Key Determinants of Postharvest Quality in ‘Gala Schniga® SchniCo Red(s)’ Apples: Firmness Retention at the Target Market After Long-Distance Transport

The objective of this study was to identify the factors that most strongly influence the postharvest quality of ‘Gala Schniga® SchniCo Red(s)’ apples under conditions of simulated transport and simulated trading at elevated temperature following long-term storage. The study was conducted over two storage seasons (2022/2023 and 2023/2024) on fruit originating from the experimental orchard of the Warsaw University of Life Sciences (SGGW-WULS) in Warsaw. The effects of harvest date (optimal—OHD and delayed by 14 days—DH), four variants of 1-MCP (1-methylcyclopropene) application: (control, Harvista™—preharvest, SmartFresh™—postharvest, and Harvista™ + SmartFresh™), controlled-atmosphere storage technology (ULO 1: 1.2% CO2 and 1.2% O2; ULO 2: 0.6% CO2 and 0.6% O2), storage period (5, 7, and 9 months), duration of simulated transport (4 or 6 weeks at 1 °C in normal atmosphere), and shelf life (0, 7, and 14 days at 25 °C) were analyzed. Five quality parameters were evaluated: firmness (F), soluble solids content (SSC), titratable acidity (TA), SSC/TA ratio, and 1-aminocyclopropane-1-carboxylic acid (ACC) content. Stepwise regression with backward elimination was applied to identify significant predictors, and partial eta squared (η2) was calculated to compare the relative strength of effects. Postharvest 1-MCP application had the greatest impact on maintaining firmness and TA (F: η2 = 75.8%; TA: η2 = 56.3%), whereas shelf life was the key factor in the deterioration of quality parameters after removal from storage (F: η2 = 55.5%; TA: η2 = 30.1) and in increasing the SSC/TA ratio (η2 = 29.6%). Harvest date strongly differentiated firmness (η2 = 51.3) and significantly affected TA (η2 = 14.4), while storage period had the greatest effect on ACC content (η2 = 14.2) and TA decline (η2 = 15.6). Preharvest 1-MCP application had a smaller effect on F and TA but significantly reduced SSC (η2 = 24.9), highlighting the importance of the timing of ethylene inhibitor application. The effects of simulated transport and preharvest weather indicators were statistically significant but relatively small compared with the effects of postharvest technological decisions and exposure time under retail conditions. The results indicate that maintaining target quality parameters throughout an extended supply chain requires precise determination of the harvest date, prioritizing postharvest 1-MCP application, and limiting shelf life under elevated-temperature conditions.