80@nitpy.ac.in nutritional and medicinal benefits for a long time. However, this process is highly dependent on solar irradiation and it requires constant supply at a longer duration (Natarajan and Elavarasan 2019a;Subramani et al. 2020). It is observed that the developing countries are facing post-harvest losses due to inefficiency in utilizing solar dryers and the mode of drying. As suggested by Belessiotis et al. drying functions on two moisture transfer mechanisms such as transfer of moisture from the mass to the surface and transfer of moisture from the surface to the surrounding air (Belessiotis and Delyannis 2011).

Solar drying of perishable crops
Vegetables and fruits are vital sources of necessary dietary nutrients but are characterized under perishable goods since their moisture content exceeds 80% (Orsat et al. 2006). Storing the product dry and moisture-free is the prominent way to sustain its quality and nutrition, but a majority of such storage mechanisms need low-temperature setups that require heavy maintenance. Around 20% of the world's perishable food supplies are subjected to drying in order to expand their shelf storage span and enhance food quality (Grabowski et al. 2003). Important quality attributes associated with vegetables include their sensory appeal, drying characteristics, microbial load, aroma, taste, retention of nutrients, and exclusion from pests and preservatives (Bhatta et al. 2020). A curtailment in the post-harvest losses of agricultural crops can majorly influence the economy of developing countries positively (Chandra and Sodha 1991). About 80% of the agricultural products in countries like India are cultivated by small-scale farmers (Murthy 2009). These farmers use the natural sun for drying agricultural and other food products due to the abundant availability of sunlight in the form of solar energy. This process is regarded as open drying. However, open sun drying has its disadvantages which affect the standard of the end product, making it inappropriate to consume. Also, certain crops are not supposed to be sun-dried as they may lose their desirable characteristics (Jairaj et al. 2009). Drying and removal of moisture content from the fruits and vegetables require a comprehensive analysis of the drying mechanism, which helps in enhancing the drying efficiency and end product quality resulting in a considerable decrease in the post-harvest losses. Hot air convective drying is still the widely recognized and accepted drying method, although it possesses several disadvantages like high energy consumption (Lewicki 2006). The features and standard of the products that are dried and their consumption are major factors considered while identifying the drying mechanism (Al-Juamily et al. 2007). An efficient drying system should be cost-effective and potential enough to optimize the energy consumption and to minimize the operational cost without compromising on the quality of drying of the product. Hence, solar dryers incorporated with evolving novel technologies are perfect options for drying perishable crops, which reduces both energy consumption and operational cost.
The adaption of solar thermal systems to preserve fruits, vegetables, and various other agricultural products is proven to be a more efficient, cost-effective, and environmentally friendly approach. The solar drying technique is a very clean and hygienic alternative that processes vegetables and fruits in sanitary conditions. It occupies less area, saves energy and time, and makes the process highly efficient concerning drying time and drying characteristics (Funebo and Ohlsson 1998;Zhang et al. 2006). In comparison to conventional open sun drying techniques, solar drying is highly advantageous since it mitigates several issues like contamination, the possibility of spoilage, lack of control, and ambiguity over-drying conditions due to longer drying duration. The solar drying method is also economical, unlike the widely used convective hot air dryer method, which has a high fuel consumption rate and energy cost. As solar energy is one of the prominent renewable energy sources and available abundantly, it is widely accepted for the dehydration of perishable food supplies. The main advantages of solar drying of agricultural crops include the possibility of an early harvest, long-term storage without deterioration, and selling a betterquality product (Kumar et al. 2022). As an added advantage, it minimizes packaging requirements and reduces transport weight. Ivy gourd drying process through solar energy is one of the cost-effective methods to produce powdered ivy gourd for making diabatic pills compared to the conventional method (Elavarasan and Natarajan 2022). The production of ivy gourd diabatic pills through this method will attract the industry for more commercialisation. Solar drying can be performed only during sunny days unless the dryer is incorporated with a conventional energy storage device. Due to limitations in solar energy collection, the solar drying process is slow compared to other conventional dryers.

