16 INHIBITION OF BROWNING BY SULFITES

B.L. Wedzicha, I. Bellion and S.J. Goddard Procter Department of Food Science University of Leeds, Leeds, LS2 9JT, Great Britain

ABSTRACT The present state of understanding of the mechanisms by which sulfites inhibit browning reactions in food is reviewed. The difficulties of specifying the composition of sulfur(IV) oxospecies in sulfited foods arise from the existence of labile equilibria between S02' US03-, 80 32and S2052-, whose position depends on concentration, ionic strength and the presence of non-electrolytes. A proportion of the additive is also found in a reversibly bound form. The main reason why sulfites are able to inhibit a wide range of browning reactions is the nucleophilic reactivity of sulfite ion. The mechanism of reactions between sulfite species and intermediates in the model Maillard browning reaction, glucose + glycine, are considered in depth and are supported by kinetic data. A most interesting feature is the fact that sulfites seem to catalyse the reactions they are added to control. Reaction products include 3,4dideoxy-4-sulfohexosulose which is formed initially and polymeric substances arise from the reaction of sulfite species with melanoidins. It is found that melanoidins from glucose + glycine react with sulfite to such an extent that one sulfur atom is incorporated for every two glucose molecules used to make up the polymer. The mechanisms of inhibition of ascorbic acid browning, enzymic browning and lipid browning are reviewed briefly. The known toxicological consequences of the formation of reaction products when sulfites are used for the control of Maillard browning give little cause for concern. Little is known of the implications of the formation of reaction products during the inhibition of other forms of browning. Consideration of the requirements for alternatives to sulfites is given. INTRODUCTION It is generally regarded that sulfite species (S02' HS0 3-, S032-) are unique in their action as inhibitors of browning in food. The mechanisms of the various known browning reactions (enzymic, Maillard, ascorbic acid, lipid) are very different from one another, and it is tantalising to consider the features of these reactions and of the additive which cause it to be so versatile. A review on this subject is

M. Friedman (ed.), Nutritional and Toxicological Consequences of Food Processing © Springer Science+Business Media New York 1991

217

timely because there is considerable interest in possible replacements for sulfites (Friedman and Molnar-Perl, 1990; Molnar-Perl and Friedman, 1990a, 1990b) in applications where those suffering from asthma may be exposed to significant levels of gaseous sulfur dioxide (Giffon et al., 1989). A detailed understanding of the mode of action is essential ~ progress towards alternatives but we will show here that a study of the reactivity of sulfites in model browning systems also provides a new route to detailed information about the mechanisms of the browning reactions themselves. When used in the context of a food additive, the term sulfite or sulfur dioxide is somewhat imprecise as it refers to a mixture of oxospecies of sulfur in oxidation state +4. The actual species present depend on many variables including the pH, ionic environment, water activity, presence of non-electrolytes and concentration of the medium in which they are dissolved. In some instances the term includes that additive which is reversibly bound to food components and no longer represents sulfur(IV) species; such bound additive is, however, released during the standard analytical procedures used to enforce the level of the additive in food. In order to simplify the specification of the amount of additive present in a food without the need to assess its detailed ionic distribution, it is normal to refer to the weight of sulfur dioxide equivalent to all the forms present. Many procedures for the analysis of the additive do, indeed, involve conversion of all sulfur(IV) species to gaseous S02. In order that the terminology be unambiguous here, the term S(IV) will be used to denote a mixture of oxospecies of sulfur in oxidation state +4 when it is not desired, or even possible, to specify the actual forms of the additive present. The terms sulfur dioxide, sulfite etc. will only be used when it is desired to refer to specific molecules or ions. We begin this review with a critical examination of the nature of sulfur(IV) species present in solution as a function of pH, ionic environment, concentration and water activity. SULFUR(IV) OXOSPECIES IN AQUEOUS SYSTEMS Sulfites are salts of sulfurous acid, the existence of which has been questioned from experimental evidence (Davis and Chatterjee, 1975) and on thermodynamic grounds (Guthrie, 1979). It is unlikely that significant concentrations of H2S0 3 exist in solution; dissolved sulfur dioxide is best regarded as S02.H20 in which any interaction between S02 and H20 is very weak and undetectable spectroscopically. The dibasic acid ionizes according to: ~

