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Preservatives are small molecules that are added to foods to stop or slow the progress of a deterioration reaction. This section is a discussion of chemical preservatives but it is important to remember that the use of any additives is prohibited where the same results could be achieved by good manufacturing practice. Chemical preservatives should only be used as part of a holistic strategy (along with preservative processes such as freezing, blanching, dehydration, cooking, salting, and packaging) to deliver the highest quality, safest foods possible.
The amount and type of preservatives permissible is legally regulated on a case-by-case basis and must be declared on the label. It is also good commercial practice to avoid preservatives where possible as "chemicals" in food may be avoided by consumers.
Different preservatives are effective against different spoilage reactions. It requires some food chemistry and usually some practical experience with a given product to identify which reactions are likely to be a problem. The more you know about the spoilage chemistry the better positioned you are to make intelligent decisions on how to stop it.
Sulfur dioxide is a useful preservative but its use is declining in foods because of its antinutritional effects (thiamin is effectively destroyed) and allergic reactions from a small population. We will look at sulfur dioxide in some detail as an illustration of the problems and advantages found in using chemical preservatives in foods.
Structure. Sulfur dioxide is a toxic, choking gas that is almost never found in foods. In fact when the gas is mixed with water a variety of salts of sulfurous acid form: sulfurous acid (in fact hydrated sulfur dioxide gas), bisulfite ions and sulfite ions.
Which species is present can be calculated from the pH of the food using the Henderson-Hasselbach equation. Most foods are in the pH range 3-7 so usually the most prevalent compounds are bisulfite (and sulfite) ions. "Sulfur dioxide" is sometimes used to describe the additive in a food but be aware there is little or no free SO2 present. Sulfites is also used and this is a little more accurate but the best term is S(IV) in recognition of the fact that the sulfur is all in oxidation state +4.
Preservative Effects. We have already seen, sulfites as effective inhibitors of Maillard and enzymic browning. In both of these cases the sulfite bound to a carbonyl intermediate group (DH or quinnones respectively) and prevented them from polymerizing to form a brown pigment. Both of these reactions depend on sulfite acting as a nucleophile to the electron-poor carbon of the carbonyl group to form a C-sulfonate:
Note the reaction is completely reversible. If the amount of free sulfite is depleted, the adduct will break down and the preservative effect will be lost.
Acting as a nucleophile is an important mechanism of sulfite preservative action. The other is that sulfite ion can oxidize:
SO32- + 1/2 O2 -> SO42-
The reaction is (usually) irreversible. It consumes oxygen and can so limit the oxidation of other compounds.
Deleterious Effects. In order to function, preservatives must be highly reactive molecules. Chemical reactivity is rarely very specific and the additive may react in an uncontrolled way with other food molecules present. An important example of this is the nucleophilic attack of sulfite ion on a thiamin molecule which destroys the vitamin activity.
Sulfites are banned in foods that are a significant source of thiamin (e.g., meat produts).
Toxicity. Sulfite ions are are absorbed into the blood stream and very rapidly oxidized to sulfate ion by the enzyme sulfite oxidase. The sulfate is safely and rapidly excreted in the urine making sulfite non-toxic even in relatively large quantities for most people. Some people are born without sulfite oxidase and this tends to be catastrophic (there are significant concentrations of sulfur dioxide in the atmosphere even if it is avoided as a food preservative).
Although sulfite is safe, sulfur dioxide is harmful in large quantities and even small amounts can trigger an attack in a certain class of asthmatics. Low pH foods are a particularly high risk for producing gaseous SO2