Melbourne, Aug 21 (IANS): The solid grape waste left over from wine-making could be used to prepare bio-fuel, researchers say.

Global wine production leaves an estimated 13 million tonnes of grape marc (the leftover skins, stalks and seeds from wine-making) waste each year, and it is generally disposed of at a cost to the winery.

Researchers from the University of Adelaide showed that up to 400 litres of bio-ethanol could be produced by fermentation of a tonne of grape marc.

"This is a potentially economic use for what is largely a waste product," said Rachel Burton.

Researchers analysed the composition of grape marc from two grape varieties - cabernet sauvignon and sauvignon blanc.

She also investigated pre-treatment of the grape marc with acid and enzymes.

They found that the majority of the carbohydrates found in grape marc could be converted directly to ethanol through fermentation with a yield of up to 270 litres per tonne of grape marc.

The leftover product was suitable for use as an animal feed or fertiliser.

Ethanol yields could be increased by pre-treatment with acid and enzymes up to 400 litres a tonne.

"Using plant biomass for the production of liquid bio-fuels can be difficult because of it structurally complex nature that is not always easily broken down," said co-researcher Kendall Corbin.

"Grape marc is readily available, can be sourced cheaply and is rich in the type of carbohydrates that are easily fermented," Corbin said.

The findings were published in the journal Bioresource Technology.