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Does a hot water treatment affect apple quality?


Depending on the cultivar and on the conditions, apples can be stored up to 10 months. However, a fungal infection before harvest may affect storability strongly by causing an early spoilage. This problem is especially relevant for organic cultivation where the application of synthetic fungicides is not allowed. However, it was shown by the MRI in a former project that a hot water treatment of apples before storage can, for instance, reduce substantially the postharvest losses due to fruit rot caused by the fungus Gloeosporium. Usually, this is already achieved by immersion the apples for 2-3 minutes in 48-52 °C warm water [1, 2]. While the fruits of some cultivars tolerate such a treatment, others develop peel browning within the following weeks. The reasons for this different behaviour are not known so far.

For this purpose, in a two-year trial, the fruits of six different apple varieties - including well-known varieties such as Elstar and Pinova as well as newer varieties such as Allurel, Ladina, Natyra and Crimson Crisp - were subjected to an immersion treatment with warm water (48 °C or 52°C) for 2 min. After the subsequent six-month storage at 1°C, it was tested how well the fruits of the individual varieties tolerated the hot water treatment. Apart from the natural variation between the growing years, it could be shown that the varieties Elstar, Crimson Crisp and Natyra tolerated the treatment well to very well, while Allurel and Ladina developed skin browning to a large extent. In the cultivar Pinova, the extent of damage varied greatly between growing years.

In the second year of the trial, the composition of the fruit peel was determined directly after treatment at 52°C and after six months of cold storage. To determine the influence of immersion in warm water on fruit quality, non-directional metabolome analyses of the fruit peel were carried out at the Institute for Safety and Quality in Fruit and Vegetables using comprehensive two-dimensional gas chromatography-mass spectrometry (GC×GC-MS) to cover a broad spectrum of compounds as good as possible. The aim of the project was to characterise the acute metabolic effect of the hot water treatment. In addition, it should be clarified whether the hot water treatment also has long-term effects at the level of the individual ingredients.

The metabolome analyses showed that the hot water treatment had both a short-term and a long-term effect on the constituent profile of the apple peel. Some of the typical metabolic processes during fruit storage, such as the degradation of acids or accumulation of sugars, were influenced by the hot water treatment e.g. accelerated or slowed down. However, these changes did not follow a simple pattern, but had a complex and partly variety-dependent effect on the content profile of the apple peel. Currently, we are investigating to what extent the changes caused by the hot water treatment provide clues as to why the individual varieties are differently susceptible to the hot water treatment.