The D-limonene or terpen it's a natural solvent existing both in the citrus fruits and in many other plants, with extraordinary technical and chemical properties. It can represent an optimal alternative to chlorinated solvents in many civil and industrial applications.
Terpene chemistry
Nearly all the terpens, contained in the essential oils of many plants, are constituted by a skeleton of carbon atoms made of isopren unities regularly linked according with the head-tail position. Some components are shown in the following formulas.
The main component of the essential citrus fruit oil is D-limonen, commonly called terpen, that is a hydrocarbon insoluble in water ; when the essential oils are used to flavor something, it's necessary to bring them is very a thin suspension in the aqueous product (soft drinks, juices, etc.) .
This purpose is reached in many ways:
- the oil is emulsified in a solution of a suitable hydrocolloid (arabic rubber, CMC, etc), with or without an emulsive agent;
- in the countries in which it's possible to use the bromurate oils (olive oil, peanut oil, etc.), these compounds (that have a high specific weight) are dissolved in the essential oil in such proportion as to obtain a lipidic phase with specific weight near to the aqueous phase to be flavored. Recently it's used the isobutirrate saccharose acetate instead of the brominates oils;
- the alcohols of the essential oils are prepared by extraction with ethanol at 65- 70% and a separation of the terpenic superior insoluble phase in ethanol. These alcohols are directly used as additives to ameliorate the soap.
The modern trends are favorable to a larger use of deterpened oils, that are soluble in aqueous solutions. The compounds obtained by deterpenation of the citrus fruit oils are oxygenated: aldehydes, alcohols, ketons, ethers, esters, ecc. It's possible to execute the deterpenation through different processes, the oldest and most diffused is the terpen separation by under vacuum distillation; than there is the steam current deterpenation for stripping the deterpened oil from the stearoterpens and others undesired waxy elements. The deterpenation itself is done by liquid extraction, through couples of solvents in which terpens and oxygenated compounds have different solubility. Lately the solvents are removed by distillation. The most modern process is in any case the deterpenation chromatographer; the kieselgur saturated with hexane is used as absorber of the oxygenated compounds that are then eluted with ethyl acetate. This solvent is then removed by distillation and the oil without terpens is recovered.
Extraction techniques
The market offers actually four different techniques for the extraction of terpen and essential oils from peels and citrus fruit pulp. The choice of the extractive technique influences both the quality of the obtained extracts and the extractive yield. None of them excel in all the categories, so the citrus fruit sector is still looking for new technological solutions.
Cold extraction. The cold squeezing technique (includes skinning and scraping) gives better quality products (as the terpenic mix is not altered by thermal treatments), but the yields are very low (normally under 0,5 % of the peel weight). Moreover with these systems the terpenic fraction is extracted with a lot of water that is separated by centrifugation.
While the essential oils obtained by simple squeezing are always liquid, the ones obtained by steam current or solvent contain also solid products at room temperature, waxes and tri terpenic hydrocarbons. The analysis of this process gave the following data:
| Process type | Process efficiency
(=% of extracted essential oils) | Quality of essential oils (content in aldehydes as citral %) | Energetic consumption in extraction phase per kg of extracted essential oil | Time of thermal treatment | | Manual pressing (excluded centrifugation) | 45 – 70% | 1,3% | 1,0 kW | - |
Steam current. This technique is correctly used for its competitive costs and for the possibility to extract nearly all the terpenic mix from the peel, but causes some variations in the mix composition that doesn't make it suitable for the deterpenation process of the mix for the valorization of its aromatic fraction.
While the essential oils obtained with the simple squeezing are liquid, the ones obtained by steam current or solvents contain products that are solid at room temperature- waxes and tri terpenic hydrocarbons.
The ratio among the components extracted subjecting 50 gr of orange peel (Citrus sinensis) to steam current from 135 to 140 degrees for one hour is reported in the following table:
| Constituents | g/100 g of oil extracts | | a - pinene | 0.5 | | b - pinene | 0.38 | | myrcene | 1.3 | | limonene | 94.4 | | linalool | 0.39 |
The terpenic mix extracted with this methodology has been determined in 2.06 grams of 100 grams of treated peel; as the peels represent the 17 % in weight of the whole fruit, it's possible to calculate the percentage content of terpenic mix compared to the whole fruit of Citrus sinensis L.: 0,3502 %.
