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History and science of roller milling wheat kernels

In a history of consuming wheat for over 10,000 years, production and consumption of roller milled grain flour only began around 150 years ago, making it a completely new phenomenon for the human civilization if you zoom out enough.


However, its effects are far reaching, beyond anyone's nascent imagination of what a bunch of metal rolls passing grains through them can do to the landscape of how wheat is bred (mostly hard grains), grown (grain size and shape suitable for roller mills), harvested (large scale commercial production), ofcourse milled (In the USA, 99% of the flour is roller milled and has been increasing in every developing country year on year), and then consumed (consumption of refined flour increased multifold via end baked products, processed food, and via sheer presence on the supermarket shelves while consumption of whole grain flour declined) by over 7 billion people on our planet.


HISTORY:


As per this resource, "The first mention of rollers to replace grindstones first occurred in 1558 with the publication of an engineering handbook by an Italian, Agostino Ramelli. His drawings illustrated a number of devices later adapted to modern milling. In 1662, another mechanical genius, G.A. Bockler, developed a mill using two corrugated rollers together with an agitating device for sifting the grind. Eventually the use of rollers for milling was widely adopted in western and central Europe. A concentration of roller mills in and around Budapest gave the name, “Hungarian,” to the process.


The invention of the steam engine by James Watt in 1769, the introduction of the more efficient roller mill system, and the application of the middlings purifier, combined to make possible model milling. The steam engine could be geared directly to the turning of millstones or employed to raise water into reservoirs, freeing the miller from his dependence on sources of natural power. Watt designed an English mill powered by steam in 1780.


Less than 30 years later, an American millwright, Oliver Evans, introduced screw conveyors to move flour and wheat horizontally and bucket elevators to lift grain and its milled products called grist. He assembled these machines, together with sifters or bolters, in the first continuous system in which wheat was milled into flour as a single uninterrupted operation. Machines were also added to clean the wheat to produce purer flour."


Manchester in the UK, Minneapolis in America and Budapest in Hungary can all lay claims of similar validity for the first installation by about 1880 of a ‘modern’ flour mill, judged by three defining factors: the use of the gradual reduction approach, full automation, and the complete displacement of millstones by roller mills.


UNDERSTANDING WHEAT GRAIN STRUCTURE


If you have ever seen a wheat kernel, you would know that it has a 'crease', which is the only reason why we need to break it open to obtain 'pure' white flour from the endosperm. As the extraction rate reaches 81-84%, the flour becomes increasingly 'contaminated' with bran, i.e. it becomes increasingly difficult to separate bran and germ from pure endosperm.


On the other hand, rice grain which doesn't have a crease can simply be 'polished' to obtain the 'pure' flour.


The whole industry and technology globally revolves around obtaining high yield high quality 'pure' white flour consistently.


TEMPERING, CONDITIONING OR HYDRO-THERMAL TREATMENT


Before the 'First Break', wheat is hydrated to 16% moisture and rested for some hours.


Moisture content increase to 16-19% in hard wheat varieties and 15-17% in soft varieties. Tempering is done for 18-72 h in tempering bins. Due to osmosis and different biochemical properties, bran and endosperm swell at different rates, and the bonds between them become weak.


As per this resource, in modern system the conditioning is done in three stages:

  • Preheating section

  • Moistening section

  • Cooling section

Conditioning can also be accomplished by direct steaming. Moistening and heating is carried out simultaneously in a single operation. The grain is heated in just 20-30 seconds to 47 degree celsius. The rapid heating weakens the bond between various parts of grain and facilitates their separation. It also has stronger action of proteins and enzymes. Conditioning results in toughening of high cellulose containing parts viz., germ and bran and softens the endosperm which facilitates size reduction. It also compensates later for the moisture loss during handling and milling.


BREAK ROLL SYSTEMS:


The process of roller milling can be broadly divided into the break and reduction systems.


The break system uses fluted rolls which exhibit a slight twist or spiral along the length of the roll, such that the roll pair imparts a scissor-like cutting action on the wheat kernel or the later part-broken stocks.

They operate with a gap between the rolls and under a differential (i.e. ratio of fast roll speed to slow roll speed) of about 2.5:1.


The precise angles and depths of the sharp and dull edges and the lengths of the land portions are matters of variation and distinction between different roll manufacturers.



The fluting breaks open the wheat kernel such that the bran tends to stay relatively intact in large particles, while the endosperm shatters into small particles, facilitating the separation of endosperm from bran by sifting.

The 'First Break' is extremely critical to getting a uniform particle size distribution.



The breakage equation for roller milling of wheat kernels in terms of the size of the input and output particles is given in its cumulative form by:



The effect of kernel hardness on wheat breakage could be adequately described by linear functions, leading to an extended breakage equation and function as follows:



REDUCTION ROLL SYSTEM:


The finest material from break rolls system resembles coarse flour and is known as middling's or farina. Middling's are ground into flour by pairs of large, smooth metal rollers called reduction rollers. They operate under pressure at lower differentials of about 1.25:1. This causes damage to starch granules and thereby affects the water absorption properties and baking performance of the flour. Each time the flour is ground it passes through sieves to separate it into flours of different fineness. These sieves are made of metal wire when the flour is coarse, but are made of nylon or silk when the flour is fine. By sifting, separating, and regrinding the flour, several different grades of flour are produced at the same time.


There can be upto four 'break rolls' and corresponding 'reduction rolls' and sifters in a whole roller milling plant setup making for multiple flour (output with particle size less than 212 micron) and bran outlets, eventually to be combined basis the 'purity' level.


In the next blog, we will be discussing the incremental improvements in roller milling technology that have taken place over the last century.


We hope this was helpful, and you could learn something new about wheat milling technology. If you have further questions and would love to know anything in specific, please carry on the discussion on our discord channel in #wheatywednesdays thread.