Drying kinetics of banana (Musa spp.)/Cinetica del secado de bananas (Musa spp.)/Cinetica da secagem de bananas (Musa spp.).
Banana is low in sodium, contains very little fat and no cholesterol. Therefore, it is useful in managing patients with high blood pressure and heart diseases (Robinson, 2006; Robinson and Sauco, 2010). Banana has the potential of neutralizing free hydrochloric acid, suggesting its use in peptic ulcer therapy. A fully ripe banana mixed with milk powder is especially recommended for ulcer patients. It is a rich source of energy. That is why it is consumed throughout the world in one form or the other (Ganesapillai et al., 2011). Research focus and interest on banana in recent time has been due to its nutritional and economic importance. Common foods obtained from banana are presented on Table I.
Drying is the removal of moisture from a food material with a view of reducing microbial activity and product spoilage, and extending storage life. Knowledge of drying kinetics of banana is essential for the control and optimization of the banana drying process, any subsequent or further process and quality of final product (Demirel and Turhan, 2003). Recent and past studies on drying characteristics of banana in different forms includes whole fruit (Nogueira and Park, 1992; Queiroz and Nebra, 2001; Dandamrongrak et al., 2002; Sousa and Marsaioli, 2004; Silva et al., 2013), slices (Sankat and Castaigne, 1992; Sankat et al., 1996; Demirel and Turhan, 2003; Leite et al., 2007; Abano and SamAmoah, 2011; Ganesapillai et al., 2011) and chunks (Mowlah et al., 1983; Garcia et al., 1988). This paper reviews the drying kinetics of banana with regards to pretreatments followed prior to drying, mathematical models used for the description drying behavior and the moisture transport mechanism during drying.
Effect of Pretreatments on Drying Kinetics of Banana
Common pretreatments applied to fruits prior to drying operation include blanching, lemon juice, ascorbic acid, sulfuring, honey dip, salt solution and osmotic pretreatment, ethyloleate, NaOH, olive oil, skin puncturing, and [K.sub.2]C[O.sub.3] (Pangavhane et al., 1999; Lewicki, 2006; Vasquez-Parra et al., 2013). Table II contains a list of pretreatments as used by various researchers prior to banana fruit drying operation.
Pretreating fruits prior to drying is essential due to its benefits. Pretreating helps keep light-colored fruits from darkening during drying and storage; it speeds the drying of fruits and minimizes the effects of drying operation on some quality parameters of fruits (Susan and Williams, 1993; Abano and Sam-Amoah, 2011; Licciardello et al., 2012). Studies have shown that pretreating with an acidic solution or sodium metabisulfite dip also enhances the destruction of potentially harmful bacteria during drying, including Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes (Di Persio, 2004). The paragraph below summarizes studies on the effect of drying pretreatments on the drying characteristics of banana as investigated by various researchers.
Effects of different pretreatments on drying characteristics of banana slices as conducted by Abano and Sam-Amoah (2011) revealed a minimum rehydration ratio of 1.215 for ascorbic acid treated slices and a maximum rehydration ratio of 1.716 for lemon juice treated samples. They concluded that lemon juice treated dried banana will reconstitute more moisture when exposed to air. Effect of ultrasound pretreatment on banana (cv Pacovan) drying kinetics under a fixed bed dryer at two different temperatures (50 and 70[degrees]C) and 3.0m x [s.sup.-1] air velocity investigated by Azoubel et al. (2010) revealed that moisture diffusivities increased with increasing temperature and with the application of ultrasound, while process time was reduced, which is an indication of an economy of energy, since air drying is cost intensive. The effects of four pretreatments (blanching, chilling, freezing, and combined blanching and freezing) on the drying rate and quality of bananas were investigated by Dandamrongrak et al. (2002). They reported that the initial drying rate was highest for the blanched treatment and the two pretreatments involving freezing resulted in the shortest drying times. The blanched sample was preferred in terms of color while the frozen samples exhibited extensive browning. The texture and flavor were significantly (P<0.05) reduced in all samples that involved blanching and/or freezing. Demirel and Turhan (2003) studied the air-drying behavior of untreated, and sodium bisulphite and ascorbic/citric acid treated Dwarf Cavendish and Gros Michel banana slices between 40 and 70[degrees]C. They reported that pretreatments and increasing temperature decreased browning while pretreatments and temperature did not affect the shrinkage.
