Over the past decade the demand for more natural food additives from consumers has resulted in an increase in the use of natural flavourings. However, this has not been reflected nearly so much in fragrances or fragrance formulations for personal care products.
Unlike food flavourings, there is no direct legislation that defines natural flavourings and processes that can be used in the preparation of natural fragrance molecules; instead legislation has been limited to restricting certain molecules that may cause adverse reactions. Guidelines, such as the recent IFRA guidance to green fragrance terminology, defines “Green”, “Green Chemistry” and “Natural” and this guidance broadly reflects EPA guidance on green chemistry and the EU food flavouring definition of natural. Market pull for the development of more natural fragrance formulations or greater use of natural extracts has been slower than that seen for food ingredients and so drivers for greener technologies have been focused on improving yields, reducing cost, minimising waste generation or providing new technologies that can deliver opportunities to produce new fragrance molecules.
Greener fragrance formulations are not just about using a greater proportion of natural and sustainable materials but also the processes used to obtain them. These processes include extraction, fractionation and conversion process, all of which can be made more efficient and cleaner with less waste generated and energy used. The commercial drivers for greater adoption of green chemistry can be divided into two groups, those that result from the growing demand for natural products and those that drive the adoption of greener solutions:
“Natural” drivers
• Consumer pressure.
• Market differentiation.
• Corporate “Green” credentials.
Green chemistry drivers
• Cost – manufacturing and waste disposal costs.
• Legislation – product, process and environmental legislation.
• Process efficiency.
• Unique routes to natural molecules.
• Functionality.
• Sustainability.
In offering solutions that can address these, the guiding principles of green chemistry proposed by Anastas in 19981 have been used to develop alternative approaches both to the extraction of natural fragrance extracts and the synthesis of fragrance molecules.
Greener extraction solvents
A wide range of natural fragrance extracts are already available including essential oils, herb, spice and flower extracts together with fragrance molecules fractionated from essential oils or botanical extracts. The technologies used to produce these traditional products are well-established but often use methods that are energy intensive or solvents that are produced from fossil fuels. Some of these solvents also need to be tightly controlled with respect to residual levels in the extracts and almost all of them raise health and safety concerns. To take this one step further and produce fragrance extracts that can be considered for organic certification2 limits the choice of process solvent still further, but the solvents that are permitted can be considered both as green and sustainable. Fragrance extracts are mostly extracted at source using a range of organic solvents such as alkanes, ethers and alcohols. In applying the principles of green chemistry clearly, water, carbon dioxide and ethanol represent both safer solvents and the use of a renewable feedstock as well as having the widest acceptability and range of solvent polarity. Combining these, such as the use of supercritical CO2 with ethanol as a co-solvent allows solvent polarity to be refined still further. Carbon dioxide can be used as an extraction solvent both as liquid carbon dioxide or supercritical carbon dioxide.3,4 Supercritical carbon dioxide is now well-established as a solvent for use in extraction for a number of reasons. It can generally penetrate a solid sample faster than liquid solvents because of its high diffusion rates, and can rapidly transport dissolved solutes from the sample matrix because of its low viscosity. Supercritical carbon dioxide is also a highly tuneable solvent capable of selective extraction of fragrance molecules from a wide range of botanical materials and is already in large scale commercial use.5 Many commercial fragrance raw materials have already been extracted using liquid or supercritical CO2 and their composition published together with the comparative composition with traditional methods of extraction described. Extraction with carbon dioxide produces an extract that will vary in composition according to the temperature and pressure used and will not produce a product identical to either hydro-distillation or to conventional solvent extraction. The ability to vary solubility as a result of changing temperature and pressure can also be used to produce products with specific composition. For example, extraction of black pepper and ginger can be carried out in such a way that fractions rich in essential oils and devoid of the pungent principles can be produced.6 This can be achieved either by total extraction at high pressure and temperature followed by reduction in pressure in sequential separators or by sequential increase in the extraction temperature and pressure. In a commercial operation, the option of extraction at a single pressure with multiple separators to recover fractions is the preferred route as this is operationally simpler and more cost effective. Water is already used as a solvent in as much as the production of essential oils by steam distillation uses water in the vapour phase to volatilise the oils. Although this process is entirely natural, the energy requirement to carry out distillation for up to three hours is significant and contributes to the production of greenhouse gases and, in most cases, the use of fossil fuels. An alternative approach to using water as an extraction solvent is to extract the botanical material with subcritical water. Sub-critical water offers an alternative solvent at the opposite end of the polarity spectrum to supercritical CO2 and can therefore be seen as a complementary technique.7 Extraction using sub-critical water does not require overly-complicated equipment as shown in Figure 2, however the recovery of the essential oil from the water needs to be considered. In almost all laboratory trials the