Hexanal Synthesis in Isolated Soy Proteins

Abstract

Sodium erythrobate (2.3 mM) and dithiothreitol (6.5 mM) caused headspace hexanal from aqueous slurries of commercial isolated soy proteins (ISP) to increase by as much as 13-fold compared with headspace hexanal in the controls. When linoleic acid was also added, headspace hexanal levels from commercial and laboratory-prepared ISP were further increased by as much as 18-and 136-fold, respectively, compared with the levels in the controls. When 13 Clabeled linoleic acid was added to aqueous mixtures of laboratory-prepared ISP along with reducing agents, 96% of the resulting hexanal contained the 13 C isotope. Commercial ISP samples had about 40% of the isotopically labeled carbon incorporated into hexanal. These large increases in hexanal were almost completely prevented by heating the aqueous ISP to 100°C or above prior to adding the reducing agent and linoleic acid. Adding cysteine or sodium sulfite reduced headspace hexanal levels from aqueous ISP slurries by more than 88%. Thiol blockers, iodoacetic acid and N-ethylmaleimide, did not affect hexanal levels.

Lipoxygenase Independent Hexanal Formation in Isolated Soy Proteins Induced by Reducing Agents

“…When the same level of DTT was added together with linoleic acid (0.16 mM), no further increase in hexanal synthesis was found in all tested samples except for ISO‐I 974C, for which the reason is unclear. Variations in hexanal synthesis among the ISP samples in response to the treatments with DTT or/and linoleic acid were observed from both current study and previous investigation (Boatright and Lu 2007). The reported protein ( N × 5.71) and lipid contents in lab ISP were 77.9% to 84.5% (Boatright and others 1998) and 4.50% to 4.87% (Boatright and Lu 2007), respectively.…”

“…Variations in hexanal synthesis among the ISP samples in response to the treatments with DTT or/and linoleic acid were observed from both current study and previous investigation (Boatright and Lu 2007). The reported protein ( N × 5.71) and lipid contents in lab ISP were 77.9% to 84.5% (Boatright and others 1998) and 4.50% to 4.87% (Boatright and Lu 2007), respectively. In addition, no differences in the iron contents from the ISPs tested in the current study were observed (data not shown).…”

“…It has been documented that incorporating ISP at a level of 2% (He and Segranek 1996) or 3% (Matulis and others 1995) together with sodium erythorbate into frankfurters significantly lowered sensory scores. Boatright and Lu (2007) reported that the synthesis of hexanal in ISP could be induced by reducing agents such as sodium erythorbate and DTT. When carbon‐13, labeled linoleic acid, together with DTT was added into ISP, they discovered that hexanal with integrated isotopic carbon atoms was indeed synthesized, which excluded the possibility that the increased level of hexanal was due to the release of pre‐existing hexanal upon action of reducing agents (Boatright and Lu 2007).…”

“…Boatright and Lu (2007) reported that the synthesis of hexanal in ISP could be induced by reducing agents such as sodium erythorbate and DTT. When carbon‐13, labeled linoleic acid, together with DTT was added into ISP, they discovered that hexanal with integrated isotopic carbon atoms was indeed synthesized, which excluded the possibility that the increased level of hexanal was due to the release of pre‐existing hexanal upon action of reducing agents (Boatright and Lu 2007). However, it still remains unknown how reducing agents facilitate hexanal formation in ISP.…”

Effect of Moisture, Lipids, and Select Amino Acid Blocking Agents on the Formation and Stability of Metastable Radicals in Powdered Soy Proteins

“…Typically, commercial ISP samples contain about 4% total lipids (mostly phospholipids) that are carried through the protein purification process bound to the protein (Boatright and Hettarachchy 1995; Boatright and Crum 1997; Boatright and Lu 2007). The reported lipid contents of laboratory prepared ISP samples are slightly lower at 3.5% to 4.0%.…”