Spices are known for their bioactive compounds and have drawn attention because of their nutritional and nutraceutical properties. They are available from different plant parts such as root, stem, leaves, fruits, flowers
etc.
(Teng et al., 2019). and in various forms including whole, roasted, pureed, paste, crushed, powdered and extractives
(Andrews et al., 1995). Compared to other forms of spices, spice extractives have better stability in terms of flavour and storage. The main two categories of extractives include, essential oils and oleoresins. They have been explored for their ability to replace the original ground spices and herbs with a standardized taste and aroma
(Serrano et al., 2020) . In order to recover these extractives, cell matrix of the respective spice has to be disrupted. Generally, dried spices are used for the extraction purposes compared to fresh spices as they have better stability and are more concentrated
(Kurup et al., 2020; Manousi et al., 2019). Dried material is crushed so that surface area is increased and solvent can very well diffuse into the cell matrices
(Saxena et al., 2018). Efficiency of extraction depends upon the extraction time, solvent used, temperature of extraction and solvent-solid ratio
(Zhang et al., 2018).
Asbahani et al., (2015) explained essential oils as aromatic oily liquids traditionally extracted by hydro or steam distillation of spices. They are complex mixtures of low molecular weight compounds and are responsible for the aroma produced
(Raut and Karuppayil, 2014). According to
Bakkali et al., (2008), there are approximately 3000 essential oils and among this around 300 are of commercial importance. Essential oil does not represent a spice completely as they contain only volatile components. Distillation is considered as the conventional method of extraction of essential oils and has limitations like thermal degradation, longer extraction time
etc.
While essential oils are the volatile components in a spice, oleoresins are the non-volatile part. They are highly concentrated forms with wholesome flavour and aroma of the respective spice. Compared to fresh and ground spice, oleoresins are hygienic
(Shaikh et al., 2006). Extraction of oleoresin involves the use of various solvents
(Aziz et al., 2018; Lucas et al., 2020; Pawar et al., 2018) and among those, acetone is the most commonly used one
(Doosthosseini et al., 2019). Solvent remaining in the extract after the extraction is removed by evaporation or distillation
(Salzer and Furia, 2009). Solvent extraction is the conventionally followed method for the extraction of oleoresin. It also has limitations like, longer extraction time, need of costly solvents
etc.
(Azmir et al., 2013; Luque-Garcia and Luque De Castro, 2004;
Sovilj et al., 2011; Stoica et al., 2016).
In order to solve the problems associated with conventional extraction methods, several non-conventional extraction techniques have been introduced. Some of those include, supercritical fluid extraction, ultrasound assisted extraction, enzyme assisted extraction, microwave assisted extraction
etc.
(Farooq et al., 2020; Strati et al., 2015; Wang and Weller, 2006;
Zhang et al., 2018). These methods are safe and efficient separation of compounds can also be ensured. They require less energy cost over the solvents and are less damaging to environment
(Gandhi et al., 2017).
Essential oils and oleoresins are widely used in food application at industrial level. They can be used where whole spice or any other forms of spices cannot be directly added. They are being used in dairy products (like yoghurt, curd, cheese
etc.), confectionary, beverages, bakery products
etc.
(Arimboor et al., 2015; El-Sayed and El-Sayed, 2020;
Melgar-Lalanne et al., 2017). Other than food, essential oils and oleoresins have application in the development of cosmetics, household cleaning and hygiene products, air fresheners, medicinal uses
etc.
This review focuses on the non-conventional extraction techniques used for the extraction of essential oils and oleoresins from various spices.
Historical perspectives of spice extraction
The record of origin of extraction from aromatic plants including herbs and spices has been lost to time. Man discovered the benefits of plants and the methods for separation of extracts very long ago. It was during Middle Ages, the understanding of extraction of aromatic plants spread to European region
(Jacobs, 2020).
Baser and Buchbauer (2019) explained that these extracts had been used in cosmetics by ancient Egyptians during 4500 BC. Mixtures of herbal preparations from various aromatic plants were used to prepare medicines and perfumes
(Elshafie and Camele, 2017) also. According to
Naeem et al., (2018), application of these oils changed with time and later on, it has been used for culinary purposes other than medicinal uses. It was during 4500 BC and 2000 BC the use of aromatic oils was first recorded in traditional Indian and Chinese medicine.
