Impact of Maturity and Peel Characteristics on the Response of Mandarin Varieties to Wax Application and Storage

Mandarins are prone to the development of off-flavors during storage that can make them less desirable to consumers.  Prior research by our group has demonstrated the existence of varietal differences in the propensity for postharvest off-flavor formation in mandarins, although the factors that determine this are poorly understood.  Ethanol, a primary metabolite of fermentation that accumulates in waxed mandarins, is commonly used as a marker of off-flavor.  Ten mandarin varieties previously found to have greatly differing rates of ethanol accumulation during storage were harvested three times during the season (early-, mid-, and late-maturity) and stored for 1 week at 20 ºC after waxing.  Measurements of internal O₂ and CO₂, as well as peel O₂ permeability, were conducted during the initial 48 h after waxing, and then internal O₂, CO₂ and ethanol after 1 week at 20 ºC.  In addition, fruit from the mid-maturity harvest were evaluated for various peel anatomical characteristics.  Internal O₂ concentration rapidly declined in the first 5 h after waxing from an average of 19% to less than 3% within the first 5 h, after which the internal O₂ began to increase again.  There was a pronounced effect of season as ethanol concentrations tended to be higher and internal oxygen lower in late- as compared to early-season mandarins after storage.  Ethanol concentration in the mid-season fruit differed greatly due to variety, ranging from a high of 1501 mg L^-1 to a low of 434 mg L^-1 for Pixie and Fina Sodea, respectively, and similar differences were found in the other two harvests.  None of the peel anatomical characteristics that were determined at harvest were correlated with the concentration of ethanol present in the fruit after storage, although measurements of the integrity of the wax layer showed relationship with the final internal O₂ concentration.  The basis for varietal differences in ethanol accumulation remains to be determined.