Sometimes I find myself standing in front of a stall for ages trying to pick the “perfect” watermelon. How do you pick your produce? Do you smell it, lightly tap it, or maybe even weigh it in your hands? It may soon get easier for clueless shoppers like me to grocery shop.
A team of researchers from Tufts University in collaboration with colleagues from Boston University, University of Illinois at Urbana-Champaign, and Princeton University have developed a sensor which can be used to monitor food quality and fruit ripeness. The sensor can be attached to a variety of foods such as on fruit skin and on cheese using water vapour, or even be immersed in liquids.
The majority of food sensors developed to date are non-disposable, rigid, flat wafers. The team’s sensor consists of gold and silk—edible and biodegradable materials, and conforms to the shape of the food. Don’t think the sensor is expensive just because it is made with gold. The amount of gold in the sensor is equivalent to the edible gold leaves or ﬂakes used on cakes and chocolates. Consumers are expected to slice off and dispose of the portion of food with the sensor.
One reason for the low cost is because it is an antenna-based passive device, basically an RFID tag which does not require power to operate.
You are probably already familiar with RFID tags used for tracking products such as library books or inventory, but they can also be used to measure remotely the physical and chemical properties of their surroundings.
The team’s RFID-like sensor consists of a gold capacitor and inductor printed on a silk film. The sensor’s response (resonant frequency) is measured remotely through a detection coil connected wirelessly to the sensor reader.
The measured resonant frequency of the sensor depends on its capacitance and inductance. The sensor can effectively measure food quality and ripeness since these chemical and physical changes vary the dielectric properties and conductivity of the sensor. The dielectric and conductivity variations change the capacitance of the capacitor, which then shifts the sensor’s resonant frequency measured.
When the sensor is attached to the skin of a banana, the resonance shifts to higher frequencies during the ripening process. As the banana ripens it softens and the sensor shape is changed as well. The ripening process also changes the dielectric properties of the banana skin. Both of these changes decrease the permittivity of the banana and shift the frequency measured.
A similar resonance shift is observed when the sensor is placed on a piece of cheese and bacterial contamination is built up overtime.
When the sensor is immersed in a plastic capful of milk, the resonance shifts to lower frequencies as the milk spoils, indicating an increase in permittivity.
Food safety is an important issue for both consumers and the food industry. The likelihood of consumers adopting this sensor for their grocery runs may depend on the availability of a portable and low-cost sensor reader (network analyzer). However, it is an economical alternative to current industrial methods used to evaluate food quality such as gas chromatography, mass spectrometry, electronic noises, and electronic tongues.
In the meantime, I’ll learn to pick the “perfect” watermelon the old fashion way—asking the savvy shoppers around me how they do it.
Tao, H., Brenckle, M., Yang, M., Zhang, J., Liu, M., Siebert, S., Averitt, R., Mannoor, M., McAlpine, M., Rogers, J., Kaplan, D., & Omenetto, F. (2012). Silk-Based Conformal, Adhesive, Edible Food Sensors Advanced Materials DOI: 10.1002/adma.201103814