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SUMMARY
Two strains of Penicillium roqueforti were successfully grown on sterilized whey with pH adjusted to 5.0-5.50. The green colour of the mould was observed to change to grey-green, grey-brown and brown as the reaction of the medium changed with time from acid to alkaline. These colour changes could be induced by adjusting the pH with ammonia or caustic sods.
The crowth of both strains in this medium was suppressed by salt concentrations exceeding 7%; below this salt level one strain grew better than the other.
The implications of the colour chances and salt tolerance in relationship to defects in blue-vein cheese are briefly discussed.
Introduction
Mocquot (1955) reported the use in France of a whey medium for the propagation of Pénicillium candidum for industria] purposes. The simplicity of this medium and ita nutritional similarity to cheese led the author to inquire whether it could be used for Peniciuium roqueforti and, if so, to study some of the aspects of this mould's behaviour related to its growth in cheese.
Experimental
Cultures: Two cultures were used - strain P108-1, kindly supplied by Dr. J. J. Jezeski, University of Minnesota, U.S.A., and strain PDB, isolated from a sample of blue-vein cheese imported from Denmark.
Medium; The first series of experiments was designed to show to what extent and at what pH the medium would support growth of the mould. The medium was prepared as follows: Fresh whey of pH 6.20 was divided into six lots. The pH of one lot was left unaltered, whilst the remaining: lots adjusted with lactic acid to pH 4.00, 4.50, 5.00, 5.50 and 6.00 respectively.
The whey was then distributed in 60 ml quantities into 500 ml flat-sided flasks, similar to Roux bottles. The flasks were plugged with cotton-wool and autoclaved at 16 lb./sq. inch pressure for 15 The whey proteins precipitated by this treatment were left in the medium. After cooling each flask was inoculated with 2 ml of a spore suspension of the mould, harvested from a malt agar slope. The flasks were then laid on their sides on shelves and incubated at room temperature.
Observations
Within 2 weeks both strains established a confluent green layer of growth on the surface of the medium. With both strains...