New corn starches

10 New corn starches P. J. White and A. Tziotis, Iowa State University, USA

10.1

Introduction: the use of corn starch in food processing

Corn provides a high-quality starch used widely in the food industry in many applications requiring particular viscosities and textures. Starch is a carbohydrate consisting of two distinct molecules; amylopectin, with anhydroglucose units linked to create a highly branched molecule; and amylose, a primarily linear glucose molecule. Starch functionality depends greatly on the molecular weight, size, and structure of its components, amylose and amylopectin, which, differing greatly in molecular-weight distribution and molecular structures, display different pasting, retrogradation, viscoelastic, and rheological properties (Morrison and Tester, 1991, Bahnassey and Breene, 1994, White et al., 1989). Normal corn starch is made up of about 25% amylose and 75% amylopectin. Amylopectin is the major component of most starches, and its fine structure plays a critical role in the characteristics of starch. Amylopectin (AP), the highly branched component of starch, consists of chains of -D-glucopyranosyl residues linked together mainly by 1,4 linkages, with 5 to 6% of 1,6 bonds forming branch points. Within amylopectin, the crystalline component is composed of parallel arrays of linear chains packed tightly in double helices (Thompson, 2000). Amylose (AM) has been defined as a linear molecule of 1,4 linked -D-glucopyranosyl units, but it is now well established that the linear molecules are slightly branched by -1,6-linkages. Amylose is the smaller of the two fractions (105±106 Da; degree of polymerization (DP) 500±5000) and possesses very few branches, 9±20 per molecule, with chain-lengths (CL) of between 4 and 100 glucose units and greater. Differences among corn starches in granule swelling (onset of viscosity), peak temperature, peak viscosity, shear

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thinning during pasting, and gel firmness during storage, have been mostly attributed to differences in amylopectin structure (Ring and Stainby, 1985, Doublier et al., 1987a, Bahnassey and Breene, 1994), whereas differences in setback and final viscosity during pasting have been attributed to amylose structure (Ott and Hester, 1965, Leloup et al., 1991, Vasanthan and Hoover, 1992). 10.1.1 Intermediate materials in corn starch Differences in granule swelling (onset of viscosity), peak temperature of gelatinization, peak viscosity, shear thinning, and firmness of gel during storage among starches have been mostly attributed to AP. Meanwhile, differences in setback and final viscosity during pasting have been attributed to AM. Lansky et al. (1949) proposed that a third component in normal corn starch exists, called intermediate material (IM), with properties different from those of AM and AP. This component could also play an important role in determining the functional properties of starch. The presence of a great number of short branch chains in this component could contribute to lower granular crystallinity, gelatinization temperature, enthalpy change, viscosity, and degree of retrogradation; and greater degree of digestibility by enzymatic hydrolysis, On the other hand, molecules with longer branch CL and a lesser degree of branching would contribute to greater crystallinity, gelatinization temperature enthalpy change, degree of retrogradation, viscosity and gel firmness (Campbell et al., 1995, Perera et al., 2001). Based on indirect evidence from iodine affinities, Lansky et al. (1949) suggested that 5 to 7 % of normal corn starch consists of material intermediate between the strictly linear and highly branched fractions. Subsequently, several types of branched polysaccharides have been recovered by various modifications of the previously described fractionation procedures. Erlander et al. (1965) recovered a low-molecular-weight component from the supernatant following AM precipitation with thymol and removal of AP by centrifugation. The polysaccharide remaining in the supernatant had a -amylolysis limit and degree of branching similar to that of AP. Perlin (1958) obtained an intermediate component following removal of AP by centrifugation and precipitation of AM with amyl alcohol. The polysaccharide remaining in the supernatant was more highly branched than AP, based on reduced -amylolysis limits, and was of lower molecular weight. With regard to starch structure in other plant species, a related highly branched polysaccharide with viscosity similar to AP was recovered from the supernatant following recomplexing of the AM fraction of starch from potato tuber, rubber seed, barley kernels, and oat kernels. A polysaccharide with a lower degree of branching than AP, but with greater average CLs and higher -amylolysis limits, was recovered from rye and wheat starches (Banks and Greenwood, 1967) and also from normal corn starch (Whistler and Doane, 1961). Another polysaccharide reported in small amounts in starch of corn (Adkins and Greenwood, 1969) is short-chain-length AM. In

