Reassessment of Thin-Layer Drying Models for Foods: A Critical Short Communication

Modeling the thin-layer drying of foods is based on describing the moisture ratio versus time data by using a suitable mathematical model or models. Several models were proposed for this purpose and almost all studies were related to the application of these models to the data, a comparison and selecting the best-fitted model. A careful inspection of the existing drying data in literature revealed that there are only a limited number of curves and, therefore, the use of some models, especially the complex ones and the ones that require a transformation of the data, should be avoided. These were listed based on evidence with the use of both synthetic and published drying data. Moreover, the use of some models were encouraged, again based on evidence. Eventually, some suggestions were given to the researchers who plan to use mathematical models for their drying studies. These will help to reduce the time of the analyses and will also avoid the arbitrary usage of the models.

Drying characteristics and prediction of best fitted drying model for coriander leaves

Drying experiments were conducted on coriander leaves as affected by drying methods (solar greenhouse drying and open sun drying), pretreatments (dipping in a solution of magnesium chloride + sodium bicarbonate + potassium metabisulphite, boiled water blanching containing sodium metabisulphite, and untreated), and loading densities (2.0, 2.5 and 3.0 kg/m2). Validity of three commonly used drying models were examined to predict the most suitable drying model for coriander leaves. The increased drying temperature under solar greenhouse dryer (42°C) increases the amount of moisture removal from the coriander leaves and reduces the drying time by increasing the drying rate as compared to open sun drying (29°C), at all the selected levels of pretreatments and loading densities. Chemically treated coriander leaves dehydrated under a solar greenhouse dryer required less drying time than other treated leaves and dried leaves. Nevertheless, drying methods and loading densities had significant effects, while treatment effects were marginal. It was found that reduction of moisture and moisture removal rate per unit time occurred mostly in the falling rate period except some accelerated removal of moisture at the beginning up to 150 minutes. Page's model was found most appropriate for drying coriander leaves among the selected models.

DRYING KINETICS OF FISH (Clarias gariepinus) SMOKED WITH BIOGAS

This is aimed at studying the aeration kinetics of catfish (Clarias gariepinus) smoked directly with biogas. Five live fresh fishes (Clarias gariepinus) were obtained from Fishery and Aquaculture Technology Department in FUTA, Ondo State, Nigeria at the age of 4 months with average weight of 900g each. The fishes were killed, de-gutted, thoroughly washed with water, cut into pieces of 3cm length. The chunks were laid in a single layer on a mesh directly exposed to biogas flame obtained from bio-decomposition of poultry waste and the weight was being monitored at 15 minutes interval until constant weight was observed. The study showed that the time taken for drying of Clarias gariepinus to reach the humidity point of around 12.43% (db.) was two and a half hours. The drying data was subjected to 10 thin-layer drying models. The compared the performances of the models using the determination of coefficient (R2), reduced chi­square (x2) and root mean square error (RMSE) between the calculated and predicted moisture ratios. The results showed that Henderson and Pabis modified model (highest R2 and lowest x2 and RMSE of 0.998, 0.00021 and 0.01386 respectively) was found to satisfactorily describe the biogas drying curves of Clarias gariepinus.

Effect of Different Drying Methods on the Drying Kinetics of Fermented Cardaba Banana Peels

Cardaba banana peels (Musa acuminata) were fermented for three days and dried using solar dryer, open sun and tunnel dryer. Nonlinear regression analysis was used to fit in the experimental data. Moisture drying was investigated using Fick’s second law. Statistical tools such as coefficient of determination (R2), reduced chi square (χ2), Mean Bias Error (MBE) and Root Mean Square Error (RMSE) were used to test the reliability of the model. Sample dried in sun had single falling rate pattern whereas samples in solar and tunnel dryer exhibited a second falling rate pattern. The values of R2 ranged from 0.872 - 0.989, χ2(1.4E-34 - 0.0624), MBE (-0.0067 - 0.0491) and RMSE (1.1E-17 - 0.2247). Effective moisture diffusivity for samples dried in solar, tunnel and sun were 2.92 E-11m2/s, 1.98 E-11m2/s and 1.09 E-11m2/s, respectively. The energy of activation in the process was 64.9kJ/mol. Page model best described drying behavior of the samples.Keywords: Fermentation, banana peels, drying, models, diffusivity, activation energy

DRYING CHARACTERISTICS, MINERAL CONTENT, TEXTURE AND SENSORIAL PROPERTIES OF PUMPKIN FRUIT LEATHER

The objective of this study was to investigate the influences of hot air (HAD: 50, 60, 70°C), vacuum (VD: 50, 60, 70°C at 300 mbar) and microwave methods (MWD: 90, 180W) on drying characteristics, effective moisture diffusivity (Deff), mineral content, texture, and sensorial properties of pumpkin pestils. MWD led to the highest drying rate and the lowest drying time in all methods. Page and Modified Page were ideally fitted to experimental results among seven thin layer drying models. Mineral content (Ca, K, Na, P, Mg, Fe, Zn, Cu, Mn) of the pestils showed higher values than non-dried (paste) mixture. Significant differences were determined between textural features of the pestils (p < 0.05). Furthermore, products dried with HAD and VD were preferred rather than MWD in terms of sensorial properties.

