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| Modified starch | Native starch
has its limitations in food applications. Some of the disadvantages include:
The native starch swells quickly and loses its viscosity almost as quickly again during continued heating and forms a viscous coherent gel. This quality is perfect in some applications, but undesirable in other applications. When you modify the starch you can adjust the properties of the starch to fit specific applications and to resist some of the actions, that the native starch yields to. |
| Cross-linking | The most
common method of modification is cross-linking. When you cross-link starch,
a chemical bond takes place between the starch chains. By cross-linking the starch you get the following properties (compared to native starch)
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 As you can see, the gelatinisation temperature is higher in a cross-linked starch than in the native starch and at the same time the relatively high breakdown is no longer seen in the X-AMYLO 100. The viscosity, especially after cooling, is also significantly higher than in native starch. During the holding period at 95°C it is furthermore evident that the cross-linked starch is stable when stirred. By cross-linking the starch you can produce a starch that under normal circumstances has almost no viscosity. If this starch is subject to either low pH, high temperature, heavy mechanical processing or all of these at the same time, the starch will, however, give some viscosity. Heavily cross-linked starches therefore “demand” a heavy treatment in order to give any viscosity whereas other starches subject to the same conditions will collapse with a weak or no viscosity as the result. |
| Stabilized
starches |
A starch that
has been heated and thereby gelatinised will after cooling begin to retrograde
(revert to its original state), which again can cause syneresis (water
will separate) = the starch loses its ability to hold water. Syneresis
is often seen as water forming on the surface. To avoid retrogradation you can stabilize the starch by adding a chemical unit that will act as a repellent and keep the starch chains from collapsing and thereby releasing water. Acetylated starches can be more or less acetylated. The acetylation causes small changes in the viscosity profile of the starch. The most important change in the profile is the lower gelatinisation temperature in the more acetylated starch. |
 When the starch is acetylated you get the following properties (compared to native starch):
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| Combinations of modifications | A combination
of processing methods is often seen in modern food industry –
with both a low pH, a relatively high processing temperature and also
a demand that the product does not form syneresis. An example of such
a product might be tomato soup that typically is homogenized and autoclaved,
the texture must be short and the product may not form syneresis. In order to function as well as possible, the starch for such a product needs to be both cross-linked and stabilized. Such a starch is often called a combination starch. In addition to the typical sequence for cross-linking you will especially notice that the gelatinization temperature - in spite of the product being crosslinked - actually is rather low compared to the native starch. This is the effect of the acetylation. By using the combination starches it is actually possible to tailor the starch to a specific application. In spite of being developed with a certain application in mind, most starches can easily be used in other applications as well. |
| Thin boiling starches | In certain
applications a high content of starch is desirable in order for the
starch to be able to form a gel as in e.g. wine gums and liquorice.
If you use native starch or the previously mentioned modified starches
as the gelling agent the paste would become too viscous during heating
as a very high starch concentration is required to form a gel. In such
applications you can with advantage use the thin boiling starches. Thin boiling starches have a low viscosity compared to native starch and this enables you to use these starches in high concentrations without the viscosity getting too high. In other applications, for example in instant soups, thin boiling starches are often used as a filler without any specific technical function. The thin boiling starches have been degraded. The most common methods of degrading the starches are oxidation, acid hydrolysing, and enzymatic degrading. |
| Oxidation | Oxidation
is the only type of modification where two different forms of modification
take place simultaneously – a degrading and stabilization like
the acetylation |
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| Hydrolysis | As mentioned
earlier, another way of degrading the starch is through an acid hydrolysis.
When you use acid hydrolysis, the starch chains are degraded almost
the way they are when oxidation is used. The difference is that the
starch is NOT stabilized as it is with the oxidation. This shows in
a much higher end-viscosity than with oxidation. The oxidized starch has a much lower end-viscosity than the acid hydrolized starch, which is due to the earlier mentioned carboxylic acid groups stabilizing the starch. When you acetylate the oxidized starch the result is a more stable starch. |
