The differences between the acid level of fruits and vegetables, between boiling water and pressure canning, between a pressure canner and cooker, between various temperature levels and boiling times, and between elevations on the earth's surface might not seem crucial in many respects, but failing to be aware of and to observe them is the "nemesis of home canners."
That was the message from University of Wisconsin Extension Service Food Safety Specialist Barb Ingham during a "Lunch and Learn" WisLine program on canning vegetables safely. She noted that many foods have a natural content of acids that control microbial growth of toxins, but that numerous vegetables are an exception.
Tomatoes, green beans, beets, carrots, peas, corn and potatoes are low in acid and must be canned in a pressure unit, Ingham stated.
"The higher temperatures obtained in a pressure canner destroy the spores of clostridium botulinium which, if not destroyed, could germinate and grow, producing the deadly botulinium toxin," she explained. "Assume that the spores are there."
An ideal "recipe for danger" in trying to preserve vegetables is to place them in canning jars with a seal to remove oxygen and then store them at room temperature, Ingham warned. "Wait a bit as the spores germinate and the toxin is formed. Your product will be decidedly deadly."
Because they have a pH of below 4.6, fruits, pickles and salsa can safely be canned in boiled water, but the vegetables above a 4.6 pH, along with meats, need to be pressure canned, Ingham emphasized.
She said Brussels sprouts, broccoli and cabbage are not candidates for canning because they would expel a heavy sulfur smell and taste when removed from the container and, therefore, are best preserved by pickling or freezing.
Pressure canning refers to temperatures 240 degrees Fahrenheit (F) and above, Ingham pointed out. Those canners are equipped with either a dial gauge, which measures incremental changes in pressure, or a weighted gauge, which measures the pressure at 5, 10 or 15 pounds per square inch, she noted.
Because of the role that landscape elevation plays, canners of vegetables need to recognize how time and pressure need to be adjusted, Ingham observed. A triangle area covering approximately the northern one-third of Wisconsin, not including the far northeast counties, is on an elevation of above 1,000 feet. (An on-line resource is available to determine's the elevation for any particular site.)
What this means at the higher elevations is that the pressure must be increased when pressure canning and the time must be increased when canning in boiling water, Ingham stated. Adding pressure increases the temperature, resulting in correlations such as a 240F at 10 pounds pressure (psi) and 250F at 15 pounds compared to a boiling temperature at 212F at sea level.
Among the common sense practices for preparing vegetables for pressure canning are to select good quality produce, wash them in room temperature rather than cold water, peel as appropriate (potatoes and beets), and add seasonings such as herbs and salt if desired, Ingham observed.
Do not try to thicken the product with flour, corn starch, butter or fat, she cautioned.
An important step for canning vegetables safely with a pressure canner is to vent the canner for 10 minutes before closing the vent port, Ingham stressed. Not getting the air out appropriately will interfere with the timing schedule and pressure levels, she indicated.
Following the recipe instructions for the particular pressure canner is also crucial, Ingham continued. This means complying with the stated limits on jar size - going down in size is acceptable, but going up is not, she pointed out. A pressure canner must be able to hold at least four quart jars and to regulate pressure at five (sufficient for tomatoes), 10 and 15 pounds per square inch, she said.
Be sure to process at the correct temperature as dictated by the elevation and realize that the process must be repeated if the pressure falls below the target at any time, Ingham emphasized. She mentioned required minimum pressures of 10 pounds at elevations below 1,000 feet, 11 pounds at all places for a dial gauge, and 15 pounds at elevations above 1,000 feet.
The Extension Service does not recommend the use of steam or pressure cookers, which are not necessarily the equivalent of presssure canners, for home canning, Ingham reported. She identified the major concern as the faster temperature changes with pressure cookers - heating too quickly and then cooling too quickly - but added that a graduate student assistant is undertaking a research project to learn more about those safety concerns.
Because there aren't any tested recipes, processing quart jars of cream style corn is another point on which the Extension Service is very cautious, Ingham indicated. "Don't do it."
For mixes such as soups that contain tomatoes, corn, black beans or onions, the processing time needs to be based on what's required for the ingredient with the longest time, Ingham stated. In this case, the time would be 85 minutes.
If the jars fail to seal during a pressure canning, the choices are reprocessing, freezing or refrigerating the vegetable for short-term use, Ingham remarked. For reprocessing, the jars need to be emptied, the contents must be reheated, new lids must be applied, and the venting and full time processing must be completed, she explained.
Another potential downfall is losing more than half of the liquid in a jar, Ingham observed. This is usually a sign of excessive fluctuation of the temperature or pressure or that underprocessing has occurred and the jars should be refrigerated, she noted.
Ingham recommends testing dial gauge testers at least annually (many county Extension Service offices have a tester) and that brand new dials should also be tested. She noted that produce suppliers also provide guidance on testing and on testing with only certain lids.
Jars and lids purchased in a previous year can be used provided that they were stored in a cool and dry location and the sealant didn't break down because of heating or freezing, Ingham indicated. Non-Ball jars and lids may be used, but the risk of not sealing increases, she added.
Once properly sealed, canned foods can be considered to be safe indefinitely, but they will begin to lose quality in about three years, Ingham pointed out. That's assuming they're being stored in a cool and dry location, she noted.
Dial vs. weighted gauges
One point which befuddles and frustrates many users of weighted gauge canners is determining what is an authentic jiggle or rock that's supposed to take place during the processing, Ingham agreed. She believes that getting acquainted with the unit that one has is the best way to solve that challenge or, when purchasing a unit, to decide on having a unit that one can "read" (a dial gauge) or can "hear" (a weighted gauge).
Ingham is happy with her older style Mirro canner, made in Manitowoc, but noted that Mirro and Presto, based at Eau Claire, have brought new units to the market. She said Mirro's 16- and 12-quart canners, are proving to be tricky on the "rock" factor, even for the top national expert on vegetable canners.
All-American canners, also made in Manitowoc, are really weighted gauge canners, although they are equipped with a reference dial gauge, Ingham explained. She said they do not need to be and should not be tested with the dial testers which are available in the Extension Service offices in many counties.
Presto's new 23-quart-tall canner allows for double stacking, Ingham reported. With that canner, be sure to place a rack both at the bottom and between the layers of jars, she advised.
The Ball company has introducted a gadget called the SureTight Band Tool for securing jar bands with a certain "finger-tip tightness," but reactions posted on web sites state that it's a confusing practice, Ingham reported. She said the Extension Service does not recommend use of that tool, in part because it has not been evaluated.
A booklet titled "Canning Vegetables Safely" (B1159) is available for purchase at Extension Service offices.
Other resources are the www.foodsafety.wisc.edu, www.uga.edu/nchfp, and www.freshpreserving.com
web sites and the "So Easy to Preserve" book and video series.
The next Wisline "Lunch & Learn" program will be presented at noon on Monday, July 23. Canning tomatoes and products containing tomatoes will be the topic.