HS0 3 -

~

SO 2-

~

3

with pKa values of 1.89 and 7.18 (25 °C, zero ionic strength) for the first and second ionizations respectively (Huss and Eckert, 1977; Smith and Martell, 1976). The fact that ionization of the acids changes the ionic charge and the number of ions present implies that the equilibria should be affected by ionic strength. The effects of 3 salts (NaCl, NaN0 3 and Na2S04) on the pKa of HS0 3- are illustrated in Figure 1 (Wedzicha and Goddard, 1988; Wedzicha and Goddard, 1991). The data are consistent with predictions of the effect of ionic strength on activity coefficients of the ions in question, using the extended Debye-Huckel formula (Robinson and Stokes, 1965). The differences between the 3 salts tried are accounted for by 218

choice of parameters in the Debye-Huckel equation and are not necessarily the result of specific salt effects. Whilst some ion pairing between Na+ and 80 32- has been identified (Wedzicha and Goddard, 1991), the significance of this is not certain. The effect of ionic strength on the ionization of S02.H20 follows the same trend as in the case of HS0 3 - and is similarly explained in terms of general salt effects. A weak association between 802 and chloride ions is also evident. The many non-electrolytes present in foods, including humectants, are likely to affect the solvation of ionic species, particularly at high non-electrolyte concentration. The effects of polyethylene glycol (PEG-400), glycerol, ethanol and sucrose on the pKa of 802 .H20 and H80 3are illustrated in Figures ~ and d respectively (Wedzicha and Goddard, 1991). These effects are seen to be substantial and opposite in direction to the effects of moderate concentrations of salts. Unlike the effect of salt, it is not possible to quantitatively reconcile the effect of non-electrolyte with any basic or empirical theory, but it is possible that the greater tendency for charged species to be solvated is the reason why non-electrolytes tend to increase the bias for less highly charged molecules. Thus, 802 is favoured in the 802/HS0 3 - equilibrium and H80 3 - in H80 3-/S03 2-. If one is to extrapolate these findings to concentrated foods, e.g. dehydrated or partially dehydrated fruits and vegetables, the view (Wedzicha, 1986) that these systems are at c. pH 5.5 and hence the predominating form of 8(IV) is H80 3 -, is very naive and possibly far from the truth. The effect of ions is to increase the concentration of sulfite ion at low to moderate ionic strength and perhaps to reduce it at very high ionic strength. On the other hand the effect of non-electrolytes would be to reduce the tendency for formation of sulfite ion at the pH of these foods. It is interesting that sucrose has little or no effect on the equilibria in question over the range of concentrations shown in Figures ~ and d, but it has not yet proved possible to measure pKa values in systems at the limit of solubility as found in dehydrated foods. 7.1

6.9 6.7

6.S

....

~

6.3

6.1

5.9

JIM

Figure 1. Effect of ionic strength, I, on the pK of a 50 roM solution of H503- at 30 °C. Ionic strength adjusted using @ NaCI, A NaN03 , El Na2so4' Reproduced from Wedzicha and Goddard (1991). 219

3.5

3.0

2.5

2.0

o

20

40

60

80

wtO!o nonelectrolyte

Figure 2. Effect of concentration of non-electrolyte on the pK of a 45 roM solution of S02 .H20 "at 30 °C. 0 Ethanol; .. Glycerol; iii PEG-400; " Sucrose. Reproduced from Wedzicha and Goddard (1991).

o

20

40

60

80

wtO!o nonelectrolyte

Figure 3. Effect of concentration of non-electrolyte on the pK of a 50 roM solution of NaHS0 3 at 30 °C. 0 Ethanol; A Glycerol; iii PEG-400; " Sucrose. Reproduced from Wedzicha and Goddard (1991). 220

The likely predominance of HS0 3- in concentrated foods renders it important to examine the formation of metabisulfite (S2052-) ion, according to: ---::. ~

°

S25 2-

In dilute solution, the equilibrium is well over to the left (equilibrium constant =0.033 1 mol-I) (Connick et al., 1982) but the degree of association of H803- increases wit~c~entration for two reasons. First, the law of mass action predicts that the position of equilibrium be concentration-dependent. Secondly, the value of equilibrium constant is dependent on ionic strength, the variation being described approximately by, K = 10(-1.398

+

0.3sJI)

for 0.15 M(I

Inhibition of browning by sulfites.

The present state of understanding of the mechanisms by which sulfites inhibit browning reactions in food is reviewed. The difficulties of specifying ...
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