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Amylose from cereal starches can form gels on cooling from a hot solution. We have been interested tofind out whether a general structure can be found for amylase gels. Studies have therefore beenperformed on purified amylose, and Fig. 6 shows a TEM micrograph of a thin section of an amylose gel.One plane of the gel is in view, and the micrograph reveals a gel structure composed of stiff rod-likestrands. We then thought it would be interesting to carry out the same investigation of the continuousphase of a wheat starch gel. Figure 7 shows part of such a continuous phase; a similar type of stiff rod-likegel structure as observed in the pure amylase gels is indeed evident. In addition, spherical particles can be

seen. These are small hydrated particles, probably composed of hydrated amylopectin, that have astabilizing effect on the amylose network. The structures found in dispersions of pure wheat starch affectthe structure and functionality of complex food products. For example, both types of starch swelling areimportant in determining the texture of pasta. In the outer layers of cooked pasta, the granules are in thesecond stage of swelling, as shown in Fig. 8. The granules have lost their original shape, and substantialamounts of amylose have leached out of the granules and can be seen as blue-stained pools outside theamylopectin-rich brownish granules. This gel-like outer layer controls the diffusion of water into the centralparts of the pasta, where the granules are in their first stage of swelling. If the pasta is heated at a lowertemperature and all of the granules are in the first stage of swelling, the pasta will be judged as spongyrather than 'al dente' regardless of the duration of heating. Apart from the elongated A-starch granules, thisplastic section reveals numerous small B-starch granules embedded in the yellow-stained gluten matrix. Acryo-section would be too thick to reveal such details. Electron microscopy has been used to show thatamylose leaches out into the gluten phase and fills small pores in the gluten network. The aggregatedamylose in the pores can be expected to have a stabilizing effect on the gluten network and to add to the

firmness of the pasta. Released amylose has a strong tendency to interact and aggregate at the surfacesof other particulate materials in foods. Amylose is known to interact with aroma compounds, emulsifiers,lipids, proteins and other polysaccharides in a manner that can affect the stability and the sensoryperception of a product. For this reason, an understanding of the interactions among starch and othercomponents is important for product development. Sometimes, the tendency of amylose to interact withlipids can give rise to quality problems, such as cloudiness in sauces, whereas starch-gluten interactioncan have a positive effect on pasta texture.

Potato starch: a close-packed system or a macromolecular dispersion?Potato starch is an example of a high-swelling starch system. Phosphate groups covalently linked to theamylopectin in potato starch increase its hydrophilic nature.

Potato starch gives high viscosity and high waterbinding capacity, and it has a low swelling temperature.Compared with cereal starches, it retrogrades less, is more transparent and has a blander taste. Owing toits slightly higher price and limited supply in some parts of the world, potato starch is not normally used forbulk applications, but used for its special properties. A substantial amount of potato starch is used inpapermaking: as a cationic derivative, it improves the binding of fillers and fibres as well as the paperstrength. During gelatinization, the granules flfSt swell like balloons, which may subsequently collapse andbecome folded23. The volume of a swollen granule is -100-fold greater than that of an ungelatinizedgranule. Cereal starch granules swell up to 30-fold their original volume during the second stage ofswelling at 90C (Refs 24 and 25). The large size of potato starch granules and their high swelling capacitylead to swollen granules that are exceptionally large in volume, which not only result in a high viscosity butalso give rise to a less smooth texture.

At concentrations greater than 4%, the gel phase of the swollen granules occupies practically the entirevolume, and the structure will be that of a close-packed system26, as depicted schematically in Fig. Ib.The granule shown in Fig. 9 is in the course of gelatinization; it can be seen that swelling has started withan influx of water into the amorphous middle of the granule, the hilum. The granules swell first in themiddle and then rupture as a result of internal pressurel3. A fully gelatinized starch paste is shown in Fig.10. The structures stained violet are the gel phase of the swollen granules, consisting mainly ofamylopectin; the structures stained dark blue are the aqueous phase into which the amylose has diffused.The granular gel phase is the dominant structure, as shown in the schematic representation in Fig. Ib. Theswollen granules are folded, intertwined and fractured. It is therefore difficult to identify individual swollengranules. Most of the blue-stained leached amylose is found in the middle of swollen granules. Potato andtapioca starches, which have a high swelling capacity, also form gels much more slowly than cerealstarches. The higher amylose content and lower molecular weight of amylose in cereal starches than inpotato starch cannot fully explain the higher rate of gel formation of cereal starches. Insufficient release ofamylose from potato starch granules might also be an important factor.

One characteristic of potato starch is the fragility of the swollen granules, which makes them prone todisperse or solubilize on heating and shearing, resulting in a macromolecular dispersion, as depicted inFig. lc. The processing of potato starch therefore readily results in overcooking. Crosslinking to variousdegrees is used to tailor the starch to a particular process. Figure II shows a macromolecular solution of an

8% starch paste that has been heated at 90C and heavily sheared until no remnants of swollen granulesremained.