New corn starches 297 normal corn starch, this linear polysaccharide has an average CL of 58 (Adkins and Greenwood, 1969). Banks and Greenwood (1975) suggested that the type and amount of IM in corn starch depended primarily on the AM percentage of the starch, although it could significantly vary among different starches. Variations in the amounts and structures of AP, AM and IM can result in starch granules with very different physicochemical and functional properties that can have a major impact on the utilization of these starches in food products (Kobayashi et al., 1986, Yuan et al., 1993). Baba and Arai (1984) found a longer average chainlength from AP and IM of amylomaize than from the AP of waxy corn. The IM from 50% amylomaize had an average degree of polymerization of 250± 300 glucose units per molecule, with four or five branches having a CL of about 50 glucose units. Wang et al. (1993) reported that about 15% of the starch in dull (du1) mutant endosperms was composed of IM, distinguished from the other components by the properties of its starch-iodine complex. Starch from du1 mutants had the highest degree of branching among a wide variety of normal and mutant kernels analyzed (Inouchi et al., 1987, Wang et al., 1993). Also, the degree of branching of AP and IM decreased when the amylose extender (ae) gene was present. The ae gene has a greater impact on the degree of branching of the IM than of the AP. The greater iodine affinity of ae IM than of AP indicated that the IM had a longer CL than did AP. On the other hand, the presence of more highly branched molecules was indicated in du1 starch. In addition, the degree of branching for IM was less than that of AP of the same starch (Wang et al., 1993). The gel-permeation chromatograms of corn starch from the dominant amylose extender (ae) mutant showed greater proportions of AM and IM than from normal corn starch (Kasemsuwan et al., 1995). Both AP and IM had a similar molecular structure, except the IM had more, shorter chains than did AP, and AP had more chains of DP 16 to 30 than did the IM. 10.1.2 Role of the environment on corn starch characteristics Corn starch and its structural features can be affected by environmental factors during the development of plants, even though the impact of the environment has not been reported to be as severe as that associated with other plant species and varieties. White et al. (1991) found differences in the differential scanning calorimetry (DSC) profiles of genotypes grown in temperate (warmer growing temperatures) versus tropical locations (cooler growing temperatures). The planting location affected the peak temperature of gelatinization and the gelatinization enthalpy change, which increased with later planting dates. Narrower gelatinization ranges were obtained from corn grown in a tropical environment than from corn grown in a temperate environment. Starch biosynthesis is subject to changes with environmental temperature, leading to the formation of different starch structures that can cause different functional properties. In a study of corn from two different backgrounds, each

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grown at two different temperatures, Lu et al. (1996) showed that kernel dry weight and density of corn decreased as the environmental temperature increased, in agreement with findings by White et al. (1991), who also showed that later planting dates were related to decreased kernel weight for corn from different lines. Lu et al. (1996) showed that starch in corn developed at a higher environmental temperature had a higher gelatinization temperature and wider temperature range than did that developed at a lower temperature, but the gelatinization enthalpy change was not affected by the developmental temperatures. A higher environmental temperature resulted in AM of smaller molecular size, and decreased iodine affinity, as well as apparent and true AM contents. The content of longer branch chains in AP increased as the developmental temperature increased, which correlated with the increase of gelatinization onset temperature. It was concluded that environmental temperature affects the enzymes that play a major role in the starch biosynthesis, such as branching enzyme I, which preferentially transfers long chains and has an optimum reactivity temperature higher than that of branching enzymes IIa and IIb, which transfer short chains and the different isoforms of soluble starch synthases.

10.2 Improving the functionality of corn starch for food processing applications: natural corn endosperm mutants Structurally and functionally, AP is the more important of the two starch fractions. Corn, and other grains, are capable of generating starch granules devoid of AM, as is observed in some mutant starches. Plants can be bred that produce starches with AM to AP contents outside the `normal' range (~25% AM and ~75% AP); for example, corn can be grown with an AM content as high as 85% (amylomaize) or as low as zero (waxy corn). Such families of starches are useful for studying the structural and functional roles of AM and AP within the granule, and also are useful in studying the synthesis and development of granules themselves. These starches generally contain AP to AM ratios that are much different than that of normal corn starch, and likely have molecules with altered structures, thus leading to an array of properties and specific applications of potential interest in food manufacturing. The normal and the corresponding endosperm mutant corn types have been produced through traditional plant breeding techniques, thus offering a natural alternative to corn developed via biotechnology techniques (genetically modified (GM)) for modifying and extending the range of functionality of a specific starch type. Native mutant starches could be marketed as `all natural' and non-GM. Many natural mutanttype corn types in which starch is affected were studied by Wang et al. (1993). A list of some of these single mutants is shown Table 10.1, along with their starch granule size distribution.

New corn starches 299 Table 10.1 Size distribution of starch granules of maize genotypes from OH43 inbred corn measured by scanning electron micrographs (Wang et al., 1993) Genotypeã

Range (m)

Average ÔSDb (m)

Normal ae bt1 bt2 du1 h sh2 su1 wx

6±17 4±11 4±9 6±19 4±11 8±22 2±9 2±10 6±14

11.6Ô4.5 7.0Ô2.1 6.1Ô1.3 10.8Ô3.4 7.8Ô2.1 13.8Ô3.6 6.3Ô1.9 5.4Ô2.6 10.3Ô2.6

ãae = Amylose extender; bt = brittle; du = dull; h = horny; sh = shrunken; su = sugary; and wx = waxy. b Average ÔSD of 30 starch granules, 15 each from two micrographs.

