What does the USDA really say about Pressure Canning Butter?

While I am aware that I risk vilification for my pressure canning butter, I am now comfortable sharing what I have researched over time.

At National Center for Home Food Preservation, under FrequentlyAsked Questions, there is a fairly in depth discussion on "processing" butter" (it's the next to the last question on the page), I read which dealt with canning butter, and it only referenced not water bathing or oven canning butter. It didn't address pressure canning.

Below is what is stated on their site in it's entirety, I am adding this because I really want you to read the information. Yet, I still recommend that you actually go to the NCHFP site and see for yourself.

Should I use directions for canning butter at home that I see on the Internet?
Indeed, there are some directions for 'canning' butter in circulation on the Internet. Most of what we have seen are not really canning, as they do not have Boiling Water or Pressure Canning processes applied to the filled jar. Jars are preheated, the butter is melted down and poured into the jars, and the lids are put on the jars. Some directions say to put the jars in the refrigerator as they re-harden, but to keep shaking them at regular intervals to keep the separating butter better mixed as it hardens. This is merely storing butter in canning jars, not ‘canning’. True home canning is when the food is heated enough to destroy or sufficiently acid enough to prevent growth of all spores of Clostridium botulinum (that causes botulism) and other pathogens during room temperature storage on the shelf.

Additionally, when you consider the economics of the process (energy costs involved with heating, cost of jars and lids, etc.), even if the butter is bought on sale, it may not be economically viable to prepare butter to store for years in this manner. Good quality butter is readily available at all times, if butter is needed for fresh use. If the concern is about emergency food supplies, there are dry forms of butter that can be purchased and stored, oils that can be used in an emergency, or commercially canned butter in tins (although we have only seen this for sale from other countries). Melted and re-hardened butter may not function the same as original butter in many types of baking anyway.

There are a few issues with the common directions circulating on the Internet at this time (Spring 2006):

1. Physical safety and food quality: In the provided directions, the jars are preheated in an oven (dry-heat), which is not recommended for canning jars. Manufacturers of canning jars do not recommend baking or oven canning in the jars. It is very risky with regard to causing jar breakage. There is no guarantee that the jars heated in this dry manner are sufficiently heated to sterilize them, as we do not have data on sterilizing jar surfaces by this dry-heating method.
2. The butter is not really being 'canned'; it is simply being melted and put in canning jars, and covered with lids. Due to some heat present from the hot melted butters and preheated jars, some degree of vacuum is pulled on the lids to develop a seal. It rarely is as strong a vacuum as you obtain in jars sealed through heat processing. The practice in these 'canned' butter directions is referred to as 'open-kettle' canning in our terminology, which is really no canning at all, since the jar (with product in it) is not being heat processed before storage.
3. Although mostly fat, butter is a low-acid food. Meat, vegetables, butter, cream, etc. are low-acid products that will support the outgrowth of C. botulinum and toxin formation in a sealed jar at room temperature. Low-acid products have to be pressure-canned by tested processes to be kept in a sealed jar at room temperature. It is not clear what the botulism risk is from such a high-fat product, but to store a low-acid moist food in a sealed jar at room temperature requires processing to destroy spores. A normal salted butter has about 16-17% water, some salt, protein, vitamins and minerals. Some butter-like spreads have varying amounts of water in them. We have no kind of database in the home canning/food processing arena to know what the microbiological concerns would be in a butter stored at room temperature in a sealed jar. In the absence of that, given that it is low-acid and that fats can protect spores from heat if they are in the product during a canning process, we cannot recommend storing butter produced by these methods under vacuum sealed conditions at room temperature.
4. Some other directions do call for 'canning' the filled jars of butter in a dry oven. This also is not 'canning'. There is not sufficient, research-based documentation to support that 'canning' any food in a dry oven as described on this web page or any page that proposes oven canning is even sufficient heating to destroy bacteria of concern, let alone enough to produce a proper seal with today's home canning lids.
   
   In conclusion, with no testing having been conducted to validate these methods, we would NOT recommend or endorse them as a safe home-canning process, let alone for storing butter at room temperature for an extended period. We do know that the methods given for preheating empty jars, or even filled jars, in a dry oven are not recommended by the jar manufacturers or by us for any food. Aside from the physical safety and quality issues, and the fact that it is not canning at all, if there happened to be spores of certain bacteria in there, these procedures will not destroy those spores for safe room temperature storage.

From my reading of the NCHFP guidelines they state do not to use boiling water bath or oven canning methods of preserving butter. While they don't state it implicitly, I interpret that since it is a low acid food, it should be pressured canned.
What I am asking is to show me a recognized document from the USDA or NCHFP that states it is unsafe to pressure can butter & why. I could have missed it, I am human. 

That is how I made my decision that pressure canning butter was safe for my family. That being said: if you want to know how I pressure can my butter here is the link: Pressure Canning Butter for Storage. 
I have also done the same research on What the USDA Really Says about Pressure Canning Milk.

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