Experimental study of the kinetics and shrinkage of ginger in convective drying ( Zingiber officinale Roscoe) † Estudio experimental de la cinética y encogimiento del jengibre en secado convectivo ( Zingiber officinale Roscoe)

In this work, the drying kinetics of ginger ( Zingiber officinale Roscoe) was studied considering the shrinkage effect. The experimental data were obtained using three different temperatures (40, 60, and 80  C). For the determination of shrinkage, two cutting orientations were employed. The volumetric and thickness shrinkage was evaluated by direct measurement. The shrinkage showed two periods during drying and had a considerable effect on the drying rate. An equation that relates the changes in the drying area as a function of the moisture content was determined. During drying, ginger showed only a falling rate period and exhibited a characteristic drying curve.


INTRODUCTION
Ginger (Zingiber officinale Roscoe) is a herbaceous perennial plant, which belongs to the Zingiberaceae family. Ginger is a rich source of active compounds (e.g., zingiberene, α-curcumene, gingerols, and shogaols). These compounds exhibit biological and pharmacological antioxidant, antilipidemic, antihyperglycemic, anti-inflammatory, antimicrobial, and anticancer properties. Ginger has been used as a spice and medicine for over 2000 years and more recently as a dietary supplement (Imtiyaz et al., 2013;da Silveira Vasconcelos et al., 2019).
Fresh gingers contain about 93% water and are perishable and sprouted during storage, which results in a decrease in the nutrients and storage quality (Ren et al., 2021). Drying is a process of removing water from foods and is characterised by simultaneous heat transfer and water movement. During the drying process, there is a loss of nutrients, and the sensory characteristics of the foods inevitably change, so it is necessary to select an appropriate drying method based on the different characteristics of the foods.
Many vegetables and fruits have been dried successfully including ginger. For instance, Thorat et al. (2012) reported an experimental study on thin-layer vacuum drying of ginger slices. Drying experiments were performed at a constant chamber pressure of 8 kPa and four different drying temperatures (40, 50, 60, and 65 C). The results showed that increasing drying temperature accelerated the vacuum-drying process. All drying experiments had only a falling rate period, no constant rate period was observed. Under the evaluated experimental conditions, the two-term exponential model provided the best representation of the thin-layer drying characteristics of ginger slices. Deshmukh et al. (2014) investigated the drying of ginger in a mixed-mode solar cabinet dryer. Freshly harvested ginger slices were successfully dried from initial moisture content of 621.50 to 12.19% (dry basis) and their drying characteristics, quality parameters, and kinetics were evaluated. Drying curves showed that drying occurred in the falling rate period and no constant period was observed. Page model was found to be most suitable to describe the drying kinetics of ginger in the solar dryer under natural convection among the tested models.
During the drying process, most of the foods simultaneously exhibit changes in volume and surface. This shrinkage phenomenon particularly affects the diffusion rate of the foods, which is one of the main parameters governing the drying process, thereby influencing the moisture removal rate (i.e., drying rate). In addition, shrinkage affects many properties of the foods. During drying, shrinkage occurs in all directions of the foods. Ren et al. (2021) reported the combinative effect of cutting orientation and drying techniques on the physicochemical properties of ginger slices. Results showed that the longitudinal cut reduced the drying time significantly, by 15.12-29.6%, compared with the transverse cut and improved the physical properties of the dried ginger slices. Buvaneswaran et al. (2022) studied the effect of drying temperature (40, 50, and 60 C) and pretreatments conditions on shrinkage and rehydration kinetics of ginger slices. Results indicated that the pretreatments and drying temperature conditions significantly affect shrinkage-deformation, and a quadratic equation, as a function of moisture content, described the shrinkage behaviour of ginger slices. The process condition had a significant effect on the rehydration process. In all combinations, the rehydration ratio of ginger slices lies between 2.5 and 5.
Although several studies have been undertaken on the drying of ginger, however, information on the shrinkage effect appears to be scanty in the literature. The main objective of this study was to investigate the shrinkage effect on ginger during convective drying at different air temperatures.

Material
Fresh ginger samples were procured from a local farmer in San Carlos, Nicaragua. After thorough cleaning and washing, the gingers free from injury were selected, manually peeled, and sliced.

Shrinkage
Among the methods in the literature to describe the shrinkage of foods, Gekas and Lamberg (1991) proposed to consider the shrinkage effect on moisture diffusion by incorporating volume changes into the effective diffusion coefficient, thus leading to: where d takes value of one (1) in the case of a one-dimensional shrinkage, the value of three (3) in the case of an isotropic three-dimensional shrinkage, and d equal to a fractal in the most general case (Sjöholm and Gekas, 1995). Thus, d may be viewed as a measure of the degree of isotropicity of the deformation and is related to linear and volume shrinkage by: where Sv and St are defined as: where V and L are volume and thickness, respectively.

