To grind something solid, many of us might think of a mortar and pestle as the original grinding machine. In fact, that technology goes back at least to 1550 BCE, when one was described in Egyptian writing. Many other hand-powered mills and grinders followed, leading to powered devices in today’s many forms.
In describing mills and grinders, Joe Porcelli, regional sales manager at IKA (Wilmington, NC), says, “There are two traditional methods: wet and dry approaches.” Wet milling uses a high-shear mixer on solids in a liquid suspension. “Wet milling helps minimize dusting, reduces heat degradation of the particles, and promotes improved flow characteristics through the mill,” says Porcelli. In the dry technique, “particles are reduced through impact on tooling surfaces or particle collision. Examples include jet mills and pin or hammermills,” he adds.
Both versions of this technology appear in many applications. “Some classic examples include wet milling of APIs—active pharmaceutical ingredients— slurries, size reduction, deagglomeration of cellulosic material in biofuels or biobased chemical conversion, and specialty pigment milling in the cosmetics and coatings industries,” says Porcelli.
Getting the right grind
Consumer concerns often impact the use of laboratory equipment. For example, recent media attention to arsenic in rice and Salmonella in peanut butter drives enhancements to food safety processes. “This work requires high quality results from the sample process,” says Kyle James, vice president of sales at Retsch (Haan, Germany). “So they are really focused on getting a grinder that produces accurate and reproducible results.”
The accuracy involves particle size and homogeneity of the particles. “With a coffee grinder,” says James, “you can get particles that range in size from 300 microns to 2 millimeters, but food safety researchers want something with more focused particle size.” To get that consistency, a researcher needs to know that a specific configuratio —grinder, rotor, sieve, timing, speed and so on—will always produce the same or similar results. “Then, if you have a problem with your analysis, you know it’s not because of the grinding of the sample,” James explains.
To help users run different tests, James points out that some grinders can store protocols—up to nine in some machines. “If you need to report the procedure, you have all of the parameters available to you,” James says.
The right grind also plays a key role in making pharmaceuticals. For example, Porcelli says, “In API wet-milling applications, as new drug materials are discovered, they are often insoluble in water. Wet milling these drug materials increases their efficacy and dissolution in the body and ease of formulation into a tablet matrix.” He adds, “A new trend in the pharmaceutical and chemical industries is the controlled crystallization and subsequent wet milling of new compounds to control particle size and material performance.”
Mills also appear in modern molecular biology research. Tim Hopkins, president and CEO of BioSpec Products (Bartlesville, OK), and his colleagues started working with yeast about 30 years ago. “We worked with hundreds of gallons of yeast, and tried to find ways to break it open,” Hopkins says. “It occurred to me that we might add sand to a bar blender and throw in some yeast.” The experiment quickly destroyed the blades on the blender, but it disrupted the yeast. That led to the so-called BeadBeater approach.
Hopkins also wanted to make a machine to work with smaller volumes. For that, he says, “We moved toward the same bead mill but with 2-ml microvials with screw caps, and we shake the heck out of them.”
At Rutgers University (New Brunswick, NJ), George M. Carman, Ph.D., Board of Governors Professor in the department of food science and director of the Rutgers Center for Lipid Research, uses mills to break open yeast cells to make extracts. He uses these extracts to get enzymes from inside the yeast cells. “There are other ways to do it,” Carman says, “but this is the easiest and most efficient way.” In short, Carman uses the mill to break open the yeast to release its intracellular contents, and then he purifies the enzymes.
It might sound easy enough to break open yeast cells, especially since the cells are usually only a few micrometers across—maybe a few tens of micrometers for the largest yeast. But as Carman explains, “Yeast has a cell wall, and that’s not easy to open. It’s a very rigid wall—harder than a plant cell wall.” He adds, “The force from the beads allows shearing to break the cell wall into fragments.”
In looking for a mill, Carman seeks several features. It must break open the cells efficiently and quickly. “It has to happen in a pretty quick manner, because too much heat would destroy the enzymes that we’re trying to extract,” Carman says. Besides breaking up the cell walls quickly and efficiently, the sample must be kept cold in other ways to protect some of the components. So Carman uses some mills that include a jacket that can be filled with ice. On the other hand, a portable mill can be used in a cold room to help keep the samples from getting too hot, and therefore damaged.
Beyond the basic technology in a mill, Carman also appreciates other features. For example, he likes that his mill lets him simultaneously process multiple microvials. “We do six, eight, 32, maybe more samples at one time,” he says. He also likes a mill with a timer. “We might shake a sample for 20 seconds, and then stop for 10 seconds to let it cool down a little before the next round of shaking,” Carman says.
From APIs to yeast, today’s mills change the possibilities of research and medicine.
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