Dry milling and fractionation of grain

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

${\displaystyle W_k=c_k(\ln d_A-\ln d_E)}$

Bond Model

${\displaystyle W_B=c_b\left(\frac{1}{\sqrt{d_E}}-\frac{1}{\sqrt{d_A}}\right)}$

Von Rittinger Model

${\displaystyle W_R=c_R\left(\frac{1}{d_E}-\frac{1}{d_A}\right)}$

In all three models:

W is the work in kj/kg;

c is the grinding coefficient

d_A is the grain size of the source grain;

d_E 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:

${\displaystyle c_k=1.151\frac{c_B}{\sqrt{d_{BU}}}}$

${\displaystyle c_R=0.5\frac{c_B}{\sqrt{d_{BL}}}}$

where d_BU = 50mm and d_BL = 0.05mm.

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:

${\displaystyle Z_d=\frac{d_{80,1}}{d_{80,2}}}$

where the d₈₀ 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.

Degermination

The objective of degermination in corn dry milling is to break down kernel to pericarp, endosperm and germ. Beall operation is used for fulfilling this goal which separates the kernels received from tempering section into tails and throughs. Beall degerminator is known for its high yield of flaking grits; however, other manufactures have lower power requirement. Pilot plant Beall has an inner cone rotating at 800 rpm. One of the advantages of Beall degermination is weight adjustment at tailgate for increment the residence time by holding back the material.

Aspiration

Aspiration is a unit operation used for separating the pericarp from the mixture of endosperm and germ, using terminal velocity which is affected by particle size, shape and density.

Gravity separation

Gravity separation is a method used for separating the components of a mixture with different specific weight. Gravity separation in dry milling is utilized in order to separate endosperm from germ.

Roller milling and sifting

Roller mills use cylindrical rollers for grinding different materials, especially grains, which can even be an appropriate substitution for hammer mill and ball mills. Sifting is used in order to adjust the distribution of endosperm particle

Oil recovery

There are two methods for oil recovery used in industry: 1) Corn expelling which is not commercially used in the US for corn germ oil recovery due to its low oil yield and presence of residuals oil in solid products; however, it is simple and cheap. 2) Extraction which is mostly used because of its high oil yield and lower residual oil although it is expensive and has explosion and safety risks.