Ester gum used as a food additive in beverages and chewing gum is prepared from resin acids of wood rosin harvested from the stumps of the longleaf pine (Pinus palustris). The resin acid composition of wood rosin can vary considerably; however, the main resin acids in ester gum are abietic acids, with smaller contents of dehydroabietic and neoabietic acids; pimaric acids, including isopimaric and sandaracopimaric acids; and palustric acid. The toxicology of glycerol esters of wood rosins harvested from the stumps of the pine tree is different from that of glycerol esters from tall-oil and gums, which are not used for the preparation of food additives. The latter are therefore not reviewed or evaluated in this monograph addendum.
Resin acids of wood rosin can react on their carboxylic acid group with salts, by esterification to ethylene glycol, diethylene glycol, glycerol, and pentaerythrol, by reduction to alcohols and aldehydes, and by decarboxylation; and also on their double bonds by isomerization (to abietic, norabietic, palustric, and levopimaric acids), by hydrogenation (dihydro- and tetrahydro- products), by dehydrogenation or disproportionation (dehydroabietic and di- and tetrahydro- products), Diels-Alder adducts (maleic anhydride and fumaric acid), oxidation and polymerization. The carboxylic group of the resin acids in wood rosin is attached to a tertiary carbon which is sterically hindered. In order to esterify this type of hindered carboxyl, higher temperatures and generally more drastic conditions are used than for other carboxylic acids. These steric effects are also responsible for the resistance of the resin acid ester linkage to cleavage by water, acid, and alkali.
Glycerol ester of wood rosin was previously considered by the Committee at its eighteenth, twentieth, thirty-third, and forty-fourth meetings (Annex, references 35, 41, 83, and 116). At its twentieth meeting, the Committee noted that in view of the stable ester bond and the anticipated stability of this material, studies of long-term and reproductive toxicity should be performed on the specific substance, as opposed to unmodified resin, before further evaluation. Specifications for food-grade material were adopted at the thirty-seventh meeting of the Committee (Annex 1, references 94 and 96), which described the material as a complex mixture of tri- and diglycerol esters of resin acids from wood rosin with a residual fraction of monoglycerol ester varying from 1 to 3%. At the forty-fourth meeting of the Committee, a full monograph was prepared on glycerol ester of wood rosin (Annex 1, reference 117). On the basis of the results of studies of faecal excretion by rats of unlabelled glycerol ester of wood rosin (identified as 'beverage-grade ester gum'), the authors concluded that hydrolysis was minor; however, the low sensitivity of the analytical chemical method used did not allow a conclusion about the stability or non-bioavailability of glycerol ester of wood rosin. The Committee was unable to establish an ADI until studies became available demonstrating the metabolic stability and non-bioavailability of glycerol ester of wood rosin under conditions resembling those present in the human gastrointestinal tract.