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Citrus fruits

The citrus fruits are of Asiatic origin (China, Burma) and, except for the kumquats that own to the Fortunella genus, they come from the Citrus genus, Aurantiacee subfamily, Rutacee family. Botanically speaking, the fruit is a hesperidium, that is a special berry with a juicy pulp divided into segments where the seeds are contained, specific of citrus fruits and related species.

The citrus fruit is formed by the following fundamental parts: the flavedo (external colored part of the peel), the albedo (white internal part of the peel), the pericarp that contains the above mentioned parts, and the pulp subdivided into segments and vesicas containing the juices and the seeds, called endocarp.

The flavedo is mainly formed by cellulosic material and contains other components as:

A. essential oils:
  1. terpens, alifatic sesquiterpens and oxygenated derivatives 
  2. bicyclic terpens and sesquiterpens 
  3. aliphatic not terpenic compounds 
  4. aromatic hydrocarbons 
  5. esters containing nitrogen 

B. components of the not volatile part of the essential oil
  1. paraffin waxes 
  2. steroids and triterpenoides 
  3. fat acids 
  4. cumarins, psoralens e flavones 

C. additional components:
  1. pigments (carotenoides, chlorophilles, flavonoides) 
  2. bitter principles (limonin) 
  3. enzymes (oxid-reducers, proteolitics, acetil-esterase, phosfatase, pectic enzymates) 

The albedo is mainly constituted by cellulosic material and also by:

A. pectic substances
  1. pectin, 
  2. protopectin, 
  3. pectic acid, 
  4. pectinic acids 

B. additional components:
  1. bitter principles (limonin) 
  2. enzymes (oxid-reducers , proteolitics, acetil-esterase, phosfatase, pectic enzymates ) 

The pulp structure is constituted by cellulosic material while the juice contains:
  1. carbohydrates (mono and disaccharides) 
  2. organic acids (citric acid and malic acid) 
  3. nitrogenous components (proteins, peptides, amino acids) 
  4. inorganic constituents (ashes) 
  5. vitamins (C vitamin) 
  6. lipids 
  7. volatile aromas (ethylic alcohol, acetone, acetaldehyde, formic acid, etc.) 
  8. pigments (carotenoides, chlorophyll, flavonoides) 

The seed
are constituted by cellulosic material with presence of:
  1. raw proteins 
  2. oils 

The composition of the terpenic mix varies depending on the typology of examined citrus fruit and of the species to which it owns. Anyway, the mix of each typology is, in different proportion, made of:

a - pinen
b - pinen
linail acetate (in the bergamot)

The incidence in percentage of the different parts that constitute the fruits vary depending on the typology of citrus fruit and is reported in the following table:

 orange            lemontangerinegrapefruit
peel %21.5 - 38.132.0-46.625.6-33.033.6-36.4
pulp %61.9-78.653.4-67.967.6-73.663.6-67.3
juice %23.8-51.021.6-27.032.5-38.630.0-33.6

The juice yield depends, besides the specie and variety, from the ripe degree, from the cultivation techniques and from the extraction method.

Basing on the actual industrial experiences, over the 60% in weight of the citrus fruits sent to industrial transformation for the production of juices is transformed in a mix of wastes (peels, part of the pulp, seeds) and is called "pastazzo". They can be used both at fresh state (es. for feedstuff ) or desiccated.

The edible pulp (endocarp) is held in a peel (pericarp) quite thick and not edible , except for the kumquats.

The citrus fruit peel is fromed by two parts.

The peripheric one (flavedo) colored in orange- yellow is composed by few cell layers that become progressively thicker in the internal part; the epidermic layer is covered with wax and contains a small amount of stoma, that in many cases are closed when the fruit is ripe.

When riped, the flavedo cells contain carotenoides (mostly xantofils) inside chromoplastides that, in a previous stadium, contained chlorophyll.

The flavedo internal part is rich in multicellular bodies with spherical - pyriform shape, that are full of essential oils.

