Crop Process Engineering

Lesson 4. Processing of Farm Crops: Processing of Cereals

 4.1 Processing of cereals

 Among major cereal crops processing of rice, wheat and maize, a detailed discussion is being given below:

 4.1.1 Processing of Rice

 Paddy (Oryza sativa L.) is most important food grains of the world and nearly 2/3of population has partially or totally adopted rice as their main food. India is the world’s second largest producer of rice. A paddy grains consists of several layers of components. The major components of paddy grain are the husk, bran layers, starchy endosperm and the embryo or the germ. Rice husk or hulls are the coating for the seeds. The hull is indigestible to humans. Bran is the hard outer layer of grain and consists of combined aleurone, nucellus, seed coat and pericarp. Along with germ, it is an integral part of whole grain, and is often produced as by-product of milling. The embryo or the germ   is rich in protein and is often removed along with bran during the milling and polishing process. The starchy endosperm is the popular white rice. A paddy grain approximately contains 20 % by weight of husk, 5 to 7 % by weight of bran layers and about 3 % by weight of embryo.

 Rice milling is the process wherein the rough rice grain is transformed into a form suitable for human consumption, therefore, has to be done with utmost care to prevent breakage of the kernel and improve the recovery. When only the husk layer is removed from paddy grain the resulting product is called brown rice. Brown rice is milled further to remove bran layers for creating a visually more appealing white rice than the brown rice.

 Method of Rice Milling

Traditionally, for milling hand pounding method was common. In a hand pound method, paddy is pounded in a traditional stone or wooden pestle and mortar. After pounding, the paddy is winnowed to separate the de-husked and un-husked kernels. The un-husked paddy grains are pounded again in the pestle and mortar. These operations are cumbersome, time consuming and require heavy labour input. By hand pounding husk and a small portion of bran are removed from the paddy. The resulting brown rice was nutritionally sound because of the bran layers.

Hand pounding became an obsolete and unpopular method after the introduction of hullers for milling paddy. The most popular model of a huller mill is the Engleberg huller. In Engleberg huller the paddy is passed two or three times get milled rice. Engleberg huller removes the rice husk and many times bran layers by mechanically cutting them between a rotating knife and a stationary blade. This is a very crude method of milling because due to cutting of husk and bran layers using metal parts, the breakage of rice is heavy and the head rice (the whole rice grain) yield is low. Furthermore, the valuable by-product, the barn, is removed along with the husk. Rice is separated from the husk and bran by sieving and bowling. Hulling using Engleberg huller reduced the human drudgery of hand pounding and is quicker than hand pounding. However, the quantity of broken rice is higher than the hand pounded rice and reduces the milling outturn.

 Bran is particularly rich in dietary fiber and essential fatty acids and contains significant quantities of starch, protein, vitamins and dietary minerals. It contains various antioxidant that impart beneficial effects on human health. Pure rice bran contains 18-21 % oil. The government felt the importance of recovering this valuable by product for extraction of oil and for other food purposes and industrial purposes. Therefore, the concept of modern rice milling came into existence. Modernization of rice milling started with a view to obtaining higher rice yields and better quality by-products such as bran and husk. The ultimate goal of the rice industry is to produce a maximum amount of whole grain rice because market value of whole kernels. It is therefore important in processing to avoid conditions that may promote breakage.

 Modern rice mills use rubber roll sheller in place of Engleberg huller. In a rubber roll sheller two differential counter rotating rolls remove the husk by frictional rubbing thus separating husk and the brown rice. In modern rice mills, the brown rice is almost always polished to remove the bran layers.

Two to four stage polishing are common in modern rice mills. Cone polishers are used for removing the bran layers.

In both the traditional and modern methods rice is milled either raw or parboiled. Parboiling refers to partial boiling of rice. Due to soaking and steaming the rice starch gelatinizes and water soluble vitamins and minerals become integral part of the rice starch. Rice becomes tougher and breakage during milling is reduced. Though there are many advantages in parboiling, it is done only base on consumer preferences. In the plant is producing parboiled rice in addition to the above it should also have a section for parboiling and drying.

 Modern rice milling processes

 The major steps involved and the definitions of each process in modern rice milling process are shown hereunder.

