Drying Protocols for Traditional Medicinal Herbs: A Critical Review

Transcription

1 Drying Protocols for Traditional Medicinal Herbs: A Critical Review Rajat Chakraborty and Tilottama Dey Department of Chemical Engineering, Jadavpur University, Kolkata, India ABSTRACT:The demand on traditional herb medicine shows a tremendous increase. Conventionally, medicinal herbs are dried at high temperatures, causing quality deterioration. In this review article, the various drying characteristics of medicinal herbs were studied, and the quality of the dried products assessed. Comparison of the different drying protocols such as air drying, microwave drying, sun drying, combined convective and microwave drying, infra red drying, freeze drying, etc. are done taking into account many important characteristics such as final moisture content, color, drying time, volatiles, etc. Experimental data as well as theoretical data have been provided to support the conclusions. INTRODUCTION: Herbs and spices are useful due to their nutrient and essential oil content. The demand of traditional herb medicine shows a tremendous increase [1]. But fresh herbs have enough moisture to degrade its quality over a span for time; thus rendering it unfit for consumption. This degradation can be prevented by drying the fresh herb. Drying basically removes the moisture from the herb which reduces and finally eliminates the microbial activity which initially causes the degradation of the herb quality. This is the most common preservation technique. Preservation is mainly important for seasonal plants with seasonal fruits, etc. Because of high water content (78 92%, w/w), herbs are dried for market, in order to inhibit micro-organism growth and prevent degradation because of biochemical reactions. Also, drying brings about substantial reduction in weight and volume, minimizing packaging, storage and transportation costs [2]. Drying is important for preserving product quality, and until recently herbs were commonly dried at high temperatures (40-60 C). Some of the active components in the product are temperature sensitive, and are not preserved by this drying method, reducing the product quality. The most ancient form of drying is open sun drying. This leaves no space for control over the process. Hence air-drying came into existence. This method can preserve the product for a longer time, but unfortunately the quality of the air-dried product reduces to a much greater extent when compared to the original sample [3]. Compared with other drying techniques; microwave drying offers opportunities as uniform energy and high thermal conductivity to the inner sides of the material, space utilization, sanitation, energy saving, precise process control and high quality of the finish product. It also reduces the drying time [4]. Vacuum drying is the best method for removal of water from herb sample compared to other methods [5]. Vacuum drying is a drying technique which is used for drying of various products; retaining colour and vitamin content [6]. Better product quality can be retained by high-degree vacuum treatment [7]. The key benefits of vacuum drying include lower process temperatures, less energy usage and hence greater energy efficiency, improved drying rates, and in some cases, less shrinkage of the product, although the cost of the process is high [8]. Another method called freeze-drying has become an increasingly important preservation technology for highly heatsensitive and delicate biological products. Freeze-drying is a specific drying method which allows obtaining dried materials with features typical of raw material. Freeze-drying is used for the reliable preservation of a wide variety of heat-sensitive products. Freeze drying is a method of dehydrating materials by sublimation under vacuum. This process is known for its ability to sustain product quality during low-temperature drying with a minimum loss of flavor, aroma, and valuable components as well as negligible shrinkage, etc. [9]. The most important in this process are time, temperature and pressure. If they are well defined they may indeed affect the quality of the final product. However, freeze drying is a high-cost drying method, due to its intensive energy demand, long drying time, and high investment costs [10]. Further studies comparing infrared drying with techniques based on air-convection show that the infrared radiation method is quicker than convection based drying methods and that it is suitable if the processing time is a prime factor [11,12]. Drying processes effect (partially or totally) the quality of the product. Various changes in physical, chemical and/or biological characteristics of sample occur. These changes alter the physical aspect such as colour and 312 Rajat Chakraborty and Tilottama Dey

