Chemical Modification of Starch

Starch is an attractive biosynthesized and biodegradable alternative suitable for film preparation and foaming. Unfortunately, starch presents some disadvantages. It is highly hygroscopic, brittle without plasticizer and its mechanical properties are very sensitive to moisture content [51]. Still, these drawbacks can be avoided by, for example, blending the starch with an appropriate biodegradable polymer. On the market 

Starch is inherently non-suitable for most applications and, therefore, must be modified chemically and/or physically to enhance its positive properties and/or to minimize its drawbacks [53]. Flexibility in adjusting the properties to the needs of the specific application by appropriately modifying the composition, low-cost blending as opposed to innovative synthetic material development, and biodegradability are some of the main advantages which strongly motivate the development of starch-based materials [9]. 

Chemical modification of starch makes it suitable for many applications in the food products (e.g., as gelling agents, encapsulating agents, thickeners) and the non-food industry (e.g., as wet-end additives, sizing agents, coating binders, and adhesives in paper industry as textile sizes in cosmetic formulations) [46]. 

Water is usually involved as reaction media for chemical modification of starch being an environmentally-friendly solvent ( green solvent ), even its use means a relatively low reactivity, or a reduced selectivity due to side reactions. As result of the chemical modification reactions, the crosslinking process of starch occurred, this way being an efficient approach to improve the performance of starch for different applications [57, 58]. Chemically modified starches were investigated by determination of the modification degree [55], and by infrared spectroscopy method [59,60]. 

Due to the brittleness of starch materials, plasticizers are commonly used. A frequently utilized low weight hydroxyl compoxmd is glycerol. Another effective plasticizer is water, although not the best because it evaporates easily. Still, starch-based materials readily absorb water and this may result in significant changes in the mechanical properties. Different routes have been explored in order to improve the mechanical properties and water resistance of starch materials. These are chemical modifications to the starch molecule, blends with polymers such as polycaprolactone [61], or reinforcement with different types of cellulose-based fillers, such as ramie crystaUites [62], and timicin whiskers [63], or montmorillonite clay particles [64]. 

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Historically, chemical modification of starch was the first widely-investigated method for producing starch-based plastics. Attempts at forming cast films from starch or amy-lose showed that the properties were highly dependent on relative humidity the films became brittle at low humidities.4 Figure 19.2 shows data for films of various amylose/ amylopectin ratios conditioned at 50% relative humidity and 23°C. Efforts to overcome... 

It is conceivable that methodology such as that used to increase starch quantity could be used to influence starch quality by manipulation of starch synthase and branching isoforms. These new starches may have greater usefulness in food and industrial processes. The production of modified "specialty starches via molecular biology techniques is promising, and perhaps more beneficial and more economical than the chemical modification of starch for industrial purposes. 

Apart from various chemical modifications of starch with urea (see, for instance, Sroczynski et a/.769), a true inclusion complex of urea with starch (17 83) was prepared, and its usefulness in urea feeding was established.770 It was reported that increases in the ammonia concentration in the blood of ruminants fed with this complex are generally much slower, and lower than in the case of the administration of urea by itself (Fig. 50). 

Limitations in possibility of chemical modifications of starch result from steric hindrance of reaction sites, solubility, viscosity of reaction medium, and susceptibility to side reactions among them, depolymerization almost always accompanies intended modification. As a rule, polysaccharides are soluble, although frequently only sparingly, in water and dimethyl sulfoxide. Polysaccharides solubilize on xanthation, i.e., on reaction with CS2 in alkaline medium, to form syrups of xanthates. On acidification polysaccharides could be recovered. Such procedure was utilized for several decades for production of artificial silk from cellulose. 

Tomasik, P. and Schilling, C.H., Chemical modification of starch, Adv. Carbohydr. Chem. Biochem., in press, 2002. 

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