Freezing effects in fruit tissue of kiwifruit observed by magnetic resonance imaging

Formation of ice, and the dynamics of freezing in immature kiwifruit (Actinidia deliciosa var. deliciosa cv. 'Hayward'; 4.7-6.8% total soluble solids) was observed by NMR imaging. Freezing was induced by subjecting detached fruit to circulating air at -40 degrees C and monitored by spin-echo imaging, Comparisons were also made between measurements of spin-spin (T-2) relaxation and the self-diffusion coefficient (D) in fresh and frozen-thawed samples, Formation of ice was visualised by loss of signal in affected areas of the fruit, freezing commencing at the epidermis and gradually progressing towards the core. Freezing tended to be asymmetric (in some fruit ice tended to move more rapidly through the core than through other tissues) and governed by the air-flow conditions (regions most directly exposed to the air-flow froze preferentially). T-2 relaxation was faster, and D coefficients greater in frozen-thawed fruit compared with fresh fruit. The significant decrease in T-2 relaxation following freezing has implications for design of on-line sensors distinguishing between damaged and undamaged fruit. (C) 1997 Elsevier Science B.V.