Oxidation and Flour Maturation

2. Enzyme-Active Soy Flour
One enzyme from soy flour, lipoxygenase, also has an oxidative effect on the protein of the gluten. During the oxidation of the lipids by lipoxygenase, peroxides are formed that have a cross-linking effect on thiol groups. However, the gluten-strengthening effect of soy flour is comparatively slight; its bleaching effect is more important. There are several types of lipoxygenases with different action patterns. While type I lipoxygenase only acts on free unsaturated fatty acids, types II and III also oxidize unsaturated fatty acid bound to the glycerol backbone. Bean flour contains mostly types II and III, which makes it an efficient agent for oxidizing all unsaturated lipids in flour. The use of lipoxygenase is limited because the enzyme creates a "green" flavour that is not desirable in this application.

3. Glucose Oxidase
The enzyme glucose oxidase (GOD) is usually derived from the mould Aspergillus (in a similar manner to amylase) and sometimes Penicillium. Honey is also a rich source of GOD, where the enzyme enters the honey from the pharyngeal glands of the bees (Molan, 1992). However, its suitability is greatly restricted by its taste.

Fig. 109: Hypothetical reactions of glucose oxidase in wheat dough

One effect of GOD in the dough is to oxidize glucose into gluconic acid with the aid of atmospheric oxygen (the slight souring that occurs in the process is negligible); its other effect is to transform water into hydrogen peroxide (Fig. 109). This oxidizing agent is not very specific and acts, for example, on the thiol groups of the gluten, on glutathione, or on ascorbic acid, all reactions resulting in tightening – or protection – of the gluten. The limiting factor in this process is the availability of oxygen.

Besides other chemical reactions that consume oxygen, yeast also takes up oxygen before starting its actual fermenting activity as it initially breathes instead of fermenting. This means that the conditions for GOD are really only good on the surface of the dough, as plenty of oxygen is always available there. The problem can be solved by technical measures during dough preparation, for example overpressure or the supply of extra oxygen (Lösche, 1996). The addition of an oxygen source, e.g. calcium peroxide, does not have sufficient effect within reasonable dosage limits. Saturation of the water with air by agitation at low temperatures improves the oxygen supply to a limited extent.

A typical GOD preparation is dosed in similar quantities to other enzymes, for example 100 - 500 ppm on flour (about 1,500 to 7,500 units of GOD per 100 kg of flour), but this depends to a very great extent on the product and process. In long fermentation and sheeted dough applications, GOD is more effective due to prolonged exposure to atmospheric oxygen.


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