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Canning Low-Acid Foods : Some Common Questions

by: Val Hillers, Ph.D. , Extension Food Specialist


Which pressure canner is more accurate, the kind with a dial or the one with a weighted control?
The weighted control tends to be more accurate.  Both types are accurate if used and cared for according to the manufacturer's instructions.  Some people prefer to see the pressure reading on a dial; others prefer the sight and sound ("jiggling" noise) of the weighted control.  The dial control must be tested frequently for accuracy.  The weighted control doesn't need this, but it must be cleaned occasionally for good service.  People who live at higher altitudes may prefer the dial because the altitude adjustment can be made more precisely.

What is the procedure for canning "mashed pumpkin"?
Canning mashed pumpkin is not recommended because the heat penetration of a thick product is poor.  Cubed pumpkin may be canned safely.  (If the pumpkin is already mashed, freeze it!)

Can low-acid food be reprocessed if it was incorrectly processed or if lids failed to seal?
If less than 24 hours have gone by since the food was processed, do one of the following:

  1. Refrigerate the food and use it in the next day or two.

  2. Freeze the food.  Drain vegetables before freezing.

  3. Reprocess the food.  Use the same jars (check jar rim and headspace) or put in clean hot jar.  Use new lids.  Process again for the full time.  The quality of twice-processed food may be poor.

What causes a lid that sealed to bulge or to come unsealed during storage?
Both bulging and loss of seal are signs of spoilage.  Discard the food--it could be dangerous.  Spoilage organisms must be growing in the food and producing gas.  The buildup of pressure inside the jar results in bulging or loss of seal.  Growth of spoilage organisms can be attributed to under processing (product was processed by wrong method or for too short a time) or to a hairline crack in the jar which permitted entry of organisms during storage.

I processed green beans in a boiling water bath.  The lids sealed.  Doesn't that mean the beans are safe to eat?
No. The fact that the lids sealed means that a vacuum was formed inside the jars.  It does not mean that spoilage organisms were killed.  Discard the beans--they could be dangerous.  The only safe way to can green beans is in a pressure canner.

What causes lids to buckle?
Putting screw bands on so tightly that air can hardly escape from jars during processing.  The buildup of pressure inside jars causes lids to buckle.  Screw bands should be tightened firmly, by hand.  Do not use a jar wrench to tighten screw bands.

What causes brown or black deposits to form on the underside of metal lids?
The underside of metal lids is protected by an enamel coating.  If there are any imperfections in the enamel, e.g., tiny scratches or pinholes, natural compounds in food can react with the metal in the lid to form brown or black deposits.  These deposits are harmless.

What are the yellow crystals that sometimes form in canned asparagus?
They are deposits of rutin, a pigment which occurs naturally in asparagus.  If the asparagus was packed in water with a high iron content, the precipitated rutin may be gray or black rather than yellow.  Either way, it is harmless.

Is it all right to can frostbitten vegetables?
Yes.  For best quality, can them as soon as possible after the frost hits.  However, the texture of the vegetables where frozen will be poor.

Is it all right to can strained (pureed) vegetables and meats?
No, the heat penetration of strained products is poor.  Instead, can vegetables or meats in the usual way and strain them just before serving.

I left home canned foods in my camper over the winter and they froze.  Are they safe to use?
If freezing did not break the jar and the lid is still tightly sealed, the food may be used.  Frozen canned foods may be less palatable than properly stored canned foods.

How long may canned foods be stored?
The recommended storage time for home canned foods is one year.  However, a period of two to three years is a reasonable storage time.  With longer storage, the eating quality and nutrient content of the food will drop, but the foods remain safe to eat if they were processed correctly and the jar is still sealed.

Can I run cold water on my canner filled with pint jars of corn at the end of the processing time to quickly cool them so I can start a new batch?
No.  Never try to rush cooling by pouring cold water over the pressure canner.  The complete processing time includes the time the pressure canner cools.  If cooling is rushed, the foods may be underprocessed.  In addition, rapid cooling may break the jars.

Can a pressure canner be used on a smooth top range?
For a pressure canner to work successfully on a smooth top range, the canner bottom must be flat and in contact with the cooktop.  Most heavy cast aluminum canners meet these specifications.  Lightweight aluminum canners have a concave bottom.  If the concavity is greater than 1/8 inch, the canner will not perform effectively.  To measure the concavity, place a straight edge across the canner bottom and measure the distance to the canner bottom with a ruler.

Can a pressure canner be used on a solid element cooktop?
Solid element cooktops require heavy gauge, flat, smooth bottom utensils that have no more than one-inch overhang beyond the element.  Most pressure canners are too large to be used on solid element cooktops.

My pressure canner is the self-venting type.  Why do your materials say all pressure canners must be vented for 10 minutes?
All pressure canners, including "self-venting" pressure canners, need to be vented 10 minutes before the pressure is allowed to rise to be certain that all air is removed from the pressure canner.

If there is air in the pressure canner, the temperature is much lower than if the canner contains only water and steam. Research has shown that under some conditions (particularly with raw-packed foods) all air may not be removed from self-venting pressure canners.

I've canned some fish and when I opened the jars it looked like there were glass particles in the fish.  The jars aren't broken so where did the glass come from?
The particles are not glass, but "struvite" crystals, which are made of magnesium ammonium phosphate.  These crystals occasionally form in seafood from substances normally present in the fish.  The crystals also occur in commercially canned fish.  A simple test to distinguish between struvite and glass is to put the material in warm vinegar for a while.  The struvite will dissolve, glass will not.

What should I do if the canner goes below the recommended pressure during processing?
To be processed correctly, the canner must remain at the recommended pressure for the entire processing time without interruption..  

Increase the heat under the canner to reach the recommended canner pressure. When the recommended pressure is reached, set the timer for the full processing time.  In other words, you will start the entire processing time over again.



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