Module 2. Cereal processing

Lesson 4

MILLING OF WHEAT

4.1 Introduction

Wheat is the one of the important cereal crop of the World, with an estimated annual production of 540 - 580 million metric tonnes. Wheat belongs to the genus Triticum of the grass family Gramineae. Common wheat (Triticum aestivum) and durum wheat (Triticum durum) are the two major wheat groups grown for food use now. Wheat is the most valuable of all food grains and is widely used in all its stages, from whole to finely milled and sifted. In the bakery, wheat flour is the most important ingredient, which provides bulk and structure to most bakery products, including breads, cakes, cookies, and pastries. Wheat is classified into two groups: hard and soft. Hard wheat is higher in protein compared to soft wheat. It yields stronger flour, which forms more elastic dough, and is better for bread making when strong elastic dough is essential for high leavened volume. Soft wheat is lower in protein, which forms weaker dough or batter, and is better for cake making.

4.2 Wheat Processing

4.2.1 Storage

Quality of wheat is to be preserved while moving from field to storage and subsequently to the processing mill. If not properly stored; insects, moisture damage, molds or other conditions may cause losses. Moisture content must be less than 20% before harvesting, and wheat is then carefully dried to moisture below 12.5%, a level which is regarded as safe for storage. The desired moisture content is achieved in kiln or in modern driers taking care of the temperature of grain does not exceed 50°C.

4.2.2 Milling

The objective of wheat milling is to grind cleaned and tempered wheat by separating the outer husk from the internal endosperm. Early processing of wheat was accomplished by means of hand grinding, grinding stones, or a mortar and pestle. Later on wheat was milled between two circular millstones, one fixed and the other mobile and rotating. Recent technology of wheat milling involves metal cylinders or rollers for milling purposes.

4.2.2.1 Cleaning

Wheat received at mill may contain certain impurities entering from field, during storage and transportation, or accidentally. Frequently encountered impurities include: straws, chaff, sticks, weed seeds, other cereal grains, shrunken and broken kernels, infected kernels, mud, dust, stones, metal objects, etc. Wheat cleaning operation makes use of certain characteristics of impurities which are different from those of wheat e.g. size (length and width), shape, terminal velocity in the air currents, specific gravity, magnetic and electrostatic properties, colour, surface roughness, etc.

The grain is initially passed through a series of screens of selected apertures that removes matter either smaller or larger in size than the wheat kernel. Gross foreign material is removed over a set of sieves (rubble separator).

In gravity separator, impurities which are similar to wheat in size but different in specific gravity are separated out. Wheat grains are then moved on tilted screen, through which adjusted air currents are drawn. Heavier materials such as stones are separated and remain closer to screen, while lighter impurities and wheat floats down the screen.

After gravity separation, series of rotating discs separators remove impurities that are similar in diameter but different in shape from the wheat. This rotating discs with indentations pick-up only those wheat kernels that fit into the pockets and allow other grains such as oats, barley to pass through.

Dry scouring of wheat kernel removes any dirt adhering to it. In the scorer wheat kernel is bounced against a wall, which may be of a perforated sheet metal, a steel wire woven screen or any emery surface.

Magnetic separators separate foreign materials such as nails, pieces of metal that could damage equipments or generate spark, which could cause a dust explosion.

In final cleaning operation, wheat is washed by water. Wheat is immersed in water (0.5 – 1.0 lit per kg) and then conveyed by means of a worm to a centrifugal machine called whizzer, where it is vigorously agitated and spun dried. Washing of wheat removes crease dust.

4.2.2.2 Conditioning / Tempering

Conditioning of wheat is carried out primarily to improve the physical state of grain for milling. In conditioning moisture content of wheat kernel is adjusted. This includes heating and cooling of the grain for definite period of time, in order to obtain the desired moisture content and distribution. At this adjusted moisture level of wheat before milling, wheat endosperm becomes mellow and bran becomes tough. Bran that absorbs proper amount of moisture becomes elastic and will not splinter during grinding to contaminate the flour with fine particles, and thus flour becomes whiter in colour. The endosperm becomes mellower and more friable, thereby reducing the amount of power required to grind it.

