# A Comparative Analysis of Thin-Layer Microwave and Microwave/Convective Dehydration of Chokeberry

^{1}

^{2}

^{3}

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## Abstract

**:**

_{eff}= 6.0768 × 10

^{−9}± 5.9815 × 10

^{−11}m

^{2}s

^{−1}), and represents the most energy effective for dehydration process (E

_{min}= 0.382 ± 0.036 kWh). A higher water-holding capacity (WHC) characterized the chokeberries obtained by the MCD method compared to the regular microwave method (MD). The mildest MCD (15 s of MD on 900 W, 7 s of CD on 180 °C) could dehydrate chokeberries with the highest WHC (685.71 ± 40.86 g H

_{2}O g

^{−1}d.m.) and the greatest evaluations for sensory attributes in terms of all properties. The results of this study provide the drying behavior of chokeberries that can help develop efficient drying methods and improve existing ones.

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Materials

#### 2.2. Methods

#### 2.2.1. Microwave Dehydration

^{−3}(30 g per tray). The MD was obtained at the microwave power (MW) power (P) 270, 450, and 900 W. The MCD was a discontinuous model: 2.30 (9 s of MW on 900 W, 12 s of CD on 230 °C), 2.00 (9 s of MW on 900 W, 12 s of CD on 200 °C), and 1.50 (15 s of MD on 900 W, 7 s of CD on 180 °C). The weight of the trays was measured at intervals of 5 min for MCD and 10 min for MD (in triplicates). The drying kinetic was based on mass losses of chokeberries [17,18].

#### 2.2.2. Models of Thin-Layer Dehydration

_{e}) was typically insufficient and could be deleted without significantly changing MR Equation (1).

_{t}and M

_{o}, respectively, represent the moisture content reached after the convective drying time t and the initial moisture content.

_{i−1}− M

_{i}) in the time between two measurements (t

_{i−1}− t

_{i}) on a specific tray during the drying process can also be used to express the drying kinetics (drying ratio, DR, Equation (2)):

#### 2.2.3. Determination of Effective Moisture Diffusivity

_{eff}) was determined. Equations (3) and (4) define the theoretical calculation model based on the product geometry (sphere is the appropriate model for the berries) [7,17]:

_{eff}is the effective moisture diffusivity (m

^{2}s

^{−1}), t is time (s), MR is the moisture ratio, J

_{0}is the roots of the Bessel function, A

_{1}(dimensionless) and A

_{2}are geometric constants (mm

^{2}), and r is the radius of the sphere (mm).

_{eff}values and a relatively long drying period, Equation (3) is derived:

_{eff}(Equation (6)).

#### 2.2.4. Determination of Activation Energy

^{−1}), and the natural logarithm of D

_{eff}versus mass load power

^{−1}was used to determine the activation energy of MD. As a result, in the measured MD power range, the plot is a straight line, indicating the Arrhenius dependence (Equation (7), [18]).

_{a}is the activation energy (W g

^{−1}), m is the mass load (g), D

_{0}is the pre-exponential factor (m

^{2}s

^{−1}), and P is the power of MD (W). The conversion factor between W g

^{−1}and kJ mol

^{−1}is 1 W g

^{−1}× M (1 g mol

^{−1}) × 1 kJ (1000 J)

^{−1}.

_{a}is the activation energy (kJ·mol

^{−1}), R is the universal gas constant (8.3143 J·mol

^{−1}K

^{−1}), and T is the absolute air temperature (K). The previous Equation (8) could be simplified into the linear equation ln (D

_{eff}) = ln (D

_{0}) − 10

^{−3}× k × (T + 273.15)

^{−1}. E

_{a}was calculated from the slope of the Arrhenius equation (Equation (9)):

#### 2.2.5. Determination of Energy Consumption of Dehydration Processes

_{2}emission during the MW and MW·C

^{−1}(1 kWh of E releases 0.998 kg CO

_{2}[17].

#### 2.2.6. Water-Holding Capacity

#### 2.2.7. Sensory Evaluation

#### 2.2.8. Statistical Methods

_{eff}, t, E, CO

_{2}, and sensory properties) that were evaluated using a covariance matrix, the principal component analysis (PCA) was applied [24]. Pearson correlation was calculated, and the level of significance of p < 0.05 was applied. R Studio 1.4.1106 program was used for the color correlation graph between the obtained mass transfer rate parameters, the WHC, D

_{eff}, t, E, CO

_{2}, and sensory properties [25].

