Ask the Produce Docs (Commodity)
I would really like to understand how to calculate the percentage of dry matter in the avocado. I know that the avocado is weighed after it gets microwaved on medium power for 6 minutes. Any help you can give me about the specifics of this process would be appreciated. (C.G.)
I believe that you are interested in understanding how to measure dry matter in avocado using the microwave technique. Here is a link to an article which summarizes information regarding the current method used in California for dry matter determination. We did a lot of work in California to ascertain that a core sample taken at the equator of the fruit (widest point for Hass) gives a very good approximation of the entire dry matter of the fruit.
In terms of the actual protocol, the exact times for drying the samples and the power setting are dependent on the microwave you are using. If you plan to do many samples you should use a commercial grade microwave, not an inexpensive one designed for home use. You should make sure that you have one that has a rotating tray so that you get more uniform drying of the samples.
In short, the protocol is the following:
1) Select your fruit sample. If you are doing multiple fruit from a grower lot, try to group the fruit by size. We normally record the entire fruit weight for reference purposes and also peel color.
2) You will need a scale that measures to at least 0.01, 0.001 is even better.
3) Take your sample. We prefer the coring method for easy sampling and less knife work. Remove the peel and any seed material including the seed coat.
4) Weigh your sample. This should be done promptly since the sample will immediately start losing weight. We try to have at least a 3 to 5 gram sample from the fruit.
5) We use watch-glasses to hold the sample. Small glass petri dishes also work. Plastic petri dishes oftentimes become warped during microwaving.
6) Place the sample in the microwave. We use 30% power for the microwave we have, the idea is not to burn the samples since this will give you erroneous readings.
7) We have found that it takes about 30 to 40 minutes at 30% power. We remove the sample and weigh after about 30 minutes. The sample is then again placed in the microwave for an additional 5 minutes. The sample is weighed again and if the weight has not changed, it is finished. If the weight has changed, then place back in the microwave for an additional 5 minutes. Continue this process until you reach constant weight. An alternative is to use a fruit dehydrator. The samples will not burn using a dehydrator but it will take 3 to 4 days for the samples to dry to constant weight. As with the microwave you will need to take the samples out after 3 days, weigh them, and place them back in the dehydrator for 24 hours before reweighing.
8) Try NOT to burn the sample, this is where the power setting becomes important.
9) Some hints:
The calculation for dry matter determination is as follows.
You need the following pieces of information:
Petri dish weight
Initial sample weight + petri dish
Final weight of sample + petri dish
The calculation is: Final weight/initial weight * 100 = Dry Matter %