Cereal structure, classification and microbiology

CEREALS

Introduction:

⮚     Cereal grains provide the world with a majority of its food calories and about half of its protein.

⮚      The cheapest source of energy.

⮚      The word “cereal” is derived from the most important grain deity, the Roman Goddess Ceres.

⮚      Botanically, however, the seed and the grain are not exactly the same despite their interchangeable use.

⮚      All the cereal grains are angiosperms, monocots, and belong to the monocotyledonous family, Gramineae or Grass family.

⮚      Also used as animal feed and for industrial purposes.

⮚      As a source of carbohydrates related to land use, Maize ranks first among the cereal grains, followed by rice, sorghum, and wheat.

⮚      Nearly all rice grown goes directly to human food. Similar amounts of corn and wheat are grown, but much of the corn is used for feeding livestock, whereas only a small portion of wheat is used in animal feed.

⮚      Over 90% of the rice is grown in Asia, where most of it is consumed too.

⮚      Although with similar uses, the pseudocereals or pseudograins are not grasses.
Ex. Amaranth, Buckwheat, Quinoa, Chia seeds.

  

Composition:

The average nutritional composition of Cereals is as follows.

Although these are typical values, compositions vary depending on varieties of the particular grain, geographical and weather conditions, and other factors. The moisture content of 10-14% is typical of properly ripened and dried grains. When the moisture content of grains from the field is higher than this, they must be dried to this moisture range, otherwise, they may mold and rot in storage before they are further processed.

Carbohydrates: Carbohydrates are the major constituents comprising about 80% of the dry matter of the cereals. The carbohydrates are customarily considered in two parts: the "crude fiber," and the "soluble carbohydrates". Knowledge of the proportion of fiber and soluble carbohydrates is of importance in relation to nutritional and digestibility studies.

Protein: The protein content of the different cereals varies and that of rice is lower than that of all other cereals. The protein content of different varieties of the same cereal also varies. Proteins are found in all the tissues of cereal grains, the higher concentration occurring in the embryo, scutellum, and aleurone layers than in the starchy endosperm, pericarp, and testa. Within the endosperm, the concentration of protein increases from the center to the periphery. The types of proteins present in cereal are albumins, globulins, prolamines (gliadins), and glutelins.
They are generally deficient in lysine content. The biological value of the proteins in germ and aleurone is higher than that of the endosperm proteins.

Lipids: More lipids are present in germ and bran than in other parts of the grain. The lipids are mostly the triglycerides of palmitic, oleic, and linoleic acids. Cereals also contain the phospholipid, lecithin.
The lipids in milled cereal products undergo two types of deterioration: hydrolysis due to the action of the enzyme lipase present in the grain and oxidation by the action of the enzyme lipoxygenase or non-enzymatically in the presence of oxygen. These changes give rise to unpleasant flavors. When the germ is separated from the endosperm as in the milling of cereals, the keeping quality of the milled products is improved.

Minerals: The husks of the cereal caryopsis of rice, barley, and lye are rich in minerals. A considerable part of phosphorus in cereals is present in the form of phytin, the calcium, magnesium salt of phytic acid (inositol hexaphosphoric acid).

→ Phosphorus and calcium present in phytin are not available for absorption.

→ Some mineral elements like copper, zinc, manganese, and iron are also present in very small quantities in cereals, oats being particularly rich in iron.

→ Rye and oats have the highest calcium content of all cereals.

Vitamins: B-group vitamins are present in all cereals, more or less to the same extent, except niacin, which is more in wheat, rice, barley, and sorghum. The distribution of the vitamins in different grains and in different parts of the same grain is not uniform. Oils from cereal grains are rich in vitamin E.

Enzymes: Cereal grains contain many enzymes and of these the amylases, proteases, lipases, and oxidoreductases. The lipases of cereals are responsible for the fatty acids appearing during the storage of cereals and their products.

General structure:

⮚      Cereal grains develop from flowers (florets) and these florets are enclosed in bracts (leaves).

⮚      Bracts are of two types Palea & Lemma.  Ovary or Ovule are developed inside the bracts into a grain.

Depending upon the structure grains are of two types.
(1) Covered Caryopsis: It is the coated grain. The seed comprises the Seed coat, Germ,& Endosperm and in addition fused palea and lemma which constitute the husk outside the fruit for ex. rice, oats, some varieties of barley, and sorghum. Thus, these grains are covered or coated caryopsis.

(2) Naked Caryopsis: These grains consist of fruit coat (pericarp) & seed only. In wheat, rye, common varieties of maize, some varieties of sorghum and barley, the lemma and palea become free from the grain at threshing and form chaff. Thus, wheat, oats, maize, and sorghum are said to be naked grains called a caryopsis.

