Sugars Unrelated to Brix Changes in Stored Citrus Fruits

Abstract

The Brix content of the fruit pulp of 'Hamlin' oranges (HO) [Citrus sinensis (L.) Osbeck], 'Robinson' tangerines (RT) [Citrus reticulate (Blanco)], and 'Palestine' sweet lime (PL) [Citrus limettiodes (Tan.)] continues to increase during postharvest storage (Echeverria and Ismail, 1987). Such an increase in Brix is not due to the loss of water and the consequential increase in solute concentration , since postharvest water loss in citrus is restricted to the peel (Purvis, 1983). Furthermore, the juice yield per fruit remained constant throughout storage, indicating that the observed changes in Brix are the result of physiological activity. Within the citrus industry, an increase in juice Brix denotes an increase in the content of mono-and disaccharides and, consequently, a superior (sweeter) taste. But this may be a misconception , since the instruments used measure total amounts of soluble solids and not necessarily the simple sugars that impart sweetness to citrus juices. In this study, we examined the content of the three predominant sugars (major contributors to sweetness) in several citrus fruit cultivars during storage to determine the degree of sugar involvement in the postharvest changes in Brix. Fruit (with the exception of PL) were purchased from a local packinghouse and delivered to the laboratory 1 day after harvest. PL were obtained from the Citrus Arboretum at the Division of Plant Industry (Florida Department of Agriculture), Winter Haven, Fla. Only fruit of uniform size and appearance were used. Fruit were washed, treated with 1000 ppm of the fungicide thiabendazole, randomly sorted, and stored in boxes (paper bags for PL) at ± 15C and 95% RH. Each box contained ≈50 grapefruit, 80 oranges, or 90 tangerines. Each bag contained 10 sweet limes. Storage times were 9 weeks for HO, 7 weeks for 'Marsh' grapefruit (MG) [Citrus paradisi (Macf.)], and 4 weeks for PL and RT. At sampling time, three individual boxes or bags (each box or bag constituted a rep-licate) were mechanically squeezed and the juice analyzed for Brix and sugars. Brix was