Monday, July 6, 2009

Vitamin C variation in citrus fruit

Been reading through my journals, starting with the oldest. I now realize that I should be starting with the newest and working backwards. This is because the latest research is the most relevant and up to date. So after this post I will alter how I am posting.

That aside there is an 1979 article which investigates how leaves of citrus plants relate to there fruit quality. In Sierra Leone of all places, I'm sure that the time it seemed really important. What they found was:
  • The higher the phosphorus level in leaves the lower the Vitamin C content. One of the major ingredients in citrus fertilizer is phosphorus. Do you think that fertilizer application has increased in the last 30 years? I would suspect so, thus our oranges would have even less vitamin C than they did a generation ago.
  • Vitamin C was directly related to copper in the leaves (R^2 0.89). Now tests have shown that copper levels in the soils, along with other trace elements, have steadily decreased over the last 3 decades. This is because as long as the trees have not got a serious deficiency disease that would affect fruit production, not trace elements are put back into the soil.
  • The variation in vitamin C between orchard sites is tremendous. One orchard produced oranges with 32 mg/100cm^3 and another nearly 3 times higher level at 82 mg/100cm^3, for the techy's the mean was 57 and SD was 16 mg/100^3 So depending on what orchard you purchased from, would greatly impact your vitamin C intake. Hence it is very much dependant on the location. This variation makes a mockery out of food tables, and even the prestigious Linus Pauling Institute states that an orange has 70mg Vitamin C per orange. It becomes difficult to break this mentality of no variation in food nutrient levels because society has brought into this concept that the nutrient levels of fruit and vegetables have been measured and are well known.
  • The vitamin C levels in tangerine is proportional to sulfur level of leaves (R^2 = 0.998). The variation was between 21 and 49 mg/100cm^3. However as only three orchards were used in this paper strong conclusions cannot be drawn. However it does reinforce the orange argument above
Full paper reference: Godfrey-Sam-Aggrey, Haque and Garber. Relation of citrus leaf nutrients to internal fruit quality in Sierra Leone. Journal of Plant Nutrition, Volume 1, Issue 2 1979 , pages 185 - 205


  1. I have heard that a huge percentage of vitamin C is lost the moment you pluck the fruit from the tree and that it continues to decline quickly over time. Hence one of the advertised reasons to take vitamin C suplimentation. Given the poor storage facilities and unseasonal seasons encountered by early sailors, how did the "very old" fruit provide sufficient vitamin C to eliminate scurvy? Can you enlighten me! Thanks David.

  2. Keith

    Great question. Tiffany recons that they put citrus tree in pots and watered them with wine.

    On a more scientific note....

    You are right to say that vitamin levels decay rapidly. Although some nutrient decay linearly, that is the same amount every day, most tend to follow exponential decay. Exponential decay is rapid to start with and then slows down. Let us take a hypothetical situation of exponential decay with a 50% reduction daily. So we pick an orange and it has 100mg of Vitamin C, tomorrow it would have 50% which is 50mg. The next day it would drop 50% so it would have 25%, next day 12.5%, next day 6.25%, 3.125 then 1.5625 and now I can't do the numbers in my head.

    Typically the decay wouldn't be 50% a day, and tends to a low but still measurable base line.
    What this example shows is that there is a low residual level that will be in the fruit for a very long time.

    Now couple this low residual vitamin C intake needed to prevent scurvy. The following details are known:

    The average person only needs 10mg of Vitamin C per day to prevent scurvy for a year. (Wohl & Goodhart pg 301)

    A man with scurvy was cured with 6.5mg vitamin C daily. (baker et al American J of Clinical Nutrition 1969)

    "The minimum requirement for Vitamin C in adults must consequently be somewhat below 10mg a day, and some test results seem to indicate that is is about 5mg a day" Uhil American J of Clinical Nutrition 1958)

    So you can see that you hardly need any vitamin C in your diet to prevent scurvy.

    I don't have any data currently upon the vitamin C decay rates in citrus stored in a unrefrigerated environment. If we take the "average" orange of 70mg exponential decay after say 6 months I would not expect there to be less than 10% remaining eg 7mg of vitamin C which would prevent scurvy.

    Also remember that when on shore the vitamin C levels would increase with the better food, so it would take a while to decrease the bodies storage levels down to the very low level that creates scurvy.

    Hope this answers your question