SOLUBLE CONSTITUENTS AND BUFFER PROPERTIES OF ORANGE JUICE

Abstract

The percentages of total soluble solids and of total sugars increased and that of acids decreased at approx. the same rate in the juice of Washington Navel and Valencia orange fruits during maturation. After the fruits reached the stage of commercial maturity, the percentages of total soluble solids, total sugars, and reducing sugars in the juice continued to increase, while the sucrose and acids decreased, as the season advanced. The rate of increase in total soluble solids, however, was less than that of the total sugars and reducing sugars. From Sept. 29 (fruit still green) until Mar. 2 (fruit well beyond the initial stage of maturity), the soluble solids of Washington Navel orange juice in California are composed of approx. 63-77% total sugars (35-40% sucrose and 27-38% reducing sugars), 23-8 % acids, and 15% other substances. The values for Valencia juice of corresponding maturity are similar. Where individual values for total soluble solids were plotted against the corresponding values for total sugars, noticeable scattering of the points occurred, but an increase in total soluble solids generally involved a corresponding increase in total sugars. This was verified by the high correlation coeffs. calculated between total soluble solids and total sugars. A similar relation between total soluble solids and reducing sugars was noted, but a considerably greater scattering of the points occurred. Although the total acids decreased during the growth and ripening of the fruit, while total soluble solids increased, large fluctuations in acid occurred without change in the total soluble solids and vice versa. Significant negative correlations were found to exist, however, between total soluble solids and total acids. Due to the high buffer capacity of the orange juice, large fluctuations in total acidity occurred without change in pH; but over a wide range of acid conc. (0.7-2.6%), the pH increased with a decrease in total acidity. The exptl. results show that the buffer capacity of orange juice is due chiefly to organic acids and inorganic salts. The soluble pectins in the juice affected its buffer curve only slightly. Boiling for 45 min., increased the initial pH of orange juice 0.2 over that of normal juice. This resulted in raising the buffer curve to a slightly higher pH level, but the shapes of the curves were the same. Although the limits of dilution were not reached in these expts., diluted (1:4) and undiluted juice had the same buffer capacity.