The use of sulfur dioxide and its salts to preserve fruits and some vegetables is a very old process. Most fruits were harvested over a very short period and had to be preserved in bulk until they could be processed into jams and jellies. Sulfur dioxide served this role very well. Fruit could be cleaned, placed into barrels, and covered with a solution of sulfur dioxide in water, or a solution of sodium bisulfite or one of its salts. After mixing in the barrels, the fruit could be kept for years. The fruit could be removed from the barrels, placed in a kettle, and boiled to remove the sulfur dioxide. After addition of pectin and sugars and boiling to produce the required moisture level, the product could be filled into consumer-sized packages. Strawberries were an important sulfited product because of the large market for strawberry jam but the sulfite treatment bleached out the red color. The chemical reaction was a simple addition reaction between the red anthocyanin pigment and the sulfite ion that was easily broken such that the boiling process reduced the sulfite content and restored the desirable red color.
The sulfite process is important today in the manufacture of maraschino cherries. Nearly all the cherries used for maraschino cherries are mechanically harvested and suffer some mechanical damage in the process of being shaken off the tree. Since cherries discolor very quickly, they are conveyed in a matter of seconds into a brine tank on the mechanical harvester. The simplest of the sulfite brines is a solution of sodium bisulfite in water, but a number of other formulations are used. The purpose of the sulfite brine is to preserve the fruit by inhibiting the enzymes that produce discoloration and to bleach out the red color. Sometimes a secondary bleach in sodium chlorite is used. A source of calcium ions is usually added to the brine to firm the cherries. The brined cherries are leached to remove the sulfite ions and any other soluble material and conveyed to a tank containing a red colorant dissolved in a sugar syrup. The cherries readily absorb the red colorant and are then drained and packaged. Maraschino, candied, and glaceed cherries are made the same way except that the sugar content is different.
Around the beginning of the twentieth century, sulfiting was the major way to preserve fruits in bulk for further processing in Europe, particularly in England because of the English people's liking for strawberry jam. The introduction of freezing provided a method of preservation that produced a superior final product from both an appearance and a flavor point of view and sulfiting decreased in importance. Sulfiting is still used in some parts of the world but the technology has been essentially displaced except for maraschino cherries.
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