The physicochemical properties of starches such as solubility, swelling, and light transmittance have been reported to be affected considerably by derivatization (Table 1). Swelling power is commonly used to investigate the phenomenon of starch leaching after heat treatment (Colussi et al., 2014; Kaur et al., 2004; Paramo-Calderon et al., 2016) and is calculated as
Swelling power (%) = [(Weight of sedimental cooked starch paste (g)]:(Weight of the sample (dry basis) (g))] X 100
The change in these properties upon modification depends on the type of chemical modification. Chemical modifications, such as acetylation and hydroxypropylation, increase while cross-linking has been observed to decrease (depending on the type of cross-linking agent and degree of crosslinking), the swelling power and solubility of starches from various sources (Singh et al., 2016). The higher swelling power of acetylated starch is caused by steric hindrance resulting in repulsion between starch molecules, thus facilitating an increase in water percolation within the amorphous regions of granules and a consequent increase in swelling capacity. The solubility of starches is calculated as:
Solubility (%) = [(Weight of the solubilized starch after cooking): (Weight of the sample (dry basis) (G))] X 100
| Starch Source | SPa (g/g) (Acetylated) | SPa (g/g) (Hydroxypropylated) | SPa (g/g) (Cross-linked) |
| Normal potato | 60-71 (Native ≈ 58-70)c,d | 33-39 (Native ≈ 28-31)e | 20-25 (Native ≈ 2831)h |
| Normal potato | NDb | ND | 23-27 (Native ≈ 28-31)i |
| Normal maize | 38 (Native ≈ 36) | ≈20 (Native ≈ 6)f | ND |
| Waxy maize | ND | ≈42 (Native ≈ 30)f | ND |
| Normal wheat | ND | 9-16 (Native ≈ 6-8)g | ND |
| Normal rice | 15-19 (Native ≈ 14-18)e | ND | ≈9 (Native ≈ 18)e |
| Waxy rice | ≈31 (Native ≈ 41) | ND | ≈14 (Native ≈ 41)e |
a SP ¼ Swelling power (g/g).
b Not detected.
c The properties of corresponding native (unmodified) starches are given in parentheses.
d Starches from different potato cultivars.
e Starches from different rice cultivars.
f Two levels of hydroxypropylation.
g Two wheat starch granule populations.
h Starches from different potato cultivars, cross-linking performed using POCl3.
i Starches from different potato cultivars; cross-linking performed using epichlorohydrin
Acetylation weakens the starch granules and the structure disintegrates followed by amylose leaching from the granule and hence the starch solubility increases (Lawal, 2004). Cross-linked starches exhibit lower solubility than their native equivalents, and solubility decreases further with an increase in the concentration of cross-linking reagent, which may be attributed to an increase in cross-link density (Kaur et al., 2006). The solubility of native, cross-linked, and hydroxypropylated potato and rice starches in dimethyl sulfoxide (DMSO) varies to a significant extent (Table 2) (Singh et al., 2016). Hydroxypropylation results in a significant increase in the solubility of potato starches in DMSO (Singh et al., 2004; Yeh and Yeh, 1993). The light transmittance (%T, absorbance at 650 nm) has been used to measure the clarity of the starch paste (Craig et al., 1989). And the starch paste clarity has been reported to vary considerably with the source of the starch and can be altered by chemical modification of the granule. Kaur et al. (2004) and Singh et al. (2004) found the light transmittance of acetylated and hydroxypropylated potato and corn starches increased because of the chemical substitution of the ‒OH groups. The acetyl moieties on the starch molecules hamper the formation of an ordered structure after gelatinization and hence retard retrogradation resulting in a more fluid paste with improved long-term clarity (Lawal, 2004). Consequently, the high retention of water enters the starch granule leading to a greater swelling power and favors the clarity of pastes and gels. Highly cross-linked potato and cassava starch pastes generally showed lower light transmittance than their counterpart native starches (Jyothi et al., 2006; Kaur et al., 2006). Contrary to acetylated or hydroxypropylated starch, incomplete gelatinization and reduced swelling of cross-linked starches are mainly responsible for their reduced paste clarity (Hazarika and Sit, 2016; Kaur et al., 2006). The clarity of starch paste is one of important factors of product qualities, as starch for thickening fruit pie filling is preferably transparent while starch used in salad dressing should be opaque.
| Starch Source | SBa (after 4 h) | SB (after 8 h) | SB (after 16 h) | SB (after 24 h) |
| Normal potato (hydroxypropylated)b,d | ≈76 (Native ≈ 57)c | ≈83 (Native ≈ 60) | ≈95 (Native ≈ 65) | ≈99 (Native ≈ 75) |
| Normal rice (hydroxypropylated)e | ≈35 (Native ≈ 10) | ≈80 (Native ≈ 20) | ≈95 (Native ≈ 35) | ≈98 (Native ≈ 55) |
| Normal rice (cross-linked)f | ≈08 (Native ≈ 10) | ≈10 (Native ≈ 20) | ≈20 (Native ≈ 35) | ≈26 (Native ≈ 55) |
aSolubility in DMSO (%).
bValues reported as % transmittance in DMSO.
cThe properties of corresponding native (unmodified) starches are given in parentheses.
dStarches from different potato cultivars used.
eTwo levels of hydroxypropylation used.
fDifferent levels of cross-linking used.