The 1.5cm steel drill swarf was found inside an onion ring.

The 1.5cm steel drill swarf was found inside an onion ring.

Hungry Jack's "doesn't know" how a piece of metal become lodged inside a onion ring sold at one of the fast food giant's Perth stores, according to its lawyers.

But the company has pleaded not guilty to selling unsuitable food over an incident earlier this year, arguing reasonable steps were taken to prevent food contamination.

"We don't know how this happened, despite all the systems being in place, despite all the investigations done, we don't know how it happened," defence lawyer Hylton Quail told the court.

But he said Hungry Jack's had taken precautions to maintain the integrity of its food and store.

"The fact that this onion ring was contaminated - which we've admitted from the start - doesn't make Hungry Jack's guilty," Mr Quail told the court.

"As we know these things happen.

"Hungry Jack's has a number of systems in place which are designed to protect food safety."

Hungry Jack's was charged after Perth woman Melissa Bayle in February chocked on an onion ring from a Morley store and spat out the piece of metal.

Mrs Bayle reported the matter to the Bayswater Council who sent health officers to inspect the premises and sent the piece of metal off to be analysed.

A microscopic examination revealed the 1.5cm piece of metal was a steel drill swarf, the Perth Magistrates Court heard during Hungry Jack's trial this week.

Prosecutors had alleged the metal came into contact with the onion ring due to maintenance work at the store a day earlier.

"At store level on the evidence it is not the case that due diligence has been shown," prosecutor David Nadebaum told the court.

Mr Quail argued more testing should have been done to establish the origin of the metal, which he said could have become lodged within the ring at the manufacturer’s level in New Zealand.

"The prosecution are really unable to prove where this steel swarf came from," he told the court.

Hungry Jack's faces a maximum fine of $200,000 if convicted, the court heard.

Perth Magistrate Elaine Campione is expected to hand down her decision on 21 December.