Ali et al., (2015) reviewed that the use of spices such as cumin, peppermint
etc. had been documented by Greek history between 500 BC and 400 BC. With the arrival of Arabs, a technique for essential oil distillation has been established. By the mid of 18
th century, around 100 oils had been invented in Europe. But chemistry of these was not known. Towards the beginning of 19th century, chemists started studying the chemistry of these essential oils, which lead to the increased production all over the world
(Essential Oil, 2019).
Continued search for better extraction techniques ended up in various conventional methods like, hydro distillation, steam distillation, solvent extraction, maceration, percolation
etc. But these methods had several disadvantages, because of which improved techniques of extraction had to be invented. So, there was an enormous interest in searching for a method that give importance to sustainable environment (Fig 1).
Supercritical fluid extraction
The concept of supercritical fluid extraction (SFE) was introduced as a substitution to the conventional extraction techniques such as steam distillation and solvent extraction
(Maharaj et al., 2018) because of its ability to extract the compounds of our interest with mild extraction conditions and minimum loss
(Yousefi et al., 2019). (Williams, 2000) explained that the SFE are of two types; static and dynamic. In dynamic mode, supercritical fluid passes through the column that has sample in it. But in static mode, supercritical fluids are absorbed by the sample and as a result, no fluid pass through the column during extraction.
(Shah et al., 2020; Yan et al., 2017) studied that extraction of a particular compound is dependent on the extraction time, pressure, percentage of co-solvent
etc.
Gandhi et al., (2017) explains SFE as the process that utilizes supercritical fluid to extract the compound of our interest (Fig 2). Supercritical fluids are those having properties similar to both gas and liquid, but cannot be distinguished as either of the two. So, this helps in the solubilization of solutes in the raw material. Density of these fluids are similar to that of liquid and viscosity to gas. This technique utilizes certain solvents at pressure and temperature above their critical point. Critical point can be explained as the point on a phase diagram at which both the liquid and gas are indistinguishable. Solvents used in this are non-hazardous, so the process can be termed as ‘green’. The most commonly used solvent is carbon dioxide as it is easily available, cheap and is GRAS for its use in food industry. Also,
Sanchez Camargo et al. (2020) reviewed that the critical conditions of carbon dioxide (30.9°C and 7.38 MPa) are easy to attain. Another advantage of using supercritical CO
2 as solvent is solvent free product.
i.e., the release of pressure by the system after the completion of extraction will help the CO
2 to convert in to gas and that will leave the sample.
But the major drawback of supercritical carbon dioxide is its low polarity, which can be solved by the use of co-solvents to change the polarity. Other than carbon dioxide, propane, ethanol
etc. can also be used as supercritical fluids for the extraction (Table 1).
Supercritical fluid extraction of essential oil
Saleem et al., (2015) reported that compared to conventional methods such as steam and hydro distillation, cinnamon oil yield was high when extracted by SFE.
Priyanka and Khanam (2018) studied that extraction of turmeric oil using SFE was found to be economical ate industrial level. Studies were carried out in order to investigate the anti-inflammatory effect of marjoram (
Origanum majorana L.) and sweet basil (
Ocimum basilicum L.) essential oil obtained by the supercritical fluid extraction
(Sanchez Camargo et al., 2020). In vitro studies showed that both essential oil had appreciable anti-inflammatory activity
(Arranz et al., 2015). Zekovic et al., (2017) reported that, highest yield of essential oil from coriander was obtained at 200 bar pressure and 55°C. Yield of extraction was high when ethanol was used as co-solvent in the SFE of
Piper nigrum by
(Nagavekar and Singhal, 2018).
Supercritical fluid extraction of oleoresin
Devani et al., (2020) studied the supercritical fluid extraction of rotten onion oleoresin using carbon dioxide as the solvent. Optimized parameters were 80°C temperature and 400 bar pressures. Study also helped in developing a method that will reduce the agricultural waste. Another study conducted by
Dutta and Bhattacharjee (2015) in black pepper was by varying the temperature and pressure of the extraction process. Different combinations of temperature (40, 50 and 60°C) and pressure (200, 300 and 500 bar) were experimented and concluded that yield was high at 60°C and 300 bar pressure.
