Influence of Infrared Drying on Drying Kinetics of Apple Slices Coated with Basil Seed and Xanthan Gums

ABSTRACT

Edible coatings can guarantee the quality of agricultural products, and performance as a low oxygen barrier, carbon dioxide, and water vapor, allowing reducing water loss or controlling water adsorption. The objective of the current work was aimed to evaluate the effects of novel edible coatings based on basil seed and xanthan gums, and infrared (IR) drying efficiency of coated apple slices. Seven empirical thin-layer models were fitted to the moisture ratio data. It was found that Page model had the best fit to show the kinetic behavior and acceptably described the IR drying behavior of coated apple slices with the lowest mean square error (MSE), root mean square error (RMSE), mean absolute error (MAE), and standard error (SE) values and the highest correlation coefficient (r) value. The values of MSE, RMSE and MAE for all experiments were in the ranges of 0.00014–0.00058, 0.012–0.024 and 0.009–0.021, respectively. The average drying time of uncoated apple slices, coated by xanthan gum and coated by basil seed gum were 48.00, 60.22, and 84.78 min, respectively. The average effective moisture diffusivity (D_eff) of uncoated and coated apple slices with basil seed and xanthan gums increased from 1.70 × 10⁻⁹ m²/s to 4.45 × 10⁻⁹ m²/s with increasing IR lamp power from 150 W to 375 W.

KEYWORDS:

• Effective moisture diffusivity
• moisture ratio
• page model

Introduction

Edible coatings can guarantee the quality of agricultural products, and performance as a low oxygen barrier, carbon dioxide, and water vapor, allowing reducing water loss or controlling water adsorption. They applied to food slices prior to drying is a technology that can improve the nutritional and sensory qualities of dehydrated products. Polysaccharide edible coatings present low water vapor barrier; however, they present good gas barrier properties, such as oxygen barrier, and could be used to minimize oxidative reactions in food during drying, pointing out the potential of using edible coatings prior to convective drying, since it could reduce undesirable changes due to large time of exposure of the food to oxygen (Fakhouri et al., 2007; Garcia et al., 2014; Salehi, 2021; Silva et al., 2015). Garcia et al. (2014) reported that edible coating by pectin reduced vitamin C losses during convective drying of papaya slices, when compared to the uncoated samples, showing that the coating protected the samples against the oxidation of this biologically active compound. Basil plant (Ocimum basilicum L.) is the mucilaginous native plant and its seeds have a high content of mucilage (gums) with outstanding useful characteristics. Basil gum is a hydrocolloid that extracted from this plant seeds (Salehi, 2019; Salehi et al., 2015). Major properties of the basil seed gum as a novel source of gum has been recently reported by Salehi (2020a).

Apple represents the fourth most important horticultural crop for human nutrition in the world. Drying is one of the important preservation methods employed for storage of apple (Salehi, 2017). One of the ways to decrease the drying time is to supply heat by infrared (IR) radiation. IR methods could be used as substitution to the current drying methods for producing high-quality dried hydrocolloids. IR heating has many advantages including high heat transfer rate, short processing time, high efficiency (80–90%), lower energy consumption, lower energy costs, and improving final product quality (Aktaş et al., 2017; Salehi, 2020c; Salehi et al., 2016). Comparing convective and IR heating as means of drying pomegranate arils was studied by Briki et al. (2019). The authors reported that the minimum times required to reach 9% moisture (w/w) starting from 78% were 510 and 94 min for convective and IR drying, respectively. In addition, Łechtańska et al. (2015) examined the IR assisted hot air drying of green pepper. They reported about 38% decrease in drying time when compared to drying using hot air.

The objective of this study was to investigate the effect of IR drying on the drying kinetics, moisture content and effective moisture diffusivity (D_eff) of coated apple slices with basil seed and xanthan gums.

Materials and Methods

Apple Slices Preparation

Slices of apple (5 mm thickness) were prepared with the aid of a cutter and a steel-made cutting tool, which was cylindrical in shape and pointed on one of the sides. The initial moisture content of the apple slices was 86% (wet basis). Moisture content of apple slices was determined in a oven at 105°C for 5 h (AOAC, method no. 934.06).

