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The juices production represents one of the main industrial application for the citrus fruit production.

This production can be supported by other activities as the essence extraction, but it creates great quantities of residues that must be recycled and valorized.

Juice production

The juices are mechanically extracted with equipments inspired to the manual skittle plant. Actually the tendency is to build small dimension plants, authomatized with low consumption and able to treat very big quantities of fruits.

The automatic treatment foreseen the following phases:
  • equatorial cut of the fruit 
  • accommodation of the two half in dedicated cups 
  • introduction of the skittle for the fruit extraction 

So the fruits, without oils, are divided in two parts by a blade. The two parts are pressed by rolls, that constitute the mobile part of the plant and squeezed against a plate. The juice is collected by a channel and the residues are thrown away with other wastes through some screw. The juice must be still worked to be suitable for the consumption:

Sieving. It's necessary to eliminate the seeds, peels fragments, and the mucilage eventually existing. This operation can be executed with finisher plants or with automatic drain centrifuge.

Homogenization. It's necessary to lower and standardize the pulp quantity. From the separated pulps it's possible to get the aqueous extracts usable for the low quality concentrated juice production.

Oil extraction. A high content in an essence can lower the juice quality because of the components alteration (particularly as regards the lemon juice).

De-aeration. It allows a better working of the heat exchanger, a most uniform filling of the juice containers

Pasteurization. It sterilizes and inactivate the enzymes responsible of the pectin degradation. To avoid the alteration of the organoleptic characteristics of the juice a flash-pasteurization is executed: it lasts maximum 1' at 90°C and meanly 40". In Italy are used mainly plate pasteurization plants in which the juice circulates in fine lamina between the two plates. The heat exchange happens through the plates, heated counter-current by hot water or low pressure steam. If the juice is destined to be stocked in tins at 90°C without being cooled and the tin is not hermetic, it needs the the addition of anti fermentation agent.

From the '80 the conserved refrigerated juice in paraffin cardboard similar to the one used for the milk have been used.

The benefits of the juice concentration are clear: decrease in packing, limited stocking space, lower transport, improved conservation. But besides these evident advantages we have a decreasing organoleptic properties of the product.

We list the main problems that happens during the juice concentration:
  • content in pulp and collides 
  • these constituents tend to gelatinize, preventing the juice from a high concentration 
  • enzymatic action 
  • the temperature used for the pasteurization don't prevent the development of the enzymatic processes. 
  • loss of perfume 

Moreover the volatile fraction are lost if a particular plant to recover them isn't used. Also the caroternoid content can be reduced and the juice can be subjected to darkening phenomena.

The plants to concentrate the citrus fruit juice can be grouped in two categories: 

  • concentrators working on high vacuum and low temperature, in which the steam consumption and water consumption are not determining 
  • concentrators working at high temperature and short time, to obtain reduced steam and water consumption. 

It's possible to ameliorate the product quality concentrating the juice till 55-63°Brix, cooling it immediately at a temperature of about 0 °C in a tank provided of a double cooling wall and lowering the density at 42°Brix (cut-back).

The concentrated juice undergoes some standardization operation and then is once more cooled at –6 °C, before being put in the final container.

It exist another concentration technique for the juice that is called crio concentration. It's not an optimal technique because a loss in flavor and color happens, in fact the suspended and soluble solids remain in the ice fraction.

The juice concentration happens through the pulverization, the freeze drying and the freezing.

In fact they are different techniques, with different application.

The pulverization starts from the concentrated juices. To accelerate the desiccation and reduce the hygroscopic behavior of the powder it's possible to put some additives in small quantities or sugar and pectin in big quantities. The most used desiccation equipment are: rotating oven, spray desiccator systems, room desiccator at 1mm. of mercury pressure.

The freeze drying is a process that foresees two main phases:

  • freezing of the juice in some trays in a chamber in which the vacuum it's applied 
  • slight heating of the trays to sublimate the ice 

The juice freezing is obtainable through the passage in a cold tunnel at – 40 °C. The frozen product is very suitable for the preparation of natural juices with water addition.

