Influence of intercellular space differences due to variety and storage upon tuber specific gravity-dry matter relationship

Abstract

At each of two locations in two years, five varieties of potatoes were examined for specific gravity—intercellular space relationships. At harvest intercellular space differred consistently among varieties and decreased during two months' storage at 40 F. Changes in weight, volume, and intercellular space were used to explain that changes in tuber specific gravity may or may not reflect changes in dry matter content and that differences and changes in intercellular space are of sufficient magnitude to account for most of the discrepancies between recently published regressions of dry matter on specific gravity developed without correction for intercellular space. A coordinated study is proposed to establish the correctness of this postulation. Estimation of dry matter content of white potatoes from specific gravity determinations is a widely accepted practice. The accuracy of the estimation has been questioned because of the relatively large error of estimation (±1.5 to 2%) usually encountered (24), the wide divergence of regressions (1, 6, 15, 16, 18, 19, 23), and the frequency with which discrepancies have been noted between specific gravity and quality (14). Recent recommendations call for “Improved accuracy in the quick estimation of solids either by specific gravity or other means” and clarification of the “effect of temperature and period of storage on changes in ... specific gravity” (25). The tests reported here were undertaken to obtain corroborating evidence for the estimation of dry matter content of sweet potatoes from root specific gravity after correction for intercellular space (IS), (8, 12). Sweet potatoes, as well as several other commodities, contain relatively large amounts of IS (8, 11) but when this is accounted for, a reasonably consistent relationship exists that appears to be nearly identical to the regression for the specific gravity of sucrose solutions (8). The work with sweet potatoes showed that 1 ml of IS produced a buoyancy equivalent to the weight in water of 2.38 g of dry matter (12). Although white potatoes contain relatively small amounts of IS on the average, amounts of 1% (1ml/100ml) and more have been reported (2, 8, 15, 16). Nissen, in Denmark, reported that varieties differed in IS and that for greatest accuracy the IS should be accounted for (15, 16). Most workers in the United States, however, have ignored the influence of IS upon the accuracy of the methol of estimating dry matter content from tuber specific gravity, although several have speculated that IS contributed to variation in this relationship (1, 18). The tests reported here indicate that varieties may differ with reasonable consistency in IS at harvest in much the same manner as sweet potatoes (17), and that these differences and changes in IS during storage may be large enough to account for most of the discrepancies observed in the relationship between tuber specific gravity and dry matter content.