Cereal grains

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Introduction
Nutritive Value of Cereals
Effect of Processing on Cereals

Introduction

Cereal grains are the seeds of grasses (gramineae). A typical cereal grain consists of the testa, the aleurone layer, the embryo and the endosperm. The embryo is the living part of the grain that is very rich in nutrients. Most nutrients (for example, protein, vitamins and minerals) are concentrated in the aleuronic layer whilst the endosperm consists mainly of starch. Cereals constitute a major source of energy in most households today and it is no surprise they occupy the base of food pyramids. Examples of cereals are wheat, maize, rice, barley oats, millet and sorghum. With the advent of agriculture in the early history of human civilisation, cereal grain consumption became prominent, especially in developing countries. This article gives an overview of this food group in terms of its nutritional values.

Nutritive Value of Cereals

Cereals and cereal products are important sources of energy, carbohydrate, protein and fibre. Though cereals generally lack vitamins A, C and B12, they contain a range of micronutrients such as vitamin E, some of the B vitamins (for example B6), magnesium and zinc. The bioavailability of the B-vitamins, however, appears to be low. For example, the bioavailability of B6 from cereal grains tends to be low, whereas bioavailability of B6 from animal products is generally quite high. With the exception of calcium and sodium, cereal grains provide good amounts of minerals needed for adequate nutrition. Since the typical Western diet is high in sodium, the low sodium content of cereal grains is desirable. The low calcium content of cereal grains does not appear to be a problem because most people in developed countries consume a mixed diet rich in calcium. It is however advisable for people living in developed countries to obtain calcium from a wider variety of sources, such as in dairy and some vegetables. Ideally, calcium and phosphorus occur in the ratio of 1:1. However, cereal grains have a low calcium/phosphorus ratio which can negatively impact bone growth and metabolism. This is because consumption of an excess of dietary phosphorus when calcium intake is inadequate or low is reported to result in secondary hyperparathyroidism and progressive bone loss.

The high phytate content of whole grain cereals may reduce the availability of calcium for absorption because the phytate forms insoluble complexes with calcium. The combined effect of low calcium content, a low Ca/P ratio, and low bioavailability of calcium via high phytate content may pose problems for healthy bone development in populations that use cereal grains as a staple food. It has in fact been reported that in populations where cereal grains provide the major source of calories, osteomalacia, rickets and osteoporosis are common, even when there is sufficient sunshine to prevent vitamin D deficiency. The high phytate content of cereals also affects the bioavailability of non-haem iron. A number of factors including phytate and fibre, tannins, lectins and phosphate may contribute to the inhibition of non-haeme iron absorption. The high levels of phytate contribute most to the inhibition of non-haeme iron. To eliminate its inhibitory effect on non-haeme iron absorption, phytate must be almost totally removed.

Some studies of zinc absorption in rats and humans have clearly demonstrated that consumption of phytate contained in whole grain cereals (for example, wheat, rye, barley, oats) inhibits zinc absorption. The bioavailability of zinc from meat is four times greater than that from cereals, and it implies that total dependence on cereal-grain and plant-based diets (for example, vegetarian diets) may lead to impaired zinc metabolism in developed countries including Australia.

Cereal protein is usually incomplete as it lacks some amino acids. In particular, lysine is limited in cereals which form the basis for many diets. Furthermore, the essential amino acid, threonine, tends to be lower in cereal-based proteins compared to animal protein sources.
Whole grain foods such as wheat, rice, oats, and barley are relatively low in energy density and are reported to help maintain energy balance.

Effect of Processing on Cereals

Cereal grains are usually processed to:

make them digestible
inactivate natural toxins and prevent bacterial growth and food spoilage
optimise the appearance, taste and texture of foods
improve convenience to meet consumer demand for quick and easy meal solutions
maximise their nutritional value.

Processing can make it easier for nutrients from grains to be digested or for nutrients to be added (fortification). There are a number of factors that determine the quality of grains and pulses for human consumption. For example, milling affects the nutritional value of grains in two ways:

The physical separation of the different grain components drastically reduces the nutrient content of the grain.
Grinding reduces the particle size which impacts on the glycaemic
index and resistant starch content of grains.

Before cereals are consumed, they must undergo some form of processing. The methods of processing cereals (e.g. milling) reduces the nutritional value of cereals. Generally, the final nutrient content of a cereal will depend on the extent to which the outer layers are removed during processing (extraction rate). Removal of these outer layers implies loss of fibre, vitamins and minerals. Extraction rate is the number of parts by weight of flour that is produced from 100 parts of the cereal (e.g. 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. Highly milled cereals such as white maize flour, polished rice and white wheat flour are therefore of less nutritive value because they have lost most of the germ and outer layers and with them most of the B vitamins and some of the protein and minerals. Flours can be produced to a range of different extraction rates, depending upon the amount of bran, germ and pericarp that is removed. Flours of high extraction rates retain many more of the micro-nutrients than those of lower extraction rates. The disadvantages of low-extraction flours to the consumer are that they contain less B vitamins, minerals, protein and fibre than high-extraction flours.

Cereals feature prominently in most breakfast cereals, bread and pasta. As a result of losses that occur during the processing of cereals, those used as breakfast cereals are often enriched.

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