As explained, certain flowers e.g., jasmine and tuberose give their greatest yield of flower oil upon extraction with cold fat (enfleurage) because their physiological activities continue for 24 hr. and longer after harvesting. During this period, the fat on the chassis absorbs the perfume emitted by these flowers.
However, the physiological activities of other flowers roses, orange blossoms, acacia, and mimosa, for instance are stopped by picking. When extracted or distilled, they yield, therefore, only as much oil as is contained in the flowers at that moment. Since no further oil develops in these flowers, the long and rather complicated method of enfleurage would prove ineffective. Hence, other methods must be resorted to, whereby a medium actually penetrates the plant tissues and dissolves all flower oil present in the oil glands.
Hesse and Zeitschel30 studied methods of distillation, cold enfleurage, maceration with hot fat, and extraction with volatile solvents as applied to various flowers, and the effect upon the yield of flower oils. Applying enfleurage to orange blossoms, for instance, Hesse found that this method yields only one-fifteenth of the amount of volatile oil obtained by steam distillation. Hesse thereby confirmed what had been known empirically in Grasse for generations.
Generations before the modern process of extraction with volatile solvents had been introduced (probably even in classical times), the perfumes of roses, orange blossoms, violets, acacia, mimosa and others had been obtained by treating the flowers with hot fat. The principle is simple :
The flowers are extracted by immersion in hot fat. In other words, the same batch of hot fat is systematically treated with several batches of fresh flowers until the fat becomes quite saturated with flower perfume. The exhausted flowers are removed and the fragrant fat, called Pomade d'Orange, Pomade de Rose, etc., is sold as such, or the pomade may be treated further by washing it with strong alcohol, exactly as with jasmine or tuberose pomades, obtained by cold enfleurage. The alcoholic extraits (Extrait d 'Orange, Extrait de Rose, etc.) may be marketed as such, or they are concentrated in vacuo, giving thereby the corresponding absolutes of pomade.
30 J. prakt. Chem. [2], 64 (1901), 250, 258; 66 (1902), 506, 513.

The process of maceration, therefore, is somewhat analogous to that of enfleurage, with the fundamental differences that, in the case of maceration, hot fat is employed, and that the actual macerating of the flowers in the hot fat is done quickly.
Maceration was an important process before the introduction of the more modern method of extraction with volatile solvents. Fifty years ago, orange blossoms, if not distilled, were treated by maceration; acacia blossoms, which do not lend themselves to steam distillation, had to be processed exclusively by maceration. Similarly, roses were macerated in Southern France because French roses, unlike Bulgarian roses, give only a very low yield of oil upon distillation. However, today, the process of maceration with hot fat is employed very little. Its products, especially those from orange blossoms, find application only in a few old-fashioned perfume formulas. Otherwise the concretes and absolutes made by volatile solvent extraction have almost completely replaced the former extraits and absolutes of maceration.
For completeness, however, we shall give a brief description of the way this old-fashioned process is carried out :
As solvent a highly purified fat base is employed. It should be prepared most carefully and in the same way as described under enfleurage.
A batch of 80 kg. of corps is heated to about 80 and at that temperature macerated with charges of 20 kg. of fresh flowers each time. This is repeated until 1 kg. of corps has been treated with about 2 to 21/2 kg. of flowers. Every extraction lasts about one-half hour, at 80, when the mass is left standing for about an hour during which it cools but continues macerating the flowers. The mass is then reheated, melted and strained through metal sieves and filter bags, whereby the exhausted flowers are eliminated. Since they retain some adhering fat, they are, while in the sieves, treated with scalding water, which liquefies the fat. The water easily separates from the fat layer. In order to remove all adhering fat, the flowers are finally packed between filter cloth, placed in a hydraulic press and submitted to pressure ranging up to about 3,750 Ib. per sq. in.. Scalding water is thrown on the filter bags during the process so that any fat still retained by the flowers is melted and expressed. Expressed fat and water again separate easily. Instead of hydraulic presses, some manufacturers employ centrifuges for removing the exhausted flowers from the fat corps. The method of maceration is rather cumbersome but it served its purpose in the old days when no better process was available. Its products (extraits and absolutes of maceration) often show a fatty "by-note" which originates from the fat corps and modifies the character of the original flower perfume. A further disadvantage consists in the fact that, on account of this fat content, absolutes of maceration easily turn rancid, thereby developing a sharp, disagreeable note. Because of their high alcohol content, the extraits are better protected against rancidity and spoilage in general.

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