Plant-derived oils.....WRT Walnut oil.....
Waiting on our resident botanist, but if I were a betting man I'd say that the tree makes but one kind of oil, regardless where it ends up.
Be a bit careful on the off-the shelf oils, because some of them have added preservatives "to retard spoilage," which is incomplete oxidation/cross-linking. They will take a while to cure. Big difference between the oils which are expressed and the others extracted by solvents. Much less likely to have the proteins that cause reaction in the solvent-extracted oil.
I did some "literature looking" before responding, and doing so made me painfully aware about how much I don't know about plant lipids!
What I
did find is that most of the studies that are reported in the botanical and biochemical/medicinal literature done on 'walnut oil' have been done on the oil found in the seed, and that it is a more healthy oil than most plant oils - high in omega-3 fatty acids. (BTW - For those of you turners with '80-grit skin' on your hands (literally), walnut oil might help re-condition the skin and help it heal more efficiently - it's used as a skin therapy compound!) I could find little, if any, information on oils extracted from fruit wall (the 'husks'), bark, or other parts of the plant. In many (most?) tree species, the vegetative parts of the plants (roots, stems, leaves) are typically not well invested with plant oils. Plant ("vegetable") oils are more commonly extracted from seeds, or in some cases, from the fruit wall [pericarp], the best example being olive oil (I tell my students that olive oil is the ONLY case of a sliding scale of "virginity" as opposed to an all or none definition found in humans and other animals!).
With respect to all plant oils being the same within one species regardless of anatomical source, there
are examples where oils found in different parts of the plant can possess different chemical compositions. One of the best examples of this is 'palm oil' versus 'palm kernel (=seed) oil'. The general fatty acid profiles of these are similar, however the proportions of the fatty acids found in these two oils differ significantly enough that they are considered two distinct oil products with different characteristics. This, despite the fact that the oil derived from the fruit ['palm oil'] is in very close anatomical proximity to the seeds, from whence cometh palm kernel oil. It seems that some plants are able to synthesize and deposit different oils in different parts of their structure; whether this occurs in walnut, I have not been able to determine. It parallels deposition of proteins, in that seed storage proteins are quite different in amino acid content/proportions than are general proteins found in the metabolically active plant.
I could not find more information (with only moderate searching 'intensity') about oils extracted (either through cold pressing or solvent extraction) from walnut or butternut (
Juglans nigra or
J. cinerea, respectively) from parts of the plants other than that from the seed.
The residual protein content (and hence, the antigenic potential) of solvent-extracted oils is certainly less than that extracted through cold pressing (and some people are quite allergic to walnut allergens - my sister is - and the allergic response results are not pretty, from personal experience). The "pasteurization" process [which is intended to kill or incapacitate living microbes, if it is true pasteurization], likely has a protein coagulating and precipitating effect, which would reduce protein content and lessen the potential for allergic response. I'm not too sure microbes are the main enemy of walnut oil in storage - oxygen is, along with heat. Storage in tightly stoppered containers at refrigerator or freezing temperatures would maximize on longevity of unused oil.
When I see Mike Mahoney in Albuquerque in a few weeks, I'll have to ask him more about the composition of his oil, and where it actually comes from.
I hope this has provided some useful information for those with "inquiring minds" that wanted to know.....
Rob Wallace