With a per capita consumption of approx. 23 kg (2014-2015; Source: Statista) apples are the most popular fruit in Germany. They are rich in valuable ingredients like sugars, organic acids, polyphenols as well as vitamins. Approx. 20,000 cultivars exist worldwide, 1,000 of these in Germany. Only 40 cultivars (e.g. ‘Elstar’, ‘Jonagored’ and ‘Braeburn’) are used in commercial cultivation. Apple cultivars do not differ only in their peel colour and metabolite profile, but also in their storability. In general, cultivars are stored at 0-2 °C under normal atmospheric conditions. Temperature-sensitive varieties that are prone to flesh browning, however, should be stored at 3 ° C and ideally in a controlled atmosphere (e.g. 3 % carbon dioxide and 1% oxygen). For some particularly sensitive cultivars like ‘Braeburn’ even lower carbon dioxide concentrations of max. 1 % are recommended. Due to the different demands on the storage conditions all new varieties have to be examined in terms of temperature and atmospheric sensitivities to determine the optimum conditions for the preservation of fruit quality. Further, cultivation conditions may cause physiological damages like bitter pit. Optimized storage conditions can delay but never completely avoid the appearance of possible postharvest damages caused by fungi like Penicillium, Botrytis, Monilia or Gloeosporium.
The Department of Safety and Quality of Fruit and Vegetables of the Max Rubner-Institut in Karlsruhe has studied for many years the change of inner and outer apple quality parameters during long-term storage at different temperatures and atmospheres and at usually 90% humidity. Peel and flesh firmness, maturity index, total soluble solids, titratable acids, antioxidative potential, total phenolics and vitamin C content are considered as basic parameters. Diseases or damages like peel browning, scab or bitter pit are visually assessed. Based on these parameters an evaluation of the long-term storability of new varieties is performed.
The concentrations of certain apple metabolites may change substantially during storage, depending on the cultivar. Furthermore, certain varieties generally display strong changes during storage, while others have a relatively constant metabolite profile for months. In order to assess the metabolic effect of storage in detail, untargeted metabolome analyses using the comprehensive two-dimensional gas chromatography-mass spectrometry (GC × GC-MS) are carried out in addition to the determination of basic quality parameters. This allows to detect well-known apple ingredients like sugars, sugar alcohols, organic acids, amino acids, catechins, phytosterols, triterpenes and wax esters, etc.) as well as unknown or unexpected compounds. The aim of the project is a better understanding of the underlying mechanisms for storage-dependent changes in the metabolite profile, and to thus create adequate storage conditions.