10.2.1 The waxy mutation The waxy (wx) mutant is unique to all other known mutants relative to its lack of accumulation of AM (Shannon and Garwood, 1984). Waxy corn, containing essentially 100% amylopectin, is the most important raw material for modified starches (Yuan et al., 1993). This mutant lacks granule bound starch synthase (GBSS), which inhibits its production of AM (Doehlert and Kuo, 1994). The introduction of the wx gene into any corn type, or the combination of any other mutant with the wx gene results in a starch granule devoid of AM (Shannon and Garwood, 1984; Wang et al., 1993). When the corn kernel is cut with a knife, its lack of AM makes the cut surface appear shiny and waxy: thus, the name, waxy, was introduced. In general, wx and normal corn development, such as starch and dry weight production, and the accompanying starch granule morphology are similar. Both types of granules have A-type x-ray diffraction patterns. 10.2.2 The amylose extender (ae) mutation The ae mutation results in a loss of starch branching enzyme IIb activity (Boyer and Preiss, 1978). Amylomaize, containing the ae mutant gene, has an apparent AM content of up to 85%, and is associated with the presence of abnormal AP (Mercier, 1973, Ikawa et al., 1978, Boyer et al., 1980, Ikawa et al., 1981). The estimation of AM content is difficult because of the presence of branched components with long external chains, which can lead to an overestimation of the AM content. On the other hand, the presence of short chain-length AM can be responsible for an underestimation. Wolf et al. (1955) proposed that the unusual properties of ae AP could be attributed to a polymer of greater linearity than normal AP and of an intermediate structure between those of AM and AP. Boyer et al. (1980) observed that starch from ae wx double-mutant corn consists of an unusual AP similar to that of the intermediate fraction of ae starches. Wolf

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et al. (1955) showed that the AP fraction in a 50% amylomaize starch had longer inner and outer chains than those of normal AP, where the inner and the outer chains were divided at the point of -1,6-glucoside bonds. Montgomery et al. (1964) supported the proposal of Wolf et al. (1955), who suggested that the AP in amylomaize starch was less highly branched than was normal AP. More recently, however, it was proposed that the abnormal AP was a result of the presence of contaminating short-chain AM. The discrepancy likely originates because the same techniques for the dispersion and fractionation of starch are seldom used among researchers (Banks and Greenwood, 1975). Although the fine structure of abnormal AP from amylomaize has not been completely clarified, there are reports showing the presence of abnormal AP in amylomaize starch (Mercier, 1973, Ikawa et al., 1978, Boyer et al., 1980, Ikawa et al., 1981). In addition to the high apparent AM content, it has been shown (Takeda et al., 1993, Klucinec and Thompson, 1998) that ae starches contain AP with a higher proportion of longer chains (DP>30) than present in the AP of common corn starch. In addition to the ae gene, the combinations of a dull-1 (du1) or sugary-1 (su1) gene with other mutant genes (except the wx gene) also have produced starches with increased AM contents (Ikawa et al., 1981, Yeh et al., 1981, Inouchi et al., 1983, 1987, Boyer and Liu, 1985, Wang et al., 1993). Wang et al. (1993) reported increased AM as well as increased intermediate material content of these starches. 10.2.3 The du mutation The mutation of du1, when homozygous in otherwise non-mutant backgrounds, results in mature kernels with a tarnished, glassy, and somewhat dull appearance, which is referred to as to the `dull phenotype'. Total carbohydrate content in mature du1 mutant kernels is slightly lower than in normal corn (Creech, 1965, Creech and McArdle, 1966). The apparent AM content in starch from du1 mutants is slightly to greatly higher than normal, depending on the genetic background (Shannon and Garwood, 1984). Starch granules from du1 mutants seem to have normal structural and physical properties, although some abnormally shaped granules are found in the mutant endosperm (Shannon and Garwood, 1984). The du1 mutation causes reduced activity in the endosperm of two seemingly unrelated starch biosynthetic enzymes, starch synthase II (SSII) and starch branching enzyme IIa (SBEIIa) (Boyer and Preiss, 1981, Gao et al., 1998). The relative AM content of starch in du1 mutant kernels is significantly greater than it is in wild-type kernels. Approximately 15% of the starch in the du1 mutant endosperm is thought to consist of AP chains that are abnormally highly branched. Mutant du1 alleles, when combined with other mutations affecting starch synthesis, result in a broad range of alterations more severe than those in the single mutants. Double mutants containing du1, together with either a wx, ae, su1, or su2 mutation, have greater amounts of soluble sugars and lower total starch content than do any of the single mutants (Nelson and Pan, 1995).

New corn starches 301 10.2.4 The su2 mutation The recessive su2 allele in corn identified by Eyster (1934) was found to reside on chromosome six. Perera et al. (2001) reported that the su2 starch granules consisted of lobes that resembled starch mutants deficient in soluble SSs, resulting in starch with a greater content of AM and a lower gelatinization temperature than that of normal corn starch (Pfahler et al., 1957, Kramer et al., 1958, White et al., 1994, Li and Corke, 1999, Perera et al., 2001). The su2 starches also have been shown to retrograde less during storage than do normal starches (Inouchi et al., 1991, White et al., 1994, Campbell et al., 1994). Li and Corke (1999) reported that the swelling power of su2 starch was significantly lower than that of normal corn starch. Starch isolated from genotypes containing su2 allele, in combination with du and su1, contained about 77% AM (Dvonch et al., 1951, Dunn et al., 1953). Also, su2 starch has an improved nutritional quality as a result of its high susceptibility to -amylase digestion; thus, its use has been suggested in improving animal feed value (Sandstedt et al., 1962). The su2 mutant also has been shown to enhance grain quality, because this allele in combination with opaque-2, resulted in kernel density nearly equal to that of ordinary dent corn (Glover et al., 1975). The su2 mutant has not been associated with any genetic lesion. Inouchi et al. (1984) and Boyer and Liu (1985) found that the su2 gene is associated with increased AM content to different extents. White et al. (1994), in describing the properties of su2 starch, found an AM content of 35%, smaller starch granules than normal starch, and suitable pasting properties for application in starch-thickened acidic foodstuffs. Inouchi et al. (1984) described starch of su2 as having an A-type X-ray diffraction pattern. The diffraction peaks, however, were broad and weak, reflecting a lower degree of crystallinity than found in normal starch. It was suggested that starch of su2 may differ from normal starch because of differences in the bonding of starch molecules or anomalous linkages within the molecules. Little difference was found in the fine structures of AP from su2 and normal starch on the basis of unit chain-length distributions (Inouchi et al., 1987). On the other hand, Takeda and Preiss (1993) determined that, compared with that of normal starch, the AP of su2 starch was composed of larger sized long B-chains, which were poorly branched, leading to an increased iodine affinity value. Several novel starches resulting from su2 alone and in combination with other alleles, such as du, ae, and wx, produced starches with properties resembling those of some modified starches (Friedman et al., 1988a,b, Wang et al., 1993). In addition, the use of starches from genotypes possessing the su2 allele, alone or in combination with other mutant genes, have been patented because of the favorable physical properties (White et al., 1994). 10.2.5 The su1 mutation Creech (1965) reported that sugary-1 (su1) corn kernels are wrinkled and have reduced amounts of dry material. The concentration of sugars is higher and the

