Stone fruit growers use various methods to determine when to harvest that can range from visual inspection of fruit background colour during fruit growth, ‘taste testing’ fruit for sweetness, measuring the flesh firmness of a small sample of fruit, a calculation based on a minimum number of days after full bloom (DAFB) or a combination of these methods, Glenn Hale and John Lopresti* write.

The best approach at present mainly depends on grower experience and subjective judgement of fruit maturity. Alternatively, novel instruments can be utilised to provide a more objective basis for determining optimum harvest maturity that removes much of the guess work.

The DA meter is a hand-held instrument that was developed approximately fifteen years ago by Professor Costa and distributed by T.R. Turoni in Italy and has slowly gained popularity for use in both research and as a practical tool to help growers determine the optimal time to harvest. There are three different models available that were designed for different crops such as stone and pome fruit, cherry and kiwi fruit.

The DA meter for stone and pome fruit (Fig. 1) measures changes in the chlorophyll concentration in the fruit mesocarp (flesh) by reflectance at two wavelengths of light (670 and 720nm). The reflectance is expressed as an ‘index of absorption difference’ (IAD). The values for stone fruit typically range from 2.0 (very immature) down to 0.0 (fully ripe). At a specific value along this scale, climacteric fruit will begin to produce ethylene which indicates the commencement of fruit ripening, with this ‘onset’ IAD value highly dependent on cultivar.

A strong inverse relationship between IAD value and fruit ethylene production has been found for many stone fruit cultivars which suggests that the DA meter can be a useful tool to monitor fruit maturity either on the tree prior to harvest, or during postharvest handling and storage. Recommended IAD values corresponding to different maturity classes for 20 stone fruit cultivars are provided in an AVR database available online.

The rate of change in IAD values and fruit maturity may fluctuate from year to year depending on climatic conditions, however fruit maturity classes remain constant across seasons and regions. Thus, monitoring the decline in IAD as fruit ripens on the tree can help to identify the optimal harvest window to ensure fruit are harvested with both sufficient storage potential and eating quality.

In support of this technology, Gaethan Cutri, CEO of Cutri Fruit in Swan Hill said, “Prior to the DA meter we ate a lot of fruit to gauge when to harvest – mostly this worked but sometimes resulted in soft or mealy fruit received by our export customers.

Figure 2. Ranges of DA meter index of absorbance difference (IAD) values in ‘Majestic Pearl’
nectarine during fruit growth at the Tatura SmartFarm during the 2021–22 season.  

“Having an objective measure of maturity has allowed Cutri Fruit to confidently delegate the decision of when to commence harvest for each variety to junior staff which is important, given we have farms across different regions.”

Another advantage of the DA meter is that it can be used anywhere along the supply chain to determine the level of fruit ripeness and consequently help predict the remaining shelf life.

Figure 3. Change in mean index of absorbance difference (IAD) value in ‘Majestic Pearl’ nectarine at the
Tatura SmartFarm during the 2021–22 season; Red dotted line indicates beginning
of ‘onset’ maturity (i.e., IAD = 1.2) and the corresponding date to begin harvesting (i.e., 26 January);
N = 320 fruit per assessment with error bars representing the standard deviation of each mean. 

During the 2021–22 stone fruit season, scientists from Agriculture Victoria Research (AVR) monitored the change in IAD values for 320 ‘Majestic Pearl’ nectarines prior to harvest over a four-week period in the Sundial orchard at the Tatura SmartFarm (Fig. 2).

Based on previously determined fruit maturity classes linked to ethylene production (Table 1), the optimal date to begin harvest in the 2021–22 season of Majestic Pearl for long term storage or sea freight was 26 January (Fig. 3). The ‘harvest ready’ window was estimated to be approximately 10 days based on a predicted linear ripening behaviour beyond the last assessment date on the 2 February (i.e. black dotted line in Figure 3).

Harvesting this cultivar prior to the onset of maturity (i.e., IAD values greater than 1.2), when fruit is still immature and not yet producing ethylene, has been shown to result in inconsistent and uneven ripening, whilst fruit may have a bland taste and lack flavour after ripening by the consumer.

On the other hand, harvesting mature fruit at an IAD value of less than 0.5 (i.e., ‘tree-ripe’), with relatively high fruit ethylene production, would result in a shorter storage potential with fruit only suitable for domestic markets, although fruit may have a higher eating quality due to a longer development period on the tree.

By utilising novel technologies such as the DA meter to assist with monitoring of fruit development and maturity in the orchard, more objective prediction of the optimal time to harvest is possible, enabling growers to supply more consistent and higher quality fruit for consumers to enjoy in both domestic and export markets.

Acknowledgement:

This work was funded by the Food to Market Program (Stone fruit) as part of the Victorian State Government.

*Glenn Hale and John Lopresti are researchers at Horticultural Crop Physiology, Agriculture Victoria Research. Contact Glenn A Hale: Glenn.Hale@agriculture.vic.gov.au