Peptidomics Analysis of Soy Protein Hydrolysates-Antioxidant Properties and Mechanism of their Inhibition of the Oxidation of Palm Olein during Frying Cycles

Abstract

This study determined for the first time the structure of the peptides (i.e., peptidomics) in soy protein hydrolysates and elucidated their effects on an oil's oxidative stability during frying cycles. The oil investigated was palm olein during 0, 4, 8, and 12 frying cycles of plantain banana chips. Proteins were extracted and hydrolyzed with two proteases. Trypsin hydrolysate (HTRY) exhibited higher anti-radical activity (DPPH, 70.2%) than the control (unhydrolyzed proteins, 33.49%) and pepsin hydrolysate (HPEP, 46.1%) at 200 µg/mL. HPEP however showed a 4.6-fold greater reduction of ferric ions (FRAP) while also possessing a higher peroxyl radical scavenging ability (716 ± 30 µM Trolox Eq/g) than HTRY (38.5 ± 35 µM Trolox Eq/g). During oil oxidative stability tests, HPEP improved the oxidative stability of the palm olein oil after 8 and 12 frying cycles, characterized by lower concentrations of hydroperoxides, and carbonyl and volatile compounds. HTRY however exerteda pro-oxidant activity. Structural data from SDS-PAGE and tandem mass spectrometry showed that the mechanism for the greater activity of the pepsin hydrolysate occurred due to unique structural features and a higher percentage of short-chain peptides. This was justified by a 25, 31, and 48% higher contents of tryptophan, histidine, and methionine, respectively (important amino acids with hydrogen atom transfer and electron-donating capacities) in the peptides identified in the pepsin hydrolysate.

Keywords: antioxidants; carbonyls; frying cycles; malondialdehyde; oxidation; palm olein; peptidomics; protein hydrolysates.

Figure 1

(A) Protein content and (B) SDS-PAGE of the soybean protein isolate and its hydrolysates. PNH: soy protein hydrolyzed with pepsin; HTRY: hydrolysate with trypsin; PNH: non-hydrolyzed soy protein. (1) Protein standard; (2) PNH; (3) HTRY; (4) HPEP. Data are means ± SEM (n = 3), and different letters indicate significant differences between means (p < 0.05).

Figure 2

Amino acid frequency of different samples. HTRY: trypsin-hydrolyzed soy protein, HPEP: pepsin-hydrolyzed soybean protein, PNH: non-hydrolyzed soy protein.

Figure 3

Antioxidant activities of samples. (A) DPPH radical scavenging activities at 12.5–200 μg/mL) with vitamin C (VITC as control); (B) FRAP (ferric reducing antioxidant power) with BHT (butylated hydroxy-toluene) as control; (C) ORAC (oxygen radical absorbance capacity) with GSH (glutathione) as control. HTRY: soy protein hydrolysate with trypsin, HPEP: soy protein hydrolysate with pepsin, PNH: non-hydrolyzed soy protein. Data are means ± SEM (n = 3), and different letters indicate significant differences between means (p < 0.05).

Figure 4

Oxidative indices of samples after 0, 4, 8, and 12 frying cycles. Cycle duration was 6 min. (A) Peroxide value; (B) thiobarbituric acid value; (C) anisidine value. HNS: unstabilized oil (no hydrolysate), HPEP: oil stabilized with pepsin hydrolysate, HTRY: oil stabilized with trypsin hydrolysate, BHT: oil stabilized with butylated hydroxytoluene. Data are means ± SEM (n = 3), and different letters indicate significant differences between means of all groups (p < 0.05).

Figure 5

(A) Total oxidation index; (B) iodine values; and (C) acid values of samples after 0, 4, 8, and 12 frying cycles. Cycle duration was 6 min. HNS: unstabilized oil (no hydrolysate), HPEP: oil stabilized with pepsin hydrolysate, HTRY: oil stabilized with trypsin hydrolysate, BHT: oil stabilized with butylated hydroxytoluene. Data are means ± SEM (n = 3), and different letters indicate significant differences between means of all groups (p < 0.05).