According to new research by Australian consumer rights group Choice magazine, frozen produce is often more nutritious than fresh due to the long periods for which fresh fruit can have been stored.

The report found that apples can be up to a year old when they are purchased and grapes can have been stored for two months.

The report found that fresh produce will often have been stored and transported chilled and in a climate-controlled atmosphere, having been sprayed with fungicides to extend the seasonal availability of fresh produce.

"Most fruit and veg can be stored for at least a few weeks if the conditions are right. But it's clearly not truly fresh," the report's authors wrote.

The nutrient content of fresh vegetables decreases during storage and the flavour deteriorates, Choice said.

In contrast, the consumer group noted, frozen vegetables retain most of their nutrients. For example, choice said, frozen peas typically have 60% more carotene than fresh peas, although the vitamin C content is comparable. Frozen vegetables "can be more nutritious than 'fresh' vegetables that have been transported over long distances," the research report noted.

Canned vegetables and fruit also retain most of their original nutrients. They have less vitamin C than fresh, but the levels of dietary fibre, carotene and folate, for example, aren't much affected by the canning process. However, Choice warned: "Watch out for added salt in canned vegetables - look for low-salt or salt-free brands."