Starch modification

Starch can be modified in various ways, which are classified by the nature of action. These include (1)  

Chemical modifications may also include the derivatization of starch by reaction of its hydroxyl groups with alkylene oxides. Other suitable substances are such that form ether linkages, ester linkages, urethane linkages, carbamate linkages, or isocyanate linkages. The degree of substitution of the chemically modified starch varies from 0.05 to 0.2. 

Biological modifications of starch include bacterial digestion of the carbohydrate bonds, or enzymatic hydrolysis using enzymes such as amylase, or amylopectase. 

The recent research focused on identifying genes and proteins that are required for the production of starch pol5uners (2). Four core groups of genes are required for the bios5mthesis of starch, i.e.  

Phosphorylases are enzymes that catalyze the addition of a phosphate group from an inorganic phosphate to an acceptor. Starch synthase is an enz5une that catalyzes the polymerization of glucose and related compoimds. 

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Until recently, the use of enzymes in the paper and pulp industry was not cortsidered technically or economically feasible. Quite simply, suitable enzymes were not readily available, except for the limited use of enzymes to modify starch for paper coatings. However, research by scientific institutions and enzyme producers has led to the development of new enzymes that offer significant benefits for the industry, particrrlarly from the environmental point of view. Two examples of applications, in addition to the starch modification, made possible by new enzyme developments are given here bleach boosting and pitch control. 

Modification, which involves the alteration of the physical and chemical characteristics of the native potato starch to improve its fimctional characteristics, can be used to tailor it to specific food applications. The rate and efficacy of any starch modification process depend on the botanical origin of the starch and on the size and structure of its granules. This also includes the surface structure of the granules, which encompasses the outer and iimer surface depending on the pores and channels, which cause the development of the so-called specific surface (Juszczak, 2003). Potato starch modification can be achieved in three different ways physical, conversion, and chemical (derivatization) (Table 10.6). 

Table 10.6 Some common potato starch modification types and preparation techniques...

Derivatization is the most widely employed method for industrial-scale starch modification therefore we have discussed this modification in more detail. 

Bemiller, J. N. (1997). Starch modification Challenges and prospects. Starch, 49,127-131. 

Lewandowicz, G., Soral-Smietana, M. (2004). Starch modification by iterated syneresis. Carbohydr.Polym., 56, 403 13. 

Starch is an abundant, inexpensive polysaccharide that is readily available from staple crops such as com or maize and is thus is mostly important as food. Industrially, starch is also widely used in papermaking, the production of adhesives or as additives in plastics. For a number of these applications, it is desirable to chemically modify the starch to increase its hydrophobicity. Starch modification can thus prevent retrodegradation improve gel texture, clarity and sheen improve film formation and stabilize emulsions [108], This may, for example, be achieved by partial acetylation, alkyl siliconation or esterification however, these methods typically require environmentally unfriendly stoichiometric reagents and produce waste. Catalytic modification, such as the palladium-catalyzed telomerization (Scheme 18), of starch may provide a green atom-efficient way for creating chemically modified starches. The physicochemical properties of thus modified starches are discussed by Bouquillon et al. [22]. 

Microwave heating has not evoked a great deal of attention in the area of starch modification. Effective starch modification is achieved when the microwave energy is coupled with hydrolytic activity originating from added mineral acid. The... 

Starch modifications can be classified as physical modifications, chemical modifications and genetic modifications.45 Physical modification of cassava starch involves application of shear force, blending and thermal treatment. A combination of thermal treatment and shear force has been widely used to produce many extruded products and snacks. Well-known physically modified cassava starch products are alpha starch or pregelatinized starch and heat-moisture treated starch. 

For chemical modification, tapioca starch is easily modified to all current commercial derivatives. There are no special precautions or equipment required beyond what already might be practiced for a particular derivative or reagent applied to other starches. Recovery of modified products is facilitated in conventional washing and drying equipment. The reader is referred to Wurzburg47 and Chapter 17 for details of starch modifications, all of which may be practiced with tapioca starch. In the preparation and evaluation of some derivatives of tapioca starch, some of its unique characteristics have been revealed. 

Mechanically-modified starches are obtained by extrusion.43 15 The high temperature and high shear forces experienced by the starch offers further options for starch modification, but applications in paper mills are rare. 

The viscosity of a starch paste is often characterized differently by the starch manufacturer and by users in the paper industry. In the manufacturing plant, viscosity tests serve to control the starch modification process. As an example, the test is used to determine when to terminate a starch oxidation reaction for viscosity reduction. In the paper mill, viscosity testing is required to characterize the flow properties of starch-based coating formulations. 

The products of starch cooking with retention are similar to those obtained by dispersion with excess steam. Retention of starch paste for several minutes under pressure requires that all steam is condensed otherwise false body of uncondensed steam will shorten the retention. Jet cooking with retention is practiced for on-site chemical starch modification. 

The cooked pastes of ETHYLEX demonstrate excellent viscosity stability over prolonged periods of time and show little change in viscosity due to temperature fluctuations. Films produced from these pastes are clear, continuous and decidedly more flexible than those produced from other starch modifications. These characteristics, and others to be discussed later, strongly recommend the ETHYLEX Gums to widespread application in the paper, textile and adhesive industries. 

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