INTRODUCTION:

Cosmetic formulations contain several phytoconstituents as they possess skin protection action against exogenous and endogenous harmful agents. They also help remedy many skin conditions. Various synthetic agents can be used in cosmetics, but they have limited use because they can lead to toxicity in humans. Herbal cosmetics provide a better alternative since they contain a mixture of phytoconstituents, which are believed to act in a synergistic manner. These provide excellent results with practically no undesirable side effects, provided its quality is assured off.

Extraction, as the term is used pharmaceutically, involves the separation of medicinally active portions of plant or animal tissues from the inactive or inert components by using selective solvents in standard extraction procedures.Medicinal plant research starts with extraction as its first basic step because the preparation of crude extracts from plants is the starting point for the isolation and purification of chemical constituents present in the plants.^[1]

However, extraction is many a time neglected or paid very little attention to and this creates several constraints on the analytical procedure. The traditional techniques of solvent extraction of plant materials is based on the correct choice of solvents and the use of heat or/and agitation to increase the solubility of the desired compounds. ^[2]Moreover, the traditional techniques require longer extraction time and as a result thermal degradation of most of the phytoconstituents may occur. ^[3]One single plant can contain several phytoconstituents. This fact necessitates the development of high performance and rapid extraction methods. ^[1,
4]

Soxhlet extraction method which has been used for almost 126 years for the extraction and separation of phytoconstituents has a major shortcoming that it requires a very long time that can be 8, 16, 24 hours or more. This leads to consumption of considerable time and heat energy and it is also labour intensive. The numbers of samples that can be processed under such conditions are also not commercially viable. The requirement of large amount of organic solvents as well as recovery step followed by evaporation to concentrate the extract make Soxhlet extraction a cumbersome process. ^{[3, 4]}

Ultrasound-assisted extraction is inexpensive, simple and efficient but the biggest shortcoming is the deleterious effect of ultrasound energy (more than 20 kHz) on the active constituents of medicinal plants through formation of free radicals and consequently undesirable changes in the drug molecules. ^[5]

Extraction techniques have been widely investigated to obtain valuable phytoconstituents from plants for commercialization. There is an increasing demand for new extraction techniques with shortened extraction time, reduced organic solvent consumption, and increased pollution prevention. Novel extraction methods such as microwave-assisted extraction are fast and efficient for extracting chemicals from solid plant matrixes. These techniques have the possibility of working at elevated temperatures and/or pressures, greatly decreasing the time of extraction. There has been a need for better and newer extraction techniques, in the herbal drug industry so that the extraction time and the cost of solvent consumption are decreased. ^{[5, 6]}Also, at present the quality and safety related problems seem to be overwhelming the potential genuine benefits connected to the use of phytoconstituents containing products and these problems are attributed to the lack of high performance, reliable extraction, analytical techniques and methodologies for establishing a standard therapeutic functionality for such products. There is an increasing trend of using pure compounds instead of crude extracts prepared from plant material.

Microwave assisted extraction (MAE) is a separation technique that has shown to provide higher quality, purity and yield of phytoconstituents when compared to the conventional extraction techniques like Ultrasonic Assisted Extraction and Soxhlet extraction. Microwave has been useful for the extraction of plant materials. Microwave Assisted Extraction process is a method used to selectively extract target compounds from various raw materials. It uses energy of microwave radiation to heat solvents quickly and efficiently. MAE is an innovative solvent-extraction technology which offers a better alternative to several thermal applications due to its efficient volumetric heat production and the fact that it has many advantages over conventional solid liquid extraction methods.^[5] Its application includes extraction of high-value compounds from natural sources, nutraceutical and functional food ingredients, and also pharmaceutical actives from biomass. This article aims to suggest its increased utilization in herbal cosmetics industry.

The history of use of microwave in extraction:

The use of microwave extraction technology has only recently appeared in analytical laboratories. In 1975, Abu-Samra et al. were the first researchers ever to use a microwave domestic oven in the laboratory, performing trace analysis of metals from biological samples. ^[5]It took almost as long as 10 years for the first publication to appear in 1986. Ganzler et al. have developed extraction protocols for lipids, anti-nutritive substances and pesticide from soils, seeds foods and feeds in a few millilitres of solvent, irradiated for 30 s up to 7 times in a domestic oven (1140 W). ^{[5, 7]}Since then microwave digestion methods have been developed for extraction of different sample types such as plants, biological, environmental, geological and metallic matrices.

