Impact of Sample Aging on Freeze-Thaw Stability of Oil-in-Water Emulsions Prepared with Soy Protein Isolates

Figures & data

Table 1 Protein solubility index (PSI) of unheated (u) and heated (h) aqueous dispersions (2.0% protein w/w in sodium phosphate buffer 10 mM, pH 7.0) prepared with fresh (SPI_F) and stored (SPI_S1 and SPI_S2) soy protein isolates

Samples	PSI
u-SPIF	93.16 ± 0.40^a,A
u-SPIS1	85.54 ± 0.97^b,A
u-SPIS2	74.80 ± 1.20^c,A
h-SPIF	96.27 ± 0.74^a,B
h-SPIS1	96.63 ± 0.68^a,B
h-SPIS2	87.50 ± 1.40^b,B

PSI values are means of two replicates (n = 2) ± standard deviation.

In the same column, mean values with different lowercase letters indicate significant differences between fresh and stored soy protein isolates.

In the same column, mean values with different uppercase letters indicate significant differences between unheated (u) and heated (h) samples.

FIGURE 1 Particle size distributions of unfrozen o/w emulsions prepared with fresh (a: SPI_F) and stored (b: SPI_S1; c: SPI_S2) soy protein samples.

Table 2 De Brouckere (D_4,3, volume-weighted) moment mean diameters and flocculation index (FI%) of o/w emulsions prepared with unheated (u) and heated (h) aqueous dispersions of fresh (SPI_F) and stored (SPI_S1, SPI_S2) soy protein isolates. D_4,3 values were obtained from particle size distributions measured in the absence (–) and presence (+) of SDS

Samples	D4,3 (μm)		FI%
	– SDS	+ SDS
u-SPIF	0.81 ± 0.01^a	0.67 ± 0.01^a	20.3 ± 0.3ª
u-SPIS1	0.90 ± 0.03^b	0.78 ± 0.01^b	15.1 ± 4.4ª
u-SPIS2	3.60 ± 0.02^c	1.10 ± 0.02^c	203.3 ± 5.6^b
h-SPIF	1.10 ± 0.04^d	0.65 ± 0.01ª	67.9 ± 9.3^c
h-SPIS1	1.53 ± 0.01^e	0.76 ± 0.02 ^b	101.6 ± 0.1^d
h-SPIS2	1.66 ± 0.05^f	1.01 ± 0.01^c	64.4 ± 5.8^c

D_4,3 and FI% values were the mean of two replicates (n = 2) ± standard deviation.

In the same column, mean values with different lowercase letters, indicate significant differences between SPI samples.

Table 3 De Brouckere (D_4,3, volume-weighted) moment mean values, flocculation index (FI%), coalescence index (CI%) and the amount of free oil (FO) for freeze-thawed o/w emulsions prepared with unheated (u) and heated (h) aqueous dispersions of fresh (SPI_F) and stored (SPI_S1, SPI_S2) soy protein isolates. D_4,3 values were obtained from particle size distributions measured in the absence (–) and presence (+) of SDS

Samples	D4,3 (μm)		CI%	FI%	FO (% w/w)
	– SDS	+ SDS
u-SPIF	228.4 ± 10.5^a,b	86.1 ± 2.0^a	12898.2 ± 200.5	165.3 ± 6.0	1.9 ± 0.3^a
u-SPIS1	267.1 ± 30.0^b	107.6 ± 2.0^b	13631.5 ± 90.5	147.8 ± 28.0	4.9 ± 0.4^b
u-SPIS2	195.4 ± 25.0^c	100.4 ± 3.5^c	9025.5 ± 15.0	94.6 ± 10.0	31.9 ± 0.6^c
h-SPIF	119.0 ± 8.2^d	0.8 ± 0.1^d	21.4 ± 5.8	13530.1 ± 80.0	1.6 ± 0.4^a
h-SPIS1	162.2 ± 7.4^c	1.3 ± 0.1^d	77.7 ± 9.4	11881.3 ± 89.3	1.8 ± 0.2^a
h-SPIS2	180.7 ± 8.0^c	70.8 ± 0.5^e	6910.0 ± 30.5	155.2 ± 8.5	32.7 ± 0.7^c

D_4,3, CI% and FI% values were the mean of two replicates (n = 2) ± standard deviation. FO values were the mean of three independent determinations (n = 3) ± standard deviation. Mean values with different lowercase letters within the same column indicate significant differences between SPI samples.