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The same type of structure can be obtained by autoclaving or jet cooking at 120C. One interesting featureis the phase separation; the amylopectin and the amylase can be seen separately within the phase of themacromolecular dispersed starch. The amylopectin appears to form a continuous phase (stained red orviolet), whereas the amylose appears to be dispersed as droplets (stained blue).

Figure 12 shows a starch paste that has also been cooked at 90C but sheared less heavilyl8. The systemis a mixture of fragments of swollen granules and a macromolecular dispersion of amylose and

amylopectin. In some application areas in paper-making, the remnants of swollen granules may bedetrimental. Potato starch for use in paper-making is therefore jet cooked at 120C, so that no remnants ofgranules remain.

Results from mixed amylose-amylopectin model systems have shown that amylopectin forms thecontinuous phase below a relative ratio of 22% amylose28.29P.o tato starches contain -20% amylosel,2,and the micrographs in Figs II and 12 imply that amylopectin does form a continuous phase. Interestingly,cereal starches normally have a relatively high content of amylose (-28%)1,2, which is greater than theinflection point at which amylose becomes the continuous phase in model mixed systems. Thisphenomenon has not been studied in the context of real starch systems, although it could underlie someeffects seen during both swelling and phase separation.

Rheology as related to microstructure The effects of the structural states can be clearly seen from thechanges in the rheological behaviour .Oscillatory viscoelastic measurements can be performed in such a

manner that the structure is not influenced mechanically by the measurement.Figure 13 shows the differences in rheological behavior between wheat starch and potato starchI8,30T. hegreatest difference is the increase in shear modulus due to the formation of the continuous amylose gelphase in wheat starch on cooling. Shear does not influence the general rheological behaviour of wheatstarch, but the shear modulus of the gel is decreased. The probable reason for this is that the highlyhydrated outer amylopectin layers of the swollen granules are sheared off, as mentioned earlier.The close-packed potato starch dispersion does not gel directly on cooling. The potato starch depicted inFig. 13 was subjected to a minimum degree of shear during preparation. The effects of shear on therheological properties are far more drastic for potato starch dispersions than for wheat starch dispersions.This is illustrated in Fig. 14 by the viscoelastic behaviour of a 5% potato starch paste sample that hadbeen subjected to a minimum degree of shear and another 5% paste sample that had been shearedbefore making the viscoelastic measurements31. The continuous line shows how the shear moduluschanges on heating and cooling under a minimum degree of shear. The squares represent shearedsamples, and illustrate a drastic decrease in the shear modulus relative to that of the low-sheared sample.At the point where the shear modulus reaches the low plateau value, amylopectin makes up the

continuous phase surrounding the remaining fragments of the granules, as shown in Fig. 12 (Ref. 18).The continuous amylopectin phase determines the rheological behaviour of sheared potato starchdispersions, and the continuous amylose phase of wheat starch gels determines the behaviour of wheatstarch systems.

Potato amylopectin: a new type of starchA completely new type of starch has recently been developed by Lyckeby Starkelsen (Kristianstad,Sweden) and Svalof Weibull (Svalof, Sweden) using genetic engineering. The synthesis of amylose inpotatoes has been suppressed using an antisense technique. The alterations in the biosynthesis of thisstarch type are basically the same as in the waxy genotypes of, for example, maize and barley. The potatoamylopectin granules are ordered as in normal starch granules, with the amylopectin formingmicrocrystallites. There is little change in the gelatinization temperatures or degree of crystallinity of thestarch granules (K. Svegmark et al., in preparation).The storage stability of potato amylopectin is high, as is the case for other amylopectin starches. The high

molecular weight of potato starch and the very low content of lipids and proteins give this starch the uniqueproperties of being both stable and viscous and capable of producing exceptionally clear solutions. Theconsistency is long and similar to that of overcooked potato starch.The absence of amylose completely alters the swelling pattern described previously for starch granules.Figure 15 shows an amylopectin granule in the course of gelatinization. Nothing is stained blue, becauseno amylose is present. The part of the granule that is still ungelatinized is clearly seen in brown. The mainstructural alteration is that the gelatinized part does not form the structures typical of swollen granules.Instead, it immediately transforms into a macromolecular solution.The immediate dissolution of the granules following gelatinization is an advantage in all applications inwhich swollen granules and fragments of swollen granules are unwanted. Amylose is believed to act as arestraint on swelling, and cereal starch granules do not show complete swelling until amylose has beenleached out of the granule. One proposed mechanism that might explain this is that amylose-lipidcomplexes within cereal starch granules dissolve at temperatures higher than the gelatinizationtemperature. The swelling pattern of potato amylopectin granules proves that amylose also restrains

swelling and maintains the integrity of swollen starch granules in the case of starches with a low lipidcontent.

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