3.5 Emulsifying properties
The emulsifying properties are critical functional attributes of food
proteins. Several factors affect the emulsifying properties, including
the type and concentration of protein, pH, ionic strength, and viscosity
of the solvating medium. Many chemical and physical factors are involved
in the formation, stability, and textural properties of oil-water
emulsions stabilized by protein (Chang et al. , 2015). The EAI is
a measurement of the interfacial area coated by protein during the
formation of an emulsion and acts as a good predictor for the surface
activity of the protein, whereas the ESI is a measurement of the
stability of the diluted emulsion over a fixed period of time (Can
Karaca et al. , 2011).
As shown in Table 4(a), the AE-IP HE control had a higher EAI value
(13.2 m2/g) than the CP control (11.4
m2/g) and the defatted CP control (8.3
m2/g) at pH 3, which may indicate a higher
hydrophobicity value of the HE products compared to the CP controls at
pH 3. Similarly for pH 7, a higher EAI value was found for the HE
control (18.3 m2/g) than the CP control (11.7
m2/g) and the defatted CP control (11.6
m2/g) at pH 3. As for pH 5, the defatted CP control
(5.0 m2/g) had a higher EAI value than the CP control
(4.1 m2/g, p >0.05) and the HE
control (3.0 m2/g, p <0.05). As for
the SE products, the defatted CP control (11.0, 9.4, and 14.5
m2/g at pH 3, 5, and 7, respectively) showed the
highest EAI values compared to the CP (5.6, 5.8, and 12.9
m2/g at pH 3, 5, and 7, respectively) and the HE
control (5.9, 6.8, and 13.9 m2/g at pH 3, 5, and 7,
respectively). The difference between the AE-IP and SE products may be
due to the different protein fractions and the nature of products
resulting from the two methods. As for pH, the EAI values tended to be
low at pH 5, ranging from 1.3-5.5 m2/g due to the low
protein solubility. Higher EAI values were found at pH 7 compared to pH
3 for all protein products obtained from unfermented control meals,
which was possibly related to the higher solubility at pH 7 than at pH
3.
During SSF, the partial hydrolysis of proteins loosens the compact
protein structure and exposes more hydrophobic sites buried inside the
protein molecule. A higher hydrophobicity value can be achieved.
Townsend and Nakai (1983) suggested a positive correlation between
hydrophobicity and the emulsifying properties of proteins. Results of
the present study showed that canola products extracted from fermented
meals have better or at least unchanged emulsifying properties compared
to the controls. Among the AE-IP products, the EAI values increased to
13.8 m2/g (A. niger ) and 21.1
m2/g (A. oryzae ) at pH 3 for the CP products
(p<0.05). In addition, only A. oryzae improved the EAI
of the HE product (17.6 m2/g), while a decrease (8.5
m2/g) was found using A. niger at pH 3. At pH
7, the AE-IP products from A. niger and A. oryzaefermented meals had either improved or unchanged EAI values. In detail,
the EAI values of AE-IP CP products extracted from A. niger andA. oryzae fermented CP meals significantly increased
(p <0.05) to 25.8 and 27.6 m2/g,
respectively. Similar to the AE-IP HE products, the EAI values were
significantly increased to 18.0 and 25.5 m2/g when
fermented using A. niger and A. oryzae . The EAI values of
the AE-IP products at pH 5 remained relatively low, ranging from 1.3-5.5
m2/g for both the AE-IP CP and the HE products,
regardless of whether fermented or not, may be due to the low protein
solubility (pH 5). In addition, a low ESI was reported for all protein
products and ranged from 1.1-4.5 min. For the SE products, A.
niger and A. oryzae improved the EAI values of the CP products
at pH 3 to 8.5 m2/g (A. niger ) and 9.7
m2/g (A. oryzae ) compared to the CP control
(5.6 m2/g), however, they were lower than the defatted
CP control (11.0 m2/g). The increases in the SE HE
product at pH 3 after SSF were also reported as 8.5
m2/g (A. niger ) and 11.3 m2/g
(A. oryzae ) compared to the HE control (5.9
m2/g). However, SSF decreased the EAI of the CP and HE
products to 1.8 m2/g (CP, A. niger ), 4.6
m2/g (CP, A. oryzae ), 4.1
m2/g (HE, A. niger ), and 3.0
m2/g (HE, A. oryzae ) at pH 5. At pH 7, a
decrease in the EAI of SE CP and HE products was also reported to 12.2
m2/g (CP, A. niger ), 14.9
m2/g (CP, A. oryzae ), 11.4
m2/g (HE, A. niger ), and 7.6
m2/g (HE, A. oryzae ). The strain A.
oryzae was able to more positively modify the EAI values of canola
protein products than A. niger while the enhanced properties were
more frequently observed in AE-IP isolates than SE.
The above EAI and ESI results reported were lower than those from
previous studies by Cheung et al. (2014) and Chang et al.(2015). Cheung et al. (2014) suggested that high solubility was
positively correlated with the ESI values due to the additional protein
precipitation and adherence to the viscoelastic film surrounding the
droplets. A lower solubility was reported compared to previous studies
on canola protein, which may result in a reduction in ESI. In contrast,
Kinsella and Melachouris (1976) explained that high protein solubility
and high fat-adsorption capacity were positively correlated with the
ability of emulsifying (form and stabilize emulsions). Wu and Muir
(2008) and Cheung et al. (2014) examined the emulsifying
properties of two major canola proteins, cruciferin and napin. It has
been reported that the emulsion prepared with cruciferin showed a
significantly higher specific surface area and a lower particle size
than that with napin. The study by Wu and Muir (2008) indicated that the
presence of napin could detrimentally affect the emulsion stability of
canola protein isolates. The differences may be a result of the canola
cultivar, the preparation of canola meals (oil extraction method) and
protein products (extraction methods and conditions), and the analytical
methods employed. These differences might result in variations in
protein fractions and the levels of protein that further affect the
emulsifying properties. In addition, the released short peptides from
fermentation may also have an impact on the protein functionality such
as a reduction in emulsion activity and stability (Kristinsson & Rasco,
2000).