Open-air sun drying in thrace region of greece: the experimental investigation of thyme and mint leaves

O. Ismail And B. Beyribey

Chemical Engineering Department, Yildiz Technical University, 34210 Istanbul, Turkey
Tel: +90 212 383 4774; e-mail: ismail@yildiz.edu.tr

Abstract— In this work, open-air sun drying of thyme (Thymus Vulgaris) and mint (Mentha Spicata)leaves was investigated. A falling rate period was occurred in drying thyme and mint leaves, starting from the initial moisture content (%73.80 for thyme and %84.70 for mint) to the final moisture content (%7.6 for thyme and %4.85 for mint). Drying times were obtained as 440 for thyme and 420 minutes for mint. L values of fresh and dried herbs were respectively found as 40.25 and 35.70 for thyme and as 44.01 and 33.08 for mint. The effective diffusivities were determined as 2.2571*10^-10 m²s^-1 for thyme and as 1.2768*10^-10 m²s^-1 for mint.

Keywords— Open-air sun Drying; Thyme; Mint; Effective Diffusivity.

I. INTRODUCTION

Thyme (Thymus Vulgaris) and mint (Mentha Spicata), two of the important members of Lamiaceae family, are native to Europe, Africa, Asia and North America. They are most widely used as medicinal and aromatic plants of increasing importance in horticulture and economics (Alçiçek, 2011; Bimakr et al., 2011; Doymaz, 2006; Gumus, 2010; Kumar et al., 2011; Safaei-Ghomi et al., 2009; Oh et al., 2008). Thyme and mint are widely used in folk medicine for the treatment of a variety of diseases including gastroenteric and bronchopulmonary disorders, anthelmintic, antispasmodic, carminative, sedative, diaphoretic (El-Nekeety et al., 2011; Saleem et al., 2000). The high antioxidant and antimicrobial activities of thyme and mint have also attracted numerous researchers (Brandstetter et al., 2009; Emiroglu et al., 2010; Kanatt et al., 2007; Kosar et al., 2005; Lacroix et al., 1997; Lee et al., 2005; Rasooli et al., 2006; Solomakos et al., 2008a, 2008b). Their essential oils and extracts are widely used in pharmaceutical, cosmetic and perfume industry, and for flavouring and preservation of several food products (Balladin and Headley, 1999; Doymaz, 2006; Safaei-Ghomi et al., 2009; Telci et al., 2010). Their essential oils containing high amount of thymol and carvacrol were reported to possess the high antiseptic and antifungal properties (Loziene et al., 2007; Oh et al., 2008).

Drying is an ancient process used to preserve food and extend the shelf life of food. Drying not only affects water content of the plants but also inhibits microorganism growth and prevents degradation because of biochemical reactions. Open-air sun drying is the most common method of crop drying in developing countries. Despite several disadvantages, open-air sun drying is still used in many places throughout the world where plenty of solar radiation is available. Although some volatile compounds evaporate during sun drying, the decrease in volatiles can be minimized when drying air temperature is 50°C or below (Akpinar, 2010; Janjai et al., 2008; Usai et al., 2011).

There has been much research on the drying kinetics of various agricultural products, such as apricot (Igual et al., 2012), bean (Hii et al., 2009), ber (Kingsly, 2007), fig (Doymaz, 2005), grape (Fadhel et al., 2005), grape leather (Maskan et al., 2002), gundelia (Evin, 2012), pepper (Tunde-Akintunde, 2011; Kooli et al., 2007), red chili (Banout et al., 2011), rosemary (Arslan and Özcan, 2008).

The aim of this study is to investigate the effect of drying time on moisture content and drying rate of thyme and mint leaves collected from Thrace region of Greece, to calculate the effective moisture diffusivity, to compare the experimental data obtained in open-air sun drying with that in solar and cabinet dryers, to consider the suitability of Thrace region of Greece for open-air sun drying in July.

II. MATERIALS and METHODS

A. Material

Fresh thyme (Thymus Vulgaris) and mint (Mentha Spicata) were collected from Thrace region of Greece and kept at 4°C prior to experiments. Samples were processed after one hour waiting time at ambient conditions to bring the samples to ambient temperature. Dry matter and moisture contents of the fresh samples were determined prior to drying experiments. Solar drying experiments were carried out during July 2011 (09.00 AM- 18.00 PM) in Greece.

