Nutritional implications of flour milling

Milling affects the nutritional value of grains in two ways:

i. The physical separation of the different grain components has the greatest impact on the nutrient content of the grain.

ii. Grinding reduces the particle size which impacts on the glycaemic index and resistant starch content of grains.

In wheat milling, the bran, aleurone and germ components are separated from the endosperm which is ground into flour.

Since dietary fibre, vitamins, minerals and antinutrients (phytic acid and phenolic acid) are concentrated in the outer bran and aleurone layers of the grain, the extent to which these layers are removed determines the nutrient content of the flour.

Extraction rate is the number of parts by weight of flour that is produced from 100 parts of wheat. The higher the extraction rate, the more bran is included in the wheat flour and hence the higher the amount of dietary fibre, vitamins and minerals in the flour. Wholemeal flour has an extraction rate of 100%, whereas Australian white flour has an extraction rate of about 78%. The graph below illustrates the impact of extraction rate on nutrient content.


Source: Aykroyd WR, Doughty J. Wheat in human nutrition. Rome: FAO;1970.


The impact of extraction rate on nutrients:

The vitamin content of flour is highly dependent on the extraction rate. As the extraction rate drops from 87% to 80%, corresponding to the outer layers and bran being removed, there is a sharp drop in the vitamin content of flour. Unfortified white flour therefore has about 40% of the thiamin and riboflavin content of wholemeal flour and around 30% of the content of niacin and folate. Since vitamin E is concentrated in the germ, the vitamin E content of white flour is significantly lower than that of wholemeal flour.

As with vitamins, the mineral content of flour drops substantially when all of the outer layers and germ are removed (80% extraction rate). White flour has around 40% of the iron content and 30% of the zinc content of wholemeal flour.

The amount of dietary fibre falls very rapidly when the extraction rate falls below 87% which is when all of the aleurone and bran layers and the germ are removed. Dietary fibre in white flour is around 30% of that of wholemeal flour.

Milling does not substantially affect the carbohydrate content of flour. Since white flour consists mainly of the endosperm which is high in carbohydrate, it has a higher carbohydrate content than wholemeal flour which includes the other grain components as well.

Protein content is not affected substantially by milling. However, the amino acid composition of the protein changes. Since lysine is present in larger amounts in the outer layers of the grain, removal of these layers and the germ reduces the lysine content of the flour.

When the grain is crushed during the milling process, some of the fat in the germ and bran is distributed to the other fractions. However, the fat content of the flour hardly changes with extraction rate.

Since most of the phytochemicals are concentrated in the outer layers of the grain, they are removed as the extraction rate drops below 80%. For instance, only traces of phytate are found in white flour.

The impact of flour milling on the bioavailability of nutrients

Phytate and dietary fibre bind minerals, particularly calcium, iron, magnesium and zinc, reducing their absorption into the body. Since white flour has a lower phytate content than wholemeal flour, the absorption of minerals is not affected to the same degree as when wholemeal flour is consumed. However, because wholemeal flour is higher in minerals, overall they provide more minerals to the body than white flour.

Nutrient content of white versus wholemeal bread


Oats, rye and barley
 Losses of dietary fibre, vitamins and minerals are small in flours milled from grains such as oats, rye and barley, where it is difficult to separate the bran from the endosperm.

Oats are essentially wholegrain products because they include the bran, aleurone and germ.

In barley, the (-glucans are in the endosperm which means they are less affected by the milling process. In oats, the (-glucans are concentrated around the periphery of the grain.

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