A review of sulfites in foods:  analytical methodology and reportes findings
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A review of sulfites in foods: analytical methodology and reportes findings

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FOOD ADDITIVES AND CONTAMINANTS, 1990, VOL. 7, NO. 4 , 433-454

A review of sulphites in foods: analytical methodology and reported
findings

T. FAZIO and C. R. WARNER
Food and Drug Administration, Center for Food Safety and Applied Nutrition, Office of
Physical Sciences, Washington, DC 20204, USA

(Received 7 August 1989; accepted 18 September 1989)

Sulphites in various forms have been added to foods for centuries. Their use became an
issue of concern when certain sensitive individuals exhibited adverse reactions to sulphite
residues in foods. Analytical methods were developed to monitor these compounds at the
regulatory limit of 10 ppm. In this report, analytical methods for determining sulphites in
foods are reviewed, along with a critique of their chemistry and procedural schemes. An
assessment of the key features of each method category is presented together with some
comparative data. The classification scheme used is based upon the fact that determination
of the sulphite content of a food is influenced more by the treatment and cleanup of the
test solution than by the final determinative step. Based on a 60-year database, the
Monier—Williams procedure still remains the method of choice.

Keywords: sulphites, Monier-Williams, sulphiting agents, sulphites methods review

Sulphites
Sulphites in various forms have been added to foods as preservative agents and

for other purposes for centuries. The use of sulphiting agents or S [IV] oxoanion
compounds has been traced to antiquity when sulphur dioxide was used by the
Egyptians and Romans to cleanse and disinfect wine vessels (Taylor and Bush 1986).
The practice of adding S [IV] compounds to wine has continued to the present day,
and food technologists have developed dozens of additional applications for S [IV]
compounds (Walker 1985). These substances have been used to maintain the
outward appearance of freshness in salad bar vegetables, assure a desirable texture
and colour in instant potatoes, and prevent the unappetizing, but apparently
harmless, 'black spot' formation on shrimp. S [IV] oxoanions such as sulphite,
bisulphite, and metabisulphite have many functional uses in food because they are
effective bleaching agents, antimicrobials, oxygen scavengers, reducing agents, and
enzyme inhibitors.

Many raw foods, when exposed to the air, rapidly discolour or otherwise lose
their natural appearance. Lettuce wilts and browns around the edges; cut apples and
peeled potatoes become brown; and avocado pulp turns black. In order to slow these
oxidative processes, retail establishment operators in the past treated foods with
chemicals called sulphiting agents. The practice of adding sulphites to foods that are
to be sold or served raw to the public was recently banned (Federal Register 1986b)
by revocation of the generally recognized as safe (GRAS) status of sulphites for use
on fruits and vegetables intended to be served or sold raw to the consumer, except
for potatoes and grapes. Also any food containing more than 10 ppm of a sulphiting
agent must have a label declaration (Federal Register 1986a).

© US Government 1990

434 T. Fazio and C. R. Warner

Concern over possible hazard from sulphiting agents also goes back a
considerable time, to an article by Kionka (1896), on the possible toxicity of
sulphites in foods. Recently the use of sulphites in foods has become an issue of
concern to both consumer and regulatory agencies (Hecht and Willis 1983).
Sulphites have been implicated as initiators of asthmatic reactions on a small subset
of sensitive individuals of the asthmatic population (Congressional Hearing on
Sulphites 1985). Asthmatic attacks can be serious, and the ingestion of foods
containing sulphites has been alleged to have caused several deaths in recent years.
(The Food and Drug Administration (FDA) has received reports of 27 such deaths
as of April 1988.)

The physiological reactions to sulphites include anaphylactic shock (which
causes constriction of the bronchial passages), headaches, abdominal pains, nausea,
dizziness, and hives. Asthmatics as a group seem to show the greatest sensitivity to
sulphites; an estimated 5-11 % of all asthmatics are affected, amounting to between
j and 1 million people in the United States (Prenner and Stevens 1976, Freedman
1977, Baker and Collett 1981, Baker and Allen 1982, Twarog and Leung 1982,
Werth 1982, Congressional Hearing on Sulfites 1985, Taylor etal. 1986). In
addition, approximately 25% of the reported cases have been people with no history
of asthma. Investigations into this potential health hazard and the knowledge of the
widespread use of S [IV] compounds in foods led to promulgation of regulations
concerning the usage and labelling of sulphiting agents (Federal Register 1986a,b).

'Sulphites' or 'sulphiting agents' are terms generally applied to a variety of
sulphur-based compounds. The six sulphiting agents most commonly used by the
food industry are sulphur dioxide, sodium and potassium metabisulphite, sodium
and potassium bisulphite, and sodium sulphite. All six of these compounds are
GRAS chemical preservatives except for use on fruits and vegetables (excluding
potatoes and grapes) intended to be served or sold raw to the consumer. These
compounds can be used in any food except meat or food that is a recognized source
of vitamin Bi. The chemical formulas of these sulphiting agents are shown in
table 1.

When added to a food matrix, some of the sulphiting agent binds to the
molecules of the food and some of it does not. The portion of the sulphiting agent
that does not combine with the food is called 'free sulphite'. This 'free sulphite' is
a mixture of sulphur dioxide, bisulphite ion, and sulphite ion in dynamic chemical
equilibrium. The percentage of each of the three chemical species in the matrix is
dependent upon the pH (acidity) of the food (Wedzicha 1984b, p. 16). Two forms
of bound sulphite have been observed, reversibly bound and irreversibly bound

Table 1. Chemical formulas
sulphiting agents.

Agent

Sulphur dioxide
Sodium sulphite
Sodium bisulphite
Potassium bisulphite
Sodium metabisulphite
Potassium metabisulphite

of typical

Formula

SO2
Na2SO3
NaHSO3
KHSOj
Na2S2O5
K2S2O5

Review of sulphites in foods 435
sulphite. Under certain conditions some of the bound sulphite molecules will
dissociate (or break apart) and form free sulphite. The portion that dissociates is
called reversibly bound sulphite. The portion that does not dissociate is referred to
as the irreversibly bound sulphite. Figure 1 shows the interconversion equations of
oxysulphur (IV) compounds and the interconversion effected by pH changes.

The widespread use of sulphites in food processing is due to the
multifunctionality of these compounds (Wedzicha 1984a). Their primary function
is to act as a preservative or antioxidant to prevent or reduce/retard spoilage and
discoloration during the preparation, storage, and distribution of many foods
(FASEB 1985). Various forms of sulphites have been used to prevent the browning
of light-coloured fruits and vegetables such as dried apples and instant potatoes.
They are also used in wine making as selective antibacterial agents which do not
inhibit the desired development of yeast. Sulphites serve a special function in the wet
milling of grain (corn), in which the additive softens the hard kernel to permit
removal of corn starch. They also have a general preservative effect, as in conserving
the carotene and vitamin C content of foods, for example. Some common foods
containing added sulphites that persist are shown in table 2 (FASEB 1985).

Since 1959 six sulphiting agents (sulphur dioxide, sodium sulphite, sodium and
potassium bisulphite, and sodium and potassium metabisulphite) have been listed
in the Code of Federal Regulations (CFR) as GRAS. This GRAS status was codified
in 21 CFR 182.3616, 182.3637, 182.3739,