Comparative Study of Three Solar Drying Methods of Adensonia digitate L. (Baobab Leaves using Drying Kinetics and Empirical Drying Models)

A comparative study of three drying methods of baobab leaves have been conducted and reported. Mixed mode solar drying, indirect mode solar drying and open sun drying of baobab leaves were conducted based on three drying models viz a viz Lewis,. Page and Henderson and Pabis models were employed in this research. Baobab leaves dried faster when dried under the mixed mode on-farm solar dryer. Drying time reduced considerably using the mixed mode on-farm solar dryer. Drying data were fitted into Lewis, Page and Henderson and Pabis models. Henderson and Pabis model (R2=0.9999, 0.9611, 0.9656; X2= 1.0297, 0.7931, 0.7710; RMSE= 0.5859, 0.6898, 0.6802 and MBE= -0.4.135, -0.4.231, -0.4176) gave the best prediction for the mixed mode drying). In the same way Henderson and Pabis model (R2=0.7450, 0.7699, 0.8243; X2= 1.9025, 0.4026, 0.2006; RMSE= 1.0684, 0.5181, 0.4058 and MBE= -0.8966, -0.3823, 0.2789) gave the best prediction for the indirect mode drying of baobab leaves. Effective moisture diffusivity of baobab leaves varied between -6.382 X 10-04 and -1.108 X 10-03 m2/s.

The effect of temperature on the okra drying process: kinetic study and physical properties of powders

This study aimed to evaluate the effect of drying temperature (50, 60, 70 and 80 °C) on okra dehydration by comparing its powder’s physical properties obtained from a sample produced by a lyophilization process. Ten drying models were adjusted to the experimental data of the drying kinetics. As a result, effective diffusivity and activation energy were determined in addition to thermodynamic parameters: entropy, enthalpy and Gibbs free energy. A physical characterization, as well as the pigments and colorimetry analyses of the aforementioned powders were made, by comparing them with samples produced by lyophilization. The powders were characterized for hygroscopicity, solubility, wettability, apparent and compacted density, fluidity and cohesiveness, pigments, colorimetric, morphological analysis (SEM) and X-ray diffraction. Midilli model was the one that best adjusted to the drying kinetic curves. There was a booster in the effective diffusion coefficient with the increase of temperature. Enthalpy and entropy were reduced with the increase of both drying temperature and Gibbs free energy. The powders presented high luminosity, and the lyophilized powder had higher pigments retention and greater solubility. All powders presented poor fluidity and intermediate cohesiveness, with amorphous, irregular and asymmetric particles. Thus, from the present study it was possible to evaluate the best drying method, the one that should be applied for the drying of okra, considering the costs involved, its quality and the final application of the product, meeting the specific needs of each consumer

Application of Drying Model to Determine Extraction Behaviours on Peanut Skin Oil Recovery by Supercritical Carbon Dioxide-Ethanol

The main objective of this study was to determine the mass transfer for extraction of peanut (Arachis hypogea) skin by using drying models as alternatives extraction models. The mass transfer was measured at the pressure ranging from 10 MPa to 30 MPa, temperature of 40 oC to 70 oC, and rate of modifier 0.075 mL/min to 0.225 mL/min. The Lewis, Page, Peleg, Henderson and Pabis, and Avhad and Macetti as drying models were modified to illustrate the extraction process and to transform as alternative empirical models. An average absolute relative deviation percentage (AARD%) of Lewis, Page, Peleg, Henderson - Pabis, and Avhad-Macetti was 9.52%, 4.67%, 19.41%, 0.26%, and 0.04%. Avhad and Macetti model offered the best fitting between experimental data and modelling data. The results showed that drying model was applicable to correlate the experimental data of extraction process due to low percentage of error and high coefficient determination.

Statistical prediction of the drying behavior of blanched ginger rhizomes

ARS-680 environmental chamber was employed in this study to determine the drying behavior of sliced ginger rhizomes. Blanched and unblanched treated ginger rhizomes were considered at drying temperature of 40 °C for a period of 2 – 24 h. Linear and non-linear regression analyses were employed to establish the correlation that exits between the drying time and the moisture ratio. Correlation analysis, root mean square error (RMSE) and standard error of estimate (SEE) analysis were chosen in selecting the best thin layer drying models. Higher values of determination coefficient (R2) show goodness of fit and lower values of SEE implies better correlation; and RMSE values were also utilized in determining the goodness of fit. The drying data of the variously treated ginger samples were fitted into the twelve thin layer drying models and the data obtained were fitted by multiple non-linear regression technique. Blanched treated sample exhibited a better drying behavior losing about 82.87 % moisture content compared with unbleached sample that lost about 62.03 % of moisture content. Two-term exponential drying model proved to be the most suitable model for predicting the drying behavior of ginger rhizome. The model exhibited high R2 values of 0.9349-0.9792 (which are close to unity) for both blanched and unbleached samples. Also, it recorded relatively low values of RMSE and SEE (3.6865 - 2.0896 and 3.6564-2.7486 respectively) for both treatments.