FIGURE 2 Microstructure of o/w emulsions prepared with unheated (u) and heated (h) fresh soy protein isolates (SPI_F) before and after freeze-thawing (F-T) in the absence or presence of sorbitol (S) at different concentrations. For u-SPI_F emulsions: a: unfrozen, b: F-T without cryoprotectant, c: F-T with S (0.75% w/w), d: F-T with S (15.0% w/w); for h-SPI_F emulsions, e: unfrozen, f: F-T without cryoprotectant, g: F-T with S (0.75% w/w), h: F-T with S (15.0% w/w). Bar = 10 μm.

FIGURE 3 De Brouckere (D_4,3, volume-weighted) moment mean diameters of freeze-thawed o/w emulsions prepared from unheated a: and heated b: fresh soy protein isolates (SPI_F) in the presence of sorbitol (S) at different concentrations (0.75-15.0% w/w in the emulsion). Values are mean of two (n = 2) independent replicates and error bars indicate standard deviation. For comparative purposes, D_4,3 values of initial, unfrozen emulsions were also included. For u-SPI_F or h-SPI_F emulsions measured without SDS, mean values with different lowercase letters indicate significant differences between samples (p < 0.05). For u-SPI_F or h-SPI_F emulsions measured with SDS, mean values with different uppercase letters indicate significant differences between samples (p < 0.05).

Table 4 Flocculation index (FI%) and coalescence index (CI%) values of freeze-thawed o/w emulsions prepared with unheated and heated fresh and stored (SPI_F and SPI_S1, respectively) soy protein aqueous dispersions with addition of sorbitol as cryoprotectant at different concentrations (0.75–15.0% w/w in the emulsion)

Sorbitol (% w/w)	u-SPIF		h-SPIF		u-SPIS1		h-SPIS1
	CI%	FI%	CI%	FI%	CI%	FI%	CI%	FI%
0.75	1.0 ± 0.2	4187.7 ± 10.0	4.5 ± 1.4	14373.3 ± 79.1	28.2 ± 1.2	4425.0 ± 396.0	71.9 ± 18.6	7988.5 ± 797.0
2.25	0	1929.2 ± 179.0	0	16075.4 ± 880.0	7.4 ± 0.1	1280.8 ± 67.8	15.8 ± 0.3	11354.1 ± 623.0
3.75	0	484.1. ± 12.0	0	1529.8 ± 141.7	4.5 ± 0.6	483.2 ± 35.2	11.7 ± 1.2	3685.8 ± 37.0
7.50	0	471.0 ± 13.0	0	668.1 ± 164.0	3.2 ± 1.0	25.4 ± 0.5	3.4 ± 0.2	229.0 ± 4.4
15.0	0	21.2 ± 1.1	0	85.2 ± 15.5	1.1 ± 0.1	19.7 ± 1.3	4.1 ± 0.7	90.1 ± 0.7

CI% and FI% values were the mean of two replicates (n = 2) ± standard deviation.

FIGURE 4 a: Aromatic surface hydrophobicity (H₀) of unheated and heated aqueous dispersions prepared with stored soy protein isolates (SPI_S1) before (U) and after freeze-thawing (F-T) in the absence (control) and presence of sorbitol (S, 3.0 and 20.0% w/w); b: Freezable water content (FW% w/w) of o/w emulsions prepared with unheated and heated stored soy protein isolates (SPI_S1) in the absence (control) and presence of sorbitol (S) as cryoprotectant (0.75–15.0% w/w in the emulsion). H₀ and FW values are the mean of two (n = 2) independent replicates ± standard deviation. For dispersions or emulsions prepared with unheated and heated SPI_S1 samples, mean values with different lowercase letters are significantly different (p < 0.05).

FIGURE 5 Coalescence index (CI%) values of freeze-thawed o/w emulsions prepared with unheated and heated stored soy protein isolates (SPI_S2) in the presence of sorbitol a: and glucose b: at different concentrations (0.75–3.75% w/w in the emulsion). CI% values are the mean of two independent (n = 2) replicates ± standard deviation.