Shrinkage determination
Slices were produced by slicing both longitudinally and transversely the ginger samples to a thickness of 3 to 5 mm using an electric slicing machine. The ginger slices were soaked in water for two days to obtain a uniform moisture content. Subsequently, the ginger slices were cut into rectangles with a length of about 40 mm and a width of about 30 mm.
As suggested by Jarquín (1996), the edges of the rectangular ginger slices were coated with an oil-based layer (e.g., an enamel paint layer) before each experiment to obtain moisture removal only on both sides of the rectangular ginger slices.
Drying was performed in a convection oven with a temperature control of about  1 C (e.g., a Fisher Scientific™ oven), which includes a metallic support structure as shown in Fig. 1. The metallic support structure includes hooks that were used as sample holders. The shrinkage experiments were performed at air temperatures of 40, 60, and 80 °C. Before starting an experiment, the oven was run for at least half an hour to obtain steady-state conditions. In each experiment, a pair of longitudinal and transverse cut ginger samples were used, with similar size and moisture content. The ginger slices were hooked evenly in the metallic support structure and subsequently were put into the oven. The length, width, thickness, and mass of the ginger slices were recorded. The ginger slices were removed from the oven to conduct data sampling every half hour during the first two hours of the drying process because moisture evaporation was faster at the beginning. The subsequent data sampling was performed every hour until the mass was invariable. All the shrinkage experiments were performed in triplicate. Post-processing of these data yields an equation that relates the changes in the drying area as a function of the moisture content [As = As(X)].

Drying
A schematic arrangement of the experimental drying apparatus is shown in Fig. 2. The equipment may be divided into four main sections as follows: gas supply and dehumidification section, heating section, drying chamber, and analysing equipment. The blower (B) supplies a gas flow--a broad range of flow rates is possible by changing the rpm setting through the frequency inverter (FI). The air passes through an adsorption column (AC) containing a dehumidificant (silica gel) to obtain a process air of low humidity content (less than 1.0% relative humidity) measured by a hygrometer. The air velocity is measured by an anemometer. After dehumidification, the air is pre-heated with electrical resistance (ER) heaters of up to 2 kW each. Temperature is controlled by means of a temperature controller (TC) that supplies heat by means of an electrical resistance heater as its final control element. Before entering the drying chamber, a static mixer homogenises the temperature by mixing the gas. The sample is put in the sample holder (SH) inside the drying chamber and is supported on a weighing balance (WB) through an oil-sealed shaft. The cross-sectional area and depth of the sample holder are 30.65 mm  109.8 mm and 3 mm, respectively. The drying chamber has a uniform cross-sectional area of 90 mm  110 mm. The sample's weight history is recorded on a computer (C). It is possible to take a reading every 12 seconds (Mendieta et al., 2015).
The drying experiments were performed at air temperatures of 40, 60, and 80 °C and a velocity of 1 m/s. Before starting an experiment, the drying apparatus was run for at least half an hour to obtain steady-state conditions. For the drying experiments, transverse cut ginger samples were used. The sample was loaded evenly within the sample holder, which covered the whole drying area. The sample holder was put into the tunnel dryer. The drying time and mass of the sample were recorded. The experiment was stopped until the mass was invariable. After drying by the apparatus above, the sample was further dried in an oven at 60 °C for 18 hours to determine its oven-dry mass (ms). The initial mass, drying mass and oven-dry mass were determined with a precise analytical balance. All the drying experiments were performed in triplicate. Post-processing of these data yields drying kinetics.

Drying kinetics
The moisture content is computed as follows: where X is the moisture content at any time (dry basis), m is the mass of the sample at any time, and t is the time. The drying rate is defined as: where Nv is the drying rate at any time and As is the drying area. Using the moisture content data as a function of time and a centred approximation of the derivative, it is possible to determine the drying rate as follows (Picado et al., 2006): The processing of the experimental data is performed using a program written in MATLAB ® , this program reads the experimental data obtained and plots the drying curves and drying rate curves according to Eqs. (5) and (7). and where  is the characteristic moisture content, f is the relative drying rate, Xeq is the equilibrium moisture content, Xcr is the critical moisture content, and Nw is the drying rate at the constant rate period. The general form of the CDC is given by f = f ().
By plotting f vs. ϕ at the different temperatures tested, a group of curves is obtained whose behaviour, if common, describes a characteristic drying curve to which a mathematical function (e.g., a polynomial function) can be determined. It is assumed that a unique relationship between f and ϕ can be found for a specific material.

