Tim Plozza, Christine Frisina and Daniele Pelliccia* 

Agriculture Victoria is collaborating with a Melbourne-based agtech company to assess the accuracy and utility of a prototype fluorescence sensor to detect and predict storage disorders in stone fruit.

If successful, the sensor and associated algorithms will be incorporated into a convenient, non-destructive handheld tool (Figure 1) for growers and packers to predict the risk of fruit developing internal disorders following cold storage, and to assess the fruit prior to sale.

Figure 1: Rubens Technologies’ handheld fluorescence spectrometer, which is commercially available for in situ estimates of fruit maturity (e.g., firmness).

Cold storage is commonly employed throughout the post-harvest chain to slow the fruit metabolic processes, allowing fruit to reach markets in a similar state to harvest.

However, in stone fruit this can result in the development of chilling injury, particularly where fruit are stored for two weeks or more which is typical for sea freight to Asian markets. Fruit with chilling injury is unacceptable at point of sale, incurring loss of income and damage to reputation.

Chilling injury commonly affects pectin metabolism within the fruit, with pectin breakdown resulting in flesh disorders such as mealiness (‘woolliness’) and internal browning (Figure 2).

Figure 2: An example of internal browning in an ‘O’Henry’ peach.

Conversely, an increase or lack of normal pectin breakdown during ripening results in rubbery or leathery texture. These disorders are generally not apparent immediately post-storage and may take several days to develop once the fruit are at ambient temperature. The external appearance of the fruit is generally not affected, making visual identification of these disorders impossible without cutting the fruit open.

This study investigated fluorescence spectrometry as a non-destructive means of detecting and predicting the development of chilling injury-induced internal disorders in peaches and nectarines, using prototype bench-top and handheld instruments and predictive models developed by Rubens Technologies.

These instruments illuminate fruit with ultraviolet (UV) light and measure the resultant light emissions (fluorescence), which are characteristic of the type, and levels of, fluorescent compounds (typically pigments) contained within the fruit.

The handheld instrument also captured visible and near-infrared reflectance spectra, providing further information about the fruit. In a parallel study by Agriculture Victoria and Rubens Technologies, fruit fluorescence spectrometry was found to accurately measure a range of fruit parameters including maturity (ethylene production and fruit firmness) and fruit colour in peaches.

The white-fleshed nectarine ‘Majestic Pearl’, which is one of the major export cultivars, and the yellow-fleshed peach, ‘O’Henry’, which is known to be quite susceptible to internal disorders caused by chilling injury, were examined.

Fruit was harvested from the stone fruit orchard at the Tatura SmartFarm during the 2020–21 and 2021–22 seasons. After harvest, fruit was placed in cold storage at 2°C, with subsets of fruit removed periodically after up to six weeks of cold storage, and destructively assessed for internal disorders following a shelf-life duration of up to six days at ambient temperature.

During assessment, fruit were rated for the presence of internal disorders (internal browning, mealiness and rubbery or leathery texture) using a scale from 1 to 5, with 1 representing no symptoms and 5 representing severe symptoms.

Fruit was scanned with a fluorescence spectrometer on the day of harvest, immediately upon removal from cold storage, and on the day of destructive assessment. In the 2020–21 season the benchtop instrument was used, and the handheld instrument was used in the 2021–22 season.

Using a machine learning approach, Rubens Technologies was able to relate the fluorescence spectra to destructive assessment results and develop models to predict the presence or absence of internal disorders.

Initial work in the 2020–21 season using the benchtop instrument showed very good accuracy for the detection of storage disorders on the day of assessment (85–91%) and on removal from cold storage (77–83%); however, it could not predict the development of storage disorders in ‘Majestic Pearl’ from scans taken at harvest, and only had a low predictive ability (61%) at harvest for ‘O’Henry’.

A revised experimental design for the 2021–22 season and the use of the newer handheld instrument, which records better quality spectra, saw similar accuracy to the benchtop instrument for the detection of storage disorders on the day of assessment and on removal from cold storage (Table 1).

Compared to the previous season's results using the benchtop instrument, there was a significantly better ability to predict the development of storage disorders from fruit scans taken on the day of harvest. More work needs to be done to further develop and validate the predictive model; however, these preliminary results show the handheld fluorescence spectrometer and predictive model could be very useful in a retail setting to identify fruit unfit for sale.

For growers and packers, the technology could enable the prediction of the cold storage potential of fruit consignments, allowing informed decisions on where to market their fruit. Rubens Technologies is currently developing a fruit grader-mounted fluorescence spectrometer to be trialled in the coming season at the Tatura SmartFarm.

Based on these preliminary results, the Rubens handheld fluorescence spectrometer shows promise as a useful tool for the identification of internal disorders at point of sale, and also as a predictive tool to assess the risk of fruit developing a chilling injury-related internal disorder when fruit are scanned before placing into cold storage.

Table 1: Accuracy of the models for predicting the correct storage disorder class (normal or storage disorder)
as well as rates of false positives and false negatives based on handheld fluorescence spectra
collected on the day of destructive assessment, after removal from cold storage, and at harvest for
storage periods of two or four weeks, for fruit harvested in the 2021–22 season.

Acknowledgments:

This study was a component of the project ‘Deploying real-time sensors to meet Summerfruit export requirements’ – a joint venture between Agriculture Victoria, the Summerfruit industry, RMIT University and two Ag Tech companies — Green Atlas and Rubens Technologies Pty Ltd. The project was financially supported by the Food Agility CRC through the Australian Commonwealth Government CRC Program with co-investment from Agriculture Victoria and Summerfruit Australia Limited. The CRC Program supports industry-led collaborations between industry, researchers and the community.

*Tim Plozza and Christine Frisina, Agriculture Victoria, AgriBio Centre For AgriBioscience, Bundoora; and Daniele Pelliccia, Rubens Technologies. Contact: Tim Plozza Tim.Plozza@agriculture.vic.gov.au