**4. Modified starches**

Briefly, chemical modification leads to a considerable change in the rheological and pasting properties of starches. Storage (G') and loss modulus (G'') of acetylated, hydroxypropylated

and cross-linked starches from different sources increase to a maximum and then drop during heating following the same general rheological pattern as native starches (Singh et al., 2007b).

Viscoelastic Properties of Starch and Non-Starch Thickeners in Simple Mixtures or Model Food 223

Several hydrocolloids can promote associations with starches and as a result, when they are added in starch pastes, an increase in G', G'' is often observed, see examples: Mandala et al.(2004a); Achayuthakan & Suphantharika, (2008); Wang et al., (2008). In such systems the question is which ingredient predominates in the overall rheology, starch or hydrocolloid. In wheat starch-hydrocolloid systems, it is the hydrocolloid that predominates in the whole

(a) In water of xanthan 0.1 wt% (□, ■) and 0.5 wt% (○, ●), starch 2 wt%/xanthan 0.5 wt% (∆, ▲), amylose 0.3

(b) In 0.1 M NaCl of xanthan 0.1 wt% (□, ■), 0.5 wt% (○, ●) and 0.8% (◊, ♦), starch 2 wt%/xanthan 0.5 wt% (∆, ▲), amylose 0.3 wt%/xanthan 0.5 wt%,( ,▼). (From Mandala et al, 2004a. Carbohydrate Polymers 58, pp 285–292, with

Furthermore, except G', G'' values, tanδ may be important in interpretations of the behavior of starch-hydrocolloids interactions. Thus, although some hydrocolloids promote an increase in G', G'' by their addition, in fact they lead to a less solid-like system (higher tanδ values) as described above. On the contrary, in some cases, hydrocolloid addition may lead

wt%/xanthan 0.1 wt% (◊, ♦), amylose 0.3 wt%/xanthan 0.5 wt% ( ,▼).

**Figure 2.** Superimposed shifted spectra.

permission).

system, according to the shift factors found.

The temperature of maximum G' drops significantly on acetylation or hydroxypropylation, while it increases after cross- linking (Kaur et al., 2004, 2006; Singh et al., 2004). Acetylated corn and potato starches showed greater values of G' and G'' under heating but lower compared to their native starch gels upon cooling of heated starch gels, confirming their lower tendency to retro gradate.

Strengthening bonding between starch chains by cross-linking will increase resistance of the granules towards swelling resulting in lower G' values in a high degree of cross-linking. Cross-link concentration and location could lead to different rheology. Botanical source also influences cross-linking and potato starches show a higher susceptibility towards crosslinking than other kinds of starches (Kaur et al., 2004).
