Alcohol soluble citrus flavour, or alcoholate, has been used for a long time in the perfume industry and above all in the food industry to give flavour to syrups and drinks. Its main properties can be summarised as follows:
• highly soluble in aqueous phase;
• marked resistance to alteration processes if compared with the essential oils from which it derives;
• high concentration of the oxygenated fraction compared with the hydrocarbon fraction (terpenes and sesquiterpenes).
The cold preparation of alcoholates (also called soluble essential oils) relies on the ability of ethyl alcohol of suitable strength to dissolve the oxygenated compounds and thus separate them from the hydrocarbon fraction, which is only slightly soluble. The oxygenated compounds are responsible for the characteristic flavour and fragrance of a citrus essential oil.
The alcoholates are generally put on the market with an indication of their flavouring strength in relation to the final syrup (1:100, 1:200, 1:400, 1:800). A general outline for the production of citrus alcoholates is as follows:
Essential oil 10 kg
Mix well for at least one hour. Decant and separate the two phases.
The oily fraction, largely containing hydrocarbons, is re-treated with a second 10 kg aliquot of alcohol of the same strength. The mixture is stirred and decanted, and the alcohol layer is combined with the one previously separated to give 20 kg of alcoholate.
The oily fraction can be further treated with alcohol for one or more washes. The aromatic alcohol obtained requires additional processing.
The separation of the hydrocarbon and oxygenated fractions becomes more selective as alcohol strength is lowered. Overall, the distribution of the oxygenated components between the two phases depends on the following main factors: alcohol strength, number and length of extractions, and temperature. The processing techniques may vary considerably and are not usually disclosed by the producers.
The apparatus used is very simple (Fenaroli, 1963): it essentially consists of a vessel with a conical bottom contained within another which functions as a thermostatic bath. The capacity of the vessel varies according to the production required. A mechanical stirrer fitted with two sets of differently angled blades is inserted into the vessel. The vessel has a lid and a drain in the bottom fitted with two taps, between which is a small glass window. Essential oil and then alcohol are pumped into the apparatus, after which the stirring process is started. Using cold water circulating in the jacket the temperature is brought to around 13 °C and maintained at that level for the whole operation. The stirrer is left to work for the required length of time, and then the mixture is left to stand. The hydrocarbon phase settles at the top, while the alcoholate is drained off and filtered.
Alternatively, in a heat-based procedure the alcoholates can be obtained directly from the flaveclo previously separated from the fruit or, more commonly, from the whole peel (Di Giacomo, 1974). To obtain the flavedo the two poles of the fruit are fixed in a machine designed for this purpose, while a special instrument pares off the part containing the essential oil glands in the form of a long, thin, continuous strip. The whole peel usually comes from a standard birillatrice. Both the flavedo and the peel, with a sufficient quantity of water added to make them fluid, are finely chopped in a grinding mill with 3 mm diameter holes. More water is added to this chopped material together with 95° alcohol, so that the final alcohol concentration of the slurry is 4°. After a certain period of maceration, the mixture is continuously pumped into a plate distillation column and after a preheating phase it reaches the high part of the plates in the column. The rectified vapours are condensed (alcohol strength: 50°—60°, according to the requirements of the end-user) and then separated from the terpenes and sesquiterpenes using Florentine oil-collecting vessels. During this heat-based procedure to obtain alcoholates, hydrolysis, oxidation, isomerisation, hydration, and dehydration reactions may take place which induce changes, particularly in the ratios between the various oxygenated components (Dugo etal., 1988). From an olfactory point of view, these alcoholates have a distinctive 'floral' fragrance.
All alcoholates, like the corresponding essential oils, have individual characteristics which indicate their origin. Sweet orange alcoholate is characterised by the presence of <5-3-carene and valencene, which are absent, or present only in traces, in alcoholates derived from other varieties of citrus fruit. Mandarin alcoholate is characterised by the presence of a-sinensal, methyl N-methyl anthranilate and thymol. Bergamot alcoholate is characterised by a high linalool and linalyl acetate content.
Lemon alcoholates have characteristic a-thujene/sabinene, Q-thujene/5-pinene, a-pinene//3-pinene and ,3-pinene/a-terpinene ratios. The citron alcoholates present carachteristic composition that allow to distinguish these from other citrus alcoholates (Dugo etal., 1988)
In order to make a sensory test of the 'strength' and the quality of the alcoholate, a drink is prepared as follows: citric acid monohydrate 2g; sucrose 80 g; water up to 1000 mL; flavour with a quantity of alcoholate between 200 and 800 pL per litre of drink and inject C02 at 3 atm at room temperature.
In the emulsion formed after essential oil is extracted, significant quantities of colloidal substances are present, especially pectin. This increases viscosity and adversely affects the centrifugal yield and, as a consequence, recovery of the essential oil.
In order to improve the conditions under which oil is separated, enzymes can be usefully employed in all the extraction processes used in industry. The use of enzymes increases essential oil yield, reduces water consumption and consequently decreases the quantity of waste effluent.
The requisite characteristics of the enzyme preparation (Janser, 1995) are as follows:
• hemicellulytic-pectolytic activities;
• high level of polygalacturonase (PG) activity;
• low level of pectinesterase (PE) activity;
• active in a very wide pH range (2.5-6.0), excellent at pH 3.0—5.5;
• excellent levels of activity at room temperature (15—40 °C);
• highly suitable for oil-water separation.
The quantity of enzyme preparation necessary depends on a number of parameters: citrus type (orange, lemon, etc.), fruit variety, maturity of fruit, extraction method, quantity of water used in the cycle, mean life of the emulsion between extraction and separation, time necessary to recycle the aqueous phase.