Factors Influencing Drying Kinetics
Drying kinetics is the description of the changes of moisture content of the material during drying. It can be expressed as a drying curve or drying rate curve. A drying curve is usually obtained experimentally by plotting the free moisture content vs drying time. This plot can be converted into a drying rate curve by calculating the derivative of the curve over time. Generally, factors influencing drying rate of agricultural products include drying air temperature, relative humidity, drying air flow velocity and composition of a food product. At a constant temperature, the moisture content of food changes until it comes into equilibrium with water vapor in the surrounding air. This is termed equilibrium moisture content. Figure 1 represents the sorption isotherm for a typical food product. Apart from the general factors, other factors such as shape, size, surface area of food product and the drying method used (Table III) could also influence the drying rate of agricultural products. The effects of the mentioned factors on the drying characteristics of banana as observed by various researchers are captured in the following paragraph.
Pereira et al. (2007) in their study on effect of microwave power, air velocity and temperature on the final drying of osmotically dehydrated bananas observed that increasing the microwave power increased the drying rate, thus making the drying time shorter. However, higher microwave power also caused temperature runaway leading to charring on the dried product; air flow cools the product surface and improves product quality by reducing charring. Ganesapillai et al. (2011) studied the drying kinetics of banana (Nendran spp.) under microwave, convective and combined microwave-convective process; they reported that microwave drying resulted in a substantial decrease in the drying time with quality product when dried at higher power (300W) level compared to other processes. Drying of bananas assisted by microwave energy at air temperatures from 25 to 55[degrees]C revealed that drying time ranges from 200-290min (Sousa and Marsaioli, 2004). Silva et al. (2013) reported a drying time of 1200 and 3265min at air temperatures of 70 and 40[degrees]C, respectively, for whole bananas. The effect of stepwise changes in drying air temperature on drying kinetics and product color in batch drying of banana pieces in a two stage heat pump dryer, as investigated by Chua et al. (2001), showed that by employing stepwise-varying drying air temperature with appropriate starting temperature and cycle time, it was possible to reduce significantly the drying time to reach the desired moisture content with improved product color. The effects of various operating parameters, i.e., drying medium temperature and pressure, on the drying kinetics and heat transfer behavior of banana as well as the energy consumption was investigated by Nimmol et al. (2007), who found that low-pressure superheated steam drying and far-infrared radiation (LPSSD-FIR) and combined far-infrared radiation and vacuum drying (VACUUM-FIR) took a shorter drying time compared to LPSSD at all drying conditions. Specific energy consumption of the vacuum pump was much higher than that of the far-infrared radiator or electric heater while specific energy consumption of LPSSD-FIR and VACUUM-FIR were lower than that of low-pressure superheated steam drying LPSSD at all drying conditions.
Drying Models for the Description of Drying Kinetics of Banana
Drying of agricultural materials usually occurs under two drying regimes, namely falling and constant rate period. Studies conducted on drying kinetics of banana by various researchers (Drouzas and Schubert, 1996; Dandamrongrak et al. , 2002; Demirel and Turhan, 2003; Ganesapillai et al., 2011) revealed that drying of banana usually takes place under the falling period and that diffusion mechanisms (movement of moisture from a region of higher concentration to a region of lower concentration) is the dominant physical mechanism prevailing during moisture removal process in bananas. However, drying of banana could also occur under the two above mentioned drying regimes in a single drying operation. For instance, Mowlah et al. (1983) reported a constant rate period followed by a falling rate period during drying of banana dices at 60[degrees]C and relative humidity of 9%. Drying kinetics and quality attributes of low-fat banana slices dried at high temperature, as investigated by Prachayawarakorn et al. (2008), revealed that the drying rate evolution occurred in three drying regimes, i.e., warming-up and two falling rate periods.
The thin layer drying characteristics of most agricultural products, which includes banana, could be described using drying models. These drying models are of two main groups, including the empirical models (Turhan et al., 2002; Diamante et al., 2010; Kaleta and Gornicki, 2010; Mundada et al., 2011; Silva et al., 2013, 2014a) and diffusion models (Karim and Hawlader, 2005; Nguyen and Price, 2007; Silva et al., 2012a, b, 2013, 1914a; Darvishi et al., 2013). According to Aguerre and Suarez (2004) diffusion models, unlike the empirical models, include the diffusion coefficient, which reflects a possible physical phenomenon that may occur during drying. Diffusion has often been described as one of the transport mechanisms for moisture transfer during drying, but the underlying basis of using the diffusion model for fitting the drying of fruits has not been fully justified, although descriptions and comparisons of various diffusion concepts and their applications are available (Chen, 2006). Common drying models available in the literature are presented on Table IV. Apart from the use of these models to describe the thin layer drying of agricultural products, they have also been suggested for the design of effective drying equipment and optimization of the drying process (Darvishi et al., 2014). Furthermore, they can be used to describe the heat penetration during drying when hot air is used. In this case heating relies on the diffusion equation, which involves the drying rate in the energy balance (Karim and Hawlader, 2005; Mariani et al., 2008; Silva et al., 2014a).
