The citrus peel, most commonly used for candying, comes from sweet and bitter oranges, lemon, citron and grapefruit. Peel is selected in relation to its thick, possibly turgid albedo which should easily separate from the carpellar membranes. Separation used to be hand made but, nowadays, it is done mechanically during juice extraction. Depending on the targeted finished product, the peel is cut in cubes or strips. Candying operation consists of the absorption on the peel (previously washed and softened) of an increasingly concentrated solution of sucrose and glucose. Candied peel should appear translucent, turgid, doughy, no mushy and homogeneous, with dry, not-cracked surface, a regular colouring, a sweet, not off-flavours or cooked flavour; all these features should remain constant keeping the product at room temperature. The best procedure implies using fresh fruit. However practical reasons such as fruit seasoning, inconstant quality of raw material, candying duration, and the eventual candying of other products as pumpkins, watermelons, cherries, peaches, flowers, etc., account for the frequent use of previously preserved peel. Brined preservation of citrus peel is one of the most widespread. The peel is plunged in a 5—7 per cent sodium chloride solution for 4—5 days. The solution is then drained and peel placed in appropriate vessels, packed in layers with salt and kept in fresh brine for about two months. The brine is then replaced with a new more diluted solution and, most of the times with the addition of a 500-1000 ppm sulphur dioxide solution. During the first phases of the brine processing, a lactic fermentation occurs which will proceed throughout the following phases. The fermentation soften the cellulose parts which enables osmotic changes between the peel and the brine. This treatment allows a translucent, soft, no mushy peel with a good turgidity that can be further improved by adding a modest quantity of calcium hydrate. A further system, which avoids the fermentation, implies that the peel is repeatedly treated with calcium hydrate and with increasingly concentrated sodium chloride solution (2, 4, 6, 8, 10 per cent), each treatment lasting for about one hour at 45 °C. This procedure allows a better keeping of the fruit albedo after desalination (Crupi, 2000). The raw material obtained from one of the above procedures is sent to candied fruit processing industries where more or less rapid systems are used all based on peel desalination in cold and/or hot water. This phase, called cooking (softening), is one of the most delicate of the entire processing. Its purpose is to enable osmotic exchanges during syrup preparation, while extending the tissues and causing fruit softening and oxidase inactivation. Slow candying process, not as common anymore, envisage peel washing and cooking followed by the arrangement in pools or small baskets and plunging in syrup which flows countercurrent from one pool to the other. During this sequence the syrup concentration will decrease, until when, in the last section the syrup is drawn, re-concentrated and recycled. The whole processing cycle will take 10—15 days. More rapid methods are performed under vacuum conditions and at lower than boiling temperatures. This allows a shorter time for candying processing (18-24 hours) as the syrup concentration is maintained and the sugar penetration in tissues is made easier by the interstice dilation. When the vacuum is interrupted the pressure increase will push a greater quantity of syrup in the peel. The above procedures are repeated and the syrups are gradually raised from 50° to 75° Bx either by adding extra solutions or, by widening the vacuum, through concentration in the same candying vessel. All the procedures of washing desalination, candying and finishing can be carried out in the same device. This generally consists of baskets set in controlled under vacuum autoclaves. The product is placed in the baskets and the syrup, which is heated by an external heat exchanger, flows on it. After candying, the product is drained and centrifuged (Decio and Lodigiani, 1990). A further system (Swisher and Swisher, 1977) envisages about one hour boiling of softened and dried peel in sugar solutions (sucrose), at increasingly higher concentration up to 50° Bx, and with a slight use of citric acid. After 20 hours standing, the product is boiled and concentrated until the temperature of 104°C is reached. Candied peel is hot drained, dried and covered with maize starch or powered sugar. The remaining humidity can be eliminated drying the product in hot air.
Nowadays, candied peel production has been, mostly, taken over by bakery and ice-cream industries.
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