Acid Inactivation of Soybean Lipoxygenase and Its Effects on Protein Solubility
Che Man, Yaakob Bin
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Permalink
https://hdl.handle.net/2142/70102
Description
Title
Acid Inactivation of Soybean Lipoxygenase and Its Effects on Protein Solubility
Author(s)
Che Man, Yaakob Bin
Issue Date
1988
Department of Study
Food Science
Discipline
Food Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Agriculture, Food Science and Technology
Abstract
This study investigated a method of inactivating soybean lipoxygenase while minimizing protein insolubilization with various acids at low pH without heating. The effects of acid on trypsin inhibitor and urease activities in the products were also studied.
Sensory scores of 10% neutralized soy slurries prepared by direct acid grinding to pH 3.0-3.8 and slurries from beans soaked for 8 hours in 0.15 M HCl showed no significant difference (P $>$ 0.05) from the blanched lipoxygenase-free control. By the chemical assay, lipoxygenase was found to be irreversibly inactivated during acid grinding at about pH 3.0. Electrophoretic pattern of SDS-polyacrylamide gel showed the band corresponding to lipoxygenase was absent from the samples treated at pH 3.0 and below. In acid soaking, the time needed to inactivate lipoxygenase decreased with increasing acid concentration and higher soaking temperature.
Full-fat soy flour from direct acid treatment retained about 90% of trypsin inhibitor activity regardless of treatment pH. More than 60% of the activity was removed from the full-fat soy material by whey separation. During soaking of soybeans more leaching was observed with higher acid concentration and soaking temperature. Urease activity was reduced to a commercially acceptable level $(<$0.2 $\Delta$pH) at the critical pH when lipoxygenase was inactivated.
Protein insolubilization was minimized on neutralized soybean products after treatment at low pH. Using HCl as the acidifying agent, the PDI of sodium, potassium and calcium proteinate forms of full-fat soy flour treated at pH 3.0 were 74.9%, 76.4% and 70.3%, respectively. Using H$\sb3$PO$\sb4$ and tartaric acid, the PDI for the sodium forms were 78.5% and 76.4% respectively. The PDI for full-fat soy material after treatment at pH 3.0 was about 52-55%, irrespective of acidifying and neutralizing agents used. Production of lipoxygenase-free full-fat soy material was studied in terms of the composition and solid component recovery when treated at pH 3.0 during extraction prior to curd separation. The protein and fat content of the material were 47% and 25% and the yield of protein and fat were 85% and 72%, respectively.
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