Gelatinise

Resistance to boiling water gelatinises in less excellent... 

The removal of zinc chloride involves an extremely lengthy procedure. The plied sheets are passed through a series of progressively more dilute zinc chloride solutions and finally pure water in order to leach out the gelatinising agent. This may take several months. The sheets are then dried and consolidated under light pressure. 

Solvent type propellant. A double base propellant in which solvent is used to assist the gelatinisation of the nitrocellulose. 

One of the important properties of starch is that it undergoes gelatinisation. Starch is insoluble in cold water and intact starch granules do not absorb cold water. However, it is possible to prepare a dispersion of... 

The gelatinisation temperature of starch is affected by the presence of sugars, fats and salts. In any practical baked product some or all of these are likely to be present, so the gelatinisation temperature will not be that observed for the equivalent pure starch in distilled water. What is important is that the starch must be gelatinised or the product will collapse. 

Another modified starch is pregelatinised starch, i.e. starch that has been gelatinised and then spray dried. The resulting product gives a starch that will absorb water and form a gel with cold water. Pregelatinised starch is normally used in instant puddings and similar products. 

The important issue is to produce a layer between the bubbles that is sufficiently stable that the bubbles do not burst either before the food is cooked or served. In bakery products the foam has to be stable until the surrounding proteins have been denatured and the starch gelatinised. 

In the extruder the flour and water will be subjected to intense agitation and will be heated to above 100°C under pressure. Extruders are constructed to work at elevated pressures, indeed the barrel of the extruder is constructed in the same way as a gun barrel. Under these conditions the starch will gelatinise but the water cannot boil because the high pressure elevates the boiling point. 

On cooking maize starch the viscosity increases when the starch begins to gelatinise. As the temperature rises towards 95°C the viscosity falls. When the paste is cooled the viscosity rapidly increases. The variation of viscosity with temperature is characteristic for each different origin of starch. Potato starch, for example, has a lower gelatinisation temperature than maize starch but has a higher maximum viscosity. When cooled the viscosity of potato starch rises less. Once again amylopectin starches do not show this behaviour as they do not gel. 

Pregelatinised Starches. These starches have been gelatinised either by extrusion or by heating in water followed by roller drying. 

Once the final expansion has occurred the proteins start to denature and the starch gelatinises. The proteins also undergo Maillard reactions, particularly at the outside of the product, which is the hottest part. 

Rye Starch. Rye starch gelatinises at or around the temperature at which a-amylase has its maximum activity (55-70°C). a-Amylase activity tends to be high in rye flour so steps have to be taken to minimise it. One step is to acidify the dough either by adding acid or... 

A product resembling bread can be made using 75% of wheat flour and 25% of flour milled from extruded rice. The importance of extruding the rice is that some at least of the starch would have gelatinised. 

Puff pastry is one of the more difficult types of pastry to make. Good puff pastry should rise well and be crisp. Puff pastry is an example of a laminated product. The uncooked dough consists of layers of dough separated by fat. When the pastry is baked the water in the pastry turns to steam that pushes the layers of pastry apart (Figure 1). The heat gelatinises the starch, causing it to set on cooling. 

The solution is to use hot or boiling water in mixing the pastry. This partially gelatinises the starch in the flour as well as melting the fat. A hard fat is used in this sort of product, either lard, the traditional material, or a hard vegetable fat. The hot water could also reduce the activity of some of the enzymes present such as amylase. 

The proteins present will start to denature and the starch will start to gelatinise. These processes cause the structure to set. 

Eclairs are an example of what is called, in English, choux pastry. The pastry is specially made so that it can be piped into shape. The process gelatinises the starch so that the paste has the necessary flow properties to be piped into shape. 

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