INTRODUCTION TO MILLING
The milling process is a fundamental step in transforming fields of corn and wheat into flour that becomes the products we love so much. The variations in milling methods, equipment, and post-milling handling contribute to the differences in functionality, nutritional content, and flavor of the final product. Steel-cut. Rolled. Stone-ground. Bolted (sifted). Many terms allude to the process behind the final flour product. Taking a moment to familiarize yourself with the most common milling techniques and the outcomes they achieve is an essential step in leveraging the untapped potential of whole-grain flour. Some researchers even argue that milling is more significant for whole wheat bread outcomes than the grain quality or recipe used!1
WHEAT MILLING BASICS
The word “processing” gets a bad rap, but most foods we eat result from some degree of processing. We process food if we sauté our vegetables or blend a fruit smoothie. We can, however, distinguish minimally processed (whole grains) from ultra-processed (a fast-food burger bun)2. In the case of whole-grain flour – grain kernels that have been physically altered only through grinding – processing is a benefit that increases digestibility and makes vital nutrients such as vitamins, minerals, and amino acids more accessible to our bodies.3
It is also relevant to distinguish refining from milling—white refined flour results from both milling and refining, and whole-wheat flour goes through only milling. Milling grinds the whole wheat kernels into smaller particles until it is fine enough to be called flour. When grain is introduced into a mill to become whole wheat flour, all of its components (endosperm, germ, and bran) stay together if it enters into single-stream processing. If its parts are separated in multiple-stream processing and then recombined in the same proportions as existed in the wheat before milling, it is considered whole wheat flour.
Refining is when the bran and germ are removed from the endosperm to create white refined flour. This process eliminates fatty acids and many beneficial micronutrients from the final flour product.4 To refine flour is common practice because removing the bran results in a whiter, lighter flour, and removing the germ minimizes rancidity, thereby lengthening shelf life.5
Table 1. Whole grain milling techniques
Grains are crushed between rollers or plates made of steel or stone. All parts of the grain are retained from start to finish.
Multiple-Stream Milling with recombination
Grains are crushed and then sifted into separate millstreams based on particle size. Larger particles typically are subject to further grinding. The final whole wheat product rejoins all separated parts together into one flour. This process is used to create uniform particle size.
Allows for treatments before recombination including bran grinding and heating to improve storage.
Uses the multiple-stream process. Because this process results in a more consistent product, it is most common in industrial production.
Uses the single-stream process. Kernels are compressed and ground between two stone plates (modern industrial stone mills have stone plates attached to metal.
The short answer: it depends. We’ll break it down (milling pun intended) below.
NUTRITION: Mixed evidence. Roller milling may be superior if the bran and germ are added back into the flour. Since it generates less heat than stone milling, roller milling is better able to preserve the composition of beneficial unsaturated fatty acids, protein, and fiber. In contrast, stone milling relies on friction to break down grains between the two plates, which generates heat, damaging these components and some of the starch.6 However, other studies have shown that stone-milling, when done at a low speed, produces less heat and creates a coarser grain, which may reduce oxidation.7
You may be asking – if heat is so impactful in determining nutritional integrity, then what happens when flour products enter a hot oven? Indeed, heat during baking does break down some nutrients, but we can enjoy the deliciousness of bread knowing that the reactions in the oven produce unique nutritional properties. During baking, proteins denature and become more available for digestion.8 Sugars can also react with protein to create the Maillard reaction, which is responsible for the yummy browning of the crust and may also have antioxidant properties.9 This reaction slightly diminishes protein quality and makes one amino acid, lysine, unavailable. Since whole grain flour is higher in protein than refined flour, it should still have higher protein content than bread made with refined flour. However, non-endosperm protein, found in whole wheat flour, may be less digestible.10 Baking can also result in resistant starch formation, which can support our gut microbiota in a similar way that fiber does.11
INDUSTRIAL PRODUCTION: Roller milled wins this one. Roller mills are the most popular commercial milling technique due to their high-capacity potential and flexibility. Producers can change the roller mill system and easily modify the end flour product by adjusting the gaps between rollers. Because particles from different parts of the grain separate during the process, they can be manipulated separately and joined back together in varying proportions to achieve a specified end product.12 Roller mills are typically made up of three parts: the break system, the sizing system, and the reduction system. Each system is responsible for breaking apart different components of the grain. Researchers found that retrieving flours from each of the different systems, rather than at the end of the milling process, can produce flours with different properties.13 They showed that flours from the sizing and reduction systems had better nutritional composition whereas flours from the break system had higher starch content and better stability.14 This is a relatively simple technique to adjust the flour to clientele specifications.
