Foods also oxidise over time when in contact with the oxygen in the air. For example, peeled apples or potatoes will quickly turn brown due to oxidation; if submerged in water they maintain their natural colour.
Although oxidation is something food producers usually strive to prevent, a degree of oxidation is important in bread flour. Freshly milled or 'green' flour produces a crumbly dough that will not rise well. Oxidation improves its gluten-forming properties. This was traditionally achieved by storing flour and exposing it to air for a period of time before it was packaged and sold. From the early 20th century onwards, chemical bleaching was introduced to save time. In many countries chemical bleaching is no longer permitted and ascorbic acid is added instead.
See also glutathione.
Want to find out more? Look at the chapter on ingredients in the book flour and water.
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