Extraction, Physicochemical, and Functional Properties of Proteins From Milk Thistle Silybum Marianum L. Gaernt Seeds

Figures & data

Table 1  Protein contents of protein fractions as well as protein isolate from S. marianu m seeds

Samples^a	Protein content (%, dry basis)	% Soluble proteins
SSP	19.95 ± 0.76^b	—
DSSP	37.52 ± 1.02^d	—
SPI	90.13 ± 1.68^g	—
SA	87.27 ± 0.81^f	74.58 ± 0.80^e
SG	85.88 ± 1.44^f	16.92 ± 0.51^d
SP	34.52 ± 1.40^c	1.01 ± 0.05^b
SGL	67.52 ± 0.69^e	7.49 ± 0.18^c
Each value (based on dry basis) was the mean and standard deviation of triplicate determinations.
^aSSP: powders of S. marianum seeds; DSSP: defatted powders; SPI: protein isolates; SA: albumin fraction; SG: globulin fraction; SP: prolamin fraction; SGL: glutelin fraction.
Values in the same column followed by a different letter are significantly different (p < 0.05) according to least significant difference (LSD) test.

Table 2  Polyphenol contents of defatted powders of S. marianu m seed, protein fractions, as well as protein isolate from S. marianu m seeds

	Polyphenol contents (%)^b
Samples^a	Total	Free	Protein-bound
DSSP	2.80 ± 0.12^c	1.43 ± 0.02^c	1.37 ± 0.08^c
SPI	3.99 ± 0.13^d	1.88 ± 0.06^d	2.11 ± 0.09^d
SA	4.37 ± 0.11^e	2.12 ± 0.09^e	2.25 ± 0.10^d
SG	4.26 ± 0.25^e	1.54 ± 0.07^c	2.72 ± 0.15^d
SGL	13.00 ± 0.15^f	1.67 ± 0.03^c	11.33 ± 0.13^e
Each value (based on dry basis) was the mean and standard deviation of triplicate determinations.
^aDSSP: defatted powders of S. marianum seed; SPI: protein isolate from S. marianum seeds.
^bThe total, free, and protein-bound polyphenol contents were expressed in g rutin equivalent/100 g of sample.
Values in the same column followed by a different letter are significantly different (p < 0.05) according to least significant difference (LSD) test.

Table 3  Amino acid composition of SSP, protein fraction, and SPI, and the FAO/WHO indicated requirements (2–5 years old) for the essential amino acids.^a

Amino acid	SSP^b	SPI	SA	SG	SGL	FAO/WHO^c
Asx^d	9.71	10.45	10.01	10.39	10.14
Glx^e	22.56	23.77	25.21	22.87	21.61
Serine (Ser)	5.35	5.56	5.48	5.86	5.67
Histidine (His)^f	2.52	2.38	2.37	2.66	2.69	1.9
Glycine (Gly)	5.92	4.65	4.80	5.36	5.46
Threonine (Thr)^f	3.90	3.27	3.10	3.39	3.78	3.4
Arginine (Arg)	10.07	8.42	8.90	8.49	8.28
Alanine (Ala)	4.28	4.28	3.97	4.57	4.75
Tyrosine (Tyr)	3.52	4.12	4.01	3.95	4.41
Cystine (Cys)	1.60	1.13	1.25	0.85	0.50
Valine (Val)^f	5.18	5.21	4.94	5.04	5.42	3.5
Methionine (Met)^f	1.23	1.76	1.95	1.58	1.79
Phenylalanine (Phe)^f	4.32	4.90	4.55	4.83	4.98
Isoleucine (Ile)^f	4.15	4.35	4.03	4.38	4.64	2.8
Leucine (Leu)^f	6.82	7.07	6.63	7.06	7.47	6.6
Lysine (Lys)^f	4.46	3.95	3.93	3.89	3.95	5.8
Proline (Pro)	4.23	4.74	4.84	4.84	4.46
Total sulfur-containing amino acids (Met and Cys)	2.83	2.89	3.20	2.43	2.29	2.5
Total aromatic amino acids (Phe and Tyr)	7.84	9.02	8.56	8.78	9.39	6.3
E/T(%)^g	32.58	32.89	31.50	32.83	34.72
^aProportion of the single amino acid to total amino acids.
^bSSP: S. marianum seed powders.
^cRecommended pattern (FAO/WHO for 2–5 years old).
^dAsx = Aspartic acid + Asparagine.
^eGlx = Glutamic acid + Glutamine.
^fEssential amino acid.
^gRatio of total essential amino acids (not including tryptophan) to total amino acids.

Figure 1 SDS-PAGE band patterns of protein fractions and SPI under reducing conditions. Lanes 1–6: (1) molecular mass markers; (2) SPI: protein isolates; (3) SA: albumin fraction; (4) SG: globulin fraction; (5) SP: prolamin fraction; (6) SGL: glutelin fraction. Protein concentration, 4 mg protein/mL; Sample load volume, 15 μL. (Color figure available online.)

Table 4  Secondary structure compositions of various protein fractions and SPI calculated from far-UV CD spectrum using the CONTIN/LL program in CDPro software

	Secondary structure compositions (%)
Protein samples	α-Helices^a	β-Strands^b	β-Turns	Random coil
SPI	0.3	61.5	11.8	26.4
SA	0.4	47.8	8.0	43.8
SG	4.0	15.4	9.3	71.2
SGL	0.1	14.2	34.3	51.5
^aCombined regular and distorted α-helices.
^bCombined regular and distorted β-strands.

Figure 2 Far-UV circular dichroism spectra of protein fractions and SPI. SA (-•-); SG (-▴-); SGL (-▾-); and SPI (-■-). (Color figure available online.)

Figure 3 Fluorescence spectra of protein fractions and SPI. SA (dash line); SG (dot line); SGL (dash dot line); and SPI (solid line). (Color figure available online.)

Figure 4 Protein solubility profiles of the protein fractions and SPI as a function of pH. The PS was expressed as proportion of soluble proteins to total proteins. Each datum point was the mean and standard deviation of triplicate determinations. DSSP: defatted powders of S. marianum seed (-■-); SA (-▵-); SG (-□-); SGL (-★-); and SPI (-○-).

Table 5  Functional properties of defatted powders of S. marianum seed.^a

	Functional properties^a
Sample	FC (mL)	FS (%)	EA (%)	ES (%)	WHC (g water/g sample)	FAC (g fat/g sample)
DSSP	102.0 ± 1.1	57.1 ± 0.8	48.1 ± 0.5	62.5 ± 1.2	3.24 ± 0.10	1.73 ± 0.08
The values were mean and standard deviations of triplicate determinations.
^aFC: foam capacity; FS: foam stability; EA: emulsion activity; ES: emulsion stability; WHC: water-holding capacity; FAC: fat absorption capacity.