Fitriady et al., (2017) also carried out experiments to optimize the temperature and pressure to get maximum yield by the SFE of ginger by using CO2. According to their study, it was at 40°C temperature and 4500 psi pressure yield was high. A method by coupling SFE by fractionation was developed by
Shukla et al., (2019) to get maximum gingerol in the oleoresin and essential oil extracted. It resulted in 5.95% oleoresin yield which is 96.15% pure and 2.71% volatile oil which is 95.94% pure.
Nagavekar and Singhal, (2019) experimented on
Curcuma longa and
Curcuma amada to study the optimization of extraction conditions of oleoresin by SFE. In the case of
Curcuma longa, maximum extraction was attained at 65°C, 150 mins and 350 bar. But for
Curcuma amada, it was at 40°C, 30 mins and 300 bar pressures. The study also reported that, addition of ethanol as co-solvent improved the extraction yield in both the varieties. SFE has been well explored for the extraction of bioactives from spices using various solvents
(De Melo et al., 2014;
Herrero et al., 2013) (Table 2).
Supercritical fluid extraction, which is a separation technique to extract bio-active components is a green extraction technology that meets all the regulations related to health and environment. Also, it has gained attention in many industries like, pharma, cosmetics, food
etc. So, this can be considered as a promising technology for the coming years.
Microwave assisted extraction of essential oil
Microwaves are non-ionizing electromagnetic waves with a frequency ranging from 300 MHz to 300 GHz and will help in improving the extraction process
(Martinez et al., 2019; Shams et al., 2015). According to
Chaturvedi (2018), microwave assisted extraction can be used to extract the nutraceuticals such essential oils, oleoresins, dietary supplements
etc. (Fig 3). Pre-treatment methods that helps in improving the extraction efficiency include, milling of the material, enzymatic pre-treatment
etc. milling is to increase the surface area and enzymatic treatment is to break the cell wall
(Vinatoru et al., 2017).
There are mainly two categories of instruments used in the microwave-assisted extraction. First one is the multimode microwave ovens that make use of closed extraction. Here, extraction occurs by controlled pressure and temperature. Second one is the focused microwave ovens in which only the part of the vessel that includes sample will be focused for irradiation with microwave
(Chaturvedi, 2018;
Manousi et al., 2019). (Chan et al., 2011) listed the factors that depends upon the yield of extraction include, solvent to plant material ratio, nature of solvent, temperature, time taken for extraction
etc.
(Arya and Kumar, 2021;
Dahmoune et al., 2015; Kaderides et al., 2019) reported that microwave assisted extraction requires lesser time. According to
Mustapa et al., (2015), use of less solvent and consumption of less energy are the major advantages of microwave assisted extraction and also it helps in reducing the thermal degradation of the extracted compounds.
Bayramoglu et al., (2008) introduced a novel microwave extraction technology without using the solvent for the extraction. They studied the extraction of essential oil from oregano (
Origanum vulgare L.) by hydro distillation and microwave extraction and concluded that there was no much difference in the composition of oil obtained from these two methods.
Extraction is carried out by combining microwave heating with dry distillation under atmospheric pressure. If the sample used is fresh, that can be directly taken for extraction. Otherwise, it has to be rehydrated and excess water should be drained off. An efficient microwave assisted extraction technique was developed by
Bener et al., (2016) for the fast extraction of curcumin from turmeric by using methanol as solvent. Similar study was conducted by
Mazzara et al., (2021) on ajowan seeds and reported that the thymol content was maximized by the use of microwave assisted extraction with reduced extraction time. In situ microwave assisted extraction of clove bud carried out by
Gonzalez-Rivera et al. (2021) explained that it is a green and economical approach that can be applied to wide range substance like, spices, herbs, seeds
etc.
Enzyme assisted extraction
According to
Cheng et al., (2015), enzyme assisted extraction is the process by which compounds of interest are extracted by the hydrolysis of cell wall. It is based on the catalytic property of enzyme to bind the cellular matrix (Fig 4). Solvent used for extraction can either be an organic solvent or water. According to
Schweiggert et al., (2008), enzymatic hydrolysis of plant material is being widely used for the extraction.
Advantage of this technique include, improved yield of extracts, reduced time, mild processing conditions
etc.