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starch content is much lower than in normal corn. Extraction of su1 endosperm by different methods resulted in different distributions of starch and phytoglycogen (Boyer et al., 1981). The su1 mutants of corn have been known for decades to accumulate, in addition to starch, a novel form of water-soluble polysaccharide, termed phytoglycogen. Summer and Somers (1944) reported that the principal polysaccharide storage product in su1 endosperms was not starch, but a highly branched, water-soluble polysaccharide of high-molecular weight, which they called phytoglycogen. Phytoglycogen has twice the frequency of branch linkages as AP, a shorter average CL (average DP is approximately 10 versus an average of 20±25 for AP), and a significantly different chain-length distribution (Yun and Matheson, 1993). Thus phytoglycogen is multiply branched and lacks the packed crystalline helices of AP. These structural alterations cause the molecule to be water soluble, whereas AP in endosperm cells is insoluble. In 1958, Erlander proposed that glycogen could be considered as a precursor of starch biosynthesis, further suggesting that AP would be produced by debranching this precursor and AM would be subsequently generated through debranching AP. Pan and Nelson (1984) first reported that the su1 phenotype is caused by the loss of the activity of one of the three isoforms of R enzyme (RE) (a pullulanase-type starch-debranching enzyme) and possesses low activities of the other two RE isoforms, suggesting that the debranching enzyme also is involved in starch biosynthesis and that the debranching enzyme participates in the organization of regularly spaced clusters within AP. Doehlert et al. (1993) reported activities of total amylase and -amylase, as well as of RE, that were lower in su1 kernels of corn than in normal cultivars. This fact contrasts with the well-accepted idea that the starch debranching enzymes (pullulanase and isoamylase) are only involved in starch degradation in conjunction with other hydrolytic activities. James et al. (1995) showed, by cloning the su1 gene, that the su1 gene of corn encodes an isoamylase-like enzyme. Recently, it was reported that the su1 gene product possesses isoamylase activity, and that su1 mutants are deficient in both isoamylase and pullulanase (Rahman et al., 1998, Beatty et al., 1999). In work by Inouchi et al. (1983), phytoglycogen had a constant distribution of CL during kernel development, and AM content of su1 starches was somewhat higher than that of normal starches during kernel development. Inouchi et al. (1987) reported no long B chains in su1 phytoglycogen. Indeed, B chains of the phytoglycogen had comparatively uniform CL, which were shorter than the CL of the other corn starches. The su1 corn contained particulate granules, made up of phytoglycogen and AM (Matheson, 1975). Yeh et al. (1981) reported widely different AM percentages in su1 starch in different backgrounds ranging from 0±65 %, likely because of different environmental conditions during kernel development, kernel age, and methods of starch isolation and AM measurement.

New corn starches 303 10.2.6 Comparisons of starch fractionation methods and functionality among mutant corn types in the same genetic background In a recent study involving most of the endosperm corn mutants just discussed (Tziotis, 2001), the fine structures of starch fractions obtained from a wild-type (normal) corn starch and amylose-extender25, dull39, sugary2, and sugary1 corn mutants in the same genetic background (ExSeed68) were isolated, evaluated, and compared by using three different fractionation procedures based on gelpermeation chromatography or alcohol precipitation methods. Starch fractions obtained from each of the three methods were enzymatically debranched and analyzed by using a high-performance anion-exchange chromatograph with a post-column amyloglucosidase reactor and a pulsed amperometric detector. No apparent differences in the molecular weight distributions of AP or of AM among the different starches were observed. The separations were performed by fractionation on a GPC column, by precipitation with 1-butanol, and by preferential precipitation with 1-butanol and isoamyl alcohol. The proportions of branch-CL of the starch components obtained by the various fractionation methods were very similar among methods for each of the starch types analyzed, such as the predominance of long branch-chains in ae25 corn and that of the short branch-chains in su2 corn. Overall, the effect of the mutations was more important in the differences observed among the starch types than was the method of analysis used. The thermal and functional properties of these starches also were evaluated and related to the structural features of the starches examined. X-ray diffraction patterns of the starches were obtained. The onset temperature of gelatinization values of starches from all mutant lines ranged from 52.0 ëC for su2 to 62.9 ëC for du39, temperatures that were all lower than that of the normal starch (64.5 ëC) as measured by using DSC (Table 10.2). The change of enthalpy of gelatinization of starch from su2 (7.7 J/g) was less than that of the wild type starch (14.1 J/g). The viscosity of the su2 starch over the cooking process was Table 10.2 Differential scanning calorimetry thermal properties of starch from ExSeed68 (wild-type) corn and mutants, dull39, amylose-extender25, sugary2, and sugary1, in the ExSeed background (Tziotis, 2001) Starch typea

