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When I buy fresh ground peanut butter, it has a very dry, paste-like texture with almost no oil. It can sit on my shelf for a week in warm weather and no oil separates out.
When I buy peanut butter in a jar, it has a layer of oil on top. Once this is mixed in, the texture is creamy and oily at room temperature.
The label claims that the jar contains only dry roasted peanuts, nothing more. This seems to be the case for all brands I've seen. Where does all the extra oil come from?
The difference may be in how finely the peanuts are ground. Depending on the stores machine, you may not be getting a perfectly smooth peanut butter which means there is still some residual oil in there.
Also, a week after fresh-ground is still very fresh. That jar of xyz brand on the shelf can be six months or older. If you leave your fresh ground out for longer, it most likely will separate.
The other bit might be that the oil comes out when the grinding processes warms up the peanuts, and your local store's machine might still keep them cool enough. You can see a similar effect with coffee beans and their roasting temperature. Speaking of which, the commercial process might also roast at different temperature.
What I have noticed is that if I let our Vitamix blender go at the peanuts a little longer, the oil does eventually get coaxed out and that batch will have its oil separate eventually.
Having recently made peanut butter by hand, (Using an hand cranked food processor).
I can say after looking at the result, over the last few weeks. My peanut butter is much more oily than store bought. It has also separated more.
The only trick to making your own peanut butter is the process it more than you thought. When you think you are done, keep going. I thought it was done when it was a kinda smoothish but crumbly texture. But I thought i would give it a extra half and hour (Hand cranked remember). Then it really became the oily smooth peanut-butter we know and love.
Peanuts are incredibly oily. But exactly how oily is going to depend on several factors. Things like growing conditions (Tempurature, soil etc), and things in preparation -- how much (if at all) were they roasted (at what tempurature), how fine was the grind, what excess oil separated off.
Commerial peanut butter can control these things by sourcing peanuts from same set of farms (or by just combining such a huge number that you have the average peanut of a nation), and by using consistent processing.
