I would like to make a large amount of multi-grain porridge mixture in one batch, to store and use small amounts of when I need it. The recipe says that the fattier seeds will go rancid if kept for too long, but doesn't say how long that might be.
The recipe includes:
Of these, the linseed and sunflower will be the most fatty.
How long can I keep these, and seeds in general, for?
Here is a site https://www.usaemergencysupply.com/information-center/self-reliance/storage-life-of-dry-foods#link7 which addresses many of the items listed, but unfortunately does not address the ones of most concern to you, the fattier items of sunflower and pumpkin seeds.
Flax they call a hard grain and put at 10-12 years with wheat and such. Rye, oats, and barley they class as soft grains and 8-10 years in dry 70 degree temps. Lower temps would extend.
I would think that pumpkin and sunflower would be considerably lower, possibly treated more like a nut. For very long term storage, canning, or vacuum sealing may be an option for reaching the 4+ year stage. I have had several year old nut meat which was canned then frozen with only minimum degrading beyond the initial softening caused by heating to can. All long term storage will cause some degrading though. To minimize, you would want cool, dry and no sunlight.
You can keep them until you dislike the taste.
The first changes in the oils start immediately after processing the seeds, so there is no real border of saying "up until that state the seed is not rancid, afterwards it is". It is a gradual change.
How long the change takes depends on things like the storage temperature, the amount of sunlight on the seeds, the amount of antioxidants they produced during growing, the degree of processing, etc. Combine this with the "gradual change" part and we cannot tell you a strict time. It is usually measured in months - not weeks, and not years. But I don't think we can be much more concrete. You have to just throw them out when they have changed to a degree that you dislike them. Alternatively, you can create a month's or 2 month supply and redo it when finished, that way it probably won't go rancid on you.
