Processing of corn for consumption as food

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Maize miller is the processing of maize (corn) for safe and palatable consumption as food. Processing can be by machine-milling in either large- or small-scale mills, or by hand-milling in domestic or community settings.

The maize is first cleaned and then "conditioned", or "tempered", by soaking the kernels in water. This facilitates the separation of the seed coat and pericarp from the endosperm. Further processing consists of separating components of the grain, and grinding it into various grades.

Cleaning and conditioning

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Cleaning and conditioning of the maize is an important step in the process and refers to the removal of foreign material and all that is not maize kernels from the to-be milled grain that lowers the quality of the product such as husk, straw, dust, sand, and everything too big or too small and lighter than a maize kernel.

It also refers to the removal of poisonous seeds, and material harmful to the milling equipment such as metal and stones. Conditioning refers to the addition of moisture to the maize to allow the bran to be peeled off in flakes during milling with plate or roller mills, allowing easy separation in a sifter and, most importantly, to add mass to the meal.

Milling and sifting

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Following this process, milling can commence and may take several forms:

• The roller mill may be a single roller mill, double roller mill or pneumatic roller mill. In a complete maize milling plant, there are several roller mills that work together, they have different functions, the first mill mainly peeling the maize skin, the second and third will grind the maize into granular sizes, and meanwhile to get some super fine flour, and the granular sized product will go to the next mill to continue grinding.
• While grinding time, use double bin sifter or square plan sifter to sift the meal from the miller, classification and sifting more super flour. In general, the sifting used to separate the flour and bran, also separate large size and small size to ensure flour quality.

Final product packing

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After the maize is processed, it will come out in different final products like flour and grits. They are different from their granular size. For the packing, a Full-auto Flour Packing Machine is adopted, and the flour is packed into 5 kg, 10 kg, 25 kg or 50 kg bags.

Best quality maize

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The best quality maize meal is therefore obtained by:

De-germinating the maize prior to milling Milling with rollers rather than hammer mills or plate mills/disc mills. Alternatively, if the budget is small, the whole maize, after cleaning and conditioning, may be milled by means of the plate mill only and then sifted (without de-germination). A significant part of the bran and germ meal would then be sifted off, resulting in a Special Sifted meal - of lower quality than roller milled meal and higher quality than hammer milled meal.

A further element of major importance is sifting. Milling and sifting, form the very essence of grain processing. The sifters in every system should be one or more of the following:

• Turbo sifters - two separation horizontally shafted with steel screens - high capacity but not sifting very fine - used in small capacity plate mill systems and as graders for samp and other primary grading after de-germinators.
• Rotary sifters - three separation horizontally shafted with nylon screens - lower capacity but very fine screening - used in small capacity plate mill systems.
• Mini plan sifters - relatively high capacity and very fine screening used as primary sifters for all mills of 1 ton per hour to 2.5 tons per hour.
• Plan sifters in various sizes and numbers of passages: Very high capacity and fine screening - used in all mills with capacity of 2.5 ton per hour and more.

Industrial type mills can be constructed over one level, two levels, three levels or more, as existing buildings determine or as practical as the solution may be. High capacity mills normally require more levels to make use of gravity in moving product between mills and sifters.

Another option is to place a small mill (500-1,000 kg/h) in a container. This is only recommended for special applications where buildings are problematic, where the mill needs to be moved from time to time to another location, where temporary power is used and where the mill is located in very remote areas.

References

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• [1] as of 15 July

The Importance of Maize Conditioning: Part 1

Charl Marais | February 3,

Conditioning is the final stage in the preparation of maize before the milling process. It plays an important role to ensure that finished product is of a high quality, and to achieve the maximum profitability in the milling process, which includes a milling gain.

The three main steps are:

• A controlled quantity of water (measured in litres per hour) is added to the maize.
• A controlled amount of maize (measured in kilograms per hour) is mixed with the water using a special dampening conveyor.
• The dampened grain is allowed to rest in a conditioning bin for a predetermined time to enable the water to be absorbed by the maize.

