3.5. Microstructure of RS3
The effects of: debranching and autoclaving on cassava starch
(Fig. 4), concentration, temperature and time (Fig. 5A), and highpressure
treatments on autoclaved samples (Fig. 5B) were monitored
using scanning electron microscopy (SEM). Fig. 4A shows
the native starch (NS) in a granular form. After debranching, the
NS loses its granular structure, and then the surface of DS appears
more fluffy which indicates that the glucan polymers only reassociate
loosely (Fig. 4B). After autoclaving the DS, a densely packed
surface region is evident in DAS (Fig. 4C). In Fig. 5A, it is apparent
that the DAS-20 incubation at 60 C for 4 h exhibits a smoother
area than DAS-10. When subjecting DAS-20 to high pressure
annealing treatments, Fig. 5B demonstrates that HPC-DAS after
six cycles (24 h of process time) has a more densely packed surface
than HPT-DAS. It clearly shows less porosity and a smoother
surface area, which would yield greater resistance to enzyme
digestion and increasing the RS content. The overall microstructural observations show that RS3 content increases as the rigid dense
crystalline structure increases.
4. Conclusion
The process employed plays an important role in accelerating
retrogradation and the transforming of native starch into RS3. In
this study, the debranching step gave more linear glucans and the
autoclaving step aggregated these to increase crystallinity. High
pressure annealing subsequently accelerated RS formation within
15 min, in contrast to atmospheric annealing (single incubation)
which required up to 8 h to result in the same yield of RS3. Thus,
process times can be drastically reduced using high pressure
annealing. Yields of RS3 fraction can be further increased following
cyclic high pressure annealing of debranched-autoclaved starch.
The highest RS yield was obtained after applying six cycles (24 h
of process time) of pressure, each alternating between pressure
application (400 MPa/60 C/15 min) to accelerate the nucleation
rate of starch crystallization, and incubation (atmospheric pressure/
60 C/3 h 45 min) for crystal propagation (41.9% RS). These
conditions gave the highest yield of RS3 from a 20% w/w solution
of debranched-autoclaved starch. Thus, the high pressure annealing
treatment is highly promising to increase RS yield. In addition, this
method intensifies the formation of RS by physical modification (i.e.
without using solvents), which is safer for food industry use