Researchers at UNSW Sydney, led by A/Prof. Rona Chandrawati, have developed a chemical sensor that changes colour as the milk spoils. This is based on the fact that milk becomes more acidic as bacteria convert the lactose to lactic acid.
The sensor is made from a combination of a polydiacetylene polymer and zinc oxide (PDA/ZnO). By altering the chemistry of the sensing molecules used, the team has been able to adjust the points where the colour changes so that it works over the precise acidity range needed to see the change from fresh milk to spoiled milk. Fresh milk shows blue, drink soon milk shows up as purple and off milk as pink.
The sensitivity of the sensor can be tuned by selecting different lengths of the basic building blocks that go together to make up the polymer from which it is composed. Designs constructed from shorter units undergo a colour change at a lower lactic acid concentration. Two new methods to tune the sensitivity of the sensor to lactic acid were explored. A system constructed from 5,7-hexadecadiynoic acid (HDDA) and ZnO were found to be best at discriminating between fresh (pH 6.8–6.0), spoiling (pH 6.0–4.5), and spoiled milk (pH 4.5–4.0) by a respective blue to purple to red colour change.
In other previous studies, the colour stability of the PDA/ZnO sensor in food was seen to be unstable. The researchers’ new method stabilises the nanocomposite by pre-exposure to the food matrix prior to locking together the structure of the composite.