The International Federation of Fruit Juice Producers and the Comité de Liaison de l’Agrumicolture Méditérranéenne are working for the standardization of the juice analysis methods.

First of all it's necessary an accurate control of the freshness, of the ripening degree and of the absence of parasite in the peel of the fruit. Then it's necessary an analytic control that shall imply at least the points below mentioned:

Organoleptic examinationGeneral aspect
Taste and flavor
Refraction index and corresponding Brix degree  
SugarGlucose, fructose, Saccharine
Total acidityCitric monohydrate acid
Ethylic alcohol  
Volatile acids 
Lactic acid 
Essential oil from citrus fruits 
Ascorbic acid 
AshesAlcalinità delle ceneri 
Potassio, Sodio, Calcio, Magnesio, 
Fosfati, Solfati, Nitrati
Formol index  
Ammoniac nitrogen 
Complete amino acidic table 
l-malic acid 
Iso citric acid 
Vitamin B1 
Anti cryptogamic residues and heavy metals  
Specific weight  

The possibility to alter the juices are various. The alteration can happen with:

  • Addition of substances constituting the juice but with different origin, as water, citric acid, sugar, vitamins, carotenoides, amino acids, mineral salts, ecc. 
  • Addition of substances as coloring agents, turbiding agents, tartaric acid, etc. 
  • Addition of juices of different origin 
  • Addition of other parts of the fruit, as albedo and flavedo. 

To discover the adulteration the turbidity tests, color test, content in pulp and raw fiber, viscosity and foam power will be useful.

Production waste

The production of drinkable juices creates big quantities of wastes that can be valorized separately.

Essential oil. The extraction of an essential oil can take place before or after the juice extraction, that means working on the whole fruit or on its peels. There are many mechanical or manual systems for the extraction. The manual methods , for economical reasons are not used anymore. We report in the table all the extraction procedures .


Manualon the whole fruit"bowl"
"circular grater"
 on the peels"sponge"
Mechanicalon the peels "sfumature"
(by pressure)
 on the whole fruit "peeling"
(by striping)

The peeling processes on the whole fruit can be classified as follows:
  • systems for the superficial striping (in this way the tissues are not damaged) 
  • systems by abrasion of the superficial layer (and this is the layer that contains the utricles full of essence) 

The peeling procedure it's the most suitable for the industrial transformation of huge quantities of fruits with a continuous process.

In Italy are used especially the equipments called peeling plants, that work by abrasion of the superficial layer and produce the "rasping", a whole of minced residues. The essence and the rasping are removed by a continuous water flow. The peeling is a process used mainly for the production of orange, bergamot and tangerine essences from still green fruits.

The "sfumature" is done after the juice extraction. It' the most effective for small-medium plants, especially for the lemon essence production of high quality. Once it was done manually ("sponge" method) and then followed the peel treatment with lime suspension, to favor the peel hardening and help the essence extraction.

Actually this method has been abandoned because too expensive and substituted by mechanical sfumature. This one require simple equipments, continuously working and relatively cheap.

The natural pressure of the essence on the utricles is naturally exploited: this pressure makes the essence coming out if the peel is compressed or bended.

In the equipment chamber the peels are strongly bended and a water flow takes the essence away from the broken utricles.

An emulsion of about 1% of essential oils is obtained and, by centrifugation, it is concentrated to 99%. The emulsion is deprived of the stearopten, waxy component, and at this point it's possible to commercialize it.

The essence extraction can happen also with a press.

The essence yield with all the extraction methods can vary depending on many factors: fruit species, ripening degree, fruits volume, used technology, etc..

The lemons have a yield varying from about 530 grams per quintal of fruits at the beginning of the season , to about 265 grams per quintal late in the season. The yield of other types of citrus fruits as tangerines and bergamots are higher, while are lower with bitter and sweet oranges.

