Evaluation of Drying Methods with Respect to Drying Kinetics, Mineral ...

Food Bioprocess Technol (2012) 5:983–991 DOI 10.1007/s11947-010-0498-y

ORIGINAL PAPER

Evaluation of Drying Methods with Respect to Drying Kinetics, Mineral Content, and Color Characteristics of Savory Leaves Derya Arslan & Mehmet Musa Özcan

Received: 22 January 2010 / Accepted: 20 December 2010 / Published online: 21 January 2011 # Springer Science+Business Media, LLC 2011

Abstract Sun, oven (50 °C), and microwave oven (700 W) drying of savory leaves (Satureja thymbra L.) were carried out to monitor the drying kinetics, changes in mineral content, and color degradation of the product. Oven and microwave oven drying shortened the drying time over than approximately 70% and 99% when compared to the sun and oven drying methods, respectively. Fresh and dried savory leaves had high amounts of K (8875.2–28468.0 mg/kg), Ca (3681.6–9852.03 mg/kg), Mg (1388.0– 3102.0 mg/kg), and P (2313.2–5045.8 mg/kg) minerals. K, Ca, P, and Mg were the most abundant elements in savory samples. Mineral content of oven-dried savory were higher than the sun and microwave dried samples. Midilli and Küçük model was shown to give a good fit to the sun and oven drying. The Midilli and Küçük, modified page and page models exhibited high coefficient of determination (R2) values ranging between 0.9995 and 0.9997, to the experimental microwave oven drying data of savory. Microwave oven drying revealed optimum color values. Oven drying resulted in a considerable decrease in color quality of savory. Keywords Savory . S. thymbra . Drying kinetics . Sun . Oven . Microwave . Mineral . Color

D. Arslan : M. M. Özcan Faculty of Agriculture, Department of Food Engineering, 22 Selcuk University, Konya, Turkey M. M. Özcan (*) Selçuk Univ. Ziraat Fak., Gıda Müh. Böl., 42031 Konya, Türkiye e-mail: [email protected]

Nomenclature a, b, c Empirical constants in drying models k, k0, k1 Empirical constants in drying models MR Moisture ratio (dimensionless) M Moisture content at any time Me Equilibrium moisture content M0 Initial moisture content N Positive integer RMSE Root mean square error r2 Coefficient of determination SSE Sum square error t Drying time (h) y Empirical constant in drying models wwb Wet weight basis dwb Dry weight basis

Introduction Satureja thymbra L. is an endemic plant of the Mediterranean region, characterized by a similar oregano-like smell. Satureja species grow mainly in the western and southern Anatolia. These species including S. thymbra have commercial importance as they are used as spice (thyme) and their essential oils are used in pharmaceuticals and cosmetics (Satil et al. 2002; Goren et al. 2004). Drying is the most important process for preserving food; however, it has some negative effects on the quality of dried products (Okos et al. 1992). Medicinal and aromatic plants contain a high level of moisture and microorganisms. Therefore, the immediate drying is the most important operation in processing to minimize post harvest quality losses, but perishable crops (Müller et al. 1989). The aromatic herbs should be dehydrated for

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Food Bioprocess Technol (2012) 5:983–991

Table 1 Mathematical models applied to the drying curves Model no.

Equation

Model name references

1 2 3 4 5 6 7 8

MR=exp(−kt) MR=exp(−kty) MR=exp(−(kt)y) MR=a exp(−kt) MR ¼ a expðkt Þ þ c MR ¼ a expðko t Þ þ b expðk1 t Þ MR ¼ a expðkt n Þ þ bt MR ¼ a expðkt Þ þ b expðgt Þ þ c expðht Þ MR ¼ a expðkt Þ þ ð1 aÞ expðkat Þ MR ¼ a expðkt Þ þ ð1 aÞ expðkbt Þ MR ¼ 1 þ at þ bt2 MR ¼ a expðkt Þ þ ð1 aÞ expðgt Þ

