Acidity level: Most microorganisms can't grow or produce their toxin in an acid environment. So, fruits with high acidity are good candidates for canning, while low-acid vegetables, such as corn and green beans, must be pressure canned. On a scale of pH acidity levels, lemons are 2.0. Peaches are 3.6. The cutoff is 4.6, and tomatoes are a 4.1 to 4.3 — just barely enough acidity to qualify them for water-bath canning.

Density of food: If the food is packed together, the heat from the boiling water can't penetrate as well.

Bacteria load: Across the country (and around the world) there may be a higher density of different types of bacteria, said Hunsaker.

"It may seem ridiculous that any microorganism will survive 40 minutes of water bath, but we don't have the same microorganisms today that they had years ago," she said. "Our environment changes over the years."

One bacteria in tomatoes withstands high temperatures for a long time, she said. This bacteria lowers the acidity level over time, so botulism can take over. This is why the USDA raised the processing time for tomatoes, and recommends adding 1 tablespoon of lemon juice, vinegar or other acid per pint.

Salsa is especially susceptible to botulism, because you're adding low-acid foods such as peppers and onions. In 1986, an Ogden woman nearly died after eating her own salsa, and a few years ago in Cedar City, a couple died after eating their home-canned salsa. So, although it doesn't happen every day, it's a real safety risk.

It's crucial to use a salsa recipe for canning that has the right ratio of acid in it. If you prefer more cilantro, chiles, etc., you can always add those just before serving. Or freeze the salsa instead.

If you forgot to add lemon juice or vinegar to your canned tomatoes, you'll need to open the jars, add the recommended amount, and reprocess them for the full amount of time. Or refrigerate the jars and use within a few days.

Jams and jellies: Sugar acts as a preservative, combining with pectin and the fruit's acidity to bind the moisture so that most microorganisms can't grow. But mold can still grow without a great deal of moisture. Even if you scrape the mold off the top, the mold spore can penetrate deeply into the jam or jelly, said Hunsaker.

USDA guidelines call for a 10- to 15-minute processing time, depending on the type of jam being made.