The internal part (albedo) with white color is composed of layers of cells generally bigger, and less compact that, near the endocarp, form a cellular tissues less dense with intercellular spaces with irregular shape. The albedo contains the great part of the vascular reticule that supply food and water to the fruit. .

The endocarp is subdivided in segments with radial disposition (segments), each of them originated by a embryonic leaf; each segment is covered by a membrane that contains many multicellular big club shape vesicas linked with a membrane through stalk.

There is no vascular connection with the vesicas and so probably the adsorb water and food by diffusion through the tissue of the stalk. It's possible to see traces of the ovules and sometimes one ot two seeds along the edge of the segments. The axis of the fruit contains moreover some radially disposed vascular strips that are connected to the peel vascular elements in both the poles.

The best orange varieties for eating have such a pulp consistence that they dissolve themselves without leaving membran traces. Moreover the peel is easily removed by the pulp. A good variety for juices shall have different characteristics: resistant membrane not easy to remove and strong cellular walls are preferred in the conservation treatments because they hardly go in the juice.

Species and varieties

Even if many specialists proposed a great number of species (Tanaka, 159, Hodgson 36, Swingle, 16), we'll speak only about 8 of them subdivided in two groups:

  • Jellow colour species: Citrus lemon (L.), Burm. F., the lemon; C. medica (L.), the citron; C. aurantifolia, Swing, the lime; C. grandis (L.) osbeck, the shaddock; C. paradisi, Macf. the grapefruit; 
  • Orange colour species: C: aurantium (L.), the sour orange; C. sinensis (L.) Osbeck, the orange; C. reticulata Blanco, the tangerine. 

Lemon. This is the most important acid fruit both for the domestic consumption and for industrial working. Even if the tree is quite sensitive to the frost, it can be cultivated in marginal areas as the acidity and the juiciness are achieved even is the ripening season is quite cold.. Some varieties (Eureka, cosmopolitan, Femminello and Monachello, in Italy) re-bloom many times and are important to equilibrate the production in different seasons. Other varieties (Lisbon, cosmopolitan, Bernia, Spain; Interdonato, Italy) are mainly winter ones as the other citrus fruits are. The acidity is between 5 and 7% (mostly citric acid).

Citron. Not edible but the peel is used to prepare candied. The fruit is used by Jewish people in the Feast of Tabernacles.

Lime. Cultivated mainly in the tropical and sub-tropical areas because of its sensitivity to the cold . The real lime are small dimension fruits (Mexican lime or West Indies lime), cultivated in Mexico, India, Egypt. The fruits of the hybrid varieties are bigger and the trees suffer less from cold (Persian or Tahiti, Bears without seeds). The lime is very acid and is used for the preparation of juices.

Shaddock. Is cultivated in Thailandia only for commercial purpose . Without any industrial application except for preparation of domestic marmelade.

Grapefruit. This relatively new specie has great importance both the juice extraction industry, and for the consumption as fresh fruit. In the first case it's used in the production of juices and in the conservation of the tinned segments. The most important variety, cultivated all over the world, called Marsh, is very much used because of its high yield, its good taste (lightly bitter), and its lack of seeds. In Texas and in Florida the rosy varieties are cultivated (Thompson and Redblush), destined mainly to the food market as the pink carotenoides give some problems in the juices preparation.

Bitter orange. The fruit is not edible and is mainly used to produce marmalade. The oil is extracted from leaves and flowers. From the bergamot, considered a hybrid of this specie (called C. bergamia Risso), an essential oil, used as basis for some perfumes is as well obtained. The best oil is obtained by the plants of the Calabrian coast where it reaches the 35-45% of esters. The chinotto (Citrus aurantium, variety myrtifolia), produces round fruits, similar to small oranges, which weight is 40-50 g. In Italy it's cultivated in Liguria and used for the preparation of sweets and drinks.

Orange. This is the most important edible citrus fruit. The different varieties have generally a double application, in fact in many cases the fruits that cannot be sold for fresh consumption will be preserved. The oranges are subdivided in: common orange, orange with dark red pulp (blood red) and navel (containing a not developed small orange at the opposite pole from the juncture one).

