3.4. Effect of extrusion cooking on water absorption index (WAI) and
water solubility index (WSI)
WAI was significantly increased (42.4–53.1%) and WSI decreased
(87.9–88.4%) by increasing FM. Higher FM during extrusion
cooking resulted in a lower degree of starch gelatinization
and probably acts as a plasticizer caused by reduced starch degradation,
also due to less shearing taking place, thus resulting in an
increase in WAI and a decrease in WSI (Hagenimana, Ding, & Fang,
2006). Increasing SS caused a decrease in WAI (6.1–23.8%) and an
increase in WSI of extruded samples (only at lower FM, 4.6–5.7%).
Again this might be attributed to the higher amount of damaged
polymer chains formed at higher shear rate, reducing the availability
of hydrophilic groups to bind more water molecules, resulting
in a decrease in values of WAI (Guha, Ali, & Bhattacharya, 1997).
Such higher SS induced higher mechanical shear leading to more
starch degradation, subsequently increasing the amount of soluble
molecules and resulting in an increase WSI values. The interaction
effect of FM and SS was significant for WSI. Storing of the samples
had no significant effect on WAI or WSI. WAI and amylose content
of starch were positively correlated; WSI and amylose content
were negatively correlated.
Compared to native flour, WAI was increased after extrusion
cooking at 50% FM (22.0–28.4%), but decreased after 20% FM
(20.1–39.7%) at both SS studied. Generally, starch granules are susceptible
to thermomechanical damage during extrusion cooking.
Damaged starch has higher water retention capacity at room temperature
and swelling compared to the native flour, resulting in an
increase in WAI values. However, after reaching a maximum with
respect to the degree of starch damage, WAI decreased under lower
FM with increasing temperature. This is probably due to dextrinization
or starch melting that prevails over the gelatinization phenomenon
(Maskan & Altan, 2011). An inverse effect was
observed on WSI. WSI of extruded flours was increased in all cases
(10.1–90.3%).