Drying models that have been used by various researchers for the description of the thin-layer drying of banana are presented on Table V. Silva et al. (2014a) used several empirical models to simulate the thin layer drying of whole bananas at temperatures of 40, 50, 60 and 70[degrees]C. Their results showed that Page and Silva et al. models were the best models to describe the drying kinetics of whole bananas. Baini and Langrish (2007) in their study on choosing an appropriate drying model for intermittent and continuous drying of bananas observed that the diffusion model, which includes the variation of moisture content and temperature throughout the banana in its solution, describes the drying kinetics of banana well for both continuous and intermittent drying. They concluded that the diffusion model is suitable for predicting the relaxation processes that occur when the drying conditions are interrupted, such as in intermittent drying. Ganesapillai et al. (2011) studied the drying kinetics of banana (Nendran spp.) under microwave, convective and combined microwave-convective process, and observed that MD (moisture diffusivity) resulted in a substantial decrease in the drying time with quality product when dried at higher power (300W) level compared to other processes. Furthermore, they reported that drying of the banana variety under the selected drying processes took place in the falling rate period, an indication that moisture removal from the banana was accomplished through diffusion mechanism. They concluded that the Midilli et al. model describes the drying kinetics of the banana variety compared to other drying models. Sousa and Marsaioli (2004) also observed that the drying of whole ripe banana of the Nanicao variety under microwave energy took place in the falling rate period, with diffusion mechanism being responsible for the moisture removal process. They, however, concluded that the drying process fitted well to a simplified diffusion model.
Relevant statistical parameters are normally used to select the best drying equation/model expressing drying curves of agricultural products, including bananas, and also to determine the consistency of the fits. The coefficient of determination [R.sup.2] is usually used to select the best equation expressing the drying curves of the samples. In addition to the [R.sup.2], parameters such as the reduced chi square value ([chi square]), root mean square error (RMSE), mean bias error (MBE), and t-value are also usually employed to determine the consistency of the fit. The basis for the determination of the best fit or model is usually the highest values of [R.sup.2] and the lowest values of [chi square], RMSE, MBE and t-values (Ganesapillai et al., 2011; Silva et al., 2014a). The statistical parameters could be calculated using Eqs. 1-4.
[chi square] = [N.summation over (i=1)] [([MR.sub.exp,i] - [MR.sub.pre,i]).sup.2] (1)
RMSE = [(1/N [N.summation over (i=1)] [([MR.sub.pre,i] - [MR.sub.exp,i]).sup.2]).sup.1.sub.2] (2)
MBE = 1/N [N.summation over (i=1)] ([MR.sub.pre,i] - [MR.sub.exp,i]) (3)
t - value = [square root of [(n -1) [MBE.sup.2]/RMSE-MBE]] (4)
where [MR.sub.exp]: experimental moisture ratio, [MR.sub.pre,i]: predicted moisture ratio, n: number of constants, and N: number of observations.
Moisture Diffusivity in Banana during Drying Process
Diffusion in solids during drying is a complex process that may involve molecular diffusion, capillary flow, Knudsen flow, hydrodynamic flow, or surface diffusion (Marinos-Kouris and Maroulis, 2006). Effective moisture diffusivity is a term used to describe the migration or diffusion of moisture in agricultural products during drying operation and it is said to be a function of material moisture content and temperature, as well as of the material structure (Abano and Sam-Amoah, 2011). According to Zogzas et al. (1996) diffusivity values fall between [10.sup.-13] and [10.sup.-6][m.sup.2] [s.sup.-1] while most values (92%) fall within [10.sup.-12] and [10.sup.-8] [m.sup.2]x[s.sup.-1]. Drying kinetics and quality attributes of low-fat banana slices dried at high temperature as investigated by Prachayawarakorn et al. (2008) revealed that the effective diffusion coefficient of banana increased with a decrease in moisture content until a certain moisture content, after which the diffusivity decreased. The influence of drying temperatures on the moisture diffusivity and quality attributes of the dried banana slices in terms of volatile compound, shrinkage, color, texture and microstructure was investigated by Thuwapanichayanan et al. (2011), who showed that the drying rate of banana slices occurred in two sub-drying periods, first and second falling rate periods. The effective diffusivity estimated by the optimization technique was found to decrease sharply with moisture content in the first falling rate period and changed slightly in the second falling rate period. Air drying of Dwarf Cavendish and Gross Michel banana slices by Demirel and Turhan (2003) revealed that effective moisture diffusivity increased with increasing temperature between 40 and 70[degrees]C in the untreated samples, while it increased between 40 and 60[degrees]C, and decreased at 70[degrees]C in the pretreated samples probably due to case hardening and starch gelatinization above 60[degrees]C. Modeling microwave drying kinetics and moisture diffusivity of banana (Mabonde variety) showed that effective moisture diffusivity of Mabonde banana variety increased with increasing microwave power (Omolola et al., 2014).
