Are the mustard seeds going into a pepper grinder, BranchingOut? So you will be using them in the kitchen?
I find that an easy way to thrash the mustard is to 1 put a tarp down on the lawn, 2 walk on the mustard, 3 separate out the larger debris, 4 climb up on my ladder with some seeds & such, 5 dump that back down on my tarp, and allow a nice breeze carry most of the debris away. The seed has good viability over several years but, I guess that I would feel the need for cleaner conditions if kitchen use was intended

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Steve