Starch gelatinisation temperature

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. 

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. 

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... 

Advantageously, the viscosity and the gelatinisation temperature are lower than that of native starch. Starch derivatives are used in the paper industry for coating and glues. 

A second stage occurs at 90°C when amylopectin-rich granules swell and deform (Hermansson and Svegmark, 1996). These temperatures are altered by starch type, and the presence of small solutes sugars elevate the gelatinisation temperature and ionic solutes exhibit varying effects related to their position in the Hoffmeister series. 

Figure 1 shows the variation in the ratio 1047/1020 cm obtained for a wheat flour/water dough, heated at 2°C/min, plotted as a function of temperature, compared to a DSC experiment performed on the same sample with identical heating rate. The decrease in the ratio 1047/1020cm-1 was closely correlated with the DSC data for flour-water mixture. This finding validated the use of FTIR to follow starch gelatinisation in situ. 

A higher amylopectin content of some starches leads to their higher gelatinisation temperatures, but the lowering which is observed in potato starch indicates the influence of phosphorylation in the latter. The relatively high pasting temperature and low peak viscosity of wheat starch has been attributed to its relatively high content of phospholipid impurities, which form helical complexes with the amylose chains as indicated above. 

Starch type Granule size ((j,m) Amylose content (%) Gelatinisation temperature (°C)... 

Acid hydrolysis is carried out by heating concentrated aqueous dispersions of starch (36—40%) with dilute mineral acids (usually with <7% HCl or 2% H2SO4) for several hours at a temperature of 40-60 °C, which is lower than the initial gelatinisation temperature. The suspension of starch is then neutralised, washed with water and the starch separated by filtration and dried. Under these conditions, amylose and amylopectin are partially hydrolysed in the amorphous regions of granules and the product is a so-called soluble starch as the damaged granules swell in cold water and collapse when heated... 

Phosphorylated starches are obtained by reaction of aqueous suspension of starch with ortho- (Na3P04), pyro- (Na4P207) and tripolyphosphates (NajPjOjg) in a weakly alkaline medium at temperatures of 120-170 °C (Figure 4.13). The degree of substitution is usually <0.25. The decrease in gelatinisation temperature versus... 

Maaruf AG, Man YBC, Asbi BA, Junainah AH, Kennedy JF, (2001) Effect of water content on the gelatinisation temperature of sago starch . Carbohydrate Polymers, v46 n4 331-337. 

A particularly profitable line of enquiry involves the exact temperatures at which order, as measured by the various techniques, is lost. Gidley [9] has looked at this in detail in order to shed light on the origin of the endotherm in starch gelatinisation. Table 9.2 shows a comparison using several techniques of the overall level of order in starches. 

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