Recipes are for guidance (except for baking which can be pretty precise). However, it's more accurate in canning to think of formulas rather than recipes.

You're putting something in a jar and forming a vacuum. That's not the same as baking a casserole as it has the potential to set up conditions that can have very serious consequences re. food poisoning.

Some modifications are possible, but they tend to fall within a limited range.

The objection to oil is that in a product where there's a possibility of botulism oil coats the spores, preventing the penetration of acid. Botulism likes a low-acid environment and the oil creates perfect little protective capsules.

The citric acid in pressure canned recipes is a reflection of fiscal reality. For example, you might have a canned tomato recipe with citric acid (or bottled lemon juice). The lab develops safe-tested times for various jar sizes. What they are aiming for is a risk-free product with optimal nutrition, flavor and texture.

Once they have the data and develop a boiling water bath time, they can extrapolate from that same data a pressure canning time as well.

To develop one time with citric acid for boiling water bath and one time without for a pressure canner requires two separate rounds of testing. It's much cheaper to let one round serve two purposes.

Also, the citric acid means the pressure canning time can be shorter, again assuring optimal quality and nutrition.

Carol

this forum seems almost paranoid in not modifying written recipes on the NCHFP website, or any other recipes for that matter.

"Paranoid" is strong a word. I think "careful" or "responsible" would be a better label. There are all sorts of "sure, go ahead and can whatever you want" websites out there. Sites that make no effort to understand the role of pH or the role of density in canning, the nature of the environment created inside a processed jar, or the long-term effects of shelf storage on some foods. They take no responsibility for the hazardous info they offer to an unknowing public. We try to.

But we always acknowledge the choice available to the reader to accept or reject the info here and we often offer some modifications to the many approved and tested canning recipe that are available. Many modifications are available IF one first understands the effect those modifications will have on the pH, the density, and the effect of shelf storage in a vacuum on that modification.

USDA/NCHFP is not the only approved source but its testing is the base from which all other safe and approved instructions are taken. It is the source for all food preservation science so yes, it is considered more reliable than using oneself or one's family for experimentation.

Carol answered your other questions well above. I'll just add that the oil/fats can coat and insulate spores and bacteria from heat penetration as well as acidification. That it can turn rancid is another, tho less important, consideration.

Assuming your question on citric acid is tomatoes related, Carol also covered it well. But don't make the mistake of assuming that just adding citric acid to other really low-acid vegetables somehow makes them safe for BWB canning. It doesn't. Acidity pH isn't the only issue. Density, higher heat than 212 degrees, and heat-under-pressure are even more important when dealing with most vegetables or mixed-vegetable combinations.

Dave

I'm no devotee of either guidelines and think we tend to over process our food. That confessed, it takes only one batch of something going funky in the basement to teach you not to take chances. It's too much work growing, harvesting, washing, peeling, and finally canning to let risk in the door. Once I find a recipe AND procedure that works well, I stick with it.

With experience you know what to look for when a batch is in progress, because you know your ingredients and your equipment so well. Still, you will always be a slave to the timer.

You've had good explanations of the reasons for adhering to the canning guidelines. I think the main point is that in most recipes, other than canning recipes, if you change them to suit your taste or inclination you aren't taking the chance of death as the possible outcome. That's pretty much the bottom line in canning. Do no harm. Smiles.