Relationship Between Fruit Maturity and Aroma Volatile Profiles of Peaches and Nectarines

Summary Australian peach and nectarine producers often harvest fruit unripe to ensure the fruit survives the rigorous supply chains required to reach both domestic and export markets. Consequently, fruit may be physiologically immature and unable to ripen adequately. One of the key selection criteria for consumers is fruit aroma. Most research on peaches and nectarines have used quality factors, such as skin colour, size, flesh firmness or sweetness, to determine maturity. This has led to the understanding that there is a large variation in overall maturity on the tree. The aim of this study was to ascertain physiological maturity, determined by the correlation of ethylene production with an Index of Absorbance Difference value (IAD) at harvest, and its impact on aroma volatile organic compounds (VOCs) produced by peaches and nectarines. The IAD measurement indicates reductions in chlorophyll-a in the outer mesocarp as the fruit develops. The changes in ethylene production were aligned with ranges of IAD values into stages: no ethylene production (NEP); onset ethylene production (OEP); and climacteric ethylene production (CEP). VOCs were evaluated using reliable existing methods (GC-MS), however the output was converted to production rates (nL/kg/hr), where most other investigations use concentrations. This study focused on some of the main aroma volatile organic compounds (VOCs) found to be associated with whole fresh nectarine and peach fruit (butyl acetate, benzaldehyde, hexyl acetate, linalool, γ-hexalactone, γ-octalactone, δ-octalactone, γ-decalactone, δ-decalactone) and how these chemicals were affected during fruit development (120 fruit x 8 cultivars), after harvest and during immediate simulated harvest shelf life (9 d at 20 °C; 4 cultivars), and after cooling and subsequent simulated shelf life (28 d at 2 °C + 3 or 6 d at 20 °C; 2 cultivars). The quality measures currently used to measure flesh firmness, sweetness and titratable acidity were also monitored during all assessments. All measurements were done on individual fruit to alleviate any influence of one fruit on another, especially with regard to ethylene production. Physiological development determined when the fruit were ready to harvest, which was different to the quality measures which indicated when the fruit were harvest ready. These points in evaluation did not match, with harvest readiness (quality) having markers in advance of the fruit being ready to harvest (physiologically mature). This in turn affected VOCs, however there was an unexpected observation of elevated VOCs production during the NEP range that was far greater than found in any other maturity stage, and it was often observed the CEP range had lower production than both other stages. There were instances of increases in VOCs production aligned with changes in ethylene production, particularly for γ-decalactone, which appeared to have higher production near the change from NEP to OEP. Observed results were not consistent between cultivars, nor were direct correlations found between chemical and ethylene production. Once harvested, physiological maturity affected the output of VOCs. Fruit harvested in the NEP range, while having some of the highest VOCs production rates at harvest, had poor production on subsequent days, indicating a lack of physiological readiness once removed from the tree. This maturity range was also observed to have unattractive textural trait normally associated with chilling injury with no cold storage undertaken. And indication that these fruit, lacking in physiological maturity, may be confused as chilling injured fruit after cold storage. Some of most important chemicals in peach and nectarine fruit aroma, linalool and total lactone production were observed to decrease with increased maturity and the length of shelf life. While those considered less attractive, acetates increased production with increased maturity and with the length of shelf life. It could be surmised that linalool and lactones were retained in the fruit as flavour. When cold stored all fruit had a lag time in producing VOCs after removal from storage. The NEP range were particularly affected with very delayed production across subsequent shelf life days. The delay in production was considered a significant effect after 2 weeks plus shelf life (nectarine) or beyond 3 weeks (peaches) regardless of maturity. The key finding from this work is that peach and nectarine fruit need to be producing their own ethylene when harvested to ensure they are physiologically ready and can progress through ontogeny. That the lack of ethylene production at harvest can affect the initiation and sustainable production of VOCs was illuminating. Even though, as a climacteric fruit, they can be induced into ethylene production by the exogenous ethylene produced by other fruit, this still did not improve the VOC production of NEP maturity stage fruit and was often aligned with poor textural quality, with or without cold storage.