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:

1. Low maturity fruit tend to burn more easily than samples from more mature fruit.
2. Very low dry matter fruit (<20%) can even ignite in the microwave. Why this is, I don’t know, but if they do, you have to start over with that particular sample.
3. If you use the coring method you will have 2 plugs from the machine from opposite sides of the fruit (preferred).  Try not to have the samples touch each other since this reduces the problem of ignition in the microwave.  Again, why… I don’t know.
4. We have found that taking opposite cores from the widest point of the Hass fruit that is approximately 220 grams will give you about a 3 to 5 gram sample, depending on the size of the seed.
5. Try not to touch the samples themselves when putting in and taking out of the microwave.  The oil from your hands can change the weights.
6. You can put multiple samples (petri dishes) in the microwave at once.  Obviously then you must be able to track each petri dish.  We have found that nail polish is a good way to label the petri dishes.  It will sustain washing of the petri dish so that each time you do measurements, you don’t have to re-label the petri dishes.
7. Make yourself an excel spreadsheet to do the calculations automatically for you.  This is what we do.

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 %

For example:
Petri dish weight = 20 g
Petri dish weight + initial sample = 25 g
Initial weight = 25 – 20 = 5 g
Petri dish weight + final sample weight = 21 g
Final weight = 21 – 20 = 1 g
Dry matter = 1/5 * 100 = 20%

I hope this is helpful to you. Since you are in Mexico, I would also check with APEAM which is the avocado growers’ association.  They should have some set protocols for you to follow as well for dry matter determination.

--Mary Lu Arpaia