Composition

Rye grain - The Kernel

The main components of cereal grains are the hull, pericarp, testa, aleuron, endosperm and the germ. Rye is, in contrast to wheat, a special grain because it is mostly consumed as whole grain flour in breads and other cereal products. The grains may also be fractionated into different types of flour during the milling process.

Microscopic picture of Rye Kernel

 

Before rye grains can be used in food production, the outer part of the grain, the hull, must be removed. After hulling, which generally occurs during threshing, the grains are used as whole, cracked or flaked, or they are ground to make flakes or flour. The starchy endosperm constitutes about 80-85% of the weight of the whole kernel, the germ 2-3% and the outer layers about 10-15%.

In the milling process the kernel can be ground and fractionated into different types of flour and bran. Ash content is a concept used to indicate the amount of inorganic minerals in food samples. Because the majority of minerals are located in the outer layers of the kernel, the ash content of flour indicates, how much of the outer layers have been included in the flour. In white wheat flour (ash content 0.7% or less), about 30% of the outer layers of the kernel have been removed.

Rye fibre

Cereals are the most important source of dietary fibre in many countries. The fibre in cereals is located mainly in the outer layers of the kernel, especially in the bran. Wheat and rye have similar bran content, but rye contains more cell walls within the endosperm, and thus has a higher fibre content. Rye is also an exceptionally good source of dietary fibre because it is very often consumed as whole grain products.

Chemical composition of rye

Down to the Molecular Level: What is Rye Composed of?

A rye kernel contains large amounts of important dietary fibre components combined with other bioactive compounds, which have numerous positive functions for our health and well-being. Rye contains both soluble and insoluble dietary fibre. The main dietary fibre component in rye is the partly soluble arabinoxylan.

In bread making, the cell wall polysaccharides in rye have a profound effect on the rheological properties of dough and bread.

The macronutrients in rye are the same as in other cereals: starch, dietary fibre, and protein. Rye generally contains less starch and crude protein than wheat, but more free sugars and dietary fibre. Of the free sugars, sucrose and fructo-oligosaccharides dominate.

The comparison between the chemical compositions of rye, wheat and oat grains are shown in the table below. All of these cereals are presented with an extraction rate of 100, which is comparable to whole grain or whole grain flour. The composition of wheat is also presented with an extraction rate of 66, which corresponds to white wheat flour. In Finland and Denmark, rye flour generally means whole grain rye with an extraction rate of 100 or close to it. In Sweden and Norway the most commonly used rye flour has an extraction rate of 80 or close to it.

Component	% of dry matter
	Rye 100	Wheat 100	Wheat 66	Oat Groat
Protein Fat Starch Ash Total dietary fibre of which soluble fibre	8-13 2-3 56-70 2 15-17 3-4	12-14 3 67-70 2 10-13 1-2	13 1 84 0,5 3 1-2	13-16 6-7 54-64 2 11-13 3-5

(Åman et al. 1997, Clydesdale 1994, Lasztity 1998, Härkönen et al. 1997, Nilson et al. 1997a,b, Welch 1995, Andersson et al. 1993, Vollendorff and Marlet 1991 and Hansen et al. 2004)

The number after the name of the cereal indicates the extraction rate.
Extraction rate is the proportion of flour derived from a known quantity of grain.
Extraction rate 100 = whole grain flour.
Extraction rate 66 = 66% of grain is milled in this flour.
Due to differences in analyzing methods the values are not totally comparable.

There are two types of dietary fibre, soluble and insoluble. Water-insoluble dietary fibre includes e.g. cellulose, and lignin, whereas xylans and b-glucan are partly insoluble, partly soluble. Soluble fibre, such as arabinoxylan and b-glucan, forms a viscous gel already at very low concentrations (0.5-2%). Rye contains more arabinoxylan and mixed-linked b-glucan than wheat, but similar amounts of cellulose and lignin.

Oats are widely known for their high b-glucan content. b-glucan has been shown to have a positive effect in reducing the risk of coronary heart disease. Whole-grain rye contains remarkable amounts of soluble arabinoxylan, which seems to have positive health effects similar to the b-glucan in oats.

The dietary fibre content of typical rye bread is about three times higher than that of white wheat bread. The reason for this is that rye bread is usually made of whole grain rye flour, while white wheat bread is typically made of wheat flour, where the outer layers of wheat grain have been eliminated during the milling process. However, even at the same extraction rate, rye flour would have a higher fibre content than wheat flour.

The outer layer of the endosperm, the aleurone layer, is rich in proteins, minerals and vitamins, especially B-vitamins. Rye is an especially good source of several minerals, e.g. manganese, iron, copper, zinc, selenium, magnesium and fluoride. The aleurone layer, i.e. the part of the grain very close to the surface, is difficult to separate from the bran (Clydesdale 1994, Åman et al. 1997).