Pre-treatments before solar drying
As reported by Sablani, changes are happening with the evaluation of quality and standards of nutrition of the agricultural products after dehydration (Sablani 2006). The agricultural crops that are using solar drying should be capable of retaining their various quality features, which include colour, texture, and nutritional values post dehydration or drying. Quality enhancement is attained by different pretreatment methods before drying. The application of appropriate pre-treatments before drying enhances the quality of drying by minimizing the time required for drying, improving drying attributes, and preserving energy-yielding higherquality end products. Several pre-treatment methods are available that are incorporated with the drying mechanism and blanching is considered the most generalized method. Dipping treatment which involves the dipping or soaking of an agricultural product mainly in organic acids (Karapinar and Gönül 1992) serves as an alternative to blanching, which helps in reducing the quantity of conventional flora and pathogenic species, and some of them like acetic acid reportedly mitigate the activity of enzymes responsible for browning (Chiewchan et al. 2010). Dipping involves soaking washed, peeled, and sliced fruit and vegetables in a suitable liquid commonly called a dipping solution. Dipping is mainly tried in fruits such as apples, bananas, peaches, and pears and it prevents it from oxidizing. Commonly employed dipping solutions consider juices that are high in vitamin C such as orange, lemon juice, pineapple, grape, and cranberry juice or solutions of honey, ascorbic acid (Vitamin C), sodium bisulphite, and sugar solution, which serve as antioxidants are found effective. The solution of ascorbic acid and water is the best way to prevent browning. Any fruit juice that has high vitamin C value is considered a dipping solution, although it might not be efficient as pure ascorbic acid. The solution of honey and water can also be used as a dipping solution for pre-treatment before carrying out the drying. Honey mixed with refined sugar solution and water as a dipping solution can enhance the taste of the dried fruit or vegetable in comparison with various other dipping solutions.

Drying characteristics: moisture diffusivity and activation energy
Drying characteristics such as moisture diffusivity and activation energy have a huge impact on the process of solar drying, which mainly determines the rate of drying and time required for drying. Drying characteristics depend on the crop properties, drying mechanism, and the pre-treatment used. Effective moisture diffusivity is stated as the rate of moisture movement and activation energy is the minimum energy required to start the drying mechanism or moisture transport.
Various research works have investigated the process of activation energy and diffusion of moisture into the thin layer drying of different food products such as onion slices (Demiray et al. 2017), apple (Aghbashlo et al. 2008), hazelnuts (Özdemir and Onur Devres 1999), potato slices (Akpinar et al. 2003), candlenuts (Tarigan et al. 2007), plums (Goyal et al. 2007), grapes (Pahlavanzadeh et al. 2001), tomato , poovan banana (Bhanu et al. 2021), red banana (Elangovan and Natarajan 2021b), and seedless grapes (Ibrahim and Pala 2002). Moisture diffusivity of the drying product should be high, which in turn increases the drying rate reducing drying time, whereas activation energy should be less in order to initialize the drying task with less amount of energy maintaining very minimum drying time. An ideal range of moisture diffusivity lies between 10 −9 and 10 −11 m 2 /s and activation energy is better to be below 35 kJ/mol to have an effective drying process (Mirzaee et al. 2009).
El-Sebaii and Shalaby (2017) dried Thymus leaves in a forced convection indirect solar dryer. The results showed that the logistic drying model best represented the drying characteristics of the samples. Yadav and Chandramohan (2018) used forced convection indirect solar dryer for their experiment. The maximum outlet temperature was reported as 2938.4 kJ/kg for the indirect solar dryer. The ambient temperature and solar radiation essay a prominent role in deciding the potential influence of solar dryers. A review done by Sandali et al. (2019) mentioned various techniques to improve the overall efficiency of the drying system. The solar dryer integrated with a chimney augments the buoyant force applied on the air stream resulting in higher airflow velocity and increases the moisture removal rate. Also, a sustainable forced convection method using a fan running with the help of electricity produced by photovoltaic panels gives out forced air circulation and increases the moisture removal rate. The usage of concentrators results in increased air temperature inside the dryer, which helps in decreasing the drying time. Saxena and Gaur (2020) developed a hybrid dryer for drying coriander and fenugreek. The results showed that the Midilli and Kucuk drying model best represented the drying characteristics of the samples. It was reported that the equilibrium moisture content of drying coriander and fenugreek reaches 3.5 and 2.5 h. Amjad et al. (2021) introduced a solar hybrid food dryer by incorporating a gas burner and evacuated tube solar collector with an inline perforation inside the drying chamber. Table 1 represents the literature review summary for different drying techniques and pre-treatments of vegetables and fruits.
The following research gaps have been identified from the cited research literature: i. The influence of pre-treatment of the ivy gourd samples on the overall drying period is not investigated. ii. The pre-treated drying characteristics of Ivy gourd have not been analyzed. iii. Sensory appeal such as aroma, taste, colour, shrinkage and texture after the drying is not estimated.
The current research is aimed to enhance the shelf life of ivy gourd through solar drying after pre-treatment with different solutions. The "Research methodologies" section presents the research design, experimental setup, and mathematical equations to support the investigational study. The "Results and Discussion" section presents the obtained results and the relevant discussions performed through the observation. The "Conclusions and future directions" section concludes the article by presenting the overview and future scope of the research.

