Measuring Vitamin C in Apples


When an apple is on the tree, it gets a continuous supply of nutrients from it during the growing season. In the apple many compounds form: esters, sugars, starch, carboxylic acids, cellulose, and other substances including vitamin C. As soon as it's picked, or the days get short, oxidation starts and the fruit begins to degrade. The rate depends on many things, but the most important factor is probably the apple variety. Apples which store well retain their nutrients much longer than early-season types like Discovery. The rate at which the fruit grows is also important; rapid-growers do not store well.

In solution, vitamin C is fairly unstable to oxidation by air. In ground-up fruit, the lifetime is reduced further by

i) enzymes which destroy it;
ii) substances from the fruit capable of reacting with it.

A ground-up apple in solution may lose its vitamin C very rapidly. Leicestershire - grown Bramleys lose half of it in dilute solution in a few minutes. Any good analysis method must allow for this.

This is not the place to give a detailed account of how the analysis is done, but here's a brief outline of the method we have devised: weigh out around 30g fruit accurately; grind it, dry, using pestle & mortar, at the same time starting the stopclock (t=0); after 60s add a little dilute acetic acid and grind for a few more seconds; add water; pour into a beaker and make up immediately to 400 ml; withdraw 40 ml; note the time; titrate with standard DCPIP solution, the titration to take no more than 60s. Repeat at 2 or 3 minute intervals a further three times. Plot titre against time and extend the graph back to t=0 to estimate the original vitamin C content.

There is little point in measuring vitamin C if the apple's ripeness is not defined accurately. References to vitamin C in apples on the internet and elsewhere are sparse, and some are quite ancient. In the ones we have found, no mention is made of the condition of the fruit - whether ripe, under-ripe, fresh or elderly. This can have a very large effect. We have decided on the following classifications of ripeness to take this into account:

1. Fruit fully swollen but under-ripe; pips white, very acidic.
2. Fruit slightly under-ripe; most pips turning colour but not fully brown; fruit a little more acidic than ideal.
3. At its peak; fresh off the tree, fruit falls without being blown off. Slight wasp marks on some fruit may be seen.
4. Fresh, but no better than top supermarket quality; a day or two old, perhaps, and not quite pristine.
5. Still in good condition but a bit more mellow; greens are beginning to fade to yellow. Some skins turning greasy. Fragrance increasing.
6. Elderly but edible, possibly shrivelled skin. Yellow and red colourations pronounced in fruit tending towards those colours. Some varieties medicinal in taste.

In these analyses, ripeness grade 3 means that we have analysed the fruit on the day of picking. The best possible grade of fruit from a supermarket or shop will therefore be 4; it's often 5.

Apples age at different rates. Discovery, for example, goes through stages 1-6 in around a week. A Cox, on the other hand, might take 3 months do the same. The pace of ripening also varies with the year, the growing conditions, the rootstock used, and other factors.

A very few apples don't develop their full flavour until they reach stage 5, but most varieties by this stage taste poor or worse.

Our results are shown here.

This work is a long term project. We intend to collect figures for a number of years to ensure reliability and to measure variation. We will also measure vitamin C levels at the various grades of ripeness.

This work is copyright and is being carried out for the National Fruit Collection. It may not be reproduced without written permission.

Nigel Deacon / David Shirley


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