Lewis Page Modified Page Henderson and Pabis Logarithmic Two-term model Midilli and Kucuk Mod. Henderson and Pabis

Ayensu (1997) Diamante and Munro (1993) Ozdemir and Devres (1999) Henderson and Pabis (1961) Yaldiz et al. (2001) Togrul and Pehlivan (2002) Lahsasni et al. (2004) Karathanos (1999)

Two-term exponential Diffusion approach Wang-Singh Verma

Sharaf-Elden et al. (1974) Kasem (1998) Wang and Singh (1978) Verma et al. (1985)

9 10 11 12

keeping quality for long periods and there are different methods for dehydrating herbs. Leaf dehydration, besides water elimination from tissues, produces changes on chemical, biological, and physical properties, as well as on the texture, color, and aroma (Hevia and Tramon 2003; Infante et al. 2010). Apart from their rich contents of vitamin C and beta carotene, leafy vegetables and herbs are an excellent source of mineral constituents whose importance in the human diet is indisputable. Some of nutrients are potassium, sodium, phosphorus, calcium, magnesium, or iron, are indispensable, in the sustainment of human health. Others such as copper or zinc are equally indispensable, but in this case the interval between the acceptable and toxic levels is limited (Kimura and Itokawa 1990; Slupski et al. 2005). In recent years, there has been a growing interest in mineral concentrations of foods, as the basic source of minerals for human (Lozak et al. 2002). The quality of many food products degrades during dehydration above room temperature. The added heat and exposure time of the product at elevated temperature affects the rate of nutrient quality degradation. Traditional drying methods such as sun and solar drying have been reported to have many drawbacks due to inability to handle large throughput of mechanical harvesters, promoting the insect and mold development due to high relative humidity during harvesting and drying. Sun drying is the most common method used to preserve agricultural products in most tropical countries. However, this technique is extremely weather dependent, and has the problems of contamination with dust, soil, sand particles and insects, and being weather dependent. Also, the required drying time can be quite long. Therefore, using solar and hot-air dryers which are far more rapid, providing uniformity and hygiene are inevitable for industrial food drying processes (Diamante

and Munro 1993; Ratti and Mujumdar 1997; Doymaz 2004). Drying of agricultural products can either be done by traditional sun drying or industrially through the use of solar dryers or hot air drying (Tunde-Akintunde et al. 2005). Solar dryers could be an alternative to hot air and open sun drying methods, especially in locations with good sunshine during harvest season (Pengavhane et al. 2002; Sacilik 2007). For heat-sensitive food products, the methods of supplying heat to the product and transporting the moisture from the product become the critical considerations for selecting the right dryer to achieve the desired product moisture content. The possibility of employing recent hybrid drying technologies for drying of foodstuffs and the ability of these technologies to minimize quality degradation in the final dried product has been considered in recent years (Chow and Chua 2001). In view of the hybridize technologies, heat pump drying, fluidized bed drying, infrared drying, microwave drying, radio frequency drying, and pressure regulating drying systems were designed to improve drying efficiencies (Chow and Chua 2001). Drying kinetics of materials may be described completely using their transport properties (thermal conductivity, thermal diffusivity, moisture diffusivity, interface heat, and mass transfer coefficients) together with the drying medium (Vagenas and Karathanos 1993; Togrul and Pehlivan 2004). Numerous studies are available regarding the drying behaviors of aromatic plants and herbs (Simal et al. 2000; Belghit et al. 2000; Fathima et al. 2001; Negi and Roy 2001; Soysal and Öztekin 2001; Park et al. 2002; Cesare et al. 2003; Akpınar 2005; Günhan et al. 2005; Guan et al. 2005; Doymaz 2006; Soysal et al. 2006; Infante et al. 2010). Pott et al. (2005) reported that high temperatures and excessive drying resulted in a noticeable increase in redness in mango slices. Soysal (2004) reported that the microwave


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Table 2 Weight loss (g) of the samples during oven and sun drying methods Time (hour)

