The Need for Alternatives
This chapter describes a large number of chemicalreactionsinvolvingS(IV)infoods and it is understandable that they have causedconcernaboutthesafetyoftheuse of sulfur dioxide as a food additive. ThetoxicityofingestedS(IV)isregardedas low on account of the very efficient sulfiteoxidasedetoxifyingsystempresentin the liver of all animals.83,84 The major reactionproductfromtheinhibitionofMaillard browning, DSH, is metabolically inert.85,86 Thedestructionofthiamin87 isthe"clas-sical" example of an adverse effect of a food additive on the nutritional quality of a food, but this loss of vitamin is not regarded as being of any significance in practice. It has, however, led to the view that sulfur dioxide should not be used in foods that are an important source of thiamin. On the other hand, the products formed when disulfide bonds in proteins react with S(IV) (Figure 8.6, reaction II) either in food or in vivo are metabolized quickly and harmlessly.84 Thus, from the available evidence, we see that despite the somewhat indiscriminate reactivity of S(IV) in foods, this is not associated with any known toxicological hazard to humans.
A critical appraisal of the evidence presented by Til and Feron,83 on whose research the acceptable daily intake (ADI) of S(IV) has been set by the European Community, indicates that there are a number of minor toxicological effects that have not yet been explained. Of particular interest is the observation that toxic effects are sometimes associated with the oxidized lipid fraction of sulfited diets, and it is speculated here that sulfite-specific oxidation products may be involved. However, the compelling reasons why much effort has turned to seeking alternatives to sulfur dioxide is the fact that some individuals are abnormally sensitive to low concentrations of SO288 in the head space above sulfited foods, or as a result of eructation after a sulfited food has been eaten and come into contact with the acid environment of the stomach.
Estimates89 of the per capita consumption of sulfur dioxide (taking account of all the possible forms in which it may be added to food) in the U.S. by reference to 782 food products reveal a daily intake that is 44% of the ADI (210 mg per person per day based on an individual weighing 60 kg). On the other hand, a total diet survey suggests that the typical intake is probably somewhat lower at 18 mg per person per day, with extreme users reaching some 64 mg per person per day. Alcoholic beverages, soft drinks, sausages, and hamburgers are the most significant contributors to the dietary intake of sulfur dioxide. Dehydrated fruits can also be a significant source of dietary sulfur dioxide for those who eat them. These levels of consumption of sulfur dioxide are among the highest intakes of food additives in relation to their ADI values. This evidence, taken together with the abnormal sensitivity of certain individuals to gaseous SO2, has been the principal reason why there has been particular interest in reducing the levels of the additive and finding suitable replacements.
There have been a number of comprehensive reviews outlining the main issues that need to be considered for replacing sulfur dioxide in foods.90 91 The role of S(IV) in controlling the diverse spoilage reactions in food as an antimicrobial agent, an inhibitor of browning, and an antioxidant are recognized and our ability to offer suggestions for replacements is based on a good understanding of the mechanisms of its preservative action. It is often said that the complete role of S(IV) in food extends beyond the obvious chemical reactions associated with its preservative action, and includes as yet unknown contributions to subtle changes in quality. For this reason there is a need to understand the full range of contributions of S(IV) to the quality of preserved foods.
Whereas S(IV) is unique in its ability to control simultaneously several forms of food spoilage, there are possible replacement food additives to control individual spoilage processes. Thus, alternative antimicrobial agents could include benzoates and sorbates, and antioxidants include citrate, tocopherol, and BHT, among the wide range of food additives currently available for these purposes.
It is often said that S(IV) is unique in its ability to control browning in food. The mechanism of inhibition of enzymic browning is the reaction of sulfite ion with the o-quinones which are formed by the enzymatic oxidation of o-diphenols. Essentially the quinones are reduced to the sulfonated phenols (Figure 8.6, reaction VI). In the case of catechol oxidation, it has been shown that the 4-sulfocatechol which is formed upon reduction of the quinone is unreactive towards polyphenol oxidase and so represents a relatively stable product.92 In general, inhibitors are expected to work in two ways. Either they inhibit the enzyme or, in the same way as S(IV), they react with the quinone intermediates. Ascorbic acid is probably the best known reagent that acts by the latter of these mechanisms. However, there has been a longstanding interest in specific inhibitors of the enzyme which work at much lower concentrations than typical concentrations of S(IV) added to food. Hydroxycinnamic and benzoic acids,93 kojic acid (5-hydroxy-2-(hydroxymethyl)-y-pyrone),94 4-hexyl-resorcinol,95 ficin,96 C3-C5 aliphatic primary alcohols,97 and even honey98 are examples of potentially useful inhibitors of varying degrees of effectiveness. Of course, where applicable, a very simple solution to prevent enzymic browning when foods are stored is the action of heat (e.g., by blanching), since polyphenol oxidase is relatively heat labile. When foods are treated in this way, only nonenzymic browning is said to occur.
The search for alternative antibrowning agents against nonenzymic browning has proven to be particularly difficult. It has long been known99 that thiol compounds inhibit the Maillard reaction in a way similar to S(IV), but the practical use of such additives was not advocated seriously until Friedman conducted a series of studies involving N-acetyl cysteine and glutathione.100-102 It is believed that thiols react with intermediates such as DH in much the same way as sulfite ion103,104 because the thiol group is similarly nucleophilic. Studies in the author's group suggest that N-acetyl cysteine is not sufficiently stable in acid solution and is converted slowly to cysteine which reacts with Maillard intermediates to form characteristic "meaty" odors. On the other hand, dipeptides of cysteine (N-substituted cysteine) with another amino acid are an excellent choice.105 Figure 8.9 shows a comparison of the rates of browning of a glucose-glycine mixture in the presence of S(IV), mercaptoethanol, the dipeptides, and glutathione. These results indicate that the dipeptides are generally as effective as S(IV) except that S(IV) is better at controlling the formation of low levels of color in the early stages of the reaction. Thiols are seen to be highly reactive towards cabbage on blanching and during dehydration,106 but unpublished evidence suggests that the mixture of products is much more complicated than that obtained when S(IV) is used as the antibrowning agent. There is as yet no evidence regarding the nature of the products formed when thiols inhibit such browning reactions.
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