Preservation under pressure

[lovebenton0]

Thank you, Mr. McGee, for extending your time and attention to us this week. I look forward to your new edition, having to admit with shame that although I have read sections in the original I do not have it in my library. I am sure this has been thoroughly covered in that compendium. However, I was not involved in the process of preserving by pressure cooker at the time, restricting myself to water bath method for appropriate foods.

The science behind preserving foods in a pressure cooker is ill-explained to the home cook by both the manufacturers and in recipes. We get general notes telling us what to preserve at certain psi and length of time involved. But why?

I understand the need for the pressure induced vacuum, but what is the science behind preserving meats and vegetables at varying psi?

Thank you.

[Harold McGee]

The pressure is significant in pressure cooking because it’s proportional to the temperature at which water boils: so the higher the pressure in a pressure cooker, the higher the temperature at which the food is being cooked. Typically a cooker maintains 15 pounds per square inch (psi) above atmospheric pressure, which corresponds to a boiling point of around 250 degrees F. This is useful for food preservation because some bacterial spores (a form of these microbes that is designed to survive harsh conditions) can survive long periods at the normal boiling point of 212 degrees. Pressure cooking kills the spores as well as the normal bacteria.

[lovebenton0]

The pressure is significant in pressure cooking because it’s proportional to the temperature at which water boils: so the higher the pressure in a pressure cooker, the higher the temperature at which the food is being cooked. Typically a cooker maintains 15 pounds per square inch (psi) above atmospheric pressure, which corresponds to a boiling point of around 250 degrees F. This is useful for food preservation because some bacterial spores (a form of these microbes that is designed to survive harsh conditions) can survive long periods at the normal boiling point of 212 degrees. Pressure cooking kills the spores as well as the normal bacteria.

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Thank you for your response.

Yes, Botulism. How does this correspond to different altitudes? If I'm at sea level (or close to it) versus someone at X feet elevation? Where the atmospheric level affects boiling point? The temperature related to the microbe's bacterial spore destruction for safe consumption would be an issue would it not?

Edited November 10, 2004 by lovebenton0 (log)

[Harold McGee]

Yes, the boiling point of water drops with altitude. I'm not sure whether the heat resistance of the spores does as well, and it's better to be safe than sorry, so in the mountains you would need to extend the time of the pressure cooking to compensate for the lower temperature.