Isolation and investigation of micro-organisms causing brown defects in mould-ripened cheeses

Browning of mould-ripened cheeses is a widespread problem in the specialist cheese industry in Australia and overseas, but little is known of the origin or mechanism of this process. A pigment-producing yeast, Yarrowia lipolytica, and two bacteria, a Pseudomonas sp. and a Coryneform, were isolated from discoloured mould-ripened cheeses. Growth of the Y. lipolytica occurred at pH 3.5-8.0, the Coryneform at pH 5.5-8.0 and the Pseudomonas sp. at pH 6.0-8.0. Pigment was produced by both the yeast and the bacteria between pH 6.0 and pH 8.0. Growth and pigment production of the micro-organisms occurred at both 10 degrees C and 30 degrees C. While growth of the micro-organisms occurred in the absence of oxygen, the requirement for oxygen in the formation of the brown pigment was confirmed. These studies indicate that the oxidative browning seen in these defective cheeses is most likely due to tyrosinase production by these micro-organisms