Dry milling of grain is mainly utilized to manufacture feedstock into consumer and industrial based products. This process is widely associated with the development of new bio-based associated by-products. The milling process separates the grain into four distinct physical components: the germ, flour, fine grits, and coarse grits. The separated materials are then reduced into food products utilized for human and animal consumption.
It is estimated that 165 million bushels of corn are dry-milled per year. Currently, dry milling is mainly focused on corn-based products for human and animal consumption, or utilized during fuel ethanol production. The main objective of the dry-milling process is to separate the endosperm, which is mainly composed of starch, from the germ and pericarp fibers as much as possible.
Features
The dry milling process includes a number of unique features: Physical separation (size/density) based on mass No use of chemicals Maximizing surface area of solids for processing Resource pulping Minimal water use, if any (short tempering) Note: Water is not used as a separation agent Low capital cost Lower separation compared to wet milling Lower concentration of starch, protein, fiber, and oil relative to wet milling
The most utilized grinding mills include pin, hammer, and disk mills, but many machines are utilized for more specific processes. To maintain a high starch extraction, the grains will go through a degermination process. This process removes the germ and fiber (pericarp) first, and the endosperm is recovered in several sizes: grits, cones, meal, and flow. It is important to note that the gluten protein matrix is not separated from the starch.
Yields
The table below is a compilation of particle size and yield of milled maize products.
Uses
Currently, products of dry milled corn products are used mostly in animal food, brewing and breakfast cereals industries.
Grits/Cones
Breakfast cereals
Snack foods
Pet foods
Corn bread
Breads
Flour
Baby foods
Baking mixes
Batters
Desserts
Frozen foods
Meat extenders
Thickening agents
Germ
Grain based oil
Vitamin carriers
Mayonnaise
Potato chips
Soups
Sauces
Livestock feed
During alcohol production, the main advantage of dry milling is the flexibility in type and quality of grain which can be utilized as substrates for the fermentation process. Dry milling can be utilized for a number of different grains with little to no alteration to machine operation characteristics.
Laws of grinding
Currently, there are three main empirical models which are used to calculate the grinding work required relative to grain size and quantity. The Kick model may be utilized for grains with diameters greater than 50 mm; the Bond model for grain diameter between 0.05 mm – 50 mm; the Von Rittinger model for grain less than 0.05 mm. The calculations are shown here:
Kick Model
Bond Model
Von Rittinger Model
In all three models: is the work in kj/kg; is the grinding coefficient is the grain size of the source grain; is the size of the ground material.
While Bond’s coefficient may be viewed in various literature, the calculation for Kick’s and Von Rittinger’s coefficients may be viewed below:
where and .
To analyze the grinding results the dispositions of the source and ground material must be computed. To quantify this characteristic the grinding degree is calculated, which is a ratio of sizes relative to the grain disposition. The grinding degree relative to grain size d80 is shown:
where the value signifies 80% mass is of size smaller than the grain.
Types of dry grinders
There are three methods used for corn dry-milling: Alkaline-cooked process Stone-ground (non-degerming process) Tempering degerming process
Tempering degerming is the most common process used for grain dry-milling in the industry.
Process overview
Corn dry milling consists of several steps. The following paragraphs describe all the steps of dry milling as well as the equipment used during these steps in detail.
Tempering
A chamber is used at this section in order to mix the corn and water and let them temper for 10 to 30 minutes. For more efficient separation, differential moisture content between germ and endosperm is desired. Tempering of kernel leads to moisture uptake. Because of the differential swelling of germ and endosperm, the germ becomes more flexible and resilient during tempering while there is no movement of material out of kernel.