essential oil is recovered either by secondary extraction with a solvent, such as hexane, or by absorption using an in-line trap. Using hexane would be undesirable from a green chemistry perspective and the cost of solid absorbents would be prohibitive on an industrial scale. Where the yield of essential oil is relatively high, physical separation or the use of a small re-boiler should be a practical and economic solution. The greenest option would be to use the aromatic extract directly and this should be possible in fragrance formulations. Extraction of essential oils from a wide range of aromatic plants and herbs, including foliage, seeds, roots and flowers has been carried out using sub-critical water and high recovery of the oils has been demonstrated. Extraction of the essential oil from marjoram leaf (Thymus mastichina)8 and coriander seed (Coriandrum sativum)9 using either hydrodistillation or subcritical water at 150°C/ 5 mPa has demonstrated that a higher yield is obtained and the extraction can be carried out in fifteen minutes using subcritical water compared with three hours for hydro-distillation. The energy required to raise the water to 150°C/5 mPa was twenty times less than that required for the hydro-distillation. In both cases, the levels of oxygenated compounds were higher in the extracts prepared using subcritical water. Unlike supercritical carbon dioxide extraction, there are so far no large-scale commercial plants that are using subcritical water to extract fragrance molecules and this is due largely to the high capital cost of the equipment required when compared with traditional steam distillation, much of which is already in place. However as can be seen from Table 1, subcritical water extraction does have a number of advantages over supercritical CO2, most notably the ability to extract green materials without drying and the lower operating costs. Greater use of both of these extraction techniques will only emerge slowly and will be driven by higher prices of conventional solvents, tighter legislation on solvent residues and consumer pressure to replace the use of solvents.
Clean synthesis of fragrance molecules
Most fragrance formulations rely on a combination of natural extracts and individual fragrance molecules to create the desired aroma. Some of these fragrance molecules can be fractionated from natural extracts and more are increasingly being produced by fermentation.10 A large number of fragrance molecules are still produced by conventional chemical synthesis which is often wasteful and inefficient. Within the IFRA guidelines, processes that can be considered as natural are defined and the most useful can be summarised as follows:
• Distillation and rectification.
• Extraction.
• Fermentation, microbiological and enzymatic processes.
• Working within these processes and mindful of the principles of green chemistry, alternative processes can be developed using the options described in Figure 3.
It is an important aspect of adopting greener processes that the purification of products that arise from primary processing is kept to a minimum rather than expending energy and resources to obtain materials of a purity that is not necessary for the application in which they are being used. For example a mixture of esters produced by fermentation could be used as a fragrance ingredient without further purification rather than attempting to isolate individual molecules. Any process that leads to the production of a natural flavouring ingredient needs natural precursors and these can be isolated from crops, waste streams that arise from crops or by fermentation. Other precursors can be obtained by enzymatic hydrolysis of macromolecules or by controlled pyrolysis. The interaction between all of these processes is shown in Figure 4. Although fermentation processes can produce a wide range of flavour molecules, yields are often low and therefore fermenter volume and cost is correspondingly high. An alternative approach is to use a solvent such as supercritical carbon dioxide in which the flavour molecules will be much more soluble4 and to carry out biocatalysis in this medium. Enzymes are generally more stable in supercritical carbon dioxide than they are in water and can catalyse a wide range of reactions that are useful in the synthesis of flavour molecules.11 High conversion rates can be achieved and recovery of both product and enzyme is straightforward. Examples of ester production,12 including chiral esters,13 kinetic resolution of secondary alcohols,14 lactones15 and enzyme catalysed oxidations16 have all been carried out in supercritical carbon dioxide. The ability to manipulate the pressure and temperature of supercritical carbon dioxide has been shown to have a significant influence on the enantioselectivity of enzymes17,18 and this has been used to achieve high yields of homochiral alcohols. Biocatalysts are not the only natural catalysts that have the potential to be used to produce fragrance ingredients, mesoporous starch based catalysts have been developed19 that can catalyse esterification of dicarboxylic acids in aqueous solutions and this has the potential to be used in combination with fermentation technologies.
Conclusion
To effectively achieve more green and sustainable fragrance ingredients and a greener approach to their production it is desirable to use only natural and renewable sources of raw material, extract flavour active compounds using permitted and preferably green solvent options and to use low energy physical processes in parallel with microbial biotransformation and biocatalysis in aqueous media or organic solvents such as supercritical carbon dioxide. The continued growth in the market for green and sustainable ingredients and consumer and corporate pressure to adopt greener process technologies will require the development and commercialisation of alternative greener approaches to the production of fragrance ingredients. The traditional approach is often inefficient, energy intensive and uses solvents that are becoming undesirable, but the investment that has been made in these existing technologies presents a considerable barrier to the adoption of cleaner and greener processes in the short and medium term.
References
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