## 3. Results and Discussion

#### 3.1. Models of Thin-Layer Dehydration

_{2}O kg

^{−1}d.m. (dry matter). The MR in dehydration measures the amount of moisture present in a food product being dried using microwave and convective energy. It is an essential parameter in drying because it determines the rate at which moisture is removed from the product as well as the final moisture content of the product. A lower MR results in faster drying and lower final moisture content, while a higher MR results in slower drying and higher final moisture content [26]. The DR as well as MR in a microwave also depend on several factors, including the type and thickness of the material being dehydrated, the strength of the microwave and the temperature of convective dehydration, and the duration of the dehydration process. Providing a general dehydration rate for microwave dehydration is difficult, as it can vary significantly depending on these factors [27]. This could be attributed to the water molecules inside the berries absorbing microwave energy, causing rapid evaporation and partial puffing. In general, materials with a higher moisture content will dehydrate faster than those with a lower moisture content. Thinner materials also tend to dehydrate faster than thicker materials [14,27]. A higher-wattage microwave and a higher-temperature range of convective dehydration will generally produce a faster dehydration ratio than a lower-wattage microwave. Finally, the duration of the dehydration process will also affect the rate at which the material dehydrates [28].

^{−k × t}) showed some mathematical regularity in the coefficient a and the coefficient k reduction, unlike slight variations in the coefficient of the polynomial model by energy growth input. In Calín-Sánchez’s research, Henderson and Pabis’s exponential models were also the appropriate model to describe the freeze and convective drying, vacuum microwave drying, and combined drying methods (convective/osmotic–vacuum microwave drying [4]).

^{2}. The maximum DR was achieved in 6–10 min of the dehydration process regardless of the dehydration model. The highest DR had the MCD models of 1.33, 1.25, and 1.17 g min

^{−1}for the models 2.30, 2.00, and 1.50, respectively. Conversely, the highest DRs for the MD models were 1.04, 0.72, and 0.39 g min

^{−1}for the microwave power ranges 900, 450, and 270 W, respectively (Figure 2).

#### 3.2. Determination of Effective Moisture Diffusivity

_{eff}, also known as the effective moisture diffusivity, is a measure of the rate at which moisture diffuses through a material. It is affected by several factors, including the properties of the material, the temperature and humidity of the environment, and the presence of any barriers or coatings on the material. In general, the effective moisture diffusion coefficient will be higher at higher temperatures and energy inputs, as the increased temperature will cause the moisture molecules to move more quickly [5]. It is not clear how microwave energy alone would affect the effective moisture diffusion coefficient, as it is not a property of the material. However, if the material is heated by microwave energy, the effective moisture diffusion coefficient will be higher at higher temperatures [18]. The experimental results in this paper confirmed this claim, where the highest D

_{eff}values had the MCD models with the highest microwave power range 900 W for 9 s and the CD temperature for 12 s (D

_{eff}= 6.0768 × 10

^{−9}± 5.9815 × 10

^{−11}m

^{2}s

^{−1}). When increasing the MW energy input or being exposed to microwave energy for a long time, the D

_{eff}will statistically significantly increase.

#### 3.3. Determination of Activation Energy

_{a}may depend on the specific reaction and conditions. However, it is known that microwave heating can cause localized heating and can lead to the formation of hot spots, which can potentially increase the reaction rate by increasing the temperature and the collision frequency of molecules [30]. The Ea value for the MD was 81.6231 ± 0.0787 kJ·mol

^{−1}and for the MCD was 92.8707 ± 0.0942 kJ·mol

^{−1}. Decreased E

_{a}indicates more effective moisture diffusivity (higher D

_{eff}) and rising moisture diffusion with sphere radius (thickness), implying that lower energy consumption causes the bond between the water molecules of the sample to break [31].

#### 3.4. Determination of Energy Consumption of Dehydration Processes

_{min}= 0.382 ± 0.036 kWh). In previous research [32], the higher temperature of chokeberry convective dehydration (70 °C) indicated a more energy-efficient process (2474.35 ± 15.74 kJ, 0.6873 ± 0.01 kWh).

#### 3.5. Water-Holding Capacity

_{2}O g

^{−1}d.m.) and less shrinkage. The previous results [33] showed a higher WHC obtained by convective dehydration (777.21–924.037 g H

_{2}O g

^{−1}d.m.). Different methods such as fluidized-bed jet milling and drying (FJMD) or freeze drying (FD) will provide the following WHC of chokeberries: 661 ± 19 g H

_{2}O g

^{−1}d.m. and 812 ± 20 g H

_{2}O g

^{−1}d.m., respectively [34].

