Many methods of preparing foods require milling or grinding raw materials, including rice, spices, and many other items. How a mill or grinder is used—the items being processed and what must be accomplished—determines the required features of a device. Here, two experts weigh in on the best methods and devices for food processing.
Beyond variations in technology, the same term “milling” can mean different things depending on what is being processed. For instance, “wheat milling comprises converting wheat kernels to flour,” says Terry Siebenmorgen, distinguished professor of rice processing at the University of Arkansas, Fayetteville. “Rice milling refers to removal of the bran layers and embryo from brown rice kernels, with the goal of leaving as many kernels fully intact as possible.” So, keep in mind the terminology when assessing this food science technology.
Various features of a device must be considered. “For grinders, the screen size—mesh size—is important in determining flour particle size distribution,” says Siebenmorgen. “Also, the type of mill—hammer mill, roller mill, et cetera—is important for handling various materials.”
When milling rice, the condition of the rotor and the screen impact the outcome. In determining the size distribution of the resulting particles of rice, however, the screen matters more, according to Siebenmorgen.
Ongoing advances in food milling technology help to overcome some common challenges. “In rice milling, a fairly recent development in commercial mills has been the conversion from single-break—single-pass—mills to triple-break mills in which brown rice passes through three sequential mills, each of different construction and milling action, to accomplish the bran removal,” Siebenmorgen says. “These later systems are more gentle and do not raise the temperature of the milled rice as much as the previous systems, thus preventing some post-milling fissuring that was previously observed.”
Rice processing also benefits from other recent advances. As an example, Siebenmorgen mentions that many commercial mills now use “water-mist milling, in which a very fine, atomized mist of water is sprayed onto rice in the late stages of milling.” As he points out, “The water serves to help remove fragments of bran on the milled kernels, thereby producing a cleaner-appearing, polished white rice kernel.”
Spice it up
One look at the spices on a grocery shelf reveals the diversity of this category of food. Nonetheless, food scientists group spices based on fiber or oil content. These categories determine the approach to processing.
With a spice with a high fiber content, such as chili, “typically a fine impact mill equipped with plate beater rotor combined with grinding track and sieve insert is used,” says Barbara Kästl, manager of new business development in the food division at Hosokawa Alpine. “This system gives the advantage of having a combination of certain impact for grinding fibers and creating a precise particle size by sieve insert.”
For an oily spice, like nutmeg or pepper, a food scientist can pick cryogenic grinding. In this process, liquid nitrogen is used to freeze a product, which makes it brittle and easier to grind. “Cryogenic grinding preserves essential oils and therefore spice flavor,” Kästl explains. This translates to a higher quality product.
Variations in spices must also be considered. One challenge arises from moisture in the raw product. “If the moisture of the feed material spice is higher compared to standard,” Kästl explains, “a significant performance loss in capacity is noticed.”
The time of year and source of a spice can also affect grinding behavior. “Spices are natural products with fluctuating properties,” Kästl says. So, a processing device should be easy to adjust as needed. “Furthermore, spice customers are producing several spices,” says Kästl.” So, the system should be easy to adjust from one spice to another.
Ensuring safe processing
When thinking of safety in milling and grinding, mechanical accidents might come to mind. With spices, processing can also start a fire through, for example, a dust explosion. “Spices are organic products, which have the ability to explode at a certain fineness under certain conditions,” Kästl notes. “Certain concepts like special designs or inertization solutions need to be considered so that a milling system is protected properly against dust explosions.”
In addition, an overloaded mill or grinder can fail, but most devices can be protected fairly simply. “Most commercial mills are monitored by installing amp meters on the electric motors that operate the rotors,” Siebenmorgen notes. “Thus, if something is lodged or overfed in the mill, preventive action can be taken before damage is done.”
Options to consider
In shopping for a new mill or grinder, various features could be worth considering. As an example, Kästl says that a mill with a higher tip speed can “produce a finer product with higher capacity.”
Access also matters. If a grinding chamber includes doors that open widely, Kästl notes, it’s easier to clean.
Beyond cleaning, mills and grinders also require maintenance. “The rotor and the grinding tool are critical spare parts and need to be replaced from time to time,” Kästl says.
The changes in mills and grinders offer a range of new opportunities in food science. The best results, though, depend on pairing the food item with the right mill or grinder, plus any needed options. With so many foods to consider, scientists often need a collection of milling and grinding devices.