Gum Extraction

Basil seeds were physically cleaned and all foreign materials were removed. Then, the pure basil seeds were immersed in water for 20 min at a seed/water portion of 1:20 at 25°C. In the next step, the gum was separated from the inflated seeds by passing the seeds through an extractor (Bellanzo BFP-1540 Juicer, China) with a rotating disc which scratched the mucilage layer on the seed surface. The initial moisture content of the basil seed gum was 99.4% (wet basis).

Coating of Apple Slices

Xanthan and basil seed gums were used to coat the fresh apple slices. A 0.6% (w/w) xanthan and basil seed gums solution were prepared at 25°C and then apple slices were immersed for 1 min in this aqueous solution.

IR Drying

The coated apple slices were dried in an IR dryer (figure 1). The distance of apple slices from IR lamp surface was equal 10 cm. The influence of IR radiation power at three levels 150, 250 and 375 W, and drying time (min) on drying kinetics of apple slices was examined. The weight changes of apple slices were measured using a LutronGM-300p digital balance (Taiwan, sensitivity of ±0.01 gr). In this study, the digital balance was placed under the IR dryer and samples were placed on its surface. Also, the weigh change data were transferred continually (every 1 min) to the computer.

Figure 1. Schematic of infrared drying of coated apple slices

Experimental Design

In this study the effects of drying parameters, including infrared radiation power (at three levels of 150, 250 and 375 W) and coating type (at three levels of uncoated, coated by xanthan gum and coated by basil seed gum) on the drying kinetics, moisture content and effective moisture diffusivity of apple slices were investigated.

Statistical Analysis

Statistical analysis of data was done in a factorial design by analysis of variance (ANOVA) using SAS 9.1 statistical software. Significant difference between data means were determine using Duncan’s multiple range test at α = 0.05 level, and it was performed to established the impact of coating type and IR radiation power on drying kinetics of apple slices. All measurement were done in triplicate.

Drying Kinetics

Numerical modeling is one of the appropriate methods for describing the drying kinetics of agricultural products (Salehi, 2020b). In order to numerical modeling the drying kinetic behavior of coated apple slices, 7 commonly used thin-layer models including Quadratic (Eq. 1(1) $MR=a+bx+cx2$(1) ), Page (Eq. 2(2) $MR=exp\left(-ktn\right)$(2) ), Newton (Eq. 3)(3) $MR=exp\left(-kt\right)$(3) , Midilli (Eq. 4(4) $MR=aexp\left(-ktn\right)+bt$(4) ), Logarithmic (Eq. 5(5) $MR=aexp\left(-kt\right)+c$(5) ), Verma (Eq. 6)(6) $MR=aexp\left(-kt\right)+\left(1-a\right)exp\left(-gt\right)$(6) and Two term (Eq. 7)(7) $MR=aexp\left(-k0tn\right)+bexp\left(-k1t\right)$(7) were examined (Akpinar and Bicer, 2005; Doymaz, 2011).

(1) $MR=a+bx+cx2$(1)

(2) $MR=exp\left(-ktn\right)$(2)

(3) $MR=exp\left(-kt\right)$(3)

(4) $MR=aexp\left(-ktn\right)+bt$(4)

(5) $MR=aexp\left(-kt\right)+c$(5)

(6) $MR=aexp\left(-kt\right)+\left(1-a\right)exp\left(-gt\right)$(6)

(7) $MR=aexp\left(-k0tn\right)+bexp\left(-k1t\right)$(7)

Where MR and t are moisture ratio and drying time, respectively. Also a, b, c, k, n, and g are coefficients of models. In these models, dimensionless moisture ratio (MR) were defined as equation 8(8) $MR=\frac{M_t-M_e}{M_0-M_e}$(8) :

(8) $MR=\frac{M_t-M_e}{M_0-M_e}$(8)

Where M_t is moisture content of the sample (gr water/gr dry matter) at time t; M_e and M₀ are equilibrium and initial moisture content (gr water/gr dry matter), respectively. In equation (8)(8) $MR=\frac{M_t-M_e}{M_0-M_e}$(8) , since M_e≪M_t and M_e≪M₀, the value of M_e is negligible and the equation was simplified to M_t/M₀ (Salehi et al., 2017).

