CELL WALL COMPOSITIONAL DIFFERENCES BETWEEN MEALY AND NON-MEALY 'FORELLE' PEAR DURING RIPENING

Abstract

Mealiness of 'Forelle' pear during ripening is a problem for the South African fruit export industry. The aim of this work was to investigate differences in cell wall polysaccharides between mealy and non-mealy 'Forelle' pears. Fruit were harvested at optimum maturity (62.8 N), cold stored for 3 to 21 weeks at -0.5°C, and ripened at 15°C for 0, 4, 7 and 11 days during which samples were taken for cell wall analysis. Only dates where mealiness occurred were used for cell wall analysis. Cell walls (CW) were extracted and de-starched with 90% DMSO and uronic acid (UA) content measured. Samples were sequentially extracted with water, CDTA, Na 2 CO 3 , 1 M KOH and 4 M KOH to determine differences in binding strength of the cell wall constituents between mealy and non-mealy tissues. UA content was measured for each of these fractions. The UA content of the water soluble fraction was lower in mealy tissues after 6 weeks of cold storage plus 11 days of ripening, and 9 weeks of cold storage plus 7 and 11 days of ripening. In the CDTA soluble fraction, UA content was also lower in mealy tissues of fruit that were cold stored for 6 weeks and ripened for 11 days. Mealy and non-mealy cell walls of 'Forelle' showed compositional differences. The lower water soluble pectin and CDTA soluble pectin without a substantial increase in the Na 2 CO 3 soluble pectin for mealy tissues suggests a more broken down cell wall in mealy tissues.