THESIS

Storage techniques and conditions of starch-based foods and products may
influence resistant starch levels. Retrogradation and restructuring of amylose occurs,
particularly with protracted storage. It is also suggested that during storage, starch
interacts with other food components, which inhibit starch-degrading enzymes and
reduce its digestibility (Kailcheusky et al. 1990).
1.4 Structure of resistant starch
The resistance of starch to digestion is influenced by the nature of the
association between starch polymers, with higher amylose levels in the starch being
associated with slower digestibility rates. Both B and C type starches appear to be
more resistant to digestion with high-amylose maize producing RS which has been
particularly useful in the preparation of foods (Brown, 2004). Retrograded starches
refer to certain structural forms of RS. Retrogradation occurs when starch is cooked in
water beyond its gelatinization temperature and then cooled. Amylose is found in the
amorphous parts of the starch crystal, while amylopectin gives starch its crystalline
structure. Upon heating with excess water and at sufficiently high temperatures, the
starch crystalline regions ‘melt’. The starch granules gelatinize and the starch is
subsequently more easily digested. However, these starch gels are unstable and upon
cooling re-form crystals that are resistant to hydrolysis by amylases (i.e. resistant to
digestion). Slow cooling of the gelatinized starch favors Type A crystallization while
slow cooling in excess water favors Type B crystallization. This process is known as
retrogradation (Topping and Clifton 2001). In general, starches rich in amylose are
naturally more resistant to digestion and also more susceptible to retrogradation.
1.5 Resistant starch as a functional food ingredient
Starch is used in multiple capacities in the food industry: for bulking,
functionality, texture and for nutritional quality, among other uses. It can therefore be
easily modified and further exploited to increase its value in disease prevention.
However, dietary intake of resistant starch has been very low. Johnson and Gee (1996)
for instance, estimated daily intake of resistant starch by an individual in the UK to be
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