#### 3.6. Sensory Evaluation

#### 3.7. Statistical Methods

_{2}(r = −0.6645 and r = −0.6637, respectively, statistically significant at p < 0.05). A lower level of negative correlation was found between the D

_{eff}and t (r = −0.5609), E (r = −0.5464), and CO

_{2}(r = −0.5458). Furthermore, a negative correlation between E and CO

_{2}and some sensory characteristics (such as consistency and taste) was also noticed. The highest positive correlations were noticed for E and CO

_{2}, r = 0.9999 statistically significant at p < 0.05). From Figure 5, the highest positive correlations were also observed for WHD and D. appearance (r = 0.9275), D. taste (r = 0.9624), D. aroma (r = 0.9507), D. total score (r = 0.9423), D&R aroma (r = 0.9373), and D&R consistence (r = 0.9696), D&R total score (r = 0.9344), statistically significant at p < 0.05). There is as well a high positive correlation between t and E and CO

_{2}(r = 0.9158 and r = 0.9161, respectively, statistically significant at p < 0.05). In addition, a high positive correlation was noticed between all sensory characteristics, r, ranging from 0.9182 to 0.9988.

_{eff}, t, E, CO

_{2}, and sensory properties (Figure 6). The angles between corresponding variables indicate the degree of their correlations, where small angles correspond to high correlations [38]. A scatter plot was created with the first two principal components (PC1, PC2) from the PCA data matrix. The first two PCs demonstrated 95.03% of the total variance in the laboratory data. This method allows for visualization of the trends in the displayed data and shows the discriminating efficiency of the used descriptors. The contribution of the variables (%) showed that WHC (8.5990%) and each of dehydrated and dehydrated/rehydrated sensory characteristics (appearance, taste, aroma, consistency, and total score with average value 8.3647–8.9748%) most participated in PC1 and D

_{eff}(19.6556%), t (27.1077%), E (22.3598%), and CO

_{2}(22.3643%) in PC2. The parting within samples could be seen from the PCA figure, where there is a clear separation of samples according to the dehydration method. Therefore, the position of the samples in Figure 6 was primarily more influenced by the type of dehydration method (MCD, MD) than the parameters of the dehydration method.

_{eff}, t, E, and CO

_{2}. Therefore, the MCD method (1.50) was characterized by the high values of the following response: sensory properties of dehydrated and dehydrated/rehydrated chokeberries except for the consistency. The MCD methods (2.00 and 2.30) were characterized by the high values of the following responses: WHC and consistency.

_{eff}values were highest for the MCD models with the highest microwave power range. As the energy input increases, the D

_{eff}also increases significantly. Lowering the E

_{a}indicates more effective moisture diffusivity (higher D

_{eff}), and increasing the moisture diffusion with sphere radius (thickness) suggests that lower energy consumption causes the bond between the water molecules of the sample to break. The experimental results discussed in the paper support the notion that the D

_{eff}values are significantly affected by the energy input, with higher values obtained for higher microwave power ranges. Understanding the factors that affect the D

_{eff}is critical for optimizing the moisture removal process in materials, particularly for industrial applications where efficient moisture removal is essential.

## 4. Conclusions

^{2}. Drying time was found to be directly dependent on the chosen dehydration method, with the MW power of 900 W for 9 s and the CD temperature for 12 s being the most effective. The dehydration process has a strong impact on energy consumption and was directly related to the duration of the drying process. The MD was less energetically demanding for the chokeberry dehydration, especially the model 2.30. The highest D

_{eff}values had the MD models with the highest microwave power range. A higher WHC characterized the chokeberries obtained by the MCD method compared to the MD method. At the same microwave energy, with prolonged effect, lower temperature, and shorter convective dehydration time, dried chokeberries will have maximum WHC and less shrinkage. MCD dried berries compared to MD berries were characterized by greater freshness, acidity, and astringency, all similar to fresh chokeberry, and affected the slightly increased crispness of dried chokeberry fruits regarding their texture. The study provides valuable information for developing new and efficient drying methods for chokeberries.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 3.**WHC in chokeberry obtained by two methods: MD and MCD.

^{a–d}Different letters in the superscript in Figure 3 indicate a statistically significant difference between values at a significance level of p < 0.05 the post hoc Tukey HSD test).

**Figure 5.**Color correlation diagram between the parameters of the independent variables and the responses of dehydration method.

**Figure 6.**PCA of independent variables and responses the WHC, D

_{eff}, t, E, CO

_{2}, and sensory properties. The red lines are the original variables in the space of the first and second components.

**Table 1.**Average values and standard deviations of t, D

_{eff}, E, CO

_{2}, and experimental models for MR and DR of MD and MCD of chokeberry.

