Flour Fundamentals

How Flour Is Milled: From Wheat Kernel to Flour Stream

It is tempting to imagine flour milling as simply grinding wheat until it becomes powder. But a modern flour mill is doing something much more deliberate. It opens the wheat kernel, separates its components, reduces them to particular particle sizes and decides which streams should become part of the finished flour. Milling is not simply about making wheat smaller. Flour milling determines what the flour will be.

Milling begins with the grain

The decisions made by the miller can begin before the wheat ever reaches the rollers (or stones). Millers may combine selected parcels or varieties of grain into what is called a grist.

Rather than treating every load of wheat as interchangeable, the miller can select different grains according to the characteristics required in the finished flour. A wheat suited to a delicate biscuit, for example, may be quite unsuitable for a strong bread flour.

The grist therefore becomes part of the flour's design. One parcel may contribute strength. Another may contribute extensibility. Another might improve milling performance, colour or another characteristic the miller is looking for.

This is one of the ways large mills can achieve consistent flour despite the natural variability of grain from farm to farm and season to season. It also makes an important point about provenance.

A bag of flour may contain wheat from several growers, districts, varieties or harvests, deliberately combined because together they produce the flour the miller wants.

Understanding the wheat kernel

Before talking about milling, it helps to understand what the miller is working with. A wheat kernel is broadly made up of three main components.

Bran

The bran forms the outer layers of the kernel.

It contains much of the grain's fibre and a considerable proportion of its minerals and other compounds. Bran contributes flavour, colour and texture to wholegrain flour.

From a baker's perspective, bran also matters physically. Bran particles absorb water and can interrupt the developing gluten network, which is one reason wholegrain dough often requires more water and behaves differently from dough made with highly refined white flour.

Endosperm

The endosperm makes up most of the wheat kernel. It consists predominantly of starch, along with proteins including those that eventually form gluten when flour is mixed with water.

Most white wheat flour is produced primarily from the endosperm. Importantly, the endosperm itself is not completely uniform. Its composition changes as we move from the centre of the kernel toward the outer layers. Milling therefore isn't simply a question of separating "white flour" from "bran". Different parts of the endosperm can produce flour streams with different characteristics.

That becomes particularly important in roller milling.

Germ

The germ is the embryo from which a new wheat plant could grow. It contains oils along with vitamins and other nutrients. Those oils contribute some of the flavour of wholegrain flour but also make flour containing the germ more susceptible to oxidation during storage.

The germ is one reason wholegrain flour generally has a shorter useful storage life than highly refined white flour.

And there is a crease

A wheat kernel is not a smooth oval with the bran wrapped neatly around it. Running along one side of the kernel is a deep longitudinal crease. This matters to the miller because part of the outer layers fold inward into that crease. Simply cracking the grain open does not neatly release all of the endosperm from the bran.

In roller milling, the grain therefore passes through a series of progressively adjusted break rolls. The aim is not to pulverise everything at once, but to open the kernel and gradually scrape endosperm away from the bran while keeping the bran in pieces large enough to separate (but we will get into that later).

The crease helps explain why roller milling requires repeated breaking, sifting and separation rather than a single trip through a pair of rollers.

What about the hull?

This is another area where terminology can become confusing.

Common bread wheat is generally free-threshing, meaning the chaff separates readily from the grain during threshing. Wheat arriving at a flour mill therefore does not normally have a tightly attached hull that needs to be removed from every kernel. Some other wheats are different. Spelt and emmer, for example, are hulled wheats and require an additional dehulling stage before milling. That distinction becomes important when comparing different grains and milling systems.

Cleaning and conditioning

Flour production does not begin with grinding. Grain first needs to be cleaned. Depending on the mill and the grain being handled, this can involve removing dust, stones, pieces of straw, weed seeds and other foreign material, along with damaged or unsuitable grain. Cleaning may take place several times between harvest and milling. The farmer may clean grain during or after harvest, and the mill will generally clean it again before processing. For us, this stage has become increasingly interesting as we work more directly with growers.

There is an important practical difference between grain straight from a farm and grain prepared to food-grade milling specifications. Cleaning, grading and handling are part of the flour story too.

Once cleaned, wheat intended for roller milling is commonly conditioned or tempered. Water is added to the grain and it is allowed to rest for a controlled period.

This changes both the outer layers and the endosperm.

The bran becomes tougher and more flexible, helping it remain in larger flakes as the kernel is broken. At the same time, moisture moves into the endosperm and changes the way it fractures during milling.