Nonlinear regression analysis (based predictive control algorithm) was done using Curve Expert software (Version 1.34, Hyams, D. G., Microsoft Corporation) to evaluate equations parameters. Mean square error (MSE), root mean square error (RMSE), mean absolute error (MAE), standard error (SE), and correlation coefficient (r) values were calculated using equations 9(9) $MSE=\frac{{\sum _{i=1}^N\left(O_i-T_i\right)}^2}{N}$(9) to 13 to evaluate the accuracy of models (Bahramparvar et al., 2014). It is noted that the highest r-value (closer to one) and the lowest MSE, RMSE, MAE, and SE values (closer to zero) represent the best model (good fitting).

(9) $MSE=\frac{{\sum _{i=1}^N\left(O_i-T_i\right)}^2}{N}$(9)

(10) $\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}=\sqrt{\frac{\sum _{\mathrm{i}=1}^{\mathrm{N}}{({\mathrm{O}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{i}})}^2}{\mathrm{N}}}$(10)

(11) $\mathrm{M}\mathrm{A}\mathrm{E}=\frac{1}{\mathrm{N}}\sum _{\mathrm{i}=1}^{\mathrm{N}}\left|{\mathrm{O}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{i}}\right|$(11)

(12) $\mathrm{S}\mathrm{E}=\frac{\sigma }{\sqrt{\mathrm{N}}}$(12)

(13) $\mathrm{r}=\sqrt{1-\frac{\sum _{\mathrm{i}=1}^{\mathrm{N}}{[{\mathrm{O}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{i}}]}^2}{\sum _{\mathrm{i}=1}^{\mathrm{N}}{[{\mathrm{O}}_{\mathrm{i}}-{\mathrm{T}}_{\mathrm{m}}]}^2}}$(13)

Where O_i is the i^th actual value, T_i is the i^th predicted value, N is the number of data, σ is the standard deviation, and T_m is given by:

(14) ${\mathrm{T}}_{\mathrm{m}}=\frac{\sum _{\mathrm{i}=1}^{\mathrm{N}}{\mathrm{O}}_{\mathrm{i}}}{\mathrm{N}}$(14)

Calculation of Moisture Diffusivity (D_eff)

Fick’s second law of diffusion can be used to describe the thin layer drying of agricultural products at falling rate period (Sacilik, 2007). According to this law, moisture ratio for different geometries including cylinder, slab and sphere is defined as equation 15(15) $\frac{\mathrm{\partial }MR}{\mathrm{\partial }t}=D_{eff}{\mathrm{\nabla }}^{2}MR$(15) :

(15) $\frac{\mathrm{\partial }MR}{\mathrm{\partial }t}=D_{eff}{\mathrm{\nabla }}^{2}MR$(15)

Analytical solution of this equation for infinite slab geometry and with assuming a constant moisture distribution, one dimensional moisture, negligible shrinkage, and negligible external resistance used to predict moisture diffusion in samples (Doymaz, 2011; Onwude et al., 2018). The dimensionless moisture content values were calculated with the equilibrium moisture content determined by dynamic equilibrium data of samples. It is given as equation 16(16) $MR=\frac{8}{{\pi }^2}{\sum }_{n=1}^{\mathrm{\infty }}\frac{1}{{(2n+1)}^2}exp(-(2n+1{)}^2\frac{{\pi }^2{D}_{eff}t}{4L^2})$(16) :

(16) $MR=\frac{8}{{\pi }^2}{\sum }_{n=1}^{\mathrm{\infty }}\frac{1}{{(2n+1)}^2}exp(-(2n+1{)}^2\frac{{\pi }^2{D}_{eff}t}{4L^2})$(16)

Where t is the drying time (s), D_eff is the effective moisture diffusivity (m²/s); L is half-thickness of apple slices which is equal to 0.25 × 10⁻² m in this study.

For long drying process period, Eq. (16)(16) $MR=\frac{8}{{\pi }^2}{\sum }_{n=1}^{\mathrm{\infty }}\frac{1}{{(2n+1)}^2}exp(-(2n+1{)}^2\frac{{\pi }^2{D}_{eff}t}{4L^2})$(16) can be further simplified to:

(17) $MR=\frac{8}{{\pi }^2}exp\left[\frac{-{\pi }^2{D}_{eff}t}{4L^2}\right]$(17)

Hence, a logarithmic form was introduced as follows:

(18) $LnMR=Ln(\frac{8}{{\pi }^2})-\frac{{\pi }^2{D}_{eff}t}{4L^2}$(18)

