Oxidation and Flour Maturation

4. Cystine
Cystine is the dimer of the amino acid cysteine in which two molecules of cysteine are linked by a disulphide bridge (Fig. 110). This sulphur bridge gives the molecule a certain oxidative effect. But at low doses it is possible that the gluten may soften, as reducing cysteine is released when cystine reacts with thiol groups of the protein. Although this has yet to be thoroughly investigated, cystine is used in spite of its high price compared to AA because it is occasionally found to have a positive effect on the properties of the dough.

Fig. 110: Chemical structure of cysteine (left) and cystine (right)

5. Dehydroascorbic Acid
DHAA is the oxidized form of AA (Fig. 111). This means that if DHAA were used instead of AA it would be possible to dispense with the initial step of oxidation. Tests have shown that this is quite possible. One reason why it is so rarely used, however, is its instability, but this could be improved by coating. A further problem is that it is more difficult, and thus more expensive, to synthesize.

Fig. 111: Chemical structure of ascorbic acid (left) and dehydroascorbic (right)

6. Bromate
The powerful oxidizing agent bromate (more precisely: potassium bromate, KBrO3) is still being used as a flour improver in many countries. Although it has a very long-lasting effect, this effect starts later than that of AA and allows better processing of the doughs, for bromate clearly oxidizes glutathione only very slowly without the need for an enzyme (cf. ascorbic acid). It results in very good fermentation tolerance and a high volume yield. In the main, bromate acts directly on the gluten. Because of doubts about its effects on health it has gradually been replaced by AA since the 1950s. A further problem is that it accelerates fire and explosion (bromate is a constituent of fireworks, especially rockets; Fig. 112).

Fig. 112: Laboratory waste bin that caught fire because ofmixed residues of potassium bromate and ascorbic acid

In countries that are now replacing bromate, combinations of AA and enzymes offer good alternative ways of achieving satisfactory dough and baking properties. Because of the low doses required (similar to AA or less) and its lower price, bromate can hardly be replaced without intervention by public authorities. Bromate is easily detected and determined semi-quantitatively with a kit in a similar manner to AA.

7. Azodicarbonamide (ADA)
Azodicarbonamide (ADA) is foaming agent (Fig. 113) used in the manufacture of expanded plastics (not only does it have an oxidative effect; it also decomposes into large-volume gases upwards of 120 °C) has been used as a temporary replacement for bromate and in some cases still is. A great disadvantage is its low dosing tolerance; a slight overdose causes the bread to split badly. The dosage is roughly the same as that of AA or bromate. The product most often used – in a correspondingly larger dose – is azodicarbonamide mixed with calcium sulphate to reduce its inflammability, usually with 23% of the pure substance.

Fig. 113: Chemical structure of azodicarbonamide

The ADA concentration of a premix can be determined by titration if no other oxidizing agent is present (AACC Method 48.71A), or by the Kjeldahl method if the carrier does not contain nitrogen. Another restriction on the use of the Kjeldahl method is the azo group (N=N) which is not fully accessible. In flour, only determination by HPLC seems to be a reliable procedure (Ahrenholz and Neumeister, 1987).

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