Traditional solvent-based extraction techniques

The process of solvent-based extraction involves the use of a liquid to selectively dissolve and remove the soluble fraction (solute) from a permeable, insoluble solid matrix, with probably one of the best known examples of solid-liquid extraction applied in the food industry being the production of fixed oils (vegetable oils) from oleaginous plants (Takeuchi et al., 2009). Traditional methods of solvent-based extraction like maceration with alcohol, Soxhlet extraction and distillation rely on the extracting capacity of various solvents and the application of heat and/or mixing. Some solvent extraction methods have specific terminology: decoction refers to extraction with the solvent at its boiling point, lixiviation is used when the target compounds are alkalis, and elution is used when the soluble solids are at the surface of the plant matrix (Takeuchi et al., 2009).

The use of these traditional extraction techniques is sometimes associated with some disadvantages like the use of harsh chemicals, overheating of the plant material sample and inactivation of important compounds, high energy, time and solvent consumption, and poor

64 extraction selectivity. Furthermore, the choice of chemical solvent and process parameters can affect the efficiency of the extraction technique to a great degree (Azmir et al., 2013). Due to the differences in the polarities of the various chemical solvents available, it is expected that the solubility and extraction efficiency of individual compounds would vary with the use of different solvents or combinations of solvents (Liu et al., 2016). Ethanol, methanol and water are polar protic solvents with dielectric constants of 24, 33 and 80, respectively, whereas ethyl acetate and acetone are non-polar and polar aprotic solvents of dielectric constants 6 and 21, respectively (Wang et al., 2011). The relative extraction efficiencies of these solvents for the recovery of phenolics from pomegranate peel are shown in Fig. 2.16. The higher the dielectric constant of the solvent, i.e. the more polar the solvent, the higher were the yields of total phenolics, proanthocyanidins and flavonoids.

Figure 2.16 The effect of various solvents on the yield of phenolic compounds from ground pomegranate

(Punica granatum L.) peels extracted at 40 °C, solvent-to-solid ratio of 15:1 (v.m-1_{) and 40 mesh particle size;}

total phenolics proanthocyanidins flavonoids. (Wang et al, 2011).

The rate of extraction of plant constituents is, to a large extent, affected by the mass transfer rate and the equilibrium state, but the Soxhlet method improves the mass transfer rate and displaces the transfer equilibrium by continuously exposing the solid matrix to an influx of fresh solvent (Wang & Weller, 2006).

Hot water extraction (HWE) refers to the sole use of H2O as solvent, and is thus less

effective at extracting non-polar compounds. For non-polar target compounds, co-solvents like ethanol or a chemical modifier are often added to increase the extraction capacity (Azmir et al., 2013). The polar nature of water enables its use as a solvent for natural water-soluble products like organic acids, sugars, proteins as well as inorganic materials (Chemat et al., 2012).

65 Another specialised type of solvent-based extraction technique, in which carbon dioxide (CO2) is commonly used, is supercritical fluid extraction (SFE), in which organic solvents are used at

temperatures and pressures above their critical points, and pressurised gas is used to collect the extract. Carbon dioxide (CO2) is a commonly used solvent for the extraction of non-polar

molecules due to its food-grade status, widespread availability, relatively low cost and low critical pressure (7.4 MPa) and critical temperature (31.1 °C) (Azmir et al., 2013; Maran et al., 2015). One of the notable disadvantages of CO2 as a solvent is its low polarity, which limits its usefulness mainly

to the extraction of lipids, fats and non-polar substances. The addition of chemical modifiers, e.g. dichloromethane or diethylamine, to enhance the polarity of CO2 has been successfully applied as a

solution to this problem (Lang & Wai, 2001; Azmir et al., 2013).

The phenolic compounds present in plant material may range from simple to highly polymerised. The different types and amounts of phenolic acids, tannins, anthocyanins and phenylpropanoids also differ between plant types, and these may interact with proteins and carbohydrates to form insoluble complexes. The complete recovery of phenolics from plant material is therefore not always possible unless the appropriate solvent and process parameters are used in conjunction with pre-treatment of the sample to promote recovery of the target compounds (Takeuchi et al., 2009). In an industrial setting, the feasibility of an extraction process will rely on the optimal combination of process parameters that would maximise the process efficiency and reduce costs. Water, ethanol, isopropanol and their use in combination have been certified with GRAS (Generally Recognised as Safe) status by the United States Food and Drug Administration, and these solvents are therefore appropriate for the manufacture of nutraceuticals (Wang & Weller, 2006).

Extraction at high temperatures may cause degradation of heat-labile chemical compounds, therefore the concurrent use of elevated operating pressures is beneficial in that it allows the use of lower temperatures to achieve the same extraction efficiency (Wijngaard et al., 2012). Under SFE operating conditions, the solvation capacity is improved due to the solvent remaining in one phase but having the properties of both gas and liquid. SFE is a useful alternative when extracting heat sensitive compounds, but disadvantages may include relatively high cost and long operating times (Shah & Rohit, 2013).

Enzyme-assisted extraction (EAE) involves treating the source material with enzyme preparations (e.g. pectinase or cellulase), which facilitates the release of the target bioactive compounds from the polysaccharide-lignin matrices which they are bound to. It may be prohibitively expensive, however, for processing larger volumes of plant material (Puri et al., 2012). The use of enzymes to enhance the extraction of polyphenols from unripe apples (Malus pumila) was also investigated by Zheng et al. (2009). Experimental results indicated that the total polyphenol content (mg gallic acid equivalents.100 g-1_{) and extraction yield were three and two times higher than in}

66

Chandini et al. (2011) investigated the effect of Aspergillus-derived pectinase and tannase pre- treatments on the quality of aqueous black tea (Camellia sinensis) extracts and found that pectinase improved the yield of total soluble solids by up to 11.5% without a concomitant improvement in the yield of total polyphenols, whereas tannase pre-treatment resulted in an improvement in both the total soluble solids and polyphenol yields (11.1% and 14.3%, respectively). The use of both the enzyme preparations, either simultaneously or in succession, was not as effective as the use of the tannase preparation alone in terms of improving the overall extract quality. In another study (Chen et al., 2011), the EAE of flavonoids from Ginkgo biloba was investigated using Penicillium decumbens cellulase, Trichoderma reesei cellulase and Aspergillus niger pectinase with maltose as the glycosyl donor. P. decumbens cellulase performed significantly better in enhancing the extraction efficiency than the other two enzymes due to its high transglycosylation capacity, which converts flavonol aglycones to more polar and therefore more soluble flavonoid glucosides. These results indicate that the use of carbohydrate-hydrolysing enzyme facilitates the release of polyphenols complexed in cell walls, and therefore may bring about a higher polyphenol yield. Examples of EAE of rooibos documented in literature are covered under section 2.3.6.