Preparation and Characterisation of Multi-functional Time-Temperature Indicators

In recent years, freeze-thaw indicators have become attractive in the food, pharmaceutical and medical industries in the monitoring of the quality and safety of perishable products, during cold-chain storage and distribution. Freeze-thaw indicators are capable of providing a clearly visible change if the product has been exposed to the freezing temperatures. The irreversible change would indicate that the product had suffered a form of thermal abuse, making it unsafe for consumption use and requiring removal from the market.
This project aimed to develop a freeze-thaw indicator using various technologies. Three possible options were evaluated for the preparation of freeze-thaw indicators.
i) Hydrogels, such as starch-based hydrogels, sodium alginate-based compositions and poly(vinyl alcohol) (PVA)-based compositions that were capable of providing syneresis upon freezing, could be used for the preparation of freeze-thaw indicators. The indicators gave a visible change through the staining of a paper caused by a water-soluble dye (Reactive Red 3) that was dissolved in the aqueous solution released from the hydrogels upon freezing;
ii) Aqueous droplets entrapped in the polymeric films through solvent evaporation of water-in-polymer solutions [i.e. poly(vinyl acetate) and ethyl cellulose solutions or the UV radiation-induced polymerisation of water-in-acrylate-based monomer emulsions]. The aqueous droplets were designed to break the polymeric films to trigger a visible change upon freezing;
iii) Microcapsules containing aqueous droplets were prepared by either the interfacial condensation polymerisation between 4, 4’–bis(chloromethyl)-1, 1’-biphenyl and hexamethylenediamine or by UV radiation-induced polymerisation of acrylate-based monomer (OTA 480) in emulsion systems. The microcapsules and a piece of water-sensitive paper (coated with a hydrochromic dye, bromophenol blue) was sealed in a container for the freeze-thaw testing. The water-sensitive paper changed colour from yellow to blue in contact with the aqueous solution released from microcapsules upon freezing.
In order to understand the feasibility of these options for the use in freeze-thaw indicators, freeze-thaw testing was carried out on the relevant hydrogels/compositions, polymeric films carrying aqueous droplets and microcapsules. The analysis showed that the polymer loading in the associated compositions and preparation conditions had an influence on the synersis of such compositions. The polymerisation process of films was monitored by FTIR and UV-Vis spectrophotometers. The formation of emulsions was studied using optical microscopy (ImageXpert evaluation). The morphology of the resulting polymeric films and microcapsules was characterised using scanning electron microscopy.