Other types occur more or less sporadically, some of which are capable of liquefying the casein of milk while at the same time they also develop lactic acid. Conn and Aikman refer to the fact that over one hundred species capable of producing variable quantities of lactic acid are already known. It is fair to presume, however, that a careful comparative study of these would show that simply racial differences exist in many cases, and therefore, that they are not distinct species. As a group these bacteria are characterized by their inability to liquefy gelatin or develop spores. On account of this latter characteristic they are easily destroyed when milk is pasteurized. They live under aerobic or anaerobic conditions, many of them being able to grow in either environment, although, according to McDonnell,[52] they are more virulent when air is not excluded. While growth of these lactic forms may go on in milk throughout a relatively wide range in temperature, appreciable quantities of acid are not produced except very slowly at temperatures below 50 deg. F.[53] From the standpoint of frequency the most common abnormal changes that occur in milk are those in which gases of varying character are developed in connection with acids, from the milk sugar. Other volatile products imparting bad flavors usually accompany gas production. These fermentations are of most serious import in the cheese industry, as they are especially prone to develop in the manufacture of milk into certain types of cheese. Not often is their development so rapid that they appear in the milk while it is yet in the hands of the milk producer, but almost invariably the introduction of the causal organisms takes place while the milk is on the farm. Numerous varieties of bacteria possess this property of producing gas (H and CO_{2} are most common although N and methane (CH_{4}) are sometimes produced). The more common forms are those represented by _B. lactis aerogenes_ and the common fecal type, _B. coli commune_. The ordinary habitat of this type is dirt and intestinal filth. Hence careless methods of milk handling invite this type of abnormal change in milk. It is a wide-spread belief that thunder storms cause milk to sour prematurely, but this idea has no scientific foundation. Experiments[54] with the electric spark, ozone and loud detonations show no effect on acid development, but the atmospheric conditions usually incident to a thunder storm are such as permit of a more rapid growth of organisms. There is no reason to believe but that the phenomenon of souring is wholly related to the development of bacteria. Sterile milks are never affected by the action of electric storms.