Oven drying

Sun drying

0 1 2 3 4 5 6 7 8 9 10 11 12

100.0 76.4 63.5 54.2 46.0 39.2 32.6 28.0 25.1 23.8 23.4 23.3 23.2

100.0 87.2 79.7 75.5 71.2 67.6 63.9 60.5 57.5 54.7 52.1 49.7 47.8

13

23.2

46.2

drying may result in some darkening of the leaf color, compared to the fresh herb. The change of color could be attributed to the browning reactions (Maillard) that occur during drying (Adam et al. 2000). However, there is not any study published on the drying characteristics of savory leaves. The aim of present work was to determine the effect of the sun, oven, and microwave drying temperatures on drying characteristics of savory plant.

sunlight at temperatures between 16 °C and 23 °C for 13 h in May in Konya, Turkey (Balladin and Headley 1999). The average value of wind velocity was 2.4 m/s and the relative humidity was 67% during the days which the materials dried. Microwave Oven Drying A programmable domestic microwave oven (Arçelik ARMD 580, Turkey) with maximum output of 700 W and 2450 MHz was used for drying experiments. The dimensions of the microwave cavity were 345×340×225 mm. Of the sample, 50 g was placed on the center of a turntable fitted inside the microwave cavity and processed until the leaves were completely dried. The microwave oven was operated by a control terminal which could control microwave power level and emission time (1 s to 100 h). The mass of the sample was measured in every 1 h during oven and sun drying (Maskan et al. 2002; Günhan et al. 2005) and every 15 s during microwave oven drying (Fathima et al. 2001) using a digital balance, measuring to an accuracy of 0.001 g (Gikuru and Olwal 2005). The tray with the sample was taken out of the drying chamber, weighed on the digital balance, and placed back into the chamber. The digital balance was kept very close to the drying unit and the weight measurement process took about 10 s. The moisture rate of the plant material was measured by drying in an oven at 105 °C for 24 h, and calculated

Material and Methods Materials Fresh savory (S. thymbra L.) was purchased from a local market in Konya, Turkey. Plant materials were kept in cooled bags while transporting to the laboratory. Moisture content of the herb was immediately measured on arrival. HNO3 used in the mineral assay was of analytical grade (Merck, Germany). Prior to each of drying experiments, the thick stems were separated from the heaves. Methods Drying of the Savory Leaves Oven Drying Of the savory samples, 100 g were distributed uniformly as a thin layer on the trays of size 0.3×0.2 m and dried in an oven (Nüve FN055 Ankara, Turkey, 55 L volume) at 50 °C for 12 h (Balladin and Headley 1999). Sun Drying Of the savory samples, 50 g were distributed uniformly as a thin layer on the trays and dried under direct

Table 3 Weight loss (g) of the samples during microwave oven drying Time (minute)

Microwave oven dried

0 0.25 0.50 0.75

100.0 89.9 79.2 65.9

1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00

57.8 49.6 43.5 38.9 35.7 33.5 32.2 31.4 30.9 30.4 30.2 30.0 29.8

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1.2

Caption: Working conditions of ICP-AES:

sun drying oven drying

0.8

Instrument: ICP-AES (Varian-Vista) RF power: 0.7–1.5 kw (1.2–1.3 kw for axial) Plasma gas flow rate (Ar): 10.5–15 L/min (radial) 15 L/min (axial) Auxiliary gas flow rate (Ar): 1.5 Viewing height: 5–12 mm Copy and reading time: 1–5 s (max 60 s)

microwave drying

0.6 0.4 0.2 0 -0.2 0

1

2

3

4

5

6

7 8 Time *

9

10

11

12

13

Fig. 1 Variations of moisture ratio as a function of time for oven, sun and microwave drying of savory. Time interval is “hour” for sun and oven dryings while it refers to “minute” for microwave oven drying. The equilibrium moisture contents were 2.40, 0.05, and 0.07 in dwb and 23.95%, 5.15%, and 6.55% in wwb for sun, oven and microwave dryings, respectively

as 77.15% (w/w). The equilibrium moisture rates were 2.40, 0.05, and 0.07 in drybasis and 23.95%, 5.15%, and 6.55% in moisture wetbasis for sun drying, oven drying at 50 °C, microwave drying at 700 W, respectively. Experiments were repeated three times and mean values were used.

