The maize milling process starts with cleaning the grain and is usually followed by conditioning the maize (dampening the maize with water and then allowing it to condition for some time in a bin)
Cleaning and conditioning
Cleaning and conditioning of the maize as 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, everything too big and 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.
Forms of milling
Following this process, milling can commence and may take several forms:
Hammer milled maize meal would generally represent an undefined class, probably closest associated with Special Sifted maize meal and represents the lowest cost option and suitable to many rural applications. This meal is often considered inferior by the trade, for the following reasons: All the bran, germ and endosperm are ground up and hammered through the aperture in the hammer mill screen. This has the result that the meal:
Has a short shelf life (the germ becomes rancid very fast after milling). The same problem as with Special Sifted maize meal. Has a great deal of "powder" which results in high adsorption of moisture and fast release of this moisture when cold, resulting in a layer of water over the "pap" in the morning and turning the porridge sour. The porridge cooks with a slightly yellow color although the raw meal may look dazzlingly white. Bran, with a higher water adsorption than endosperm apparently gives a bloated feeling to the eager eater.
Maize may also be de-germinated or polished prior to milling through a Hippo hammer mill with 1.6mm screen providing a very good quality Special Sifted meal.
For good quality maize meal, de-germinate the maize at either 3 tons per hour, 1,5 tons per hour or 500 kg per hour capacity. 2. Then mill the samp and chips (the de-germinator or polisher separates the bran, germ meal and endosperm) by means of a plate mill or a hammer mill or preferably a roller mill. 3. Sifting the meal if milled through a plate mill or roller mill or sifting is not required if milled through a hammer mill.
Best quality maize
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 (see above) 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 to consider, 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.