|MadSci Network: General Biology|
Hi, Bryony. Vitamin C or ascorbic acid content in oranges does vary to begin with, being affected by variety of the fruit, maturity, growing conditions and storage conditions. The level of Vitamin C in the orange will decrease over time which makes sense when one looks more closely at the characterstics of the vitamin. Vitamin C is a strong antioxidant and is easily oxidized itself in the presence of O2. It degrades with time and increased temperature (ie. in boiling vegetables or and canning juices). Therefore, you will find that the thicker skinned orange varieties (ie. Valencia)will lose Vitamin C more slowly than the thinner skinned varieties (ie. tangerine). The skin helps to retain vitamin and moisture content, as well as protecting the inner fruit against pests and temperature fluctuations. Vitamin C is usually determined by High Performance Liquid Chromatography (HPLC) or by a colorimetric titration. These methods may not be available to you, but if you want to monitor the trend of what is happening in your oranges - not absolute values - , you could just try total titratable acidity. Assemble several oranges of each of the types to be tested. Determine the average initial total acidity, as ascorbic acid, for three or four oranges of each variety. Keeping each variety identifiable (label on each orange), store them under uniform temperature and humidity conditions. The drawer in a refrigerator would be fine. Then, at selected intervals determine the total titratable acidity for each. When doing total titratable acidity, ((no. mL N/10 NaOH titrated x 0.017612)/sample wt in g)x 100 = % ascorbic acid At each test interval, look at the percent Vitamin C/ascorbic acid and at the percentage of loss based on the intial level for each. Then look for a pattern that you can correlate to the skin thickness. Carol Crouse Food Technologist The Food Chain Ltd.
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