The essential oils are constituted by terpens, mainly d-limonene (about 90%). The limonene mustn't be confused with limonin, a mixture that give the bitter flavor to some citrus fruits and hasn't any commercial importance.

The quality control of the essential oils mechanically extracted must be discussed apart.

To control this quality the following determination are performed: organoleptic exam, specific weight at 15°C, optical rotation and refraction index of the first distilled tenth, acidity index, esters, aldehydes, alcohols, dry residue, solubility in alcohol. Hereafter we report the purity limits.

ParametersSweet orangeBitter orangeBergamotLemonTangerine
Specific weight at 15 °C0,8480 - 0,85100,8520 - 0,85600,8800 - 0,88800,8560 - 0,86100,8525 - 0,8570
Optical rotation a 15°C+98° a + 100°+88,80° a +96,8°+8° a + 30°+57° a + 65°+65° a +75°
Refraction index at 20 °C1,4725-1,47501,4730-1,47501,4660-1,46801,4740-1,47551,4745-1,4780
Rotation of the 1° distilled tenth Superior no more than 2° to the directMore elevated than the direct Inferior of no more than 5° to the directSuperior no more than 2° to the direct
Acidity indextill 1,40 till 2till 1,40 
Aldehydes  %0,8 – 1,40,7 – 1,2 3,0 – 4,00,50 – 0,85
Esters %  30 - 45  
Free alcohols%  15 - 30  
Dry residue %1,4 - 4,03,0 – 4,74,5 – 6,51,8 – 3,62,3 – 3,5
Solubility in alcohol 85°  till 1 : 1  
Solubility in alcohol 90°  till 1 : 0,4  

Other tests, based on the gas chromatography, on thin layer chromatography, on the high performance liquid chromatography, on spectrophotometry in UV and IR, should be performed to express a complete opinion on the essence quality. These tests are able to find any adulteration and contamination on the essence.

The citrus fruit essences are composed by three main groups of chemical substances: oxygenated compounds (aldehydes, ketons, alcohols and esters), that give the particular essence smell, terpens and sesquiterpens, components of the not volatile residue.

The essences deprived of the terpenic hydrocarbons are called deterpened, while the ones without terpenic and sesquitepenic hydrocarbons are called desesquiterpened. They're essences obtained by fractioned distillation under vacuum or by column chromatography of finally by solvent counter-current distribution (one polar and the other not polar). These concentrated essences are more soluble in alcohol and more stable, even if deprived of some olfactory marks typical of the fresh fruit.

The citrus fruit aromatic compounds are located exclusively on the peel. Other parts of the tree as flowers or leaves or young branches can give essential oils (neroli from flowers, petit-grain from young branches). To obtain these essences the steam current distillation process is suggested, because the combined action of heat and water steam swell the cellular walls and help the passage of the essential oils outside.

Seed oil. The raw oil obtained by simply squeezing of desiccated seeds has a yellow pale color and a pleasant scent. The refining happens by washing or treatment with active adsorbing earth. It can be used in the agro food sector.

Cellulosic fraction. The cellulosic fraction of the citrus fruit pulp contains many components that can be valorized separately through opportune industrial processes. The cellulosic fraction is also usable without separation processes for the feedstuff materials.

Feedstuff. After juice and oils extraction, about 50% in weight of the fruits is constituted by peels.

The dumping of these working residues, the citrus fruit pulp, can become a real problem in addition a an economic loss. The desiccated citrus fruit pulp can be considered the main components of excellent animal feedstuff.

In fact the lab tests showed that the citrus fruit pulp has a nutritional level comparable to the desiccated beetroot one and can be also compared to the barley.

Some data are reported in the table:

Raw protein - 6,50 % 
Raw fat - 2,00 % 
Raw fiber - 16,0 % 
Ashes - 8,0 % 
Lime - 5,0 % 
Total carbohydrates - 60,0 %

Moreover it has been estimated that the air pollution from citrus fruit dust is reduced of the 90 %.

For this kind of use in the feedstuff, the working foresees the neutralization of the hydrophilic pectin properties, the mincing of thee material and the addition of lime. This operation consent the formation of calcium pectate.