(Wijesinghe and Jeon, 2012).
Das et al., (2021) reported that, enzyme assisted extracted is purely dependent on the morphology and biochemical aspects of the plant material used for extraction and also the physio-chemical properties of the compounds to be extracted. In depth knowledge regarding these factors will aid in the right selection of enzyme. Enzymes are proteins with specialized catalytic functions and are produced by all organisms and are essential for all the metabolic activities. Prior to conventional methods of extraction, these enzymes are used for the pre-treatment of the plant material used for extraction by which the yield can be improved
(Baby and Ranganathan, 2013).
Enzyme assisted extraction of essential oil
According to the study conducted by
Sowbhagya et al., (2010) the yield of volatile oil from celery seeds was higher when it was pre-treated with enzymes and extracted oil did not have any physical or chemical changes. Similar study was carried out by
Sowbhagya et al., (2011) on the extraction of cumin oil using enzyme assisted extraction. Study revealed that the pre-treatment of cumin seeds with enzymes led to improved yield of oil compared to the conventional methods. Evaluation of volatile oil indicated that compounds such as a-pinene, b-pinene and r-cymene was found to be slightly changed, but no change was observed in cuminaldehyde content. Enzyme pre-treatment of garlic also resulted in the improved yield of garlic oil and this was explained by
Sowbhagya et al., (2009). Enzymes used include cellulase, pectinase, protease
etc.
Enzyme assisted extraction of oleoresin
A novel technology for the production of paste-like and spray-dried ginger condiment was developed by the enzyme assisted liquefaction of ginger rhizome by
Schweiggert et al., (2008). Varakumar et al., (2017) studied on the enzyme assisted three phase partitioning of oleoresin from ginger rhizome and reported that the yield of oleoresin was higher compared to conventional solvent extraction. Three phase partitioning is basically used for protein separation and recently has been applied for the oleoresin extraction also. The three phase consisted of water, ammonium sulphate and
t-butanol.
Kurmudle et al., (2011) studied on the enzyme assisted three phase partitioning for the extraction of oleoresin from turmeric. Ammonium sulphate and
t-butanol was added to the slurry of oleoresin pre-treated with enzymes. Study revealed that the yield was improved and the time taken for extraction was lower compared to the conventional acetone extraction.
All these studies show that, the use of enzymes at optimum concentration will result in the enhancement of yield of essential oils and oleoresins. Because of which, enzyme assisted extraction can be considered as a better extraction technique compared to the conventional methods.
Ultrasound assisted extraction
Ultrasound assisted extraction is considered as a green technology and has been proved to be efficient as it uses fewer toxic chemicals. It provides wide range in the selection of solvent as toxic ones can be replaced by GRAS solvents
(Rao et al., 2021). Solvent selection is another factor that is of higher importance when it comes to extraction and it depends upon the solubility and polarity of the compounds to be extracted.
Ultrasound waves can be successfully used for the extraction of oleoresin from various spices
(Melgar-Lalanne et al., 2017;
Morsy, 2016;
Muhammad et al., 2021). According to the study conducted by
Li et al., (2005), ultrasound can cause cell disruption and particle size reduction resulting in a larger contact area between the solid and liquid phases. In comparison to conventional procedures, this resulted in improved solvent access within the substance to be extracted (Fig 5).
Arya and Kumar (2021) reported that use of ultrasound assisted extraction resulted in improved extraction yield as it uses lower temperature and short time. Ultrasonic effects are induced within a material with the help of intense pressure and temperature gradient generated by high intensity shock waves. This will lead to the solvation of chemical constituents in the material used for extraction.
Ultrasound assisted extraction of oleoresin
Morsy, (2016) studied the difference between conventional maceration with ultrasound assisted extraction of oleoresin from nutmeg. GC/MS analysis indicated a great difference between the two oleoresins obtained and oleoresin obtained by maceration had less amount of myristicin compared to the other. Another study by
Supardan et al., (2012) reported that ultrasound assisted extraction can be used to obtain oleoresin from ginger and GC analysis of the same indicated that there was no difference in the composition of oleoresin.
Martinez et al., (2019) compared Soxhlet extraction with ultrasound assisted extraction and explained that the latter required shorter time and yield was quite higher. Thus it helps in fast-tracking the targeted extraction.