Pests of stored grains

Insect infestation occurs in stored grains and grains products to variable extent depending upon the storage conditions. Certain beetles and moths are notorious stored-food pests in home, as are some mites. Most food products are contaminated by insects and insect parts to some degree.

Most pests are imported into the house with foodstuffs already infested, if only the egg is present, it will not be obvious that the food is infested.

The granary weevils is one of the oldest known insect pests, is a universal feeder upon grains, and is cosmopolitan, having been carried by commerce to all parts of the world. It prefers a temperate climate and is more frequently found in the Northern States than in the South.

Larvae hatch from eggs, feed and develop and adults emerge in the product. If the infested food is not put into sealed containers after purchase other food in the kitchen or pantry may become infested.

Pests can bore through wrappers or cardboard containers, providing entry for other pests. Adults of some species feed in the same food that the eggs were laid in; others leave the food to pupate somewhere in cupboard or crevices in walls.

Life cycles of stored products pests vary, but management and control of these pests is about the same for all species.

The losses of nutritional compounds are proportional to the extent of insect infestation, nature of insect feeding and distribution of nutritional chemicals in different seed components.
Pests of stored grains

The most common grain weevils

Grain insects are widely distributed in all grains, cereal products, and animal feeds, and are very common where grain is stored (silos, warehouses, storage bins, barns, and houses).

Grain weevils are in the family Curculionidae and contain three highly changing pest species: Sitophilous oryzae (L.) rice weevils, S. granarius (L.) granary weevils and S. zeamais Motschulsky (maize weevils).

The rice weevil is the most common. Maize weevils are the less common. Adults are 0.3-0.6 cm in length and have an elongated snout containing the mouth parts that are typical of all weevils. Large numbers of them will appear whenever the stored produced is slightly disturbed. Grain weevils have a worldwide distribution but none thrive in tropical and subtropical regions.

Sitophilus granarius

All weevils have similar habits and inflict the same type of damage on grain. They attack primarily whole grain and seeds and typically do not reproduce on fine products such as flour, but can infest formed cereal products such as pasta. The female chews a hole into a kernel of grain, inserts her ovipositor and deposits an egg at the bottom of the hole.

As the ovipositor is withdrawn, she secreted a mucouslike substance that fills the hole to the surface of the grain and dries into hard plug which protects the egg and later the young larva desiccation.

Adults live a long time and females can lay a large number of eggs over their lifetime. Larvae develop and pupate within the seed and after eclosion adults chew out of the seed. Because larvae pupae and adults can occur inside whole kernels, these species can be more difficult to detect and this contribute to fragment counts in processed grain products.
The most common grain weevils

Grain weevil: Sitophilus granarius

Sitophilus granarius is a grain weevil, and is largely restricted to temperate regions, including cool highland areas in tropical latitudes. The Anglo-Saxon word wifel, from which weevil is derived referred to the grains weevil, which has infested stored grain since prehistoric times.

The adults and larvae of this weevil will feed in grain and cereal products, but it is primarily a pest of whole grain. It breeds in chickpeas, and all common grains such as maize, oats, barley, rye, wheat, kafir, buckwheat and millet. The egg is laid and the larva lives inside cereal grains, hollowing them out and causing serious damage if uncontrolled.

It can be destroyed by fumigation in suitably constructed storehouses.It also infests many foodstuffs prepared from flour. This weevil was a huge problem in the days of sailing ships because the staple food on board was a thick, hard cookie in which the grain weevil thrived.

Some sailors broke their cookies and tapped them on the table to dislodge the grubs and beetles; others ate them as they were and were possibly better nourished as a result.

The Sitophilus granarius can be recognized by the presence of a prominent snout at the front of the head. Adult are 2.5 – 45 mm long, dark brown to blackish brown, and without pale markings on the elytra. Pronotum has punctures that are distinctly elongate or oval and not round or irregular-shaped, as in S. oryzae and S. zeamais.