The basics of microwave energy:

Microwaves are non-ionizing electromagnetic waves of frequency between 300 MHz to 300 GHz or between wavelengths of 1 cm and 1m. They are positioned between the X- ray and infrared rays in the electromagnetic spectrum. Microwaves are made up of two oscillating perpendicular field’s i.e. electric field and magnetic field and the former is responsible for heating. Unlike conventional heating which depends on conduction – convection phenomenon with eventually much of the heat energy being lost to the environment, in case of MAE, heating occurs in a targeted and selective manner with practically no heat being lost to the environment as the heating occurs in a closed system. ^[8]This unique heating mechanism can significantly reduce the extraction time (usually less than 30 min) as compared to Soxhlet.

The principle of heating using microwave is based upon its direct impact with polar materials/solvents. It is governed by two phenomena which are ionic conduction and dipole rotation. Mostly these occur simultaneously. Ionic conduction refers to the electrophoretic migration of ions under the influence of the changing electric field. The resistance offered by the solution to the migration of ions generates friction, which eventually heats up the solution. Dipole rotation means realignment of the dipoles of the molecule with the rapidly changing electric field. ^{[7, 8]}

Heating is affected only at a frequency of 2450 MHz and hence this is the most commonly used frequency for commercial microwave instruments. The energy output associated with it is of 600- 700Watts. The electric component of the wave changes 4.9 × 104 times per second thus creating an intense heat that can escalate as quickly as several degrees per second (estimated as 100C/s at 4.9 GHz). ^{[5, 8]}The solvent molecules try to get aligned with the electric field to be in the same phase, but with the electrical component of the wave changing at such a rapid speed, the molecules fail to do so and start vibrating thus generating heat through frictional force. At frequencies greater than 2450 MHz the electrical component changes at a much higher speed. As a result the molecules do not get sufficient time to get aligned with the external field and in this case no heat is generated. At frequencies lesser than 2450 MHz the electrical component changes at a much lower speed and the molecules get sufficient time to get aligned thus there occurs no heating.

The above mechanisms clearly indicate that only dielectric material or solvents with permanent dipoles get heated up under microwave. The efficiency with which different solvents heat up under microwave depends on the dissipation factor (tan δ), which indeed is the measure of the ability of the solvent to absorb microwave energy and pass it on as heat to the surrounding molecules. ^[8] The dissipation factor is given by the equation: tan δ = ε’’ / ε’, where ε’’ is the dielectric loss which indicates the efficiency of converting microwave energy into heat. ε’ is the dielectric constant which is the measure of the ability of absorbing microwave energy.

The basics of microwave assisted extraction:

Plant cells contain minute microscopic traces of moisture that serve as the target for microwave heating. The evaporation of moisture present inside the plant cell on heating due to microwave energy generates a tremendous pressure on the cell wall due to swelling of the plant cell. The pressure pushes the cell wall from inside, stretching and ultimately rupturing it. This facilitates leaching out of the active constituents from the ruptured cells. This phenomenon can be even more intensified if the plant matrix is impregnated with solvents with higher heating efficiency under microwave (higher tan δ value). Higher temperatures attained by microwave radiation can hydrolyse ether linkages of cellulose, which is the main constituent of plant cell wall, and can convert it into soluble fractions within 1 to 2 min ^[5]and this in turn helps solvent molecules to easily reach the phytoconstituents inside the cell.

Microwave assisted extraction offers the advantage that it leads to the disruption of weak hydrogen bounds that are promoted by the dipole rotation of the molecules. Solvents generally used include a wide range of polarities, from heptane to water. The sample may be extracted using a single solvent or mixture of solvents that absorb microwave energy strongly. During extraction, the solvent volume must be sufficient to ensure that the solid matrix is entirely immersed. Generally, a higher ratio of solvent volume to solid matrix mass in conventional extraction techniques can increase the recovery.^[5]However in some cases only selective heating of sample matrix is brought about by immersing the sample in a microwave transparent solvent (hexane, chloroform). This approach is particularly useful for thermo labile components to prevent their degradation. Sometimes the matrix itself interacts with microwaves while the surrounding solvent possesses a low dielectric constant which is advantageous for thermo-sensitive compounds and hence, useful for extraction of essential oils. Solvent free MAE (SFMAE) has been designed, where the moisture content within the plant matrix itself serves extraction and no solvent are used. ^[5]

Instrumentation for MAE:

MAE can be used in two different modes that are closed vessel operation under controlled (elevated) pressure and temperature and another is open vessel operation performed at atmospheric pressure. These technologies are named as pressurized microwave assisted extraction (PMAE) and focused microwave assisted extraction (FMAE), respectively.^{[5, 8]}Closed vessel systems are generally advised for digestions or acid mineralisation or for extractions under drastic conditions, since the solvents may be heated to 100°C above their atmospheric boiling point. Both extraction speed and efficiency are enhanced in this procedure. The closed vessel system is most suitable for volatile compounds. Open cells are quartz vessels topped by a vapour condenser. The system works at atmospheric pressure, and the maximum temperature is determined by the boiling point of the solvent used. The solvent is heated and refluxed through the sample, and in this case the microwaves are focused on the sample placed into the vessel allowing homogeneous and very efficient heating. The sample to be extracted can be placed into a Soxhlet-type cellulose cartridge in order to avoid filtration steps, or may be directly dipped into the solvent. Open cells offer increased safety in sample handling and they allow larger samples to be extracted.