Pre cleaning	:	Sieve separator, magnetic separator and winnowers	Removing all impurities and unfilled grains from paddy. This is also involves removing the metal parts from the paddy.
De-stoning	:	Vibratory separator	Separating from paddy
Parboiling	:	Partially boiling paddy by soaking, steaming and drying. Helps in improving the nutritional quality by gelatinization of starch inside the rice grain. It improves the milling recovery
Dehusking	:	Rubber roll sheller	Removing husk from paddy with minimum damage to the rice grain
Husk aspiration	:	Aspirator	Separating the husk from brown rice/unhusked paddy
Paddy separation	:	Specific gravity separator	Separating the unhusked paddy from brown rice
Whitening	:	Cone polisher	Removing all or parts of the bran layer and germ from brown rice using cone polishers
Polishing	:	Improving the appearance of milled rice by removing the remaining bran particles and by polishing the exterior of the milled kernel using cone polishers
Grading	:	Vibratory sieve grader	Separating small – large brokens from head rice
Color sorting	:	Colour sorter	Removing the discolored rice from the white rice
Bagging	:	Preparing the milled rice for transport to the customer

 Cleaning and Grading

 The paddy must be cleaned off the chaff and lighter impurities, foreign grains of different sizes, sand, stones, debris and metal pieces that may present. The different cleaning operations and the machinery used for such are shown in Table 2.1.

 Table 4.1 Machinery used in cleaning paddy

Machinery	Operation
Vibratory sieve	For removing foreign of different sizes
De-stoner	For removing stones
Magnetic separator	For removing the metal parts in the paddy bulk
Winnower	For removing lighter impurities

 Cleaning paddy becomes a must for delivering quality and consumer acceptable rice in the market. Removing metal parts and stones become inevitable because they cause damages to the milling machines. Many of these operations can be performed a single or with two machines.

 Parboiling

 Parboiling is an optional and pre-milling process given to the paddy. It is hydrothermal process in which paddy soaked in water followed by gelatinization by steaming. The step process involves soaking, steaming and drying. There are different methods in parboiling such as household method, single steam method, double steam method, hot water soaking method and pressure parboiling method etc. Among these methods single steaming, double steaming and hot soaking methods are adopted in commercial scales. Some popular methods of parboiling are listed below:

       i. Single steaming method

In single steaming method paddy is soaked in water at room temperature for 1-3 days in cement cisterns. After complication of soaking the water is drained and steamed in batches.

       ii. Double steaming method

In double steaming method raw paddy is steamed first and added in batches into the water for soaking at room temperature in a cement tank. The steamed paddy increases the temperature of the soaking water to above 50 to 60 ⁰C and slowly cooled to room temperature in a day. After 24 hours, water is drained and paddy is again second time steamed.

iii. Hot Soaking Method

In this method, the raw paddy is added to hot water at 90 to 95 ⁰C. The water is circulated so the temperature is maintained at 65 to 70 ⁰C for 5 to 6 h. After soaking, the water is drained and steamed in the same vessel.

Steaming and Drying

In modern rice milling the streaming is done separately using a steaming unit. The steam is produced usually from a boiler unit under pressure. The soaked paddy is then dried. In modern rice mills usually LSU driers with hot air from steam heated exchangers is used for drying.

The purpose of this to produce physical, chemical and organaleptic modifications in rice with economic, nutritional and practical advantages.

Advantages of Parboiling

1.  The milling yield is higher and the quality is improved as there are fewer broken grains.

2. The grain structure becomes compact and vitreous, even if some kernels were entirely or partly chalky.

3. The milled rice becomes translucent and shiny.

4. Parboiled paddy and milled parboiled rice keep longer and better than in the raw state as germination is no longer possible and the kernel is hard making it resistant to attacks by insects and to absorption of moisture from its environment.

5. The grains remain firmer during cooking and less likely to become sticky.

6. A greater amount of water is absorbed during cooking causing the rice to swell.

7. After cooking the rice absorbs less fat from added conditions.

8. When cooked, the rice keeps longer and it will not go rancid so easily.

9. Parboiled rice retains more proteins, vitamins and minerals which have spread during parboiled in the endosperm.

10. After cooking parboiled rice is more digestible.

11. Less solids are kept behind in the cooking water.

12. Shelling of parboiled paddy is easier.

13. Rice bran obtained from parboiled rice gives higher percentage of oil.

 Disadvantages of parboiling

1. The heat treatment during parboiling destroys some natural anti oxidants, hence rancidity developed in parboiled during storage is more than in raw rice.

