The major prerequisite of the process used for production of essential oils is that the product obtained must resemble the natural aroma and fl avor of the original source, which is a combination of different com- pounds of varying organoleptic characteristics. Oxygenated organic com- pounds like aldehydes, ketones, alcohols and esters are the dominant con- tributors to the overall aroma and fl avor. The essential oil produced should ideally have all these components in the same proportions as in the original natural product in order to match the natural aroma and fl avor. For example, steam-distilled rose oil contains less than 1 wt% phenyl ethyl alcohol (PEA) whereas the solvent-extracted rose oil contains greater than 60 wt% PEA.
It is a common experience that the steam distillation condensate has an
odor similar to that of the oil. Thus, this condensate has some value. The sale of rose water (otto of rose) for use in weddings in the sub-continent is probably the only way by which the distiller realizes the value of the conden- sate. In practically all other cases, the condensate is wasted. Table 2 gives estimates of the values of wasted oil in the condensate in India for some essential oils. The estimate is conservative because it does not include oil physically carried with the condensate. Even this conservative estimate is a mind-boggling number and is particularly important in the social context of developing countries like India where marginal farmers are main contributors to the overall produce. The value of the recovered oil from central distilla- tion facilities and pro rata distribution of the value will be a big bonus to the small farmer and can certainly stop the downslide. Table 2 shows a major contribution from Mentha arvensis. If the relatively high value fl avor sector is included, for India alone the value of wasted oil can easily reach US$ 100 million. On a rough estimate, the combined number for the South East Asian countries can be upwards of US$ 160 million. It must be noted that these numbers are simple statistics. They do not refl ect in any way the high value that can be gained when the recovered oil is blended with the main distilled oil fraction to obtain a premium grade of the respective essential oil.
Table 2: Loss of essential oils in distillation condensate water. Production fi gures are for India only
Essential oil
Production, 2006, million tonnes
Unit price, 2006
Volume, 2007
Oil lost in water, kg*
Value of oil lost in water,
$/yr
Arvensis 28,000 $ 14/kg 35,000 MT 5.6-7x106 46-58x106
Basil 100 $ 8/kg 100 MT 1x104 8.2x104
Citrodora 100 $ 8/kg 100 MT 1x104 7.4x104
Citronella 300 $ 8/kg 300 MT 3x104 2.52x105
Peppermint 450 $ 23/kg 450 MT 4.5x104 1.3x105
Spearmint 250 $ 23/kg 300 MT 2.5x104 7.2x104
Total 29,200 36,200 MT 5.72-7.72x106 47-59x106
* Data refer to 100 kg water per kilogram oil. Solubility of oil in water is 1000 ppm.
8.6.1 Polymeric Adsorption Process
Various techniques, such as cohobation, poroplast extraction and adsorption, which can be used to recover the dissolved substances, have been discussed in the literature. Polymeric adsorbents can be advan- tageously used to recover dissolved essential oil components. Several in- vestigators have established the utility of adsorption in this context beyond doubt. One study showed that although cis-rose oxide could not be detected in the condensate, this valuable component was found in the recovered oil
in signifi cant proportions. These investigations show that more than 95% of the oil in the condensate can be recovered. The polymeric adsorbents used are hard cross-linked macroreticular beads which can be used in adsorption- regeneration cycles practically indefi nitely. The technique is simple to use and does not require sophisticated instrumentation as the breakthrough can be judged from the smell of the water coming out of the adsorbent bed.
The regeneration of the spent bed can be done using low-boiling alcohols or ketones, and the eluate can be distilled in a relatively short distillation column to obtain a relatively high boiling oil fraction.
8.6.2 Pervaporation Process
Membrane separation processes have been receiving increas- ing attention particularly for situations involving recovery from relatively dilute (~1000 ppm) aqueous solutions. Pervaporation is one such process which yields very high (~1000 ppm or more) selectivity in the very dilute solution range. Essential oil components which have high affi nity for organophilic polymers can be recovered at very high selectivities. One study showed that silicone rubber membranes yielded bold menthol crystals when the Mentha condensate water was studied under the pervaporation mode. Similar re- sults were also obtained for basil water. Subsequent studies showed that the high selectivity of properly selected membranes results in a permeate concentration far exceeding the solubility limit of the organics, resulting in phase separation.
Figure 3: Recovery of dissolved organics using pervaporation
The separate oil layer can be directly recovered to blend with the main oil fraction to obtain premium grade oil. Figure 3 shows a sche- matic of pervaporation-based recovery of dissolved essential oils in the con- densate. It is evident that this technique consists of a closed-loop operation
with only treated water going out of the battery limits. This treated water has very low biochemical oxygen demand (BOD) and chemical oxygen demand (COD), which is another bonus for the processor.