THE OBJECTIVES OF CONDITIONING MAIZE

If maize is milled in its dry state without any conditioning, the kernels will be pulverised on the degerminators and roller mills. This will produce a low quality product that&#;s a mixture of endosperm, germ and bran with low commercial value.

The objectives of conditioning maize are:

• To toughen the bran skin and make the bran more elastic. The bran should be elastic so it won&#;t shatter and can be easily removed.
• Toughening the bran will also allow the tip cap to be removed more easily. If the tip cap is not completely removed, it will be ground up on the roller mills causing black specks in the meal.
• To make the germ more pliable or elastic. This will allow for easier separation from the endosperm in one piece. This in turn will stop the fat (oil) from being released into the endosperm.

• To lower the adhesion between the bran skins and the endosperm. This will allow for the bran to be peeled more easily from the endosperm.
• To leave the endosperm as hard as possible. This is very important, since a soft endosperm will result in the manufacturing of a finer meal. Granulation plays a very important role in maize milling, especially in the manufacturing of grits, rice and super maize meal. Mills that produce a high percentage of samp would also require a hard endosperm.
• To increase the moisture for achieving a commercial gain. This is a bonus as the milling gain contributes to the technical profitability of the mill, along with the extraction rate.

FACTORS INFLUENCING THE PENETRATION OF THE WATER INTO THE MAIZE KERNEL

• Kernel hardness. Water penetrates the endosperm of soft maize more rapidly than that of a hard endosperm variety. This makes it more difficult to condition soft maize, because if the endosperm is too soft, an excessive amount of fine meal will be produced.
• The initial moisture level. The normal moisture of maize should be between 11 to 12%. Low moisture would be below 10.5%, while high moisture would be in excess of 13%.
• Lying time. The lying or conditioning time controls the depth of penetration of the water into the endosperm. This is a critical aspect of conditioning and will depend on what type of milling process the maize will be subject to (roller mills or degerminators followed by roller mills).
• Temperature. Raising the temperature of the maize by means of hot water or steam, will allow a faster penetration of the moisture into the endosperm. However, this system comes with extra expenses such as boilers, and complicates a fairly simple process.
• Relative humidity. The relative humidity in the area where the maize mill is located (coast or inland) will have an effect on the time the water penetrates into the endosperm. The relative humidity and temperature should also be considered when determining the final moisture of the maize meal. The hotter and more humid the climate, the less the final moisture should be.

CONDITIONING STAGES

Water is normally added in two stages. On short systems, a single conditioning system can be used.

• Short systems, single stage conditioning. The conditioning takes place after the maize has been cleaned and all the impurities have been removed. Between 6% and 8% moisture is added, and the maize is allowed to lie for approximately 10 minutes, depending on the kernel hardness. Obviously the softer the kernel, the shorter the lying time as the endosperm must remain as hard as possible for aforementioned reasons. The water that&#;s added should not penetrate the endosperm and is only to loosen the bran from the endosperm.
• Double conditioning. Here, two conditioning stages are used. Moisture is added according to the calculation below, and the maize is allowed to lie for 3 to 12 hours, depending on the kernel hardness and mill design. In the 2nd conditioning stage, an addition of 6% to 8% water is added with a short lying time of 10 to 20 minutes. The moisture must not penetrate into the endosperm, as this would make it too soft. The moisture must mainly be just under the bran and germ layers so that the bran/germ can be easily removed (almost like peeling a banana).

 When milling soft maize as found in central and east Africa, the Target       Moisture Value on the 1st conditioning stage could be reduced. This also   applies to high moisture maize. If the moisture is 13.5%, the addition of   water on the first conditioning should be left out.

Maize with a moisture content higher than 13.5% should not be milled, and the miller should ensure that the moisture content of maize meal should never exceed 14%.

Read the next installment on The importance of maize conditioning: Part 2, where we cover the equipment used during conditioning and also include a handy calculation.

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