Table 2.

Effects of main extrusion parameters on starch digestibility.

Extrusion Parameter	Botanic Source	Effects on Starch Digestibility	Possible Mechanisms	Reference
Feed moisture	Brown rice and pinto bean composite flours	RDS increases while RS decreases with lowered feed moisture	The lower the processing moisture, the higher the degree of mechanical degradation, resulting in a decrease in RS	[100]
	Buckwheat flour	Low moisture increases starch digestibility	Lower moisture content enhances the degree of starch molecular degradation, and thus increases starch digestibility	[97]
	Rice flour	RS increases with the increase in feed moisture	High feed moisture increases the mobility of starch molecules, leading to gelatinization, and enhances the tendency of starch to retrograde, which leads to the formation of resistant starch	[102,103]
	Corn and potato starches	High moisture decreases starch digestibility	Water has a lubricating effect inside the extruder that reduces the starch degradation. Water decreases the feed material temperature during extrusion, resulting in less gelatinization and slower starch digestion rate	[104]
Extrusion temperature	Corn starch–guar gum mixtures/Corn and potato starches	Starch digestibility increases with the increased extrusion temperature	Higher temperature results in greater starch gelatinization and thus higher starch digestibility	[59,105]
	Rice flour	RS increases with increasing temperature	High temperature causes melting of crystallites and formation of reorganized crystallites that are more stable to heat	[102,103]
	Novel wholemeal wheat flours	No relationship observed between the starch digestibility and extrusion temperatures	Starch was completely gelatinized and had no retrogradation as the samples were dried immediately after extrusion	[106]
Screw speed	Whole grain oat flour	Moderate screw speed (300 rpm) led to higher SDS and less RDS	Low screw speed means long residence time. High screw speed produces high shearing on starch molecules. Therefore, both low and high screw speed can reduce SDS	[101]
	Green banana flour	Samples extruded at lower screw speeds have higher RS content	Starch degradation is relatively less at lower screw speed, and longer residence time is favorable	[40]
	Mixture: chickpea flour, maize flour, oat flour, corn starch, onion powder	As the screw speed increases, RS decreases	Shear effect at higher screw speeds makes starch molecules more sensitive to enzymes	[107]