Sample preparation
The experimental procedure starts with preparing the sample. Fifteen kilograms of fresh ivy gourd is purchased from the market. Washing, peeling, and slicing of ivy gourd to the required size are done. Dipping pre-treatment is performed by immersing the ivy gourd samples for 10 min in five types of dipping solutions, namely ascorbic acid, lemon juice, sugar solution, honey, and control solution categorizing it into five different samples. One kilogram of each (a total of 5 kg) is taken for three different experimental setups.

Solar dryer setup and temperature monitoring
The three major solar drying setups used for experimental analysis are illustrated in Fig. 1, which includes an open sun dryer, a natural convection dryer, and a forced convection dryer. The natural convection dryer and forced convection dryer are equipped with a drying chamber made of a two-layered galvanized iron sheet (GI) of dimensions 1290 mm × 850 mm and thickness 1.5 mm. The top portion of the chamber is covered with a pane glass of 5 mm thickness. The dryers were designed in a way that the heat gets trapped within the drying chamber effectively. In order to insulate the setup, coconut husk and thermocol are used with the GI sheets, which act as an insulating medium and prevent heat transfer within the surfaces. The samples are kept inside the chamber over a mesh of dimensions 1190 mm × 750 mm. There are inlet (Ø22mm) and outlet (Ø26mm) pipes placed for the circulation of ambient air. The forced convection dryer includes a fan blower in extra to complete the setup, which is powered and can be controlled. Solar radiation and temperature are monitored using a pyranometer and two different thermocouple configurations, respectively. Nine thermocouples of K-type (chromel/constant wires and − 270 to 1260 °C) configurations are employed for determining the temperature inside various segments of the dryer and a thermocouple of J-type (Copper/ Nickel wires and − 270 to 760 °C) is used for determining the ambient temperature. Pyranometer (SR20-TI, secondary standard (ISO9060) having a sensitivity of 14.77 × 10 −6 V/ (W/m 2 ) is used for determining the solar radiation (Kumar Natarajan et al. 2019). The thermocouples and pyranometer are connected to a data acquisition unit where the data is decoded and collected and then monitored using a data logger (Agilent 34972A, Keysight) (Natarajan and Elavarasan 2019b).

Mathematical correlations to calculate the drying characteristics
Moisture content can be calculated by measuring the wet weight (W w ) and the dry weight (W d ), where the wet weight is defined as the weight of the ivy gourd before drying and dry weight is taken post drying. The equation given below defines the moisture content based on the drying characteristics by the following equation (Ullah et al. 2020;Ahmad et al. 2021).
The drying process of food material is mostly governed by diffusion mechanism as the rate of falling period. Therefore, Fick's 2nd law of diffusion is applied to find the effective moisture diffusion and is governed by (Elangovan and Natarajan 2021a) Eq. (2): where MR represents the moisture ratio, D eff is defined as the effective moisture diffusivity (m 2 /s), t is the corresponding drying time (hrs), and L is the thickness of the ivy gourd sample (m). The plot of ln (MR) against drying time gives out a straight line with a slope governed by the mathematical expression as shown in Eq. (3): From the above equation, the effective moisture diffusivity can be evaluated by assuming negligible shrinkage and constant temperature during the drying process (Yogendrasasidhar and Setty 2019; Daş et al. 2021). Also, MR can be determined using the expression presented in Eq. (4), where M t defines the amount of moisture content present for a given time t, M o represents initial moisture content, and M e is moisture content at equilibrium. The activation energy is determined by plotting ln (D eff ) against 1/T, where the effect of effective moisture diffusivity on temperature can be determined. This is governed by an expression called Arrhenius Eq. which states: where E a is the activation energy (kJ/mol), R is the universal gas constant (8.314 kJ/mol K), T is the temperature (K), and D o is the diffusion factor (m 2 /s). The plot of ln D eff against 1/T gives a straight line of slope K where the relation between E a and diffusivity coefficients can be defined through linear regression analysis and activation energy (E a ) is evaluated by (Daş et al. 2021):

