Calculating Dry Matter Percentage
Adapted from sources listed at bottom of page
Introduction: How do you tell when the fruit is ripe?
Unlike many fruits, avocado maturity does not coincide with ripeness. An avocado becomes ripe - softens, and so can be eaten - only after it has been picked from the tree, requiring several days at room temperature. It is difficult to know in advance whether an avocado fruit is mature enough to harvest because external appearance of avocado fruit gives only subtle clues to the stage of maturity. The skin loses some of its glossiness, becoming duller; surface russeting increases; varieties that turn black on ripening may become partly so on the tree. When cut open on picking, a mature fruit will slice smoothly, without a crackling sound or rough cut surface. Also, the seed coat will usually be thin and brown instead of fleshy and white.
The ultimate test is picking a fruit and letting it ripen. Mature avocado varieties vary widely in pleasantness of flavor, but an immature fruit of any variety will likely shrivel as it ripens and have a watery, bland, or even "grassy" flavor. You do not want to release such fruit commercially. Therefore, a simple test (closely correlated with taste testing results) was needed to help keep immature fruit off the commercial market.
It has long been recognized that there is a close relationship between the oil content and the development of an avocado fruit. Basically, oil content increases as the fruit matures. The problem was the methods for determining oil content
- Extraction with petroleum-ether in a Soxhlet apparatus and gravimetric determination of oil content; although accurate, it is tediously long and definitely not simple.
- Refractive index (RI) determination of an oil extract with chloronaphthalene; this method had questionable accuracy and chloronaphthalene is suspected of being a health hazard.
- Determination of oil by nuclear magnetic resonance (NMR); accurate but requires expensive equipment.
Then it was discovered that, as oil content increases, the amount of water in the fruit decreases. Percent dry weight increases during fruit development and the increase is mainly due to the increase in oil. Since it is far easier to analyze percent dry weight than percent oil content, a simple method was soon developed so that any person can easily determine the maturity of his avocado fruit. Minimum maturity standards based on Percent Dry Matter have been determined for many avocado varieties. Click here for the amended dry weight procedures, Department of Food and Agriculture.
Percent Dry Matter
Spatula or spoon
Petri dish or other small glass dish
Food processor with chopping blade
Knife & potato peeler
Select your avocados carefully. Pick them from the shady part of the tree, from different parts of the grove (if you have a grove). The most immature appearing avocados representing the regular crop should be chosen. Off bloom fruit should not be tested. The fruit must be hard to be tested.
- Fruit are first cut in half (through the seed). Next, lay the flat side down and cut a wedge out of the middle (approximately 1/8th of the fruit). Do this for the other half of fruit. Use these wedges for sampling.
- Using the potato peeler or knife, remove the skin down to flesh. Remove the seed and all traces of the seed coat.
- Cut these wedges into smaller pieces and place into the food processor with a chopping blade. Run the food processor until the avocado has been chopped into fine pieces (starts to stick to the side of the food processor container). Will have the size and consistency of grated Parmesan cheese.
- Reserve several tablespoons of sample in a plastic bag in case the sample needs to be rerun.
- Weigh an empty dish and record its weight... this is the TARE WEIGHT.
- Place the empty dish on the scale and add avocado sample until you have added 5 grams. Record this weight on the data sheet. (Does not have to be exactly 5 grams, but within 0.3 on either side... just remember to record the exact weight). This is the WET WEIGHT.
- Place the dish containing the finely chopped avocado into the microwave oven.
- Since microwave ovens vary, it is critical to start at a low power setting and gradually work up to higher settings to prevent scorching. Suggested setting might be 40% power for 15 minutes. After weighing, microwave the sample again for 3 minutes at 40% power then reweigh. This process is repeated at one minute intervals until no further weight loss is observed (after several times of doing this, you can determine the proper power setting and approximate time... just be sure NOT to burn the sample).
- After no further weight loss, remove the sample and weigh... this is the DRY WEIGHT.
Calculating the Percent Dry Matter
So, if your empty glass dish weighs 3 grams (TARE WEIGHT), the weight with fresh avocado is 8 grams (WET WEIGHT) and your weight after drying is 4 grams (DRY WEIGHT), the calculations would look like this:
Sources of Information
Bergh, B. O., J. Kumamoto, P. Chen. 1989. Determining maturity in whole avocados. Calif. Avocado Soc. Yearbook. 73:173-176.
Lee, Seung-Koo. 1982. A review and background of the avocado maturity standard. Calif. Avocado Soc. Yearbook. (1981) 65:101-109.
Rosenthal, I., U. Merin, G. Popel, and S. Bernstein. 1985. An analytical assay for the determination of oil content in avocado. Calif. Avocado Soc. Yearbook. 69:133-136.
Amended dry weight Procedures, Department of Food and Agriculture. Title 3. California Code of Regulations