"Eat to Live, Not Live to Eat"
John H Weisburger, 2000
The following report is a familiar one in this series, namely to what extent does processing of whole fresh fruit influence their nutritional properties? Fresh strawberries have a very short storage life because of their high water content, metabolic activity and fungal load. As a result the enthusiastic home grower very often has to find ways to store them for longer periods to avoid wastage. Jams have filled this spot for ages now, not only because they store so well without any special needs such as refrigeration, but also because they're universally enjoyed and long ago became part of our national dietary habits, eg scones with cream and jam.
Strawberries, like all berries, are exceptionally rich in phytochemical antioxidants, the plant components that are thought to play such a prominent role in the health benefits of all fruits and vegetables. Following is a summary of a US study (International Journal of Food Sciences and Nutrition, (2010) 61: 759–769) showing a dramatic 96% decrease in total antioxidant activity when cooked up as a jam. Even frozen strawberries had only 59% activity remaining.
The present study was conducted to determine differences in antioxidant levels of fresh, frozen, and freeze-dried strawberries, and strawberry jam. Hydrophilic (water soluble, free) antioxidant activity (HAA) and lipophilic (lipid soluble, bound) antioxidant activity (LAA) were measured using the ABTS/H2O2/HRP de-coloration method. HAA and LAA were then summed to calculate the total antioxidant activity (TAA). Mean differences in TAA were expressed on an 'as consumed' and 'dry weight' basis and analysed using one-way analysis of variance and pairwise comparisons. The mean TAA based on dry weight for fresh strawberries (43.6 mcmol/g Trolox equivalents) was significantly higher than for freeze dried (30.1), frozen (25.6), and jam (1.6). Results agree with previous studies reporting that strawberries are a valuable source of antioxidants for consumers.
When TAA was expressed on an 'as consumed' basis, the TAA levels were fresh (3.4), freeze-dried (30.1), frozen (2.7), and jam (1.2). Fresh fruit are a low calorie food because they are mainly water, between 80 and 95%. When given amounts of phytonutrients are spread throughout this water the resulting concentrations will predictably be an order of magnitude lower than those more solid forms like dried fruit, where the moisture level may have been reduced to 5-10%. This simple dilution effect means only smaller amounts of dried fruit need be eaten to obtain a certain amount of vitamins, minerals and phytonutrients compared to the fresh form. But expressing activities on an 'as consumed' basis is not the best way to compare the nutrient density of different fruits or fruit products. This is best done using dry weight because moisture content of fresh fruit depends so much on horticultural factors like plant genetics, cultivar, climate, soil nutrition, harvest timing and precipitation. In late maturity and ripening stages, fruit weight and size can double in a couple of days with rainy conditions, but in this case it's almost all water, and phytonutrient amounts in the whole fruit will usually have changed very little. In addition to considering this simple dilution effect on concentrations, there can also be chemical changes in individual phytonutrients during processing steps as a result of oxidation or enzymatic action, and these changes must be taken into account in the final product. Cooking fruit for sufficient time to give the required degree of softening for a paste-like jam to be made commonly causes some nutrient decomposition.
Another important consideration when comparing the relative merits of a range of fruits is the chemical properties of the different antioxidants. The hydrophilic phytonutrients are easier to extract and assay than the lipophilic, and in many of the early studies these were the only ones reported. But most fruits have some in the lipophilic form and in many this type may contribute most of the activity. Consequently more thorough recent studies, as in the above, have reported both for a better evaluation.
From a separate nutritional perspective, a key negative of jams is the large quantity of added sugar, frequently 50%, to prevent microbial growth and deterioration. And this addition is to a fruit that's already sufficiently sweet to appeal to most palates. Pure sugar is calorie-dense and nutrient-empty, and it's the last thing we should be adding into our diets; current levels of obesity illustrate we're already overloaded. It produces a food that has a high glycemic index which predisposes to diabetes amongst other unwanted physiological and medical outcomes. Strawberries are best eaten fresh when possible. If just picked they may last for 1-2 weeks at 0-4°C in the refrigerator, less if you bought them and they've already been in the postharvest transport and marketing stages for some time. Enjoy them for their delicious flavour and aromas and be comforted in the knowledge that they're also good for you.