B. Drying Process

27.38 g thyme and 26.83 g mint were used for the drying experiments. Two wire mesh trays of 0.4*0.25 m² size were used on which to place the thyme and mint leaves. The leaves placed in a sieve tray were put on a portable digital balance (Alfais, I2000-1) to determine the weight loss by time and exposed to the sun. Four of samples were dried in an oven (Heraus, FT420) at 105°C for 24 h due to determine the initial moisture content (AOAC, 1990). Before drying, dry matter and moisture contents were specified for the samples.

C. Color Measurements

Color is one of the most important quality criteria for food. An unfavorable color change of food can decrease its quality level and market opportunity. Color measurements of fresh and dried leaves were done with Minolta CR-200 Chroma Meter (Minolta Co., Osaka, Japan). The values of L (lightness), a (greenness) and b (yellowness) before and after drying were measured for both thyme and mint leaves as given in Table 1. Five pieces of leaves for each herb were used in the color experiments.

Table 1. The average values of color parameters for fresh and dried herbs (thyme and mint)

D. Modeling

The moisture ratio and drying rate of mint and thyme during drying experiments were calculated using the following equations (Akpinar et al., 2003, Lee and Kim 2008; Doymaz 2011):

Moisture ratio:

(1)

where; MR is the moisture ratio (dimensionless), M_t is the moisture content at a specific time (g water/g dry matter), M_o is the initial moisture content (g water /g dry matter), Me is the equilibrium moisture content (g water/ g dry matter), respectively.

The equilibrium moisture content (Me) was assumed to be zero for sun drying and the MR equation (Eq. 1) was simplified as Eq. 2 (Soysal et al. 2006; Özbek and Dadali, 2007):

(2)

Drying rate:

(3)

where; DR is the drying rate (g water/g dry matter * min), Mt+dt is the moisture content at t + dt (g water/g dry matter), and t_m is the drying time (min).

Effective moisture diffusivity:

The effective moisture diffusivity was therefore calculated by the following equation (Crank, 1975):

(4)

where, D_eff is the effective moisture diffusivity (m²/s); L_t is the thickness of the slab (m), t_s is drying time (s) and n is the positive integer. Only the first term of Eq. (4) can be used for long drying times.

For long drying times; n=1, then Eq. (4) can be written as:

(5)

Equation (5) can be rewritten as (Sacilik; 2007; Wang et al., 2007, Kadam et al., 2011):

(6)

The slope (k₀) is calculated by plotting ln(MR) versus time according to Eq. (6) to determine the effective moisture diffusivity for different temperatures (Doymaz, 2006).

(7)

III. RESULTS AND DISCUSSION

Open-air sun drying experiments were carried out during July 2011 (09.00 AM- 18.00 PM) in Greece. The variation of ambient air temperature during open-air sun drying of thyme and mint leaves under natural convection on a typical day is shown in Fig. 1. During the drying experiments, the temperature of ambient air ranged from 36 to 46 °C. The highest air temperature was reached between 11:30 a.m. and 14:30 p.m.

Fig. 1. Variation of ambient air temperature during open-air sun drying of thyme and mint leaves on a typical day of July 2011 in Thrace region of Greece

Figures 2 and 3 demonstrate the drying curves of thyme and mint leaves. It is seen that moisture ratios of both thyme and mint leaves decreases continuously with drying time. As seen in these curves, no constant rate period was observed in drying of thyme or mint leaves. However a falling rate period occurred, starting from the initial moisture content (%73.80 for thyme and %84.70 for mint) to the final moisture content (%7.6 for thyme and %4.85 for mint). The results are generally in an agreement with some literature studies on drying of various food products (Simal et al. 2006; Lee and Kim, 2008).

Fig. 2. Moisture ratios versus drying time for thyme and mint.

Fig. 3. Drying rates versus drying time for thyme and mint.

A. Color Characterization

The results show that L*, a* and b* values of dried thyme and mint leaves are lower than those of fresh leaves. In other words, a darker lightness, an increase in greenness and a decrease in yellowness were seen in the colors of the dried thyme and mint leaves. In generally, high value of L and the lowest ratio of -a/b are desired. Chroma and color difference (ΔE) were calculated by Eqs. 8 and 9 (Jarad et al., 2007).

