Subject: To deliver peaches of high quality with a long shelf life, fresh, sufficiently ripe fruits are stored at low temperatures. The aim is to slow down the ripening process and prevent the spread of rot and mold. However, deviations from the ideal cooling temperature can cause chilling injury in the stored fruit. The damage is usually not discovered until the goods have reached the customer. To prevent or minimize food loss, an early warning system is needed, and the researchers Farcuh and Hopfer have successfully used aroma-active secondary metabolites in the fruit as markers for chilling injury. The fruit's metabolism, which is active during the ripening process, reacts to chilling injury. The tell-tale signs are that concentrations of emitted aroma-active volatile organic compounds (VOCs) change. The aroma profile can be monitored using HS-SPME-GC-MS to determine if anything has changed.

About the study: Farcuh and Hopfer examined 384 specimens of the luscious golden peach “Red Haven” (Prunus Persica) . The researchers randomly divided the fruits into three groups, each of which was stored at three different temperatures: at 0 °C, the ideal cooling temperature for peaches, at 5 °C, known for the formation of cold lesions, and at 20°C. Since no cooling damage is to be expected at 20 °C, this fruit selection served as a control group. The aroma profile of the fruit was then determined over several days and correlated with observed chilling injury.

Sample preparation: 50 gram samples of peach were each mixed with 50 mL of ice-cold saturated calcium chloride solution and 50 µL of the internal standard naphthalene-d8 and homogenized. The homogenates were transferred to individual glass bottles, sealed and placed in ice. Ten minutes after the foam had separated from the liquid, a 1 mL aliquot of each sample slurry was transferred to a 20 mL headspace glass vial, sealed with a steel screw cap with a Teflon/silicon septum, snap frozen in liquid nitrogen and stored at -80 °C until the VOCs were determined.

The Analysis: Prepared peach samples were analyzed by HS-SPME-GC-MS and the aroma profiles were recorded. A 7890B GC with 5977B MS (both Agilent Technologies) were used, the sample extraction and -introduction steps were automated using a GERSTEL MultiPurpose Sampler (MPS robotic). 

Sample Preparation: Samples were thawed at room temperature for one hour and placed on the MPS. Each sample was equilibrated for five minutes at 40 °C and then extracted using HS-SPME for 30 minutes at the same temperature. A mixed phase preconditioned SPME fiber (DVB/CAR/PDMS, Supelco) was used. The fiber was thermally desorbed at 250 °C for ten minutes in splitless mode with a constant flow of 1.2 mL/min of high-purity helium as carrier gas. Separation of the desorbed volatile compounds was performed using a capillary GC column (Rtx-Wax 30 m × 0.25 mm × 0.25 μm, Restek) in a temperature programmed GC oven (35 °C [1 min] - 5 °C/min - 150 °C - 30 °C/min – 250 °C [10 min]). The MS ionization source, quadrupole, and interface were set to 230 °C, 150 °C, and 250 °C, respectively. Analytes were detected across the 35 to 350 amu range at 8.1 spectra per second; retention indices (n-alkanes C8–C23) were compared with values from literature, MS spectra with library entries from the NIST mass spectral library (National Institute of Standards and Technology (NIST) version 17) and, whenever available, with authentic standards.

Results: In their SPME-GC/MS investigation into aroma compounds released by peaches of the “Red Haven” variety during post-harvest storage, out of a group of 30 volatile aroma compounds, Farcuh and Hopfer identified a total of nine that were predictive of chilling injury damage. These are γ-hexolactone, γ-octalactone, γ-decalactone, δ-decalactone, 2-hexenyl acetate, benzaldehyde, 3-methylbutyric acid, γ-damascenone and linalool. According to Farcuh and Hopfer, those volatile aromatic substances have the potential to detect chilling injury in peach fruits of the “Red Haven” variety at an early stage, i.e. before symptoms such as mealiness and leatheriness become apparent.

Conclusion: Monitoring of the determined markers could prove helpful when developing optimal post-harvest storage and handling practices of peaches. Food produce loss caused by chilling injury could be prevented or minimized. The researchers have reservations concerning the use of the determined markers for other fruit types. To evaluate the universal robustness of their method for commercial detection of early-stage cold injury development, future studies should expand the range of peach cultivars examined.

Source: Farcuh M, Hopfer H. Aroma volatiles as predictors of chilling injury development during peach (Prunus Persia (L) Batsch) cold storage and subsequent shelf-life. Postharvest Biology and Technology 195 (2023) 112137.