Mathematical Modeling of Drying Curves For mathematical modeling the equations in Table 1 were tested to select the best model for describing the drying curve equation of savory during drying. The moisture ratio of savory during drying was calculated using the equation; MR ¼ ðM Me Þ=ðM0 Me Þ (Midilli and Küçük 2003). The regression was performed in Statistica computer program (Statistica for Windows 5.0). The coefficient determination (r2), sum square error (SSE), and root mean square error (RMSE) were calculated in order to evaluate the goodness of fit to the models. The lower the SSE and RMSE values and the higher r2 values indicate the high fit of the model (Doymaz 2003). Determining the Mineral Composition About 0.5 g dried and ground sample was put into a burning cup and 15 mL pure HNO3 was added. The sample was incinerated in a MARS 5 Microwave Oven (CEM Corporation, USA, 3100 Smith Farm Road, Matthews, NC, USA) at a temperature of 200 °C and the solution was diluted to 50 mL with distilled water. Mineral concentrations were determined by inductively coupled plasma atomic emission spectrometer (ICP-AES; Skujins 1998).

Copy time: 3 s (max 100 s)

Color Measurement Color of savory samples was determined by Minolta Chroma meter CR 400 color meter (Minolta Co., Osaka, Japan) before and after drying. A sample size of 5 g of whole leaves was used for color measurements. The color meter was calibrated against a standard calibration plate of a white surface and set to CIE Standard Illuminant C. The L*, a*, b* values are average of ten readings. The color brightness coordinate L* measures the whiteness value of a color and ranges from black at 0 to white at 100. The chromaticity coordinate a* measures red when positive and green when negative, and chromaticity coordinate b* measures yellow when positive and blue when negative (Soysal 2004; Doymaz et al. 2006).

Results and Discussion Drying Characteristics of Savory Leaves The weight loss of samples during different drying methods is given in Tables 2 and 3. According to sun drying, oven 2.5 2 Drying rate

Moisture ratio

1

sun drying oven drying

1.5

microwave drying

1 0.5 0 0

1

2

3

4

5

6 7 Time *

8

9

10

11

12

13

Fig. 2 Drying rates of oven, sun and microwave drying of savory. Time interval is “hour” for sun and oven dryings while it refers to “minute” for microwave oven drying


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Table 4 Results of statistical analysis on the modeling of moisture contents and drying time for the oven dried savory Model no.

Parameters

r2

SSE

RMSE

1 2 3 4 5 6 7 8

k=0.4184 k=0.3782, y=1.0960 k=0.4118, y=1.0960 k=0.4211, a=1.0065 k=0.2985, a=1.1511, c=−0.1689 k0 =0.4210, k1 =0.4210, a=0.5033, b=0.5033 k=0.3627, a=1.0001, b=−0.0477, n=0.6877 k=0.4210, a=0.3355, b=0.3355, g=0.4210, c=0.3355, h=0.4210

0.9935 0.9945 0.9945 0.9935 0.9980 0.9935 0.9999 0.9935

0.001335 0.001134 0.001134 0.001328 0.000418 0.001328 0.000004 0.001328

0.036538 0.033675 0.033675 0.036435 0.020453 0.036435 0.002087 0.036435

9 10 11 12

k=0.5095, a=1.5496 k=0.4184, a=0.1728, b=1.0000 a=−0.3044, b=0.0238 k=−1.0181, a=−0.0002, g=0.3923

