Mal d 2 is a minor allergen from apple which shows a high conformational stability due to its eight conserved disulfide bridges. Chemical reduction of disulfide bridges and linearization of Mal d 2 lead to enhanced IgE reactivity in vitro and indicate a higher potential for allergenicity. Since food preservatives such as sulfur and vitamin C are reducing and denaturing agents, their influence on Mal d 2 allergenicity was verified by simulated food processing conditions. The immunoreactivity of purified Mal d 2 was investigated after different treatments in vitro and in vivo using IgG/IgE Western blotting, mediator-releasing cell assay, and skin prick and oral smear tests. The conformational changes of Mal d 2 upon addition of 1% and 5% vitamin C were also monitored by attenuated total reflectance Fourier transform infrared spectroscopy. The results show no positive skin and oral smear test reactivity to native, heated, or vitamin C-treated purified Mal d 2. Furthermore, the results confirm that sulfur in combination with heat treatment can influence the structural integrity and thus the allergenicity of Mal d 2, while vitamin C is too weak as a reducing agent to change allergenicity.