Dry milling and fractionation of grain

Dry milling of grains is mainly used to manufacture raw materials into consumer and industrial products.This process is broadly relevant to the development of new bio-based related by-products.The milling process separates the grain into four distinct physical components: germ, flour, fines and coarses.The separated material is then reduced into food for human and animal consumption.An estimated 165 million bushels of corn are dry milled each year. Currently, dry milling is mainly focused on corn-based products for human and animal consumption, or for fuel ethanol production.The main purpose of the dry milling process is to separate the endosperm, which is mainly composed of starch, from the germ and peel 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)

• 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 commonly used grinding machines include pin mills, hammer mills and disc mills, but many machines are used for more specific processes.In order to maintain a high starch extraction rate, the grain will go through a degermination process.The process begins by removing the germ and fiber (peel) and then recovers the endosperm in several sizes: grits, cones, meal and runny. It is important to note that the gluten matrix has not separated from the starch.

Uses

Currently, dry ground corn products are mainly used in the animal food, brewing and breakfast cereal industries.

Grits/Cone

• Breakfast cereal

• Snack foods

• Pet food

• Cornbread

• Bread

Flour

• Baby food

• Baking mixes

• Hitter

• Dessert

• Frozen food

• Meat extenders

• Thickener

• Germs

• Grain oils

• Vitamin carrier

• Mayonnaise

• Potato chips

• Soup

• Sauces

• Livestock feed

The main advantage of dry milling during alcohol production is the flexibility in the type and quality of the grains that can be used as a substrate for the fermentation process.Dry milling can be used on many different grains with little change to the operating characteristics of the machine.

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 determination 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 form 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 to grind different materials, especially grains, and can even be a suitable replacement for hammer and ball mills. Sieving is to adjust the distribution of endosperm particlesOil 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.