Peak onset temperature (ëC)

Gelatinization range (ëC)

Change in enthalpy (J/g)

Retrogradation (%)

Wild-type du39 ae25 su2 su1

64.5a 62.9b 62.5b 52.0d 60.4c

8.4e 9.8d 16.8a 13.1b 11.3c

14.1b 16.1a 15.7ab 7.7c 8.5c

58.5 33.3 57.5 33.1 38.1

a

b

Values reported are means of four replicates. Numbers followed by the same lower-case letter within each column are not significantly different at P < 0.05. b Value reported for ae25 starch is the sum of enthalpy change obtained from this peak and another peak not reported in the table.

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relatively stable as measured by Rapid Visco Analyser (RVA), showing only a small breakdown of the peak viscosity during cooking, and suggesting high stability of the starch granules against mechanical shear. The su1 mutant starch formed the strongest gel among all starch-gel samples during measurements of both fresh and stored gels evaluated by using a Texture Analyzer. 10.2.7 Influence of genetic background on functional properties Variations in the functions of mutant starches related to structural differences were noted when the mutant was placed in corn with different genetic backgrounds. For example, Li and Corke (1999) developed five different corn inbred lines and evaluated the thermal, pasting, and gel textural properties of each isogenic line consisting of normal corn along with its du and su2 mutants. Differences were reported among the same mutant but in a different background, in the AM percentage (varying between 29.5 and 43.2% for su2, and 29.0 and 37.6% for the du mutants); swelling power; solubility; digestibility; onset, peak, and conclusion temperatures of gelatinization; change of enthalpy of gelatinization; pasting properties; and gel firmness and adhesiveness. These values compare with the work of Tziotis (2001) just mentioned (Table 10.2). The gelatinization onset temperature (To) values of starches from mutant lines all placed in the same experimental background line of ExSeed68 were all lower than that of the normal starch, which agrees with relative values reported by others for these mutants in the same (Perera et al., 2001) and other genetic backgrounds (Pfahler et al., 1957, Kramer et al., 1958, Brown et al., 1971, Ninomya et al., 1989, Campbell et al., 1994, Ng et al., 1997, Li and Corke, 1999). The change in enthalpy of gelatinization, gelatinization range values, and retrogradation percentage values reported by Tziotis (2001) also reflected differences among the mutants that were similar to results from these other studies. Both absolute and relative thermal property values of corn starches can vary, however, based on their genetic backgrounds, as indicated by the following reports on To values. One should bear in mind that the values, although all obtained by DSC, did not necessarily result from the same heating parameters and starch moisture contents. The du mutant starch in the IA5125 inbred line had a To of 61.0 ëC, the ae mutant starch had a To of 68.8 ëC, and normal starch a To of 64.2 ëC (Sanders et al., 1990). Similarly the To for the ae starch in the W64A background (70.6 ëC) was greater than that of normal starch (63.9 ëC) (Krueger et al., 1987). The su2 mutant in an Oh43 background had a variable To, which decreased with increased dosage of su2, reaching 58.3 ëC (normal 67.3 ëC) for the complete su2 mutant background (Campbell et al., 1994). Alternatively, Inouchi et al. (1991) reported that normal corn starch in the Oh43 background had a To of 61 ëC, with the mutants as follows: ae (65 ëC), du (64 ëC), and su2 (45 ëC). Thus, all mutant starch values in that study, except for su2, were greater than that of the normal starch. Wang et al. (1992) also reported DSC values for corn mutants in the Oh43 inbred line with slightly greater

New corn starches 305 relative and absolute To values than those of Inouchi et al. (1991). The To of the ae, du, and su1 mutant starches were 68.7, 67.2, and 64.6 ëC respectively, with the normal counterpart having a To of 67.2 ëC. Li and Corke (1999) studied corn starch from five different genetic backgrounds (A632, Oh43, Hz85, Hz101, and Hz47) consisting of the du and su2 mutants and their normal counterparts. In all cases in that study, the properties of du mutant starch were very similar to those of the normal starch, as noted by Wang et al. (1992), and the su2 mutants had To values approximately 10 C lower than those of the normal. The su1 mutant corn starch in the P39±5XP51±B background had a To of 62 C, with the normal counterpart at 65 C (Ninomya et al., 1989).

