Retrogradation and gelation, of starch

Modifications of the native starch form can be made by means of thermal and mechanical treatments where the amount of water has an essential role. Referring to Equation 2.1, when starch is heated at a water volume fraction above 0.9 a pure gelatinization phenomenon (and afterwards gelation and retrogradation) or jet-cooking occurs. On the contrary, when the water volume fraction is low (e.g. below 0.45), a real melting of starch crystallites occurs [11,27] and thermoplastic starch can be obtained [27]. 

Native A-starch occurs predominantly in cereal grains, B-starch is found in certain tuberous plants, C-starch is a rare form found in some plants and may actually be a combination of A- and B-starches. Partial conversion of B-starch to A-starch can be accomplished by adjusting temperature and humidity conditions, but complete conversion has never been achieved. When starch is dissolved in hot water, it spontaneously undergoes gelation and subsequent crystallization in the well-known process of retrogradation.(13) The polymorphic form of retrograded starch is B starch, irrespective of the form of starch initially dissolved. 

Retrogradation - The ordered reassociation of starch molecular strands by hydrogen bonding that occurs after paste formation. The hydrogen bonds form junction zones which can lead to higher viscosity, gelation, and precipitation. 

In this survey, gelation is considered as a rapid and random association of linear molecules, while retrogradation is comparatively slow and crystalline. The differentiation is an arbitrary one, since the same forces are responsible. Thus corn and wheat starches are prone to retrograde as compared with tapioca and potato, while waxy maize is completely stable. 

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