D

Journal of Agricultural Science and Technology A 4 (2014) 60-68 Earlier title: Journal of Agricultural Science and Technology, ISSN 1939-1250

DAVID

PUBLISHING

Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.) Turhan Koyuncu1 and Fuat Lule2 1. Faculty of Technology, University of Adiyaman, Adiyaman, Turkey 2. Department of Vocational High School Kahta, University of Adiyaman, Adiyaman, Turkey Received: September 11, 2013 / Published: January 20, 2014. Abstract: In this research, convective and microwave drying characteristics, energy requirement and color changes of dill leaves (Anethum graveolens L.) were reported. Dill leaves were dehydrated in a computer connected parallel air flow type dryer and in a microwave oven dryer. Samples of freshly harvested dill leaves were dehydrated under three air temperatures of 50, 60 and 70 °C, and at three microwave power levels of PL-1 (90 W), PL-2 (160 W) and PL-3 (350 W). Selected drying air velocity was 0.30 m/s for all temperatures. Dill leaves were dehydrated from the initial moisture content of 735 (percentage dry basis) to a final moisture content of 8%-10%. During convective drying experiment, products were weighted automatically by the balance per 5-10 min. Data were transferred to the computer and processed by software. During microwave drying, the products were weighted, and data were recorded manually per 15-60 min. The influence of drying method, drying air temperature and microwave power level has also been studied. Hunter L, a, b values system was also used to evaluate changes in total color difference (ΔE) on dried products. The results showed that convective drying air temperature and microwave oven power levels influenced the total drying time, total energy requirement, specific energy requirement and color difference for dill leaves. The minimum specific energy requirement was determined as 10.72 kWh/kg and 18.72 kWh/kg for 70 °C and PL-3, respectively. 70 °C drying air temperature and PL-3 were found to yield better quality product in terms of color retention of Hunter L, a, b and ΔE. As a result, to reduce drying energy consumption and to keep better color retention, convective drying can be recommended for this application. Key words: Dill, drying, convective, microwave.

Nomenclature A c

Drying air flow surface area, m2 Specific heat of air under adiabatic conditions,

kJ/kg·K

Dt E kg (c )

Total drying time, h

Energy requirement for drying 1 kg of product and for convective drying, kWh/kg

E kg (m )

Energy requirement for drying 1 kg of product and for microwave drying, kWh/kg Et (c ) Total energy requirement for a charge of the convective dryer, kWh Et ( m ) Total energy requirement for a charge of the microwave dryer, kWh I Electric current, A

PM db

The moisture content on dry basis expressed as

percentage, % Corresponding author: Turhan Koyuncu, professor, research fields: energy, solar energy applications and energy in agriculture. E-mail: [email protected].

U v Wd Wo ∆T ρ

Electric voltage, V Drying air speed, m/s Weight of dry matter in product, kg Initial weight of undried product, kg Temperature differences, K Air density, kg/m3

1. Introduction Drying or dehydration is the oldest method in food conservation, and its object is to remove most of the water present in the product by evaporation. The reduction of moisture content inhibits or decreases microbial and enzymatic activity, which otherwise would produce food damage. Besides, dehydration makes food product handling easier, owing to the volumetric shrinkage and weight losses products undergo during process [1]. Natural open-air sun

Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.)

drying is practiced widely in hot climates and tropical countries. Considerable savings can be obtained with this type of drying, since the source of energy is free and renewable. However, this technique is extremely weather dependent and has the problems of contamination, infestation, microbial attack, etc.. Also, the required drying time for a given load is approximately 2-4 times longer than greenhouse, cabinet and parallel air flow type dryers [2-4]. In addition, the drying methods and dryer types strongly affect the color retention of the product. In recent years, much attention has been paid to the quality of foods during drying. Both the methods of drying and physicochemical changes that occur in tissues during drying affect the quality of the dehydrated products. More specially, the method used for drying affects properties such as color, texture, density, porosity and sorption characteristics of materials [5]. Color plays an important role in appearance, processing and acceptability of food materials. Color is perceived as part of the total appearance, which is the visual recognition and assessment of the surface and subsurface properties of the object [6, 7]. The first judgement of quality made by a consumer on a food at point of sale is on its appearance. Appearance analyses of foods (color, taste, odor and texture) are used for the maintenance of quality throughout and at the end of processing. Color is perhaps the most important appearance attribute, because abnormal colors, especially those associated with deterioration in eating quality or with spoilage, cause the product to be rejected by the consumers [8, 9]. Color deterioration has been studied by several researchers for a number of products. Studying the effects of five methods of drying: conventional, vacuum, microwave, freeze and osmotic drying on color of apple, banana, potato and carrot, Krokida et al. [5] measured the color characteristics by Hunter Lab chromameter and reported that the changes in redness (a) and yellowness (b) followed a first order kinetic model; Lopez et al. [9] studied the influence of drying

