a. Address here. Queens University Belfast, School of Chemistry and Chemical Engineering, Stranmillis Road, BT95AG, UK

* corresponding author: andrew.mills@qub.ac.uk

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Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

www.rsc.org/

Novel time-temperature, 'consume-within' indicator based on gas-diffusionA. Mills*D. Hawthorne A. Graham and K. Lawrie

The novel time-temperature indicator label comprises an ammonia sensitive indicator layer film pressed onto a second film, with an ammonia-generating, adhesive layer. When separated the blue-coloured indicator film reverts back to its original (ammonia free) yellow form at a controllable, temperature dependant rate. The labels are easily made and stored.

Temperature is one of the primary controllers of the rates of food spoilage through various degradation pathways, including: microbial growth and enzyme and non-enzyme based oxidation reactions1. It is not surprising, therefore, that time-temperature indicators, TTIs, are playing an increasingly important role in food packaging, especially in the distribution chain of chilled or frozen goods1-6. A TTI shows an easily observed, time-temperate dependent change that reflects the temperature history of the temperature-sensitive product to which it is attached1,2. For example, the MonitorMark TTI3, manufactured by 3M, comprises a blue dye in a fatty ester which melts at a defined threshold temperature and then diffuses along the wick at a temperature dependant rate. This type of TTI is routinely used to monitor the thermal exposure of pallets of temperature sensitive products, including food, vaccines and drugs. In contrast, the Fresh-Check® Freshness, 'You can See' labe4l, is based on a temperature-dependant polymerisation which causes the label to darken with time and is used on individual packaged foods, to identify those that have not been refrigerated correctly and so are not fresh. Other chill or frozen chain TTI monitors include OnVu5

(notable for being UV activated), from Bizerba GmbH, and Timestrip, from Timestrip6 (like MonitorMark – a liquid diffusion based indicator). In many cases (e.g. MonitorMark and Timestrip) the TTI is activated by breaking a seal, whereas others, so called 'full-history' TTIs, like Fresh-

Check4, are always active, and so need to be made and stored below their threshold temperature. The main barrier to the widespread use of TTIs on individual packages in the chill or frozen chain is cost, with MonitorMark and Timestrip indicators typically costing > $1 ea3,6, making them usually inappropriate for use with individual commercial packages. The printable Fresh-Check® indicator is much cheaper4, but even when 1-2 M indicators are produced, their cost is > $0.05 ea.7, which, for the food packaging industry at least, is usually considered too expensive for use with individual packages. In addition, the latter labels need to be stored frozen, which adds to the cost of their production, distribution and application2,4. This communication describes an alternative TTI label, that is readily stored at ambient temperature, inexpensive to produce and easily activated and which works via a mechanism that is completely new to TTI technology. The label comprises an ammonia-sensitive indicator layer film pressed onto a second film, with an ammonia-generating, adhesive layer. The ammonia-sensitive indicator layer was created using an ink containing: 27 mg of the dye bromophenol blue, BPP, dissolved in 0.95 g of methanol and 6.4 g of ethanol. To this were added 0.5 g of the polymer, polyvinyl butyral, PVB, and 0.5 g of the plasticiser, tributyl phosphate,TBP. The BPB ink was applied onto a 45 μm thick poly(ethylene terephthalate), PET, substrate using a draw-down method and a K-bar No. 8, to give a dry, yellow-coloured film, with a thickness of ca. 8 μm, as measured by SEM. The formulation of the ammonia-sensitive indicator can be summarised as: PVB/BPB/TBP in which the components were present in the ratio: 100/2.7/100 parts-per-hundred resin, pphr, respectively. The ammonia-generating layer was made using an ink containing: 2.0 g of ammonium bicarbonate dissolved in 10 g of water, to which were added 10 g of the pressure-sensitive, water-based adhesive Tackwhite AP03 HP (50% solids; Adhesive Technical Services). This ammonia-generating adhesive was coated onto a 45 μm thick PET fim using a K bar number 5, to yield a tacky dry coat with a thickness of 5 μm and a component ratio, in pphr, of 100/40 for the formulation; Tackwhite AP03 HP/NH4HCO3.

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1

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Upon pressing the two films together to create the label, which we shall refer to as the 'charging step', the initial yellow colour of the

indicator film changes to blue, as the ammonia, generated by the gradual decomposition of the ammonia bicarbonate, i.e.

List of Figures

Figure 1: Time vs. absorbance data (at 602 nm) recorded for a 100/2.7/100 BVP/BPB/TBP TTI film in its charging stage at 22 oC. The inset diagram and photographs illustrate the UV-vis spectral changes and colour changes exhibited by the label as a function of time.

Figure 2: Time vs. absorbance data (at 602 nm) recorded for a 100/2.7/100 BVP/BPB/TBP TTI film in its activation stage at 22 oC. The inset diagram and photographs illustrate the UV-vis spectral changes and colour changes exhibited by the label as a function of time.

Figure 3: Normalised absorbance, nA, (at 602 nm) decay profiles recorded for a charged 100/2.7/10 BVP/BPB/TBP indicator film at the following temperatures (from left-to-right): 22oC, 15oC, 10oC, 7.5oC, 5oC, and 2oC. The broken horizontal line identifies the different values of t(green), when nA = 0.35, for the different temperatures.

Figure 4: Photographs of an opened package containing beef, refrigerated at 5 oC with the 'consume within' indicator after: 0, 48 and 72 h.

2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx