Smart labels keep you away from spoiled food

Journal of China Science and Technology Journalist Zhao Guangli

Foods, beverages, cosmetics... Are these products thrown or used in the last few days of shelf life? This is a problem.

Some nutritionists point out that the definition of shelf life is [the quality guarantee period of products under normal conditions", which means that during the warranty period, the merchants are responsible for the quality of the products, but the shelf life does not equal to the last available time. For example, rice, coffee and other foods are still edible after the expiration date.

Yan Chunhua, an academician of the Chinese Academy of Sciences and a professor of the School of Chemistry and Molecular Engineering of Peking University, did not think so: [In most cases, the shelf life of the product packaging is stated in the following: 'This product is always under proper conditions during transportation, storage and sales ( If the temperature and humidity are appropriate, the package is not damaged, the product is not contaminated, etc.) Presumed under the premise of preservation, since the product inevitably undergoes changes in temperature during these processes, especially for medical supplies such as vaccines. The shelf life indicated on the product packaging does not have sufficient credibility, which may pose a threat to public health."

The two points of view are diametrically opposed, but there is one point where the two sides reached a consensus: The shelf life is not the only criterion for determining whether food is deteriorating. This in turn makes friends who are entangled in [throw or use" even more confused: how do we judge whether it is deteriorating?

The research on "color-changing smart labels" led by Yan Chunhua et al. proposes a solution.

This is a [color-changing smart label" that purportedly [indicates the real-time quality of perishable packaging products." Researchers cleverly use the common color-changing reactions in chemistry to indicate the quality of packaged products in real time.

What did the researchers do about the label?

Yan Chunhua took E. coli in milk as an example. According to a reporter from the China Science Journal, in the normal temperature range (0°C to 35°C), the reproduction rate of E. coli accelerated with increasing temperature; similarly, large The rate of most chemical reactions also increases with temperature. If a specific chemical reaction can be found and its reaction rate at each temperature is consistent with the propagation rate of E. coli, the two are mathematically equivalent.

"Thus, if a smart label is prepared with this chemical reaction, the label is placed on the packaging box when the perishable product such as milk is shipped - this guarantees that the two will experience the same temperature history and can use the chemical reaction in the label. Tracking, simulating and instructing the quality change process of the product," said Yan Chunhua, adding that in order to facilitate the reading of relevant information, chemical reactions that change color over time should be used.

In other words, the design of the smart tag is based on the fact that the process of degrading the milk and the process of changing the color of the label are synchronized at any temperature.

In search of label materials, the researchers selected the "Gold Nanorods Epitaxial Silver Shell Growth" chemical reaction. According to reports, the gold nanorod aqueous solution is red, and after the silver source (silver nitrate) and the reducing agent (vitamin c) are introduced into it, the resulting elemental silver is deposited on the surface of the gold nanorods to form a "core-shell structure" ( Nano-materials - such as silver particles, nano-scale ordered assembly structures formed by chemical bonds or other forces that coat another nano-material. As the "silver shell" thickness increases, the solution changes from red to orange, yellow, green, and even blue and purple.

Yan Chunhua told reporters that at room temperature, the time required for this chemical reaction process can be adjusted within minutes to months (or even longer). In addition, the researchers also found a simple and easy way to adjust the sensitivity of the reaction to temperature changes (ie, the apparent activation energy, the apparent activation energy can be adjusted in the range of 50 ~ 190kj/mol or even wider) .

[The range of these parameters covers the metamorphic kinetic parameters of the vast majority of perishable products, and thus can track, simulate and indicate the real-time quality of the vast majority of perishable products." Yan Chunhua said that researchers have used E. coli as an example. Synchronization of food spoilage and label discoloration has been achieved at multiple temperatures. "This is expected to be achieved at any time by the color of the label, which can be used to reverse product quality without even having to open the product package."

Yan Chunhua and associate professor Sun Jingdong's research group completed the study in collaboration with the research group of the Faculty Fellow Chen Xing of the Faculty and Wang Jianfang's team of the Physics Department of the Chinese University of Hong Kong. Once this study was reported at the 247th American Chemical Society Spring Conference and Expo 2014, it received extensive attention from academia, industry, the media, and the public. According to reports, although the follow-up work of the research is still in progress so far, there have been a number of well-known foreign companies that have sent in letters to consult relevant results and discuss industrial cooperation.

The reporter learned that there have been some technologies that have been dedicated to solving the problem of monitoring the quality of perishable foods, such as using a data logger to track and record the temperature changes experienced by the product. These technologies are often costly, and it is difficult to completely cover the entire process of the product from the manufacturer to the consumer, and it is difficult for consumers to read the information recorded therein intuitively.

In comparison, the "color-changing smart label" is more reliable and safer, and its raw materials have no toxic or side effects. Yan Chunhua is full of hope for its productization: [This label is expected to expand the current product quality monitoring range from 'manufacturer to consumer', and advance monitoring accuracy to a single package level with high potential. Value."