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conditions on hazelnut browning, and evaluated the color changes by CIELab system. The rate of pigment formation was determined from the total color values with zero-order kinetic model; Rocha et al. [10] studied the effect of pretreatments on drying rate and color retention of basil; Negi and Roy [11] studied about the effects of different blanching and drying treatments to establish the retention of β-carotene, ascorbic acid and chlorophyll in leaves of savoy beet (Beta vulgaris var bengalensis), amaranth (Amaranthus tricolor) and fenugreek (Trigonella foenum graecum). Dill (Anethum graveolens L.), a biennial or annual herb of the parsley family (Apiaceae or Umbelliferae), is native to Southwest Asia or Southeast Europe and cultivated since ancient times. The leafy tops can be clipped and used in cottage cheese, potato salad, cream cheese, tomato soup and salads [12]. Dill grows up to 90-120 cm tall and has slender branched stems, finely divided leaves, small umbels (2-9 cm diameter) of yellow flowers, and long spindle-shaped roots. In general, dill leaves (dill weeds) and seeds (small fragrant fruits) are used as seasoning. The leaves could be used in eggs, meat, salads, seafoods and soups; the seeds could be used in bread, flavouring pickles and soups. Dill essential oil, extracted from both leaves and seeds, could also be used in chewing gums, candies and pickles. The plant is native in Southwest Asia and is cultivated in Europe, India and the United States. It is also successfully cultivated in Taiwan. Literature demonstrates that dill leaf consumption could lower the risk of cancer and reduce the level of cholesterolaemia. Moreover, dill leaf, seed and its essential oil could provide good antioxidant activities [13]. In recent years, the demand for seasoning vegetables has grown, reflecting the increase in their consumption. The variety of dill classed as a seasoning vegetable is being applied more and more widely in modelling the flavour of numerous food products. It can be used as an ingredient in dried

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Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.)

seasoning mixtures; in the production of cheeses, fish and vegetarian dishes; as an admixture in “ready-to-eat” and more recently “do-it-for-me” dishes; and also as a basic constituent of soups and sauces which are very popular in Central and Eastern Europe. The use of dill for various culinary purposes depends to a great extent on its vegetative development as measured by the plant height. The value of dill for processing also depends on the stage of its growth. Young delicate plants can be used for drying, the older ones for freezing, while those at a more advanced stage of growth can be used for the preparation of stock or extracts. The intensity of color and its shade, and hence the attractiveness of the raw material, depend on the level of chlorophyll pigments and their proportions. In green plants the chlorophyll pigments are accompanied by carotenoids, which affect the color of raw material and products obtained from it, and also enhance their vitamin content. The last statement chiefly concerns β-carotene. Like vitamin C and polyphenol compounds, carotenoids are also classified among the basic constituents of the antioxidative effect. Green vegetables including dill, are a rich source of these substances [14]. In order to store dill leaves, it is possible to use different methods such as traditional method, cold storage and drying depending on the technical opportunities, food consumption and food processing ways [15]. In different literatures, it is possible to see some information about mathematical modelling and experimental studies of thin layer drying process of various vegetables, such as garlic, red pepper, purslane, eggplant, broccoli and onion. However, there are not enough reports concerning the convective and microwave drying characteristics, heat energy requirement and color retention of dill leaves during our literature survey [12]. Therefore, fresh and cleaned dill leaves were dehydrated in a microwave oven dryer at different power levels and in a computer connected parallel air flow type dryer at various temperatures and selected most suitable velocity to

determine the drying kinetics, energy requirement and color retention for drying in this experimental investigation. Besides, the other aim of this study was also to investigate the effect of various drying methods and drying temperatures on the color of dehydrated dill leaves.

2. Materials and Methods Dill leaves grown in Black Sea Region of Turkey were freshly harvested manually and used for the investigation. Dill leaves were cleaned in an air screen to remove all foreign material such as dust, dirt, pieces of branches and foreign leaves. Dill leaves were dried in a computer connected parallel flow type dryer and in a microwave oven dryer. The convective dryer equipped with an electric heater (air heating duct), temperature adjuster, centrifugal fan (blower), air speed adjuster (regulator of variable transformer), corrosion resistant chromium mesh, corrosion resistant chromium sheet, glass wood insulator, a 0.01 g sensitive Precisa BJ 600 D digital balance, RS232 connection, a PC, specially designed Balint data processing software, drying air inlet and outlet channels as well as thermostat, temperature indicators, wattmeter and free wheels (Fig. 1). The microwave oven dryer mainly consisted of magnetron tube (source of radiation), oven cavity, filter, step-up transformer, power plug, wave guide, mode stirrer and oven tray (Fig. 2). The products were placed on the chromium mesh as a thin layer. In order to produce different temperatures and fix up the velocity, the electric current of the heater and the rotation of the fan were adjusted manually. The system was also controlled by the thermostat automatically. To measure the power consumption, air speed, relative humidity and drying air temperatures at different points, several digital devices such as watt meter, hot-wire anemometer having in the measurement sensitive of 0.1 m/s, Testo AG 309 type relative humidity and temperature sensors and thermocouple were connected to the drying system. In addition, it must

Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.)

