3.8. Potential application of the TTI (Activation test in milk)
The pH of fresh milk is slightly acidic, ranging between 6.6 and 6.8, with an average of 6.7 at 20 °C or 6.6 at 25 °C. When milk is at a temperature that is above the maximum described for as ideal for cooling (7 °C), it begins to decompose, which changes the medium's pH, reducing the stability of the milk. Many different factors, such as microbial contamination and subsequent accumulation of lactic acid due to microbial metabolism, can cause the pH of milk to lower. Therefore, milk spoilage can be detected and monitored through a colourimetric pH monitoring system such as the TTI developed in this work.
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.
Table 4 shows the average values and standard deviations of colour parameters L* deviations, a*, b*, and the total colour variation (ΔE) on the CIELab scale of triplicate samples of the TTI film according to activation by contact with pasteurized milk that was kept at a temperature above the cooling temperature for time intervals ranging from 0 to 4 days