Amylose ordering processes

Ordering Processes of Amylose-Iodine Solutions. A second feature observed in earlier ORD and CD measurements with amylose-iodine solutions was a time dependent increase of the Cotton effect (U. As mentioned above, intra- and intermolecular ordering processes may be considered. In order to differentiate between these two possibilities, we initially carried out studies in very dilute solutions to prevent association as far as possible (Figure 7). 

Starch phase transitions occur in a wide temperature range. The phase transition process starts at temperatures as low as 35-40 °C, depending on the type of starch. In contrast to what was previously believed, it is now understood that amylose and/or amorphous phases also play significant roles in the phase transition process (Ratnayake and Jackson, 2007 Vermeylen et ah, 2006). Theories that describe gelatinization and phase transition in terms of crystallite melting, therefore, are unlikely to adequately explain the phenomena. In summary, it is evident that starch gelatinization is not an absolute result of crystallite melting. Hence, it should not be considered a simple order-to-disorder phase transition of starch structures. 

It is well known that rice taste is a function of chemical constituents such as protein, moisture, amylose, fatty acid, and minerals. It is impractical to use the results of time-consuming chemical analyses to control the blending process. In order to overcome this problem, rice taste analyzer based on NIR principles was developed several years ago [15]. At present, there arc five different types of analyzers, as shown in Table 2, which are commercially available. A total of more than 300 analyzers are being used in the milling plants as well as experimental stations at present. 

The behaviour of cereal starch granules during gelatinisation in excess water has been widely studied. This process commences at around 65 °C when crystalline order is lost. Solubilisation of amylose starts in the amorphous centre of the granules (Figure 18.2). 

In order to effect complete dissolution of starch at temperatures below 100°, Bauer and Pacsu recommended the use either of dilute alkali or of dilute acid solutions. Accoixling to their so-called alkali process, defatted corn starch is dissolved in 0.5 to 1.0 N alkali at room temperature. Sufficient mineral acid is added to the resulting solution to bring its pH within the limits of 10-4. After saturation with (for example) Pentasol (a mixture of primary amyl alcohols), the system is heated for several minutes at a temperature between 60° and 100° on cooling, an amylose precipitate is obtained which can readily be separated in an ordinary, industrial centrifuge. Addition of excess methanol to the supernatant liquor causes precipitation of the amylopectin. Different kinds of starches can be fractionated by this method. Starch concentrations of up to 5% are claimed to give about a 24% yield of amylose (showing an iodine value of 16.0%) and a 76% yield of amylopectin (with a 0.9% iodine absorption). 

Bryce and Greenwood studied the kinetics of formation of the major volatile fraction from potato starch, and its components. They limited their interest to the temperature range from 156 to 337 and to the formation of water, as well as of carbon mon- and di-oxide. The results revealed the following facts. Stability toward pyrolysis within the first 20 minutes of the process falls in the order amylose < starch < amylopectin < cellulose. Autocatalysis is absent, as shown by Puddington. Both carbon mon- and di-oxide are evolved as a consequence of each of two first-order reactions. The initial one is fast, and the second is slow. The reasons are not well understood, but they probably involve some secondary physical effects. The amount of both carbon oxides is a direct function of the quantity of water produced from any polysaccharide, which, furthermore, is independent of the temperature. The activation energy for the production of carbon mon-and di-oxide reaches 161.6 kJ/mol, and is practically independent of the polysaccharide formed. At the limiting rates, the approximate ratios of water carbon dioxide carbon monoxide were found to be 16 4 1 for amylopectin, 13 3 1 for starch, 10 3 1 for amylose, and 16 5 1 for cellulose. 

Some models on the molecular configuration change of amylose, 1, 21) a mechanism on the fibril formation similar to the amylose retrogradation (1 ) have been proposed on the basis of the results observed by the static techniques and the meas= urements of slow reaction, however, the initial fast nucleation process in such a short time range, milliseconds to seconds, has not been found and pointed out at all. Thus, the fast inter= molecular process (second-order rate constant, in order of 1.5 X... 

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