CALORIFIC VALUES OF THE SOFT PARTS OF THE TELLINID BIVALVE MACOMA-BALTHICA (L) AS DETERMINED BY 2 METHODS

Calorific values of the ash-free dried soft parts of Macoma balthica (L.), sampled monthly during 1 year at a tidal flat station in the Dutch Wadden Sea, have been estimated by two methods. The indirect method started from an analysis of the biochemical composition and used conversion factors for the various components to calculate the calorific value of the total tissue. The direct method employed a micro-bomb calorimeter and a furnace for separate ash determinations. The indirect method yielded estimates that were consistently higher than those from the direct one. Two sources, explaining most of the difference, could be traced, viz. incomplete drying of the samples used in the calorimeter and a too high conversion factor for weight to calories applied conventionally for lipids. This conversion factor was found to depend on the method of extraction of the lipids. but was always significantly below the commonly used 9.3 to 9.7 cal · mg-1. The best estimates for the annual mean of the calorific value at complete combustion of the ash-free and well-dried soft parts of Macoma were 5.47 for the direct and 5.59 cal·mg-1 ash-free dry wt for the indirect method. Both values are higher than estimates for Macoma published earlier by various authors. Calorific values at physiological oxidation amount to only about 4.6 cal·mg-1 or less, due to incomplete protein oxidation. For this reason, it is concluded that biologically significant calorimetry cannot dispense with biochemical analysis. On the other hand, calculation of calorific values from biochemical composition should be accompanied by some calorimetry, as the conversion factor for the lipid fraction, at least, may deviate significantly from commonly applied values. © 1979.