Citrozym CEO, a food grade enzyme preparation produced by Aspergillus niger, is an enzyme preparation tailor-made for this application.
When optimised, this enzyme technology allows the aqueous phase to be recycled back to the extractors after the first centrifugation. Thus the consumption of fresh water and production of waste water are drastically reduced, thereby minimising oil loss. The increase in the yield of essential oils is in the range of 5—15 per cent. The overall process is improved and becomes more economical.
Processes based on the use of supercritical solvents enable temperature labile compounds to be extracted in relatively mild conditions. They potentially offer some undoubted advantages over traditional processing methods in the case of some particular classes of compounds.
In particular carbon dioxide, which above the critical point (31 °C and 74 bar) assumes precise supercritical characteristics, and is preferable to other fluids for a number of reasons: it is odourless, tasteless, and colourless; it is chemically inert; it presents no danger of explosion; it is an apolar solvent; it is easy to remove.
In the essential oil sector, it has been found that the use of supercritical C02 at low temperatures in a system equipped with columns operating in countercurrent allows problems relating to quality to be addressed that traditional processes, such as distillation, are unable to resolve. Traditional methods are inadequate at maintaining the stability of some elements of the bouquet, avoiding loss of volatile components and at achieving a sufficient degree of selectivity towards various components.
It should be remembered that the main oxygenated components of essential oils (esters, alcohols, aldehydes and ketones) of low and medium molecular weight are soluble in liquid C02 and in supercritical C02. Apolar compounds of low molecular weight (hydrocarbons) are similarly soluble. The solubility of these compounds, within the same family, generally decreases as molecular weight increases. Since chlorophyll, caretonoids, sugars, aminoacids and the majority of inorganic salts are insoluble, it is evident that this property can be exploited in the extraction process to obtain considerable selectivity which discriminates against compounds that contribute nothing to the aroma and whose presence in the aromatic extract may be counterproductive.
Interesting applications forecast in the citrus area are: in the extraction of essential oils from flowers and from leaves; the enrichment of certain classes of compounds (deterpenated and desesquiterpenated oils); and the elimination of undesirable components (for example furanocoumarin from bergamot essential oil).
The prospect of employing supercritical solvents in the extraction of essential oil directly from the peel, however, is of little practical interest given that cold-press systems currently used in industry already enable high quality cold-pressed essential oil to be prepared at a reasonable cost.
Ultrafiltration and reverse osmosis
The citrus industry has an interest in limonene recovery techniques and in the application of membranes.
Commercial ultrafiltration (UF) and reverse osmosis (RO) membranes are used to concentrate limonene present in coldpressed oil centrifuge effluent and molasses evaporator condensate (Braddock, 1982). UF membrane rejection rates are 78—97 per cent for mixtures with initial limonene concentrations from 0.04—0.6 per cent v/v. RO membrane rejection rates of limonene range from 87—99 per cent for feed streams containing 0.06—0.23 per cent limonene. Initial membrane flux rates for centrifuge effluents are in the range of 10—100kg/m2/h. Evaporator condensate fluxes are higher, 25-400, while pure water rates range from 25 (RO) to 1,000 kg/m2/h (UF).
Contact with limonene adversely affects membrane flux rates in decreasing order of severity as follows: polysulfone > cellulose acetate > teflon-type.
In countries in which artificial colourings in drinks are prohibited there is a special demand for a type of sweet orange essential oil with an elevated content of carotenoids (ca l,000ppm in terms of 5-carotene), while, usually, significantly reduced levels are found in the essential oil obtained industrially using the most common extraction processes, even from late season fruit.
For some years technological strategies have been studied to enable essential oil to be enriched with carotenoids to reach the levels expressly demanded by the market (Rispoli and Di Giacomo, 1968).
One procedure adopted is as follows: fresh flavedo from ripe oranges is cut into fine strips using a Citromat-type machine, then finely ground without adding water in a grinding mill with 1.5 mm sieve openings. The product obtained in the form of a paste is rendered fluid with the necessary quantity of an essential oil obtained using one of the traditional methods and stirred for several hours. The quantity of essential oil may vary depending on the carotenoid content of the flavedo. Some operators use orange pulp in place of flavedo. The mass obtained is pressed and the liquid obtained is centri-fuged to separate impurities which might be present.
Using this method there is a significant increase in the waxy fraction—the vehicle through which the carotenoids are transferred to the essential oil. The finished essential oil when compared to common essential oil, has significantly higher refractive index, specific weight and evaporation residue values; at the same time the optical rotation value is lower.
The possibility of producing bergamot essential oil bergaptene free is a major issue for the citrus industry in Calabria which must face the danger that this essential oil may be used less and less because of problems associated with the phototoxic properties of this component.
Three conditions must be fulfilled so that furanocoumarins free essential oils can be used in the same way as ordinary oils:
• the properties of the fragrance must be maintained;
• the storage life must not be affected;
• the physicochemical parameters, especially those detectable by instrumental analysis, should not be altered, with the exception of those which obviously derive from the reduction of the furocoumarin content.
The ideal process should aim at the specific elimination of bergaptene, without modifying the content of all coumarin and psoralene compounds. The overall composition of the non-volatile fraction certainly should not be compromised.
The process currently used in Italy is based on the treatment of the essential oil with an aqueous alkaline solution. The bergaptene level can be substantially reduced without compromising the quality of the essential oil by appropriate control of the following variables: the concentration of the alkaline solution, the type of stirring apparatus, the contact time between the two phases, and the temperature.
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