Complex question. First because the way you canned the tomato juice doesn't meet the guidelines. The guidelines call for 35-40 min of pressure canning when you add additional low acid vegetables to tomato juice. The added acid covers the tomatoes but not the added vegetables. So that's the answer to the question "Other than taste and texture, can anyone provide a chemical/biological reason why adding the juice would be unsafe?" Technically it was under-processed. Consequently adding it now to something else now only compounds the problem.

Second, as you mentioned already even if it had been canned per the guidelines adding it still wouldn't be recommended by the guidelines for 2 reasons: (1) the quality decline that happens naturally as it is stored effects the quality of the current product it is added to. Whether that decline is in the form of higher pH (since it rises over time), color, texture, or flavor it is "less than ideal". And (2) the waste of energy the duplicate processing uses and the further decline in quality it causes.

All that said, it is your choice depending on the level of risk you are comfortable with. I would want to know the pH of the 2015 jars before using and if it was still ok then I would boil it separately before adding it to anything new.

Now add to all that are the issues with canning tomato soup in the first place. I don't know what recipe you have found to use but do keep in mind that tomato is one of the few soups which has only 1 tested and approved recipe. No milk, no fats or dairy, no thickeners, no pastas or rice, dried herbs only and like all soups it has to meet the 1/2 and 1/2 rule in the jar for BWB canning. That's almost impossible to do with tomato soup. Otherwise it is pressure canning only. The guidelines call for canning tomato sauce and then turning it into tomato soup only after the jars are opened.

So it boils down to a do-at-your-own-risk issue.

Hope this helps.

Dave

Thank you Dave for your comprehensive advice - much appreciated! The NCHFP recommendations for tomato juice is 10 minutes @ 15 psi. The amount of onions and garlic I added is so small that I was comfortable extending the time to 15 minutes with acidification.

I went ahead and added the juice to my soup batch, added the full amount of citric acid (so the juice portion was double acidified), and pressure canned for 45 minutes at 15 lbs. I never follow approved recipes but always make sure my tomato/low acid vegetable ratio is within that of approved recipes, always acidify, and always pressure can @ 15 lbs. I've done a lot of research on botulism and have been canning for over 40 years, so I understand the risks and accept them as being negligible.

Thanks again for your information and advice!

Tom

"I never follow approved recipes but always make sure my tomato/low acid vegetable ratio is within that of approved recipes, always acidify, and always pressure can @ 15 lbs."

As I said, that is always your option, your risk to take. But for the benefit of other readers I always try to point out the potential flaws in that approach.

In this case, canning at 15 lbs. still doesn't nullify the rise in pH that develops over time. The only way to cover that aspect is to fully acidify for ALL the ingredients to begin with as under-acidified foods can quickly rise into the unsafe zone during storage. Nor is botulism the only concern, tho the most potentially serious. Listeria grows in a much wider rage of pH than even botulism.

And it does still leave you with the other issues.

" (1) the quality decline that happens naturally as it is stored effects the quality of the current product it is added to. Whether that decline is in the form of higher pH (since it rises over time), color, texture, or flavor it is "less than ideal". And (2) the waste of energy the duplicate processing uses and the further decline in quality it causes."

There is laboratory research to support all the guidelines but choosing to ignore or modify them remains the individual's choice.

Dave