2. Parboiled rice takes more time to cook than raw rice and may have characteristic off flavour which may not be liked by raw eaters.

3. Parboiling process needs an extra investment of capital.

4. Inspite of extra expenditure involved in parboiling, the higher out turn of head yield from the process brings in additional profit to the miller and ensures cheaper price to the consumer. Apart from this, the higher nutritive value of parboiled rice can by-itself justify large scale practice of parboiling. Generally scented the fine varieties of paddy has good milling quality are not parboiled.

 Traditional method of parboiling

The traditional method process consists of soaking in water at room temperature for 24-28 hours or more steaming in kettles under atmospheric pressure and drying in the sun. In the boiling method paddy is soaked in ordinary water for 24-72 hours and then steamed. In double boiling method steam is first injected into raw paddy in the steaming kettle before soaking. Hot paddy raises the temperature of soaking water to 45-50˚C, which helps to reduce the soaking time to 24 hours. Therefore soaked paddy is steamed. Sometimes the soaking water itself is heated about 50˚C before the raw paddy is dumped in it and in this case first steaming is not required. Traditional method of parboiling has the following disadvantages:

 During prolonged soaking, fermentation sets in and an undesirable smell is developed.

1. It is a conductive to development of mycotoxins which are quite harmful.

2. Sun drying is quite uncertain.

3. Loss due to manual consumption by birds, rodents and insects in the process of drying significant.

4. Unhygienic conditions associated with the process of drying.

5. It requires large drying yard and

6. Labour requirement is high.

 The modern methods used in different countries have been very successful in overcoming the above limitations.

 Modern method of parboiling (CFTRI)

Parboiled tanks are filled with clean water is heated to a temperature of about 85˚C by passing steam through the coils already placed inside the tank. Sometimes hot water is prepared in a separated hot water tank before being pumped into the parboiled tanks. Paddy is dumped into the hot water as quickly as possible. The resulting temperature of paddy water mixture in the tanks stays around 70˚C. Soaking water can be re-circulated into the hot water tank to maintain a constant temperature of 70˚C. After getting the paddy soaked for 3-3.5 hours, the soaking water is drained out and the water discharge value kept open to remove water that condenses during steaming. Soaked paddy is exposed to steam at a pressure of about 4 kg/cm² through the open steam coil. Soaking and steaming of paddy is done in the same tank. Splitting of the husk is the indication of completion of parboiling process. The parboiled paddy is taken out by opening the bottom door and can be dried either in sun or by a mechanical dryer.

 Milling

The dried paddy is milled to produce rice. In the modern rice mills, rubber roll shellers are used. A rubber roll sheller is also associated with cone polishers. Usually 2-4 cone polishers are used depending on the size of the rice mills. In this method of milling the husk is removed by rubber roll sheller and the bran and germ portions of rice are separated during polishing in the cone polishers.

During milling, the husk and bran layer are removed. After removal of husk, the polishing is done to more than 5 % to obtain the write rice. Due to soft nature of kernels, the raw milling results in higher percentages of brokens with reduced out turn compared with parboiled rice. The milling yield slightly varies and depends on the quality of raw paddy being milled.

 4.1.2 Processing of Wheat

Wheat the basic ‘bread grain’ of the European civilization has been the chick source of human food, for at least the past 6000 years. Wheat is the second important food grains of India.

It is estimated that 10 percent of the crop is lost either during harvesting, threshing transportation or storage. With increasing production this amount of loss is very significant. The losses in storage are both quantitative and qualitative. It is necessary that every effort is made to reduce the losses particularly in storage. The slogan that “grain saved is a grain produced” should be popularized among the farmers the traders.

Next to rice, wheat is the second most important food crop. With reduced acreages under coarse cereals such as sorghum, minor millets, pearl millets, finger millets and barely, the food habits of the people are gradually shifting in favour of rice and wheat. The importance of wheat is increasing in modern society because of its flexibility in making ready to eat or easy to make or other convenient foods. Even in the traditional rice eating states wheat is popular. A number of milling, baking and confectionary units are coming up. Different type of foods requires different qualities of wheat. Thus wheat consumption will continue to rise and subsequently the demand for different kinds of wheat.

