I’m making potato soup with Russet potatoes, barley, onions, and water. As the pot boils, purple foam is coming to the surface of the water. What causes this and is it safe to eat?

The purple foam that you're seeing while making potato soup with Russet potatoes, barley, onions, and water is likely due to a chemical reaction between the potatoes and the water. While it's not common for the foam to turn purple, it could be related to the type of potatoes you're using or some other factors.

The purple color could be the result of pigments or compounds naturally present in the potatoes reacting with the boiling water. For example, some varieties of Russet potatoes may contain purple pigments that can leach into the water when they are boiled. Additionally, the pH level of the water and the type of utensils or cookware you're using can sometimes influence the color of the foam.

In most cases, the purple foam is harmless and safe to eat. It's unlikely to be a sign of spoilage or contamination. However, if you're concerned about the color or if there are any unusual odors or other signs of spoilage, it's a good idea to exercise caution and consider discarding the foam or the top layer of soup if you have doubts about its safety.

To prevent or minimize the purple foam, you can try the following:

1. Rinse the potatoes before adding them to the soup to remove any excess surface starch.
2. Use a stainless steel or non-reactive pot to cook the soup, as reactive materials can sometimes affect the color of the food.
3. Check the pH level of your water; if it's highly alkaline or acidic, it can affect the color of certain foods. Using neutral or slightly acidic water might help.

Remember that food safety is essential, so if you ever suspect that something might be off with your soup, it's better to err on the side of caution and not consume it.