Figure 8.1 Cross section of a citrus fruit, showing: (a) flavedo with oil glands; (b) seeds; (c) juice vesicles; (d) central axis; (e) albedo; (f) segment; (g) segment membrane.
• the endocarp, which is the edible portion of the fruit, made up of segments containing the juice vesicles which have ultra-thin walls. The segments are covered by a thin carpellary membrane and are distributed around a central axis which has the same composition as the albedo;
• the seeds, found around the central axis embedded in the endocarp.
In the epicarp, and more precisely in the region immediately below the epidermis of the fruit, the oil glands containing the essential oil are found. These glands are between 0.4 and 0.6 mm in diameter and have no walls as such, but are enclosed instead by the remains of decayed cell matter, with no openings or excretory outlets. The oil glands are surrounded by cells containing an aqueous solution which is rich in salts, sugars and colloids. When the peel comes into contact with water, the higher osmotic pressure inside these cells causes them to absorb water and become turgid, thus putting the oil glands under pressure from all directions. Under these conditions, if the peel is subjected to a mechanical process such as pressing or grinding, the oil spurts out with considerable force when the oil glands are ruptured. Turgor of the peel can be attained more readily in freshly-picked fruit. The degree of maturity of the fruit is also important. On the other hand, as already mentioned, the tissues around the oil glands are spongy and can absorb liquid rapidly, and so they will tend to soak up any essential oil in their vicinity. Essential oil extraction methods take these factors into account and so the mechanical process is always performed under strong jets of water, which wash the oil away before it can be reabsorbed by the fruit.
DETERMINATION OF THE ESSENTIAL Oil. CONTENT OF THE FRUIT
None of the mechanical or manual processes used to extract essential oils from fruit or peel is able to recover the total quantity of these important derivatives. The maximum attainable yield of oil from the raw materials depends on a number of factors, some of which relate to the nature of the fruit and others to the nature of the machinery used to extract the oil. These factors will be discussed in some depth later on, but it is in any case essential to have a method at our disposal with which we can determine the total essential oil content of the fruit. Such information provides us with an indispensable point of reference when we need to make decisions about the quality of the fruit (from the point of view of oil yields) and about the efficiency of the machinery employed for the extraction and recovery of the oils.
In general, distillation is the best method to determine the essential oil content of the fruit. In order to achieve reliable results, the fruit must be representative of the batch under investigation and there must be no oil loss during the grinding process. A convenient procedure is therefore to take a sharp knife and remove the thin outer layer containing the oil glands from weighed fruit (the quantity should be sufficient to provide not more than 2 mL of essential oil). The peel should be soaked for at least 12 hours in an aqueous sodium chloride solution (ca 20 per cent). The softened peel is ground in a standard meat grinder, care being taken to sluice thoroughly, and about 2 L of slurry are collected in a 3L, narrow-neck, round-bottom flask. A Clevenger (1928)
oil separator (Figure 8.2), filled with water up to the graduated part, and a condenser are fitted to the flask and the mixture is brought to boiling point. The mixture is refluxed for about an hour at the rate of 50 drops per minute. The distillation is then interrupted, the separator is removed from the flask and water is drained off through the tap provided until the essential oil reaches the graduated part. The quantity of essential oil is measured from the base of the lower meniscus to the highest point of the upper meniscus. To calculate the weight in grams, the measured volume is multiplied by the mean specific gravity of the distilled oil. Duplicate determinations must not give rise to differences greater than 0.1 mL.
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