The humidity is reduced till 70-75%, in a rotating press. A liquid and a panel of cellulosic pressed material are so obtained. The liquid can be further concentrated and give the citrus fruit molasses to be fermented to produce industrial alcohol. The panel vice versa is desiccated to obtain dry animal feedstuff, that with integrators addition is ready to be consumed.

(example of feedstuff in pellets)

Pectine extraction. The orange peels contain about the 3% of pectin (of the fresh peel ) and in this way they constitute, together with the apple squeezed pulp, the raw material to produce pectin. The peels are deprived of the oils, cut into little pieces and washed carefully to eliminate all the undesired soluble solids (sugars, citric acid glucosides). Then the pectolitic enzymes are inactivated with a short thermal treatment if the peels aren't immediately treated. 

The insoluble pectin (protopectin), existing in the peels, is so partially hydrolyzed at boiling temperature, with an acid treatment (usually with HCl) at pH 2 for about one hour. The filtration of the pectin solution is quite complicated even with the use of filtering supporting agents.

The clear sieved solution, that contains about 1% of pectin is concentrated under vacuum till 4%. Then this solution is precipitated, purified, grinded and standardized (normalized).

The precipitation stadium is obtained by adding isopropanol or acetone; the precipitate is sieved and pressed with an hydraulic press and the solvent is recovered by grinding the aqueous solution. With this method big quantities of solvent are lost. Nowadays most part of pectin is precipitated with Al3+ at pH 4. This precipitate, after filtration and pressing, it's suspended in ethanol, acidified with HCl (to remove the Cl-ions). Finally the normal pectins are filtered again, pressed, desiccated (till 2-5%) at high pH and low temperature. The pectin with low content of methossiles are sensitive to calcium pectate with small quantities of scarcely soluble calcium, differently from the acid-pectin - sugar gelatins obtained with the pectin at high methossilic content.

The main use of the pectin in the agrofood industry (that is the gelatin production) is similar to the domestic one (jam production): in fact the cooking of the fruit liberates the pectin that, in presence of high saccharose concentration the formation of a gel.

The highly methosilated pectin with about 70% of galatturonic esterified acid, are used as they are for the preparation of sugary jams and gelatins.

The scarcely methosilated pectin (esterification degree about 30%) that jellify with the presence of Ca++ ions and absence of saccharose are used always more to prepare gelatins at low caloric content.

Pectin are used largely also for other agro food products as the instantaneous powders, as stabilizers in the frozen sweets, in the milk products, and to ameliorate the frozen fruit aspect. In other fields they're used to prepare pharmaceutical products for the diarrhea, for the controlled release of medication contained in capsule, and from the industrial point of view as stabilizer in oil-water emulsions.

The lab tests developed within the Life Terpene project showed that the use of the PIDIC dynamic plant compared to the traditional filtration techniques ameliorates the pectin extraction thanks to the texturization of the citrus fruit pulp that makes possible the attack of the acid reagent. Within the project the tests for the pectin extraction have been developed only at preliminary level and so for the moment it's not possible to give other details.

Yeast production. From the liquid obtained by the pressing of the citrus fruit peels it's possible to obtain an alimentary yeast with a high tenor in proteins and vitamins.
The production process includes the following phases:

  • Dilution of the sugars till the concentration of about 0,5 %
  • Addition of yeast "Torula utilis"
  • Adjusting of pH and T
  • Mass aeration (to help the fungi growing, minimizing the alcohol production)
  • Addition of nourishing agents.

The desiccated yeast contain a percentage in raw protein varying from 44 to 55 % and can be considered an excellent source of vitamin of complex B (tiamin, riboflavin, niacine, pantotenic acid, ergosterol).

