| According to a nutritional comparison of fresh, frozen and tinned vegetables carried out in 2007, by the time vegetables are consumed, fresh, frozen, and tinned versions may have similar nutritional values (1). |
While nutrients like vitamin A, minerals, and fibre are stable, others, like vitamin C, react with oxygen after the vegetable is picked and change chemically so that they no longer function the same way in our bodies. This is called oxidative degradation.
The major frozen food companies claim that the majority of their vegetables are harvested, cleaned, cut, and frozen within hours.
This cuts down on nutrient loss due to short-term storage. But because the vegetables are usually blanched before the flash-freezing process, certain heat-sensitive nutrients, such as thiamin, can decrease. Long-term storage of frozen vegetables (6 to 12 months) can further diminish vitamin C because the vegetables are still exposed to oxygen, even in the freezer.
Overall, the comparison in the study above showed losses of vitamin C due to the entire freezing process ranging from 10 to 80 percent, with averages around 50 percent.
When foods are canned, they are exposed to a lot of heat. When foods are frozen, they are exposed to a short burst of heat and then cooled. When it comes to fresh, they aren’t heated at all (although they may be chilled) until they are cooked at home by the consumer.
When food is heated (and this includes cooking it), as in canning, the bacteria and enzymes that contribute to spoiling or decay are destroyed or at least slowed down. Freezing food does the same thing. These actions decrease the levels of some nutrients, but in rare instances, also simultaneously increase the levels of others. Tomatoes, for example, are one exception. When tomatoes are heated or cooked, levels of lycopene are increased (an antioxidant) but in the process some vitamin C is lost.
Processing also stops moisture loss and interaction with oxygen but canning can involve adding additional ingredients like sodium, sugar and water that may affect nutrient levels.
Another factor in the overall equation is when the product is harvested. Fresh produce is typically picked before it's ripe, so unlike frozen or tinned products, it often doesn't come with a full load of nutrients from the beginning. What nutrients the produce does have will start to diminish over time. Even if it looks fresh, a premium appearance can be deceiving. Generally, freezing and canning slow nutrient loss.
So it’s all about balancing act. Any fruits and vegetables are better than no fruits and vegetables. Tinned and frozen food might be somewhat depleted during processing, but once that initial loss is over, they can start to make gains on fresh food. This means that a 6-month-old tin sitting on the shelf could be healthier for you than its fresh counterpart over in the fruit and vegetable aisle, languishing for weeks waiting to catch a customer's eye.
For peak flavour and good value, fresh fruit and vegetables in season are always a good choice. So try to choose fresh as often as possible, keep frozen veg in the freezer for emergencies and only choose tinned foods that are low in or free from added salt and sugar.
|
| 1. Rickman et al. J Sci Food Agric 87:930–944 (2007) |
