Fruit quality headed for nirvana

John Lopresti with the Nirvana

At a glance

• Two handheld devices are being used to measure sweetness and maturity of stone fruit without damaging the fruit.
• Findings will help direct tree management to produce fruit that meet high customer expectations.
• Best suited to growers targeting high-value export markets.
• Recent study highlighted a large variability in sweetness, and ripeness, in stone fruit harvested from the same tree.

It may sound like science fiction, but Victorian fruit growers may one day be using infrared waves to judge when is exactly the right time to harvest fruit, thanks to research being conducted by the DPI's Premium Fruit Project.

DPI scientist John Lopresti has been busy testing various technologies that can be used by Victorian growers to measure the maturity and sweetness of their fruit without destroying it. His two tools of choice — the Nirvana and the DA-meter.

"We are focusing on using these technologies to collect data for individual fruit," Mr Lopresti said.

"Then we can start linking orchard temperature, and the effect of tree canopy management, with fruit quality. In this way we can find out what's actually happening to fruit during growth and their position or location in the orchard."

He said that modelling based on this data would offer growers a better understanding of how their fruit was developing on the tree, and also ideas for how to better manage their trees.

"Whether that's through increasing light exposure within trees or thinning of fruit early in the season, we hope to come up with tree management principles that will be useful for growers, depending on the market they're aiming for."

Staying in front

Stone fruit can attract a premium price in Asian markets

According to Mr Lopresti, the modelling will give Victorian fruit producers a better chance of staying competitive on the world stage.

"We're competing with markets such as Chile where at the moment they're producing similar quality products at a much lower cost," he said.

"If we can guarantee a certain level of sweetness and maturity then this will give us an edge in certain markets."

Mr Lopresti said it was important to improve on current techniques for measuring fruit maturity and quality, where fruit is taken back to a lab and destroyed.

"Once you've destroyed the fruit you can't take any more measurements. You want to be able to measure sweetness, and maturity, on the tree through the development process and then pick it at the right time."

Currently growers tend to use colour to decide when to pick fruit, but this is often misleading, according to Mr Lopresti.

"Many growers basically go to a few trees in a single row, and decide that since the colour is right and it tastes okay, I'll harvest tomorrow.

"I don't blame them, they're very busy people, but such basic and limited sampling of fruit is not going to give you an accurate idea of the diversity of maturities present at any one time in your orchard block."

Nirvana

So how do these two handheld devices work?

The Nirvana — developed by Sydney company Integrated Spectronics — shoots near-infrared red (NIR) waves into the fruit, then measures the change in the waves reflected back. This difference can be used to estimate the sugar level, or sweetness, in the flesh of the fruit.

It may sound straightforward, but the programming behind it is certainly not.

"You have to develop calibration models that link actual sugar levels in fruit with the spectra that the machine's reading," Mr Lopresti said.

"The model then needs to be calibrated for different types of fruit."

Mr Lopresti said the results could be used not only to decide when to harvest, but also to identify areas where management techniques could be improved.

"On a nectarine tree there might be 200 fruit at harvest, but there's likely to be a large variation in the sweetness and maturity. We can then ask why are we getting this variability and how can it be minimised."

He said sugar levels of fruit within a single tree could vary by up to 10 per cent as a result of many factors, including how much light each part of the tree was receiving.

The machine can also be used to choose when to strategically harvest parts of trees.

Although Mr Lopresti has been working with the Nirvana for a while, he said last season was by far the most successful, thanks to fine-tuning of calibration models and greater experience with the machine.

DA-meter

The DA-meter complements the Nirvana, measuring fruit ripeness rather than its sweetness.

Mr Lopresti said that there was an important distinction between the two, despite popular belief otherwise.

"Also, fruit within a tree will be ripening and becoming sweeter at different speeds and this is a major cause of variability in fruit quality at harvest."

Developed by an Italian company, TR Turoni, the DA-meter uses visible-near infra red spectrophotometry (vis-NIRS) technology to measure chlorophyll degradation, that is, the change in colour of fruit flesh from green to yellow.

This colour change can then be linked to the ethylene produced by the fruit which is an excellent indicator of when fruit is beginning to ripen.

DPI is working with the University of Bologna in developing calibration models for stone fruit, apples and tomatoes for use with the DA-meter.

Figure 1: Sweetness as measured by soluble solids content (SSC) of 25 Black Diamond plums harvested from within each of five individual trees

SCC was measured destructively using a digital refractometer. Horizontal red line represents the generally accepted minimum SSC for stone fruit in the Australian market. The number above each column of data points is the average SSC of fruit within that tree. SD is the standard deviation of the average SSC and describes the overall variability of SSC within that tree.

The latest results

This year, DPI followed 100 fruit in two nectarine cultivars during the four weeks prior to harvest.

Mr Lopresti said that unlike previous research, by using the DA-meter and the Nirvana, he was able to follow individual fruit during growth on the tree at harvest and during cool storage, rather than simply applying averages based on tests on fruit that was picked and destroyed.

Unfortunately it wasn't the best year for such a study due to high summer rainfall and brown rot levels, with almost half of the fruit Mr Lopresti was tracking dropping prior to harvest.

The findings, however, will add to DPI's understanding of what leads to good quality fruit.

"When we know what the important factors are, and how they interact, we can focus on these variables," Mr Lopresti said.

He said the results using plums and nectarines indicated that variability in sweetness, as measured by soluble solids content, was relatively high within individual trees studied.

As an example soluble solids content in 25 plums ranged between 11–18 per cent in five highly-managed plum trees that would be expected to produce fruit with a low variability in sweetness (see Figure 1).

This season the DA-meter was used to track the maturity of nectarines to determine the right time to pick fruit as it begins to ripen. For the nectarine cultivar 'Summer Flare 34' it was determined that a DA-meter reading below 0.6 indicated that fruit was beginning to ripen (see Figure 2).

As an example, the maturity of five fruit of similar size and colour was followed for six weeks prior to harvest. At commercial harvest only two of the five fruit were found to be physiologically mature even though other harvest maturity indicators such as colour and firmness were similar between fruit.

Figure 2: Change in nectarine maturity during the final 40 days of fruit growth based on chlorophyll degradation as measured non-destructively using a portable DA-meter. Maturity measurements were taken on the same five fruit within a single tree

Is it worth investing in the technology?

As the technology develops, the price for these devices will decrease, with further drops expected over the next five years.

Ultimately, Mr Lopresti said deciding whether to invest would really come down to the size of the orchard and the market they're aiming at.

"If you're targeting the export market and you're a large grower, you probably want to use it," he said.

For those that do invest, DPI can offer detailed advice on how to best use these technologies.

"For a particular block of trees a grower might want to know when should I harvest to maximise the sweetness?" Mr Lopresti said.

"We can give them a sampling plan to choose 50 fruit across an orchard on which to take measurements that will give them a fair idea of harvest maturity on that day."

The plan is simple to use, Mr Lopresti said.

Once the Nirvana readings are entered, the spreadsheet comes up with some basic statistics, including the average sweetness and range of sweetness values enabling growers to alter management techniques or strategically harvest depending on their needs.

And what should we expect in the future?

"Fundamental research is continuing on calibration of these technologies with fruit quality with the objective of extending their capabilities to other quality parameters such as dry matter content," Mr Lopresti said.

"Further validation of the sampling plan using these technologies will also be conducted over the next few years to improve its usefulness to growers."