The D_eff was calculated through Eq. (18)(18) $LnMR=Ln(\frac{8}{{\pi }^2})-\frac{{\pi }^2{D}_{eff}t}{4L^2}$(18) by using the method of slopes. From Eq. 18(18) $LnMR=Ln(\frac{8}{{\pi }^2})-\frac{{\pi }^2{D}_{eff}t}{4L^2}$(18) , a plot of experimental drying data in terms of lnMR versus time gives a straight line with a slope (K) of:

(19) $Slope(K)=-\frac{{\pi }^2{D}_{eff}}{4L^2}$(19)

Results and Discussion

Drying Time

Symmetrical temperature sharing by IR improved final product quality (Baeghbali et al., 2019). Statistical analysis of experimental results (data) demonstrated that the coating type (uncoated, coated by xanthan gum and coated by basil seed gum) and IR power, have a significant effect on the evolution of drying time of apple slices (p < .01) (table 1). The effects of coating type and IR radiation power on the moisture content (%) of apple slices are shown in figure 2. The average drying time of uncoated apple slices, coated by xanthan gum and coated by basil seed gum were 48.00, 60.22 and 84.78 min, respectively. As expected, the moisture content decreased with increasing the power because of the increasing temperature and heat transfer gradient between the air and samples. The average drying times of coated apple slices with basil seed gum were 135.67, 69.00 and 49.67 min at 150, 250, and 375 W, respectively. With increasing IR intensity, due to the increase in samples temperature and increasing evaporation rate and the decrease in drying time, the specific energy for drying of coated apple slices decreases. The average drying time of coated apple slices with xanthan gum reduced from 90.67 to 39.33 min when the IR radiation power was increased from 150 to 375 W. The effect of IR pretreatment on low humidity air drying of apple slices was investigated by Shewale and Hebbar (2017). They observed that the pretreatment with IR waves decreased the drying time approximately 23 and 17% in low-humidity air and hot air drying, respectively.

Table 1. Results of analysis of variance for drying time parameters of coated apple slices

Sources of changes	Degrees of freedom	Sum of squares	Mean square	F	P
Coating type	2	6314.9	3157.4	30.11	0.000
Power	2	17613.6	8806.8	83.99	0.000
Coating type × Power	4	1930.2	482.6	4.60	0.010
Error	18	1887.3	104.9
Total	26

Figure 2. Variations of moisture content (%) of coated apple slices with drying time at different: a) Coating type (150 W power); b) IR power (Coated apple slices with basil seed gum)

Numerical Modeling

In order to estimate drying kinetics of food products, numerous empirical models have been used by researchers. In this study, 7 thin-layer equations (Quadratic, Page, Newton, Midilli, Logarithmic, Verma and Two term) were selected and fitted to experimental data to choose the best and most suitable equation. The model with the highest r value and the lowest MSE, RMSE, MAE and SE values was selected as the best suitable model describing the IR drying processes of uncoated and coated apple slices. It was found that Page model has the best fit to show the kinetic behavior. The estimated parameters (fitting data) of the Page model including drying constants, k and n, are tabulated in table 2 along with corresponding statistical data (MSE, RMSE, MAE, SE, and r) for all experiments conditions. The values of MSE, RMSE, and MAE for all experiments were in the ranges of 0.00014–0.00058, 0.012–0.024, and 0.009–0.021, respectively. Also, the values of r and SE for all experiments were in the ranges of 0.996–0.999 and 0.013–0.025, respectively. This model has been proved for other IR dried products such as apple (Zhu and Pan, 2009), apple pomace (Sun et al., 2007), mushroom (Salehi et al., 2017), banana (Pekke et al., 2013) and onion (Sharma et al., 2005).

Table 2. Model constants of the page model for all experimental data

Coating type	Power (W)	k	n	MSE	RMSE	MAE	SE	r
Uncoated	150	0.0147	1.2163	0.00058	0.0241	0.0211	0.025	0.996
	250	0.0233	1.3976	0.00027	0.0165	0.0140	0.017	0.998
	375	0.0304	1.3437	0.00033	0.0181	0.0143	0.019	0.998
Xanthan	150	0.0111	1.2481	0.00014	0.0120	0.0088	0.013	0.999
	250	0.0160	1.3181	0.00039	0.0197	0.0156	0.021	0.997
	375	0.0288	1.2627	0.00036	0.0188	0.0151	0.021	0.997
Basil	150	0.0100	1.1725	0.00034	0.0186	0.0160	0.019	0.998
	250	0.0191	1.2038	0.00038	0.0195	0.0165	0.020	0.997
	375	0.0270	1.2027	0.00048	0.0220	0.0187	0.023	0.997

Figure 3 shows comparison of fitted moisture ratio data by Page model with experimental results. These results indicate that Page model is appropriate in describing drying characteristics of uncoated and coated apple slices with basil seed and xanthan gums under the various IR drying conditions.

