Taro Leaves Drying Kinetics and Monolayer Moisture Content

INTRODUCTION |1

A study on the drying kinetics and monolayer moisture content of taro leaves

This research was aimed to develop dehydrated products based on Taro leaves and finding out the effect of drying parameters such as loading density and temperature on that control the drying kinetics. To determine the end point of drying studies on the sorption isotherm was conducted. From the moisture sorption isotherm data, the monolayer moisture content was estimated by Braunauer-Emmett-Teller (BET) equation using data up to a water activity (a_w) of 0.52 and monolayer moisture content was found to be 8.92 g water per 100 g solid for taro leaves. By using another most important model, GAB (Guggenheim-Anderson-DeBoer) model, using data up to a_w=0.9, the monolayer moisture content of taro leaves was found to be 19.78g water per 100 gm solid.

INTRODUCTION

It is estimated that by 2020, the population of Bangladesh will be as high as 200 million, which means that there is a need to produce more food from the limited land resources. In this context, there is a need to explore alternate food crops, which could supply food in food insecurity situations. “Taro” can be the alternative to the other vegetables for developing and under developed country. Apart from acting as cheap energy and dietary source, this crop provides other micronutrients, vitamins and dietary fiber as well.

In Bangladesh, taro is used as vegetable throughout the country. Corms and cormels are used as starchy vegetables whereas leaves and leaf stalks are used as ‘shak’. During famine, a large number of people reportedly survive simply on food materials made by boiling the corms, cormels, stolons, leaf stalks and leaves of different varieties of Taro.

The subfamily Colocasioideae of family Araceae consists of three edible tubercrops, namely ‘taro’ (Colocasia esculenta Schott), `tannia’ (Xanthosoma spp.) and giant Taro’ (Alocasia spp.). Among these crops, taro and tannin are cultivated to a larger extent, while giant taro is not as common as commercial crop like the other two. In general, these are crops of third world countries, particularly grown in Africa and Asia. About 88 % of the total world acreage is in Africa which produces about 80% of total production (Onwueme, 1978). Among the three crops, taro is more common in South-East Asia. It is one of the ancient crops with an interesting history blending with the evolution of agricultural systems (Gopalan et al., 1974).

A large number of horticultural varieties of taro are widely cultivated in Bangladesh and still larger varieties grow wild. During the rainy season when other vegetables are in scarcity in Bangladesh this taro goes a long way to meet the demand for vegetables. The leaves, petioles, stolons, corms and cormels, and indeed all the parts of some taro are taken as food in large quantities by the rural population in our country. Hence the use of Taro as vegetables, both leaves and roots, in the diets of the people of our country assumes special and added importance.

The taro has also medicinal value. Processed “Bishknchu” is used in Ayurvedic medicine for the treatment of rheumatism. Juice from petioles and whole leaves are used as antiseptic to check bleeding from minor injury in the rural areas of Bangladesh (Chowdhury, 1975).

The possibility of wider use of the Taro leaves as vegetables crops in our country may be ascribed to their unusual environmental adaptability and ease of cultivation. The lowland types grow in standing water which is rarely possible for other crops. The taro can be produced with minimum capital investment. Growing of this crop does not require any special technological skill. Their keeping quality in most cases is excellent.

The best way of preserving the leasfy vegetables is drying or dehydration. This process costs less, then other preserving methods and require simple instrument. The type and conditions of the blanching treatment prior to drying affect the retention of ascorbic acid, carotene, and ash in the dried vegetables. The sun-dried vegetables had inferior color, texture and acceptability compared to the vegetables dried in the cabinet dryer.

In the mechanical dryer, desired temperature and airflow could be maintained. Compared to sun/solar drying, higher airflow and temperature can be used in mechanical drying. This leads to high production rates and improved quality products due to shorter drying time and reduction of the risk of insect infestation and microbial spoilage as well as minimum nutrient loss. Since mechanical drying is not dependent on sunlight so it can be done as and when necessary.

Based on the above information, the present experiment was broadly aimed to study on development of shelf stable taro (Colocassia esculenta) leaves product. The specific objectives of this study are as follows:

• To determine the composition of fresh and processed Taro leaves
• To develop the isotherm
• To study the drying characteristics of taro leaves during mechanical and vacuum drying
• To study the storage stability of processed taro leaves

Materials and methods

3.2.3 Sorption isotherm studies

The moisture sorption properties of dried Taro leaves were determined at room temperature under conditions of various relative humidity (11-93% RH) in the vacuum desiccators. The various RH conditions were achieved in vacuum desiccators using saturated salt solutions.

The following salt solutions (Table. 3.1) of known water activity were used for the study (Islam, 1980).

