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To pick an example, this green bean canning recipe gives the same processing times for both hot and raw pack. Presumably even with raw pack, the 20-25 minute processing time is enough to thoroughly cook the beans. Why would you cook them additionally first? Is it just for people who like very well-cooked vegetables?

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According to Pick Your Own, the main reason is that in raw packing, more air is introduced into the jar. This may lead to discoloration over time, which is an aesthetic issue, rather than a safety issue.

Regarding hot packing, they describe the benefits:

This practice helps to remove air from food tissues, shrinks food, helps keep the food from floating in the jars, increases vacuum in sealed jars, and improves shelf life. Preshrinking food also permits filling more food into each jar.

They provide the following guidelines regarding which method to use:

  • Use the hot-pack method, especially with acid foods to be processed in boiling water rather than a pressure canning method.
  • The Raw pack method is fine for pickles and for vegetables to be processed in a pressure canner.

See also: National Center for Home Preservation describing the differences, benefits, and reasoning in great detail.

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In this case, it was a pressure canner recipe, so... is there actually any reason? (The NCHP page isn't terribly clear about this - like Pick Your Own, it says raw pack is good for pressure canned vegetables. But it doesn't really explain, for example, is discoloration is not a problem?) –  Jefromi Aug 1 '13 at 21:48
    
YEs, preshrinking the vegetables allows more to fit in the jar, and there is less discoloration which is due to the volume of air in the jar oxidizing the pigments in the food. –  SAJ14SAJ Aug 1 '13 at 22:18

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