Dry matter testing can be carried out using either a conventional or microwave oven and an accurate set of scales.
Keep records of the harvest date and maturity test results for future reference.
- Select five avocados representative of fruit you intend to harvest. Note that fruit on the northern side of the tree and higher will be more mature than fruit low on the southern side, so consider your harvesting plans when selecting representative fruit. Also consider that there will be variability within the orchard; to make an informed decision on maturity in an orchard it is better to do several samples of five fruit from around the orchard than to only do one sample of five fruit from a few places in the orchard.
- Cut each fruit longitudinally into quarters — stem end to base and select two diagonally opposite quarters from each fruit.
- Remove the seed as well as any adhering seed coat and peel the fruit sections.
- Shred the flesh of the 10 quarters (that is, the two diagonally opposite pieces from each avocado) using a fine kitchen grater — ideally the shreds should be less than 1mm thickness which is achieved on graters with five cutters per square centimetre.
- Weigh a shallow, heat-proof (or microwave-proof) dish and record this weight. Make sure the dish is completely dry.
- Mix all shreds thoroughly and spread at least 100g of the avocado flesh uniformly over the shallow heat or microwave-proof container. Weigh the container plus avocado flesh and record this weight.
Pre-heat oven to 100–110°C. Place dish in centre of oven and leave undisturbed for five hours. During drying, be careful to avoid burning the avocado flesh and minimise opening the oven door. An easily-read oven thermometer is useful to monitor the temperature.
After five hours, allow to cool. Weigh to determine the dried avocado weight. With your first tests you should redry the sample for a further 30 minutes and then reweigh to ensure that the sample is fully dried. Record the dried weight.
Place the dish in the microwave and run microwave at medium/low for 20 minutes. You may need to adjust power setting and time depending on conditions, microwave type etc. Avoid burning the flesh and dry to a constant weight. Record this weight then repeat the procedure until there is no change in weight of the dish.
By reheating and reweighing you will establish when the sample is fully dry and can adjust the time required for future tests. When first using the microwave technique, keep a constant watch on the sample until you are confident of the settings and time needed.
(Note: To avoid possible damage to your microwave, it is advisable to have a container of water near the back corner to maintain humidity in the oven. Avoid moving this container after drying your avocado sample until it has cooled.)
Calculating dry matter
Use the following calculation: weight of dried avocado sample (less weight of container), multiplied by 100, divided by weight of fresh avocado sample (less weight of container).
Weight of fresh avocado sample (less weight of container) = 125g
Weight of dried avocado sample (less weight of container) = 30g
30g multiplied by 100 divided by 125g
= 24% dry matter
Avocados Australia's Best Practice Resource contains articles on avocado maturity testing, harvesting and handling.