	(8)
	(9)

As it is seen in Table 1, better results in color quality were achieved for dried thyme leaves according to dried mint leaves. Higher color difference is seen upon drying of mint in comparison with thyme.

B. Determination of Effective Moisture Diffusivity

Effective moisture diffusivity is in general functions of material moisture content and temperature, as well as of the material structure (Marinos-Kouris and Maroulis, 1995). The effective diffusivity was calculated using the method of slopes. Effective diffusivities are typically determined by plotting experimental drying data in terms of ln(MR) versus time (Lomauro et al., 1985). From Eq. (7), a plot of ln (MR) versus time gives a straight line with a slope (k₀). This slope is the measure of the diffusivity. Figure 4. illustrates the linearity of the relationship between ln(MR) and drying time for both thyme and mint. The values of Deff for both thyme and mint are presented in Table 2. Upon drying of thyme and mint leaves, the effective diffusivities are found as 2.2571*10^-10 and 1.2768*10^-10 m²s^-1, respectively. The effective moisture diffusivity values in foods are in the range of 10^-12 to 10^-6 m²/s and the accumulation of the values is in the region 10^-10 to 10^-8 m²/s (75%) (Madamba et al., 1996 and Erbay and Icier, 2010). As seen in Table 3, the observed Deff values are within in the general range for food materials.

Fig. 4. Linear relationship between ln(MR) and drying time for thyme and mint.

Table 2. Moisture diffusivity and its linear equation for thyme and mint

Table 3. The comparison between present work and other studies for effective diffusivities of thyme and mint.

Drying times for thyme and mint exposed in a sample tray in the sun were determined as 440 and 420 minutes, respectively. Other studies about thyme and mint dried with solar and cabinet dryers are summarized in Table 4.

Table 4. The comparison between present work and other studies for drying time of thyme and mint

As referring Doymaz (2011), the lower Deff values are obtained for both thyme and mint. The reason for this, the air temperature ranged between 36-46 °C upon drying the presented experiments, while Doymaz (2011) set the temperature to a stable value of 50 °C in his experiments. Moreover, air velocity of 2 m s^-1 is also stable in his experiments; however, it is not possible to stabilize the ambient temperature in open-air sun drying.

The lowest drying time for mint was achieved as 200-175 minutes for drying at 40-50 ^oC air temperatures by Ethmane Kane et al. (2009). On the other hand, Doymaz (2006) and Kadam et al. (2011) dried mint at the temperature of 45 °C and they observed different Deff values. One was higher than our observation, while the other one was lower. This difference can be attributed to the difference in drying times, drying methods and stability of the drying temperature. While the drying time of mint is 420 minutes in the temperature range from 35 to 46 °C in the presented open-air sun drying, it is respectively 600 and 285 minutes at 35 and 45 °C in the cabinet dryer (Doymaz, 2006) and 390 minutes at 45°C in the laboratory model tunnel dryer (Kadam et al. 2011). Furthermore it is obvious that drying time decreases by increasing the air temperature. In the present work, comparable results were obtained without any energy requirement.

IV. CONCLUSION

In this study, open-air sun drying of thyme and mint leaves was investigated in July 2011, in Thrace region of Greece. Drying times for thyme and mint were obtained as 440 and 420 minutes, respectively. Successful L values of fresh and dried herbs were respectively found as 40.25 and 35.70 for thyme and 44.01 and 33.08 for mint. The effective diffusivities were determined as 2.2571*10^-10 m²s^-1 for thyme and as 1.2768*10^-10 m²s^-1 for mint. The results obtained in open-air sun drying experiments are comparable with those in solar and cabinet drying experiments. Unlike drying with solar and cabinet dryers, open-air sun drying also eliminates the energy cost of heating. Therefore, it is considered that Thrace region of Greece is extremely convenient for open-air sun drying in July and August.