Error analysis
Errors and uncertainties in the experimental analysis can be introduced because of inappropriate instrument selection, inaccurate calibration, errors in analysis and observation, and test planning. While performing drying experiments in a solar dryer for drying Ivy gourds, the relative humidity, moisture diffusivity, and activation energy were measured using appropriate instruments. The uncertainties in the experimental analysis are measured to determine the accuracy of the analysis according to experimental data. The analysis of effective moisture diffusivity of drying ivy gourd is given as: Rearranging the above Eq. (7): where x = MR ; y = D ef f Differentiating Eq. (8) with respect to y, the above equation is transformed as: Differentiating Eq. 8 with respect to L, the diffusivity is given as: Differentiating Eq. 8 with respect to t, the equation is now given as: The drying rate of the process is evaluated as shown in the equation below.
where σ 1 =0.577, σ 2 = 0.005&σ 3 = 0.577 The activation energy of drying ivy gourd is defined as: Rearranging Eq. 13: where y = D ef f ; x = E a Differentiating Eq. 14 with respect to y, Eq. 14 is transformed as: Differentiating Eq. 14 with respect to D o and Eq. 14 becomes: Differentiating Eq. 14 with respect to T and Eq. 14 becomes: Now, the drying rate is given as: where σ 1 =0.42, σ 2 = 0.25&σ 3 = 0.577 The drying rate of drying ivy gourd is given as: Rearranging Eq. 19: where m = m i −m d Differentiating Eq. 20 with respect to t and the Eq. 20 becomes: Differentiating Eq. 20 with respect to m and the equation is given as: where 1 =0.577 2 = 0.577  The measured uncertainty for temperature and solar radiation is ± 0.04 °C and ± 5.67 W/m 2 , respectively. Considering the above parameters, the uncertainty of drying rate and kinetic parameter of drying ivy gourd in a SSSD is about ± 0.05 kg/s and ± 0.39 m 2 /s, ± 0.17 kJ/mol, respectively.
The uncertainty analysis during the drying experiment of ivy gourd is illustrated in Table 2.

Solar dryer performance during forced and natural convection
The performance of the natural convection and forced convection solar dryer was tested in terms of solar radiation per unit area, ambient temperature, the temperature of drying air, and temperature of the absorber plate. Figures 2 and Gatea (2011) and Elavarasan and Natarajan (2022), where a solar drying system was fabricated and designed for analysing the solar collector efficiency used for the purpose of drying. The solar drying system acquired a maximum collector temperature of 71.4 °C for solar radiation of 750 W/m 2 .

Moisture ratio
The moisture ratio of all the samples and its variation with respect to drying time is illustrated from Figs. 4, 5, 6, 7, and 8. It can be inferred from the results that Ivy gourd with a moisture content of 1000 g before drying was reduced to 120 g after drying in 11 h when dipped in a control sample, ascorbic acid, and lemon juice. The same drying process took 12 h when Ivy gourd was dipped in a sugar solution sample and 14 h when dipped in a honey sample for an open sun drying method. Similarly, for a natural convection dryer, the moisture content reaches the minimum value in 9 h for the control sample, ascorbic acid, and lemon juice, whereas it takes 10 h for sugar solution and 11 h for honey sample. For forced convection dryer, the duration was less, and the moisture content was reduced to the minimum value in 7 h for the control sample, ascorbic acid, and lemon juice and in 8 h for sugar solution and honey sample. It was also observed from the results that forced convection dryer  Madan et al. (2014), the moisture content decreases with respect to time at a continuous rate irrespective of drying air temperature. A steep increase in the moisture removal rate is observed between 12 and 13 h for forced convection and between 13 and 14 h for natural convection, which gradually decreases with the reduction in moisture content.