Eggs are laid inside grain of cereal and the larva remains in the grain, feeding and pupating, before the adult emerges through a small exit hole. This activity produces large quantities of dust and faecal material known as ‘frass’ and may cause significant economic loss.

Infestation can go in detected for a period of time, and can be introduced into new areas when grain is moved.
Grain weevil: Sitophilus granarius

Fusarium mycotoxin in cereal

Five toxic fungal secondary metabolites (mycotoxins)are considered to be economically and toxicologically important worldwide: aflatoxin, ochratoxin, deoxynivalenol and derivatives, zearalenone and derivatives, and fumonisins.

Mycotxins are toxic secondary metabolites produced by filamentous fungi (molds). Mycotoxins are very powerful protein synthesis inhibitors, so they can affect the DNA, which is a problem in and of itself. Because they affect the DNA, the cells start multiplying abnormally which can eventually develop into cancer. That mycotoxins can and do cause oesophageal cancer, kidney cancer, liver cancer, prostate cancer.

Mycotoxins production in grain can begin in the field an continue throughout storage. Mycotoxin production is dependent mainly in both well defined ranges of temperature and water activity. But in turn, the optimum climatic conditions for mycotoxin in infected grains depends on the substrate, Fusarium species and isolate.

Experts of FAO estimate that 25% of the world’s food crops are affected by mycotoxins each year with substantial impact of Fusarium species to food contamination. Fusarium species infection of cereal ears causes three undesired effects:
*Reduction of grain yield and quality
*Economic losses in livestock fed with contaminated cereals and the consequent reduced animal production
*Mycotoxins carryover to food products resulting in potential toxicity to human.

Fumonisins are carcinogenic mycotoxins by some Fusarium, primarily F. verticillioides growing in maize. At least 28 fumonisins (fumonisin B1, fumonisin B2, and fumonisin B3) analogs are now known and three of these occur naturally in maize world wide.
Fusarium mycotoxin in cereal

Cereal species

Cereals are members of the grass family Gramineae, being monocotyledonous angiosperm.  Based on botanists’ approximation, there are about 350,000 plant species, of which about 195,000 species are economically important flowering plants.

Cereals can be defined as grasses of which the grain is used for food; they may be cultivated or the grain is collected from wild plants.

Nearly 50 species are cultivated worldwide and as few as 17 species provide 90% of human food supply and occupy about 75% of the total tilled land on earth.

The cereal grains such as wheat, rice, corn, barley, oat, rye, sorghum and millet provide 50% of the food energy and 50% of the protein consumed on earth. Cereals are comparatively easy to grown and harvest, to store and to process into versatile and popular foods.

Wheat, rice and corn together make up three-fourths of the world’s grain production. The cereal species used in the greatest volume by the food industry is wheat. The types cultivated include hard (H) and soft (S) grained, red ® and white (W) grained and winter (W) and spring (S) sown variants.
Cereal species 

Deterioration of grain during storage

Deterioration of grain during storage is accompanied by loss in weight of the grain because some of the grain is being consumed.

Some of variables are temperature; moisture; oxygen; storage structure; physical, chemical and biological properties of grain bulks; microorganisms and insects, mites, rodents and birds.

Respiration
Respiration is a means of releasing energy from stored nutrients (mainly carbohydrates) and a consequence of long storage is a loss of weight.

Germination
Deterioration results from loss of weight due to enzymes activity and a loss of quality resulting from excessive enzyme activity in the products of processing.

Biochemical reactions
Deterioration changes in grain lipids may be either oxidative resulting in typical rancid flavors and odors, or hydrolytic, resulting in the production of free fatty acids.

Microbial infestation
During storage they respire and given adequate moisture, temperature and oxygen they grow and reproduce, causing serious deterioration in grain.

Insects and arachnids
Insects that infest stored grains belong to the beetle or moth orders: they include those capable of attacking whole grain and those feed on grain already attacked by other pest.

Initially, the rate of deterioration is slow but as the favorable combinations of variables are set and the storage period is prolonged, a very high loss in grain quality and quantity occurs.
Deterioration of grain during storage

Grain Processing: Milling

Milling is a series of operations involving the grinding of grains to produce starch or flour, most commonly from wheat, oats, corn, rye, barley or rice.

