It is citric acid, not ascorbic acid for tomatoes. Don't use ascorbic acid. Ascorbic acid is to preserve color, not for added acid for the safety of your tomatoes to prevent botulism.
You can get citric acid online or in some canning sections of stores. Walmart may have it. I order mine online. Also, you can get it at wine making shops. Here there is a sort of farm/feed store called Wilco that has it.

Thanks for the info Linda. I double checked my recipes and you're right, it's citric acid. I wonder how I got that mixed up? Where might they stock citric acid at walmart? Pharmacy or baking isle?

Has anyone used lemon juice? Does it change the flavor of the tomatoes noticeably?

Is the acid really neccessary? My coworker says that she's never used acid with her canned tomatoes and they always turn out fine.

Yes, the acid is necessary. Tomatoes are on the margin of high-acid and low-acid. Ripeness, exposure to light, growing conditions, variety can all affect the natural acidity and bump the tomatoes below that safe level. The additional acid is a fail-safe.

I have found jars of citric acid crystals in the pharmacy and in the natural foods/health foods/vitamin section of the big multi-purpose stores.

Carol

I found it at a local beer/wine making hobby store. You can also order it online if you don't want to search for it. I drove past the hobby store all the time so it was no problem to stop in. I actually originally bought it to make fizzy bath bombs with my girls.

Thanks everyone, I was able to find it at a local health food store where they stock it in bulk. In fact, I just finished putting up my first 7 quarts of tomatoes!

thanks again.

Ascorbic acid (better know as vitamin C) is used ONLY for color retention for those foods that tend to easily oxidize and turn brown. Canning peaches are a good example. Without the ascorbc acid, they will quickly turn brown even in an air tight jar. In commercial canning, I have seen the use of pickle crisp (Calcium Chloride) used to help keep whole packed tomatoes from breaking down too much during storage. The vitamic C in pill form usually also contains corn starch as a binder. Measuring an exact amount can be difficult that way, so pure ascorbic is necessary. Fruit Fresh is one name brand too.

What is everyones experience with using lemon juice instead? I put up my first 4 qts of crushed tomatoes and used 2 Tblsp of lemon juice instead, I hope it works ok?

Susan in NC

was it bottled lemon juice? ... that's okay. the acidity of bottled lemon juice is standardized while the acidity of fresh lemons varies.

Lemon juice is OK, if bottled, as mentioned, but you can also be assured that citric acid measured into each jar prior to filling will give you the same acidity necessary to be safely canned.

Some feel the citric acid is less noticeable in the final product than lemon juice.

Carol