Investigating milk spoilage and development of time temperature indicator film for milk.

Time–temperature indicators (TTIs) are devices with a time–temperature-dependent change
to reflect food quality status and temperature history. There is increasing interest in the
application of TTIs to monitor food cold chain logistics for food safety and quality. TTIs
must be easily activated and then exhibit a reproducible time-temperature dependent change
which is easily measured, this change must be irreversible and ideally mimic or be easily
correlated to the food’s extent of deterioration and residual shelf-life. The incorporation of
TTI on plastic milk packets has the potential to replace/supplement stamped expiry. As an
effective indicator of milk quality, pH measurement method was adopted in which, the
reduction in pH with time at three different temperatures viz. 5°C, at room temperature (RT)
and 37°C. was observed. Also, sensory method such as heating the milk to check curdling
was performed simultaneously. Generally curdling of milk is considered as spoilage of milk,
though, the standard for “spoiled” milk can be subjective. Bacteria in milk produce lactic
acid as by product which leads to the decrease in pH of the milk causing curdling of milk.
Thus, the pH correlation with bacterial growth was also studied but no correlation was
observed between pH drop and bacterial growth. Once, the pH measurement was done,
activation energy for milk shelf life was determined by plotting a graph of pH against time at
three different temperatures and using Arrhenius equation. We then developed a method for
incorporating bromocresol green, a pH-sensitive colour-changing dye, into a methylcellulose
film. This film can be added to existing packaging for milk with detectable pH changes. This
TTI was activated using 0.5ml of four different concentrations of lactic acid (1.5M, 2M and
2.5M) and was used as an external TTI for detection of milk spoilage. In the vapour diffusion
of lactic acid, an irreversible colour change of a chemical chromatic indicator (from blue to
yellow) clearly and progressively occurred due to the pH reduction. The activation energy of
this TTI was also determined. When the activation (Ea) values of the TTIs and the Ea
associated with food quality losses are similar or less than 40KJ/mol, these TTIs could be
considered as good candidates to monitor food quality losses. The TTI activated using 1.5M
lactic acid was found to be applicable as milk spoilage detector at all three temperature
conditions under which milk spoilage was studied. It can be used with existing packaging for
milk with detectable time temperature changes.