10.3 Chemically modifying corn starches for use in the food industry The use of starch in many industrial applications depends on its granular structure; cold water solubility; colloidal dispersability during heating; film forming, binding, adhesion, thickening and stabilizing abilities; and textural contribution (Jarowenko, 1986). In addition to the altered structures of starches from the naturally occurring corn endosperm mutants, chemical modifications serve as a powerful tool to expand the spectrum of corn starch properties and uses. Chemical modifications usually involve the addition of moieties on the linear chains of the glucose units of the starch molecules, thus changing the molecular size and viscosity characteristics of the starch. The response of starch to chemical treatments depends on its origin, type, and history. Chemical modification of starch, by placing substituent groups along its polymeric backbone, decreases gelatinization temperature, and increases translucency, viscosity, freeze-thaw stability, solubility, swelling power, hardness, cohesiveness and adhesiveness of the starch gel (Betancur-Ancona et al., 1997). 10.3.1 Succinylation Succinylation of starch, by the addition of succinic anhydride (dihydro-2,5furandione) to its molecules, may be desirable in the food industry because the modification improves properties, such as decreasing gelatinization temperature; increasing freeze-thaw stability, thickening power, viscosity stability, clarity, ability to swell in cold water, and stability in acid and salt; and reducing the tendency to retrograde (Tessler and Wurzburg, 1983, Trubiano, 1987, Bhandari and Singhal, 2002), although this chemical process is used more in industrial applications than in food applications. The beginning material for starch succinylation is generally waxy starch, because the combination creates very useful starches. The peak viscosity increases slightly with increased treatment level (with succinic anhydride), whereas the final viscosity decreases, and the low-temperature stability improves. Cooked, unmodified corn starch forms a gel

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Starch in food

on cooling, whereas starch succinates give smooth, stable, high-viscosity colloidal suspensions. Extrusion also has been employed to prepare starch succinates with succinic anhydride (Tomasik et al., 1995). Bhandari and Singhal (2002) compared the swelling power of corn starch succinates to that of the native corn. They reported that the swelling power of native corn starch increased from 2.1 to 11 over the 45 to 95 ëC temperature range, whereas the swelling power of corn starch succinate with degree of substitution (DS) was 2.14 at 45 ëC and 17 at 95 ëC for corn starch succinates of DS 0.20. They considered that the increase in swelling power with the DS could be due to easy hydration, an indication of the increasing number of hydrophilic groups incorporated in the starch. Regarding the Brabender characteristics of corn starch succinates, the peak and cold paste viscosities increased with an increase in DS from 0.05 to 0.20. When studying the freeze-thaw stability of the same starches, native corn starch showed 66% syneresis after ten cycles, whereas with an increase in DS, resistance to syneresis improved. Also, percent syneresis decreased with an increase in DS. For example, a corn starch succinate with 0.20 DS demonstrated excellent freeze-thaw stability. In addition, the corresponding gelatinization temperature dropped with increased treatment level. Corn starch treated with succinic anhydride produces starch pastes that are relatively clear, stable to viscosity changes, but gummy in texture, which is not always desirable in food applications. To avoid the gummy texture, succinic anhydride can be reacted with lightly cross-linked granular starch so that the resulting texture of the cook is short, smooth, and shiny, with retention of the stability and much of the original clarity. Cross-linking also improves the resistance of the derivative to breakdown at high temperatures. Succinate derivatives are used in the food industry as binders and thickening agents in soups, snacks, canned, and refrigerated food products. Their properties also are desirable in pharmaceuticals, where they are used as tablet disintegrants. Crosslinked high-AM starch (Hylon VII) was introduced a few years ago as an excipient (ContramidTM) for controlled drug release (Ispas-Szabo et al., 2000). This starch swells in water to form an elastic gel. Its ability to regulate the swelling controls drug release in aqueous media as a function of cross-linking density. So this hydrogel is particularly suitable as a pharmaceutical excipient. These properties are strongly dependent on the degree of the cross-linking of the starch. Best release properties and highest mechanical hardness were obtained from cross-linked high-AM starch matrices with low-to-moderate crystallinity, where the V- and the B-type structures coexist with amorphous regions (Ravenelle et al., 2002). 10.3.2 Acetylation In a study, Wilkins et al. (2003a) evaluated the variability of starch acetylation caused by hybrid influence. Six wx corn hybrids grown in 1998 and five wx corn hybrids grown in 1999 were wet-milled in the laboratory and modified, with the reaction efficiencies monitored. Reaction efficiencies were highly variable (47