The theoretical determination of effective moisture diffusivity of banana using the solution to Fick's second law has been widely used by various researchers (Doymaz, 2009; Thuwapanichayanan et al., 2011; Silva et al., 2014a; Omolola et al., 2014). Eqs. 5 to 7 summarize the solution of Fick's second law of diffusion, where MR: moisture ratio, [D.sub.eff]: effective moisture diffusivity ([m.sup.2] x [s.sup.-1]) and L: half-thickness (m) of banana slices
MR = 8/[[pi].sup.2] [[infinity].summation over (n=0)] 1/[(2n + 1).sup.2] exp(- [[pi].sup.2][(2n + 1).sup.2]/4[L.sup.2] [D.sub.eff]t)
Eq. 5 is based on the assumption that the moisture diffusivity is constant, the banana slices represent infinite slab geometry and the initial moisture distribution is uniform (Demirel and Turhan, 2003; Abano and Sam-Amoah, 2011; Thuwapanichayanan et al., 2011). It could further be simplified to a straight line equation:
ln (MR) = ln (8/[[pi].sup.2]) - ([[pi].sup.2][D.sub.eff]/[L.sup.2] t) (6)
The plot of experimental drying data in terms of ln (MR) against time gives a straight line with a negative slope ([phi]) of
[phi] = [[pi].sup.2][D.sub.eff]/[L.sup.2] (7)
A two-dimensional numerical solution in generalized coordinates of the diffusion equation with boundary condition of the third kind, obtained through the finite volume method for the description of moisture diffusivity in whole bananas has been extensively discussed by Silva et al. (2014b).
However, the determination of effective moisture diffusivity of agricultural products, including banana, during drying could also be determined experimentally. Experimental methods for effective moisture diffusivity determination include concentration-distance curves, permeation methods, sorption kinetics, electrospin resonance, nuclear magnetic resonance, radiotracer methods and drying techniques that include regular regimes and simplified methods (Crank and Park, 1968; Assink, 1977; Schoeber and Thijssen, 1977; Luyben, 1980; Naesens et al., 1981; Windle, 1985; Hendrickx et al., 1986; Coumans and Luyben, 1987; Gros et al., 1987; Moyne et al., 1987; Eccles et al., 1988; Marinos-Kouris and Maroulis, 2006).
Understanding drying kinetics of banana with regards to mathematical modeling of the drying behavior and moisture migration or transport mechanism is important for the description of drying behavior, design of effective drying equipment, optimization of drying process and description of heat penetration mechanism during drying process. Several drying models are available in the literature for the description of thin layer drying kinetics of agricultural products out of which models such as page, two-term, diffusion, Silva et al. and Midilli et al. adequately describe the drying kinetics of some banana varieties. Furthermore, relevant and adequate pretreatment is essential and required prior to drying of bananas as this result in a good quality dried banana product in terms of color, microbiological and shelf stability and reduction in drying time, thereby reducing energy consumption during drying.
Received: 06/15/2014. Modified: 04/28/2015. Accepted: 05/04/2015.
Adewale O. Omolola. B.Tech., M.Sc. in and PhD Candidate in Food Science and Technology, University of Venda (UNIVEN), South Africa. Address: Department of Food Science and Technology, UNIVEN, Thohoyandou, P.M.B x5050, 0950, South Africa. e-mail: firstname.lastname@example.org,.
Afam I. O. Jideani. B.Sc., Obafemi Awolowo University, Nigeria. M.Sc., University of Ibadan, Nigeria. Ph.D., University of Leeds, RU. Professor, UNIVEN, South Africa. e-mail: email@example.com.