Table 3.1: Water activity of saturated salt solution

Petri dishes were used for preparing saturated salt solution. The various salts were put in the Petri dish and water is added to give a saturated condition. The method involved putting a small accurately weighed about 1g sample in a previously weighed Petri dish into desiccators contained saturated salt solutions. The sample and the solution was separated a perforated plate to avoid mixing. The desiccators were evacuated to less than 50 Torr. At various intervals, the vacuum was broken with air, the sample weighed and replaced in the desiccators, which was then re-evacuated. The sample was weighed daily in the initial period and less often, as the sample started to reach equilibrium. Weighing was continued until the sample weights were constant two days in row.

In the mid-1970s, water activity came to the forefront as a major factor in understanding the control of the deterioration of reduced moisture, drugs and biological systems (Labuza, 1975). It was found that the general modes of deterioration, namely physical and physicochemical modifications, microbiological growth, and both aqueous and lipid phase chemical reactions were all influenced by the thermodynamic availability of water (water activity) as well as the total moisture content of the system.

Control of initial moisture content and moisture migration is critical to the quality and safety of foods. Ideally, food manufacturers develop products with defined moisture contents to produce a safe product with optimum shelf- life. Quality and safety factors that the manufacturer must consider are microbial stability, physical properties, sensory properties, and the rate of chemical changes leading to loss of shelf-life. Water activity or the equilibrium relative humidity of a system is defined as:

Where

Vapor pressure of water in equilibrium with the dry system

Saturation vapor pressure of pure water at the same temperature.

Sorption properties of floods (equilibrium moisture content and monolayer moisture) are essential for the design and optimization of many processes such as drying, packaging and storage (Muhtaseb et al., 2002).The moisture sorption isotherms show the equilibrium amount of water sorbed onto a solid as a function of steady state vapor pressure at a constant temperature (Bell and Labuza, 2000).

There are many empirical equations that describe this behavior, but the water sorption properties at various RHs should be experimentally determined for each material. The general shape of the isotherm, specific surface area of the sample, reversibility of moisture uptake, presence and shape of a hysteresis loop provide information on the manner of interaction of the solid with water (Swaminathan and Kildsig, 2001).

Sorption properties are important in predicting the physical state of materials at various conditions, because most structural transformations and phase transitions are significantly affected by water (Roos, 1995).

Langmuir (1917) developed an equation based on the theory that the molecules of gas are adsorbed on the active sites of the solid to form a layer one molecule thick (monolayer).

The Brunauer-Emmett-Teller (BET) sorption model (Brunauer et al.1938) is often used in modeling water sorption particularly to obtain the monolayer value (Eq. 2.10) which gives the amount of water that is sufficient to form a layer of water molecules of the thickness of one molecule on the adsorbing surface (Bell and Labuza 2000, Roos, 1995).

The BET monolayer value has been said to be optimal water content for stability of low-moisture materials (Labuza, 1975 and Roos, 1995). The BET equation was developed based on the fact that sorption occurs in two distinct thermodynamic states; a tightly bound portion and multilayer having the properties of bulk free water (Zografi and Kontny, 1986). The BET equation is:

Where,

= the measured moisture at water activity

= the monolayer moisture content (the optimal moisture content for maximum storage stability of a dry food);

c = the isotherm temperature dependence coefficient (energy constant)

Vanchy (2002) determined the moisture sorption isotherm of Whole milk powder (WMP). The WMPs were stored at 20 and 35°C under 11%, 22% and 33% relative humidity . The monolayer moisture content was 4.8%, (solids not fat basis) at 0. 11 using the BET equation and 5.1 % at 0.23 according to the GAB equation.

Nikolay et al. (2005) determined the moisture equilibrium data (adsorption and desorption) of semi-defatted (fat 10.6 % wet basis) pumpkin seed flour using the static gravimetric method of saturated salt solutions at three temperatures 10°C, 25°C, and 40°C, found that the equilibrium moisture content decreased with the increase in storage temperature at any given water activity. They fitted the experimental data to five mathematical models (modified Oswin, modified Halsey, modified Chung-Host, modified Henderson and GAB).

The GAB model was found to be the most suitable for describing the sorption data. The monolayer moisture content was estimated using the Brunauer-Emmett-Teller (BET) equation. The BET model (Brunauer et al. 1938) gives the best fit to the data at a_w of up to 0.5 (Bell and Labuza 2000, Roos 1995).

Guggenheim-Anderson-de Boer (GAB) sorption model (Anderson 1946, Boer 1953, Guggenheim 1966) introduces a third state of sorbed species intermediate to the tightly bound and free states. The GAB equation has a similar form to BET, but has an extra constant, K (equation 2.11). BET is actually a special case of GAB.

The GAB equation is:

Isotherm equations are useful for predicting the water sorption properties of a material, but no equation gives results accurate throughout the entire range of water activities. According to Timmermann (2003), the GAB monolayer value is always higher than the BET monolayer value. Prediction of water sorption is needed to establish water activity and water content relationship for materials (Roos, 1995)

Where

m = the measured moisture at water activity;

= the monolayer moisture content (the optimal moisture content for maximum storage stability of a dry food),

=the GAB multi-layer constant;

c=the isotherm temperature dependence coefficient (energy constant).

