Safety evaluation of certain food additives - ipcs inchem

SULFITES: ASSESSMENT OF DIETARY EXPOSURE 223
(INS 227), potassium bisulfite (INS 228) and sodium thiosulfate (INS 539). Sulfites
are added to foods such as processed meats, dried fruit, wine, beer, fruit and
vegetable juices, water-based flavoured drinks and processed fish and seafoods,
with the major functions of preservation and inhibition of browning reactions. The
ability of sulfites to prevent further oxidation of polyphenols is especially important
in wine production, where these components are considered to contribute to the
taste, texture and colour of the final product (Burroughs, 1981; Ough, 1986). Sulfites
may also occur naturally in some foods, especially in fermented ones, as a result
of endogenous formation by yeast during the fermentation process (Taylor et al.,
1986).
Sulfites may be present in food as sulfurous acid, inorganic sulfites and a
variety of reversibly and irreversibly combined forms. They react rapidly with a
variety of food constituents, including reducing sugars, aldehydes, ketones and
proteins, to form various combined sulfites, such as the highly stable
hydroxysulfonate adducts. The amount of compound in each of these states is
dependent on a number of factors, including the food matrix and the pH. Sulfites
reversibly bound may dissociate into free sulfite when the food pH is raised above
10 or when acidified solutions are heated until boiling (Ough, 1986; Fazio & Warner,
1990; Lück & Jager, 1997). Adducts irreversibly bound are usually formed by the
reaction of sulfites with alkanes or aromatic compounds, giving rise to sulfonic acids,
which are not recovered when the sulfited food is submitted to alkaline conditions
or upon distillation in acid medium. The fraction of sulfites that does not bind to food
constituents is called “free sulfite”, constituting a mixture of sulfur dioxide, bisulfite
and sulfite ions in a dynamic equilibrium. This fraction is rapidly converted to
molecular sulfur dioxide when the sulfited food is acidified (Wedzicha, 1992). As
sulfites react with a variety of food constituents and some are lost during processing
and storage, their use levels may not represent the amount needed to ensure an
effective residual level through the shelf life of the product and may not reflect
the concentration remaining in a food at the time of ingestion (Taylor et al., 1986;
Annex 1, reference 137). The fate of added sulfites is highly dependent on the
chemical nature of the food, the type and extent of storage conditions, the
permeability of the package and the level of addition. The combination with organic
constituents, the equilibrium between the various inorganic forms, the volatilization
of sulfur dioxide and the oxidation to sulfates are all important reactions, and their
relative importance will depend mostly on the food involved (Fazio & Warner,1990).
2.2 Analytical methods
Analytical methods for the determination of sulfites in food have been
reviewed (Fazio & Warner, 1990; Karavicová & Simko, 1996). Published methods
are generally based on the known chemistry and reactivity of sulfites with the matrix
and require some means of recovering sulfur dioxide. They fall into two basic
categories: methods that require an initial distillation of the test sample to free the
sulfur dioxide, and those that use a non-distillation reaction to achieve the same
end-point (Wedzicha et al., 1984; Wedzicha, 1992). Several methods use acidic or
basic solutions (AOAC International, 1995a, 1995b, 1995c, 1995d, 1995e, 1995f),