Organic milk often undergoes "ultrapasteurization," in which the milk is heated to a very hot 280° F (137.8° C) for 2 seconds, rather than the 161° F (71.7° C) for 15 seconds used in conventional pasteurization. By killing more bacteria, the extra heat extends the milk's expiration date. It's not clear whether organic-milk producers ultrapasteurize as an additional safeguard against bacteria, since organic cows consume no antibiotics, or simply to extend shelf life. Properly refrigerated, ultrapasteurized milk has a shelf life of 40 to 60 days unopened, compared with 15 to 17 days for milk pasteurized the regular way. But you should consume all milk within 7 to 10 days after opening.