Extractives in Douglas-fir forestry residue and considerations for biofuel production

Abstract

Forestry residues are a plentiful, low environmental impact feedstock for biofuels and bioproducts. Douglas-fir is the most prevalent tree species in the timberlands of western North America, with approximately 5 million tons of sustainably harvestable forestry residues available each year. These forestry residues are an important potential biomass feedstock containing holocellulose, lignin, protein, ash, and phytochemicals commonly identified as “extractives”. The phytochemical extractive category make up 5–25 % of the dry weight for different tissues of Douglas-fir, but are rarely represented with molecular detail in feedstock models of residues for biofuel or other bioproduct. These extractives contain both primary and secondary metabolites and represent potential revenue sources as side products from processing, but also includes species that are astringent, toxic, endocrine disruptors and/or reactive in similar chemical processes. Within the “extractives” category are phytochemicals such as proanthocyanidins, phlobaphenes, waxes, flavonoids, terpenoids, phytosterols, lignans and many more. This review first identifies phytochemical molecules found in different Douglas-fir tissues, then quantities these by category and individual molecular species, to the extent allowed by the literature. We combine the literature into a quantitative, molecularly detailed, mass conserving model for a particular Douglas-fir forestry residue (“slash”). This model is used in a sulfite/bisulfite biofuel process simulation for understanding the molecular partitioning of extractives in different process streams. Model results are used to explore some implications for extractive species in the production of sugars and waste products from Douglas-fir forestry residue feedstock.

Appendix: Sapwood, heartwood, and bark detailed models

Molecularly specific compositional models for softwood, hardwood, bark, and slash were assembled and used to come up with the specific compositional estimates in Table 13. This was done by using a combination of literature data, heuristics, and mass balances to estimate the concentrations of individual molecules for extractive classes that available literature can justify to be at least 0.1 % of the forestry biomass (o.d. basis). This excludes classes such as triterpenes, lignans, and sesquiterpenes (Dellus et al. 1997; Conner and Foster 1981; Erdtman et al. 1968). These models are presented in Table 16.

Table 16 List of individual molecules in Douglas-fir for classes expected to be >0.01 % of the o.d. mass

The concentrations of the most commonly studied sugars are included for the different tissue types (i.e., glucan, mannan, xylan, galactan, and arabinan). Analyses of other units of hemicellulose such as, 4-O-methyl-glucuronic acid, glucuronic acid, galacturonic acid, and rhamnose have been carried out for the heartwood and sapwood (Kaar and Brink 1991) and values for these components are included; these compounds are not regularly reported in studies of the bark. Lignin values reported here come from studies that measured acid soluble and insoluble lignin (Kaar and Brink 1991; Robinson et al. 2003). Protein values have been reported for the sapwood, heartwood, and cambium (Kaar and Brink 1991; Dziedzic and McDonald 2012). The value for protein in the cambium has been used here to estimate the protein composition of the entire region we label “bark”. We used reported ash values for sapwood, heartwood and bark (Kaar and Brink 1991; Leu et al. 2013).

In many instances data for the composition of specific extractive compounds were not directly quantified in the literature for sapwood, heartwood, and/or bark. When this was the case, estimates were made based on a synthesis of the best available data. In the cases of flavonoids and phytosterols in the full model, we found data for the total amount of that extractive category in a tissue, and then estimated the individual molecular composition using mass-conserving heuristics and other molecular characterizations we found in diverse literature reports. With the diterpenes and monoterpenes, we use reported values for components that have been quantified in the sapwood and heartwood (Foster et al. 1980) and estimated other components from their known relative abundance compared to the quantified compounds.

Dellus et al. (1997) reported the total polyphenol extractives (polymeric polyphenols and flavonoids) in sapwood and heartwood as 0.7 and 2.4 % of the dry mass respectively. The authors were also able to individually identify the species responsible for 52 and 86 % of the polyphenols in each tissue type, respectively. We took the individual species identified, and had them represent the whole polyphenol category (excluding phlobaphenes though) by scaling their values up proportionally to meet the total measured polyphenol values. To account for the complex polymeric polyphenols in a manageable way in this model, we separated the polymeric polyphenols into water soluble “condensed tannins” and water insoluble “phlobaphenes”. Specific types of condensed tannins or phlobaphenes are lumped into one these categories rather than using the diverse nomenclature associated with these compounds. For example, the procyanidin measurement of the sapwood from Dellus et al. (1997) are assigned to the condensed tannin category in the full compositional model Table 16 here, and the phlobatannins from Graham and Kurth (1949) were used for condensed tannins in the heartwood (~0.2 %). The polymers of dihydroquercetin, procyanidins, and other polyphenols Dellus et al. (1997) reported in the heartwood are assumed here to be comprised of condensed tannin and phlobaphene. The phlobaphene value in the heartwood was found by taking Dellus et al. (1997)’s adjusted values for “complex polymers” and subtracting Graham and Kurth (1949)’s value for phlobatannin (condensed tannin). In the bark, condensed tannins have been reported to be in the range of 7.5–18 % (Kurth 1953) and an intermediate value of 13 % is assumed here to be similar to a recent report on bark characterization (Pan et al. 2013). Studies have been undertaken to identify more polyphenol species in the bark (Hergert 1960), but because of limited quantitative data, the only bark flavonoid value in our model is for dihydroquercetin/dihydroquercetin-glucoside [a value of 6 % assumed here (Kurth 1953)].

Fats and Waxes values come from reports interested in the wax or tall oil and mass balances when total quantities of a category were known. Ferulic acid esters have been reported to be 1.4 % of the dry weight of the bark (Laver and Fang 1989), but reported data was not found for sapwood and heartwood. Ferulic acid esters and triglycerides show up in the hexane soluble wax of the bark (5–7.5 % by dry mass (Kurth 1953), an intermediate value of 6 % assumed here) so the portion of hexane soluble wax not attributed to ferulic acid esters was used for the Triglycerides/Free Fatty Acid section of our bark model (Kurth 1953). Foster et al. (1980) reported triglycerides in the sapwood were 58 % of the neutral section of the diethyl ether extract (corresponding to 0.319 % dry weight), and this same proportion of triglycerides in the neutrals was assumed for the heartwood (0.4 %; Foster et al. 1980). The free fatty acids also reported by Foster et al. (1980) for sapwood and heartwood were added to the triglycerides for the Triglycerides/Free Fatty acid portion of the full compositional model. For further information on the constituents of fats and waxes, see (Pan et al. 2013; Laver et al. 1977; Foster et al. 1980).

Foster et al. (1980) identified values for acidic diterpenes (resin acids) in the sapwood and heartwood and Pan et al. (2013) in the bark with their values used in our model. The resin acid data was paired with a study by Erdtman et al. (1968) which reported the chemical composition of a wood resin sample to estimate the non-acidic diterpenes and monoterpenes. Values for the total phytosterols (including phytosterol esters) in the heartwood and sapwood come from Foster et al. (1980), i.e. 35 % of their “neutrals” class in the sapwood and heartwood are used as phytosterols. We assume that 50 % of these phytosterols are sterol esters before saponification. This is combined with a report from Fischer et al. (1981) which identified the individual species of sterols present after saponification. The sapwood phytosterols values are also used as estimates for those in the bark since resin components are known to be similar in the sapwood and bark (Back 2000).

To complete the mass balance, the unaccounted remainders of the compositional models are treated as soluble carbohydrates such as pectin and called “Extractable holocellulose” in Table 16.