MARKETING: Stone milled has the advantage here. Health-conscious consumers associate “stone-ground” with preserving the integrity of the whole grain.15 Further, stone milling is the oldest milling technology.16 Since no parts of the grain are separated during the milling process, the components of the grain remain in natural proportions, appealing to consumers that favor “natural” products, even though this can be duplicated in the roller milling process.17 Some producers have capitalized on this marketing opportunity by first processing with stone milling and then finishing with roller milling.18 Although artisanal bakers and organic millers tend to use this method,19 which reinforces the idea that stone-milled is the most “natural” milling technique, some bakers prefer stone-ground because of perceived differences in texture and flavor.20
HOME MILLING: Choose whatever works best for you. Consider your budget (prices range from $100-$350+), kitchen space, existing equipment (there are some attachments for stand mixers), how fine you want the end product to be, and how much grain you think you’ll want to mill at a time. Although you will need to make the upfront investment in a mill, purchasing the whole grains is usually less expensive per weight and grain kernels, and if stored properly, can keep for a longer time, reducing potential waste.21 Mills meant for home kitchen use are usually classified into two groups: burr or impact mills. Burr mills (which can be either stone or steel) grind the grain between two plates, whereas impact mills work at high speed to break apart grains in a stainless-steel chamber of raised fins.22 If you are most concerned with the nutritional quality of the wheat, you may want to pay attention to the heat generated, but remember that by milling grains at home, you are already maximizing the nutritional quality of the flour by not letting it sit on a shelf and degrade! Pleasant Hill Grain, located in Nebraska, offers a short guide to the grain mills they sell here, but you will likely want to spend some time looking at reviews on each of the manufacturers’ websites. Here at the Food Lab, we use a Mock Mill and a KitchenAid attachment. One of our colleagues owns a WonderMill, a reasonably priced, not-very-pretty workhorse that has lasted for twelve years of milling several times per week.
STORING FRESH WHOLE GRAIN FLOUR
It might seem odd that a product that we habitually see sitting on grocery store shelves (for who knows how long) and in the backs of our pantries (also for who knows how long) is actually not as “shelf stable” as we may think. Especially whole wheat flour. Before you throw in the towel about storing whole wheat flour properly, consider that the beauty of whole grain flour – all of the goodness contained in the grain’s outer shell and germ – is precisely what makes it go bad more quickly. And it is what makes a little fussing over storage absolutely worth it. Grain needs to be stored properly to fully appreciate and reap the benefits of its wholesome beauty.
Why does whole-grain flour spoil more quickly?
Whole grain flours are packed with healthy oils [link to nutrition benefits page], making them more nutritious but also less stable than refined flour. Heat, light, and moisture degrade the nutrient content of whole grain flours, which require cold temperature storage to slow the degradation of nutrients, flavor, and taste. While the fat composition of wheat is relatively low (approx. 2-3%), whole wheat flour contains more fat (also known as lipids), than refined flour. The fat distribution within the grain is approximated as follows: 63% in the endosperm, 34% in the germ, and 3% in the bran.23 Unfortunately, wheat grains also contain enzymes that break down these fats, releasing what are called “non-esterified fatty acids,” which can make the flour taste bitter and rancid.24 Some enzymes that degrade fats are more plentiful in the bran and germ, potentially exacerbating lipid oxidation in whole grain flour.25 Oxygen in the environment can also get to the fatty acids and break them down. The breakdown of fats is of concern because it can negatively affect the bake by diminishing gluten’s ability to hold gas and stretch.26 While the breakdown of fat is the main storage concern, changes in proteins and carbohydrates can also occur over time which can affect baking behavior. Vitamin E and carotenoids, which have desirable antioxidant properties, also degrade over time.27 This is to say, all bakers using whole-grain flours should treat fresh flour as a perishable product, using it as soon as it is milled, or freezing it if necessary.