New corn starches 307 to 73%), and were significantly lower for 1998 hybrids (50.0%) than for the same hybrids grown in 1999 (62.7%). Regarding the pasting properties of the modified starches analyzed by using an RVA, acetylated starch from 1999 had increased peak, trough and final viscosities and increased reaction efficiency as compared with acetylated starch from 1998. Differences in setback were observed among 1998 hybrids for acetylated samples. Differences in trough and final viscosity were observed among 1999 hybrids for acetylated and native (unmodified) samples, whereas differences in breakdown among 1999 hybrids were observed for native samples. Also, Wilkins et al. (2003b) evaluated the differences in the pasting properties and reaction efficiencies of acetylated dent corn starch from ten dent corn hybrids grown during 1998 and nine dent corn hybrids grown during 1999, all wet-milled in the laboratory. Acetyl content (reaction efficiency) was measured by a spectrophotometric method and ranged from 35 to 56%. Reaction efficiencies for starches from 1998 hybrids were lower than for starches from 1999 hybrids. Differences in peak viscosity, trough viscosity, final viscosity, setback, and pasting temperature occurred among 1998 hybrids, whereas differences only in trough viscosity, final viscosity, and breakdown were found among 1999 hybrids. Wang and Wang (2002) studied the effects of the catalyst used in acetylation, including sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2), on the chemical and physicochemical properties of acetylated waxy corn starch. The Ca(OH)2-catalyzed acetylated starch had a slightly higher pasting temperature and a lower -amylolysis limit than did the acetylated starch prepared under NaOH or KOH catalysis, but there were no differences related to their thermal properties. The isoamylase-debranched acetylated starches catalyzed by Ca(OH)2-and their -limit dextrins showed an elution profile, analyzed by high-performance size-exclusion chromatography, that was different from those of the other two acetylated starches with a greater proportion of saccharides eluted at a longer retention time. However, the differences in pasting temperature, -amylolysis limit, and carbohydrate profile among the acetylated starches diminished when ethylenediaminetetraacetic acid (EDTA) was added. The results suggested that calcium might induce intermolecular cross-linking through chelation with oxygen of the anhydroglucose units and that this type of cross-linking was promoted in acetylation catalyzed by Ca(OH)2. 10.3.3 Hydroxypropylation The effects of modification sequence on chemical structures and physicochemical properties of hydroxypropylated and cross-linked waxy corn starch were recently reported (Wang and Wang, 2000), where the chemical structures of dual-modified starches and their -limit dextrins were characterized with high-performance liquid-chromatography. The hydroxypropylatedcross-linked starch had higher Brabender viscosity than did the cross-linkedhydroxypropylated starch at both pH 7 and 3; but both starches has similar

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Starch in food

gelling properties. The hydroxypropylated-cross-linked starch had significantly higher onset and peak gelatinization temperatures, gelatinization enthalpy, and lower retrogradation. Also, the latter starch exhibited a significantly higher amylolysis limit and higher content of low molecular weight saccharides in its isoamylase-debranched starch, suggesting a structure more accessible to enzymatic digestion than that of the cross-linked-hydroxypropylated starch. Further structural analyses revealed different distribution patterns of modifying groups between the two modified starches. The results indicated that the modification sequence was responsible for changing the susceptibility to enzymes, the locations of substitution, and the physicochemical properties of the hydroxypropylated and cross-linked wx corn starches. Shi and Be Miller (2002) recently evaluated the aqueous leaching of the hydroxypropylated common corn starches at different times and temperatures. Results indicated that the greater the modification the easier it was for AM to leach out and that the preference for leaching of derivatized AM decreased as molar substitution of the whole starch increased. McPherson and Jane (2000) studied the effects of extrusion variables on granular morphology and molecular weight of the starch components of native and cross-linked (0.0 to 0.028% POCl3) hydroxypropylated (8%) corn starches. Extrusion of starches caused substantial morphological changes in granular structure. Starches extruded at 60 ëC showed distorted and fragmented granules, whereas the extruded starches at 100 ëC showed no granular structure and were completely amorphous. Extrusion conditions affected the molecular weights of the extruded starches. Increased starch moisture content reduced AP degradation during extrusion. Also, cross-linking prevented AP degradation. However, the magnitude of AP degradation increased at higher levels of cross-linking as shear increased. Increased temperature of extrusion decreased AP molecular weight of extruded native and hydroxypropylated corn starches, whereas the opposite effect was observed in cross-linked hydroxypropylated starches, likely related to the glass transition temperature. Bae and Lim (1998) performed the hydroxylation of normal (25% AM) and high-amylose (70% AM) corn starches to 0.1 degree of molar substitution with propylene oxide in an alkaline-ethanol medium (70% ethanol). Stearic acid, glycerol monostearate, or lecithin (3%, based on starch) was added to each mixture to examine the effects on the physical properties of the extrudate. Highamylose, alone and with all additives, showed lower die swelling in extrusion than did normal corn starch, whereas hydroxypropylated normal corn starch and hydroxypropylated high-amylose starch showed higher die swelling than the corresponding unmodified starches. Hydroxypropylation increased the water absorption for both starches (high-amylose extrudates from 22 to 35% and normal corn starch extrudates from 68 to 97% at 25 ëC). Differential scanning calorimetry showed that during extrusion, the lipid additives formed a helical complex with AM in normal and high-amylose starch, but that the complex was weak with their hydroxypropylated derivatives. The extruded strands of highamylose starch, alone and with additives, exhibited higher tensile and bending

