3:Milling of wheat
3.1.Introduction1
Most cereal grains are ud either for the production of animal feed or for the milling offlour for human consumption.
If ud as an animal feed,the grains have only to be crushed to obtain a granulated product.Any type of mill may be ud for this purpo.In most cas it will be a hammer mill,which in one processing step fragments the grains and pulverizes them by a beating action of hammers.
The grinding of wheat for human consumption,however,has to meet higher standards and is called milling.The main purpo of milling is to completely parate bran and germ from the mealy endosperm and to thor-oughly pulverize the mealy endosperm into middlings,molina andflour (definitions of the substances are given in Section3.4).This is achieved through the shearing and scraping action of millstones(in windmills and watermills)or mill rollers(in a modernflour-milling factory).In this process it is esntial to extend slightly thefloury particles to such a degree that their internal structure is made loo.In German this is called‘Auflösung’.The water absorption capacity at doughmaking will be higher in aflour with a loo structure and it will form a more elastic and stronger d
ough than in a flour with a compact structure.Milling offlour for human consumption is achieved in three steps:界面设计原则
1.cutting open the grains,
2.scraping bran from molina and otherfloury substances,and
3.2.Conditioning or tempering
In European mills,grains of soft-kerneled wheats are usually brought to mois-ture contents of16%and hard-kerneled wheats to17.5%.Higher moisture levels are required for milling extremely hard wheats.Duration of tempering varies from about8hours(for soft wheats)to24hours(for hard wheats).
A tough bran and a friable mealy endosperm are prerequisites for an ef-ficient paration of the two.As the bran becomes progressively tougher and less brittle with increasing moisture content,milling will produceflour that 1For more information e Honey(1986);Bass(1988).loe
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is less contaminated by bran dust,thus being whiter and having a lower ash content.The mealy endosperm is rendered mellower and more friable with the addition of the correct amount of moisture.However,excessive moisture causflaking,whereas insufficient moisture leads to undue pulverization. Since an ideal moisture distribution is achieved when the bran is slightly damper than the endosperm,a cond light tempering(about0.5%moisture) is often applied shortly(less than1hour)before milling.
To improve the physical state of the grains for milling,the actual milling process is preceded by conditioning or tempering of the wheat.This process involves the addition or removal of moisture(wetting or drying,respectively) for definite periods of time,in order to obtain the desired moisture content in the mass of grain,and the desired distribution of moisture throughout each kernel.
3.3.Milling
3.3.1.Milling in a traditional mill
In windmills and watermills,cereals are ground between a pair of horizontally placed millstones.The lower or bed stone isfixed to thefloor of the milling room.The upper stone rotates on a central axis.Th
e two stones are approx-imately1.5m in diameter,but the thickness of the stones may vary from mill to mill.Wheat is poured from a bin in the upper storey of the mill into a hole (the eye)in the centre of the upper stone(Figure3.1).In this way the wheat is positioned centrally between the two millstones.The upper-side of the lower stone and the under-side of the upper stone arefluted(Figure3.2).
The distance between the two stones is in the centre about1cm,in the periphery less.During thefirst few rotations the grains are cut and broken by the edges of theflutings,which work like scissors.Thereafter,the milling good goes in a spiral movement towards the periphery of the stones,where the next step of the process takes place:the inner part of the he mealy endosperm,is parated from the outer he bran.The large and medium-large endosperm particles that are relead during this process are called molina.The diameter of the particles can vary from about300µm to750µm.
Finally,the ground particles end up in the outer15cm of the millstones, where the distance between the stones is the smallest.Here,the molina is rubbed into dust-like particles:flour.Floury particles ground on a millstone have a diameter of about200µm or less.
If the stones are properly t,the tough bran will remain intact.Theflour that is ejected from between the millstones will be a mixture of bran,pure伦敦奥运会闭幕式辣妹组合
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aggravated
Figure3.1.Driving gear of a stone-mill.
