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Key to top quality fish is exhausting

Posted: Sunday, July 28, 2002

Many Alaskans take pride in the salmon they can. Unfortunately, there are Alaska residents teaching people in our area campgrounds how to can fish in cans and taking a shortcut, skipping an important step. Their students then end up in our Extension Office worried about the safety and quality of their canned fish.

The shortcut I am talking about is the step of exhausting cans.

Because skipping this step happens so frequently, I asked University of Alaska Fairbanks Cooperative Extension Food Safety Specialist Dr. Kristy Long to elaborate on the importance of this step of the process.

"Cans of fish are exhausted by heating prior to attaching the lid to remove air by replacing it with steam," she said. "The fish expands as it is heated, which drives out gases that are either dissolved or held in the product. The exhausting also reduces the amount of air in the headspace of the can prior to attaching the lid."

Once lids are attached using a can sealer, air cannot move in or out of the can. Therefore, the air and gases need to be removed before the lid is attached. If the fish in the can is not exhausted before attaching a lid, a vacuum will not form in the can. A vacuum maintains the "lid ends in" during storage.

"Most bacterial spoilage results in the production of gases which cause jar lids or can lids to bulge outward," Long said. "The bacteria that causes botulism can be an exception.

"Most people who have found canned foods with bulging lids in a grocery store or on their kitchen shelf at home don't buy or use them because they know this could mean the food is spoiled. 'Ends in' on a can is an indicator that the food is safe to eat. 'Ends out' indicates spoilage.

"A vacuum also reduces the amount of oxygen in the fish and in the container. Oxygen over time can cause off-flavors in fish. So, exhausting helps maintain a higher quality fish product over a longer period of time.

"A third reason for exhausting fish in cans prior to attaching the lid is that the lids can be permanently distorted (usually bulging) during the canning process. If the oxygen and other gases in the fish and can have not been partially removed by exhausting, the gases could expand enough to cause the lid ends to bulge outward because the gases aren't able to get out of the can."

The directions provided by Cooperative Extension Service are the safest way to can fish in cans and the directions include exhausting the fish in the cans before attaching the lid, Long said.

"All commercial processing plants that can fish in cans exhaust the product before attaching the lid. I think that if there were a safe method that excluded exhausting, the commercial canning industry would be using it."

For a copy of the publication written by Dr. Kristy Long entitled, "To Can Fish in Cans," contact the Cooperative Extension Service office.

Linda Tannehill is an agent at the Alaska Cooperative Exten-sion Office. The office is at 43961 Kalifornsky Beach Road, Suite A, Soldotna, AK. The phone number is 262-5824 or (800) 478-5824.



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