After more than two weeks sitting at room temperature in a plastic container, a McDonald's cheeseburger did not grow mouldy despite cheeseburgers from competing brands doing so, an informal Herald test has found.

The Herald bought three cheeseburgers each from McDonald's, Burger King and Carl's Jr. Once they were cooled, one of each group was placed in open air, one in an airtight container at room temperature and one in an airtight container in a fridge.

After 15 days, the condition of the burgers was compared (scroll down to see pics of the results).

The group with the most variation was those placed in airtight containers at room temperature in the Herald newsroom.

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All of these were soft and moist at the end of the informal comparison, but while the Burger King sample erupted into a forest of mould and the Carl's Jr burger had medium-sized spots of mould on the bun, the McDonald's one looked unchanged.

The group of burgers placed in the fridge had no mould, although small white specks were starting to form on the bottom of the Carl's Jr and Burger King samples. The burgers placed in open-air conditions dried out and went completely hard. The Burger King and Carl's Jr ones did not get mould on the outside, but mould was visible on the Carl's Jr burger meat when it was cut open.

A small spot of mould grew on the bottom of the McDonald's burger where some condensation had formed between the bun and the plastic lid it was sitting on.

A McDonald's spokesman declined an opportunity to come and see the Herald's informal test, but said: "Without understanding the conditions in which the New Zealand Herald carried out its experiment, there could be a number of reasons why the level of mould varies between the cheeseburgers tested."

He said the company's bun supplier used a unique process that "enhances the bun's resistance to mould".

"It is also possible that, due to McDonald's and our suppliers' strict food safety standards, at no point during the baking and shipping process did mould spores come into contact with the cheeseburgers.

"For mould to grow, spores need to have been present at some point." He said there were three preservatives used in cheeseburgers.

"Preservative (262), commonly used in breads, is used in our buns; preservative (200), commonly used in cheeses to prevent mould growth is used in our signature cheese; and preservative (211) is used in our pickles." The meat patties were 100 per cent beef and "as they are fully cooked they would be more likely to dry out than mould". Burger King and Carl's Jr did not respond to requests for comment.

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