From bibliography it results that the quantity of essential oils in the citrus fruits can very from 0,3 to 0,7% of the weight of the whole fruit.
The analysis of this process gave the following results:
| Process type | Process efficiency
(=% of extracted essential oils) | Quality of essential oils (content in aldehydes as citral %) | Energetic consumption in extraction phase per kg of extracted essential oil | Time of treatment | | Extraction under steam current | 98-100%
| 1,8% | 130 kW
| 1h |
Supercritic CO2. This technique has an excellent yield and produces a terpenic mix of high quality because the used temperatures are very low, but its costs are so high that it's never used. The analysis of this process gave the following results:
| Process type | Process efficiency
(=% of extracted essential oils) | Quality of essential oils (content in aldehydes as citral %) | Energetic consumption in extraction phase per kg of extracted essential oil | Time of thermal treatment | | Extraction with supercritical CO2 | 98-100% | 1,3% | 100 kW | 1h |
Solvents and greases. This technique isn't well applicable to the citrus fruit pulp because it needs a great quantity of solvents and it presents costs and problems similar to the steam current extraction ones. Moreover this technique can present problems for the environmental management, the plants safety, etc..
The extraction process has been compared at lab level with the other thermal-mechanical processes actually on the market, through tests on citrus fruit pulp obtained by the squeezing of Citrus sinensis (containing the 0,4845% of tepenic mix in weight).
The analysis of this process gave the following results:
| Process type | Process efficiency
(=% of extracted essential oils) | Quality of essential oils (content in aldehydes as citral %) | Energetic consumption in extraction phase per kg of extracted essential oil | Time of thermal treatment | | Solvent extraction | 98-100% | 1,0% | 250 kW | 4h |
Uses.
The entire D–limonene world production is estimated to be 36.000 tons/year. Naturally not all the D–limonene is destined to the industrial use.
The terpen can be used to substitute the aromatic/chlorinates solvents used in the pickling sector and in the cleaning of electronic cards sector but also in the inches and pains for particular application.
A partner of the project Life Terpene "Master Green" created the following products based on terpen:
Wall paining and lime marmorine: it's used to paint the wall inside and outside. Is adapt to all kind of new plaster or ld ones already treated with lime. Without dilution to produce marmorine.
Glazed wax paste (neutral or colored): it's used for the treatment and maintenance of wood furniture and floors.
Wood fillers: it's used as protective stratum for surfaces that successively must be treated with wax.
Finishing liquid wax: for the finishing of wood, marble and terracotta tiles surfaces.
Protective oil (without desiccative agents): used for the treatment of wood and terracotta tiles floors, for kitchen surfaces subjected to solicitations. It's resistant to the use. The protective oil, even allowing the transpiration, has an excellent impermeability.
Wall paint with natural resin: it's a washable paint for the treatment of plasters new or old, for the cork panels, not treated wall paper, wood panels, laminated products, plasterboard.
Transparent lake for interior walls: finishing for interior parts (walls, doors, window frames) for all kinds of wood not subjected to solicitations. Regarding very adsorbent woods it's better to treat with impregnation agent.
Natural enamel for wood: it's used to cover all kind of wood and metal surfaces, for exterior and interior use.
Paint for interior and exterior walls: it's used for all kind of interior and exterior wood surfaces, even subjected to solicitation as parquet is. Usable on adsorbent woods preventively treated with impregnating agent.
Impregnating agent for interior and exterior parts (neutral or colored): it's used on all kinds of adsorbent woods.
Anti-rust: it's used to protect exterior metallic surfaces.
Consolidating agent for stone and friable surfaces: it's a consolidating and proofing agent, colorless, for the treatment of friable surfaces, stone, plasters scarcely consistency. It's used to isolate smoke spots.
Natural enamel for radiators: it's used to cover the radiators.
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