Heat Stress and Fruit Maturity Affects the Volatile Composition of Apples During Storage

Heat treatments of apple fruit can be used as quarantine treatments or to impart physiological changes to prolong fruit storage life.  Stress imparted by heat can impact the volatile composition of apple fruit and thus its flavor and quality.  Many factors can affect the response of fruit to heat stress including stress severity, fruit maturity, cultivar, and storage duration.  The objective of this study was to determine how these factors affect volatile composition and fruit quality, of ‘Jonagold’ and ‘Cortland’ apples.  Fruit harvested at 3 maturities, immature (pre-climacteric), commercial harvest maturity (CHM) and post climacteric maturity (PCM, CHM plus 4 weeks) were subjected to heat stress treatments of 46 °C air for 0, 4, 8, or 12 h. Following heat treatments, fruits were stored in air at 0 °C and evaluated after 0, 1, 2, or 3 months.  Headspace volatiles released from whole fruit were collected on adsorbent tubes and analyzed using thermal desorption and GC-MS.  In addition, peel and flesh browning, firmness, titratable acidity, and soluble solids were measured.  Esters, which provide fruity aroma notes, were the most abundant class of volatiles account for 60% and 86% of the total volatiles measured in ‘Cortland’ and ‘Jonagold’ apples, respectively.  Heat treatments of 4 or 8 h increased the ester content of immature ‘Jonagold’ apples 2.5- and 1.8-fold 24 h after treatment, but the 12 h treatment reduced ester content 50%.  In contrast, ester content of ‘Cortland’ apples subjected to the same treatments was reduced 30% to 65%.  The effects of heat treatments on more mature fruit and following longer storage times tended to decrease fruit ester content when compared to untreated control fruit.  After 3 months of storage, ester content of PCM ‘Cortland’ and ‘Jonagold’ fruit that had been treated for 12 h was only 9% and 18% of controls.  Heat stress-induced reduction of ester content was often not accompanied by any visual loss of quality.  However, fruit with the greatest loss of ester also developed skin and flesh browning.  Heat stress that caused a loss of volatiles was often accompanied by a decrease in fruit titratable acidity, but no effect on soluble solids content was observed.