There are, of course, different types of canning: Water -bath canning, pressure canning, and oven canning. The following foods should not be canned in any way, shape or form:

Butter
Butter is a dairy product and shouldn’t be canned.  Milk and butter are “low acid foods”, this means they can support the outgrowth of C. botulinum and toxin formation in a sealed jar at room temperature.  Unique fats in milk and butter (the dairy fats) can protect botulism spores from the heat of even a pressure canner.  SO, if you buy butter from the store and there was one person at the dairy where that butter came from, that was not following cleanliness procedures, you could get sick from canning it.Botulism spores (commonly found in dirt) are not killed by the pasteurization process of milk so they can still be in there at any time.  While undesired, this usually isn’t a big deal because if ingested they can pass through a healthy adult without causing any harm (or a resulting product recall).  However, if you take that same dairy product containing the spores and put in a jar where you seal it off from oxygen for an extended period of time at room temperature…..it can grow undisturbed into one of the many strains of Clostridium botulinum, and produce an extremely lethal neurotoxin until you open it up and eat it.How lethal you may ask?

Milk or Milk Heavy Products like Condensed Soups
I’ve articles claiming that the only reason why it’s not recommended to can milk is solely because of the resulting unfavorable taste and texture of the milk.  In truth that is only part of the reason: Dr. Elizabeth L. Andress, National Director of Home Food Preservation, University of Georgia Department of Foods and Nutrition and Dr. Andress, a Professor and Extension Food Safety Specialist have recently commented “the amount of heat that would have to be applied to kill harmful bacteria” would be “extremely detrimental to its quality.”But it’s not just the taste and texture of the milk, Dr. Andress states, “milk is a finely balanced emulsion of proteins in water.  If the milk proteins are over-heated, they drop out of suspension and the milk separates” the visible separation is an indication that the same ‘life sustaining’ proteins of the milk are no longer intact defeating the purpose of canning milk.  Also, milk is a LOW acid food (see above explanation) and dairy fats can protect botulism spores from the heat of canning.  So if your milk somehow does have spores in it and you put it in a jar and seal it off from oxygen for an extended period of time at room temperature…..it can grow into one of the many strains of Clostridium botulinum, and produce an extremely lethal neurotoxin.

Pureed or Mashed Squash and Pumpkin and Pumpkin Butter
In purée form these two should not be canned, not water bathed or pressure canned.  However in cubed form it is safe to “pressure” can them.  Here is why: Pumpkin is a low acid food (low acid foods can support the outgrowth of C. botulinum) when pumpkin and squash are pureed it’s really thick (dense).  Through recent testing several universities have discovered that not even a pressure canner can produce enough heat to reliably penetrate the jar to the center of the purée, it is just too dense.  When pumpkin and squash are cubed and suspended in water the pressure canning process is reliable every single time.  The reason why canning purée pumpkin or pumpkin butter was recommended by the USDA and now it’s not, is because further testing was done which indicated that sometimes the pressure canning process worked and sometimes its did not, even with the addition of lemon juice the tests were still coming back with positive for Botulism spores (Source).  With one minor change to the process (using cubes and not a purée) it was 100% reliable so the recommended recipes for purée pumpkin and squash were withdrawn and replaced by cubed recipes.  This change was not made to annoy everyone it was made with our safety in mind.

Rice and Bread (and pastas)
Adding rice, bread, or pastas to canned food recipes, like soups, is also not recommend.  Canning rice or bread or cake products straight is also not recommended.  It changes the density and increases the pH of the food, this may alter it to the point where you would not get adequate heat penetration to kill off the nasty bugs.  There is also the problem with these of ingredients expanding.  If you are canning with a jar that is flawed you run the risk of an exploding jar ruining your entire batch.  Food manufacturers produce canned soups with these ingredients with high temperature/pressure retort sterilization machines, but we do not have this capability for our home canned goods.  Canned cakes and breads are a favored item of the “anything can be canned” crowd.  Not only do they meet the botulism criteria of being moist and low acid, the airless center of the cake just doesn’t get hot enough to kill botulism that could get there a number of ways.  It doesn’t matter if the oven temperature is at 350 degrees; the internal temperature will not get anywhere near required 240 degrees for 30 minutes that is required for it to be safe.  Since rice, wheat, and cake mix store so easily dry – I recommend dry canning them (click here to read an article) for separate storage. Can make soups without rice and noodles – and simply add them later when you go to use the can of soap.

Eggs 
It is not recommended to can eggs at all. I know people who would fight me on this until their death, because, they claim they have found ways to make it ‘safer’ but here are the facts. Eggs are a low acid and a high density food.  Hard boiled eggs have a pH around 6.8, so it is possible for botulism to grow because the vinegar does not penetrate deeply into the egg.  You can, however, pickle eggs then refrigerate them and they will then last an unnervingly long time this way. There are no safe tested recipes for canning hard-boiled eggs, pickled or otherwise.  Food manufacturers get away with canning eggs because their capabilities are greater than what our home pressure canners can do – this isn’t a conspiracy it’s just a fact of life.  If you are wondering why pickled eggs can sit at a saloon out of the fridge and be safe, the answer is because saloon patrons usually eat the eggs within a couple of days.  When eggs are pickled, if some nasty bug survives in the middle of the egg away from the vinegar it will grow undisturbed at room temperature until the egg is eaten, the longer it sits at room temperature the more it grows. That is why it’s wise to refrigerate all pickled eggs. Now if you want to do your own research and part ways with the safety guides here that is up to you. Knowing what I know now about the science behind canning – I am sticking with the safety guides on this one.

There are other no-can items as well and you can go over to American Preppers Network to find them. You can also learn about “Research-Based” home canning,  Ball Blue Book, or The Complete Guide to Home Canning (the USDA guide) , and why you should not blindly try canning items that others have not yet tried.

Please, do not be discouraged but know there are good and bad ways of canning. The good ways are perfect for preppers and survivalists. Do your research!