If you are making veggie stock, avoid adding vegetables such as Tomatoes, Beetroot, Cabbage, Potatoes, Turnips, Zucchini, Asparagus, Kale, Brussels Sprouts etc. Starchy Vegetables such as Potato and Turnips will make your stock gummy. Tomatoes make it too pulpy, Zucchini, Kale, Brussels Sprouts, Cabbage can make it taste bitter, and Beetroot turns your stock red.