Food Science
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On page 54 of the Nuffield Food Science book, there is a contradiction about ascorbic acid promoting or inhibiting the formation of disulphide bridges in bread. However, in biology food science it states that ascorbic acid promotes the formation of disulphide bridges. So I am tempted to follow the majority. However, when I come to think about it, we know that ascorbic acid is an antioxidant by getting oxidised itself, so how can ascorbic acid oxidise adjacent SH groups to form a disulphide bridge, resulting in ascorbic acid getting reduced?
Thank you very much for you time,
Igloo writes ..........
The amino acid cysteine has “-SH” groups in its structure. Glutenins – one of the types of protein found in flour – contain cysteine amino acid units at regular intervals along the polypeptide chains.
In the coiling process, some of the intermolecular forces operating (giving rise to the tertiary structure) are due to the interaction of two -SH groups to form what is known as a disulphide bridge. This process involves oxidation, with water as the by-product.
In dough production disulphide bridges are continually being formed and destroyed since the process is reversible.
However the addition of oxidising agents to flour encourage oxidation of -SH to -SO3H groups, which prevent disulphide bridges being formed. These added oxidising agents are known as known as “flour improvers”.
During dough production, it is beneficial that fewer disulphide bridges are formed, so that the dough can expand more readily, enabling a greater volume of loaf to be formed eventually.
Potassium bromate(V), potassium peroxodisulphate(VI) and potassium iodate(V) are strong oxidising agents and are extensively used as flour improvers, though some countries prohibit the use of some of these agents.
As you quite rightly point out, ascorbic acid isn’t an oxidising agent – it’s a reducing agent of course – but the enzyme ascorbic acid oxidase, which naturally occurs in flour, soon oxidises the ascorbic acid to dehydroascorbic acid.
This is a reversible reaction, and the dehydroascorbic acid is the oxidising agent actually responsible for converting the –SH groups to –SO3H groups. In this process ascorbic acid is regenerated.
I hope that this helps to sort out your problems
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updated: 25 June 2007
