Controlled atmosphere storage of apples: Cell wall composition and fruit softening

SummaryClimacteric apple fruits cv. Golden Delicious were stored for up to six months (a) at low temperature, (b) in controlled atmosphere (3% CO2 and 3% O2), and (c) in ultra-low oxygen (3% CO2 and 1% O2). The temperature, relative humidity and ethylene concentration were maintained at 1°C, 94% and 100 (µl 1-1 respectively in all three storage regimes. Samples of fruits from each treatment were analysed after four 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). Cellulose content fell slightly during storage without much differences between 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 did not vary significantly in each of the storage conditions but cellulose-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 only a slight shift towards lower molecular weights. The possibility is discussed that the enzymatic cleavage of linkages between hemicelluloses and pectins is involved in the softening of apple fruits during storage.