In the traditional method of harvesting ripe wheat is cut and bound in to sheaves, which are stood in stacks in the field. The action of wind and sun reduces moisture of the wheat from between 16 percent in a few days. When the moisture is sufficiently reduced sheaves are collected, stacked and threshed. The threshed grain with moisture of 14 % or less can be stored safely.

Most important event, in processing is milling of wheat, the term milling in reference to the process involving food grains is a trade name used for reduction of grains into flour or meal or any other consumable form for human beings or animals. In India, a large population of wheat is used as atta and maida.  The scope of milling covers a wide range of process depending upon the grain and products.

Structure of wheat grain

The grain of wheat consists of outer covering the pericarps and testa which is hard and indigestible, an aleurone layer, which contains a higher proportion of protein than flour, an embryo attacked to a small structure, the schutelum at the lower end of the grain, and finally the endosperm comprising of 85 percent of the whole grain from which the flour is derived.

It is important to mention that the milling of wheat is physical process. The composition of bran germ and endosperm in wheat is given below:

Bran – 12%

Germ – 3%

Endosperm – 85%

Purpose of flour milling

The purpose of flour milling is to first separate the endosperm from bran and germ in large chunks as possible and then reduce the size of the endosperm chunks flour sized particles through a series of milling steps. The germ can be rather easily removed because it contains oil which makes it putty-like and hence it will flatten under the force of rolls. However, the case with bran is quite different. Bran like endosperm is ground very easily into a fine powder and hence it is quite impossible to separate it from a fine powder and powdery endosperm. Flour contaminated with wheat bran is brownish rather than white is not desirable for food purposes.

The effective separation of bran form endosperm depends upon the principles:

(i) When wheat is soaked with water, the bran becomes tough and rubbery while the endosperm which is in the interior becomes soft and friable.

(ii) When wheat berry is sheared by the corrugation of first roll or break will spilt open releasing some endosperm and flour and thus export the remaining endosperm of the bran in successive break rolls.

Most of wheat mills obtained about a 70 % field of flour from a theories yield of about 85 percent total endosperm. The by-products of milling- the grain and shorts and bran together comprise about 30 % of the mill stream.

Wheat milling process

The wheat milling process consists of five main parts. They are;

(i) reception and storage of wheat

(ii) cleaning of wheat

(iii) tempering or conditioning

(iv) milling of wheat into flour and its by products.

(v) storage of the finished products

Reception and storage of wheat

Wheat selection is important to the flour miller without sound wheat unspotted, and free from insect contamination the miller is powerless to pro an adequate product. Whether wheat is to be bread production or for cakes, coke, pastry or biscuits depends primarily upon the protein content of the wheat. So the flour contains a lower percentage of protein then the grain from which milled, the wheat is brought at about one percent higher protein content wanted in the finished flour.

It should be pointed out that wheat as other cereal grains is subject to govern inspection and grading. Such a grading system is of great help to the grain. Since it not only takes into consideration the soundness of wheat for milling pulse but also limit the amount of contamination of other cereal seeds, weed seed that foreign matter.

Wheat is stored at the mill in large concrete line. Before storage the wheat undergoes a preliminary cleaning and drying to remove the grass impurities foreign material and to remove moisture if necessary do that the wheat can stored without heating. Heat production results from the growth of microorganisms in the grains and is caused by the heat of their respiration process and these that, wheat must be kept dry normally no higher than 14.5 percent moisture.

The drying process is coupled in three stages.

(i) The wheat is preheated to desired temperature. The temperature of grain is not allowed to exceed 48 ⁰C when the moisture content is 17 percent and 39⁰C when moisture content is under 17 percent.

(ii) Water is removed by heat vaporization and

(iii) The grain is cooled by water evaporation.

Cleaning of wheat

Since the grain impurities have already been removed at the storage the techniques used in the cleaning house is more refined. Wheat cleaned by removing loose foreign material and dirt adhering to the surface of the grain.

Wheat is weighed as it enters the cleaning house and therefore goes through a separator, which is set to remove fine impurities and dust. Small pieces of sticks, stones, sand and dust are shifted away and light impurities such as wheat chaff are removed by air current. After this wheat passes over magnetic separators to remove by adventitious metal which may have formed their way into the wheat mix. Another cleaning machine used to remove other cereal grains and weed seeds is a disc separator. In such a disk separator different types of plates can be set to discharge wheat at any place along the route.

