If, after emptying your wine juice bags, you could find a dark sediment. This is a naturally occurring falling out of solution of nutrients, tannin, or natural pigments.
What is dark sediment?
“Dark sediment” in a wine juice bag could refer to various substances that can accumulate at the bottom of the bag over time, such as dead yeast cells, grape skins, seeds, or other particulate matter. This is normal and can happen when the wine juice is fermented, as the yeast cells break down the sugars in the juice and produce alcohol and carbon dioxide. The solid materials that are left over can settle at the bottom of the container.
If you find this sediment in your wine juice bag, rinse the bag out with a little water, shake it, and add it to your fermentation.
This is normal and is a good thing to reintroduce into your wine.