Industrial alcohol. The alcohol can be produced from the liquid called indusial molasses obtained by the pressing of the citrus fruit peels destined to the feedstuff production. 
In fact the such obtained industrial molasses contains some sugars - from 42 to 52% (molasses of Californian lemon) and this helps the alcoholic fermentation. The saccharose is mainly converted by the invertasi in glucose and fructose in accordance with the following reaction:

C6H12O6  2 C2H5OH + 2CO2

Actually the fermentative process is much more complex and foresees different step. The industrial alcohol is obtainable also as by product from the citric acid working.

Carotenic extracts. The carotenic extracts are natural alimentary coloring agents. They're obtained from ripe orange or tangerine, treated with suitable solvents, possibly using a reverse-current flow. The solvents can be re-collected and reused.

Other components. Also other less important constituents of the citrus fruits can be valorized from the economical point of view and be also profitable.

Limonine. The limonine is an excellent natural food bitter. It's used especially in the alcoholics and bitter soft drinks. It's possible to obtain it from the liquid extracted by the citrus fruit pulp through selective absorption on polyamides powders or other plastic materials, or by extraction with solvents.

Flavonoides. Over on century ago, the researches on the scurvy demonstrated that the lemon juice relieves and treated the illness. Then it was discovered that the therapeutic factor of the juice was C vitamin. But, when it was possible to produce synthetic vitamin C they realized that the vitamin alone didn't eliminate all the illness symptoms, especially the capillary fragility. In fact in 1936 they found that another group of molecules inside the lemon juice, having curative and preventive effect against scurvy: the flavonoides. These molecules own to an important vegetal pigment group and present the classical chemical structure similar to the anthocyanins, responsible for the blue, purple, violet and red color in the plants. 

Many substances have been studied, as the esperidin, eriodictiol, the quercitrin, the rutin and the epicatechin. The esperidin is the most important commercial source for P vitamin. It's soluble in alkali at pH 11. It's filtered and acidified at pH 6 with hydrochloric acid. Then it crystallizes. The flavonoides are molecules having anti-inflammatory properties. They are used in the treatment of the capillary fragility. They can be obtained from the citrus fruit pulp by extraction at pH 11 and subsequent precipitation at pH 4,7.
The fruits partially ripe have higher yield.

Citric acid. The best raw material to produce citric acid at industrial level are the citrus fruits juices, especially the ones obtained from lemon, bergamot, and lime. The citric acid is used in the alcoholic drinks and in many agro food products, in the pharmaceutical industry, in dyeing industry, etc. 

The process to obtain the citric acid from the citrus fruit juice is called Scheele Process, from the chemist that for the first time in 1784 isolated the citric acid. The process can be reassumed in the following equations:

2 C6H8O7 +3 Ca (OH) 2    Ca3 (C6H5O7) 2 + 6H2O
Ca3 (C6H5O7) 2 + 3H2SO4    2 C6H8O7 + CaSO4

The citric acid as calcium salt precipitates and is successively decomposed with sulphuric acid. The citric solution that comes out is sufficiently pure to crystallize. In fact , in the not treated citrus fruit juice the crystallization is impossible because of the impurities presence as sugars, pectin, inorganic salts, etc. Actually the production of natural citric acid is decreasing, nearly supplanted by the citric fermentation with Aspergillus niger coming from the beetroot or sugarcane molasses.

Lactic acid. The lactic acid can be produced by fermentation of the citrus fruit juice by the Lactobacillus delbruckii bacterium. The whole process needs from eight to ten days and a temperature of about 50°C. The yield is 71-84 % referred to the transformed sugars. The obtained lactic acid is neutralized with calcium carbonate because the lactobacilla activity is inhibited when the percentage of lactic acid exceed 1,5 %.

Essence extraction

The essences obtained from the citrus fruit juices shows, by one side, an ester content superior to the one contained in the peels, and by the other side a reduced content in aldehydes. This peculiar composition determines the special "fruity" scent.

Moreover only from essences produced from citrus fruit juices it's possible to separate the valencene, an sesquiterpenic from which it's possible to obtain the nootkatone, bicyclic sesquiterpenic ketone, used in perfumery to exalt or valorize other scents.