MadSci Network: Chemistry

Re: What is the actual temperature which destroys vitamin C?

Date: Mon Feb 8 13:15:29 1999
Posted By: Jean Weese, Faculty, Food Science , Auburn University
Area of science: Chemistry
ID: 917468475.Ch

	The reason you could not find a simple one temperature to answer 
your question is that there is no one temperature that will apply to all 
foods and cooking processes.  The destruction of the vitamin is dependent 
on the food or source of the vitamin.  Nature has put many things in the 
food to protect the food from losing the vitamin.  Take for instance an 
orange.  As long as you keep the orange intact with the skin you will keep 
most of the vitamin.  But as the fruit ripens and the cell walls start to 
break down the vitamin is gradually destroyed.  This happens less rapidly 
if the orange is keep refrigerated.  This is partly due to the fact that 
the cell wall  destruction is slowed down, so the two work together.   Also 
another factor is the pH of the food or the source of the vitamin.  Vitamin 
C or Ascorbic Acid is a weak acid and is more stable (not destroyed as 
rapidly) if it is in this environment.  Like the orange.  There are many 
acids in addition to ascorbic acid in the orange.  This help to keep the 
vitamin from being destroyed.  

Therefore, temperature is only one factor that will result in the 
destruction of Vitamin C.  There are others, like oxidation.     After you 
cut the orange the cut part is now exposed to oxygen.  Oxygen causes the 
destruction of the vitamin C.  After a short period of time the vitamin in 
the orange will be destroyed without any change in temperature.  

Now let us put them all together into a food.  Like oranges.  We squeeze 
the juice from the orange.  This starts the destruction of vitamin.  We 
know from research studies that this actually results in a greater loss 
than the short time high temperature exposure from the pasteurization used 
by most juice companies.  If the juice is exposed to a lower temperature 
for a long time you will actually destroy more of the vitamin C than if you 
flash pasteurize short time high temperature.  So the temperature is not as 
important as some of the other factors that you need to control in cooking 
foods.  We know that cooking food in small amounts of water is less 
destructive to the vitamin than in large amounts because the vitamin is 
water soluble.  So cooking in a microwave for a short time is less 
destructive than cooking a food in hot water for a long time even though 
the temperature may be higher in the microwave.  

One more example: Potatoes are a very good source of vitamin C.  But most 
of our potatoes are eaten as french fries.  These potatoes are cooked at a 
very high temperature.  But because they are not in a water solution (FAT) 
the vitamin is not destroyed as rapidly as if you boiled them in water to 
have mashed potatoes. In one medium potato that is peeled and boiled you 
will get about 10 milligrams of vitamin C .  If you take this same potato 
and French Fry it you will receive about 15 milligrams.  

Vitamins are very interesting.  There is a lot more too it than you think. 
Also if you take the vitamin out of the food source altogether and place it 
in a vitamin pill the vitamin is very easily destroyed.  So if you take 
vitamin pills and have had a bottle on your counter you may not have any of 
the vitamin C left in the bottle if it has been there for a long time.  If 
you have any other questions e-mail us again and I will try to answer any 
other questions you might have.   	

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