Complex sentence: Thus, older recipes that use sour milk may have been written prior to the availability of pasteurized milk, when it would be milk that had naturally acquired a sour taste through bacterial fermentation at room temperature.
Split into simpler sentences: Thus, older recipes that use sour milk may have been written prior to the availability of pasteurized milk. Before pasteurized milk became widely available, sour milk would have been unpasteurized milk that had naturally acquired a sour taste through bacterial fermentation at room temperature.