The Technological Process of Wheat Grinding Machine
The technological process of wheat milling machinery.
(1)Water regulation
The wheat grains received by the flour mill must be removed before milling due to various impurities mixed in the process of harvesting, storage and transportation. In addition to the impurities such as dust, fluff and microorganism adhering to the surface of wheat grain can be cleaned by friction and water washing of wheat beater and wheat washing machine, the differences in size, shape, suspension speed, specific gravity and magnetism between wheat grain and other impurities such as weeds, wheat straw, wheat husk, dust, stone, broken iron, pests and other grains are mainly used for cleaning. In production, vibrating screen, plane rotary screen and other screening machines are generally used to separate impurities different in size (thickness and width) from wheat through appropriate screen holes; Buckwheat, barley and other grains with different length from wheat were separated by drum cleaner and disc cleaner; The spherical grains of buckwheat and pea were separated by spiral separator; Magnetic metal impurities are removed by permanent magnetic roller and magnetic fence; The stone and slag whose specific gravity is larger than wheat grain are separated by specific gravity stone remover and wheat washing stone remover. Due to the different moisture content and physical characteristics of wheat grains from different varieties and regions, some are dry and hard, some are wet and soft. After cleaning, wheat grains still need to adjust their moisture content, that is, to dry the wheat grains with high moisture content, and add water appropriately to the wheat grains with low moisture content, so as to achieve the optimal moisture content, so as to have good milling properties. Water conditioning can be carried out at room temperature. After moistening wheat (adding water to wheat and putting it in storage for a certain period of time), the wheat grain cortex and endosperm can be separated easily, and the endosperm is loose and easy to grind; As the toughness of the surface increases, it can avoid crushing and affect the powder quality, thus providing conditions for the good and stable process and the water content of the finished product to meet the standard. Heating regulation is a kind of water heat treatment equipment, which can add water to wheat grains, heat them, and then moisten them for a certain period of time. This is not only more conducive to milling, but also can improve the baking performance. The specific operation techniques vary with wheat varieties and hardness. The time of moistening wheat at room temperature is generally 12-30 hours, and the optimum moisture content of ground wheat is 15-17%. Generally speaking, the moistening time and water content of durum wheat are higher than that of durum wheat. In the process of wheat cleaning, in order to meet the quality requirements of producing various kinds of food, the wheat from different producing areas and varieties are often processed in proportion by wheat blender. The more perfect process of wheat cleaning and water regulation is shown in the figure.
Modern flour mills are generally 5-6-storey buildings, which are equipped with grinding machines, flat sieves and cleaning machines, as well as conveying equipment such as lifting, flat transportation and gravity pipes. The whole system consists of skin mill, slag mill, core mill and corresponding classification, cleaning and other subsystems, forming a continuous production process.
(2)Leather mill
Its main function is to strip the grain and scrape the endosperm from the bran. Generally, it needs 4-5 grinding processes. After the first grinding, the wheat grains enter the sieving machine for grading. The coarsest is bran, which is first separated from the sieve surface (coarse sieve) with the largest sieve hole, and then separated into medium-sized wheat residue (larger endosperm particles with wheat bran), wheat core (smaller endosperm particles mixed with wheat bran), coarse flour with small particle size and the finest finished flour. The bran is grinded by the next skin mill. Wheat dregs and wheat cores are selected by the flour cleaner to separate wheat bran, endosperm with wheat bran and pure endosperm grains. Then, the pure endosperm grains were put into the heart mill to produce high quality flour. In the bran grinding system, the residual endosperm can be separated by beating, rubbing or brushing.
Grinding is generally carried out by roller mill. The classification is carried out with a flat screen whose screen body moves in a plane circle. Then, the reciprocating screen surface and the airflow through the screen hole are used to clear the flour, so that the light and more wheat bran containing materials can be separated as sieve materials. The pure and fine endosperm grains pass through the front screen surface, and the slightly inferior and coarser endosperm grains pass through the back screen surface. The flour cleaning system is relatively perfect in flour mills which mainly process hard wheat and produce high-quality flour; Flour mills that process soft wheat tend to be simplified, even without the use of flour cleaners.
(3)Slag mill
The main function is to deal with the endosperm grains with wheat bran separated from the skin milling system, and further separate the wheat bran from the endosperm by slight stripping and scraping of the grinding roller, and then recover the purer endosperm grains by sieving and milling. The structure of grinding machine for slag grinding is the same as that of skin grinding system. Generally, it needs 1-3 passes of grinding, depending on the size of the flour mill and the hardness of the processed wheat.
(4)Heart mill
The function is to grind the endosperm with different particle size into powder. The grinding roller adopts smooth roller, which can make a small amount of wheat bran and wheat germ be rolled into flakes while the endosperm grains are ground into powder, so as to separate the materials on the screen during sieving, avoid mixing into flour after crushing, and reduce the quality of finished products. However, the endosperm particles will also be pressed into powder pieces, so they must be crushed by a loosening machine and then sieved. The material is grinded one by one in the core grinding system until the Endosperm on the bran chip is grinded clean and a certain flour yield is obtained. The number of grinding passes is usually 6-9, and more passes are needed to process durum wheat.
The existing milling technology can not completely separate the endosperm from the wheat husk. The higher the flour yield of wheat, the more wheat bran mixed in wheat flour. The ash (mineral) content of flour with 70% flour yield is the lowest, which is close to that of endosperm; When the flour yield increases to 85%, the ash content of flour increases to 0.92%, which indicates that there are more epidermis in flour and the quality is inferior (see table).
(5)Finished product preparation
In the milling process, all the flour produced by each system is mixed to obtain the uniform flour with different flour yield, such as the uniform flour with flour yield of 72% or 85%. If the flour produced by each system is divided into several grades according to the quality, it is called grade flour. The best flour has the whitest pink, the least wheat bran and germ, and the ash content is 0.4%. It can be used to make high-quality bread, biscuits and cakes; The first-class pink color is inferior to the first-class powder, the ash content is 0.7%, and the baking property is poor; The second-class pink color is better, mixed with more wheat bran and wheat germ, ash content is 1.2%, baking property is poor. The proportion of various grades of flour may vary according to market demand and production practice. If the total flour yield of wheat is 75%, 60% of the first grade flour and 10% of the second grade flour can be produced.
Due to the different edible and processing properties of hard wheat and soft wheat, flour mills now process them into flour respectively, and then mix them according to the user's needs. According to the quality requirements of different foods, flour mills can also process various special flour, such as bread flour, pastry flour, noodle flour and general flour. In addition, various kinds of flour can be processed, such as adding vitamins and minerals to flour to increase nutritional value and make fortified flour; Add bleach to improve color; Adding modifiers to improve baking properties.
The by-products of wheat flour with certain flour yield are bran, bran meal feed and wheat germ. Wheat germ was raised separately in the milling process, accounting for 0.5-1.2% of the weight of wheat.