Next step in the cleaning operation is the removal of dirt from the surface of the wheat by scouring. The scourers differ widely in design. Usually the wheat is removed by paddles against an emery coated surface, the severity of the treatment being controlled by the clearance between the paddles and stationary emery surface. The dirt and outer coating of wheat bran are removed by air aspiration.

The final cleaning step is a water work. The water dissolves the dirt and permits stones and bits of metal to sink. In some cases it appears to reduce microbiological contamination of the wheat and in all cases worker tends to add about one percent water to the original moisture content.

Tempering or Conditioning

Tempering refers to the addition of water to the bran and endosperm. The bran becomes tough and rubbery while the endosperm becomes less vitreous. This improves milling efficiency.

Tempering involves adding water to raise the moisture to 15 to 19 % for hard wheat and 14.5 to 17 % hours. During this time, the water enters the bran and diffuses inward causing the bran to loose its friable characteristics and to become leathery in textures.

Conditioning, in contrast to tempering always the use of heat since quick diffusion of water into endosperm as well as the bran is the purpose. Wheat conditioners involve four sections. The section heats the heat to the proper temperature (as heat affects gluten quality, normally a temperature of 45 ⁰C can not be exceeded). The second section adds moisture and holds the wheat for the proper temperature. The third section cools the wheat at room temperature and the final section provides a holding bin where the moisture in wheat is allowed to equilibrate before milling.

Here all the conditioning water can be added at once. The wheat goes through the preheat section and the cooling section in 1.5 hours or less. However, it is held in holding bins from 8 to 18 hours the longer times being used for the harder wheat.

In one method of conditioning the wheat is heated very rapidly but the direct injection of steam and held at a temperature of 49⁰C for about one min. Following this, there is a rapid cooling plunging the wheat into cold water and hence to a centrifugal machine to remove the surface water. This method is method is extremely fast and has advantage that the cooling process not tends to dry the bran coat since it is not done by cool air or evaporation.

Wheat grinding

The grinding of whet is done between pairs of rolls. These rolls, since they are moving opposite direction moving at different rate of speed one from the other and set with an appreciable gap between them do not grind the wheat primarily by crushing. Rather, the reduction of wheat size is by shearing for which because of the set of the roll runs about 25 times faster that the slower one and at speed from 250 to 450 rpm.

Here, the roller milling area is divided into two sections, the break section and the reduction section. In the first area system, the bran is broken open and the endosperm is milled away. This system quit often involves from proceeding one. After each break the moisture of free bran, free endosperm, free germ and bran containing adhering endosperm is shifted. The bran having endosperm still attached goes to the next break roll and the process is repeated until as much as endosperm has been separated from the bran as is possible. The freed endosperm is section to the reduction rolls where it is further reduced to the proper particles size for flours.

Rolls in the second system are reduction rolls which are similar to the break roll expect that their surface is normally smooth to rather than corrugated. After each reduction of endosperm channels which are returned to the second roll and the process is repeated.

Component of wheat mill

1. Break rolls

The surface of the break rolls is always fluted to obtain the necessary grinding effect. The saw tooth flutes run spirally around the roll and the number of flutes per unit length increases from the first to the fourth break. The first break run 4-5 flutes per cm on the fourth about 12 flutes.

2. Break sifting system

After each set of break roll there is a sifting system, called scalping. The system can be divided into two parts.

(a) Plan sifier:  It is flat sieves pilled in tries one above the other the action of the sifter is rotary in plane parallel with floor. As the sifter moves in about 9 cm circle, the small sized particles spill through the sieve below while the oversized particles travel across the sieve to a collecting through and are removed. As many as 12 sieves can be piled on the top of the other and there are four separate compartments in one Plansifier.

In Plansifiers larger pieces of bran with adhering endosperm are first remove at the top and are sent to the next break roll. The next sieves are finer and remove bran and germ. The next layers finer jets remove endosperm middling and the bottom troughs of flour. Wire mesh screens are used for the coarse separations and cloth for finer separations. The finest sieves run upto 196 mesh per inch and have opening of 0.060 min.

(b) Purifier: The middlings still contain minute size bran particles which are removed by sending the product through a purifier where air currents carry the bran away. A purifier is essentially a long oscillating sieve inclined downward and becoming coarser from head to tail. Air is blown through the sieve from the bottom which causes the flour to stratify into endosperm chunks of different size. On the bottom are the smallest pure middlings. Next are the larger middlings. Next come endosperm and bran particles. Next the large pieces of endosperm still adhering to bran, and endosperm chunks. On the top there are heavier and finally the light bran particles.