2 structure. Undesirable reactions cause degradation of aroma compounds and nutritional substances [12]. These changes cause reduction in product quality [13]. The loss of volatiles in herbs and spices during drying depends mainly on drying conditions and the biological characteristics of the herb. Some volatile compounds evaporate during drying, while others are partially retained. Some compounds arise as oxidation products during drying. The choice of right drying technique directly affects the product quality. MATERIALS and METHODS: Rosemary (Rosmarinus officinalis L.) is an aromatic, evergreen, shrubby herb typical of Mediterranean countries. Dried rosemary leaves are used in fried chicken, salads, baked products, condiments, perfumes and soaps. Besides, essential oils of spices and/or herbs could also be used as functional ingredients [14]. Numerous studies have documented the antioxidant, antimicrobial, antiviral, anti-inflammatory, anti-ulcerous, anti-carcinogenic properties of plant essential oils (14, 15]. Origanum is one of over 200 genera in the Lamiaceae (mint family), and the genus includes culinary, fragrant, medicinal and ornamental plants. Sometimes called the prince of herbs [16], Origanums have been valued for their culinary, fragrant and medicinal properties. These qualities are the result of naturally occurring chemicals in the plant. One source for these qualities is the volatile or essential oil. The stinging nettle (Urtica dioica L.) is an herbaceous flowering plant native to Europe, Asia, Northern Africa, and North America, and is the best known member of the nettle genus Urtica. It is recommended by the herbalists of many different cultures for a wide variety of purposes in herbal medicine. It is a very nutritious food that is easily digested and is high in minerals (especially iron), vitamin C and pro-vitamin A [16]. Nettle is a vegetable which rapidly perishes after harvest and which is currently consumed only in season. Because of their high moisture content, nettle leaves should be dehydrated for a better and longer preservation. Mint (Mentha spicata L.) has been used as a medicinal and aromatic plant since ancient times. Its leaves are used for flavoring, tea infusions and spicing [17]. Mint leaves are refreshing, antiseptic, anti-asthmatic, stimulative, diaphoretic, stomachic, and antispasmodic. Mint leaves can be used as a medicinal and aromatic plant. It helps in colds, flu, fever, poor digestion, motion sickness, food poisoning, rheumatism, hiccups, stings, ear aches, flatulence and for throat and sinus ailments. It is rich in calcium and phosphorus. Basil (Ocimum basilicum L.), belonging to the hamiaceae species, is an annual, herbaceous, 20 60cm length and white-purple flowering plant which comes from India and Iran. It is cultivated in Mediterranean countries and in various regions with temperate and hot climates. Fresh basil is widely used in the Mediterranean kitchen such as tomato products, vegetables, salads, pizza, meat, soups and marine foods. It is commonly known that the presence of essential oils and their composition determine the specific aroma of plants and the flavour of the condiment. Numerous studies have documented that sweet basil contains high concentrations of phenolic compounds (especially rosmarinic acid and caffeic acid), which are characterised by high antioxidant capacity [18,19]. Chard (Beta vulgaris subsp. cicla), is a leafy green vegetable. While the leaves are always green, chard stalks vary in color. Chard has been bred to have highly nutritious leaves. Chard is, in fact, considered to be one of the healthiest vegetables available and a valuable addition to a healthy diet (not unlike other green leafy vegetables). Swiss chard is high in vitamins A, K and C containing 214%, 716%, and 53%, respectively, of the recommended daily value. It is also rich in minerals, dietary fiber and protein. Sometimes chard is consumed for its iron content: the stalk retains more iron compounds than the leaves, hence their rosy color. All parts of the chard plant contain oxalic acid. DRYING METHODS: Table 1 summarizes different drying methods. Table 1: Drying methods DRYING DRYING TREATMENT METHOD MINT OREGANO NETTLE ROSEMARY BASIL 313 Rajat Chakraborty and Tilottama Dey