The purpose is not simply to make separation easier. Conditioning can also influence milling behaviour, particle size, starch damage and ultimately how the resulting flour absorbs water and behaves in dough.

Different mills use the terms conditioning and tempering somewhat differently, and the exact process varies with the wheat and the mill.

The important point is that the miller begins influencing the flour before the grain is actually ground.

What a flour mill does

At its simplest, milling reduces grain into smaller particles.

But producing good flour involves much more than making wheat small. A miller is trying to control which parts of the grain end up in which flour, how finely they are milled and how the resulting flour behaves.

There are many mill designs, but most wheat flour encountered by Australian bakers will have been produced using either roller milling or stone milling. They are different approaches, but we don't think one should automatically be regarded as superior to the other.

They are tools.

What matters is the grain being milled, how the mill is operated and what the miller is trying to produce.

Roller milling

A modern roller mill does not simply crush wheat into flour in one pass. Instead, it progressively takes the kernel apart.

Breaking the grain

The first stages are known as the break system. Pairs of rollers open the grain and begin separating pieces of endosperm from the bran. The aim is not initially to make fine flour. It is to release the endosperm while keeping the bran relatively intact so the two can be separated.

After the first break, the material is sifted. Larger pieces return for another pass through rollers with different settings. More endosperm is removed and the remaining particles become progressively smaller.

The crease in the wheat kernel is one reason this gradual approach matters. Some endosperm remains attached around and within the folded outer layers, so several passages are needed to remove it efficiently without simply grinding the bran into the flour.

Sifting and purification

After each grinding stage, the material is sorted. Plansifters contain multiple sieves that separate particles according to size. But size is not the only useful distinction.

In larger roller mills, purifiers can use airflow together with differences in particle shape and density to separate relatively pure pieces of endosperm, often called semolina at this stage, from bran and other material of similar size.

This is an important part of why roller milling offers so much control. The mill isn't simply producing "flour" and "bran". It is continually creating, separating and redirecting streams of material with different characteristics.

Reduction

Once relatively clean pieces of endosperm have been separated, they pass through further rollers that progressively reduce them into finer particles.

After each reduction stage, the material is sifted again.

Some has become flour.

Some remains too coarse and returns for further reduction.

Some streams may contain greater proportions of bran, germ or outer endosperm and can be directed elsewhere.

Eventually, the miller reaches a point where further grinding produces material that is no longer desirable for the flour being made.

When does endosperm become flour?

This raises an interesting question. At what point does a piece of ground endosperm actually become flour?

There isn't a magical chemical transformation. Particle size is part of the answer. As endosperm is progressively reduced, it eventually becomes fine enough to hydrate appropriately and perform as intended in a dough or batter.

Which means particle size is not simply a matter of appearance, it is a part of flour functionality,

Flour streams

Roller milling therefore creates multiple flour streams rather than a single homogeneous flour pouring directly from the mill. Those streams do not necessarily have identical composition or baking properties. These streams can be combined (or kept separate) to produce flour with particular characteristics.

That is one of the great strengths of roller milling: control.

A mill can produce highly refined white flour, higher-extraction flour, wholemeal flour and potentially several other products from the same wheat. This also means that saying a flour is "roller milled" tells us surprisingly little by itself. We still need to know what wheat went into the mill and what the miller chose to put back together.

Stone milling

Stone milling takes a different approach. The grain passes between two stones, with one usually rotating against the other. The gap between the stones and the rate at which grain is fed through determine how the kernel is broken down. Because the kernel is milled together rather than progressively separated into streams first, stone milling naturally incorporates the different parts of the grain into the milled product.

The resulting meal can then be used as wholegrain flour or sifted to remove some of the larger bran particles. This makes stone milling particularly well suited to producing characterful wholegrain and high-extraction flours.

But again, stoneground is not itself a quality standard. Stone diameter, stone dressing, feed rate, milling speed, grain moisture, particle size and sifting all influence the final product.

A finely milled and carefully sifted stoneground flour can be quite different from a coarse unsifted wholemeal flour even though both were produced on stones.

Roller Milling vs Stone Milling: The wrong question

When we first wrote about flour several years ago, we tended to think about stone milling and roller milling as two relatively distinct categories of flour.

The more we have learnt, the less useful that distinction has become on its own. Neither milling system guarantees good flour. And neither guarantees bad flour.

A roller mill gives the miller considerable control over separation and flour streams; while a stone mill can retain more of the grain together and produce flours with distinctive flavour and character. Both can be used thoughtfully.

Rather than asking only: Is this stoneground or roller milled?

