When the process of making one thing results in a second product as well, that second thing is called a byproduct. Molasses, for example, is a byproduct of refining sugar.
Food production results in many byproducts: wheat germ is a byproduct of wheat milling, and orange and lemon oil are byproducts of citrus fruit being processed into juice. Sawdust is a byproduct of the lumber industry, and feathers are a byproduct of poultry processing. In each of these cases, the byproduct is important and useful, but secondary to the initial product. The word has been around since the mid-1800s, and in the UK it's spelled with a hyphen: by-product.