RESULTS AND DISCUSSION
3.1 Shrinkage characteristics Figure 3 presents how volumetric shrinkage (Sv) depends on the moisture content (wet basis) of the ginger samples at 60 °C. In a convective drying process, shrinkage normally occurs due to cellular collapse because of the cell wall and membrane damage by heating. As can be seen from Fig. 3, the shrinkage showed two periods during drying. The first period corresponded to a falling period in which the shrinkage of the ginger slices is higher due to high mass transfer. The second period corresponded to a period in which the shrinkage was essentially constant due to the internal resistance to moisture migration and low free moisture. This could also be due to the transition from a rubbery to a glassy state and case hardening (Lin et al., 2020). This shrinkage behaviour is similar for all the temperatures (40, 60, and 80 °C) and longitudinal and transverse cut samples.
It was observed that the shrinkage of the transverse cut samples was higher than the shrinkage of the longitudinal cut samples. This could be due to the cutting orientation of the internal fibres of the ginger samples that left empty spaces within the ginger slices and between the fibres, thus causing the drying air to penetrate and produce higher moisture evaporation. In addition, the shrinkage was progressive towards the centre, thus making the ginger slices take a concave shape. The results of Fig. 3 suggest that the volume shrinkage of the ginger slices equals the volume of removed water.
Analysing the relationship between the volume change (Sv) and the thickness change (St) experimentally obtained, exponent d in Eq.
(2) was calculated by linear regression of ln(Sv) vs. ln (St). A typical plot of such results is shown in Figs. 4 and 5. Longitudinal cut samples showed a d-value of 1.55. A fairly good fit was obtained (see Fig. 4) as represented by a correlation coefficient of 0.983 (for all temperatures). The degree of shrinkage of the transverse cut samples is shown in Fig. 5. The d-value is 1.65 with a correlation coefficient of 0.989 (for all temperatures). As is shown, the exponent d slightly changed with the drying temperature and cutting orientation. The results suggested an anisotropic behaviour (related to the change of ginger structure during drying). Further, the results confirmed the general assumption of onedimensional moisture transport.
These equations enable to consider the shrinkage effect on the drying rate of the ginger samples.

Drying characteristics
The variation of moisture content with drying time at air temperatures of 40, 60, and 80 °C and an air velocity of 1 m/s for ginger are shown in Fig. 8. The moisture content of the ginger samples decreased exponentially with time. As expected, an increase in drying air temperature reduces the time required to reach any given level of moisture content. This can be explained by increasing temperature difference between the drying air and the ginger samples and the resultant moisture (water) removal.
Drying rates were estimated based on Eqs. (7) and (11) and are shown in Fig. 9. An important influence of air temperature on drying rate is observed. As expected, an increase in air temperature increases the drying rate because higher air temperature causes a higher reduction of moisture content -in other words, at high temperatures the transfer of heat and mass is high and moisture loss is excessive. As can be seen from Fig. 9, no constant rate period was observed in the drying of ginger; the drying process took place at a falling rate period. These results are in good agreement with earlier observations (Thorat et al., 2012;Deshmukh et al., 2014). Shrinkage during drying was considered in calculating the drying rates.  The drying rate and moisture content are normalised by the constant drying rate and critical moisture content, respectively. However, for ginger no constant drying rate period was observed and another approach must be found to normalise the drying data. In this case, the critical moisture content and the maximum drying rate were determined at the beginning of the falling rate period (Bellagha et al., 2002;Picado et al., 2006).
The drying rate and moisture content are normalised by the constant drying rate and critical moisture content, respectively. However, for ginger no constant drying rate period was observed and another approach must be found to normalise the drying data. In this case, the critical moisture content and the maximum drying rate were determined at the beginning of the falling rate period (Bellagha et al., 2002;Picado et al., 2006).
In Fig. 10, experimental drying data are plotted to represent f = f (). Figure 10 shows that all drying curves obtained with the characteristic moisture content () and relative drying rate (f), for the different tested conditions, fall into a tight band, thus indicating that the effect of variation in different conditions is small over the range tested. The regression analysis was performed using MATLAB ® 's Curve Fitting Tool to find the best equation for the ginger characteristics drying curve. A polynomial equation was found to fit the best the experimental data:

CONCLUSIONS
The drying of ginger (Zingiber officinale Roscoe) considering shrinkage was experimentally studied at air temperatures of 40, 60, and 80 °C. For the determination of shrinkage, two cutting orientations (longitudinal and transverse) were employed. The volumetric and thickness shrinkage was evaluated by direct measurement. The shrinkage showed two periods during drying. The first and the second period corresponded to a falling period and a constant period, respectively. The shrinkage of the transverse cut samples was higher than the shrinkage of the longitudinal cut samples. The shrinkage was progressive towards the centre, thus making the ginger samples take a concave shape. The results confirmed the general assumption of one-dimensional moisture transport. An equation that relates the changes in the drying area as a function of the moisture content was determined. As expected, an increase in drying air temperature reduced the time required to reach any given level of moisture content. No constant rate period was observed in the drying of ginger. The drying took place at a falling rate period. Ginger exhibited a characteristic drying curve.