Production Technology of Citrus Essential Oils

Regardless the system for recovering oils from citrus fruits, the next operation is the separation of oils from water; this operation is accomplished by centrifugation. Water - oil emulsion always contains peel fragments and other debris, so it is necessary to clean it before feeding separators and, also in this case, finishers are used.
Centrifugation also eliminates fine mud remaining in the emulsion after finishing;  are used truncated cone disk centrifuges where phases are separated according specific gravity difference. Solid sediments deposit at bowl periphery, that has to be periodically cleaned, while water and oil go to different outlets.

Centrifuges allow automatic waste elimination without stopping the machinery, through an hydrodynamic device that temporarily moves bowl discovering a certain number of holes through which mud is eliminated; in the meantime machinery still rotates at the same speed.  
Usually separation is performed in two phases, in the fist one a self-cleaning separator takes out mud and increases oil content of the emulsion; in the second stage water is completely eliminated in smaller size disks machinery or through "candle supercentrifuge". Usually second stage separator bowl revolves at higher speed respect to the first stage. Water solubilizes a certain percentage of oxygenated compounds of the oil, so it is usually recycled into extractors and renewed few times a day or just once a day. In this way finished oil has an higher content of oxygenated components and it is possible to recover most part of the oil remaining in the water by distillation. 
On the other side, recycle may cause following disadvantages :

  • development of microflora with consequent poor oil quality
  • higher plugging chance for extractors nozzles with yield lowering
To more conveniently recycle water, it is used a decant system known as "Florentine vessels" ; in the first vessel remain higher amounts of solids , while to extractors goes, through a pump, water from last vessel. Periodically, and at least at the end of processing day, all the system is cleaned and water is distilled to recover dissolved oil. Distillation is made just if it is economically convenient, i.e. just for mandarin oil, lemon oil and grapefruit oil. 
Citrus oil easily undergo to change caused or catalyzed by oxygen, light, heat, moisture and some metals like iron and copper. It is mandatory that storage conditions are absolutely suitable to assure a prolonged preservation of organoleptic and analytical characteristics that are fixing commercial value.
These conditions are:
  • complete water absence in the oil
  • use of stainless steel vessels
  • complete vessels filling to minimize contact with air
  • storage temperature close to 0°C
Water is almost eliminated with accurate centrifugation, anhydrification is obtained with anhydrous sodium sulfate. To avoid air contact, and it is not possible to completely fill vessels, it is recommended to ripple with inert gas (nitrogen or better carbon dioxide, in the latter case the simplest way is to use dry ice). 
During storage, citrus oils tend to separate waxy insoluble that can be eliminated by centrifugation. Precipitation is helped by low temperature. The simplest way to eliminate waxes uses high and reduced diameter vessels with conical end having three or more outlet valves at different heights. After wax decant, oil is collected from higher outlet valve and, after, from other valves up to waxy layer that is completely separated by low temperature centrifugation.

Distilled Oils

These oils are obtained through under vacuum steam distillation. Machinery is built in stainless steel and it is made by three basic components : boiler, condenser and collecting separators.

In the hemispherical bottom of the boiler lies an holed steam distributing ring ; condenser may be tubular or coil and uses water as condenser while separators are glass cylinders with bottom discharge valves and vent valves at the top. Vacuum is obtained through a liquid ring pump and it is regulated to obtain a distillation temperature ranging between 45-55°C. Boiler is loaded with liquid, using vacuum made in the equipment, up to 50 % of total boiler volume, then steam is opened while vacuum pump is always on. Distilled oil is collected in one of separators together with water, that is decanted trough drain valves and, finally, oil is collected. While using one of two separators, distillation continues on the second. For every load of about 600 - 1.000 liters, distillation lasts about one hour. 
Recently have been developed continuous distillers showing following advantages :
  • continuous distillation
  • steam saving
  • water saving
  • lower distillation temperature
  • higher hourly output
  • shorter holding time with consequent better product characteristics
Machinery is made by four main parts; a feed pump brings product in through a tubular heat exchanger using exhausted product as heating media; then product go to distillation column and falling goes in contact with steam coming from the bottom of the column; steam sweeps oil and the mixture is cooled by a coil condenser while exhausted product goes in the heat exchanger. All operation is made under vacuum produced by a vacuum pump.
Separators collect distilled oil and water that separates for gravity in two layers; eliminating alternatively vacuum in separators, it is possible to drain water and to collect distilled oil. Distilled oil is colorless, has a lower specific gravity respect to cold pressed oil, has lower aldheydes content and extremely low fixed residue. 

Folded Oils

These oils are obtained through hi-vacuum fractional distillation of cold pressed oils.
Depending from working confitions, i.e. according distillation time, and other physical parameters, final oils may contain different amounts od terpenes. So, it is possible to obtain oils 3-4-5 fold and also Terpeneless oils. From Terpeneless oil, for further distillation, but made in different conditions, are obtained Sesquiterpeneless oils. Terpeneless oils are produced in distillation systems made by :
  • boiler with a false bottom for heating the oil
  • an high rectification column, made by raschig rings or by a number of perforated disks having, over the top, a reflux system with water cooling
  • a condenser
  • two collecting separators to collect distilled terpenes
Vacuum has to be lower than 15 mm Hg and distillation has to last up when it is no more possible to collect terpenes. This mean that in the boiler are just contained components that are not able to cross rectification column, it means oxygenated products, Sesquiterpenes and waxes. This product is transferred in an another distillation system like the first one but without rectification column.
Distillation is continued using live steam to speed up the system when the process slack off; distillation is continued as long as only water distills.
In these conditions only waxes remain in the boiler; this means that the oil distilled in the second distillation has to be redistilled one more time until it is recovered the expected product; process is controlled by checking with several chemical tests like solubility in different concentration alcohol. According to analytical results, fractions are put together.
What we have describes is one of the methods used for folding oils, any how it is important to say that every producer has his own system, according personal experience and tactics adopted with double intent to simplify operations and obtain expected products. In other words, folding oils is a difficult matter that require a deep knowledge of oils composition and of boiling points of different classes of substances.