0.9947 0.9935 0.9939 0.9993

0.001092 0.001335 0.001259 0.000151

0.033050 0.036538 0.035478 0.012280

drying processe is more affective and and weigh loss is very high than that of sun drying. Plots of the moisture ratio versus time and drying rate versus time curves are shown in Figs. 1 and 2 which represent the experimental curve of drying characteristics of savory herb. The moisture content of the material was very high during the initial phase of the drying which resulted in high drying rates due to the higher moisture diffusion. By using oven drying method, the drying time needed up to the moisture content of 23.3% wet weight basis (wwb) was 4 h, while 13 h needed to reach the same moisture content by sun drying and approximately 90 s by microwave drying. This shows oven drying method shortened the drying time by 69.2% when compared to sun drying. The time taken to reach a moisture content of 5.1% (wwb) for the oven drying was 7 h, a moisture content of 23.65% (wwb) for the sun drying methods was 13 h and a moisture

content of 5.25% (wwb) for the microwave oven drying was 315 s. The time was longer for sun drying due to the low and fluctuating temperature during the drying period. Kowalski and Mierzwa (2009) reported that although time could be constraints but it gives a better preservation of nutrients as opposed to high intense heating. However, Łupińska et al. (2009) reported that microwave-assisted drying of biological material can be very risky in order to obtain the best quality of expected product as microwave ability to concentrate in the moistest material regions can lead to destruction of sensitive biological tissue. Drying of agricultural products can either be done by traditional sun drying or industrially through the use of solar dryers or hot air drying (Tunde-Akintunde et al. 2005). Solar dryers could be an alternative to hot air and open Sun drying methods, especially in locations with good sunshine during harvest season (Pengavhane et al. 2002; Sacilik 2007).

Table 5 Results of statistical analysis on the modeling of moisture contents and drying time for the sun dried savory Model no.

Parameters

r2

SSE

RMSE

1 2 3 4 5 6 7 8 9 10 11 12

k=0.2146 k=0.2096, y=1.0135 k=0.2141, y=1.0135 k=0.2113, a=0.9847 k=0.1536, a=1.0898, c=−0.1395 k0 =0.2113, k1 =0.2113, a=0.4923, b=0.4923 k=0.2516, a=0.9997, b=−0.0196, n=0.6793 k=0.2113, a=0.3282, b=0.3282, g=0.2113, c=0.3282, h=0.2113 k=0.2413, a=1.3984 k=0.2759, a=−129.450, b=0.9979 a=−0.1556, b=0.0063 k=−0.3895, a=−0.0005, g=0.2034

0.9919 0.9920 0.9920 0.9922 0.9964 0.9922 0.9995 0.9922 0.9923 0.9924 0.9874 0.9964

0.001334 0.001330 0.001330 0.001305 0.000600 0.001305 0.000076 0.001305 0.001278 0.001265 0.002100 0.000610

0.036529 0.036465 0.036465 0.036127 0.024498 0.036127 0.008707 0.036127 0.035751 0.035565 0.045827 0.024705

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Table 6 Results of statistical analysis on the modeling of moisture contents and drying time for the microwave dried savory Model no.

Parameters

r2

SSE

RMSE

1 2 3 4 5 6 7 8

k=0.2509 k=0.1457, y=1.3437 k=0.2385, y=1.3437 k=0.2657, a=1.0663 k=0.2160, a=1.1250, c=−0.0843 k0 =0.2657, k1 =0.2657, a=0.5331, b=0.5331 k=0.1429, a=0.9902, b=−0.0007, n=1.3394 k=0.2657, a=0.3554, b=0.3554, g=0.2657, c=0.3554, h=0.2657

0.9899 0.9995 0.9995 0.9923 0.9967 0.9923 0.9997 0.9923

0.001762 0.000072 0.000072 0.001349 0.000973 0.001349 0.000065 0.001350

0.041975 0.008472 0.008472 0.036735 0.031186 0.036735 0.008076 0.036736

9 10 11 12

k=0.3635, a=1.8866 k=0.4663, a=−234.807, b=0.9965 a=−0.1760, b=0.0076 k=0.1286, a=−18.159, g=0.1331

