Designs of sensors for smart food packaging

Abstract

The Food and Agriculture Organization of the United Nations estimates that one-third of food produced (1.3 billion tons annually) is lost or wasted along the food supply chain despite being still fit for human consumption. At the consumer level, misunderstanding of how expiry dates relate to food quality or safety is one of the main reasons leading to unnecessary food waste and also presenting food safety risks. The use-by date is solely used as a guide to indicate peak quality, and in many cases products may be safely consumed past their use-by date. Conversely, for products that have not passed their use-by date, food mishandling such as temperature abuse conditions or contamination can lead to foodborne illnesses. Therefore, an effective indicator for spoilage could eliminate consumer confusion and reduce food waste. In light of this, the present study aims to develop different sensors that can detect meat spoilage at low temperatures, as well as detect the presence of ethylene, which has applications in the monitoring of fruit ripening. During the spoilage of high-protein foods such as meat and fish, several gases, especially ammonia, are released due to the decomposition activity of bacteria. The presence of ammonia in a food package can thus imply that the food has been spoiled. In this study, 10,12-Pentacosadiynoic acid (PCDA) monomers were used as sensing material to detect the presence of ammonia. To render the sensor more practical, chitosan and cellulose nanocrystals (CNC) were used to form a flexible film, like a sticker. By fabricating PCDA/CNC/chitosan composite, the sensor was able to detect ammonia gas at low temperatures, such as in fridge (4 degrees Celsius) and freezer (–20 degrees Celsius) temperatures. By fabricating a PCDA/CNC/chitosan composite, the sensor could successfully detect ammonia gas at low temperatures. The sensor could also detect the spoilage of meat samples that were stored in fridge and freezer temperatures. For some climacteric fruits (e.g., avocado and kiwi), the ripe and unripe stages are barely discernible. People usually check the firmness of a fruit to determine its real-time ripening stage, which generally corresponds to the rate of ripening. To this end, colorimetric sensors are particularly useful. In this study, PCDA was functionalised with the Lawesson reagent, which replaced carboxylic headgroups with the thiol (-SH) headgroups of the PCDA. Chitosan and CNCs were used for the same purposes as the sensor that was developed for detecting ammonia. The newly developed PDA–SH/PDA/CNC/chitosan composite could detect the presence of ethylene through the Michael addition reaction. The sensor’s sensing capacity can be extended so that it is applicable in different environments that require the detection of ethylene. With the same detection aim, PCDA can be further functionalized with N-heterocyclic ligands such as 2,2′-Dipyridylamine (DP). DP can form complexes with Cu+, which has great affinity with ethylene. Due to ethylene–Cu+ binding, the Cu+/PDA- DP/PDA sensing solution changed colour from blue to red when exposed to ethylene. The sensor exhibited high specificity towards ethylene rather than the common interfering gases including carbon dioxide and nitrogen. These results can imply that a sensing material can be diversely functionalised for the same sensing purpose. The sensor capacity can be further extended by incorporating different polymers, such as the two studies mentioned above. Overall, the developed sensing platforms demonstrated great sensitivity and specificity to food quality and safety markers such as ethylene and ammonia. The multi-sensing platforms provide different sensing options and could be further functionally extended for use in different sensing applications.