Patrick F. Kapila. B.Sc. Agric., M.Sc. and Ph.D. in Agricultural Engineering, UNIVEN, South Africa. Senior Lecturer, UNIVEN, South Africa. e-mail: Patrick.firstname.lastname@example.org
The authors acknowledge the financial support to AOO from the Research fund project number SARDF/14/FST/01 and also the Work Study Programme of the University of Venda, Thohoyandou, South Africa.
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TABLE I COMMON PRODUCTS OBTAINED FROM BANANA Banana Products References Yoghurt Aurore et al. (2009) Jam Espiard (2002) Muffins and DAFF (2011); Juarez-Garcia et al. (2006) confectionaries Beer and wine Carreno and Aristizabal (2003) Chips Carreno and Aristizabal (2003), Lemaire et al. (1997), Anyasi et al. (2013) Infant foods Ganesapillai et al. (2011) Fruit bar Mohapatra et al 2011, Anyasi et al., 2013 Noodles and pasta Anyasi et al. (2013) TABLE II COMMON PRETREATMENTS FOR BANANA Pretreatment References Ascorbic acid Demirel and Turhan (2003); Abano and Sam-Amoah (2011); Thuwapanichayanan et al. (2011) Lemon juice Abano and Sam-Amoah (2011) Salt solution Abano and Sam-Amoah (2011) Honey dip Abano and Sam-Amoah (2011) Citric acid Demirel and Turhan (2003); Sousa and Marsaioli (2004) Sodium metabisulphite Demirel and Turhan (2003) Ultrasound Azoubel et al. (2010) Blanching Dandamrongrak et al. (2002) Chilling Dandamrongrak et al. (2002) Freezing Dandamrongrak et al. (2002) TABLE III DRYING METHODS FOR BANANA FRUIT Drying Method Reference Spouted bed Bezerra et al. (2013) Kiln Baini and Langrish (2007; 2008) Fixed bed Azoubel et al. (2010) Freeze Pan et al. (2008) Cabinet tray Silva et al. (2013); Demirel and Turhan (2003) Heat pump Dandamrongrak et al. (2002) Microwave Sousa and Marsaiolli (2004); Ganesapillai et al. (2011) Oven Leite et al. (2007); Abano and Sam-Amoah (2011); Ganesapillai et al. (2011) Combined Ganesapillai et al. (2011) microwave-convective TABLE IV DRYING MODELS FOR THE DESCRIPTION OF DRYING CHARACTERISTICS OF AGRICULTURAL MATERIALS Model Equation Page MR = exp (-[kt.sup.n]) Henderson MR = a exp (-kt) Modified MR = a exp (-kt) + b exp (-gt) + c exp (-ft) Henderson Logarithmic MR = a exp (-kt) + c Wang and Singh MR = 1 + at + [bt.sup.2] Diffusion MR = a exp (-kt) + (1-a) exp (-kbt) Verma MR = a exp (-kt) + (1-a) exp (-gt) Two term MR = 1 exp (-kt) + u exp (-gt) Two term MR = a exp (-kt) + (1-a) exp (-kat) exponential Midilli et al. MR = c exp (-kt(tn)) + bt Silva et al. MR = [e.sup.-at-b[square root of t]] Model References Page Lahsasni et al. (2004) Henderson Doymaz (2009) Modified Ganesapillai et al. (2011) Henderson Logarithmic Togrul and Pehlivan (2002) Wang and Singh Miranda et al. (2009) Diffusion Ganesapillai et al. (2011) Verma Ganesapillai et al. (2011) Two term Lahsasni et al. (2004) Two term Doymaz (2009) exponential Midilli et al. Midilli et al. (2002) Silva et al. Silva et al. (2013) MR: moisture ratio; a, b, c, k, g, h, l, u and n: model constants; t: time. TABLE V DRYING MODELS FOR THE DESCRIPTION OF DRYING KINETICS OF BANANA Variety Model References Cavendish, Page Dandamrongrak et al.(2002); Cavendish Silva et al (2013) cv. nanica Cavendish Two- term Dandamrongrak et al. (2002) Dwarf Cavendish, Diffusion Baini and Langrish (2007); Gross Michel, Demirel and Turhan (2003); nanicao Sousa and Marsaioli (2004) Cavendish Silva et al. Silva et al. (2013) cv nanica Nendran spp. Midilli et al. Ganesapillai et al. (2011)