When milling wheat into flour, two types of milling systems can be used — the classic, or conventional, long flow or the compact, or short flow, mill. Each system will produce flour, but each has its advantages and disadvantages.
Before making an investment in either type of mill, consider several factors, including the mill's location, experience of the operators, amount of money to be invested, the availability of wheat varieties, the types of flour required and the needs of the end user, whether it is a bread baker, cookie and biscuit maker, cake producer or other food processor. Ultimately, economics and return on investment will determine which milling system to use.
A long flow mill is the classic gradual reduction system, made up typically of somewhere between 14 to 17 steps — five breaks, nine to 12 reductions — using roller mills, sifters and purifiers throughout. (A classic mill diagram was featured in the September, October and November 1998 issues of World Grain.)
A short flow unit reduces wheat to flour using the roller mill as the principle grinding medium, but in fewer steps than a conventional mill, usually about half. Short flow units typically have less capacity, though they can be built to provide any capacity required, using the same full-size, traditional machinery as a conventional mill, just less of it.
Nearly every milling engineering company worldwide produces its own version of a conventional flow and may offer an abbreviated flow. (Shortflow is a copyrighted term for a patented process held by Kansas State University, Manhattan, Kansas, U.S.)
The long flow is to the short flow as a limousine is to a small, run-about-town car —both can move from A to B, yet the former has the greater flexibility.
SHORT FLOW ADVANTAGE. The short flow mill has a number of generic names, such as "compact," "mini," "abbreviated" and "short." Advantages include a simpler diagram, lower energy consumption, lower cost per tonne and modular construction, allowing factory pre-assembly to expedite installation.
Because short flow mills require less equipment and need less space, the overall capital cost is much less than a conventional, long flow mill, and construction time is reduced. Short flows also require less supervision, less power and have lower maintenance costs.
Modular short flow units — plants that are not only partially pre-assembled or pre-tested and containerized for shipment, but also capable of being added to for a quick increase in capacity — are sometimes known as "platform" mills. These plants are frequently erected inside single-story buildings such as warehouses or farm buildings, and can be clad or have siding attached directly to the structure to enclose the process, reducing the cost of the final installation.
Another advantage is that a short flow facility can be located close to or on a customer's premises, allowing for a variety of business arrangements between the miller and baker. It is always cheaper to transport wheat than flour since wheat is easier to handle, more free flowing, does not need specialized trucks for transport and has a higher bulk density than flour. Collecting, storing, cleaning and tempering wheat in a central location and then supplying satellite short flow mills also saves money by not having to duplicate equipment.
Return on investment is further enhanced on short flow mills because delivery, erection, installation and commissioning are quicker than long flow mills. Another advantage of short flow plants is their ability to be moved and re-erected elsewhere should the local market dry up or disappear.
Because there is less equipment involved, short flow mills use less energy. They can be driven by diesel generators, hydroelectric power units, steam turbines or other locally available power converters.
Capacity of short flow mills ranges from a few hundred kilograms per hour to more than 9 tonnes of wheat per hour. With PLC computer controls, compact mills can be run without personnel at night and on weekends, as is the case with large, scale mills.
Many large conventional mills seeking to increase capacity or produce specialty flours have installed short flow units. Short flow mills are occasionally used to grind specialty wheats, such as organic wheat, to prevent cross contamination, and can switched to grind maize, rice, rye, barley, sorghum and malt (for brewing) with a few minor adjustments.
Mobile mills with low capacities exist for either military or other purposes. On contrast to both the conventional mills and the abbreviated flow units, mobile mills usually utilize miniature equipment to reduce the capacity produced and physical size of the facility. Pre-erected on a trailer and powered by the tractor engine, a mobile mill can grind just enough flour for field kitchen baking.
As far as quality is concerned, the larger, more sophisticated short flow plants come fairly close to conventional mills. The ultimate test is one of customer satisfaction and consistency.
Although milling durum for pasta is not normally considered an option with compact mills because of the high quality restrictions, it can be done. Many short flow mills in North Africa are being used to produce cous cous.
BENEFITS OF LONG FLOW. The classic long flow mill has capacities from 15 tonnes/hour to 50 tonnes/hour and beyond. Probably the most important benefit of the long flow is flexibility. In the wrong hands this can be a disadvantage, but that is a management — not an equipment — problem.
The long flow mill has the edge when it comes to yield or extraction (the percentage of flour milled from wheat). A conventional mill has more "tail-end" milling capabilities, so any missed grain or under-milled product has a chance of being caught and milled. A short flow process does not allow for "missing" the product at the head end with a safety net of grinding it at the tail-end of the mill. A mill with twice the number of milling steps will typically produce a uniformly higher extraction, based on similar wheat and product specifications. Extraction becomes more valuable the greater the monetary gap between wheat cost and market value of bran/millfeed.
A conventional plant is set to mill more gently and less aggressively than the short flow type, thereby creating a lower mineral or ash content. Particle size can be verified and controlled on a wider basis, according to demand. This can be an important benefit.
Wheat germ is easy to extract in large mills, but is difficult, sometimes impossible, from the compact variety. In certain areas, wheat germ is a highly prized and valuable commodity. Most conventional plants can extract good quality germ — up to 0.75% by weight — from wheat.
Because blending varieties and qualities of wheats is a function of the bin capacity of the facility, larger conventional mills also may have the advantage here unless the compact mills also have adequate bin capacities. However, long flow mills do have a greater tolerance for milling lower quality wheats.
Water absorption of flours can be troublesome in any mill. Many believe that long flow plants have the clear advantage because the wide variety of equipment can adjust to almost any circumstance. However, recent blind tests by the American Institute of Baking in Manhattan, Kansas, indicate better absorption with short flow flour, one manufacturer said.
Increasing capacity of large, conventional mills has been made easier by the introduction of the 2-high rollermill and the debranning process. The "bobtail" or "booster" concept increases capacity of an existing flow with the addition of extra machinery. The entire milling system is then run simultaneously.
Debranning the extra wheat to be ground produces wheat kernels stripped of 65% to 70% of bran skin. The remainder is in the crease. This process diminishes the need for extra break roll machinery and matching sifter surface.
At the design stage of a "bobtail" addition, ensure that the original plant can still be run without the bobtail addition. This allows the mill to run according to fluctuating market demand.
Just-in-time requirements for customers — highly desirable for that elusive competitive edge — are manifestly easier with short flow, lower capacity units. Both long and short flow mills comply with sanitation and compliance standards, though the smaller the unit, the easier this is.
Before choosing which type of mill flow is most appropriate for your purposes, analyze your customers' needs. Visit short flow and long flow mills to judge performance, and seek advice from suppliers to determine which method is the most cost effective and still fully meets your criteria.