Both the above-mentioned systems are available as multi-mode and single-mode or focused systems. ^[8]A multimode system allows random dispersion of microwave radiation within the microwave cavity, so every zone in the cavity and sample it contains is evenly irradiated. Single mode or focused systems allows focused microwave radiation on a restricted zone where the sample is subjected to a much stronger electric field than in the previous case. Even a modified multimode domestic microwave oven operates as an open vessel extraction system. Principle elements of a microwave device comprise four major components:^{[8, 9]}

(a) Microwave generator: magnetron, which generates microwave energy using alternating current from domestic power lines at frequency of 60 Hz (this is then stepped up to 2450 million Hz by a transformer). The magnetron operates at 4000 to 6000 volts.

(b) Wave guide: this is used to propagate the microwave from the source to the microwave cavity that holds sample to be heated.

(d) Circulator: this allows the microwave to move only in the forward direction.

However, the applicator in case of multi- mode system can be a closed cavity inside which microwaves are randomly dispersed. Uniform distribution of microwave energy inside the cavity can be achieved by using beam reflectors or turntable that makes heating of the sample independent of the position. In focused microwave systems, the extraction vessel is however kept directly in a microwave waveguide and that acts as the applicator. The bottom few inches of the vessel are directly exposed to the microwaves, whereas the upper region of the vessel remains cool as glass is transparent to microwaves and hence does not get heated up in the process. This results in an effective condensing mechanism inherent in the design.

Microwave ovens can have mono-mode or multimode cavity. The mono-mode cavity can generate a frequency, which excites only one mode of resonance. The sample can be placed on the maximum of the electrical field as the distribution of the field is known. The multi-mode cavity is large and the incident wave is able to affect several modes of resonance. This superimposition of modes allows the homogenization of field. Homogenization systems such as rotating plate are added.

Following are the examples of commercially available microwave extraction models: ^[9]

a) CEM solvent extractor

b) Microwave assisted extractor

c) Microwave reflux

d) Sub-500 W microwave extractor

e) Drydist model of milestone

f) Solvent free extractor

g) Monolithic equipment for microwave assisted extraction

h)Closed vessel mono-model of CEM Co

Advantages of MAE:

The main advantages of microwave assisted extraction over the conventional extraction techniques are that it reduces solvent consumption (10mL versus 250 mL for Soxhlet), it has a shorter operational time (15 to 30 minutes), and it possesses moderately high recoveries, has a good reproducibility and requires minimal sample manipulation for extraction process. It also provides a high sample throughput (8 to 16 samples in one extraction run), exact reaction control by temperature and pressure sensors, and depicts the possibility of automation and documentation for GLP requirements.^{[8, 9]}The volumetric heating or heating of the bulk as opposed to transferring heat from the surface, inwards, is more efficient, uniform and less prone to overkill or supererogation. In processing applications, the ability to instantaneously shut the heat source makes enormous difference to the product quality and hence the production economics. The very nature of heating through the involvement of the raw material under processing brings about quality consistency as well as positive environmental impact. MAE is a viable candidate for performing extractions due to its applicability over a wide range of sample types because the selectivity can be easily manipulated by altering solvent polarities.

Disadvantages of MAE:

Compared to supercritical fluid extraction (SFE), an additional filtration or centrifugation is necessary to remove the solid residue during MAE. Furthermore, the efficiency of microwaves can be very poor when either the target compounds or the solvents are non-polar, or when they are volatile. ^[9] Commercial scale-up poses certain limitations but these can be technically overcome with technological development.

Applications of MAE:

Following are some examples of reported applications of MAE.

