60. Discoloration in New Zealand Cheddar Cheese. Muddy, Pink and Bleached Defects. I. Bacteriological Investigations

During the past two years, discoloration, owing to its prevalence in Cheddar cheese, has been the subject of much investigation by the New Zealand Dairy Division, and in the present paper are incorporated details of the observations made in this laboratory and in cheese stores in New Zealand and England, together with short reports on the same subject from English laboratories.

To avoid confusion it must be emphasised that the defects dealt with are “bleached,” “pink” and “muddy” discolorations only. The observations do not apply to “black spot” (due to lead or other metallic contamination), nor to “red spots” (due to faulty annatto), nor to “acid cut” bleaching which seems to follow the outline of the original pieces of curd. Muddy or dark discoloration occurs in white and in coloured cheese, but the bleaching and pink discoloration dealt with here are confined to coloured cheese.

150. The effect of certain metallic contaminants on the Cheddar cheese making process

Traces of certain metallic contaminants (copper, iron, lead, zinc, aluminium and manganese), when added at the rate of 3 and 7 p.p.m. to milk used for the manufacture of Cheddar cheese, cause certain flavour (and colour) defects in the matured cheeses, of which, however, no indication is given when the cheese is plugged and graded at about 14 days after manufacture.

The distribution of copper, iron, manganese and lead between curd and whey during the cheese-making process is recorded and the action of traces of various metals on the acidity development in a typical "starter" culture has been followed.

The discolorations noted for copper- and iron-contaminated cheeses are considered to be due not to the formation of their sulphides (although this is the case for lead discoloration), but to the atmospheric oxidation of a colourless metal: protein complex.