Irradiation uses ionising radiation, which alters food’s molecular structure, but leaves it looking the intact. Substances produced by exposure to radiation are known as radiolytic products. Their presence indicates whether or not a product been irradiated (Chauwan, S. et al. (2008). Detection methods for irradiated food. Comprehensive Reviews in Food Science and Food Safety, http://onlinelibrary.wiley.com/doi/10.1111/j.1541-4337.2008.00063.x/full).
Different foods react differently to radiation. The changes in them mean that Irradiated produce is processed; it is no longer fresh.