Model | T (min) | D_{eff}(m ^{2} s^{−1}) | Henderson and Pabis Model | Polynomial Model | E (kWh) | CO_{2}(kg) |
---|---|---|---|---|---|---|

100%, 900 W | 42 ± 4 ^{a} | 4.4891 × 10^{−10} ± 4.4518 × 10^{−12}, ^{f} | y = 1.1181 × e^{−0.0730 × x}R ^{2} = 0.9881MSE = 0.0025 | y = 2.5657 × 10^{−7} × x^{5} − 3. 3384 × 10^{−5} × x^{4} + 1.6315 × 10^{−3} × x^{3} − 3.6273 × 10^{−2} × x^{2} + 0.3231 × x + 0.0850R ^{2} = 0.9706MSE = 0.0165 | 0.624 ± 0.059 ^{b} | 0.623 ± 0.059 ^{b} |

50%, 450 W | 75 ± 6 ^{b} | 1.7492 × 10^{−10} ± 2.0241 × 10^{−12}, ^{e} | y = 1.0134 × e^{−0.0366 × x}R ^{2} = 0.9999MSE = 0.0002 | y = 1.5705 × 10^{−8} × x^{5} − 3.6291 × 10^{−4} × x^{4} + 3.0319 × 10^{−3} × x^{3} − 1.0989 × 10^{−2} × x^{2} + 0,1513 × x + 0.0029R ^{2} = 0.9950MSE = 0.0059 | 0.640 ± 0.062 ^{b} | 0.638 ± 0.062 ^{b} |

30%, 270 W | 192 ± 15 ^{c} | 3.6891 × 10^{−9} ± 3.3214 × 10^{−11}, ^{a} | y = 0.9432 × e^{−0.0147 × x}R ^{2} = 0.9979MSE = 0.0001 | y = 3.4667 × 10^{−10} × x^{5} − 1.5067 × 10^{−7} × x^{4} + 2.3610 × 10^{−5} × x^{3} − 1.5861 × 10^{−3} × x^{2} + 0.0394 × x + 0.0393R ^{2} = 0.848MSE = 0.0069 | 0.866 ± 0.081 ^{c} | 0.865 ± 0.081 ^{c} |

2.30 | 24 ± 2 ^{a} | 6.0768 × 10^{−9} ± 5.9815 × 10^{−11}, ^{b} | y = 1.3174 × e^{−0.1795 × x}R ^{2} = 0.9338MSE = 0.0344 | y = 1.8931 × 10^{−5} × x^{5} − 1.0469 × 10^{−3} × x^{4} + 0,0186 × x^{3} − 0,1157 × x^{2} + 0,2901 × x + 3.2426 × 10^{−10}R ^{2} = 0.9999MSE = 0.0893 | 0.382 ± 0.036 ^{a} | 0.381 ± 0.357 ^{a} |

2.00 | 26 ± 2 ^{a} | 1.5412 × 10^{−9} ± 1.5374 × 10^{−11}, ^{d} | y = 1.2133 × e^{−0.1167 × x}R ^{2} = 0.9560MSE = 0.005 | y = 1.1634 × 10^{−6} × x^{5} + 9 × 10^{−5} × x^{4} − 1.8603 × 10^{−3} × x^{3} + 0.0042 × x^{2} + 0.1998 × x − 0.0030R ^{2} = 0.9970MSE = 0.1205 | 0.396 ± 0.036 ^{a} | 0.395 ± 0.363 ^{a} |

1.50 | 30 ± 3 ^{a} | 1.3565 × 10^{−9} ± 1.2355 × 10^{−11}, ^{c} | y = 1.1058 × e^{−0.0948 × x}R ^{2} = 0.9872MSE = 0.0014 | y = 1.3537 × 10^{−6} × x^{5} − 1.2626 × 10^{−3} × x^{4} + 0.0044 × x^{3} − 0.0709 × x^{2} + 0.4884 × x − 0.0010R ^{2} = 0.9917MSE = 0.6254 | 0.451 ± 0.048 ^{a} | 0.450 ± 0.447 ^{a} |

^{a–f}Different letters in the superscript in Table 1 indicate a statistically significant difference between values at a significance level of p < 0.05 (the post hoc Tukey HSD test).

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**MDPI and ACS Style**

Petković, M.; Filipović, V.; Lončar, B.; Filipović, J.; Miletić, N.; Malešević, Z.; Jevremović, D.
A Comparative Analysis of Thin-Layer Microwave and Microwave/Convective Dehydration of Chokeberry. *Foods* **2023**, *12*, 1651.
https://doi.org/10.3390/foods12081651

**AMA Style**

Petković M, Filipović V, Lončar B, Filipović J, Miletić N, Malešević Z, Jevremović D.
A Comparative Analysis of Thin-Layer Microwave and Microwave/Convective Dehydration of Chokeberry. *Foods*. 2023; 12(8):1651.
https://doi.org/10.3390/foods12081651

**Chicago/Turabian Style**

Petković, Marko, Vladimir Filipović, Biljana Lončar, Jelena Filipović, Nemanja Miletić, Zoranka Malešević, and Darko Jevremović.
2023. "A Comparative Analysis of Thin-Layer Microwave and Microwave/Convective Dehydration of Chokeberry" *Foods* 12, no. 8: 1651.
https://doi.org/10.3390/foods12081651