Fats and oils can create safety problems so generally home canned foods should be as fat free as possible. One way the fat will interfere with canning is by blocking the heat penetration when processing the food. Fat molecules surround bacterial spores which ends up helping the bacteria survive.
When it comes to sealing lids, leaving fat can also increase the risk of an unsuccessful seal. This is because the fat in foods can climb up during processing and overflow in between the lids and the rim of the jar. One thing that you should keep in mind is white vinegar cuts through grease. So when you know there's some fat in your jar, wipe the rims clean with white vinegar instead of water before adding the lid.
Another big reason to avoid fat in your jars is rancidity. This occurs because the fats and oils go rancid over time resulting in an off taste in the food. As you can see it is important to trim off as much fat as possible before canning your food.
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