South African health authorities have stepped up random tests on meat and other foods in supermarkets after a study showed that donkey, goat and water buffalo have been added as unlabelled ingredients in meat products.



The investigation started last month in response to a scandal in Europe where horse meat was labelled as beef, health department spokesman Popo Maja said.



It took on more urgency after University of Stellenbosch researchers said this week that unconventional and unlabelled ingredients were found in nearly two thirds of South African hamburgers and sausages tested.



"Investigations in the form of random tests are under way to ensure everybody complies with the food labelling regulations," Maja told Reuters.



Shares in major South African retailers fell on worries that an industry-wide investigation could trigger product recalls.



"Retailers have been under pressure recently because of valuation issues but the meat scandal is the reason for today's sell-off," said Paul Chakaduka, a trader at Johannesburg-based Global Trader.



Major food retailers Shoprite, Pick n Pay and Woolworths said they believed all of the meat for sale at their stores was properly labelled.



"We do not believe that our meat products are implicated in this study," said Woolworths.



Shoprite, which imports some pork cuts and oxtail from Germany, Argentina and Australia, said it would penalise any supplier should any of their products be implicated in the university's study.



"We do not believe that any of our suppliers, who are reputable companies, would transgress food standards and labelling regulations," said Whitey Basson, the company's chief executive officer.



The European horse meat scandal led to recalls of ready meals and damaged confidence in the continent's complex food industry. It also raised concerns worldwide over the risks to human health from a complex and nebulous meat supply chain.