NOMENCLATURE

a	greenness
b	yellowness
Deff	effective moisture diffusivity (m2/s)
DR	drying rate (g water/g dry matter * min)
k0	slope
L	lightness
Lt	thickness of the slab (m)
Me	equilibrium moisture content (g water/ g dry matter)
Mo	initial moisture content (g water /g dry matter)
MR	moisture ratio (dimensionless)
Mt	moisture content at a specifictime (g water/g dry matter)
Mt+dt	moisture content at t + dt(g water/g dry matter)
n	positive integer
tm	drying time (min)
ts	drying time (s)

REFERENCES
1. Akpinar, E.K., Y. Biçer and A. Midilli, "Modeling and experimental study on drying of apple slices in a convective cyclone dryer," Journal of Food Process Engineering, 26, 515-541 (2003).         [ Links ]
2. Akpinar, E.K., "Drying of mint leaves in a solar dryer and under open sun: Modelling, performance analyses," Energy Conversion and Management, 51, 2407-2418 (2010).         [ Links ]
3. Alçiçek, Z., "The effects of thyme (Thymus vulgaris L.) oil concentration on liquid-smoked vacuum-packed rainbow trout (Oncorhynchus mykiss Walbaum, 1792) fillets during chilled storage," Food Chemistry, 128, 683-688 (2011).         [ Links ]
4. Arslan, D. and M. Özcan, "Evaluation of drying methods with respect to drying kinetics, mineral content and colour characteristics of rosemary leaves, Energy Conversion and Management, 49, 1258-1264 (2008).         [ Links ]
5. AOAC, "Official method of analysis," Association of Official Analytical Chemists, Arlington, USA (1990).         [ Links ]
6. Balladin D.A. and O. Headley, "Evaluation of solar dried thyme (Thymus vulgaris Linné) herbs," Renewable Energy, 17, 523-531 (1999).         [ Links ]
7. Banout, J., P. Ehl, J. Havlik, B. Lojka, Z. Polesny and V. Verner, "Design and performance evaluation of a double-pass solar drier for drying of red chilli (capsicum annum l.)," Solar Energy, 85, 506-515 (2011).         [ Links ]
8. Bimakr, M., R.A. Rahman, F.S. Taip, A. Ganjloo, L.M. Salleh, J. Selamat, A. Hamid and I.S.M. Zaidul, "Comparison of different extraction methods for the extraction of major bioactive flavonoid compounds from spearmint (Mentha spicata L.) leaves," Food and Bioproducts Processing, 89, 67-72 (2011).         [ Links ]
9. Brandstetter, S., C. Berthold, B. Isnardy, S. Solar and I. Elmadfa, "Impact of gamma-irradiation on the antioxidative properties of sage, thyme, and oregano," Food and Chemical Toxicology, 47, 2230-2235 (2009).         [ Links ]
10. Crank, J., The mathematics of diffusion, 2nd ed.. Oxford, UK, Clarendon Press (1975).         [ Links ]
11. Doymaz, I., "Sun drying of figs: An experimental study," Journal of Food Engineering, 71, 403-407 (2005).         [ Links ]
12. Doymaz, I., "Thin-layer drying behaviour of mint leaves," Journal of Food Engineering, 74, 370-375 (2006).         [ Links ]
13. Doymaz, I., "Drying of thyme (Thymus vulgaris L.) and selection of a suitable thin-layer drying model," Journal of Food Processing and Preservation, 35, 458-465 (2011).         [ Links ]
14. El-Nekeety, A.A., S.R. Mohamed, A.S. Hathout, N.S. Hassan, S.E. Aly and M.A. Abdel-Wahhab, "Antioxidant properties of thymus vulgaris oil against aflatoxin-induce oxidative stress in male rats," Toxicon, 57, 984-991 (2011).         [ Links ]
15. Emiroglu, Z.K., G.P. Yemis, B.K. Coskun and K. Candogan, "Antimicrobial activity of soy edible films incorporated with thyme and oregano essential oils on fresh ground beef patties," Meat Science, 86, 283-288 (2010).         [ Links ]
16. Erbay, Z. and F. Icier, "A review of thin layer drying of foods: Theory, modeling and experimental results," Critical Reviews in Food Science and Nutrition, 50, 441-464 (2010).         [ Links ]
17. Ethmane Kane, C.S., M.A.O. Sid'Ahmed and M. Kouhila, "Evaluation of drying parameters and sorption isotherms of mint leaves (M. pulegium)," Revue des Energies Renouvelables, 12, 449-470 (2009).         [ Links ]
