Remember to Vent

A critical step to achieving proper pressure inside a pressure canner is allowing it to VENT. What does this mean?

Venting is also “exhausting” the canner, a process of letting steam (and air) come out of the canner through the vent pipe for a period of time before beginning the pressure processing time.  Air trapped in a canner lowers the processing temperature and results in under processing of low-acid foods.  To be safe, the USDA recommends that all types of pressure canners must be vented 10 minutes before they are pressurized.

WHY VENT? 

It is STEAM, not water, that does the processing in a pressure canner. Low-acid foods (foods with a pH of 4.6 or higher such as all vegetables excluding tomatoes, meats, seafood, soups and sauces) are not acidic enough to destroy bacteria, their spores, and the toxins they produce or prevent the growth of botulinum bacteria in a vacuum.  The heat-resistant spores produced by C. botulinum can only be destroyed with the correct combination of temperature, pressure, and tested time. Temperatures in the range of 240°F to 250°F (115°C to 121°C) are needed to kill spores (USDA 2015). Water can get no hotter than the boiling point (212ºF, 100ºC), but steam can. Steam trapped in the canner increases the atmospheric pressure inside the canner causing the boiling point of water to increase to 240ºF-250ºF, the temperature needed to destroy bacteria and C. botulinum, that would otherwise be free to grow in a vacuum sealed jar.

In order to reach the optimum temperature to destroy botulinum bacteria, air inside the canner must be exhausted to allow space for a pure steam environment to build. There is a vast amount of air in a canner due to the space between the water level and the lid as well as the air that escapes from inside the jars and from the water. The most “jar air” comes from those with raw-packed foods.

Image Source: USDA Complete Guide to Home CAnning

HOW TO VENT

The vent or petcock is a short hollow pipe that sticks up above the canner lid.  When open, it allows air and steam to escape from the canner.  When closed, it holds steam inside. To vent a canner, leave the vent port uncovered or manually open the petcock (some older models).  After placing jars inside the warm canner and securing the canner lid, set the burner on high. Watch for steam to escape from the vent pipe. When a strong, visible, funnel-shaped steam cone emerges, set a timer for 10 minutes and let the canner continuously steam. After the 10 minutes, add the weight or counterweight to the vent or close the petcock to pressurize the canner.

WHAT HAPPENS DURING VENTING?

As the water boils inside the canner, the empty spaces become a mixture of steam and air. As steam increases, it pushes the air out creating a pure steam environment.  USDA processing times are based upon a pure steam environment which makes venting so very important.

In addition to venting, remember to adjust for altitude. Most recipes list processing time based on altitudes near sea level. To ensure the health of those who enjoy your foods, always use a tested recipe and follow instructions. Remember to VENT for 10 minutes to ensure that any and all microorganisms are destroyed.

Source: National Center for Home Food Preservation and USDA Complete Guide to Home Canning (2015)

Marlene Geiger

I am a graduate of the University of Nebraska-Lincoln with a BS in Home Economics Education and Extension and from Colorado State University with a MS in Textiles and Clothing. I enjoy spending time with family and friends, gardening, quilting, cooking, sewing, and sharing knowledge and experience with others.

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5 thoughts on “Remember to Vent

  1. The instructions for my pressure cooker/canner say to set the heat on high until a steady column of steam comes from the vent for ten minutes before putting the pressure regulator on. I was processing 6 pts. of stewed tomatoes (10 lbs. pressure for 15 min.) The water boiled for 20-30 minutes and some steam was escaping but still no steady column of steam. Is such a long period of time normal? I had the Power Plus Burner on Super Boil. Finally, I put the pressure regulator on anyway, adjusted the heat for the recommended “jiggles per minute” and processed for the proper time. All the jars sealed. Can I assume the food will be safe?

  2. Hi Diane, it usually is not hard to tell when the canner is venting properly as there is a steady head of steam coming from the vent pipe; I’m not sure why you were unable to see the steam but you certainly allowed more than enough time for venting. (I am not familiar with Power Plus Burner on Super Boil so have no idea what this means or how it played into your process.) I suspect that if you let it boil for 20-30 minutes, there was adequate time for venting and you may have also lost more than the normal amount of liquid from the canner during this time. However, since you indicate that the weights jiggled properly for the recommended time, it is quite likely that the food in the jars is safe. One must NEVER assume that a sealed jar is a safe jar. A safe jar is only achieved when the canning method was properly done so that the internal temperature of the contents reaches the needed temperature (240 degrees) and time to kill bacteria. If bacteria such as Clostridium botulinum is not properly killed in canning, it will continue to grow in an anaerobic environment, a sealed jar.

  3. Hi Sharon, to my knowledge, there are no home tests to detect botulism–one cannot see, taste, or smell it. The USDA ARS has developed field test kits for botulism for the purposes of bio-security and testing BOTOX.
    Botulsim risk is low when foods are properly canned using tested recipes, following the directions carefully, and processing appropriately. Botulism cannot live in an acidic environment so foods that are naturally high in acidity (fruits and pickled items with added vinegar) are not a risk for botulism making them safe to be processed in a water bath canner which is sufficient to stop other forms of spoiling.
    Foods that are naturally LOW in acid (meats and vegetables) are at risk for botulism. For these foods, high heat is the enemy and should be processed in the high heat of a pressure canner. Botulism spores are stopped at temperatures above 240 degrees Fahrenheit. This is only achievable in a pressure canner. No matter how long you boil water, it will not reach this temperature. That is why you need the pressure, from a pressure canner, to reach that level of heat.

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