Oxidation and Flour Maturation

The present necessity for oxidative treatment might be regarded as a disadvantage of the fast and gentle processing of grain into flour. Natural ageing of the flour by exposure to the atmosphere alone is no longer possible, so maturation has to be speeded up with oxidative preparations. Oxidation primarily affects sulphur containing amino acids that are constituents of the gluten. The oxidation of two adjacent hydrogen sulphide (thiol) groups results in the formation of a disulphide bridge between different sections of the long gluten molecule or between different gluten molecules. This causes a hardening of the protein.

1. Ascorbic Acid
By far the most important substance for this purpose is ascorbic acid (AA). Using a complex biochemical method starting with starch as the raw material it is produced in a very pure form and sold as a fine or crystalline powder in various concentrations to facilitate dosing. Less often, AA of purely biological origin is used. The most common product is Acerola fruit powder, the dried juice of the Acerola cherry, with 17 - 19% pure AA. However, this more natural variant is up to 50 times more expensive than the synthetic product. Other substances on the market are AA obtained from rose hips and mixed preparations, some
of them containing AA of biochemical origin. At the mill, flour is treated with typically 0.5 – 3 g of pure AA per 100 kg. Very soft glutens or flours for certain applications (mainly frozen dough) require a larger dose of 6 - 10 g. AA does not act on the protein directly; it may be seen rather as an agent protecting against the loss of protein stability by counteracting glutathione, a reducing (softening) agent, that occurs naturally in the flour. This is only possible if AA is oxidized to dehydroascorbic acid (DHAA) at the beginning of the kneading process with the aid of the flour's own enzymes (i.e. ascorbate oxidase and glutathione dehydrogenase). In this process, glutathione is oxidized to glutathione disulphide, thus eliminating the gluten-softening effect of glutathione (Grosch and Wieser, 1999; Fig. 107).

Fig. 107: Reaction of ascorbic acid in wheat dough (modified from Grosch and Wieser, 1999)

Proof of an adequate quantity and homogeneous distribution of the product in the flour can easily be obtained with Tauber's reagent in conjunction with a Pekar test (Fig. 108). A convenient and storable set with the two solutions required is available on the market. Titration with iodine, which is more accurate but less convenient, is still common practice as well. In biochemical laboratories, test kits with ascorbate oxidase and also HPLC are used to determine AA very accurately.

Fig. 108: Wet Pekar sample with Tauber's reagent for determination of ascorbic acid



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