Can previously canned foods (either from the grocery store or home canned) be used to make a new batch of home-canned foods?

Often people think that they can save time by buying larger containers of canned food, transferring the contents (or leftovers from the first use) to smaller jars and re-processing it. Or buy canned foods to replace ingredients that they cannot get fresh. Others wonder if this is a way to save leftovers from any size can for a longer time than they will keep in the refrigerator.

There are several problems with these practices:

1. The USDA's National Center for Home Food Preservation (NCHFP, located at the University of Georgia) has not determined safe conditions or process for this. They found that in some cases, the way the heat is distributed throughout the jar during canning will be very different if you start with already canned/cooked food than with fresh. Excessively softened foods will pack more tightly into a jar, or arrange themselves differently and the process time recommended for fresh foods will not be enough for the already canned foods. Underprocessing can lead to foodborne illness or at the very least, spoilage and loss of product. You definitely could not just transfer the food and "seal" the jar. You would need some heat treatment known to destroy any organisms transferred with the food.
2. Most likely the quality of the food will be greatly reduced in canning the food for a second time. The heat of canning does cause loss of some nutrients, and a second round of canning will further reduce the nutritional value as well as a substantial decrease in texture and taste.

Without tested processes for re-canning foods, there is no way to know how to reduce the canning process and the default (although not a recommendation) is to process for the full time and temperature as if starting from scratch. When you consider you are not even saving money and resources, it does not seem worth the loss of food quality to practice this re-canning of commercially canned food. The recommendation of the NCHFP is not to re-can foods.

That's the official view from the food scientists.  Interesting to me is what they did not say.  For example, canned tomato paste is already cooked until it is a dense mush. Recooking it won't likely change it a bit.  And adding a small amount of a canned food to your batch likely also wouldn't have an impact, such as adding one can of tomato paste to a batch of tomato sauce to thicken it.  Or using canned fruit to make jam.  In this case, the canned fruit is reduced to a near liquid and is also high in acid.

So my take is, their advice steps a bit into the realm of overkill. I agree that simply re-canning already canned foods is just plain dumb. But using some canned foods as a partial ingredient in a new batch, can make sense in some circumstances. A little common sense can go a long way here.