Effective moisture diffusivity
Fick's 2nd law as, stated in Eq.

Activation energy
Arrhenius expression, Eq. (5) (Chen et al. 2013), was applied to calculate the activation energy (Ea) as shown in Fig. 14 where the plot of ln D eff against 1/T gives a slope equal to Ea/R from which the activation energy is calculated using linear regression analysis. The activation energy ranges from 12.7 to 110 kJ/mol for most of the food products (Rizvi 1986;Mirzaee et al. 2009). According to the results, the average value of activation energy varied from 21.12 to 34.96 kJ/mol for different samples and drying methods (Tables 4 and 5). The lowest activation energy is observed for ascorbic acid samples when dried using the forced convection dryer followed by control, lemon juice, sugar solution, and honey samples. The lower viscosity of ascorbic acid results in lower activation energy, which means the amount of energy required to activate the molecular diffusion mechanism is low. The high viscosity of honey, when compared to other dipping solutions, results in a higher amount of activation energy requirement for the samples during the drying process. The obtained activation energy value for drying ivy gourd samples was lower than already existing literature for drying red banana (Elangovan and Natarajan 2021b), apple (Aghbashlo et al. 2010), tomato (Elavarasan et al. 2021a), ivy gourd (Gilago and Chandramohan 2022a, b), and poovan banana (Bhanu et al. 2021) of 5 mm same thickness.

Sensory characteristics
Sensory analysis is done with the help of 5 individuals who verified, smelled, and tasted each sample to comment on the sensory appeals of each represented by Table 6. The conclusion is made by giving individual ratings out of 5 in terms of appearance (colour, shrinkage) and taste (aroma, taste). The lemon juice sample is found to have better sensory appeal in terms of colour (darkness) and shrinkage followed by honey, ascorbic acid, and control sample, whereas the honey-dipped  sample offers a better taste followed by lemon juice dipped samples, control, and ascorbic acid-dipped samples, respectively. A similar observation was reported by existing literature for drying of pre-treated banana (Abano and Sam-Amoah 2011) and pineapple slices (Abano 2010).

Conclusions and future directions
The effect of pre-treatment with dipping solutions on different solar drying processes of ivy gourd samples is investigated through an experimental study. It is observed that the proposed pre-treatment worked in an effective manner compared to the control sample. A significant variation is observed in terms of high moisture diffusivity and low activation energy among the tested samples. The following observations have been made from this study and they are the following: • In open sun drying, control samples are exhibited higher moisture diffusivity, whereas the ascorbic acid samples shown higher moisture diffusivity and low activation energy followed by control, sugar solution, lemon juice, and honey samples in natural and forced convection. • The highest moisture diffusivity and the lowest activation energy are observed in ascorbic acid-based samples compared to other in three different drying processes. • The sensory appeal is found best in the lemon juice sample in terms of colour (darkness) and shrinkage, whereas the honeydipped sample has a better aroma compared to other samples. • It can be declared that the ascorbic acid is the best dipping solution for the pre-treatment of ivy gourd in terms of drying characteristics, whereas lemon juice is the best choice if sensory appeals are given as priority. • Forced convection drying is found as the best method to observed better increment in the shelf life of ivy gourd. Thus, solar drying of food products to be cost-effective, environmentally safe, and sustainable for food industries.
Furthermore, the above study will be extended to investigate on the nutritional and physicochemical properties, and sustainability analysis of the pre-treated solar-dried ivy gourd samples is required since the drying process and conditions significantly influence the chemical composition and nutritional values. The influential factors include the type of food, drying method, dipping solution, operating conditions, and storage conditions. The dipping pre-treatment mainly affects the nutritional values, which is out of the scope of this paper and requires future work.
Author contribution Elavarasan Elangovan: data curation, conceptualization, investigation, formal analysis, writing original draft preparation. Kumar Natarajan: resources, validation, project administration, writing-review and editing, supervision, conceptualization. Data availability The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Ethics approval and consent to participate Not applicable.

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Competing interests The authors declare no competing interests.