The effect of calcium infiltration on structural changes in cell walls of stored apples

Apples of cv. Golden Delicious were vacuum infiltrated with 300 mM mannitol, with or without 50 and 150 mM CaCl2. The fruits were stored under low temperature of 2-4 degrees C for 100 d and sampled every 25 d for flesh firmness and transmission electron microscopy. The infiltration of the fruits with 150 mM CaCl2 resulted in higher retention of flesh firmness throughout the storage period, but with 50 mM CaCl2 this could be retained only up to 25 d. The content of insoluble calcium in the flesh increased with increase in the storage period. A large amount of the infiltrated calcium accumulated in walls. However, no relation could be observed between flesh firmness and the amounts of calcium associated with free, ionically bound and covalently bound pectin fractions of the walls. The transmission electron micrographs revealed that most of the changes appeared to involve the middle lamella, and flesh firmness can be judged from the visual inspection of the micrographs. It appears that the effect of calcium in maintaining an intact middle lamella may not be its direct ionic effect.