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 orig in or mechanism of this process. A pigment-producing yeast, Yarrowia lipoly tica, and two bacteria, a Pseudomonas sp. and a Coryneform, were isolated f rom 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 occur red at both 10 degrees C and 30 degrees C. While growth of the micro-organi sms occurred in the absence of oxygen, the requirement for oxygen in the fo rmation of the brown pigment was confirmed. These studies indicate that the oxidative browning seen in these defective cheeses is most likely due to t yrosinase production by these micro-organisms.