Researchers have been working to develop creative solutions to the short shelf life of whole-grain flour. Attempts to stabilize lipids have included: incorporation of additional antioxidants or metal ions into flour, limiting oxygen, heating, and gamma irradiation.28 These approaches haven’t been super successful, but the good news is there are a few simple and low-tech solutions to keep your whole grain flour fresh, detailed in the next section.
Purchasing and storing your fresh whole milled flour
We encourage you to get the freshest flour possible to maximize nutritional benefits and flavor. Unfortunately, as Jonathan Stevens of Hungry Ghost Bread in Northampton, MA laments, “The problem with almost all the whole wheat flour that you can get in the store is that it’s rancid already, and you won’t really know that until you try to make a loaf of bread and it just doesn’t perform right, it doesn’t taste quite special. And you blame yourself.” Luckily, “now we’ve got a whole renaissance of local mills” and the technology for home mills is becoming more accessible (Stevens). Below, we offer tips on how to source the freshest whole grain flour, and if you need to purchase from the grocery store, we’ll tell you how to maximize the freshness of that flour, too.
Option A. Buy locally milled flour.
Visit your local mill or store that stocks freshly milled whole-grain flour or whole grains that they can mill for you. If this option is available to you, we highly recommend it – it allows you to support local growers, millers, and/or store owners, feel more connected with your food, and guarantee freshness. The internet is your friend when it comes to hunting down local mills!
How to buy: Check out our New England Mills [link to that section] guide to find a mill or store close to you. If there is not a location close to you, some will ship to you, but still try to buy as locally as possible to minimize shipping costs and time.
How to store: If you plan to use up your whole grain flour quickly, it will keep in airtight containers on a cool, dry pantry shelf for 1 to 3 months. We suggest tasting the flour as it ages because some varieties may age, which is to say become bitter, more quickly than others. If you plan to use your flour more gradually, then freezer storage is recommended to maintain freshness for 2 to 6 months.29 The refrigerator is a good option if freezer space is low. You can empty the whole flour bag into your storage container of choice or wrap the original flour bag in an airtight plastic bag.30
Option B. Buy the whole intact grains and mill them at home.
– How to buy: You can buy whole intact grains locally [link to New England Mills], but if that is not an option, whole grains are commonly available in the baking aisle or bulk section of the grocery store or food co-op. In both contexts, try to determine how long the grains have been in the store before making your purchase. Buying grains from bulk bins is a fine option if there is a high turnover rate at the retailer. Check for a faint sweet scent or neutral aroma before purchasing. If the grains smell oily or musty, keep searching—these grains are past their peak.31
– How to store: When selecting pre-bagged grains, check for a “sell by”, “use by”, or “packaged on” label—these will clue you into the freshness of the grain and how long it will stay fresh after purchase. Whole grain kernels do not spoil as fast as ground flours because the insides of the grain are not exposed to oxygen. You can store intact whole wheat berries in the pantry for about 6 months; in the freezer for about 1 year. You can mill at home [link to home mill section] as needed to maximize freshness.
Option C. Buy whole grain flour at the grocery store and store it with care.
– How to buy: Pay close attention to the “best by” date on the bag.
– How to store: Just like with locally milled flour, you’ll need to store it in an airtight container in a dark, cool space (see “Option A”).
General tips for storage:
– Smell your flour when you bring it home and as you continue to use it. Does it have a sweet or neutral odor, or has it begun to smell musty? If the smell starts to seem off, you may need new flour soon. You can still bake with the older-smelling flour to avoid food waste if there are no other signs of spoilage; your baked goods may just taste a bit different.