63

Fig. 1 Schematic presentation of the computer connected parallel flow type dryer.

Fig. 2 Schematic presentation of the microwave oven dryer.

be noted that the experimental drying studies we conducted showed us that the maximum length of the drying chamber was approximately 1 m depending on the drying air temperature distribution during the length of the dryer. Thus, the drying chamber was selected less than 1 m long. During these studies, it was also seen that when the length of the drying chamber was more than 1 m, there were important temperature and relative humidity differences between the beginning and the end of the drying chamber [16, 17]. The moisture content (percentage dry basis) of fresh products at harvest was approximately 735% (Eq. (1)) [18]. The moisture content of the products was determined by using an air oven set at 105 °C, and kept until reaching constant weight [1, 19]. For safe and long-term storage, the moisture content should preferably be less than 10%. For that reason, the fresh products with moisture content of 735% were dehydrated until the moisture content became

8%-10% in the dryer. During drying time, the mass of dill leave samples were weighted automatically by the balance per 5-10 min and all tests were replicated three times. The dryer was installed in conditions that were a relative humidity of 60% (± 3%) and a temperature of 20 °C (± 1 °C). This air was heated by the heater and directed to the drying chamber. Three different temperatures such as 50, 60 and 70 °C and a selected air velocity of 0.30 m/s were used for experimentation. This was coming from the fact which was understood from the preliminary studies that the temperature less than 50 °C and the air speed more than 0.30 m/s extremely increased the drying time and energy requirement for these products. In addition, three power levels (PL-1, PL-2 and PL-3) of microwave oven dryer were also used for products drying. The products were placed on the tray of the oven dryer that technical features given in Table 1 for drying. During experiments, drying characteristics, total

64

Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.)

Table 1 Technical features of microwave oven dryer. Data/power level (P-L) (written on the oven by the manufacturer) Voltage (V) Current of fan and lamp (A) Current of magnetron tube and tray motor (A) Current of all components (A) Power consumption of fan and lamp (W) Power consumption of magnetron tube and tray motor (W) Total power consumption (for all components) (W) Active period time of fan and lamp, only (s) Active period time of all components (s) Total time of a period (s) Real power consumption for different levels (measured) (W)

drying time, total energy needed for drying of one charge of the dryer, total energy requirement for drying 1 kg of wet product (specific energy requirement) and color retention for different convective drying temperatures and for microwave drying power levels were found (Eqs. (2)-(5)) [20]. When drying was completed, the average moisture content of each sample was analyzed according to the vacum oven method [19] (Eq. (1)), and the Hunter L, a, b values of dehydrated dill leaves were determined to study the color of the samples. The color was evaluated by measuring Hunter L (brightness, 100 = white, 0 = black), a (+, red; -, green) and b (+, yellow; -, blue) parameters by means of a reflectance colorimeter (CR 400, Chromometer, Minolta, Japan). A white tile (No 19633162) was used to standardize the instrument. From the instrumental Hunter L, a, b values, the color difference (ΔE) were calculated according to Eq. (6).

 W − Wd  PM db =  o  × 100 W d   Et (c ) = A.v.ρ .c.∆T .Dt U .I .Dt 1000 Et ( c ) E kg ( c ) = Wo Et ( m ) E kg ( m ) = Wo Et ( m ) =

[

ΔE = ( ∆L) 2 + ( ∆a ) 2 + ( ∆b) 2

(1) (2) (3) (4)

(5)

]

1/ 2

(6)

