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

Flesh firmness as an important harvest maturity parameter is currently determined on nectarines with a penetrometer fitted with an 11 mm plunger. In late cultivars that are harvested relatively firm the use of the 11 mm plunger proved to be problematic since firmness readings are highly variable. The aim of this trial was to determine if 8 mm plungers can be used to provide accurate flesh firmness readings for nectarines.

Four nectarine cultivars were harvested at regular intervals starting 4 to 7 days prior to the expected commercial harvest date in 2016 and 2017. In 2018, three additional cultivars were added. In total there were three harvest maturities for each cultivar. At each harvest the flesh firmness was determined with penetrometers fitted with plungers of 11 mm and 8 mm in diameter. In 2016 and 2017, standard deviations were determined for flesh firmness at each of the harvest maturities to determine the variability in the firmness readings. In 2018, a regression analysis for flesh firmness with the two plungers was done. After harvest the fruit were cold stored for 4 weeks at -0.5 °C and evaluated before and after a shelf life of 5 days at 10°C, for flesh firmness, decay, shrivel and internal quality.

Generally, the standard deviation in flesh firmness at harvest was higher when the 11 mm plunger was used compared to the 8 mm plunger. This indicates that the variation in flesh firmness for fruit harvested at specific sizes or skin colour will be less when the 8 mm plunger is used. For Alpine, August Red and September Bright nectarines, the fruit from the first harvests generally developed less internal problems and decay compared to fruit from the last harvest. The 8 mm plunger as a tool to determine flesh firmness at harvest could potentially allow producers to harvest early without incurring rejections due to variation in flesh firmness at harvest. Summerfire (non-melting type) did not soften over the harvesting period and remained firmer than the maximum firmness level set by DAFF.

Flesh firmness was determined accurately with the 8 mm plunger and variation between readings was less compared to the 11 mm plunger. A conversion factor of 1.8 is suggested to covert firmness readings from the 11 to the 8 mm plunger. On firm fruit, the 8mm plunger could be more accurate as fruit will not crack, as often occurs when the 11 mm plunger is used. If there is a need to change to the 8mm plunger DAFF regulations should be changed accordingly.