Raw packing refers to the canning procedure of filling jars (or cans) with completely uncooked fruits or vegetables. This is in contrast to other canning methods like hot packing, where fruits or veggies may be boiled for a couple of minutes and then immediately placed in cans and sealed. Each method can be employed for a variety of different foods, but some foods are better suited to raw packing than to hot packing, and vice versa.
The main thing to remember about raw packing is that food should be packed very tightly together, and in most cases it is usually covered with canning water or canning liquid that is heated. So it’s not the case that raw packing avoids placing extremely hot liquid in the canning jar. This heat, in addition to canning jar sterilization, minimizes risk that food will become contaminated. In general though, this form of canning is great when you want to preserve veggies or fruit in semi-uncooked form. It works particularly well for things that will be served crunchy, like pickled cucumbers, or other pickled vegetables.
If you’re canning fruit, raw packing may not be quite as effective, especially if you plan to store canned items for more than a few months. Most fruit when exposed to air will begin to exhibit some discoloration within a few hours of being sliced. When you pack raw, you leave some of the air that the food already contains, and this can cause the raw packed food to discolor in a couple of months. Hot packing may be more appropriate because boiling the food for a few minutes can help preserve color and eliminate some of the air that the fruit or veggies naturally contain. A good book on canning can help you determine whether the food your want to preserve will work best with a raw or hot pack method.
When you are canning at home, a principle concern should be food safety. For this reason, the United States Department of Agriculture (USDA) gives guidelines on how to safely practice raw packing. To keep your food fresh, enjoyable and free of contamination, paying attention to these guidelines is very important. They are available online in PDF format and most up to date cookbooks will also observe and explain food safety protocol, whether you raw pack, hot pack or choose other food preservation methods.