It is a well known fact that bacteria play a large part in the success or failure of the satisfactory production of dairy products, but the role of yeasts and moulds should not be overlooked. These living organisms, commonly known as fungi, are the next higher form of life in the vegetable world after the bacteria stage. Their form of growth resembles the growth of plants in that they reproduce by budding, and their spores, analogous to the seeds of plants, are the means whereby many species propagate further generations. The yeast cell is much larger than the ordinary bacterium, so that it is possible to study them with the aid of much lower magnifications. When grown on solid media the yeasts give colonies not unlike those of bacteria except that the edges of the colonies are less defined, the colonies themselves project well above the surface of the media, and their surfaces are usually of a rough appearance. A good example of mould growth is that of the ordinary “green mould.”—Yeasts usually prefer to grow on the surface of liquids, and moulds are found to grow most vigourously on solid or semi‐solid media, such as meat, cheese, butter, etc. The growth of bacteria in the media hinders the simultaneous growth of the fungi, so that it is only after the media has become too acid for the growth of bacteria that yeasts and moulds are able to grow. In support of this theory it has been found that fungi will grow on the surface of sterile milk, but ordinary fresh milk containing bacteria is not a suitable media as the fungi cannot compete with the bacteria. It is found, therefore, that only bacteria proliferate in fresh milk. However, when milk has become sour bacterial growth is arrested, and it is then that mould growth becomes perceptible. The fungi tolerate a relatively large amount of acid. Media used for their cultivation is generally standardised to a ph of about 4.5. The optimum temperature for their growth is in the region of 75°–90° F. Some species will grow at 32° F., others even below this temperature. Low temperatures are not lethal to the fungi, so that when infected products are removed from cold storage growth may occur. The temperatures required to kill them and their spores generally falls between 130°–180° F. Most yeasts are killed at temperatures above 120° F., while their spores may have to be exposed to higher temperatures.
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