The first time I bought peanut butter without all those stabilizers and icing sugar in it, I used a butter knife and then a
spoon, to slowly try to mix the hardened material with the oil. It was a monotonous task.
I buy it when it goes on sale. It may be near expired, but really, the stuff lasts just about forever. I think it goes cheap because it separates to the point where it's hard to reincorporate the oil.
Now I use a whisk. First the jar goes into a bowl of warm
water, where it sits for a while. Then I use the whisk in a straight up and down motion as you might with a potato masher. The thin wires slice through the meat of the nuts and allow the oil to penetrate.
After just a couple minutes of this, I stop and let it sit for a few minutes or half an hour or until the next day. It doesn't matter. By the time I come back to it, the oil has worked its way into the more solid material, and only a couple of minutes of light agitation, makes it all into a homogeneous nut butter. A butter knife may still be required just at the very bottom edge of the jar, where a rounded whisk does not fit.
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Sometimes, a very old jar may have quite solid material in the bottom. In this case, it could be penetrated with a butter knife and the top of the knife swirled around in a circular motion, while the bottom remains quite stuck.
This can be done in half a dozen spots, and then let it sit overnight. The oil will do it's magic, and the whisk can be used.
I've tried an immersion blender with some success, but when you consider clean up time, this seems better.
Seems like a lot of words to describe a simple process. Hope it helps.