"Eat to Live, Not Live to Eat"
John H Weisburger, 2000
Traditional methods for extending the storage life of perishable foods such as fruit, vegetables, milk, meat and fish were based overwhelmingly on techniques available at the time, eg salting, smoking, fermentation and solar drying with little knowledge of the effects these processes had on nutritional properties. Nowadays we have a far wider range of techniques including freezing, high pressure, spray and drum drying, and vacuum and microwave drying, but the question for each remains – how do they affect healthful nutrients and are some techniques better than others? The answers vary greatly depending on what is intended, the particular fruit and cultivation conditions, its state of maturity, how pre-treatment may influence processing, storage conditions etc, so let's just consider one example – thermal drying of apricots.
This product is commonly known and enjoyed everywhere as about half the total world crop is sold dried, usually by solar heating. Most consumers prefer them when in a çhewy rather than crisp state and this means they are processed to a moisture content that is higher than that required to totally prevent microbial spoilage. This is prevented usually by 'sulphuring', where either burning sulphur or bisulphite is used as a preservative. However some people are hypersensitive to the resultant SO2 and have unwanted inflammatory reactions. Plus, SO2 totally destroys vitamin B1. Leaving this as a separate issue, to what extent does the solar or hot air drying cause breakdown of valuable nutrients?
All fruits have high moisture content, usually around 90%, and as a result their nutritional qualities depend mainly on vitamins and similar acting phytonutrients, fibre type and content, and to a lesser degree, minerals. Minerals are the least affected by drying, fibre properties are changed because cellular structure is compromised as the fruit dries out, and vitamins/phytonutrients are the most sensitive to processing conditions. It should be realized that in achieving extended storage life with drying, products are subjected to longer, possibly degrading conditions, than they ever would be in the fresh state. Storage losses, due for example, to oxygen and light can then sometimes outweigh those from the processing operation itself. Most breakdown of vitamins and phytonutrients in drying occurs enzymatically or thermally, the latter often by oxidation. Pre-treatment blanching (eg 100°C for 1-3 minutes followed by rapid cooling) is one way to inactivate enzymes and minimise losses from this source.
Carotenoids are responsible for the yellow colour of many fruits and vegetables, and are a major nutritional feature of apricots. Many of these can be converted into vitamin A in the body, ie they are pro-vitamins. In a recent Italian study (Plant Foods & Human Nutrition, 2013, 68, 241-246) it was found that apricot halves dried in a dehydrator at 60°C or 70°C with no other pre-treatment took 21 and 15hrs respectively to reduce moisture content from 87% to 20%. Carotenoids are regarded as being relatively heat stable whereas many of the other common vitamins, vitamin C in particular, are notoriously more prone to degradation. Nevertheless, in this study carotenoids still decreased by 28 and 43% respectively compared to the fresh fruit.
In general, newer techniques such as freeze-drying lead to better nutrient retention than that obtained in hot air or sun dried products. Solar drying is still a major process in the developing world because it doesn't involve large capital costs and it's energy-free. But sufficient space is needed, labour is required to turn and rake fruit, processing is not well-controlled and needs warm sunny weather, fruit have to be protected from rain and over-night moisture, there can be foreign matter (eg dirt) contamination, and fruit need protection from insects, birds and microbial spoilage given the process can take several days. There is also a risk of aflatoxins being produced which are carcinogenic. In addition, it involves exposure of fruit to strong light which can cause breakdown of phytonutrients such as vitamins B2and C.
Surveys show that, on average, consumers place hedonic qualities of fruit, ie taste, flavour, aromas, appearance and texture before nutrition. And dried fruits are normally most desirable because their sweetness etc has been concentrated through loss of moisture. However, if nutrition is important to you then fresh is almost always best. Dried products do serve a valuable role in providing important out-of-season foods which can also be distributed to locations other than where they're grown. The challenge to the modern processing industry and the home processor, is to adopt those techniques that provide the tastiest, healthiest and safest products.