Ask A Scientist Chemistry Archive

name         Christian G.
status       student
age          11

Question -   How do you determine how can I figure out how much water is in an apple?
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Christian,

Assuming you are not interested in the water content of the core and seeds, here is an 
approach that will give you a good idea as to the apple's water content:

Wash and dry the apple. Cut it into quarters and then into very thin slices. Carefully 
remove the parts considered core and seeds. Use a fairly accurate scale to weigh the 
remaining slices. Spread the slices in a single layer on a wire cooking rack supported 
on a cookie sheet. Place the sheet in an oven set to about 215 F. In time, the slices 
will wrinkle and become very brittle and dry.

Remove the sheet, allow it to cool, then carefully place ALL the dried pieces of apple 
on the scale and weigh them again. This time they should weigh much less than when wet 
because the oven treatment will have driven off the water.

Divide the dry weight by the wet weight and then multiply the result by 100. The number 
thus produced is close approximation of the dry percent apple. Subtract that number from 
100 to get the percent water that was in the original apple.

Regards,
ProfHoff 687
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If you determine the amount of volatile material in the apple, or any other fruit, you will 
have a pretty accurate measure of the amount of water present because the only other 
volatiles in the fruit are the substances that produce the odor of the particular fruit 
and some other minor ingredients.
    You will need a little bit of equipment to do the experiment, but it is not very 
	expensive. You will need some sort of scale to measure the weights (I am assuming 
	you are doing this at home, rather than at school,  and do not have access to any 
	lab equipment.) An inexpensive scale is a postal scale which will measure up to 
	about 2 kg or so with a precision of 1 gm.
They vary in price from $10 - $30 depending upon the model. *Make a weighing "boat" 
out of heavy duty aluminum foil. It should be about 2-4 in diameter with "walls" 
about 1/4 - 1/2 in. deep. You can make these by forming the aluminum foil around a 
glass or a cup. Trim off any excess foil because you do not need any "scrap" on the 
dish. Weigh each boat you make. Postal scales usually have a "tare" function that 
allows you to cancel out the weight of the empty boat, but it is a good idea to weigh 
the empty boats
anyway. Using the "tare" function is preferred, if the scale has one.
**Cut the apple into pieces and remove the seeds and core. You can also peel the apple 
if you do not want to include the skin.
***Mash the apple into a "mush". A food processor or high speed blender is ideal for 
doing this, if you have one available.
****Spread the apple mush in a thin even layer -- I would guess about 1/8 in deep-- 
across the entire bottom of the weighing boat and weigh it. Before you do this make 
sure you have weighed the empty aluminum boat or have "tared" it using that function 
on the scale. Be sure NOT to reset the tare if you have "zeroed" the weight of the 
empty boat. Record the weight of the "apple sauce".
*****Pre-heat the oven of your kitchen stove to about 140 F. = 60 C. Place the weighed 
sample into the oven for about 60 minutes The oven temperature should be high enough to 
evaporate all of the water without getting any "popping" or "spattering" of the 
"apple sauce". If it does spatter you will need to change the procedure as follows.
        Do not pre-heat the oven. Place the sample in the cool oven and set the oven 
		temperature to about 140F. and let the sample warm up as the oven pre-heats to 
		about 140F. This slows the initial rate of evaporation of the water so that it 
		does not "spatter". When the oven is up to temperature let the sample dry for 
		30 min.
******Remove the sample from the oven and let it cool back to room temperature. Then 
weigh the dried sample. The difference in this weight and the initial weight before 
drying (Subtract the tare if you did not use the "tare" function on the postal scale.) 
is the weight of water that has evaporated.
******* The %water = 100*{ [Initial weight] - [Dry weight] }/ [Initial weight].
********If you want to make sure that your result is accurate, return the sample to the 
oven for another 15 - 20 minutes and repeat the weighing. The weight of the dried sample 
should not have changed appreciably. If it has changed by more than a gm or so,
keep putting it back in the oven until you get a constant "dried" weight. If you want you 
can increase the oven temperature at this point to about 160 F = 70 C. since most of the 
water will have been evaporated.

NOTE: If you want to do what chemists typically do, use three samples rather than a single 
sample. All of them should give the same percent water, even if the individual initial 
weights of the samples of "apple sauce" are different.

What I have written here sounds a lot more complicated than it really is. You will get the 
"hang" of it after you practice a few times. You can apply this general method to any fruit 
or vegetable. What you want to make sure of is that the samples are "mush" so that water is 
not trapped in lumps of the sample.

Vince Calder
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Christian,

First, weigh the apple on a scale to the nearest tenth of a gram.  Next place the apple in 
an oven at say 200 degrees Fahrenheit until it shrivels up.  Take the apple out of the 
oven carefully because it will be hot.  Let the apple cool down to room temperature for 
two to three hours.  Then again weigh the apple. Then take the original weight of the 
apple and subtract the shriveled up apple
weight and the difference will be roughly the amount of water that was in the apple.

Sincerely,
Bob Trach
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