We now think the better questions are:

What grain was used?
Was it a single variety or a grist of several grains?
How was it prepared?
How much of the grain remains in the flour?
How finely was it milled?
Was it sifted?
Which flour streams were included?
Was it blended with other flour streams or wheat varieties?
What was the miller trying to achieve?

That brings us to one of the most useful concepts in understanding flour.

Extraction rate: how much of the grain became flour?

Millers often talk about extraction rate. Put simply, extraction describes the amount of flour produced relative to the amount of grain milled.

If 100 kg of wheat produces 75 kg of flour, we could describe that as roughly a 75% extraction. The remaining material is not necessarily a waste product. It may include bran, germ, pollard and other milling fractions with their own uses. As extraction increases, more of the original grain generally finds its way into the flour. At the upper end, wholegrain flour aims to retain the edible parts of the grain in approximately their original proportions. At lower extraction rates, progressively more of the outer portions of the kernel are excluded, producing lighter and more refined flour.

This gives us a continuum rather than just two categories of white and wholemeal. A flour can be lightly sifted, moderately extracted or very highly refined. That can have a substantial effect on flavour, colour, mineral content, water absorption and dough behaviour.

Ash: another way millers describe flour

This brings us to a term Australian home bakers do not encounter particularly often: ash.

Baker’s Note: Working with higher-extraction flour

Higher-extraction flour usually contains more of the outer portions of the grain, so it often behaves differently from a highly refined white flour.

A few practical things to expect:

It may need more water. Bran and other outer grain components absorb more moisture.
Give it time. A short rest or autolyse can help the flour hydrate more fully before you decide whether the dough feels too dry.
Expect a different dough feel. Higher extraction can make dough feel firmer, rougher or less immediately extensible.
Gluten development may be less straightforward. Bran can physically interrupt the gluten network, so strength may build differently even when the protein percentage is similar.
Flavour and colour will usually be more pronounced. That is part of the character of the flour, not necessarily a sign that anything has gone wrong.

A useful starting point: keep your usual recipe, but be prepared to add a little more water and give the dough more time before making major adjustments.

Ash does not mean ash has been added to flour. It is a laboratory measurement. To determine the ash content, the lab will heat the flour sample until its organic material has burned away. The mineral material remaining is weighed and expressed as the flour's ash content.

Why is that useful?
Minerals are not distributed evenly throughout a wheat kernel. Their concentration tends to increase toward the outer parts of the kernel. As a result, highly refined flour taken predominantly from the inner endosperm generally has lower ash than flour containing more of the outer portions of the grain.

Broadly speaking:

Lower ash = generally more refined
higher ash = generally more inclusion of the outer portions of the grain

But ash is not a quality score. A low-ash flour is not automatically better than a high-ash flour, and a high-ash flour is not automatically better for bread.
It tells us something about the flour's composition. Extraction and ash are related concepts, but they are not the same measurement. Two milling runs at a similar extraction can still differ in ash depending on which parts of the kernel entered the flour. This makes ash useful to millers trying to understand and reproduce a particular flour.

White flour is not one thing

Once we understand gristing, flour streams, extraction and ash, the term white flour becomes much less precise than it first appears. Often determined by the mill, there are a lot of potential differences.

One flour might be a very refined drawn predominantly from the central endosperm, while another might retain considerably more of the outer endosperm despite still appearing relatively white.

One might be made from a single wheat variety, while another may have been deliberately milled from a grist containing several varieties or parcels of grain.

All could reasonably be described as white flour, yet they may absorb water differently, have different flavours and colours and behave differently during fermentation.

So, simply dividing flour into "white" and "wholemeal" can miss much of what can be described as the character of a flour.

Finding the flour that works for you

Understanding how flour is grown and milled is useful, but the practical question is still what happens when you take it into the kitchen. A flour may suit the way you bake because of its flavour, the way it absorbs water, how easily the dough develops, how it handles during fermentation or simply because you like the finished result.

When you find a flour that resonates with the way you bake, get to know it. Learn roughly how much water it likes. Notice how quickly it develops strength. Pay attention to how it behaves through a long fermentation and how the dough feels when it is ready to shape.

But the next batch may not be exactly the same.

A new harvest, a different milling run or a change in extraction can alter the way a familiar flour behaves. That does not necessarily mean the flour is worse. It may simply need a slightly different amount of water, a little more time or a change in handling. Recipes are useful starting points, but flour is an agricultural ingredient rather than a perfectly fixed industrial material.

The aim is not to find one flour that behaves identically forever. It is to find flour whose character suits what you are trying to make, then learn how to work with it.