GRINDING PROTOCOL: University of Minnesota Forage Quality NIRS Laboratory[1]
Objective: To accurately represent field samples for laboratory or NIRS analysis.
· The entire dried field sample is coarse ground (approx. 150 g) using a Wiley mill # 4 with a 4-6 mm screen. Alfalfa leaves easily break off from the stem making it difficult to subsample a portion of unground material. The larger screen size quickly breaks up the sample.
· Any material left in the grinder is added to the sample. An air hose or vacuum cleaner is used to clean out mill between samples.
· The coarse sample is placed in a large blender for one minute to remove stratification of material after grinding.
· Pour the sample from the blender cup into a tub. Spread out the sample into a uniform layer. Then use a 4 oz. brown plastic bottle to subsample from three different areas of the material.
· Regrind sample using a Tecator cyclone mill with a one mm screen. Pour sample unto tray of grinder and grind back into the same bottle. Use air pressure or vacuum to clean between each sample.
· Sample has become stratified again due to the fine grinding and needs remixing. A 15-gallon plastic drum is used as a sample tumbler.
· Place 70 – 100 samples into tumbler for 15 minutes. The drum rotates at 15 rpm and contains a rod that lifts and drops the bottles to provide a random tumbling.
In tests using NIR, subsamples from the same bottle were nearly identical. Standard errors for laboratory chemical procedures also decreased due to the more uniform subsampling from the tumbler.
Whirlpaks can also be tumbled by sealing open end with tape and providing an air space for sample movement.
If samples sit longer than 4 weeks before analysis, retumbling is repeated due to possible moisture stratification within the sample.
· Grinder maintenance: We grind only a few thousand samples each year and therefore resharpen the Wiley blades once a year. On the cyclone mill we change screens once a year or whenever the holes appear oblong. We change the grinding rings every other year. This noticeably improves grinding speed and may be cost effective to replace every year. We replace seals quite often and we would like to find an alternative that is cheaper than original equipment.