Maize Mill Products

1.    Primary Products

1.1    Flaking Grits

These are the prime products from a full degerming system consisting of the largest pieces of broken maize endosperm free of husk and germ.

1.2    Maize Rice

Of smaller particle size than flaking grits but similar in specification and uses to 'samp' but not normally flaked.

1.3    Grits

These are the equivalent to the Semolinas in wheat milling. Different grades according to granularity and fat.

1.4     Polenta, Bakers Cones

This is a fine granular product.

1.5     Maize Flour

(Not to be confused with high fat content maize flour taken from the degerming section nor corn flour produced from the wet process).

This is the finest of the "low fat" fractions.

1.6     Maize Meal (known as mealie meal in southern Africa)

This is made in many different grades depending on market requirements. It can be made as a straight run product up to an extraction determined by the maximum fat content allowed or as part of a divide when other lower fat products are taken off.

1.7    Maize Bran (known as hominy chop the U.S.)

This consists of the outer layers of the maize and is used for animal feed.

1.8    Maize Germ

This is a valuable by-product being rich in oil. It has a pleasant nutty taste when fresh but can quickly become rancid if not treated within a short time after milling.

Its main value is in the oil which can be extracted or it can be used as animal feed without oil extraction.

The untreated life of maize germ is very short if it has a high moisture and the ambient temperature is also high (up to 2 days).

For economical oil extraction the germ should have maximum moisture content of 12% to avoid formation of free fatty acids during storage. Whole germ with undamaged cells is less susceptible to formation of free fatty acid. A minimum fat content of 14% is also required. A dry degerming system can produce over 10% germ with a fat content of 18 - 25% depending on the maize used.

Germ should be dried to a maximum of 6% moisture for lengthy storage. It is important that germ is cooled following drying and prior to binning.

The oil yield drops with extended storage prior to the expellers.

Pelletting is preferred as this stabilises the fat and improves handling.

Note: Free Fatty Acid (F.F.A.) - Free fatty acid value is a measure of the extent to which fats have become rancid. The free fatty acid value is defined as the number of m.g. of sodium hydroxide (alkali) required to neutralise the free fatty acid in 1 g of sample.

1.09   Maize Germ Meal

In some rolls the maize germ and bran are mixed together and sold or processed as maize germ meal with a fat content of 9-10%.


2.    Some Maize Processed Products

2.1    Cornflakes

Where flaking grits are used, these are cooked with steam and additives until gelatinised and flaked between a pair of horizontal 450 mm diameter rolls before drying and toasting.

Note: An alternative method developed by Henry Simon Limited uses the Cooker Extruder process. In this method the feed material can be any granular maize product; granularity size is not important although the fat content must still be quite low, certainly below 1.5%.

The meal is fed to the Cooker Extruder together with any additives and colouring required and emerges as a continuous strand of about 5 mm diameter.

This strand is chopped into small pieces which are then fed to a flaking roll, the flakes so produced being dried and toasted.

2.2    Pre-cooked Meals

In certain parts of the world and in particular the northern counties of South America, there is a great demand for pre-cooked maize meal known as "AREPAS" which is used for making "Tortillas" or a sort of porridge.

The traditional way of making this type of meal is very similar to the Cornflakes process described earlier.

The coarse grits above about 12w, from a conventional degermer and gravity table plant are cooked, flaked and dried.

The dried flakes are then reduced to a course meal on a hammer mill and reduced still further to a fine meal on a rollermill and sifter flow.

Again, as with the Cornflakes process, the Cooker Extruder can be used in the same way to produce a flake which can easily be broken up to make the "pre-cooked" meal. The fat content can be up to 1.2 - 1.4% .

2.3    Maize Oil (Corn Oil)

This is obtained from the maize germ or maize germ meal using either mechanical or chemical means.

The amount of potential oil available remains much the same regardless of the input quantity; in other words, there is almost a direct inverse relationship between the extraction rate of the germ meal and its fat content.