· Nutraceutical products from plant sources

· Numerous biologically active such as taxanes, azadiractine related limonoids, glycyrrhizic acid, tanshinones, artemisinin^[9]

· anti-oxidative phenolic compounds

Following are some examples of commonly used herbs in cosmetics. These have also been mentioned in the Ayurveda: ^[10]

· Indigo (as bindi/tika)

· Madder Root (to beautify lips and cheeks)

· Hibiscus Rosa Cynensis/Jaswand/Shoe Flower(to blacken and maintain hair colour)

· Raktachandan (as bindi/tika)

· Aloe Vera(to prevent aging and regeneration of new cells)

· Chandan and Vertiver/Usheer(as scrubs and face packs to remove dead cells, regeneration of new cells)

· Haldi/Turmeric (as a face pack and also as an antiseptic)

Following are some examples of phytoconstituents that are used in herbal cosmetics. It is suggested that these can be effectively extracted using MAE:

· Retinoic Acid (naturally occurring form of Vitamin A)

· Alpha hydroxy acid: ( Examples: glycolic acid and lactic acid)

· Aleuritic acid

· Baswellic acid

· Vitamins (Vitamin C, Vitamin E):

· Co-Enzymes Q-10 (Ubiqinone)

· Cosmarinic acid

· Ursonic acid

· Curcumin

· Resveratrol

· Tea polyphenols

· Silymarin

· Quercetin

· Luteolin

· Catechins

· Ascorbic acid

· Genistein

· Noedihydroguaiaretic acid

· Carnosic acid

· beta-carotenoids

· alpha-tocopherol

· Cafeic acid

· Ferulic acid

· beta-santalol

· Allicin

· Arganyllipofrutyl

· Argantensyl

Following are some examples of phytoconstituents that are used in herbal cosmetics and have been reported to be extracted using MAE: ^[7]

· Polyphenols from

· Green tea carotenoids from Paprika powders

· Glycyrrhizic acid from liquorice root

· Carnosic acid from Rosemary

· Canola oil from Canola

· Oil from evening primrose and borage seeds

· Oil from olive seeds

· Lipids from several oleaginous seeds

· Essential oil from Mint Leaves

· Essential oil from Cuminumcyminumand Zanthoxylumbungeanum

· Essential oil from Lippia alba

· Pigments from Capsicum annum

· Aroma extraction

The application of MAE for herbal cosmetics:

Herbal cosmetics can be defined as those beauty products which possess the ability to produce desirable effects such as healing, smoothening, rejuvenating, enhancing and conditioning because of herbal ingredients i.e. phytoconstituents. These active ingredients serve many purposes including increase in skin elasticity, delay in skin ageing by reducing wrinkles, protection against UV radiation by antioxidant property and checking degradation of collagen, etc. Cosmetics may be categorized into the following classes: cosmetics for enhancing the appearance of facial skin, cosmetics for hair growth and care, cosmetics for skin care, shampoos, soaps, powders and perfumery, etc.^[10] Herbs can be used in cosmetics in a number of ways such as infusion, decoctions, extract, tincture, flower water, oil soluble extract, etc.

Herbal extracts and intermediate products of cosmetics are better prepared using MAE. Herbal cosmetics thus prepared are found to be better in terms of quality.

CONCLUSION:

Many herbs have been scientifically evaluated for their cosmetic potential and for the most suitable method for extraction, purification and characterisation of cosmetically beneficial phytoconstituents. However, there is a lack of scientific review of phytoconstituents that are used in cosmetic preparations. The most preferred and efficient method of extraction for various plant materials must be identified. Also, the rising number of Green engineering regulations calls for more efficient energy usage and more environment friendly raw materials as well as effluents. Electric heating technologies such as radio frequency, microwave, ohmic and infrared are fast emerging, among them microwave shows a highly promising future. Microwave energy for heating has been in commercial use since 1950.But it is only recently that its benefits as an environmentally friendly source of thermal energy have been widely appreciated. ^[9]

Certain shortcomings of herbal products overshadow their advantages. The case of herbal cosmetics is similar. Quality and purity concerns are sensed. Each ingredient in the herbal cosmetics has an array of chemical constituents with complex molecular formulae. This requires a qualitative finger-printing tool so as to assure safety, purity and quality of the final product. The herbal extracts used in cosmetic preparations must be prepared using a rather selective, reproducible and analytically answerable technique likes MAE.

Microwave assisted extraction (MAE) techniques have shown immense potential and advantages over conventional extraction techniques. Yet it is not fully utilised to its capacity. The extraction of phytoconstituents by microwave provides a vast scope of research exploration and a myriad of hidden benefits of MAE may be discovered. Moreover, the adherence to regulatory and safety aspects of herbal cosmetic preparations which are often overlooked or paid very little attention to, can be assured by the use of MAE due to the attributes of like selectivity, reproducibility, higher levels of purity and quality it offers. Hence MAE’s application in herbal cosmetic industry should be explored.

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Received on 30.01.2013 Accepted on 12.03.2013

Res. J. Topical and Cosmetic Sci. 4(1): July –Dec. 2013 page 54-58