There are several ways to process castors to lure grade use. From simple preparation to more complex formulating. The important thing to do is to clean the castors and oil sacs of all remaining tissue, fat and membrane as best you can. As this material in even small amounts at times can and does change the final odor result.Genuine clean castor will last a longgg time but rough cut castors will spoil/sour draw bugs etc. and cause some undesirable results in a formula due to their odor change.
I prefer a surface dried finished product before processing. At this time you can also remove any distinquishable undesirable waste left on them more easily.From this point you can do as you wish with them.Freeze, grind, scrape etc.Freeze and label is what I do until use. Others that have a specific formuation purpose I will grind,preserve and let age until needed.
It is a wise practice to stir your ground and preserved material occasionally to allow better contact with your preserving slurry. Most materials expand and compress after grinding to some degree. Intermitent stirring ensures good contact with all the material and will prevent dead spots in your mix.
If all components are clean,well blended and then preserved properly, just about any combination can be achieved to develop a desired end result odor.
Preserving and stopping is extremely important to maintain and sustain good quality of your product.If done right it will last forever in a sealed container.