On my Canning Mustard post, a reader named Dannie asked a very good question. She wrote:
I just purchased my first water bath canner and my daughter and I plan to do canning together this summer. Being new at this I just wondered if you could tell me why you had to heat the mustard before you put it in the glass jars. It seems like double cooking to me but I know there is a logical reason.
I realized I had no suitable answer to this question. I have raw-packed foods before (notably bacon) but most of the time I hot pack foods. But why hot pack? I never gave it much thought. My favorite canning reference book, Putting Food By, mentions hot food packs better and requires less processing time. Somehow this didn't seem like the whole story.
So I turned to an expert named David Blackburn at a wonderful website called CanningUSA.com and posed the question. Here is his response:
In the case of mustard (and all canned products,) temperature control has to do with viscosity and kinetic energy. Basically, the thickness of a liquid, solid or gas, determines the combination of time and energy it takes to change its temperature. In canning, we are most concerned with the lowest core temperature.
The easiest example of this is canning whole tomatoes, which are not viscous. If you cold pack them, they need to be processed in pint jars for 85 minutes in a water bath canner. If you hot pack them, it's 40 minutes. The former cooks at the temperature of the waterbath canner, or the boiling point of water - 212 degrees at sea level, after it's brought both the water in the canner and glass jars up to temperature. The latter process heats the tomatoes using a wood, electric or gas heat source before canning. Depending on the energy source, this temperature is nine to fifteen times higher than the boiling point of water. So, it's a lot faster and more energy efficient to heat the product up to its boiling point first, can it and then process in the canner. When there is an option between the two processes, I only publish the least in overall time and energy consumption.
Let's go back to your mustard question. Imagine heating a pint of water and a pint of mustard on a stove top. For the water, you can put it in a pan on any heat source and at any temperature and it pretty much takes care of itself. You don't have to worry about it burning or heating unevenly because it has a low viscosity. For mustard, which is much more viscous, so you have to heat it more slowly and stir it constantly to make it heat evenly and to ensure it doesn't stick or burn. I haven't calculated it, but you could sumise it takes more energy and time to heat mustard than water. If you placed the mustard in a jar inside a waterbath canner, it would take an eternity for its lowest core temperature to reach the appropriate canning processing temperature. As far as your bacon is concerned, it isn't viscous and the paper doesn't impede the processing.
I also noted a question on your post about electric tops and pressure canners. Electric heat sources do not heat evenly, so you can't control the temperature. The glass tops are not flat, which makes it worse. We recommend buying a small gas burner and external tank, such as propane, which burns a lot hotter than natural gas.
Wow, I think you'll agree that Mr. Blackburn's reply is a lot more intelligent than anything I could ever dream up! I'm grateful he took the time to explain the process.
Thanks, Dannie, for asking such an excellent question.