(PL-1) 90 W 232 0.23 6.92 7.15 53.36 1,605.44 1,658.80 17 4 21 359.21

(PL-2) 160 W 232 0.23 6.92 7.15 53.36 1,605.44 1,658.80 15 6 21 512.07

(PL-3) 350 W 232 0.23 6.92 7.15 53.36 1,605.44 1,658.80 12 9 21 741.35

3. Results and Discussion During a drying process, two periods can be distinguished. The first is called constant drying rate period. The second drying stage is called the falling drying rate period. During the first period, the surface of the product behaves as the surface of the water. The rate of moisture removal during this period is mainly dependent on the surrounding conditions and only affected slightly by the nature of the product. The end of the constant drying rate period is marked by a decrease in the rate of moisture migration from within the product below that sufficient to replenish the moisture being evaporated from the surface. The falling drying rate period is dependent essentially on the rate of diffusion of moisture from within the product to the surface and also on moisture removal from the surface. For agricultural products, the duration of each of the drying regimes depends on the initial moisture content and the safe storage moisture content. Especially for fruits and most vegetables, the drying would take place within both the constant and falling rate periods that can be seen easily. Both the external factors and the internal mechanisms controlling the drying processes in the two main rate regimes are important in determining the overall drying rate of products [18, 21]. For these reasons, the changing of the moisture content of dill leaves must have two periods depending on the drying time. The moisture content of the products as a function of

Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.)

drying time are presented in Figs. 3 and 4 for different convective drying temperatures and microwave power levels. As seen from these figures, all lines have two stages. The moisture content rapidly reduces and then slowly decreases with rising of the drying time. In addition, it is obvious from the figures that drying temperature and microwave power play an important role on the total drying time (Figs. 3-5). The least drying time (0.42 h) was obtained at PL-3. The highest drying time (11 h) was found at 50 °C. The total energy requirement for a charge of each dryer and energy needed for drying 1 kg of products can be seen from Figs. 6 and 7, respectively. There is a strict correlation between these two figures, because of the fact that the values in Fig. 7 were obtained from values in Fig. 6 by calculation (Eqs. (2)-(5)). As it is understood from these figures that the minimum heat energy (10.72 kWh/kg) is needed for drying of 1 kg products at temperature of 70 °C for dill leaves. The maximum energy (61.68 kWh/kg) is needed at PL-1. Color is an important quality attribute in food to most consumers. In addition, color analysis is important in foods, especially quality criterion for the production and trade. Color is also an important parameter of quality index of food for universal acceptability [12]. L, a, b and ΔE values are commonly used as an index to report the color quality. The changes in color parameters of dehydrated dill leaves are presented in Figs. 8 and 9. Samples dried at

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70 °C and PL-1 showed the highest Hunter L value, whereas the samples being dried at 60 °C and PL-2 gave the lowest L values. Different authors have reported that, decreases in L value correlated well with increases in browning of foods [22, 23]. So the dill leaves dried at 60 °C and PL-2 experienced an extensive browning. The redness (a) value decreased in comparison to fresh samples and the highest decrease was found in the drying temperatures of 50 °C and power level of PL-2. The yellowness (b) value was the highest for the samples dried at 60 °C and PL-3. The color changes in ΔE were also obtained for convective drying temperatures and microwave oven power levels. The color difference was relatively high for the samples dried at 50 °C and PL-2. Finally, the color changes in samples dehydrated at 70 °C and PL-3 were lower than the other drying temperature and power levels. No significant difference was found in color changes among the temperature of 70 °C and PL-3. There were about 11% and 5% decline in Hunter b values, compared with the fresh products for 70 °C and PL-3, respectively. This may be caused by good retention of carotenoids in the samples dried at 70 °C and PL-3.

4. Conclusions Dill leaves were successfully dried in a computer connected parallel air flow type convective dryer and in a microwave oven dryer at different temperatures of

Fig. 3 Moisture content as a function of convective drying time for temperature of 50, 60 and 70 °C.

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Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.)

Fig. 4 Moisture content as a function of microwave drying time for power level of PL-1, PL-2 and PL-3.

Fig. 5 Total drying time of product at different temperatures and power levels.

Fig. 6 Total energy requirement for a charge of convective and microwave dryers at different temperatures and power levels.

Fig. 7 Energy requirement for drying 1 kg of product for convective and microwave dryer.

Convective and Microwave Drying Characteristics of Dill Leaves (Anethum graveolens L.)

67

Fig. 8 Color changes in Hunter L, a, b and color difference (∆E) of dill leaves dried at temperature of 50, 60 and 70 °C.

Fig. 9 Color changes in Hunter L, a, b and color difference (∆E) of dill leaves dried at microwave power levels of PL-1, PL-2 and PL-3.

50 °C, 60 °C and 70 °C, air speed of 0.30 m/s and power levels of PL-1, PL-2 and PL-3. It is found from the results of the experimental investigation that the drying air temperature and power levels play an important role on the total drying time, specific power consumption and dill leaves color changes. The main conclusion of this study is that dill leaves must be dried in convective type dryer at temperature of 70 °C and air velocity of 0.30 m/s to minimize the energy consumption and to keep the higher quality and color retention for drying of dill leaves.

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