Threshing removes the grain from the stem. However, the grain is not usually ready for cooking until the husk (glumes) has been removed.

The exact details of grain processing depend on two main factors; the physical structure of the grain concerned and the nature of the flour product being made from that given grain.

Wheat, sorghum and pearl millet usually thresh naked that is without the glumes. Maize is on a cob covered with a sheath: the sheath must be removed and the grain taken from the cob.

A combine harvester threshes the maize free from both sheath and cob. Barley, oats, rice and most of the small millets thresh with the glumes adhering tightly to the kernel so that even a modern threshing machine or a combine harvester cannot remove the grain from this husk.

The first step in any milling process for this latter group is to remove the glummer. This is done by rubbing the grains one against the other in a mortar or by simple rotary machine, by splitting off the glumes in a disk huller, or by use of rubber rollers rotating at a differential speed.

The objective at this stage of milling is to obtain as high an out-turn as possible of whole grains.

Once free from glumes, all grains may be treated in the same way. Cereals have an outer layer and a gem, known collectively as bran, which is rich in lipids that rapidly oxidize to give off-flavors. The bran also tends to be less easily digested than the endosperm.

Milling of wheat into flour has two principle aims: to separate the bran from the endosperm; and to reduce the endosperm particles to assize sufficiently small to be rapidly hydrated on subsequent processing.

The objective of the second stage of milling is, therefore, to remove the bran. In the case of rice, the grain is required whole, so great care is taken not to break the grain.

In the process of rice polishing, separation of bran from endosperm is achieved simply by abrading the outer bran layers away leaving the intact endosperm.

Most other cereals are eaten as flour or meal (a coarse flour). The more of the bran that is removed, the better the flour will keep.

Grain Processing: Milling

The economic importance of cereal grains

Of all the plants on which humans have depended for food, those that produce the cereal grains are by far the most important, as they have been since earliest recorded time.

Cereals are widely cultivated and produce annually. Cereal grains are the seeds of cultivated grasses that include wheat, corn, oats, barley, rye, rice, sorghum, and millet.

All cereals belonging to the taxonomic family know as Gramineae. Other globally important crops in the Gramineae family include sugar cane and bamboo.

There are a number of reasons why cereals have been so important in our diet. They can be grown in a variety of areas, some even in adverse soil and climatic conditions.

They give high yields per acre as compared to most other crops, and once harvested, their excellent stored stability combined with their nutritional value makes them the most desirable of foods for holding in reserve. Cereals are the most important crops in the world, with total annual yields in excess of 2000 million tons compared with less than 700 million tons for root and tuber crops and about 380 million tones for legumes and oilseeds.

They are easy to package and transport and they can be used to produce a large variety of highly desirable foods both for humans and animals, as well as beverages for human consumption.

Cereal grains are the most important source of the world’s total food. Rice alone is reported to supply the major part of the diet for more than one half of the world’s population. In general cereals represent 60% of the calories and proteins consumed by human being worldwide.

Cereal grains are the staple food of the peoples of developing countries, providing them, with about 75% of their total caloric intake and about 67% of their total protein intake.

Cereal crops are energy dense, containing 10000-15000 kJ/Kg, about 10-20 times more energy than most succulent fruits and vegetables. Nutritionally they are important sources of dietary protein, carbohydrates, the B complex of vitamins, vitamin E, iron, trace minerals and fiber.

Per capita consumption of cereal grains has been rising in the developing countries of South and Southeast Asia, cereal consumption has reached a plateau.

The grains are eaten in many ways, sometimes as a paste or other preparation of the seed, more often milled and further processed into flour, starch, oil, bran, syrup, sugar, dried breakfast forms, and so forth.

From a culinary the point of view, consumers see a great variety of grains included in menu offerings: from soup and salads to desserts.

They are also used to feed the animals that provide us with meat, eggs, milk, butter, cheese, and a host of other foods.

The economic importance of cereal grains