Strain sweep measurements
Fig. 5 shows storage module (G0) as function of strain amplitude
for formulation 1. For those samples with added stabilizer (Fig. 5a),
two different regions were observed linear viscoelastic region
where G0 was constant and nonlinear region in which G0 started to
decrease with increasing strain. Within the linear viscoelastic
region, the G0 values were independent of the applied strain
meaning that the samples behaved like a viscoelastic solid (Fig. 5a).
The length of the linear section is closely related to product stability
since it is thought to account for the formation of physical
entanglements between proteins and carbohydrates at the
oil/water interface providing structure to the liquid (Franco et al.,
1995). Increasing the homogenization pressure resulted in higher
values of G0 when compared with the control sample, meaning that
HPH increased the strength of the network. This was evidenced
both by the increased magnitude of G0 plus extended linear range
indicated by the critical strain which is determined as 85% of G0 at
the linear range. The highest strength of the network judging by the
magnitude of G0 values was obtained at 45 ± 3 MPa/92 C and the
application of higher homogenization pressures (78e286 MPa) did
not increase the strength of the network (Fig. 5a). The yielding
point or critical strain is associated with the breakdown of the
secondary network of particles and it marks the transition from
viscoelastic solid to viscoelastic liquid (Hesarinejad et al., 2014). In
the case of samples without added stabilizer, the G0 values as
function of strain amplitude were too low and rather scattered
regardless of the HPH conditions. Magnitude of the G0 for products
without stabilizer was one logarithmic scale lower. The linear
ranges were not as easily visible due to the detection limits of the
instrument; although, some exhibited better linear ranges.
Strain sweep graphs for formulation 2 are given in Fig. 5b. The
magnitude of the G0 for the HPH treated samples with added
stabilizer was larger and exhibited longer linear ranges. Increasing
the homogenization pressure not only extended the length of the
viscoelastic region but also yielded higher values of G0 compared
with the control sample (1.94 and 0.11 Pa, respectively). For
formulation 2 without added stabilizer, the strain sweeps for HPH
treated reveled a relatively short viscoelastic region for all samples
spanned from 1.3 to 46.3% within a relatively low G0 values, ranging
Fig. 4. Mean particle size as function of homogenization pressure for different
formulation with and without the addition of stabilizer. (a) formulation 1 and (b)
formulation 2.
orifice valves (Schultz et al., 2004). Parameter b has been used as an
indicator for turbulent fragmentation (Hakansson, 2015) since it
combines the Kolmogorov's theory with dissipation rate of
turbulent energy. The prediction of mean particle size at pressure
levels below the threshold value was fairly well described by a
power law fashion according to Kolmogorov's theory. At high
pressure levels, velocity gradients of large magnitude and protein
stability may shift the balance of adsorption and coalescence.