Flour Treatment : Enzymes

Fig. 119: Hemicelluloses

2. Hemicellulase, Pentosanase and Xylanase
Wheat flour with an ash content of about 0.5% contains about 2.5% pentosans (typical rye flour about 7%) that can bind up to ten times their weight of water. These pentosans belong to the category of the hemicelluloses, "relatives" of cellulose (Fig. 119), and are made up of different sugar molecules (including glucose, xylose and arabinose). The prevailing polymer consists of a xylose backbone with arabinose side chains (Fig. 120) and is therefore called xylan or arabinoxylan, and the enzymes accordingly xylanases or arabinoxylanases, or – less specifically – pentosanases. Approximately one third of these pentosans are soluble in water, while two thirds are larger molecules that are water-insoluble.

Fig. 120: Enzymatic hydrolysis sites in wheat xylan

Xylanases break these substances down. This process initially leads to the formation of more soluble molecules from the water-insoluble pentosan, and this increases the binding of water and thus viscosity. These molecules are broken down still further as the process continues; water is released and the viscosity reduced.

It is assumed that pentosans form a network with gluten (e.g., Hoseney and Faubion, 1981), for instance by crosslinking via ferulic acid residues on the xylan molecule (Fig. 121); the more pentosans are involved, the firmer is the network. This is one reason why darker wheat flours and mixtures containing rye flour have a lower volume yield.

Fig. 121: Cross-linking of gluten and hemicellulose (modified from Hoseney and Faubion, 1981)

The volume yield can be increased considerably by adding hemicellulases which are only present in minor amounts in flour. The beneficial effect of hemicellulases (an enzyme family comprising pentosanases, xylanases and other enzymes acting on hemicelluloses) on dough properties and the volume yield of baked bread was discovered about two decades ago. Xylanases are now probably the most important "volume enzymes" for baked goods. Xylanases differ largely in respect of their specificity towards the arabinoxylan molecule. Proper selection will result in dryer or stickier dough surfaces, less or more water absorption, softer or stiffer dough, finer or coarser crumb structure, and – last but not least – a larger volume yield.

These have only a limited effect on the Falling Number, but their activity can sometimes be recognised very clearly in the Amylogram (lower gelatinization temperature and maximum viscosity) and also in the Alveogram where some hemicellulases cause a change in the curve similar to that produced by cysteine but without any breakdown of the protein (Fig. 122). It should be mentioned that the softening effect is the sum of the loosening of the pentosan-protein network and the release of water from the pentosan gel, which makes the water available for further hydration of the gluten and hence softening.

Fig. 122: Effect of a xylanase from Trichoderma ssp. on the Alveogram. 
Left: no enzyme added; right: with 30 ppm xylanase on flour

Many of these enzymes are also derived from Aspergillus species, just like fungal amylases, but these are strains that have been selected for or specialised in the production of hemicellulases. More recently, bacterial strains, either conventional or GMM, have been developed to produce very specific xylanases with excellent baking properties. A gene of a new type of xylanase has been obtained from algae and transferred into a bacterial production strain. The xylanase from this organism has a very low affinity to cereal xylanase inhibitors, e.g. TAXI (Triticum aestivum xylanase inhibitor). Hence, its effect should not be influenced by natural fluctuation of the inhibitor level, and results will be more uniform in different wheat lots (Gebruers et al., 2002; Frisbæk, 2003).

Hemicellulases are mostly sold in compounds with amylase. It is not possible to give a general dosage recommendation as there is no standard method of determining hemicellulose activity. The available methods are usually based on determining the release of reducing sugars, the reduction of viscosity or the breakdown of synthetic or coloured molecules and are very difficult to relate to each other. Moreover, even the use of a standard method for different hemicellulases does not necessarily permit conclusions in respect of baking properties. Presumably the points at which hemicellulases of different origin attack the pentosan molecules are too various.

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