The Truth About Potassium Bromate

In 2007, Chinese authorities pulled a batch of imported snack chips from store shelves because they believed the chips contained potassium bromate, a food additive banned in China. The chips' country of origin? The United States.

Potassium bromate is also illegal in the European Union, Canada, Brazil and elsewhere because it causes cancer in rats and mice. In the United States, however, it has remained legal since it was first patented for use in baking bread, in 1914.

Bread dough's beige and shapeless appearance belies the staggering complexity of the molecules that hold it together. Gluten — the wheat protein infamous for its indigestible components that some people have to avoid - is the glue that binds bread dough to itself. (Gluten-free bread relies on other thickening agents such as xanthan gum).

But in order for two gluten molecules to bind to each other, molecular bridges have to form between them.

Such bridges do not form spontaneously. Rather, they're the product of oxidation. Historically, bakers relied on oxygen in the air to form the molecular bridges in dough — they "aged" flour after milling it by exposing it to open air for weeks, and then slowly mixed the flour in dough, all the while allowing ambient oxygen to do the hard work of bridge building.

But potassium bromate is a powerful oxidizing agent that chemically ages flour much faster than open air. Potassium bromate bleaches dough, and enhances its elasticity by strengthening its network of molecular bridges, which makes for the formation of tiny, thin-walled bubbles as the bread rises. The end product is fluffy, soft and unnaturally white.

Potassium bromate (KBrO3) is an oxidizing agent that has been used as a food additive, mainly in the bread-making process. Although adverse effects are not evident in animals fed bread-based diets made from flour treated with KBrO3, the agent is carcinogenic in rats and nephrotoxic in both man and experimental animals when given orally. It has been demonstrated that KBrO3 induces renal cell tumors, mesotheliomas of the peritoneum, and follicular cell tumors of the thyroid. In addition, experiments aimed at elucidating the mode of carcinogenic action have revealed that KBrO3 is a complete carcinogen, possessing both initiating and promoting activities for rat renal tumorigenesis. However, the potential seems to be weak in mice and hamsters. In contrast to its weak mutagenic activity in microbial assays, KBrO3 showed relatively strong potential inducing chromosome aberrations both in vitro and in vivo. Glutathione and cysteine degrade KBrO3 in vitro; in turn, the KBrO3 has inhibitory effects on inducing lipid peroxidation in the rat kidney. Active oxygen radicals generated from KBrO3 were implicated in its toxic and carcinogenic effects, especially because KBrO3 produced 8-hydroxydeoxyguanosine in the rat kidney. A wide range of data from applications of various analytical methods are now available for risk assessment purposes.