After coming into contact with the milk, a visible colour change
was detected in the TTI samples (Fig. 8), which at pH 6.7 (unspoiled
milk) showed a dark grey colour that, as the pH decreased from 6.7
to 5.0, gradually became clearer. At pH 4.6 (spoiled milk) the TTI
changed to a dark pink colour, clearly indicating milk spoilage.