Dry weight or ash-free biomass

The mass of one C-mol biomass depends only on the carbon content according to Equation (39). Although it is tempting to calculate Mx from the elemental formula as the sum of the mass of its elements, this estimation is incorrect since some atoms have not been analyzed, in particular those that are found in ash. A/x,ash-free coiTesponds to the mass of the elemental formula without ash, i.e. to an hypothetical biomass, 

Since dry weight measurements always include ash, only M calculated according to Equation (39) should be used to calculate elemental or energy balances and never the hypothetical ash-free molar mass. 

The sulfur content of biomass varies largely from one microorganism to another the mass fraction is 0.28 g per 100 g biomass for Saccharomyces cere-visiae and up to 0.49 g per 100 g in Methanobacterium thermoautotrophicwn (Table 5). Similarly, the phosphorus and potassium content of biomass showed large variations among different organisms as illustrated in Table 5 and Table 6. Phosphorus is mainly found in biomass as phosphate groups in RNA and DNA whereas potassium is present as ion. The potassium content of M. thermoautot-rophicum is 4.3%, twice that of S. cerevisiae. 

Rbntgen fluorescence spectroscopy and plasma atomic emission allow quantitative measurement of S, P as well as many other elements. The ion mass fraction of Kluyveromyces marxianus and M thermoautotrophicum are listed in Table 7. The large difference in iron and nickel content is related to the metabolism. In fact, these two ions are cofactors of key enzymes in methanogene-sis [35]. The total mass of ions found in biomass is 6.36 g per 100 g biomass for M. thermoautotrophicum and 3.14 for K. marxianus (phosphorus is not taken into account since it is not present as free ion in biomass). The values are in good agreement with [36]. 

M thermoautotrophicum and K. marxianus. Adding the mass of oxygen bound to P to the total mass of all trace elements (Table 7) results in 17.59 g per 100 g of biomass for M. thermoautotrophicum and 7.33 g for K. marxianus. These values are very close to the corresponding mass fraction of ash (7.1 and 17.35 g per 100 g biomass), suggesting that no external oxygen has been taken up during combustion of biomass, and therefore the measured mass fractions of ash only takes into account elements that were originally present in biomass samples. 

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