• Output: Visible Signal

Several Technologies already exist which address the problem of monitoring the thermal exposure of products in the cold chain. One particular family are Temperature Time Integrators (TTIs).⁵
The key aspect of a TTI is that they are based on a phenomenon which can act as a signal to a consumer for example, eg. a colour change. For Cell By Date we would like a visible signal that tells the consumer that as a result of the thermal history of the beef it is not fit for consumption e.g. it's been left out of the fridge for too long. To Determine the point at which ground beef is not good to eat we have to look at the biology of the spoilage process.The dominant organisms leading to the spoilage of beef depend of the beef's composition and the environmental conditions under which the beef is stored. For refrigerated packaged beef Pseudomonas spp. were dominate areobically while Lactobacillus was dominant anaerobically.¹

• Lifespan : 7 days

There also seems to be a general rule for beef that when the bacterial count reaches 10⁷ cm^-2, off odours and slime production occur and the beef is considered off. ² For controlled isothermal conditions in a laboratory environment the time taken for beef to reach this spoilage point seems to be at most 7 days.³ This implies that the shelf life of our system needs to be at least 7 days.

• Health & Safety : Non-living , non-infectious
  

Because bacteria are responsible for the spoilage of beef it is unwise to use a bacteria based device eg. by using e.coli/yeast as a chassis as this would further add to the spoilage.

• Activation Energy : 30 +/- 20 kJ/mol
  

The Gompertz's model is widely used when considering beef spoilage as it has been shown to fit growth data very well. ¹ Using the Gompertz model we can get the specific growth rate, Lag phase duration (LPD) and maximum population density (MPD) for bacterial growth at a particular constant temperature. And then using these we can determine the Activation Energy (Ea) for the beef spoilage reaction. For U2 grade Argentinian beef stored in polyethylene and SARAN PVC, the Ea ranged from 80kJ mol^-1 to 220kJ mol^-1 for a range of bacteria. ⁴

One contrary value for the Ea of the beef spoilage rxns is given by Leak ² who calculated Ea = 30kJ mol^-1. The difference between Leak's value and that of Giannuzzi ⁴ probably lies in their packaging methods, which is of importance as the project aims to target aerobically fresh gound beef.

In order for Cell by Date to act as a TTI & accurately report the spoilage rxn of beef, the activation energy of the two rxns needs to be similar. For example a difference between the two Ea's less than 20kJ mol^-1 would result in the TTI estimating the thermal history of the beef to be within 1 degree C of the actual history.⁶. For Cell by Date we would like our Activation Energy of our system to be 30 +/- 20 kJ mol^-1 to yield an accurate thermal history of the most sensitive spoilage rxn found by Leak, which is most likely for aerobically fresh beef.

• Response Time: Less that 1 Hour
  

In addition, to correctly couple the Ea of our system to that of the dominant spoilage reaction in beef, we also have to consider the response time of our sytem. Our system needs to have a rapid response time, so in other words it needs to be able to quickly switch between states eg. low output and high output. This is so our system can capture small variations of temperature in the cold chain and report them in a meangingful way. Specifically, a response time in the order of a few hours would ensure that if there are any problems in the cold chain this will arise in our system very quickly.