~Rusty spot.~ This name is applied to the development of small yellowish-red or orange spots that are formed sometimes throughout the whole mass of cheddar cheese. A close inspection shows the colored points to be located along the edges of the curd particles. According to Harding,[216] this trouble is most common in spring and fall. The cause of the difficulty has been traced by Connell[217] to the development of a chromogenic bacterium, _Bacillus rudensis_. The organism can be most readily isolated on a potato surface rather than with the usual isolating media, agar or gelatin. ~Other pigment changes.~ Occasionally, with the hard type of cheese, but more frequently with the softer foreign varieties, various abnormal conditions arise that are marked by the production of different pigments in or on the cheese. More frequently these are merely superficial and affect only the outer layers of the cheese. Generally they are attributable to the development of certain chromogenic organisms (bacteria, molds and yeasts), although occasionally due to other causes, as in the case of a blue discoloration sometimes noted in foreign cheese made in copper kettles.[218] De Vries[219] has described a blue condition that is found in Edam cheese. It appears first as a small blue spot on the inside, increasing rapidly in size until the whole mass is affected. This defect he was able to show was produced by a pigment-forming organism, _B. cyaneo-fuscus_. By the use of slimy whey (lange wei) this abnormal change was controlled.