Functionality Of Starch

Starch is a versatile and cheap, and has many uses as thickener, water binder, emulsion stabilizer and gelling agent. Starch is often used as an inherent natural ingredient but it is also added for its functionality.

It is naturally found tightly and radially packed into dehydrated granules (about one water per glucose) with origin-specific shape and size (maize, 2-30 μm; wheat, 1-45 µm; potato, 5-100 μm ). The size distribution determines its swelling functionality with granules being generally either larger and lenticular (lens-like, A-starch) or smaller and spherical (B-starch) with less swelling power.

Granules contain 'blocklets' of amylopectin containing both crystalline (~30%) and amorphous areas. As they absorb water, they swell, lose crystallinity and leach amylose. The higher the amylose content, the lower is the swelling power and the smaller is the gel strength for the same starch concentration. To a certain extent, however, a smaller swelling power due to high amylose content can be counteracted by a larger granule size .

Although the properties of starch are naturally inconsistent, being dependent on the vagaries of agriculture, there are several suppliers of consistently uniform starches as functional ingredients.

Of the two components of starch, amylose has the most useful functions as a hydrocolloid. Its extended conformation causes the high viscosity of water-soluble starch and varies relatively little with temperature. The extended loosely helical chains possess a relatively hydrophobic inner surface that is not able to hold water well and more hydrophobic molecules such as lipids and aroma compounds can easily replace this.

Amylose forms useful gels and films. Its association and crystallization (retrogradation) on cooling and storage decreases storage stability causing shrinkage and the release of water (syneresis). Increasing amylose concentration decreases gel stickiness but increases gel firmness. Amylopectin interferes with the interaction between amylose chains (and retrogradation) and its solution can lead to an initial loss in viscosity and followed by a more slimy consistency.

Mixing with κ-carrageenan, alginate, xanthan gum and low molecular weight sugars can also reduce retrogradation. At high concentrations, starch gels are both pseudoplastic and thixotropic with greater storage stability. Their water binding ability (high but relatively weak) can provide body and texture to foodstuffs and is encouraging its use as a fat replacement.

A significant proportion of starch in the normal diet escapes degradation in the stomach and small intestine and is labeled 'resistant starch' (for a recent review see ), but this portion is difficult to measure and depends on a number of factors including the form of starch and the method of cooking prior to consumption.

Nevertheless resistant starch serves as a primary source of substrate for colonic microflora, and may have several important physiological roles. Resistant starch has been categorized as physically inaccessible (RS₁), (raw) ungelatinized starch (for example, in banana; RS₂ ), thermally stable retrograded starch (for example, as found in bread, especially stale bread, mainly amylose; RS₃) and chemically modified starch (RS₄). Resistant starch should be considered a dietary fiber.

Although not exactly quantifiable due to its heterogeneous nature, some is determined by the official Association of Official Agricultural Chemists (AOAC) method. Starch with structure intermediate between the more crystalline resistant starch (for example, RS₃ in staled bread) and more amorphous rapidly digestible starch (for example, in boiled potato) is slowly digestible starch (for example, in boiled millet). Slowly digestible starch gives reduced postprandial blood glucose peaks and is therefore useful in the diabetic diet.

Many functional derivatives of starch are marketed including cross-linked, oxidized, acetylated, hydroxypropylated and partially hydrolyzed material. For example, partially hydrolyzed (that is, about two bonds hydrolyzed out of eleven) starch (dextrin) is used in sauces to control viscosity.