Rice starches modification

Properties of rice starches are changed by chemical modification in the same way as the properties of other starches (see Chapter 18). Starches prepared via the alkali method have been modified to provide additional pH and shear stability. In general, hypochlorite-oxidized rice starch has a lower gelatinization temperature and lower maximum paste viscosity producing a softer, clearer gel. Hydroxypropylated rice starches have lower gelatinization temperatures, whereas crosslinked rice starch has an increased gelatinization temperature, increased shear resistance and acid stability. 

Bourtoom and Chinnan (2008] studied the WVP of rice starch-chitosan blend films with different type and concentrations of plasticizer. They observed that the WVP increased from 5.45 to 8.68 gmm/m day kPafor sorbitol 14.53 to 28.73 gmm/m day kPa for glycerol and finally from 14.70 to 19.10 g mm/m day kPa for polyethylene glycol. This tendency was due to possible structural modifications of the polymer network and also because the network could become less dense as a result of increasing the mobility of the polymeric chains or by the free volume of the film. Also, the increase of WVP could be related to the hydrophilicity of the plasticizers because the presence of this material increases the concentration of polar residues in hydrocolloid based film... 

As indicated in Table 4.12, four regions which constitute the catalytic regions of amylolytic enzymes are conserved in the starch-branching isoenzymes of maize endosperm, rice seed and potato tuber, and the glycogen-branching enzymes of E. coli.286,281 It would be of interest to know whether the seven highly conserved amino acid residues of the a-amylase family listed in bold letters in Table 4.12 are also functional in branching enzyme catalysis. Further experiments, such as chemical modification and analysis of the three-dimensional structure of the BEs, would be needed to determine the nature of its catalytic residues and mechanism. 

Depending on the relative amylopectimamylose ratio found in the rice, the starch can exhibit a variety of gelatinized textures and strengths, as well as resistance to acid. Overall, there are several basic factors that affect starch performance. These factors, which include rice variety, protein content, method of starch production and modification, are described below. 

Genetic modification of many dicotyledoneous crops such as potato using gene transfer via A. tumefaciens is, at present, an efficient and reliable technique the application of the antisense route to limit or neutralize the action of undesirable genes has also been applied successfully to potatoes. Conversely, for monocotyledoneous crops such as maize, wheat, and rice, transformation can be accomplished by the much less efficient particle gun technique. This technique has been improved, resulting in rapid progress in the development of maize and later wheat with improved agronomic properties and/or altered starch composition. 

---