Preface: I have never really took much notice in this matter prior to very recently when I noticed the epidemic of thick-skinned navel oranges at the college dining halls. I often heard grumbles of “Why is the orange peel so thick?” as students dug into their navel oranges. Then, I remembered, I have encountered especially thick skinned lemons back when we had a lemon tree at home. This led me to wonder what causes different peel thicknesses in citrus fruit, and does this say something about the nutritional quality of the fruit?
Let me first clarify the obvious: with oranges, there are all sorts of varieties which result in; thin rind oranges that are harder to peel but more suitable for juicing, or thick rind naval oranges that are easier to peel. But usually, the fruits of each varieties e.g. satsuma, mandarin, tangerine, or naval, almost always have identical peel thickness. I will be talking about in this post, the causes of different thickness peel in the same variety citrus fruit.
So what causes this?
Often, thick rinds in citrus fruits are caused by either too much nitrogen or too little phosphorus. To put simply, too much nitrogen will affect the plant’s ability to absorb phosphorus, and thus causing a phosphorus deficiency (When there is more or less of one of these substances, the other will also be affected). When there is too much nitrogen or too little phosphorus the plant will almost always bear small fruits with thick rinds and very little juice.
Backyard gardeners often make this mistake by having added too much nitrogen-rich fertilizer into the soil.
Effect on nutritional quality
Since thick rinds in citrus fruits are most likely caused by nutritional imbalance in the soil, there is potential that the nutritional profile of the fruit will be affected. Thick-rind, dry fruits can be thought of as defect products from factories. Nevertheless, oranges with slightly thick peels but normal interior should not be wasted as this is not an indicator of severe nutrition deficiency.
In fact, all kinds of fruit varieties, including oranges, will contain different traits and nutritional value i.e. vitamin C and citric acid content. For example, check out this chart on vitamin C content in apple varieties: http://www.naturalhub.com/natural_food_guide_fruit_vitamin_c_apple.htm
So how do we tell which piece of fruit is more nutritious? Contrary to popular beliefs, the sourness of a citrus fruit does not indicate high vitamin C content. In fact, a citrus fruit’s sourness depends on its citric acid level. After all, how could a guava which has no sour taste whatsoever contain almost 4 times as much vitamin C as naval oranges per 100 g? The best way to eat fruit and obtain the optimum nutrition is when it is still fresh (vitamin levels decrease over time after the fruit has been picked, and even more so at room temperature), recently picked, and has been stored well. That is local and naturally grown fruits.
Conclusion: thick peel citrus fruits are simply fruits from a tree grown in unfavorable condition. As long as the flesh taste and look fine, they should still be quite nutritious.
Now, I know the story behind the epidemic of thick rind naval oranges at the dining halls. Perhaps the dining services have changed fruit suppliers, or the changes in season has effected the orange tree’s fecundity. Whatever the problem was: save the oranges!