Few people realize the old mills that highlight historic towns throughout the country once offered farmers the finest available milling technology for transforming grain into flour. Business boomed for centuries, and these water-powered mills anchored regional economies and helped lay the foundation for today's modern food industry.
While the vision of a waterwheel driving a rotating millstone may seem a quaint memory of a bygone era, the engineering concept behind the technology remains valid and in action to this day as one of many effective milling methods. Back then, choosing the ideal milling method was easy.
Today, food manufacturers, processors and ingredient suppliers enjoy access to a wide range of high-tech milling methods. Each one offers a variety of advantages for achieving required product characteristics such as desired particle size and moisture content, along with other considerations such as cost, footprint and the need for ancillary equipment. Each method has also given rise to a diverse range of size reduction equipment, and, in many cases, many types of machines representing several different milling methods may all be well-suited for processing the identical product and for meeting the identical size reduction goals (Figure 1). In fact, several different technologies offer overlapping capabilities, and there may be more than one right solution when selecting milling equipment.
Size Reduction Goals
To help make sense out of the dizzying array of milling options, start at the end by first considering — not the milling machine — but the end product. The most important step in selecting the right size reduction equipment is to establish the target size reduction goal for the end product at the outset. Is it a coarse salt product or an ultra-fine confectionery sugar? This target is typically based on the particle size required and the particle size distribution desired, typically expressed in terms of milling the material down to a given particle size distribution in microns.
For example, a potential target for corn starch may be expressed as achieving particle sizes down to D97 of 75 µm. This means that after milling, 97 percent of the particles measure 75 µm or less in diameter. This is also referred to as the top size or top cut to represent the top, or largest, size particles being produced. A secondary target often involves a median cut or median size. Commonly expressed as D50 of 28 µm, for example, this means that after milling, 50 percent of the particles measure more than 28 µm and 50 percent measure less than 28 µm. A bottom cut or bottom size may also be incorporated into the target to minimize the amount of particles that are smaller than desired. Expressed as D10 of 15 µm, for example, this means that after milling, 10 percent of the particles measure less than 15 µm. As the relationship among these three types of cuts or sizes changes, the tighter or looser the acceptable range for the size becomes and the easier or more difficult it becomes to meet the criteria for on-spec product. Similarly, the wider the acceptable range for the size of the end product, the wider the processing window for the milling operation and the less complex and costly the milling machinery required.
End products that demand the vast majority of particles consistently measure the same size, such as cake flour, often require a different approach to milling than end products that are intended to include a range of different sizes, such as cookie crumb or chopped walnuts. Any wholesale baker would manage to decorate cakes with nearly any size cookie crumb or chopped walnuts, but if the flour fails to meet the required specifications the baker is in for a long day. The particle size directly affects the amount of surface area and the interactions among the flour, water and other ingredients. For cakes and other baked goods to achieve a desired delicate, heavy or fluffy texture every time, the baker needs the flour to meet the same particle size every time.
The particle size of food products also requires a very human consideration that doesn't apply when selecting milling equipment for chemicals, plastics and other products. Since food products are to be consumed, the effect of particle size on taste and mouthfeel needs careful consideration. For most people, the tongue begins sensing the existence of particles at 18 µm. As the particle size increases, the tongue begins to sense the smooth texture becoming more and more gritty. By approximately 75 µm, the tongue may find the texture negatively impacts both mouthfeel and taste. Similarly, a single product or ingredient may require different particle sizes when applied to different products based on the product's functionality. Fortifying an energy bar with whey protein, for example, may require a different particle size than when adding it to a protein shake or smoothie. In the bar, the particles need to be sized to mix evenly with the other ingredients and produce a desired taste and mouthfeel after baking. In the smoothie, evenly dispersing the particles may be more important than its performance when heated.
Arriving at the target size reduction goal may seem challenging but it serves as a vital first step for choosing the right milling equipment.
Testing and Particle Size Analysis
Despite the importance of determining the required particle size, many people involved with product development simply do not know the size of their existing products or the particle size needed for new products. When asked about the target particle size, “something like flour,” “as fine as possible” or “just like cereal” are common responses. In these cases, consider checking with a milling machinery manufacturer that offers a substantial track record with the product.
