New strategies to reduce the content of the allergen Mal d 1 in apple fruits during long-term storage

The key objective of this work was to find strategies to reduce the Mal d 1 content in apple fruits. Through this, individuals allergic to apples might have the chance to find ways to tolerate the consumption of apples. All the studies carried out, showed the complexity of this issue. The Mal d 1 synthesis is affected by several factors like cultivar, storage conditions, how apples are consumed as well as the interaction of the allergen with polyphenols. All these factors result in a complex interaction of influencing the Mal d 1 synthesis. 10 cultivars were analyzed in the different experiments of this study to get a representative overview of the different available cultivars. The fruits of these cultivars were stored for short time periods as well as for a period of 44 weeks to observe the development of the allergen Mal d 1. The results of each experiment can be summarized as follows:

1. The Mal d 1 content of eight cultivars was analyzed during a storage period of 36 weeks. In this period the Mal d 1 content increased in all tested cultivars independently of the storage condition. However, differences between the Mal d 1 contents of the two storage conditions were inconsistent. ‘Boskoop’ was the only cultivar that had a significant higher Mal d 1 content when stored in CA compared to cold-stored fruits. In contrast to this, ‘Elise’ had a higher Mal d 1 level after storage in a cold chamber. The effect of storage conditions on the Mal d 1 content of the cultivars ‘Elstar’, ‘Gala’, Golden Delicious’, ‘Jonagold’, ‘Pinova’ and ‘Topaz’ was not clear.
2. To investigate the influence of 1-MCP during long-term storage on Mal d 1 half of the fruits were treated with 1-MCP. Afterwards the fruits were stored for 36 weeks in a cold chamber as well as for 44 weeks in CA. The results of the Mal d 1 analysis showed that all fruits treated with 1-MCP had a significant lower Mal d 1 content than the untreated fruits, with exception of the cultivar ‘Boskoop’. Further, it was noted that cultivars with a high Mal d 1 content showed greater differences between the treated and the untreated fruits compared to cultivars with a lower Mal d 1 content.
3. Separated Mal d 1 analysis of the peel and the pulp resulted in a significant higher Mal d 1 content in one gram peel compared to one gram pulp. On average of all sampling dates and tested cultivars the Mal d 1 content was 11.5-times higher in the peel compared to the pulp. Since the peel makes up only 10% of the whole apple fruit, the absolute Mal d 1 content of the total peel was not highest for every variety compared to the Mal d 1 content in the total pulp. However, peeling an apple can reduce the Mal d 1 content significantly.
4. Analysis of catechin, epicatechin and the total phenol content resulted in only little differences between the sampling dates, but the concentration differed for the tested cultivars. Correlation analysis showed that naturally occurring catechin and epicatechin had no impact on the extractable as well as on the bio-available Mal d 1 content. But the extractable Mal d 1 of ‘Diwa’ and ‘Boskoop’ was strongly correlated with the total phenols. Furthermore, there were high correlations between Mal d 1 and the activity of the polyphenoloxidase.
5. Comparing the results of all tested cultivars showed that the Mal d 1 contents differ widely. The cultivars ‘Boskoop’, ‘Elise’, ‘Pinova’, ‘Santana’, and ‘Topaz’ were identified as cultivars with a relatively low Mal d 1 content. Further, it is to note that these cultivars are not a cross of ‘Golden Delicious’, with exception of ‘Pinova’. In contrast to this, cultivars with a higher Mal d 1 content have ‘Golden Delicious’ as a parent.