In general terms, it is possible to extract between 1.6 and 2.2 kgs of oil from every 100 kgs maize, depending on the process used and this could be obtained from 10% (by extraction as mill) of germ at 20% fat or from 20% of germ meal at 10% fat.

The refined maize oil is an excellent source of unsaturated fat and is used for salad dressings and cooking. It is also used in the production of margarine, as a vitamin carrier by the pharmaceutical industry and to improve other inferior edible oils.

The oil is also used by the synthetic rubber, cosmetic and paint industries.

Because the oil has a high value, it is well worth extracting. The high cost of the equipment needed is not economical on a smaller type of maize mill of 2-3 tonnes/hour.

Notes:

1.    For larger mills of up to 12-15 tonnes/hour, mechanical expelling using screw pressing can be employed. However, there are two serious drawbacks; the process leaves 4-6% of fat in the residue cake and the screw press fibres need very frequent stripping and cleaning because of the carbonisation of the relatively high starch content.

Where this process is used, the germ is first cooked at 100°C and then transferred to the screw press which forces the oil bearing material into a narrow slotted barrel or cage. The pressure generated, forces the oil through the slots and it is then filtered and held in storage tanks ready for refining.

2.    The most efficient way of removing the oil is by using the solvent extraction method which leaves about 1% residual oil in the de-fatted meal. The solvent used is Hexane.

Because this process is costly, the minimum amount of germ material which can be handled economically is 4 tonnes/hour, which represents the product from a mill in excess of 20 tonnes of maize/hour.

In some cases it is possible for a number of smaller maize mills to provide germ to a single oil extraction and refining plant.

The solvent plant works best on maize germ which has been pelleted on a conventional feed mill type pellet press arrangement. This serves to stabilise the free fatty acids, improves storage and handling and also assists in the actual extraction process itself.

Germ meal is very easy to pellet but steam is desirable to help make a firm pellet, normally 6 mm in diameter.

This is obtained from the maize germ or maize germ meal using either mechanical or chemical means.

The amount of potential oil available remains much the same regardless of the input quantity; in other words, there is almost a direct inverse relationship between the extraction rate of the germ meal and its fat content.

In general terms, it is possible to extract between 1.6 and 2.2 kgs of oil from every 100 kgs maize, depending on the process used and this could be obtained from 10% (by extraction as mill) of germ at 20% fat or from 20% of germ meal at 10% fat.

The refined maize oil is an excellent source of unsaturated fat and is used for salad dressings and cooking. It is also used in the production of margarine, as a vitamin carrier by the pharmaceutical industry and to improve other inferior edible oils.

The oil is also used by the synthetic rubber, cosmetic and paint industries.

Because the oil has a high value, it is well worth extracting. The high cost of the equipment needed is not economical on a smaller type of maize mill of 2-3 tonnes/hour.

Notes:

1.    For larger mills of up to 12-15 tonnes/hour, mechanical expelling using screw pressing can be employed. However, there are two serious drawbacks; the process leaves 4-6% of fat in the residue cake and the screw press fibres need very frequent stripping and cleaning because of the carbonisation of the relatively high starch content.

Where this process is used, the germ is first cooked at 100°C and then transferred to the screw press which forces the oil bearing material into a narrow slotted barrel or cage. The pressure generated, forces the oil through the slots and it is then filtered and held in storage tanks ready for refining.

2.    The most efficient way of removing the oil is by using the solvent extraction method which leaves about 1% residual oil in the de-fatted meal. The solvent used is Hexane.

Because this process is costly, the minimum amount of germ material which can be handled economically is 4 tonnes/hour, which represents the product from a mill in excess of 20 tonnes of maize/hour.

In some cases it is possible for a number of smaller maize mills to provide germ to a single oil extraction and refining plant.

The solvent plant works best on maize germ which has been pelleted on a conventional feed mill type pellet press arrangement. This serves to stabilise the free fatty acids, improves storage and handling and also assists in the actual extraction process itself.

Germ meal is very easy to pellet but steam is desirable to help make a firm pellet, normally 6 mm in diameter.