Are fresh vegetables healthier than frozen ones?

by Jennifer Bowden / 13 September, 2017
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Shiny, crisp produce is surely better than the packaged frozen variety –  isn’t it?

QUESTION: In a previous column, Teddy Tahu Rhodes talked about frozen veges being as healthy as fresh ones. Can you elaborate?

ANSWERFrozen fruit and vegetables are a convenient alternative to fresh produce; they’re quick and easy to prepare and often significantly cheaper. But those benefits aside, it can be hard to accept that frozen veges on the dinner plate and frozen berries in the morning smoothie pack the same nutritional punch as fresh produce.

Fruit and vegetables are important sources of vitamins, minerals, dietary fibre and other health-promoting nutrients. These include: folate, found in green, leafy vegetables; provitamin A (carotenoids), in yellow, orange, red and green vegetables such as corn and carrots; vitamin C in berries; and potassium in many vegetables and fruit.

These vitamins and minerals are required for thousands of chemical reactions within the body that reduce the risk of non-communicable diseases such as heart disease, stroke and some cancers.

Ideally, we would all eat freshly harvested fruit and vegetables. However, most produce is grown kilometres from consumers.

Cellular respiration and oxidation can cause nutrient losses during packaging and shipping. To stop spoilage and destroy pathogens, vegetables that aren’t destined to be sold fresh typically go through two stages before packaging: blanching, then freezing.

Blanching involves briefly immersing the produce in hot water to deactivate enzymes that would otherwise cause undesirable changes to the flavour, colour, texture and nutrient content. In the process, heat-sensitive vitamins such as folic acid, thiamine and vitamin C can be damaged. Some manufacturers use “steam blanching” to reduce vitamin loss.

So how do frozen fruit and veges stack up against fresh produce? Research suggests they compare very well.

In a 2015 study published in the Journal of Agricultural and Food Chemistry, US researchers assessed the levels of vitamin C, riboflavin (vitamin B2), alpha-tocopherol (vitamin E) and beta-carotene (converted by the body into vitamin A) in fresh and frozen corn, carrots, broccoli, spinach, peas, green beans, strawberries and blueberries.

Samples of each fruit and vegetable were harvested, processed and then analysed for nutrient content within 24 hours of harvest; again at three and 10 days for fresh samples; and at 10 and 90 days for the frozen samples. The process involved blanching then freezing for the vegetables, while berries were simply frozen.

Vitamin C levels were less degraded in the frozen-stored samples than the fresh-stored samples. None of the eight foods showed vitamin C losses during frozen storage; in fact, vitamin C levels were higher in the frozen samples of corn, green beans and blueberries than the fresh samples.

Riboflavin, too, was well retained in the frozen samples, with carrots, corn, broccoli, blueberries and green beans all retaining similar levels to fresh samples. Only frozen peas lost riboflavin during storage.

Vitamin E levels benefited most from the blanching-freezing process, the researchers found. Frozen peas, carrots and corn had significantly more alpha-tocopherol than fresh samples; the remaining foods had similar levels whether fresh or frozen.

Beta-carotene levels were lower in frozen peas, carrots and spinach, and there were no significant differences in levels between samples of green beans and broccoli.

Overall, the vitamin content of frozen produce was comparable and occasionally higher than that of fresh produce. The exception was beta-carotene, whose levels fell significantly in frozen peas, carrots and spinach.

Clearly, frozen fruits and vegetables are a convenient and nutritious option that we can enjoy as part of a healthy, balanced diet.

Boil those imported berries

The Ministry for Primary Industries recommends boiling frozen imported berries at 85°C for at least one minute. Frozen berries have been linked to a hepatitis A outbreak in New Zealand and other food-borne illness outbreaks in Sweden, Australia, the US and Ireland.

This article was first published in the August 19, 2017 issue of the New Zealand Listener.

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