Several methods are employed to condition the wheat. Heating the wheat, application of warm water, application of live steam, or just intensive mixing of wheat and water are some of the methods used to increase the amount and rate of water penetration into kernel.

Three factors affect the rate and level of water penetration into the kernel: temperature, amount of water (% moisture content) and time. The ideal water and wheat temperature for tempering condition is 25°C. Higher temperature will increase the rate of penetration into the kernel. Temperature above 50°C will change endosperm starch and protein characteristics.

Typical moisture contents of tempered wheat and tempering times are as follow:

Table 4.1 Typical moisture contents of tempered wheat and tempering times

Type of wheat	Optimum moisture content of tempered wheat	Tempering time (Hrs)
Hard spring wheat	16 – 17%	36
Hard red winter wheat	15.5 – 16.5%	24
Soft wheat	14.5 – 15.5%	10
Durum wheat	16 – 17.5%	6

4.2.2.3 Milling / Separation of flour

Objective of wheat milling is to separate the branny cover and germ of the wheat kernel from the endosperm Fig. 4.1 .

Wheat flour milling is a process that consists of controlled breaking, reduction and separation, Wheat flour milling involves three basic processes:

i). Grinding: Fragmenting the grain or its parts

ii). Sieving: Classifying mixtures of particles based on its particle size

iii). Purifying: Separating bran from endosperm particles based on their terminal velocity, by means of air currents.

Grinding of the wheat occurs between two cast rolls (break rolls) that rotates against each other. These rollers are fluted and they are not in contact with each other. The upper roller rotates two and a half time for each rotation of the lower one. Hence, the grain is engaged between fluted serrations of the rolls and broken or cut by the faster roll as it is held back by the slower roll. This initial stage in milling process is referred as ‘breaks’. The breaks are used in the grinding steps to separate the bran, germ and endosperm from each other. The grist coming out from the rolls is sifted through a plansifters. The plansifter is a machine consisting of a vertical nest of horizontal sieves, the whole assembly gyrating in a horizontal plane. A single plansifter consist of four or five different mesh sizes may yield five or six fractions of different particle size.

The series of break rolls and sieves converts the grain into semolina, which is small granule made up of endosperm. The outer husk is collected separately as bran. The semolina is separated into three grades: fine, medium, coarse in an operation called ‘gradual reduction system’. Here the rolls are smooth and one rotates only one and a quarter times for each rotation of the other.

These three streams are then put through purifiers. Purifier consists of a long, narrow, sieve set. The sieves become coarser progressively in size of mesh from head to tail. The sieve section is connected to a fan and the air is drawn up through each sieve section to draw off branny particles.

The number of parts of flour by weight produced per 100 parts of wheat milled is known as the flour yield, or percentage extraction rate. The wheat grain contains 82% of white starchy endosperm, but it is never possible to separate it out fully from the bran. Extraction rates of different flours are as follow:

Table 4.2 Extraction rates of different flours

Sr. No.	Flour	Extraction rate (%)
1	Wholemeal flour	95
2	Brownish flour	85
3	Creamy flour	80
4	White flour	70

4.3 Air Classification of Specialty Flour

Air classification of flour is used where there is a demand for extremely precise specification of granulation and protein content of flour. Flour with a narrow range of particle size has the advantage of increasing the tolerance of oven temperature and water absorption during the baking of cakes. Commercial flour particles granulation is between 0-150µm. A flour fraction of 1-17 µm contains a high level of protein. A flour fraction of 17-40 µm will usually be marked as to its higher starch content and lower protein level. It is not practical to separate particles of less than 73 µm with conventional sieves. Accordingly, particles are segregated by air using differences in particle shape, specific gravity and size.

4.4 Milling By-Products

The by-products from wheat milling process are known as ‘wheatfeed’. They comprise bran, the coarse residue from break grinds, fine wheat feed, accumulated particles from the purifiers and reduction grinding. Bran and fine wheat feed are used in compound animal feeds.