Undesirable microorganisms like bacteria, moulds and yeasts are hard to remove from the surface of fresh food. Washing, peeling, and blanching fruits and vegetables during preparation of a canning recipe help to reduce numbers of these microorganisms, but using the correct canning technique at the correct processing time is the safest way to preserve food for long term pantry storage. 
Clostridium botulinum is a bacterium that exists naturally in soil and water, often for many years. Botulinum spores are found (harmless) on the surfaces of fresh food because they only grow when air is not present. However, in the “ideal environment” – low-acid, low oxygen, moist, and a temperature of 4-49°C (40-120°F) – these bacteria multiply spores to produce vegetative cells, which multiply to produce a toxin within as little as 3-4 days. The toxin produced by Clostridium botulinum causes botulism, a deadly form of food poisoning. 
The acidity of food determines the preserving technique – low acid food (pH >4.6) must be processed in a pressure canner; high acid food (pH <4.6) is processed in a boiling water bath canner. Acidic food can be naturally acidic (like most fruits) or have acid added to tomatoes and vegetables - like vinegar, lemon juice or citric acid - to create pickles, chutney, relish and salsa. Low acid food does not stop bacterial growth.  The acid in acidic food halts bacterial growth (and helps destroy these bacteria when heated during canning, too).
Oxygen is present in all jars when they are filled, in the headspace (the air pocket between the food level and the jar rim). As jars are processed in a boiling water bath (or pressure canner if low-acid food), the food heats, expands and boils, the air pocket expands, and oxygen is pushed out of each jar. After jars have processed, this air pocket shrinks, pulling lids downwards to create a strong vacuum seal.
The vacuum seal also prevents moisture entering (or escaping) jars of canned food during storage – all of the moisture present in jars has been heat treated (if pressure canned) and/or acidic enough (in the case of water bath canned jars of food).
Boiling water has a maximum temperature it can reach, i.e. 100°C (212°F) - which is not high enough to destroy botulinum spores. This is why low-acid foods (the “ideal environment” for the spores to multiply) must be processed in a pressure canner, to reach 115°C (240°F) and be held at this pressure/temperature for the processing time required (i.e. 20-100mins) to destroy bacteria. As water boils at a lower temperature when altitude increases, a higher processing time (and higher pressure level for pressure canning too) is required to reduce the chance of bacterial growth in canned foods.
Therefore, it is important to prepare and process canned food correctly, exactly as the recipe states. Where possible, store jars of home canned food below 35°C (95°F), ideally at 10-21°C (50-70°F) to retain high quality preserves.
Author: Megan Radaich            
Image Credit: Megan Radaich
Publication: www.foodpreserving.org

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