The GAB model can be used to a maximum water activity of 0.9. The following procedure is suggested by Biozt (1983) to fit data on water activities and equilibrium moisture content.

Equation (2.11) can be transformed as follows:

Where

Equation (2.12) indicates that GAB equation is a three-parameter model. The water activity and equilibrium moisture content date are regressed using equation (2.12) and values of three coefficients, and are obtained. From these coefficients, the values of k,, and c can be calculated.

To overcome this weakness of the GAB equation, modifications of the equation have been proposed (Schuchmann et al. 1990; Timmermann and Chirife 1991). Timmermann and Chirife (1991) used one additional parameter in the GAB model and studied the so-called third stage of sorption using experimental data of starch with satisfactory results.

Isotherm equations are useful for predicting the water sorption properties of a material, but no equation gives results accurate throughout the entire range of water activities. According to Timmermann (2003), the GAB monolayer value is always higher than the BET monolayer value. Prediction of water sorption is needed to establish water activity and water content relationship for materials (Roos, 1995).

Results and discussion:

The sorption isotherm is an extremely valuable tool for food scientist because it can be used to predict potential changes in food stability, for selection of packaging, for selection of ingredient and for predicting drying time. A sorption isotherm for dehydrated taro leaves obtained by Vacuum oven drying (VOD) was established to determine how the taro product will behave in a confined environment. To obtain the moisture sorption isotherm, moisture content (dry basis) versus water activity were plotted on linear graph paper (Figure 4.1).

The results shown in Figure 4.1 (tabulated data given in Appendix-II, Table 2.1), indicate that samples absorb little water particularly at lower a_w (<0.52) Water activity above 0.52, the samples begin to absorb water than others. At the higher water activity considerable absorption takes place. These results were also compared with those obtained by other author (Islam, 1980 and Kamruzzaman, 2005) and a good agreement, with some differences, was found between them. Possibly, the little differences are due to the use of different varieties of sample or different criteria followed to determine the equilibrium.

Figure 4.1 Graphical presentation of sorption isotherm of Taro

The water sorption isotherm of taro follows the shape of the sigmoid type isotherm. The resultant curve is caused by the combination of colligative effects (physical properties of solution), capillary effects, and surface-water interactions (Bell and Labuza, 2000). A distinct “knee” usually indicates a formation of a well-defined monolayer.

The monolayer moisture content was estimated using the Brunauer-Emmett-Teller (BET) equation. The BET equation is an extension of the Langmuir relationship that accounts for multilayer coverage.

BET equation was used (eq. 2.10) to calculate monolayer moisture content (m_o) and energy constant (C). m_o represents the optimal moisture for maximum storage stability in the dry state. Results obtained from BET equation are shown in Table 4.2.

Table 4.2 Data for BET and GAB methods

From the slope and intercept of BET equation (Appendix II, Figure 2.1), monolayer moisture content and energy constant of taro leaves calculated for VOD samples. The monolayer moisture content of taro leaves was found to be 8.92g water per 100 g solid (Table 4.2). The calculated monolayer moisture content are greater than those found by Islam (1980) who reported 5.5 for potato slice and 6 for potato powder and by Kamruzzaman (2005) who reported 7.52 for aroids.

Another important model of sorption isotherm behavior stated by GAB (Guggenheim-Anderson-DeBoer) in equation 2.11 and 2.12 to determine the monolayer moisture content of food products. This is very important for safe level of storage of food.

Dry foods are usually considered to be most stable to chemical reactions if their moisture content is at or near the BET monolayer (Labuza et al., 1970). Usually air dried products are dried to moisture content corresponding to a_w 0.6 (Nickerson and Sinskey, 1977).

From this study it is seen that VOD taro leaves give 25% (Figure 4.1) moisture content at 0.6 a_w. From this standpoint, freeze dried products are considered best for sorption studies (Islam, 1980). It may be mentioned here that the current study was concerned with adsorption isotherm so as to avoid risk due to hysteresis effect. At same moisture content adsorption path gives higher water activity than desorption path. Thus product dried to safe a_w level according to adsorption isotherm will be even safer when it follows desorption path.

After fitting data (Appendix II) the following figure was developed and from the developed equation the monolayer moisture content of taro leaves were found for GAB model.

Fig. 4.2 Graphical presentation of GAB model of sorption isotherm

From the developed Figure (4.2) and equation (4.1) the coefficients found, and were -0.121, 0.114 and -0.003 respectively (Table 4.2). Taking k= 0.9 and found the monolayer moisture content 19.78gm water per 100 gm solid. It is shown that the standard GAB equation is adequate to describe experimental data for water activity values up to 0.90 but fails to adequately describe the experimental data when data in the range of aw 0.9-1.0 are included in the calculation.