Abstract: Apple is the fourth most important fruit produced and eaten around the world with a production of 84 million tonnes in 2014. The modern cultivated apple (Malus × domestica Borkh) is probably the result of interspecific hybridization and, at the moment, ten thousand or more apple cultivars are listed in the European Apple Inventory. The large number of cultivars reflects also a broad range of variability in the quality traits. “Fruit quality” is a dynamic concept changing on the basis of the consumer needs and perceptions that reflect socio-cultural evolution. Almost all the characteristics determining apple quality can be measured or classified. Consumers assess apples by its appearance (colour, size, shape, absence of defects) and then by its eating quality, although the latter may determine the willingness to buy the product again. Furthermore, the high level of polyphenols confer apples relevant nutraceutical properties. Apple consumption has been deeply encouraged and a broad range of varieties are competing in the market, but the indisputable quality of a commodity is proved by the repeating purchase by consumers. In this review, the most common worldwide utilized external and internal quality parameters (i.e. firmness, starch, soluble solid content, titratable acidity) and conventional techniques as well as some new approaches to assess them are explored. New quality metrics recently developed, like IAD and dry matter, are discussed too. The current review discusses the impacts and potentials of both environmental conditions, and agronomic factors. Environmental and agronomic factors along all the growing season strongly affect the final apple quality including the nutraceutical aspects. Temperature and light contribute to several modifications of external and internal apple quality like red overcolor and dry matter accumulation, but also can trigger unwelcome disorders like sunburn. Orchard design, training system and pruning can dramatically impact the skin overcolor and the maturity of the fruit. Crop load and thinning can determine physiological adjustments that benefit the dry matter accumulation in the fruit. Irrigation and nutrition can modified the overcolor and the chemical composition of the flesh.