Whether the product matches existing test data or is starting from scratch, today's milling equipment manufacturers offer so many types, configurations and custom options that are capable of meeting such tight specifications, that testing the actual product on lab size and/or full size machinery is highly recommended. The first step in testing often involves evaluating a sample of the product on a particle size analyzer that uses pneumatic sieving to determine the particle size distribution and arrive at a target end point. Then other product characteristics can be addressed such as the size at the infeed, bulk density, flowability, hardness, moisture content, heat sensitivity, toxicity and even explosivity. All of these, and others, play key roles in determining the most effective and cost-efficient way to meet the target particle size goal.
Now armed with critical product knowledge, the type of milling equipment can be considered.
Milling Selection: Hammer and Screen Mill
First, check if a low-cost technology can meet the requirements. One of the least costly types of milling machinery for food products is also the most frequently used with thousands of installations worldwide. While the old millstone relied on shear to grind wheat into flour, this hammer and screen mill style uses the impact of swinging hammers that rotate at high speeds inside a round housing to reduce the particle size. When the particles meet the targeted size, they fall through a screen en route to collection. Screens are available in a variety of sizes and configurations to suit a wide range of materials whether dry, sticky, fibrous or abrasive. Today's most effective hammer mills grind down to D90 <75 µm. If a finer end product is required then more advanced impact systems may be considered.
Pins, Knife and Attrition Mills
One such advanced system achieves fine grinding down to D97 <35 µm. It replaces the hammer rotor with a pin disc style rotor and forces the material to collide with a series of pins set in concentric circles protruding from high-speed rotating discs. This style excels in milling crystalline, fatty and oily products such as spices that may buildup and blind screens. The latest models allow these fatty, oily and other heat-sensitive products to be processed without separate cooling equipment. Other systems replace the hammers with knife rotors that granulate the material. These perform well for coarse granulation to medium fine size reduction and excel with softer products. For finer milling of soft and/or heat-sensitive products, attrition mills offer an alternative. These mills grind the product between two, tapered plates that rotate at high speeds, reducing the particle size as the product moves from the infeed at the center to the periphery for discharge and collection.
Air Classifying Mills
When the target demands very narrow particle size distributions with grinding down to D97 at <20 µm, air classification mills may be considered. Bringing a higher level of engineering to the process, the latest air classification mills combine impact milling with dynamic air classification. Material is fed from a silo or fed into the hopper and automatically conveyed first into a milling chamber where hammers rotate at high speeds for size reduction. In this clever design, particles are constantly recirculated from the outer liner back into the rotating hammers. When the particles meet the desired size, air carries them to an internal classifier. On-spec particles exit the mill for collection and any oversized particles are rejected and captured for further milling. Due to its ability to reduce very large particles such as 6 mm grains of rice to very small particles under 50 µm in a single pass with minimal fines when other mills would take several passes, this type of mill is quickly becoming the preferred system for companies that focus on efficiency and lean operations. Its inherently easy-to-clean design and flexibility for processing multiple products on the same system also explain its demand in the food industry.
Other types of milling equipment such as traditional ball mills and advanced fluidized jet mills perform similar ultra-fine size reduction but rarely offer the same combination of capabilities and low cost for processing food products as the machinery previously described. And still, more than one of the systems described may effectively meet the target specifications. Testing the product on more than one system may prove the point. Most reputable manufacturers offers access to a testing laboratory with pilot scale milling equipment to ensure — and even guarantee — it will work as specified. Then the initial cost must be considered, along with secondary factors such as footprint, maintenance needs and versatility. In many cases, the initial cost needs to be weighed against the gains in product quality, production speed, dust control and worker safety along with energy savings and the payback period. Even the resale value needs to be considered given that milling machines may be quickly sold when growth demands upgrading to machines that offer additional capabilities.
With numerous milling options available, food processors face a challenging landscape when sifting through the range of methods and machines. Further, many mills require an array of ancillary components upstream and downstream of the actual mill from storage silos and feeders to conveyors and dust collectors. In these cases, consider whether the milling equipment manufacturer has experience integrating all of the elements as an integrated mill system. An ill-advised purchase may lead to costly unplanned downtime, disruptions in production and failure to meet product quality specifications. Selecting the right milling equipment requires a thorough understanding of the product and its desired characteristics along with careful testing and expert guidance to ease the process and ensure a successful purchase.