CONTROLLED-ATMOSPHERE STORAGE OF APPLES - CELL-WALL COMPOSITION AND FRUIT SOFTENING

Climacteric apple fruits cv. Golden Delicious were stored for up to si x months (a) at low temperature, (b) in controlled atmosphere (3% CO2 and 3% O-2), and (c) in ultra-low oxygen (3% CO2 and 1% O-2) The tempe rature, relative humidity and ethylene concentration were maintained a t 1 degrees C, 94% and 100 mu l l(-1) respectively in all three storag e regimes. Samples of fruits from each treatment were analysed after f our and six months. Treatment (c) gave the firmest fruit, followed by (b) and (a). The cell wall fraction per unit dry weight did not differ quantitatively for each treatment, but showed qualitative differences . The decrease in total pectin and hemicellulose contents with storage period was least in (c), intermediate in (b) and greatest in (a). Cel lulose content fell slightly during storage without much differences b etween each of the storage conditions. The free pectin content of the wall fractions increased least in (c) followed by (b) and (a), and was complementary to wall's covalently bound pectin content and firmness of the fruits. Ionically associated and hemicellulose bound pectins di d not vary significantly in each of the storage conditions but cellulo se-bound pectin decreased gradually. Changes in the molecular sizes of polyuronides were not evident up to six months in any of the storage conditions. Gel-filtration chromatography of hemicelluloses showed onl y a slight shift towards lower molecular weights. The possibility is d iscussed that the enzymatic cleavage of linkages between hemicellulose s and pectins is involved in the softening of apple fruits during stor age.