Greek yogurt's popularity has extended far beyond its original homeland and is now enjoyed by growing numbers of people all over the world.

Greek yogurt production in the state of New York, for example, has nearly tripled from 106,000 to 315,000 tons between 2007 and 2012.

However, producing the creamy delicacy generates huge volumes of food waste in the form of liquid whey.

Now a team of researchers from the United States and Germany have developed a technique which harnesses the abilities of simple bacteria to turn the leftover sugar and acids into a product that can be used in biofuels or animal feed.

The new method is described in a study published in the energy science journal Joule.

"To be sustainable, you want to convert waste streams where they are made, and upstate New York is where the cows are, where the dairy farmers are, and where the Greek yogurt craze began in the United States," said Lars Angenent, senior author of the study and professor at Cornell University and the University of Tübingen.

"That's a lot of acid whey that has to be driven to faraway locations for land application, but we want to produce valuable chemicals from it instead."

The waste whey from Greek yogurt mostly consists of lactose (a type of sugar found in milk), fructose (another sugar, found in fruit) and lactic acid (a product of fermentation). The team use bacteria in a bioreactor to turn the discarded liquid into an extract containing two useful compounds – caproic acid and caprylic acid.

Large carbon footprint

With further processing, these compounds can be turned into a 'drop-in' biofuel - that can be mixed into jet fuel, for example. Alternatively, they can be also used as a kind of 'green antibiotic' that could be fed to livestock. Both these options have economic, environmental and social advantages.

"The agricultural market might seem smaller, but it has a very large carbon footprint and turning acid whey into a feedstock that animals can eat is an important example of the closed cycles that we need in a sustainable society," Angenent said.

"The fuel market, of course, operates at a lower price but its demand is virtually unlimited."

The researchers hope to improve on their methods and boost capacity.

"There is much more that can be done to optimise the extraction process and to scale up in an economical way," said Angenent.

"We can also learn more about the nature of the microbiomes and the biology involved and start investigating whether this technology can be translated to other waste streams."