Date of Award
Doctor of Philosophy
Food Science and Human Nutrition
Zivko L. Nikolov
Two enzyme forms (PAO-I and PAO-II) of porcine liver aldehyde oxidase (aldehyde: oxygen oxidoreductase, EC 184.108.40.206) were purified to homogeneity using affinity chromatography. The heat treatment and ammonium sulfate fractionation steps resulted only in a partial purification. Both enzyme forms, PAO-I and PAO-II, have similar pIs of 5.8 and molecular weight of 262,000 and 255,000 daltons, respectively;The two enzyme forms exhibited different substrate specificities. Compared to PAO-II, PAO-I showed higher affinity for the medium-chain aldehydes (pentanal and hexanal) which are mainly responsible for the greeny and beany off-flavor of soybean products. The PAO-I enzyme form, which is more specific in utilizing medium-chain aldehydes, could be used for reducing the off-flavors associated with soybean proteins;PAO-I was selected to study pH and temperature stability. Purified PAO-I was stable between pH of 7.1 and 10.7 and at temperatures up to 45°C. Energy of denaturation for PAO-I (158.1 kJ/mol · K[superscript]-1) was more than three-fold higher than the energy of activation (47 kJ/mol · K[superscript]-1). The enzyme was more stable at alkaline pHs than at neutrality and repeated freezing and thawing did not inactivate the enzyme;The two pK values of 6.2 and 11.3 of PAO-I indicated the presence of histidine and guanidine residues in the ES complex. The estimated pK values of 7.5 and 9.9 for the n-pentanal binding to the free enzyme suggested the involvement of cysteine, lysine, tyrosine, and arginine as possible amino acid residues;Gas chromatography analysis showed that more headspace volatiles were present in the water extract of soy proteins at pH 7.0 than at pH 9.0. Schiff-base formation between aldehydes and soy protein increased at alkaline pH and reduced the amount of headspace volatiles at pH 9.0. The incubation of a soy protein extract and PAO-I reduced the headspace pentanal and hexanal by as much as 90%. The sensory panelists perceived lower beany flavor (p < 0.01) of the aldehyde oxidase treated soy protein extract.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Maheshwari, Peeyush, "Characterization and application of porcine liver aldehyde oxidase in the removal of off-flavors from soy proteins " (1995). Retrospective Theses and Dissertations. 11015.