      1.            Pericarp: It is known as the part of the fruit formed from the wall of the ripened ovary surround the seed coat (the testa). Pericarp sometimes consists of 3 layers: the Epicarp, Mesocarp, & Endocarp.
→ The innermost layer of the pericarp tears during the ripening of the seed & in mature grain they are represented by a layer of branching hyphae-like cells known as tube cells.

      2.            Seed Coat: (Testa) It can be a thin single or double layer. The inner layer of the testa of wheat is deeply pigmented which gives the grain its characteristics color.

      3            Aleurone: It is a protein found in protein granules in matured seeds.
→ The term aleurone refers to the aleurone layer. It is the outermost layer of endosperm.
→ It contains about 20% each of protein, oil, and minerals & 10% of the total sugars present in the grain.
→ The cells are rectangular with thin cell walls consisting of one or three cell layers.

      4.            Endosperm: It is the storage house of grain and consists of starch and protein.
→ Starch having spherical granules are embedded in a matrix of protein.
→ The granules in Wheat, Rye, Barley, Maize, & Sorghum are simple whereas those in rice are compound i.e., about 60 granules are compound together.
→ If the cereal grain germinates, the seedling uses the nutrients provided by the endosperm until the development of green leaves that allow photosynthesis to begin.

      5.            Embryo: (Germ) The embryo (or germ) is a thin-walled structure, containing the new plant. It is separated by the scutellum (which is involved in the mobilisation of food reserves of the grain during germination) from the main part of the grain, the endosperm.

Fig.: The general structure of a grain

Contamination of Cereals:

⮚      Freshly harvested grains contain a few thousand to millions of bacteria per gram and from none to several hundred thousand mold spores. Bacteria are mostly in the families Pseudomonadaceae, Micrococcaceae, Lactobacillaceae, and Bacillaceae.

⮚      Most of the microorganisms are removed with the outer portions of the grain during milling. The milling processes, especially bleaching, reduce numbers of organisms, but there then is a possibility of contamination during other procedures, such as blending and conditioning.

⮚      Bacteria in wheat flour include spores of Bacillus, coliform bacteria, and a few representatives of the genera Achromobacter *, Flavobacterium, Sarcina, Micrococcus, Alcaligenes, and Serratia.

⮚      Mold spores are chiefly those of aspergilli and penicillia, with also some of Alternaria, Cladosporium, and other genera.

Spoilage:

⮚      The major factors involved in the spoilage of stored grain by molds include microbial content, moisture levels above 12 to 13 percent, physical damage, and temperature. Numerous different molds can be involved, but the most common are species of Aspergillus, Penicillium, Mucor, Rhizopus, and Fusarium.

Preservation:

⮚      Cereals usually have lower water activity (a_w) so it is not very difficult to prevent the growth of microbes as long as these products are kept dry.

⮚      The recommended storage temperature is  4.4 to 7.2^o C for dry products.

⮚      Insecticides and fumigants, ammonia (2 percent) and propionic acid (1 percent) reduce mold growth in high-moisture corn.

Storage of cereal grains:

After harvest, correct storage of the grain is important to prevent mold spoilage, pest infestation, and grain germination. If dry grains are held for only a few months, minimum nutritional changes will take place, but if the grains are held with a higher amount of moisture, the grain quality can deteriorate because of starch degradation by grain and microbial amylases (enzymes). Cereal grains, although stored in the dormant state, continue to respire producing heat, water, and carbon dioxide. This facilitates the growth of molds which are invariably present in the grain. The growth of molds produces many enzymes that cause chemical deterioration of grains.

Processing Aspect of Cereals: 

Slightly different milling processes are used for the various grains, but the process can generally be described as grinding, sifting, separation, and regrinding. The final nutrient content of cereal after milling will depend on the extent to which the outer bran and aleurone layers are removed, as this is where the fiber, vitamins, and minerals tend to be concentrated.

Toxicity:  Recently, acrylamide (described as a probable carcinogen) has been found in starchy baked foods.

No link between acrylamide levels in food and cancer risk has been established and based on the evidence to date, the UK Food Standards Agency has advised the public not to change their diet or cooking methods. However, the Scientific Committee on Food of the European Union (EU) has endorsed recommendations made by the Food and Agriculture Organisation/World Health Organization which include researching the possibility of reducing levels of acrylamide in food by changes in formulation and processing.

Health benefits:

There is evidence to suggest that regular consumption of cereals, specifically whole grains, may have a role in the prevention of chronic diseases such as coronary heart disease, diabetes, and colorectal cancer. The exact mechanisms by which cereals convey beneficial effects on health are not clear. It is likely that a number of factors may be involved, e.g. their micronutrient content, their fiber content, and/or their glycaemic index. As there may be a number of positive health effects associated with eating whole grain cereals, encouraging their consumption seems a prudent public health approach. To increase the consumption of wholegrain foods, it may be useful to have a quantitative recommendation. Additionally, a wider range of wholegrain foods that are quick and easy to prepare would help people increase their consumption of these foods.

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