Paper sensor measures food freshness

By harnessing the material properties of paper, a new sensor could help save vast amounts of food from being wasted...

Forget sell-by dates; this new food freshness sensor can instantly tell whether or not a pack of meat is still edible. 

The ‘printed electrical gas sensor’ is made of paper, costs less than a penny, and it might save some of the millions of tons of edible food we throw away every year.

Dr. Firat Güder from Imperial College London is the engineer who invented the device. He says it can even connect to a mobile phone, and he hopes it start to appear in packaging for poultry and fish.

“We have a very inexpensive sensor that performs really well. To me, it was just dead obvious that this would be a really useful technology in sensing spoilage and freshness of meat products.”

Food waste is a major problem - 30% of all food produced for human consumption is thrown away. The UK alone throws away 7 million tons of household waste a year - and an estimated 60% of that is still safe to eat.

The problem is best-before dates. They are approximations, and they often don’t reflect the reality of what’s happened to the food or how it was processed.

The printed electrical gas sensors can tell freshness more accurately by detecting ammonia - a gas which is released by meats when they decompose.

Dr. Güder said that the breakthrough was harnessing an unlikely material: paper. “Everybody used paper as just a carrier of materials. People were not exploiting the intrinsic properties of paper itself.”

Even when it looks dry, paper carries a surprising amount of water. The cellulose fibres inside paper absorb moisture from the air. Gases from the air can then dissolve in this thin layer of water.

It is then possible to detect dissolved gases - like ammonia - by measuring how well the paper conducts electricity. When ammonia dissolves, it turns into ions; and the more ions present in the thin layer of water, the better the conductivity.

Dr. Güder said that his sensors are biodegradable, non-toxic, and comparable to or better than most commercial ammonia sensors - but a fraction of the cost. “And through high-volume manufacturing we estimated that we can decrease the price by about a thousand times.”

In future the sensors could be included into “smart” food packaging, to help supermarkets reduce waste. A lot of packaging already includes electronic NFC tags, to help with inventory management. These new sensors could be integrated with the tags and connect directly to smartphones.

As well as saving plastic and food that would otherwise be thrown away, it is hypothesised that the sensors could be used to detect kidney disease by measuring the levels of ammonia in a patient’s breath.