When you examine the jars of jam or pickled veggies prepared for the winter, sometimes you may notice that a dry, dark crust forms on top. Here’s why jam turns dark and what you can do to prevent this.
The top layer of jams and pickles dries out and turns dark due to the process of oxidization. When you filled the jars, perhaps you left too much space under the lid, the oxygen darkened the contents, and the top dried out. This happens especially with jars of quince, apricot or plum jam, which contain less sugar syrup.
The darkened part is not unfit for consumption. It is just oxidized fruit or vegetables, and the taste does not change during the process. Nevertheless, the sight is unpleasant enough so the top layer is most often discarded.
Another reason why jam and pickled vegetables in jars turn dark is because they don’t contain enough liquid or syrup, and the slight evaporation causes the top to dry out. The third and last reason is related to the fact that perhaps you didn’t sterilize and boil the jars well enough, and the enzymes continued the oxidation process.
When the jam or vegetables in the jars spoil
Usually once the dry, darkened part is removed, the rest of the jam or pickles can be consumed safely.
Health hazard is only present if you see black or white mold under the lid. In this case, the process is no longer just about oxidation, but the entire content of the jar is spoiled, potentially toxic, and should be discarded altogether.
How you can prevent oxidation
The next time you prepare jars of jam or other homemade canned food, don’t leave too much space between the food and the lid. Try filling the jar to the brim, adding a layer of sugar syrup on top for sweet preparations.
Also, no matter how well you tighten the lids, it is recommended to boil the jars in a large pot for 25-30 minutes from the moment the water boils to sterilize them completely.