Thursday, August 15, 2013

The nutritional value of canned food

A few days ago I caught up with a couple of friends, and the topic turned to the value of preserving food. One of my friends was wondering whether canning and preserving was actually worth it. She questioned whether too many vitamins might get destroyed in the canning process.


I am an avid fan of canning and preserving as part of our push to greater food security, lower food costs and better nutrition for our household. 

There is no doubt in my mind, that fresh produce straight from the garden and eaten within a few hours of being picked is better than any other alternative. 

Second best is food canned from home grown organic produce which has been picked and processed as soon as possible after harvesting. 

Farmers' markets and direct farm outlets also provide another good source of fresh produce, but those are still quite rare in Australia. 

So-called "fresh" food from the supermarket, on the other hand, may not be so fresh at all. 

Let's have a look what nutritional science tells us.

The moment fruit and vegetables get harvested vitamins start breaking down. Vegetables kept in storage will have lost a significant proportion of their vitamin content by the time they get eaten:
"Nearly half the vitamins may be lost within a few days unless the fresh produce is cooled or preserved. Within one to two weeks, even refrigerated produce may lose half of its vitamins." (William Schafer, University of Minnesota Extension)
Similarly, cooking vegetables or canning fruits destroys a substantial proportion (between 30 and 50 per cent) of some vitamins, particularly vitamins A, C, B1 (thiamin) and B2 (riboflavin). In other words, after just one week in the fridge, "fresh" produce has a similar vitamin content as canned food. 

Given that most supermarket produce in Australia takes at least several days from harvest to the farm gate, the comparative nutritional superiority of fresh produce is already lost by the time consumers put their fruit and vegetables into their shopping trolley.


Is canning and preserving worthwhile from a nutritional standpoint?

Let's look at the example of tomatoes. I preserve many bottles of tomatoes every year, usually enough to last us until the next harvest. In summer, when the tomatoes are ripe and can be freshly picked and eaten straight away, there is no need for canned tomatoes in cooking. However, in winter - at least in the cold climate zones of Australia, such as Canberra region and Tasmania - the amount of food available in the garden can be very limited, and canned food is a great addition to a varied winter diet.

Australian supermarkets stock "fresh" tomatoes all year round. Sometimes they are even on special in the middle of winter. It is quite reasonable to ask whether "fresh tomatoes" from the supermarket could be nutritionally superior to my home canned produce. To my mind, there is little doubt that my sun-ripened canned tomatoes taste far better than the cardboard varieties on offer in the shops, but lets focus on nutrition alone for a moment.


Almost all of the food sold as "fresh" in our big supermarkets is at least a few days old even if it has been transported as quickly as possible from the farm to the produce market in Sydney and from there to the supermarket shelves. However, fruit and vegetables on supermarket shelves can easily be more than a week old, as reported by Choice Magazine:

Australian grown fresh tomatoes straight from the farm may be already 10 days old by the time they hit the supermarket shelves:

"About 75% of Australian tomatoes are produced in Queensland, where they grow year round. In the Bowen region they’re harvested from May to early November while in Bundaberg they can be harvested for most of the year. In summer they’re also produced in NSW and Victoria. Western Australia is self-sufficient, relying on winter tomatoes from Geraldton and Carnarvon and summer ones from the Perth area. 
So, depending on where you live, tomatoes can be well-travelled by the time you eat them. Not surprisingly, it can take 10 days or longer from when tomatoes are picked to when they arrive at your supermarket — and they can be there a few days more before you actually buy and eat them. All this time they’re losing flavour." (Choice Magazine October 2009)

In the case of tomatoes, canning could even improve the quality of the fruit. Researchers from Ohio State University found that the amount of phytochemicals such as lycopene in tomatoes can be enhanced through the process of cooking. Research into the significance of lycopene in the prevention of cancers is ongoing, but initial findings of a number of studies found that 
"dietary intakes of tomatoes and tomato products containing lycopene have been shown to be associated with decreased risk of chronic diseases such as cancer and cardiovascular diseases."

What about other fresh produce?

Other "fresh foods" are often a lot older than 10 days. The Sydney Morning Herald reported in January 2008 that
"APPLES on sale in supermarkets are up to 10 months old".
The Sydney Morning Herald asked food researcher Dr. Stephen Morris from the independent Sydney Postharvest Laboratory about the quality of apples that old. In the same article, Morries is quoted as saying that 
"Apples can be kept for six months and they will still be of very good quality. After nine months the quality is going to start to be affected and at 10 and 11 months you are not going to get such a good apple."
Fresh food storage technologies are now so sophisticated that it is increasingly difficulty for consumers to work out how fresh the produce is that we take home from the shop.

To last the distance, food is also treated with a range of chemicals, fungicides and in the case of imported fruit and vegetables, fumigated with methyl bromide to comply with quarantine rules. It may still look fresh, but it is definitely past its best:

    "English spinach, for example, retains only 53% of its folate and 54% of its carotene after just eight days stored at fridge temperatures. Apples held in cold storage for three months contain lower levels of antioxidants. With extended storage, they also lose flavour and aroma; they can go floury quickly unless kept in the fridge." (Choice Magazine, November 2010)
And of course, taste and texture suffer as well. Unfortunately for the consumer, it is often only at home when cooking or eating the produce that the inferior quality becomes obvious. Buying organic at the supermarket does not necessarily mean we get fresher produce, either.

It therefore comes as no surprise that research conducted at the University of Minnesota found that
"Vegetables handled properly and canned promptly after harvest may be more nutritious than fresh produce held many days after harvest under abusive conditions."
Happy canning!














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