Retailers will this week be asking searching question about the security of their supply chains following the recall of more than 50 ready meal and potato salad lines.

Tesco and Sainsbury’s initially pulled about a dozen potato salads over fears they might contain pieces of metal but the situation escalated later in the week with the recall of 45 ready meals and vegetable dishes.

Sainsbury’s has been hit hardest, pulling 48 products and revealing a “very small piece of metal” had been discovered in a pack of its Be Good To Yourself Fish Pie. Tesco has recalled six lines, and Waitrose and Morrisons one each.

The FSA said initial investigations suggested a piece of equipment broke up on the manufacturing line, resulting in fragments of metal going into a batch of potatoes used in the ready meals.

The FSA has not named any companies, but potato processor Swancote Foods issued a statement saying it was working with its customers to investigate a “potential contamination issue”. “Swancote Foods is committed to maintaining the highest standards of safety and traceability,” it added.

Food safety experts said metal detectors would have been used throughout the supply chain (Swancote itself announced six months ago it had replaced detectors as part of a £2.5m investment), so the contamination raised urgent questions.

“I can’t think of any large food manufacturer that doesn’t have metal detectors, so you have to ask why they didn’t work,” said Professor Chris Elliott - who led a government inquiry into food supply chains following Horsegate.

His views were echoed by Brian Smith, co-founder of consultants Booth Smith Food Technology, who said he was “amazed” the metal wasn’t detected by equipment at the potato processor or later in the production process. “There has clearly been a breakdown somewhere in the chain.”

Limitations of metal detectors

Metal detection experts say it is difficult to totally eliminate the risk of metal contamination, particularly in the case of stainless steel, a core component in a lot of food manufacturing equipment.

Stainless steel can be harder to detect than ferrous metals such as steel or non-ferrous metals such as copper. In a wet food such as mashed potato, a sphere of stainless steel three or four times larger than a sphere of ferrous metal would be required to give a similar signal to detection equipment.

“There are limitations to how small a piece of metal can go before it can be detected - based on factors including type of contamination, size and shape of contamination and quantity and density of product,” said Phil Brown, sales director at metal detector supplier Fortress Technology.