In the first group we find: Salustiana, Cadenera, Comune and Bernia form Spain, Biondo and Ovale from Italu, Hamlin and Pineapple from Florida, Pera from Brasile, Shamouti (Jaffa) from Israele, Valencia (cosmopolites). The varieties Pineapple and Valencia are particularly suitable to be conserved.

In the second group we can find the blood red orange, the sicilian and spanish (that contain anthocyanin), which working present some difficulties because the color of their juice is not easy to be maintained.

The third group includes the Washington Navel and similar varieties, which cultivation is extended to all the citrus fruit areas because it's a very good fruit to be consumed fresh but not easy to be conserved because of its bitter late flavor.

Tangerine. There are many varieties of this fruit, with the peel not adherent to the pulp, generally used fresh even if they don't tolerate very well the transportation and the stocking. This fruit is conserved in big quantities only in Florida and Japan. The Japanese tangerine are of the Satsuma variety (classified also as C. unshiu Marcovitch). In Japan there is a diffuse market for the tinned segments.

In Florida mainly the tangerine of the variety Dancy are well preserved. The tangerine is the real C. reticulata. Other tangerine varieties are cultivated in the Mediterranean area and used for fresh consumption; they have a yellow lanceolate leaf (classified also as C. deliciosa Tenore). Finally , to complete this group, we shall speak about the hybrids. The hybrids of orange- tangerine include the clementine and the temple, with dark orange color, both very important. In Florida the hybrids of tangerine-grapefruit are as well cultivated (Tangelos). Generally they are all destined to the fresh consumption.

Fortunella spp. The Kumquats are "new" fruits of little commercial importance. They are used to prepare candies when the sweetish peel is edible, differently from the one of the real citrus fruits, and the fruit is completely eatable.

Ripening and ripe standard

The citrus fruits are picked up from the tree when ripen; when they're picked they don't ripen, they only increase their juiciness and they change their colour (if the fruit is green).

Except for the hydroids of the disaccharides that become mono saccharides, and consequently an increasing of sweetness, the amount of sugar doesn't grow in the fruits because the green fruit doesn't contain any starch reserve.

The modification that happen on the tree during the ripening season on a edible citrus fruit are mainly an increasing of the percentage of the total insoluble elements (SST) and a reduction of the percentage of citric acid. For the opposite tendencies of these two parameters , the ratio SST/acid (or Brix/acid) constitute a very sensitive ripening parameter, especially in the first phases. Some ripening standards have been fixed for the edible fruits in different countries. The standards in Florida are the most precise and diversified depending on the species, varieties and destination of the fruit (fresh for consumption or canned). The juice analysis helps to judge if the minimum standards have been respected by the early fruits and allows to calculate the juice liters and the SST kilograms obtainable from a lot of fruit, to fix an adequate price for the fruit to be bought.

The minimum ratio SST/acid for orange and tangerine vary from 8.5:1 to 9.5:1, for the grapefruit from 6:1 to 7:1; the higher is the SST percentage, the smaller can be the ratio SST/acid. Late in the season, the values grow considerably till becoming quite high when the fruit is too ripe.

In the case of the the acid citrus fruits, the above mentioned ratio hasn't got any meaning and as the acidity is not a problem, the ripening standards for lemons and limes are decided with the juice percentage that it's possible to obtain (lemon 25-30%, lime 42-50%) . Generally in lemons the percentage of citric acid doesn't decrease with the ripening of the fruit as happens in the edible fruits, and probably it increases (even if the data on this matter are quite scarce and not completely convincing). Other ripening parameters , es the ratio citric acid/ malic acid (that in the oranges increases during the ripening) or the ratio Brix-arginin (both Brix and arginin increase in the orange during the ripening of the fruit) have been proposed but not yet in practice. Any serious test to link the ripening with the aromatic volatile compounds have never been performed.