Proton spin-spin relaxation time of peel and flesh of navel orange varieties exposed to freezing temperature

Proton spin-spin relaxation times (T2) of peel (albedo and flavedo) and juice sacs (flesh segments) of navel oranges were measured at 10 MHz using a Bruker Minispec PC 110 NMR spectrometer. The oranges were subjected to chilling (5 ◦ C) and freezing (−7 ◦ C) temperatures for 20 h and warmed to room temperature before peeling for T2 measurements. The exposure to chilling or freezing temperature did not affect the T2 values of peel, but freezing caused an appreciable decrease (∼15%) in the T2 values of flesh segments of the varieties of navel oranges studied. When the peel was exposed to −20 ◦ C, the T2 showed a drastic reduction suggesting that the peel did not freeze at −7 ◦ C. The possible cause of reduction in the T2 values when exposed to freezing temperature may be damage to the juice sac membrane and leakage of juice out of the sac. The difference in the T2 values between juice sacs of freeze-affected and normal oranges can potentially be used for detection of freeze-damaged fruits.  2005 Society of Chemical Industry