18. Evin, D., "Thin layer drying kinetics of gundelia tournefortii l.," Food and Bioproducts Processing, 90, 323-332 (2012).         [ Links ]
19. Fadhel, A., S. Kooli, A. Farhat and A. Bellghith, "Study of the solar drying of grapes by three different processes," Desalination, 185, 535-541 (2005).         [ Links ]
20. Gumus, T., "Determination of the changes of antifungal properties of satureja hortensis, thymus vulgaris and thymbra spicata exposed to gamma irradiation," Radiation Physics and Chemistry, 79, 109-114 (2010).         [ Links ]
21. Hii, C., C. Law, M., Cloke and S. Suzannah, "Thin layer drying kinetics of cocoa and dried product quality," Biosystems Engineering, 102, 153-161 (2009).         [ Links ]
22. Igual, M., E. García-Martínez, M.E. Martín-Esparza and N. Martínez-Navarrete, "Effect of processing on the drying kinetics and functional value of dried apricot," Food Research International, 47, 284-290 (2012).         [ Links ]
23. Janjai, S., N. Srisittipokakun and B.K. Bala, "Experimental and modelling performances of a roof-integrated solar drying system for drying herbs and spices," Energy, 33, 91-103 (2008).         [ Links ]
24. Jarad, F.D., B.W. Moss, C.C. Youngson and M.D. Russell, "The effect of enamel porcelain thickness on color and the ability of a shade guide to prescribe chroma," Dental Materials, 23, 454-460 (2007).         [ Links ]
25. Kadam, D.M., R.K. Goyal, K.K., Singh and M.K. Gupta, "Thin layer convective drying of mint leaves," Journal of Medicinal Plants Research, 5, 164-170 (2011).         [ Links ]
26. Kanatt, S.R., R. Chander and A. Sharma, "Antioxidant potential of mint (mentha spicata l.) in radiation-processed lamb meat," Food Chemistry, 100, 451-458 (2007).         [ Links ]
27. Kingsly, A.R.P., H.R. Meena, R.K. Jain and D.B. Singh, "Shrinkage of ber (Zizyphus mauritian L.) fruits during sun drying," Journal of Food Engineering, 79, 6-10 (2007).         [ Links ]
28. Kooli, S., A. Fadhel, A. Farhat and A. Belghith, "Drying of red pepper in open sun and greenhouse conditions," Journal of Food Engineering, 79, 1094-1103 (2007).         [ Links ]
29. Kosar, M., H.J.D. Dorman and R. Hiltunen, "Effect of an acid treatment on the phytochemical and antioxidant characteristics of extracts from selected lamiaceae species," Food Chemistry, 91, 525-533 (2005).         [ Links ]
30. Kumar, P., S. Mishra, A. Malik and S. Satya, "Insecticidal properties of mentha species: A review," Industrial Crops and Products, 34, 802-817 (2011).         [ Links ]
31. Lacroix, M., W. Smoragiewicz, L. Pazdernik, M.I. Kone and K. Krzystynik, "Prevention of lipid radiolysis by natural antioxidants from rosemary (Rosmarirzus oficinalis L.) and thyme (Thymus vulgaris L.)," Food Research International, 30, 457-462 (1997).         [ Links ]
32. Lee, J.H. and H.J. Kim, "Drying kinetics of onion slices in a hot-air dryer," J. Food Sci.Nutr., 13, 225-230 (2008).         [ Links ]
33. Lee, S.J., K. Umano, T. Shibamoto and K.-G. Lee, "Identification of volatile components in basil (ocimum basilicum l.) and thyme leaves (thymus vulgaris l.) and their antioxidant properties," Food Chemistry, 91, 131-137 (2005).         [ Links ]
34. Lomauro, C.J., A.S. Bakshi and T.P. Labuza, "Moisture transfer properties of dry and semi- moist foods," Journal of Food Science, 50, 397-400 (1985).         [ Links ]
35. Loziene, K., P.R. Venskutonis, A. Šipailiene and J. Labokas, "Radical scavenging and antibacterial properties of the extracts from different thymus pulegioides l. Chemotypes," Food Chemistry, 103, 546-559 (2007).
36. Madamba, P.S., R.H. Driscoll and K.A. Buckle, "The thin-layer drying characteristics of garlic slices," Journal of Food Engineering, 29, 75-97 (1996).         [ Links ]
