An Introduction to Pressure Canning

[British Red]

My take on pressure canning is this can result in long term stored food with botulism, which I believe is not noticeable like mould etc would be. The USDA produce guidelines which result from something like 100 years of lab research. This is why a canner is required, not just a pressure cooker. The only difference seems to be repeatability, a pressure cooker certainly might be safe for canning, but without using a known weight to seal, and taking into account your elevation above sea level as they do, you won't know if you're replicating the same pressure, time and procedure the USDA have shown to be safe.
Since you asked me to, in the USDA The Complete Guide To Home Canning, ISBN 9798631975507 chapter on tomatoes and tomato products it says acidification is required (adding citric acid) both with boiling water or pressure canning.
For salsas it says do not alter the veg quantities in each recipe, use tomato quantities after peeling and coring, do not drain them, do not change the proportions of veg to acid and tomatoes because it might make the salsa unsafe. For their spag bol sauce do not increase the veg proportions.
They have quite different times between smoked fish, fish, beef and chicken, in quart jars sweet potatoes want 90 minutes, white potatoes 40, beans 75, beets 35, mushrooms 45 and their mixed veg recipe 90 as examples.

I was trying to explain the underlying science and probably didn't explain myself clearly.

I'll include two extracts from the "Ball complete book of home preserving" that probably explain better than I can.

The first describes which canning methods are safe according to acidity levels

Pressure canning guide by English Countrylife, on Flickr

As you can see, high acidity foods can be safely water bath canned but low acidity foods are safely preserved via pressure treated.

Now on the subject of botulism, the key factor is spores. The basic bacteria is easily killed by normal boiling but the spores are not, they require a heat above 100C to destroy them. However once the temperature is reached the food is rendered safe

Destruction of botulinum by English Countrylife, on Flickr

It's important to note that this temperature must be reached in the interior of all food stuffs so large chunks of different foodstuffs take different times to achieve sterilisation as the centre has to heat up, hence different canning durations. However recipe variations in broadly similar foods are generally okay in my view (e.g. beef stew and beef in wine sauce are canned identically, I would be quite comfortable using a different sauce with the same method). If you prefer to stick to the exact recipes given, then it's certainly fine to do so.

My purpose in posting this isn't to dispute the safety of tested recipes but to help people understand the underlying science. I hope that helps

 

[grizzlyj]

My purpose in posting this isn't to dispute the safety of tested recipes but to help people understand the underlying science. I hope that helps

Me too. I felt your blanket times for 55 or 75 minutes in your previous post goes against the safe way to can according to the very specific guidelines the USDA provide and wanted to point out the variation they give within the produce you mentioned a single time for. Between the USDA and the Ball group it would seem any other cook book recipes just incorporate those, and even then stick to recent ones as their advice changes. If you deviate from those recipes then that may be a risk which some reading this may not realise. YMMV.
Happy canning