Figure 3. Comparison of fitted data by page model with experimental results

Moisture Diffusivity

Effect of IR drying systems on the D_eff of some fruits and vegetables was studied (Salehi, 2020c). The D_eff values lie within in range of 10⁻⁸ to 10⁻¹⁰ m²/s for fruits and vegetables in this dryer. The D_eff values are determined by plotting experimental drying data in terms of lnMR versus time. The values of D_eff at different condition drying of uncoated and coated apple slices obtained by using Eq. (19)(19) $Slope(K)=-\frac{{\pi }^2{D}_{eff}}{4L^2}$(19) and calculated values are shown in table 3. The results of such fitting gave a average regression coefficient of 0.93 indicating that the quality of such fitting was satisfactory. The D_eff values of coated apple slices with basil seed gum and xanthan gum were ranged from 1.30 × 10⁻⁹ to 3.47 × 10⁻⁹ m²/s and 1.74 × 10⁻⁹ to 4.20 × 10⁻⁹ m²/s, respectively. D_eff values increased with increasing IR radiation power because of the rapid movement of water at high temperatures (Doymaz, 2011). The average D_eff increased from 1.70 × 10⁻⁹ m²/s to 4.45 × 10⁻⁹ m²/s with increasing lamp power from 150 W to 375 W. These D_eff values are comparable with the reported values of 1.00 × 10⁻⁸ to 3.72 × 10⁻⁸ m²/s for dried quince in IR system (Mehrnia et al., 2017) and 0.87 × 10⁻⁹ to 2.64 × 10⁻⁹ m²/s for dried pomegranate arils in IR system (Briki et al., 2019).

Table 3. Effective moisture diffusivity values (D_eff) of coated apple slices at different IR drying conditions

Coating type	Power (W)	Effective diffusivity (m^2/s)	r
Uncoated	150	2.06 × 10^−9	0.941
	250	4.92 × 10^−9	0.954
	375	5.68 × 10^−9	0.900
Xanthan	150	1.74 × 10^−9	0.946
	250	3.06 × 10^−9	0.914
	375	4.2 × 10^−9	0.916
Basil	150	1.3 × 10^−9	0.933
	250	2.54 × 10^−9	0.943
	375	3.47 × 10^−9	0.908

Conclusions

In this study, the influence of coating type (uncoated, coated by xanthan gum and coated by basil seed gum) and IR radiation power on the drying kinetics of apple slices were studied. The coating type and IR lamp power influenced the drying time of coated apple slices. The average drying times of coated apple slices with basil seed gum were 135.67, 69.00 and 49.67 min at 150, 250 and 375 W, respectively. The drying characteristics were satisfactorily described by Page model with the highest r value (greater than 0.996) and the lowest MSE, RMSE, MAE and SE values (a good fit). The D_eff values of coated apple slices with basil seed gum and xanthan gum were ranged from 1.30 × 10⁻⁹ to 3.47 × 10⁻⁹ m²/s and 1.74 × 10⁻⁹ to 4.20 × 10⁻⁹ m²/s, respectively. In addition, the average D_eff of uncoated and coated apple slices with basil seed and xanthan gums increased with increasing IR lamp power.

List Of Symbols

D₀Pre-exponential factor (m²/s)

D_effEffective moisture diffusivity (m²/s)

kDrying rate constants in models (1/s)

KSlope

LHalf slab thickness of the samples (m)

M₀Initial moisture content (kg water/kg dry matter)

MeEquilibrium moisture content (kg water/kg dry matter)

M_tMoisture content at time t (kg water/kg dry matter)

nNumber of constants

NNumber of observations

rCorrelation coefficient

tDrying time (min)

Disclosure Statement

We have no conflict of interest to declare.