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The rapid development of cold-chain transportation necessitates consumers to present high requirements on safety and freshness of fresh food in recent years. The quality and taste of fresh food can be monitored and controlled through the intelligent packaging technologies and new food packaging materials such as time-temperature indicators (TTIs), Radio Frequency Identification (RFID), biological composites and polymer nanocomposites. Based on different packaging materials, indicators and sensors are employed in food packaging for real-time detection of information about freshness, temperature, microbiological, and shelf life of products in the supply chain. Wide varieties of packaging materials are suitable for providing intelligent and smart properties for food packaging, such as oxygen scavenging capability, antimicrobial activity, and recording the thermal history. Due to the special properties of prepared materials, TTIs are used to point out the remaining shelf life of perishable products throughout the supply chain. Compared with others, they have the advantages of low cost, small size and convenient indication. Additionally, the TTIs can effectively solve food quality and safety problems caused by temperature fluctuation in supply chain. Since the irreversible color change of TTIs, the food safety situation would be shown intuitively. Currently, the TTIs were widely used in application of food packaging by providing safety information. However, the application is also accompanied with some deficiencies such as the accuracy of monitoring, migration of toxic substances, stability and expensive cost etc. This review will deeply discuss the preparation of various types of TTIs based on different package indicating materials with a particular emphasis on how to improve their accuracy and stability, control the migration of toxic substances and to develop new TTIs.

Color reproduction of fluorescent full-color prints depends on many factors, such as preparation of luminescent inks, ratio of luminescent inks to each other, printing technology and so on. In order to make color expression more abundant on fluorescent full-color prints, reconstruction of fluorescence emission spectrum is particularly significant. As opposed to custom methods, principal component analysis has been applied to color science permanently. The method was applied to emission spectral reconstruction in this work and the up-conversion luminescent inks were selected. 336 samples were composed of single ink halftone at a quarter, half, 75%, and 100% surface coverages. The samples were firstly superimposed in one ink and two inks on the blank paper. Moreover, their emission spectral was measured and the procedure for principal component analysis was also performed. The emission spectral was reconstructed by using 1 nm interval from 351 nm to 748 nm. Ultimately, the accuracy of recovery spectral was evaluated through CIEDE2000 color difference evaluation. The obtained results indicated that principal component analysis can be used to reconstruct emission spectra. Besides, the method can also be used for color estimation between different printing materials.

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