Any recipe not know to be safe to can. That means any recipe that the USDA, it's scientists and it's lawyers haven't given the nod to.
What makes a recipe unsafe to can in a water bath is low acid. In a pressure canner, the inability to reliability heat the entire contents of the jar to the required temperature without turning the product to mush.
As time has gone on less things have canning recommendations, which I believe is a refection of improved food science. It also is a reflection of our decreasing tolerance of perceived risk, an increase in our demand for the "quality" of the canned product and a bias toward letting "experts" do everything for us.
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