3 Shade-dried Leaf samples arranged in a monolayer and kept in shade for 7 days; room temperature ranged from 29 to 32 C with relative humidity of 70% to 90%. Sun-dried Mint leaves were distributed uniformly as a thin layer onto the stainless steel trays of size m and dried under direct sunlight at temperatures between 20 and 30 C for 13 h. Air dried The drying of mint leaves was investigated in cabinet dryer at 35, 45, 55 and 60 C were carried out, relative humidity of 8 40%. Oven dried Mint leaves were distributed uniformly as a thin layer onto the stainless steel trays of size m and dried in an oven at 50 C for 12 h. Leaf samples airdried at 20 C and 40 C in an air fluxdrying oven (Novatech International, Houston, tx). Vacuummicrowave Drying was performed in a domestic digital microwave oven at five different microwave output powers, being 180, 360, 540, 720 and 900 W. Leaf samples arranged in a monolayer for 8hrs temperature ranged from 31 to 50 C with a relative humidity of 50% to 75%. The dryer temperature was set at 48 c with an air flow rate of 2.3 m 3 /s and relative humidity of 25% for 11.5 hr in the dryer. After a vacuum of 27 in of Hg was achieved, the magnetron was powered at 3.2 kw for 12 min, followed by 1 kw for 6 min and then 0.5 kw for 5 min. Ambient airflow rate through the chamber was 3 l/min. Drying treatments in air-drying were 50, 75 and 100 C, and 1 m/s fan speed. The oven has the capability of operating at nine different temperature stages, being 50, 75, 100, 125, 150, 175, 200, 225 and 250 1C at 1 m/s air velocity. Different microwave output powers, 160, 350, 500, 650, 750 and 850 W were used in the microwavedrying. Convective method was operated at 60 C with an air velocity of 1.2 m/s. The dryer was operated at 480 W. Rosemary samples were placed in a cylindric container of pressure inside from 4 to 6 kpa. Additionally an air stream of 22 C at a velocity of 1 m/s is provided. Leaves were dried by sun drying (temperatures between 30 and 35 C for 2 days). Leaves were airdried using a conventional convective drier, where air at 40, 50 and 60 C was blown at an air velocity of 1.2 m/s. Oven drying (50 C for 15h) A sample (600 g) of fresh basil leaves was placed in the drying drum of a 4 kw maximum power. After a vacuum of 27 in. of Hg was achieved, the magnetron was powered at 3.2 kw for 12 min, followed by 1 kw for 6 min and then 0.5 kw for 5 min. Ambient air flow rate through the chamber was 3 L/min. 314 Rajat Chakraborty and Tilottama Dey

4 Combination of convective pre-drying and vm finish-drying (cpd * vmfd # ) Convective predrying (cpd * ) followed by vm finish drying (vmfd # ) at 360 or 480 W. Nine different combination levels were formed in combination drying curves by combining microwave output levels of 160, 350 and 500 W, and air temperatures of 50, 75 and 100 C. Two runs of the combined method were performed: (1) pre-drying until a moisture content of 0.44 followed by finish-drying at 360 W, (2) predrying until 0.44 kg kg - 1 and VMFD at 480 W, and (3) pre-drying until 0.18 kg kg - 1 and VMFD at 360 W. Infra red dried The sample was dried for a period of h in a wooden static bed infrared dryer (lamps of 500mm, 1000mm and 1500mm in length with spectrum ranging from a wavelength of <2µm to 4-5µm). Freeze dried The product was dried for a period of 14 h at Pa (freeze drying at high pressure, FD-HP % ) and for 12 h at Pa (freeze drying at low pressure, FD- LP $ ) with the heating plate kept at 17 C. Leaf samples were freeze-dried under vacuum (1.6 mm hg). Chamber and condenser temperatures were 20 C and -55 C, respectively. * convective pre-drying; # vacuum microwave finsh drying; % freeze drying-high pressure; $ freeze drying-low pressure DRYING KINETICS Evaluation of models For mathematical modeling the equations in Table 2 were tested to select the best model for describing the drying curve equation of herbs during drying. 315 Rajat Chakraborty and Tilottama Dey

5 Table 2: Mathematical models applied to the drying curves Sl. No. Model Name Equations Reference 1. Newton Model Zheng and Wang, Page Model Pereira, Modified Page Model Ozdemir, Handerson and Pabis Henderson, 1961 Model 5. Logarithmic Model Yaldiz, Two-term Model Togrul, Midilli and Kucuk Lahsasni, 2004 Model 8. Mod. Henderson and Karathanos, 1999 Pabis Model 9. Two-term exponential Model Sharaf-Elden, Diffusion approach Kasem, Wang and Sing model Wang and Singh, Verma Model Verma, Third degree polynomial Antal, 2010 Model 14. Sigmoid Model Antal, 2010 For air-drying (cabinet), only Lewis, Page, Henderson-Pabis and Logarithmic models were used. All the models gave consistently high coefficient of determination values. This indicates that all the models could satisfactorily describe the air-drying of mint leaves. Among the drying models, the logarithmic model may be assumed to present the thin-layer drying behavior of the mint leaves. A detailed study showed that the Page model gave higher values than the Lewis and Henderson and Pabis models. As observed in Fig1, the logarithmic model provided a good agreement between experimental and predicted moisture ratio values [20]. Moreover, drying curves of mint which were dried in a convective drier at 30, 40 and 50 C and 0.5 and 1.0 m/s air velocities were fitted using the Page equation with best results [21]. Fig 1: Influence of air-drying (cabinet) temperature on drying curves and simulation of drying curves by using the proposed logarithmic model. 316 Rajat Chakraborty and Tilottama Dey

