Routine upkeep keeps them grinding
That is changing, says Eric Clouser, a sales specialist at Omni International (Kennesaw, GA). “Laboratory bead milling, especially for small sample volumes, is a relatively new idea. A number of companies produce ball mills, but until recently the devices have been too large for laboratory use.”
Like their much larger cousins, bench-scale mills employ beads of stainless steel, ceramic, or glass to reduce the volumes of sample particles. “It’s an aggressive form of homogenization suitable for almost any sample,” Clouser says. After milling, tubes are suitable for further manipulation such as extraction, reagent addition, and filtration.
Mills as cell disruptors
Because there is a wide range of marketed cell disruption technologies, one might not consider ball milling bacteria, yeast, or suspended animal cells due to their small size. Yet this form of size reduction, which releases cellular contents undamaged, is fast and efficient. “We sell an ultrasound homogenizer as well, but a bead mill is superior for cells,” Clouser tells Lab Manager. He predicts that within a few years ball mills will replace traditional homogenizers, including rotorstator, ultrasonic, and even pressure homogenizers, for many lab applications.
Ultrasound of microorganisms is slow, may chemically disrupt target analytes, and adds heat that can denature proteins and genes. Bead milling reduces the likelihood of losing sample through mechanical or thermal events. Since sample tubes are disposable, bead milling eliminates crosscontamination or artifacts due to improper cleaning of sonication heads or propeller blades. Both tubes and beads are intended for a single use.
“In the life sciences, shaking-type bead mills are superior to other types of mechanical disruption because the sample is securely enclosed within a small vial and the integrity of each sample is protected,” says Tim Hopkins, president of Biospec Products (Bartlesville, OK). Compared with a French press or a mortar and pestle, bead milling processes much smaller volumes, down to 0.1 mL, which conserves samples and reagents. “You lose too much of your sample if you try to homogenize volumes in the microliter range in conventional homogenizers,” Hopkins adds.
Although milling is not terribly complex conceptually, adhering to good operating procedures is essential. “We too frequently learn that customers have bought a used mill but not the operator’s manual, and they end up damaging the unit or cannot achieve their size-reduction goals because they don’t understand the unit,” says Greg Boyer, marketing manager at Hosokawa Micron Powder Systems (Summit, NJ).
Users should also inspect mills/grinders periodically for wear or damage. How frequently? “There’s no blanket answer to that question,” Boyer acknowledges. A pharmaceutical grinder might require cleaning several times per day to ensure sanitation, or it might need to be shut down in order to inspect for sample quality. By contrast, a unit grinding rock samples might be cleaned less frequently but checked for damage more often. Usage patterns also dictate more- or less-frequent inspection and cleaning practices, Boyer adds.
Economics and performance are the two main factors when purchasing milling/grinding equipment, according to Boyer. Customers know their samples and usually have a good idea of the extent of size reduction they require, but there is more than one way to skin this particular cat.
Potential purchasers should always test a mill before buying, to confirm in-spec particle characteristics. Top vendors are quite open about testing samples and advising customers. “Our application engineers have experience with most materials and can quickly identify the right milling system,” explains Boyer. He recommends submitting a few samples beforehand, which his colleagues will test to customer specifications with several possible mills—an air classification, universal, or hammer mill, for example. Hosokawa invites customers to its facility to discuss results in person with the firm’s technical staff.
“We give customers options based on outcomes and performance,” Boyer says. “One mill may be right if they’re looking for high production rates and low energy costs, whereas a different mill might be suitable for ultrafine or coarse granulation.” Boyer admits that at times, given a particular material and performance criteria, “we may not even offer the right type of equipment.”
Buyers should avoid mill/grinder vendors that lack experience in their specific field. “A company known for pharmaceutical milling may not fully understand the requirements for foods,” Boyer says. For example, regulations for processing drugs are quite stringent with respect to particle size, size distribution, and process temperatures. Over- or undermilling can be disastrous for realizing a product’s key quality attributes.
Hopkins provides several tips on purchasing a high-capacity bead mill cell disruptor. Sample capacity (number, not volume) and cost are the first features to consider. Next comes vial orientation. Vials positioned horizontally with agitation in the same direction disrupt more efficiently than vials held vertically, thanks to greater shaking room within the mill. High shaking speeds and throw (distance the vials move during oscillation) are critical for optimal performance. Skimping on either parameter can lengthen disruption times by up to a factor of ten.
If your lab works with microplates, make sure the mill you purchase can handle deep-well versions that users can seal appropriately for the vigorous shaking that bead milling entails. “Not all microplates qualify,” Hopkins warns. Several bead mill suppliers sell microvials with beads preinstalled. Although this can save the user some time, it increases disposable materials cost about tenfold. Furthermore, Hopkins says, there is nothing “magical” about the bead mix in precharged vials, and beads are easy to self-load.
Not all workflows require highly efficient ball mills. PCR preps can often get by on partial cell disruption, since the genes are amplified afterward. But proteomics investigators need all the material the sample holds, which is achievable only through 100 percent lysis.
For additional resources on laboratory mills, including useful articles and a list of manufacturers, visit www.labmanager.com/mills-and-grinders