What is damaged starch in flour How is it damaged and how is it measured What is its importance in baking

Starch granules in flour have a flattened, roughly spherical shape which is sometimes described as lenticular. They range in size from about 10 to 50 ^m. Each starch granule has a surface or skin.

Within the developing wheat grains the starch granules are embedded in a protein matrix in the endosperm. During the flour milling process the endosperm is fragmented by the action of the milling' s rolls or stones. Some of the starch granules are exposed to high pressures during the milling process and their surfaces may be become mechanically ruptured or damaged. The damage to starch granules typically occurs during the reduction (smooth rolls) stage of roller milling. Here the gaps and speed differentials between the rolls may be adjusted to give more or less starch damage according to the requirements for the final flour.

Damaged starch is susceptible to attack by a-amylase (see 4.4) and this action provides the basis for the different methods that have been and continue to be used for the measurement of the damaged starch level in flours. A longstanding method based on the enzymic hydrolysis of starch was that devised by Farrand (1964) and for many years the level of damaged starch in flours was referred to in Farrand Units (FU). More recently the most important methods of measuring damaged starch are:

• the Megazyme method based on a two-stage enzymic assay (Gibson et al., 1992);

• the AACC method (Donelson and Yamazaki, 1968; AACC, 1995) based on digestion of the damaged starch by fungal a-amylase with the value expressed as a percentage.

The importance of damaged starch is mainly for breadmaking. Damaged starch absorbs twice its own weight of water in contrast with undamaged starch which only absorbs around 40% of its weight. This high water-absorbing capacity means that the damaged starch may account for about 16% of the total flour water absorption, a value similar to that for the protein itself (Stauffer, 1998). The contribution that damaged starch makes to flour water absorption has made it an essential element of flour specifications.

The upper limits for starch damage are not well defined or understood. The link between damaged starch and a-amylase activity is an important one since excessive amylase activity leads to dextrin formation and the release of water into the dough which, in turn, causes softening. Breadmaking processes employing periods of bulk fermentation are more likely to experience such problems than most no-time systems.

Very high levels of starch damage may lead to loss of bread quality, including a more open (larger average size) cell structure and greying of the crumb colour. Farrand (1964) observed such quality losses and related the starch damage and flour protein levels. His premise that the damaged starch level should not exceed protein2 divided by 6 is no longer relevant but the principle that the higher the flour protein, the higher the starch damage that can be accommodated remains a relevant 'rule of thumb'.

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