Apple flavor is a complex matrix of physical and chemical sensory experiences. The consumer often confounds the textural feel of eating an apple with the chemical aspects of flavor. For this discussion, however, we will ignore any textural aspect and concentrate on the chemical components of apple flavor.
The first aspect of flavor is the collection of complex, volatile compounds produced by the fruit. These compounds have been studied extensively in the food chemistry arena. For example, Rowan, et al. (2009) studied the heritability of 89 volatiles which contribute to apple flavor and found that there is a genetic foundation to the complex and variable nature of apple volatiles. This work confirms the underlying genetic basis for varietal differences in flavor. Mehinagic et al. (2006) studied three apple cultivars and found about 30 volatiles contributing most to flavor. Further, there appears to be approximately 15 principal volatiles, and these vary significantly among varieties. Mehinagic et al. (2006) also studied the relationship between ripening and flavor volatiles and showed that some volatiles increased in concentration greatly during ripening and others did not.
Apples are a climacteric fruit, meaning that a mature apple will experience a dramatic rise in respiration when ripening is initiated. Many other changes occur as soon as ripening is initiated, softening being the most obvious textural change. It has long been known also that flavor volatiles increased in biosynthesis during the ripening process, contributing to the development of flavor. Other important chemical changes affecting flavor occur during the ripening process. Most importantly, organic acids break down making the fruit progressively less acidic, and starch is converted to sugar making the fruit progressively sweeter. The quantities of acid and of sugar are important, but their relationship also affects flavor.
A number of years ago, we were assessing a number of varieties, and one was an old English variety, Ashmead’s Kernel. We regularly measure the amount of sugar (as percent soluble solids in the juice), and we measured soluble solids before we had ever tasted the fruit of Ashmead’s Kernel. It had one of the highest sugar contents that we had ever measured, so the research technician set one aside to eat after he was finished with other tests. When he tasted the fruit, his response was not as expected. Ahsmead’s Kernel has very high sugar, but when ripe can also have very high acid content as well.
The sugar-to-acid ratio is one of the most important aspects of apple flavor. It increases dramatically during ripening, but it also is a major distinguishing characteristic among varieties. A “sweet” apple has a high sugar-to-acid ratio, and a “tart” apple has a low sugar-to-acid ratio. These distinctions regularly do not relate to actual sugar content or acid content.
Wesley Autio, University of Massachusetts, Amherst
References
Mehinagic, E., G. Royer, R. Symoneaux, F. Jourjon, and C. Prost. 2006. Characterization of odor-active volatiles in apples: Influence of cultivar and maturity stage. J. Agric. Food Chem. 54:2678-2687.
Rowan, D.D., M.B. Hunt, P.A. Alspach, C.J. Whitworth, and N.C. Oraguzie. 2009. Hertiability and genetic and phenotypic correlations of apple (Malus x domestica) fruit volatiles in a genetically diverse breeding population. J. Agric. Food Chem. 57:7944-7952.