6 Fig 2: Influence of vacuum and microwave wattage on drying curves and simulation of drying curves by using the proposed modified Henderson Pabis model. Fig 3: Influence of vacuum and microwave wattage on drying curves of chard leaves through semiempirical Page s equation The different MR models used to determine the moisture content as a function of drying time are enlisted in Table 2. Page, Modified page, Midilli and Kucuk, Diffusion approach and Verma models gave the best values for the sun drying and Page, Modified page, Midilli and Kucuk and Two-term exponential models expressed good fit to microwave oven drying methods. The Page, Modified page, Midilli and Kucuk, Two term exponential and Diffusion approach models were best fit for oven drying of mint leaves [22]. Table 3: Comparative study of air drying, microwave drying and freeze drying of mint, oregano and nettle. PARAM ETERS Final moisture content Drying time (h) 317 Rajat Chakraborty and Tilottama Dey DRYING TREATMENT MINT OREGANO NETTLE ROSEMARY BASIL AD MW FD AD MW FD AD MW FD VM FD AD 11.5% 10% % 9% % % 9.1% 15% % % %

7 AD- air dried;mw- microwave;vm- vacuum microwave;fd- freeze dried; Hence, the table clearly shows that freeze drying reduces the moisture content to a much lower level when compared to air-drying and microwave drying. But microwave drying has much lower drying time as compared to air-drying and freeze drying. EFFECTIVE MOISTURE DIFFUSIVITY From Table 4 it can be seen that the values of D eff increased greatly with increasing temperature. These values are consistent with the present estimated D eff values for mint and nettle leaves. The calculated effective diffusivity (D eff ) values (m 2 /s) of mint leaves for the sun, oven and microwave oven drying process were , and , respectively. Microwave drying yield the highest D eff values which were higher than that of sun and oven (50 C) drying. The value of D eff for the sun drying was higher than that for the oven drying, which indicates that open sun drying process had better mass transfer efficiency than conventional convective style oven drying without air velocity application. Table 4: Values of effective diffusivity obtained for mint leaves at different temperatures and relative humidity for mint and nettle MINT NETTLE Velocity, m/s Drying conditions Diffusivity coefficient, D eff 10-9 Temperature, C m 2 /s Relative humidity Diffusivity coefficient, D eff 10-9 m 2 /s Relative humidity 40% 55% 70% 40% 55% 70% It can be seen that the values of D eff increased greatly with increasing temperature. Similar variations were also observed during drying of carrot, black tea and aloe. The calculated effective diffusivity (D eff ) values (m 2 /s) of mint leaves for the sun, oven and microwave oven drying process were , and , respectively. Microwave drying yield the highest D eff values which were higher than that of sun and oven (50 C) drying. The value of D eff for the sun drying was higher than that for the oven drying, which indicates that open sun drying process had better mass transfer efficiency than conventional convective style oven drying without air velocity application. The values obtained in the present study are in agreement with the results of Park et al. 1. Maskan, M., 2000, Microwave/Air And Microwave Finish Drying Of Banana, Journal Food Engineering, 44, pp Carroll M. B., and Churchill S. W., 1986, A Numerical Study of Periodic On-Off Versus Continuous Heating by Conduction, Numerical Heat Transfer, 10, pp Khraisheh, Mcminn, And Magee, 2000, A Multiple Regression Approach to the Combined Microwave and Air Drying Process, Journal of Food Engineering 43, Ratti C., 2001, Journal Of Food Engineering 49, pp Feng H., Tang J., 1998, Microwave finish drying of diced apples in a spouted bed, Journal of Food Science, vol. 63, pp Maskan, M., 2000, Microwave/air and microwave finish drying of banana, Journal of Food Engineering, vol. 44, pp Rajat Chakraborty and Tilottama Dey

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