Hamburgers made from horse meat?

The Tesco burgers that contained up to 29 percent equine DNA were likely to have been made with high-protein powders derived from horse rather than fresh meat, the Guardian has been told.

The main focus of efforts to trace the source of adulteration in the Tesco economy burgers has now shifted from the meat itself to additives used in the manufacturing process. The Irish processors ABP have pointed the finger at suppliers of the "beef ingredient products" it uses to make cheap burgers. The Tesco burgers were only 63 percent meat and 37 percent of other ingredients.

Economy burgers are typically bulked out with additive mixes of concentrated proteins extracted from animal carcasses and offcuts. Industry sources said the 29 percent horse DNA was more likely to have originated with these high-protein powders from rendered horses rather than any fresh horse meat. ABP declined to comment on its ingredients or on the companies it uses for additive mixes but they are believed to be in the Netherlands and Spain. The processor said it had stopped work at its Silvercrest Foods plant in Co Monaghan, Ireland, until further notice.

Tesco said it could not comment on the source of the horse DNA while it was investigating with its supplier and the authorities.

All the major retailers have been given until Friday afternoon to present to the Food Standards Agency (FSA) a full list of the sources of meat and other ingredients used in their burgers. The FSA is negotiating with local authorities to test samples of beef products from retailers around the country for the presence of DNA from other species, including horse and pig, because it does not have the capacity to sample and test nationwide. However, some local authorities have had to cut their budgets for food standards testing drastically as their finances have been squeezed. The FSA is expected to announce it will make some DNA checks on meat.

The Food Safety Authority of Ireland earlier this week said it had found significant contamination and adulteration of beefburgers with horse and pig in supplies sold in Ireland and the U.K.

On Thursday MPs criticized Tesco in the Commons for not checking its meat supplies and asked why the contamination was spotted in Ireland and not by U.K. watchdogs.