Evaluation of the 8 mm penetrometer plunger to determine harvest maturity on plums

The objective of this trial was to establish if an 8 mm penetrometer plunger can accurately determine flesh firmness as a harvest maturity parameter and also as an indicator of flesh firmness of plums after arrival in the market. The standard 11.2 mm plunger as currently used in South Africa was used as reference. The aim is to standardise the method used to determine flesh firmness in South Africa, with other stone fruit producing and importing countries.

Flesh firmness was determined on Fortune, Laetitia and African Delight plums with penetrometer plungers of 8 mm and 11.2 mm diameters. Firmness was determined in the 2019 season at three different harvest maturities with an 8 mm plunger and 11.2 mm plunger on each cheek, while in the 2020 and 2021 seasons it was determined at harvest, after cold storage (SOS) and after shelf life (EOS). At each of the 2020 and 2021 evaluations there were three sub samples and firmness was determined on both cheeks of the same fruit, sample 1 with an 8 mm plunger and 11.2 mm plunger on each cheek, sample 2 with a 8 mm plunger on both cheeks and sample 3 with a 11.2 mm plunger on both cheeks. Fruit quality was also determined at SOS and EOS without statistical analysis.

The three cultivars tested exhibited slight variations in the regressions comparing the 8 mm and 11.2 mm plungers at the three evaluations, namely: Harvest 1, 2 and 3 for the 2019 season and at harvest, SOS and EOS for the 2020 and 2021 seasons. In 2019 the average the conversion factors varied between 1.8064 (R² 0.7236) for Fortune, 1.7691 (R² 0.7910) for Laetitia and 1.7016 (R² 0.6322) for African Delight. In 2020 the conversion factors varied between 1.7215 (R² 0.7122) for Fortune, 1.8095 (R² 0.9159) for Laetitia and 1.6656 (R² 0.6964) for African Delight. The average across cultivars was 1.73 which is similar to the 1.76 of the 2019 results and comparable to the conversion factor of 1.8 that was previously established for nectarines. The average conversion factor in 2021 for Fortune (Figure 14) was 1.7688 (R² 0.7286), Laetitia (Figure 16) 1.7534 (R² 0.8709) and African Delight (Figure 18) 1.7061 (R² 0.7267). The average conversion factor across cultivars was 1.74 compared to a conversion factor of 1.73 & 1.76 in the previous two year’s of plum work and 1.8 that was previously established for nectarines (De Kock, 2019). Taking all 3 seasons in consideration, the average conversion factor across cultivars was 1.745.

The flesh firmness distribution curves established for the 8 mm and 11.2 mm plungers for each of the cultivars showed similar trends and there was a normal distribution of flesh firmness readings in all cases.

Flesh firmness comparison in 2020 with 8 mm plungers (data not shown) between both cheeks of same fruit exhibited an average conversion factor for Fortune of 0.9974 (R² 0.5985), Laetitia 0.9869 (R² 0.933) and African Delight 1.009 (R² 0.7667), with the higher accuracy for Laetitia with a R² value of 0.933. Similarly, in the 2021 season the average conversion factor for Fortune was 0.9765 (R² 0.5112), Laetitia 0.9736 (R² 0.8221) and African Delight 0.9764 (R² 0.6048), with the higher accuracy on the Laetitia with R² value of 0.8221. The low R² values were due to variation in flesh firmness between the cheeks of the same fruit and not due to the

plunger diameter used and this was evident from the scatterplots.

Like the 8 mm plunger, in 2021 (Figures 26,28,30) the conversion factor between the two 11.2 mm plunger flesh firmness readings for both cheeks per fruit across cultivars were ± 1.0. This means that on average there are no differences.

The milestone 3 that was altered after the 2019 research was achieved in 2020 and 2021 and it was established that both the 8 mm and 11.2 mm penetrometer plungers tested could measure flesh firmness accurately with R² values in 2020 of between 0.6964 and 0.9159 and R² values in 2021 between 0.7267 and 0.8709.

The results indicated that a change can be made from the 11.2 mm to the 8 mm penetrometer plunger to determine flesh firmness on plums at harvest and after storage. It will be important to test a wider range of cultivars per production region to fine tune the conversion factors and possibly develop more specific conversion factors for each cultivar as the data base is expanded. Across three seasons, in this study, the average conversion factor across cultivars was 1.745. The outcome of this project must be shared with the SFJMF for decision making and thereafter can be shared via the Fresh Quarterly and SA Fruit Journal.