Skin greasiness in apple is caused by accumulations of liquid waxes: Evidence from chemical and thermodynamic analyses

The surface waxes of some apple (Malus x domestica) cultivars can become greasy during storage. Our previous work suggests the accumulation of fluid esters cause the increasing wax fluidity. To provide evidence for the inference, we revealed the effects of chemical changes in the apple epicuticular waxes on their thermodynamic properties. First, the chemical compositions and thermodynamic properties of the epicuticular waxes on 'Jonagold' apples were measured using GC-MS and differential scanning calorimetry, respectively. Then, a simulation experiment was performed in which alkanes (representing the epicuticular waxes) were mixed with different proportions of farnesyl linoleate. As skin greasiness in 'Joangold' develops, the linoleate and oleate esters (mainly the farnesyl linoleate) increase significantly, which is the most obvious chemical change in the epicuticular waxes. Meanwhile, the thermodynamic properties including the melt temperatures and the enthalpy decrease. In the simulation, the melt temperatures and the enthalpy of alkanes decreased with the increasing farnesyl linoleate. This confirms that the accumulation of farnesyl linoleate in 'Jonagold' can cause the decrease in thermodynamic properties of the epicuticular waxes. The results also suggest the esters may raise the wax fluidity of apples, in a manner similar to that of the plasticizer in the polymers.

Automated summary

The accumulation of oleate esters in the surface waxes of apples can lead to a decrease in their thermodynamic properties, causing the wax to become greasy. These results suggest that esters may act similarly to plasticizers in polymer in increasing the fluidity of apple waxes.