37. Marinos-Kouris, D. and Z.B. Maroulis, "Transport Properties in the Drying of Solids," Handbook of Industrial Drying, Ed. A.S. Mujumdar, 2^nd Edition, Marcel Dekker Inc., New York. 113-160 (1995).         [ Links ]
38. Maskan, A., S. Kaya and M. Maskan, "Hot air and sun drying of grape leather (pestil)," Journal of Food Engineering, 54, 81-88 (2002).         [ Links ]
39. Oh, S., J. Ko, S. Jeong and J. Hong, "Application and exploration of fast gas chromatography-surface acoustic wave sensor to the analysis of thymus species," Journal of Chromatography A, 1205, 117-127 (2008).         [ Links ]
40. Özbek, B. G. and Dadali, "Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment," Journal of Food Engineering, 83, 541-549 (2007).         [ Links ]
41. Rasooli, I., M.B. Rezaei and A. Allameh, "Ultrastructural studies on antimicrobial efficacy of thyme essential oils on listeria monocytogenes," International Journal of Infectious Diseases, 10, 236-241 (2006).         [ Links ]
42. Rodríguez, J., J. Cárcel, G. Clemente, R. Peña and J. Bon, "Modeling drying kinetics of thyme (Thymus vulgaris)," In proceedings of Euro Drying, Palma, Mallorca (2011).         [ Links ]
43. Sacilik, K., "Effect of drying methods on thin-layer drying characteristics of hull-less seed pumpkin (Cucurbita pepo L.)," Journal of Food Engineering, 79, 23-30 (2007).         [ Links ]
44. Safaei-Ghomi, J., A.H. Ebrahimabadi, Z. Djafari-Bidgoli and H. Batooli, "Gc/ms analysis and in vitro antioxidant activity of essential oil and methanol extracts of thymus caramanicus jalas and its main constituent carvacrol," Food Chemistry, 115, 1524-1528 (2009).         [ Links ]
45. Saleem, M., A. Alam and S. Sultana, "Attenuation of benzoyl peroxide-mediated cutaneous oxidative stress and hyperproliferative response by the prophylactic treatment of mice with spearmint (Mentha spicata)," Food and Chemical Toxicology, 38, 939-948 (2000).         [ Links ]
46. Simal, S., M.C. Garau, A. Femenia and C. Rossello, "A diffusional model with amoisture-dependent diffusion coefficient," Dry Technol., 24, 1365-1372 (2006).         [ Links ]
47. Solomakos, N., A. Govaris, P. Koidis and N. Botsoglou, "The antimicrobial effect of thyme essential oil, nisin, and their combination against Listeria monocytogenes in minced beef during refrigerated storage," Food Microbiology, 25, 120-127 (2008).         [ Links ]
48. Solomakos, N., A. Govaris, P. Koidis and N. Botsoglou, "The antimicrobial effect of thyme essential oil, nisin, and their combination against Escherichia coli O157:H7 in minced beef during refrigerated storage," Meat Science, 80, 159-166 (2008).         [ Links ]
49. Soysal, Y., S. Oztekin and O. Eren, "Microwave drying of parsley: Modelling, kinetics, and energy aspects," Biosystems Engineering, 93, 403-413 (2006).         [ Links ]
50. Telci, I., I. Demirtas, E. Bayram, O. Arabaci and O. Kacar, "Environmental variation on aroma components of pulegone/piperitone rich spearmint (Mentha spicata L.)," Industrial Crops and Products, 32, 588-592 (2010).         [ Links ]
51. Tunde-Akintunde, T.Y., "Mathematical modeling of sun and solar drying of chilli pepper," Renewable Energy, 36, 2139-2145 (2011).         [ Links ]
52. Usai, M., M. Marchetti, M. Foddai, A. Del Caro, R. Desogus, I. Sanna and A. Piga, "Influence of different stabilizing operations and storage time on the composition of essential oil of thyme (thymus officinalis l.) and rosemary (rosmarinus officinalis l.)," LWT-Food Science and Technology, 44, 244-249 (2011).         [ Links ]
53. Wang, Z., J. Sun, F. Chen, X. Liao and X. Hu, "Mathematical modeling on thin layer microwave drying of apple pomace with and without hot-air pre drying," Journal of Food Engineering, 80, 536-544 (2007).         [ Links ]

Received: March 16, 2012
Accepted: June 7, 2012
Recommended by Subject Editor: Mariano Martin