Canned and frozen foods tend to be seen as less nutritious than fresh versions — but that’s not always the case.
According to a recent Which? report, frozen and canned foods may contain more nutrients, as well as being less wasteful since they don’t go off before you’ve had a chance to use them.
‘Though, in theory, fruit and veg fresh from the field are the most nutritious, as the vitamins and minerals are all intact (they decline naturally over time), this might not be the case after produce has been transported to the store, sat on the supermarket shelf for a while and then languished at the back of your fridge for a week,’ says Helen Bond, a dietitian based in Derbyshire.
‘In fact, many foods that are frozen contain just as many nutrients as fresh produce because they are frozen quickly so nutrients are retained.’
Strawberries are best frozen as all the vitamin C from the fresh fruit is retained
And although the canning process — which involves heat-sterilising food in a sealed container to kill micro-organisms — can reduce some nutrients that are damaged by heat, others stay stable until you open it.
A 2015 study in the journal Nutrients found that people who consumed more than six canned food items a week had higher amounts of 17 essential nutrients, including potassium (good for maintaining healthy blood pressure), calcium (for strong bones) and fibre (needed for good digestive health) than those who ate fewer than two canned foods a week.
So which option is really the best? We asked Helen Bond to assess the nutritional content of common foods that can be consumed fresh, tinned or frozen — and we then picked a winner.
Strawberries, canned - halves the vitamin C content
As well as counting as one of your five-a-day, seven (80g) fresh strawberries supply more than half (57 per cent) of an adult’s daily vitamin C, needed for healthy skin and immune system, and a quarter of the daily vitamin B9 (folate), which helps make healthy blood and reduces tiredness. But strawberries lose 30 per cent of their vitamin C within three days of harvest, according to research published in 2015 by the University of California. Check the strawberries are firm and glossy as this is a sign they are higher in vitamins.
Nine canned strawberries count as one of your five-a-day because they tend to be smaller than fresh. Also the canning process roughly halves the vitamin C content and slashes levels of folate by 90 per cent as these compounds get damaged by heat. The main issue with canned strawberries, though, is the syrup most are kept in — a typical half-can serving eaten with the syrup contains seven teaspoons of sugar (some comes naturally from the strawberries but the majority is added) — a third of an adult’s daily limit, which can contribute to tooth decay and weight gain.
While frozen strawberries win due to their preservation of vitamin C
Frozen - WINNER
All the vitamin C of the freshly harvested fruit is retained because the strawberries are frozen at source — and the nutrients are intact three months later, according to the 2015 University of California study. Freezing may also increase the amount of anthocyanins — plant pigments that give the fruit its red appearance and have been linked with healthier blood vessels. Consume quickly once thawed and don’t miss the nutrient-rich juice that drips out.
Fresh - WINNER
Fresh, skin-on boiled new potatoes contain 2.2g fibre (for gut health) per 120g portion (about three new potatoes), roughly a seventh of your daily vitamin B1— important for releasing energy from food and maintaining a healthy nervous system — and 13 per cent of your daily folate, important for psychological wellbeing.
This portion has a reasonable amount of vitamin C, 10 per cent of the RDA needed to maintain a healthy skin and immune system.
Fresh, skin-on boiled new potatoes win over canned and frozen as they offer roughly a seventh of your daily vitamin B1
Canned potatoes have a lower glycaemic index
These Asda frozen potatoes have 0.5g salt per 100g
Canned potatoes have a lower glycaemic index than fresh, meaning they don’t raise blood sugar levels as much, which is better for people with type 2 diabetes.
This happens as cooking then cooling the potatoes during the canning process increases levels of resistant starch, which isn’t broken down into sugar.
However, levels of B vitamins are much lower — a 120g portion has just 2 per cent of our daily vitamin B1, and folate is reduced by around 50 per cent of fresh food levels. Fibre levels are also lower, as the potatoes are peeled.
These Asda frozen potatoes have 0.5g salt per 100g — less than 10 per cent of the daily limit, but still more than in unsalted, fresh boiled spuds. As with canning, freezing creates more resistant starch in potatoes, so they raise blood sugar less than fresh ones.