Figure3.2.Carves on the milling surfaces of the two millstones(H.Titulaer,Mühlsteinbau, Geldern1-Veert D).
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flour and molina that has not been fully ground.Such a mixture is referred to as whole-wheatflour.This whole-wheatflour may be led through a round sieve(a bolter)in order to obtain pureflour.
3.3.2.Milling in a roller-mill
Around1850the idea aro,first in Switzerland and later in Hungary,to u porcelain or steel rollers instead of millstones.The diameter of the rollers is25or30cm and their length may vary between100and150cm.The rollers are positioned horizontally in pairs and rotate at different speeds in opposite directions.The distance between the rollers can be adjusted.Por-celain rolls are not ud any more.Roller-mills have certain advantages over traditional stone-mills.The capacity of a roller-mill is much larger than that of a windmill or watermill.The range of products from a roller-mill is larger and better geared to the needs of industrial processing.Moreover,a roller-mill does not require the laborious and time-consuming sharpening of the furrow edges of the millstones.It is,therefore,not surprising that many modernflour-milling factories have been created since the1880s,in Europe as well as in North America.Life without the mills is hard to imagine nowadays.Roller-milling is a cleverly designed process.Although most of the work in a modern mill is done by special llers and plansifters)considerable milling experience and skill are still required to obtain good results.Roller-milling compris veral grinding steps,each being followed by a sifting operation.Broadly speaking,the grinding stages may be grouped into two successive sytems:the break system and the reduction system.
The break system.Primarily,the break system aims to bring about a virtual paration between the br
an and the mealy endosperm.It is carried out with the aid of corrugated orfluted iron rollers,called break rollers(Figure3.3). They operate in pairs,revolving in opposite directions and at different speeds (the speed differential is approximately1:2).The process includes successive milling steps,in which the corrugations of the rollers become progressively finer and the milling gap(distance between the rollers)smaller.Under each pair of rollers a t of horizontal sieves(plansifters)is placed so that the milled product is graded according to size.End products of the break system are mainly coar offals(bran),germs,coar endosperm particles(molina, 300–750µm and middlings,125–300µm)and a certain amount offlour (diameter less than125µm).
march是什么意思The reduction system.Primarily,the reduction system aims to reduce coar endosperm particles(molina and middlings)toflourfineness.It involves a ries of reduction steps in which,as in the ca of breaking,the rollers are t
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Figure3.3.The milling of wheat with two break rollers. progressively clor at each successive processing step.The reduction rollers, also,operate in pairs.They have smooth,slightly roughened(mat)surfaces; and speed differentials are4:5(in Europe)or2:3(in USA and Canada).Each reduction is again followed by plansifters,through which the ground material is sifted according to particle size.The reduction process is repeated veral times until ultimately most of the mealy endosperm has been converted to flour.
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vertically>nightwish主唱End products of the reduction system are mainly:fine offals(shorts)and fine endosperm particles(flour).
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To illustrate the milling steps in some more detail,the simpleflow diagram of the experimental laboratory mill in Figure3.4will be discusd here.
mutualConditioned wheat is fed to a pair of corrugated chilled-iron rollers known as thefirst break-rollers,one of which revolves at two-and-a-half times the speed of the other.The space between thefirst break-rollers is such that the grains are broken mainly into relatively coar pieces with a minimum of crushing,so as to avoid powdering the bran(powdered bran cannot be pa-rated fromflour).
The material relead from thefirst break-rollers pass to a sifter machine (plansifter),which parates the particles according to size by means of a stack of horizontal sieves of increasing degrees offineness.The sieve sizes are(from top to bottom)750,300and125µm,thus dividing the grinding stock into four fractions:coar materials(larger than750µm),molina (between300and750µm),middlings(between125and300µm)andflour (smaller than125µm).
Thefinest material(flour)leaves the mill and pass directly to theflour bag.Intermediate-sized particle
s,tho larger thanflour particles,are nt to thefirst reduction rollers(molina)or(the smaller particles,middlings)to the third reduction rollers.
