Thermoduric bacteria can survive the pasteurization process and cause your milk to spoil even when refrigerated. Some thermoduric bacteria also present health hazards to anyone consuming the infected milk. As per Science Direct: "The thermoduric bacteria commonly found on farm dairy equipment and in raw milk are limited to a few species of five groups of bacteria, viz., streptococci, micrococci, coryneform bacteria, erobic spore formers and occasionally Gram-negative rods."