In the years before pasteurization this was true. Bacteria, proteins, and some enzymes present in natural milk can inhibit the growth of yeast. Scalding the milk, or bringing it to a point of steaming, but not boiling, kills the bacteria, denatures the proteins, and stops those pesky enzymes from getting in the way of yeast. Nowadays the pasteurization process most of our milks go through solves this problem for us and we can skip the scorching step. However, if you use unpasteurized milk, please follow this guide to scalding milk. Then allow it to cool to at least 110°F before adding your yeast.