Some recipes will call for blanching vegetables (boiling or steaming and then rapidly cooling them). One of the reasons for blanching is to kill off some harmful microorganisms.

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      First, let’s look at how different microorganisms are affected by heat. See the table “Processing and Storage Temperatures for Food Preservation” for an illustration. There are three things that can cause your food to spoil: enzymes, fungi (molds and yeasts), and bacteria.

       Enzymes promote the changes that are a necessary part of that plant or animal’s life cycle. Eventually, however, these enzymes change the food’s color, flavor, and texture and make it unappetizing.

       Molds are fungi that grow on foods and look like fuzz. Some molds are harmless, but many more are not. These harmful molds produce mycotoxins, which can cause illness, and they thrive in high-acid environments, meaning they may eat the acid in a food that would otherwise protect that food from spoilage by bacteria. Molds are easily destroyed when heated to temperatures between 140 and 190 degrees Fahrenheit.

       Yeasts are fungi that cause food to ferment. This fermentation—while good in certain circumstances, such as those used in making pickles, bread, and beer—will often make food unfit to eat. As with molds, yeasts are easily destroyed when heated to temperatures between 140 and 190 degrees Fahrenheit.

       Bacteria are very different and can be more difficult to destroy. Some bacteria, notably the dreaded Clostridium botulinum, which causes deadly botulism poisoning, require a low-acid environment to live but can withstand temperatures much higher than yeasts, molds, and enzymes. As we said earlier, some bacteria thrive in environments where there is no oxygen. So, sealing the jar on these guys would just make them happy. And, bacteria produce spores, which—if not killed during processing—become bacteria that produce deadly toxins. While some bacteria will die at the same temperature as yeasts and molds, those pesky spores are much more resilient and require significantly higher temperatures to succumb.