The Anatomy of Cereal Seed: Optimizing grain quality involves getting the right proportions within the seed

By Andy Hammermeister, Ph.D., P.Ag.

Knowing about grain quality starts with knowing the anatomy of a single grain. Whether the grain is to be used for feed or for human consumption, the key characteristics of a grain still apply. Have you ever wondered how the anatomy of a grain affects its quality characteristics? The figure below shows the anatomy of a typical grain. Here we will discuss the parts of seed from the inside out starting with the embryo.

The embryo, also known as the germ, is the beginnings of a New plant, including the genetics, and early plant structures (leaves and roots) that will get the plant started. The embryo contains various protein, oils, enzymes and vitamins. It makes up approximately 3% of the seed. The enzymes it contains helps to trigger the release of nutrients from the remainder of the seed when the embryonic plant starts to grow.

The germ is typically removed during milling of refined flour because it can influence bread making quality, and the oils in the germ can go rancid if the flour is stored for a long time. Some millers/bakers such as Speerville Mill and the Dover mill, will mix the germ back into the flour (along with the bran discussed below) when making whole wheat bread.

The endosperm makes up 75-83% of the seed. It contains the starch which is held in a matrix of protein. This is the energy source of the seed for germination, and the proportion of protein and starch in the endosperm will dictate its characteristics for feed and food processing.

Surrounding the endosperm is the aleurone layer which makes up a relatively small part of the seed. The aleurone contains enzymes that can start the reaction that changes starch into sugar, a key step in using grains for specialized uses such as brewing beer.

The next layer is the seed coat which is often referred to as the bran. In wheat, the bran accounts for approximately 14% of the seed. The bran is the skin of the seed and helps to protect it from disease and pests. The bran is mainly made up of fiber and contains some vitamins. The bran can affect protein content and milling quality, so it is typically removed when making refined flour but may be added back in to make various percentages of whole wheat. Small seeds will have a larger amount of bran compared with the inside of the seed, whereas larger or plumper seeds will have a smaller ratio of surface area (bran) to inside. This means that larger seeds are often more desirable, especially from a milling perspective.

Cereals are typically enclosed (while still in the head of the plant) by two fibrous sheaths, the hull, that protect the seed (not shown in the figure). The larger one is called the lemma and the smaller one is the palea. For wheat, rye, and hulless forms of oats and barley, the lemma and palea fall off the seed during harvest and become part of the chaff. But the lemma and palea are closed tightly on hulled forms of oats and barley. The lemma and palea are mostly fiber, and do not add much value in terms of feed. They account for up to 30% of the yield of hulled grains. This means that a hulled variety of oats (or barley) may yield 20-30% higher than a hulless oat, however, the energy and protein of the oats will be 20-30% lower, resulting in lower feed value.

Food processors typically prefer hulless varieties of crops because it eliminates the step of removing the hull before (eg. rolling the oats for oatmeal). As mentioned above, the hull does help protect the seed from disease and damage. Hulless varieties of oats and barley are more susceptible to disease during crop establishment and damage during harvesting. Damaging the seed can reduce quality and increase losses.

 

Posted August 2008