The sieves are so designed that only the endosperm particles pass through and go on the reduction rolls. The cover tails are composite particles of bran and endosperm and pure bran which either go back to a break roll or mill feed.

As many as purifiers are quite normally found for 4 break rolls in the scalping system. For example, the fine middlings from the first, second, third and fourth breaks normally go through a double purification step while the coarse middlings need only one purification treatment.

3. Reduction Rolls

The reduction system comprises two parts roll mills and sifting machines. The purpose of reduction rolls is to reduce endosperm middlings to flour size and facilitate the removal of the last remaining particles of bran and germ. The roll stands in reduction system are further divided into coarse rolls and fine rolls depending upon the clearance between the rollers. The coarse rolls are used only to produce middlings of uniform size for layer reduction to flour. Hence these rolls are often called sizing rolls and the middlings sent to them are called chunks. Middlings sized on the sizing rolls are then sent to the fine rolls to be ground to flour.

After each reduction the resulting product is sent to sifter s where finished flour is removed and over sized material is sent back to the reduction rolls for further processing.

4.Reduction sifting system

Plansifier are used behind the reduction rolls and their purpose is to divide the stock into coarse rolls and the fine middlings are returned to fine rolls while the flour is removed from the middling system.

Purifiers are often used behind the coarse reduction rolls. The purpose in this case is size grading rather than purification.

One is must to be careful to over ground the flour in the reduction system. Over grinding damages starch granules and makes the flour unsuitable for baking. Over grinding can be detected by high maltose value.

5. Scratch system

In this system, if the mil is working properly, , the scratch system can be by passed and if it is not, this system is used to maintain proper release of endosperm from bran. The scratch syatem is an extension of the break system and thus used as stand by system only.

4.1.3 Processing of Corn

Corn (Zea mays) is the crop used either human and animal consumption or industrial use. Corn is processed in to different food and feed ingradients, beverages and industrial products. It is usually harvested when its moisture content is in the range of 18 to 24 %. The maize kernels must be dried to safe moisture levels of about 12 % to avoid losses. During mechanical harvesting and shelling, sometimes result in external damage, like breaking of the pericarp and the germ results attack of biotic factors like insects, mites, pathogens, fungi, microorganisms etc. during storage before processing. Low moisture content and storage temperatures reduce the deterioration and microbial growth. Maize is consumed in different forms like maize grits, polenta, corn bread, popcorn and maize flakes. Maize is also widely used to make beverages. Corn kernel is composed of endosperm, germ, pericarp and tip cap.

Milling of corn

Dry milling and wet milling are the most common methods are used for corn milling. The different end products after dry milling process like germ used for oil extraction while husk and de-oiled germ used for cattle feed, grits for breakfast whereas starch, germ and feed are the products of wet milling.

Dry milling method of corn

Dry milling method is divided in to two methods; non de-germing method (traditional) and de-germing method (modern). In traditional method, the whole kernel is ground in to meal by stone grinder without removing germ having high fiber and high protein. Then, the hull and germ are removed by sifting from the meal. In de-germing method, the kernel are moistened with a less amount of water and then tempered for equilibrium. Then, the moistened kernel are allowed to dry and then the hull, germ and tip cap are removed from the kernel to get corn grits. The germ is used for oil extraction and de-oiled germ and hull are used for cattle feed purpose which is known as hominy feed. The yield of endosperm products and horny feed are about 70 per cent and 30 per cent respectively.

Tempering – De-germing (T.D.) method of dry milling

The major objectives of this method are:

a) to remove essentially all germ and hull so that endosperm contains as low as and fibre as possible,
b) to recover a maximum amount of the endosperm as large clean grits without any dark speck, and
c) to recover a maximum amount of germ as large and pure particles.   

