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Determination of dry matter (DM)

The principle of the method is that, by heating a certain sample, it is possible to eliminate all its water content, and then dry material is obtained. This technique requires  a balance and a heating device, usually an oven.  A recent development is to use a "Micro-wave" oven which makes it possible to dry the sample within 15 to 30 m minutes (larger the sample more time required)

Instructions

1/ Weigh a dry and clean container (a beer can cut in half) (C)

2/ Add the original, fresh sample to the container, and weigh both of them together (C+S).  This operation is made at least in duplicate of the same sample 

3/ Dry the sample in the micro-wave oven for 15 minutes then weigh the can and contents (DS), return the can and contents to the oven for 5 minutes more and weigh again. Repeat this process until the last weight is the same as the preceding one, which implies that all the water has been eliminated.

Calculations

                              % DM = (DS-C)/(CS- C) * 100

     Taking into account the weight of the container (C), then

   S = Weight of the original sample, and (CS) =  weight of sample + the can

   DS = Weight of the dry sample plus the can

Example

  The weight of a sample of fresh leaves plus a container was 15.12g (CS) and 13.40g (DS) in the fresh (original) and dry state, whereas the weight of the container was 12.88 g (C). Then :

% dry matter = (13.4-12.88)/(15.12-12.88)*100 = 23.2%

The DM content from a duplicate of the same sample was 22.8%. Then the average DM content of the sample of leaves was.

Average % DM = (23.2 + 22.8)/2 = 23.0.

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