Japanese food group Maruha Nichiro Holdings has launched an investigation into how a number of its products became contaminated with a pesticide, sparking a recall of 88 products in the country.

The company was forced to recall 6.3m frozen food products last week (29 December) after testing prompted by customer complaints detected the pesticide malathion, which is commonly used in agricultural production.

In an emergency press conference, Maruha Nichiro revealed malathion had been detected in a range of products such as frozen pizza. The highest concentration of malathion was found in the company's corn cream croquettes. Matuha Nichiro emphasised malathion has a low toxicity level but can cause nausea and diarrhoea.

The contaminated products were produced at one of Maruha Nichiro's factories in Japan's Gunma Prefecture. The company subsequently recalled batches of all 88 products produced at the site, 42 of which were sold into retail.

The company is now investigating how the incident occurred and announced today (3 January) that it has formed an investigation committee to look into the incident. The committee is made up of senior management and quality assurance personnel, including Maruha Nichiro group managing director Hiroshi Imamura.

Japan's Ministry of Health, Labour and Welfare said malathion was not detected at the Gunma facility. Maruha Nichiro said: "As a result, it is considered that it is not likely to have been contaminated during the manufacturing process."

The company is now looking at the "contamination route" - from raw material storage to molding and packaging. It also plans to interview all 300 employees at the site, with interviews beginning tomorrow.