The basic unit operation involved in Tempering and De-germing method is discussed hereunder;

a) Cleaning

Cleaning is the first and most important operation essential for the subsequent milling operations. The dry cleaner are consisting of aspirators and sieves and wet cleaner consisting of a washing destining unit and a mechanical type dewatering unit known as whizzer are used for cleaning of corn. The iron particles are removed by using magnetic separator.

b) Hydrothermal / conditioning treatment

Cold or hot water or steam of predetermined quantity is added to corn kernel either in single or two or three stages with appropriate times after each stage. This tempering period is vary according to the hydration methods.  So also tempering temperatures vary from room temperature to about 50°C accordingly. Either cold or hot water is used for the addition of moisture. A little heat in the form of open steam is added as and when necessary The optimum moisture content for de-germing in de-germer is 21-25 %.

c) De-germing

De-germing is the most important operation to remove germ, hull and tip cap from the kernel and to get grit. After de-germing, the products received consists of mixture of kernel components, freed from each other to varying degrees with the endosperm particles varying in sizes from grits  to flour. Beall de-germer consists of a rotating cast iron conical roller mounted on a horizontal shaft in a conical cage.

d) Drying and cooling of degermer stock

The degermer products are to be dried to 15 to 18 per moisture content for proper grinding and sifting.Generally rotary steam tube dryers are used for drying the product. Rotary Louver type dryer can also be employed. The stock is heated to about 50°C. Counter-flow of cross-flow rotary, vertical gravity or fluidised bed types of cooler can be used for cooling the dried products.

Rolling and Grading

Recovery of various primary products is the next step. Further release of germ and husk from the endosperm product occurs during their gradual size reduction roller mills.

The germ, husk and endosperm fragments are then separated by means of sifters, aspirators, specific gravity table separators or purifiers. Sifting is an important operation and is variously referred to as scalping, grading, classifying, or bolting depending upon the means used and purpose. Sifting is actually a size separation ration on sieves. Scalping is the coarse separation made on the product leaving a roller mill or degermer. Grading or classifying is the separation of a single stock (usually endosperm particles into two or more groups according to particle size. Bolting is removal of hull fragments from a corn meal or flour.

Wet Milling method of corn

The raw corn for wet milling should contain 15-16 per cent moisture and it should be physically sound. Insect and pest infested, cracked and heat damaged corns (treated at temperature around 75° C during drying) are unsuitable for wet milling. The heat damaged corn affects the quality of oil extracted from its germ.Sufficient amount of moisture is added to the corn during steeping in the wet milling process in order to prepare the corn for subsequent degerming, grinding and separation operations.

The wet milling process consists of the following steps:

i)   cleaning

ii)  soaking

iii) germ separation and recovery

iv) grinding and hull recovery and

v)  separation of starch and gluten

Cleaning

Impurities such as dust, chaff, cobs, stones, insect-infested grain and broken and other foreign materials are removed from corn by screening and aspirating. The clean grains are conveyed to the storage bins.

Steeping

The major objectives of steeping are to soften the kernel for grinding, to facilitate separation of germ and gluten from the starch granules and  to remove solubles, mainly from the germ. Water impregnated with SO₂ (i.e., acidulated water with H₂SO₃) is used for stepping; it helps in arresting certain fermentation (luring long sleeping process. The   steeping is carried out at about 50° C   for   a   period varying from 28 to 48 hours in different plants. The steeped corn attains a moisture content of about 45 %.

Germ recovery

The wet and softened kernels having about 45 % moisture are conveyed to the de-germing unit. This machine consisting of stationary metallic plate and rotating plate with projected teeth for tearing the soft kernels apart and freeing the germs without grinding them. The pulpy mixture containing germ, husk, starch and gluten is separated by floatation (old) method but in modern method, mixture passed through the hydroclones where the germ is separated as it is lighter by centrifugal force.

Milling and fibre recovery

After separation of germ and screening of the coarse particles, the mixture contains starch, gluten and hulls. The horny endosperm and hulls are then ground to release the rest of the starch. Material to be ground enters the machine through a spinning rotor and is thrown out with a great force against the impactors at the periphery of the rotor and also against a stationary impactor   resulting in considerable reduction in particle size. The starch is readily released here with a ery little size reduction of hulls. The milled slurry, containing the ground starch, gluten and hulls is passed through a series of hexagonal reels where the coarser hulls and fibres are removed.

Starch-Gluten separation

The slurry containing starch and gluten is concentrated an then gluten particles are separated by high speed centrifugal separator as it is lighter than the starch particles. This process is carried out in two stages and the second stage of centrifugation is performed by a number of hydroclones types of equipments. The starch obtained is filtered and then dried o produce dry starch.

 References:

http://www.ddgs.umn.edu/articles-proc-storage-quality/2001-Davis-%20Processing.pdf