Sunday, August 13, 2023

Causes of Canning Jar or Contents Darkening


Four Reasons Canning Jar Contents Darken

First, how pressure canners work to keep food safe.

Pressure Canners basically heat the jars to a temperature high enough to destroy the microorganisms that could cause food spoliage and or foodborne illness.    The heating process also removes air from the product and creates a vacuum which helps to prevent recontamination by harmful microorganisms.

While the canning process remains one of the safest ways to preserve food, every practices comes with its problems and home canning is no exception.

Sometimes the jar or its contents may turn to a brown, black or a dark color.  Here are the most common reasons that that occurs and how to prevent experiencing them.

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Food Darkens in the Top of the Jar - Five Causes

1. Cause:  The liquid did not cover the food product.

Prevention: That's easy enough to prevent. Completely cover the food product with liquid; brine, syrup, juice or water before adjusting the two-piece cap.

2. Cause: Food was not processed long enough to inactivate enzymes.

Prevention: Process each food for the recommended length of time.

3. Cause: The manner of packing the jar and processing did not expel enough air.

Prevention: This is a common problem that happens when using the wrong packing method. If the recipe calls for the food to be hot packed, follow that instruction.

4.  Cause: Proper headspace was not used.

Prevention: Follow the recommendations closely.

5.  Cause: Headspace was too large or bubbles weren't removed prior to processing.

Prevention:  Don't forget to debubble the jar contents by sliding a nonmetallic spatula between the food and the jar to release trapped air bubbles.  Skipping this step can cause this problem and a myriad of others.

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Fruit Darkens After Removed From The Jar

Cause: Fruit wasn't processed long enough to inactivate enzymes.

Prevention: Process each fruit by the recommended method and for the recommended length of time. 

For fruit processing, remember that time is counted when the water reaches a rolling boil in the boiler-water canner.

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Corn Turns Brown After Processing - Three Causes

Cause: The variety of corn processed was not suitable for canning or the corn was not harvested at the correct time.

Prevention: Select only varieties of corn recommended for preserving and choose ears of corn with plump, shiny kernels filled with milk.

Cause: Liquid did not cover the corn.

Cause: Jars were processed at too high a temperature.

Prevention: If recommended pounds of pressure was used in processing, the dial gauge could be faulty and should be checked.

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Food in Jars Become Black, Brown or Gray

Cause: Some foods contain natural chemical substances such as sulfur compounds, tannins and some acids which reacts with minerals in the water or with metal utensils used in preparing the food.

Prevention: Use soft water. Avoid using brass, copper, iron, aluminum, zinc utensils if the tin plate is worn.

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Black Spots on the Underside of the Metal Lid

Causes: The same natural chemical substances in some foods can cause a black or brown deposit on the underside of the lid. This deposit is harmless and does not mean the food is unsafe to eat.


The home canning process has been time tested and is always improving. It's also one of the most enjoyable sustainable acts you can learn. Armed with a well-rounded knowledge of the canning process can save you disappointment, time and spoiled food.

I hope you found this article helpful.   Enjoy the canning process!  Canning and preserving food is a great step toward a more sustainable lifestyle!


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