Our knowledge regarding the exact nature of these indefinite faults is as yet too inadequate to enable many of these undesirable conditions to be traced to their proper source; but in many cases the taints observed in a factory are due to the abnormal development of certain bacteria, capable of evolving unpleasant or even putrid odors. Most of them are seeded in the milk before it comes to the factory and are due to careless manipulation of the milk while it is still on the farm. Others gain access to the milk in the factory, owing to unclean conditions of one sort or another. Sometimes the cheese-maker is able to overcome these taints by vigorous treatment, but often they pass on into the cheese, only to detract from the market value of the product. Most frequently these "off" flavors appear in cheese that are cured at too high temperatures, say above 65 deg. F. ~"Gassy" fermentations in cheese.~ One of the worst and at the same time most common troubles in cheese-making is where the cheese undergoes a fermentation marked by the evolution of gas. The presence of gas is recognized by the appearance either of spherical or lens-shaped holes of various sizes in the green cheese; often they appear in the curd before it is put to press. Usually in this condition the curds look as if they had been punctured with a pin, and are known as "pin holey" curds. Where the gas holes are larger, they are known as "Swiss holes" from their resemblance to the normal holes in the Swiss product. If the development of gas is abundant, these holes are restricted in size. Often the formation of gas may be so intense as to cause the curds to float on the surface of the whey before they are removed. Such curds are known as "floaters" or "bloaters." If "gassy" curds are put to press, the abnormal fermentation may continue. The further production of gas causes the green cheese to "huff" or swell, until it may be considerably distorted as in Fig. 33. In such cases the texture of the cheese is greatly injured, and the flavor is generally impaired. [Illustration: FIG. 33. Cheese made from gassy milk.] Such abnormal changes may occur at any season of the year, but the trouble is most common in summer, especially in the latter part. This defect is less likely to occur in cheese that is well cheddared than in sweet curd cheese. When acidity is produced, these gassy fermentations are checked, and in good cheddar the body is so close and firm as not readily to permit of gaseous changes. In Swiss cheese, which is essentially a sweet curd cheese, these fermentations are very troublesome. Where large holes are formed in abundance (blaehen), the trouble reaches its maximum. If the gas holes are very numerous and therefore small it is called a "nissler." Sometimes the normal "eyes" are even wanting when it is said to be "blind" or a "glaesler." [Illustration: FIG. 34. Block Swiss cheese showing "gassy" fermentation.]