You might lose more vitamins than with fresh as prepared this way, the potatoes are heated twice.
An 80g portion (about three tablespoons) of boiled peas has around 16 per cent of your daily vitamin C and half your daily vitamin B1 (thiamin), needed for a healthy nervous system. But these figures are for recently harvested peas, not those that have been hanging round for a few days. One analysis showed vitamin C levels drop by 15 per cent seven days after picking, due to natural declines after produce is cut (which can be hastened by light or being kept out of the fridge). So if you pick them yourself, consume on the day. If buying, softening or wilting of fresh produce indicates nutrients in decline.
Canned peas offer more iron but contain very little to none vitamin C
There's more iron in canned peas than frozen or fresh ones, with three tablespoons (80g) providing 11 per cent of your daily recommendation, which helps fight fatigue. Canning pretty much wipes out all the vitamin C in the peas, but folate levels are very similar to fresh boiled peas — a tenth of your daily needs is found in an 80g portion. It is thought that peas used for canning may be more mature than those that are picked to be frozen, which could explain the differences in iron, folate and vitamin C levels.
Frozen - DRAW WITH FRESH
Frozen peas are blanched and frozen within hours of harvesting. The blanching process — plunging the peas into boiling water to inactivate the enzymes that could cause spoilage — affects vitamin C a little, but levels are still high and stay that way. Levels of folate are marginally higher than fresh or canned, with an 80g serving providing 12 per cent of your daily needs, versus 11 per cent in cooked fresh peas. An 80g serving of fresh, canned or frozen peas counts as one of your five-a-day.
Frozen peas are blanched and frozen within hours of harvesting keeping vitamin C levels high
Fresh spinach is a good source of folate, with almost half your daily amount in an 80g raw portion (80g is the amount needed to count as one of your five-a-day), though this drops by nearly 30 per cent if boiled. A boiled portion also supplies about 11 per cent of your daily vitamin E, which protects cells from damage; 16 per cent of your bone-building calcium and 9 per cent of your daily iron.
Levels of vitamin C in spinach can drop dramatically in the fridge — by 75 per cent in seven days, according to research.
Putting them in the salad drawer, where humidity is higher, is best for preserving vitamin C, or store in an airtight container.
Levels of vitamin C in fresh spinach can drop dramatically in the fridge
Canned spinach, which works well stirred through pasta, has similar levels of nutrients such as iron and vitamin A, important for immunity and eyesight, and vitamin E, as fresh cooked leaves.
Vitamin C levels can be higher in canned than in fresh, boiled spinach
However, vitamin C levels can be higher in canned than in fresh, boiled spinach, because there’s less water for it to leach away into the can.
Canning cuts the folate levels in raw spinach because this vitamin is sensitive to heat — though folate is also depleted when fresh spinach is cooked. An 80g portion has 29 per cent less calcium than fresh raw, but still supplies levels similar to half a glass of milk.
Frozen - WINNER
Many of the nutrients of fresh spinach are preserved when the leaves are fast-frozen at source. Vitamin C levels are higher in frozen spinach than when it is stored at 4c (refrigerator temperature) for more than ten days. You’ll get a bit more vitamin A from frozen, too — an 80g cooked serving provides your daily requirement, compared to 64 per cent from fresh.
Frozen spinach is lower in folate than freshly harvested spinach — unless the latter has been left hanging about for days.
Cook direct from frozen to reduce nutrient losses.
Frozen spinach is lower in folate than freshly harvested spinach
Portion of fresh mushrooms provides 15 per cent of your daily vitamin B2
The B vitamins in mushrooms are reduced by the canning process
Fresh - WINNER
An 80g portion of fresh, raw mushrooms provides 15 per cent of your daily vitamin B2, and 12 per cent of your daily vitamin B3 — which are both important for releasing energy from the food we eat and for maintaining healthy skin. This serving also supplies almost a quarter of your daily selenium, an antioxidant mineral that’s needed for a healthy immune system and thyroid function.
The B vitamins in mushrooms are reduced by the canning process — some quite considerably. For example, while vitamin B2 is 7 per cent lower in canned mushrooms,