During squeezing, the essential oil coming from epicarp passes in the juice. If the oil quantity is limited, no more than 0.15 per mil, it participates to the bouquet formation, of the typical flavour which characterizes the species; a higher content is a negative factor also for juices with short shelf life stored at low temperature.
In addition to the enzymatic degradation occurring in juices with mild or no thermal processing, the acid pH causes chemical alterations, most of all at the expense of monocyclic and bicyclic terpenes (Dugo etal., 1990; Verzera etal., 1990). The single strength lemon juice taste, even if stored at low temperature, can show significant degradations depending upon its essential oil content.
The oxygen absorbed in the juice during technological operations is another cause of decay: ascorbic acid is oxidized and ¿/-limonene is attacked with formation of terebenthic taste.
The recent trend of industry is to reduce to a minimum these inclusions with the use of adequate equipments, like extractors modified in order to avoid oil excess; for what concerns oxygen, it is recommended to reduce the close contact between juice and air.
The essential oil excess can be removed centrifuging the juice with de-oiler centrifuges or through partial distillation in vacuum: a limited juice rate, between 5 and 6 per cent, is transformed in vapour at about 50 °C; in these conditions, the 80 per cent of essential oil is removed together with the air present in the juice. The non-terpenic aromatic fractions, condensed again, are reintroduced in the juice after the oily phase has been separated by means of decantation or centrifugation.
When the juice must not be confectioned but concentrated, the operations of de-oiling and de-aeration occur inside the system of concentration in vacuum, in a different way depending on the constructing scheme of the used evaporator. Several systems are used in the case of freeze-concentration and of osmotic concentration, depending on the productive cycle.
The contemporary reduction of essential oil and oxygen from citrus fruit juice is obtainable also with the use of a spinning cone column, constituted by a vertical stainless steel cylinder in vacuum, which has a series of inverted cones inside; alternatively, a cone is soldered to the cylinder wall and is therefore fixed, while the following one is linked to a rotating shaft. At the top of the column the juice to be treated enters, flowing because of gravity on the upper surface of the first fixed cone, and from here it drops on the following cone connected to the rotating shaft. Because of centrifuge force, the juice ascends toward the brim of the rotating cone and flows on the brim of the third cone, which is fixed, and so on until the juice, de-oiled and de-aerated, reaches the bottom of the column. Due to rotation, alternate with fixed cones, a thin turbulent film is generated. From the bottom of the column a gas current rises, generally water vapour, which helps the stripping of volatile components; they go out from the head of the column and are condensed. Operating at 27 °C, more than 90 per cent of oil and more than 95 per cent of oxygen present in the juice are removed (Schofield, 1994; Schofield etal., 1998).
The oxygen present in citrus fruit juices can be strongly reduced by means of glucose-oxidase which catalyses the oxidation of glucose to gluconic acid.
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