– Mark the date of purchase and milling.
– As PJ Hamel of King Arthur Flour writes, “Three words: Airtight, cold, and dark.”32 See her full article on whole grain storage here.
– Some grains have slightly shorter or longer shelf lives than the general “2 to 6 months in the refrigerator/freezer and 1 to 3 months in the pantry” guidance above. For more specific shelf life guidelines for your grain of interest (say, barley or oats), the Whole Grains Council has put together an excellent table.
– If you are making bread, you may want to intentionally age your flour for a short period of time. See our full section on flour aging below.
AGING FLOUR: FRESH VS. AGED FLOUR: WHICH IS BETTER?
At the Tufts Friedman School of Nutrition Science and Policy, we are most obviously concerned with nutrition. But that’s not the only thing we care about! The Tufts Food Lab wants to make sure that our research and recommendations ultimately translate to food that is delicious and accessible. If that means that we sacrifice a small bit of nutrition for a net gain, then we understand. Technically, from a nutrition science perspective, fresh, straight from the mill, whole grain flour offers the most beneficial nutrient composition.33 It also provides the most flavor since the grain’s nutrients are the source of the flavor.
The practice of aging flour goes back centuries. Historically, bakers would store flour for months, allowing oxygen to get to it, which in turn would improve its strength and gas-trapping abilities, ideal for breads.34 You’ll notice in our Storage section above that we offer tips on how to help prevent the oxidation of flour, contrary to the purpose of the aging process. This is because there is a fine line between flour that is purposefully aged and flour that is rancid, and there are variables including the variety of wheat and the temperature at which the aging occurs. As soon as the grain is split open, its nutrients become vulnerable to degradation and rancidity.
When flour is aged for several months, oxidation restructures the proteins within the flour while starch remains rather consistent. With aged flour, the influence of gluten becomes more pronounced, forming stronger bonds which lead to a more elastic dough.35
Some bakers prefer using freshly milled flour, citing the health benefits, extra vigor, and special flavor provided by its nutritional quality;36 others prefer to age their flour for better gluten development in breads.37 Short-term aging of flour results in molecular bond changes among gluten proteins that benefits breads.38 Research has shown that flour aged for about 2-3 weeks showed higher bread loaf volume, higher water absorption, and increased viscosity.7 However, much of this research has been conducted on refined flour, and given that whole grains spoil more quickly, there is more research needed on whole grain flour storage specifically.39
To age your own freshly milled whole-grain flour, you can let it sit out in a non-airtight bag, such as a paper bag, for approximately one to three weeks.40 One baker recommends making sure that your grains or flour has been frozen for 72 hours before aging, to make sure insect eggs have been killed.41
Freshly milled whole grain flour
Aged whole grain flour
Less gluten development, resulting in a denser bread in some cases
Allows for more gluten development which creates a lighter texture
The nutrients in the kernel that have been exposed to oxygen from milling have little to no time to degrade
Nutrients exposed to oxygen during the milling process begin to deteriorate immediately
Faster: This flour is more “alive” due to increased nutritional content, so you’ll want to keep a closer eye on your sourdough starters and proofing breads
A big reason why many people use freshly milled flour is its flavor, which can vary with the variety of wheat, and its flavor complexity
Note: You have probably seen flours in the grocery store labeled “bleached” and “unbleached.” In the refined flour world (where flour contains only the starchy endosperm), aged flour is commonly known as unbleached flour. Both bleached and unbleached refined flour experience oxidation; for bleached flour, oxidation is achieved through chemical bleaching agents such as benzoyl peroxide and chlorine, whereas for unbleached flour it is achieved by age.42 Bleaching is associated with a softer flour; aging results in a denser texture, more gluten strength, and higher elasticity.43
For more information on sourcing, storing, and utilizing freshly milled flour, check out Maine Grain Alliance Director Tristan Noyes’ conversation with James Beard Award-winning cookbook author and baker Sarah Owens: Stone Ground Flour with Sarah Owens & Tristan Noyes