0.9990 0.9993 0.9984 0.9968

0.000163 0.000123 0.003784 0.000855

0.012748 0.011069 0.061514 0.029239

Evaluation of the Models Twelve different mathematical equations given by various authors for the drying curves were used for fitting to the data and to determine the moisture content as a function of drying time (Table 1). The statistical values are given in Tables 4, 5, and 6. These models exhibited high coefficient

of determination (r2) values ranging between 0.992 and 0.999. So, all these models could be used to describe the drying of savory. It is clear that Midilli and Küçük, Verma, and logarithm models gave the highest r2 values between 0.9964 and 0.999 and the lowest SSE and RMSE values for the sun and oven drying methods. The r2 values obtained from the Midilli and Küçük, modified page and page

Table 7 Mineral contents of fresh, oven, sun and microwave oven dried savory (mg/kg) Minerals

Fresh

Oven dried

Sun dried

Microwave oven dried

Al

63.03±11.33a Bd

189.16±8.21 Af

192.55±9.55 Af

196.57±6.77 Af

B Ba Ca Cd Co Cr Cu Fe K Li Mg Mn Na Ni P Pb Sr V

52.98±14.81 Cd 11.45±5.37 Cd 3681.64±363.26 Cb 0 1.39±0.72 d 94.99±16.26 Bd 1.01±0.48 Bd 311.08±73.24 Bd 8875.24±573.70 Ba 6.01±0.69 Bd 1387.97±175.08 Bcd 19.84±2.07 Cd 233.07±94.40 Cd 28.06±8.51 Bd 2313.23±727.72 Bbc 0 38.00±5.06 Bd 0.44±0.14 Bd

101.78±12.42 Af 20.56±3.16 ABf 9372.17±438.08 Bb 0 1.39±0.31 f 142.81±10.81 Af 5.83±0.02 Af 482.44±20.25 Ae 27470.72±600.92 Aa 15.27±2.22 Af 2965.61±56.64 Ad 44.55±3.12 Bf 443.16±51.33 Be 48.46±6.09 Af 4895.48±154.72 Ac 0 100.17±0.67 Af 0.76±0.12 Af

73.99±1.79 Bf 24.28±6.03 Af 9663.28±326.27 Bb 0 2.31±0.37 f 150.57±10.44 Af 6.97±0.59 Af 576.01±35.65 Ae 29186.12±672.62 Aa 18.53±1.64 Af 3331.35±155.27 Ad 56.09±0.67 Bf 713.91±114.62 Ae 50.72±2.48 Af 5185.34±725.29 Ac 0 107.16±3.21 Af 0.79±0.21 Af

75.91±3.94 Bgh 16.98±2.17 Bh 10521.52±219.23 Ab 0 2.54±0.22 h 140.78±2.97 Afg 5.91±0.19 Ah 522.91±10.57 Ae 28747.66±332.78 Aa 19.37±0.13 Ah 3007.98±73.34 Ad 73.34±1.26 Agh 517.34±76.94 Be 47.19±1.42 Agh 5056.59±105.96 Ac 0 112.28±0.91 Afgh 0.82±0.09 Ah

Zn

8.03±1.41 Cd

25.47±0.30 Bf

32.50±0.85 Af

27.78±0.01 Bh

a

Mean±standard deviation

Different upper case letters in a row show statistically significant differences between treatments at 5% level probability Different lower case letters in a column show statistically significant differences between mineral values at 5% level probability

Food Bioprocess Technol (2012) 5:983–991 -1

40 36

ab

-3

b

b

32

a*

b

28

b

b

-6 -7

20 fresh

sun dried

oven dried

microwave dried

fresh

sun dried

oven dried

microwave dried

-0,1

14

a

a

a

12

ab

b

-0,3

a*/b*

b*

-4 -5

24

10

a

-2

a

L*

Fig. 3 Effects of different drying methods on L*, a*, b*, and a*/b* values of savory. Bars with different letters are significantly different (P