New corn starches 309 strengths (37.1±58.4 and 2.16±5.07 MPa, respectively), than did the normal corn starch strands (12.4±59.3 and 1.06±4.10 MPa, respectively) at the same moisture content (7.5±8.5%). Both tensile strength and percent of elongation of the starch strands were reduced by the presence of a lipid additive. Hydroxypropylation increased elongation and flexibility of the extrudates. Also, hydroxypropylated high-amylose starch exhibited the greatest mechanical strength and flexibility among all starches. 10.3.4 Phosphorylation Liu et al. (1999) reported the preparation of phosphorylated starches with sodium tripolyphosphate at pH 6, 8, and 10 from wx (3.3% AM), normal (22.4% AM), and two high-amylose (ae, 47 and 66% AM) corn starches. The resulting starches had a decreased gelatinization peak temperature and an increased pasting peak viscosity except for wx, which showed a slight increase in gelatinization temperature. There was a substantial effect of phosphorylation pH on paste viscosity. Also, more cross-linking was found in ae starches with phosphorylation at pH 10. It was indicated that sodium ions decreased the paste viscosity of all the phosphorylated starches, whereas only slightly affected the paste viscosity of native starches. Phosphorylation increased swelling power of some of the starches, with greatest swelling power at phosphorylation pH 8 and least at pH 10. In the case of wx starch, maximum swelling power was obtained after preparation at pH 8 and minimum swelling power at pH 6. With phosphorylation, the clarity and freeze-thaw stability of all the starches was greatly increased compared to the native starches. Digestibility of phosphorylated ae starches was increased, but phosphorylation had little effect on wx and normal starches. All phosphorylated derivatives showed an increase in the adhesiveness, springiness, and cohesiveness; additionally, the hardness of 47% ae and wx starches increased, and that of normal starches decreased. Enthalpy of gelatinization decreased after phosphorylation with the ae starches exhibiting the greatest decrease. Lim and Seib (1993a) produced phosphorylated corn starches with 5% sodium tripolyphosphate (STPP) and/or 2% sodium trimetaphosphate (STMP). All phosphorylation reactions were done using 5% sodium sulfate and adjusting the initial reaction pH by adding aqueous sodium hydroxide or hydrochloric acid to the pre-reaction slurries. The degree of phosphorylation decreased 40 to 50% with STPP as reaction pH increased from 6 to 11, whereas it increased by 100% with STMP. Corn starch phosphate with the most desirable pasting properties was obtained at an initial pH of 11 with STPP and contained 0.16% P (including 0.02% from lipid). When corn starch was treated with a mixture of 5% STPP and 2% STMP, the best product when pasted at 95 ëC was obtained at the initial reaction pH of 9.5. Paste clarity of the phosphorylated starches indicated that cross-linking accelerated rapidly above pH 8 with STMP, but above pH 10 with STPP.

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10.3.5 Cross-linking Cross-linking is the most widely used technology for the improvement of the properties of native starches, which are usually sensitive to shear, high temperature, and acid treatment when cooked in water. Cross-linked wx cereal starches generally show a `short' spoonable texture, higher paste stability, and resistance to cooking shear, temperature, and low pH as compared to native starches (Whistler and Be Miller, 1997). On the other hand, cross-linking reduces paste clarity and stability to cold storage. In that case, the undesirable properties can be improved by further modification treatments, such as esterification or etherification. Cross-linked starch is essential in the manufacture of foods to thicken, stabilize, and provide texture. The creation of cross-linked starch originated from the need for starch granules, which resist disintegration on cooking with water. To avoid a thick, pasty mass, a process was designed to chemically treat native starch with acid chlorides including phosphorous oxychloride (POCl3) in water (Felton and Schopmeyer, 1943). Other researchers followed suit with novel chemical approaches to cross-linking starch using other reagents such as epichlorohydrin (EPI) or sodium trimetaphosphate (STMP) (Konigsburg, 1950, Hofreiter et al., 1960, Lloyd, 1970). Waxy starches (lacking AM) are often used as the base for cross-linked starches because AM retrogrades on cooling and forms an irreversible gel (Katzback, 1972). The mechanism of the reaction is as follows: cross-linking agents bind neighboring anhydroglucose units in the amorphous regions of the wx corn AP. The cross-links prevent the granules from fully swelling and ultimately disintegrating. The covalent cross-link network also makes the granules less susceptible to pH extremes and high shear processes common to food manufacturing. The extent of the effects of cross-linking on swelling and viscosity depends both on the treatment conditions of raw starch and on how the starch is prepared in the final application. Major factors in the cross-linking reaction include chemical composition of reagent, reagent concentration, pH, reaction time and temperature (Lim and Seib, 1993b). The degree of crosslinking for food starches is very low, making the extent of reaction and yield of cross-linked starch difficult to measure chemically, consequently requiring the measurement of physical properties. Maximum extent of cross-linking reaction for EPI with corn starch, assuming that the percentage of reacted EPI that results in cross-links is constant, was reported at 90% (42 hr at 25 C) by Hammerstrand et al. (1960). No such quantitative values are reported for POCl3 or STMP. Cross-linked starches are produced when unswollen, native granules are mixed in an aqueous system with reagents capable of reacting with at least two of the hydroxyl groups of neighboring molecules (Wurzburg and Szymanski, 1970). The type of reagent used and cross-linking conditions determine the ratio of mono and di-type bonds (esters with phosphorous based agents and glycerols with epichlorohydrin) caused by the cross-linking reaction mechanism and available starch hydroxyls (Koch et al., 1982). Starch thickening properties can be controlled by changing the degree of cross-linking and manipulating the extent of swelling. A relationship between

New corn starches 311 rheological properties and swelling capacity of starch granules has been demonstrated (Evan and Haisman, 1979, Bagley and Christianson, 1982). The flow behavior and textural properties of cross-linked starch are very complex due to the effects of starch concentration, heating rate, heating temperature, and amount of shear, as well as competition with other dissolved solutes and polymers (Doublier et al., 1987b, Steeneken, 1989, Gluck-Hirsch and Kokini, 1997). Relative effects that the different cross-linking agents have on physical properties have been studied (Evans and Haisman, 1979 Eliasson, 1986, Steeneken, 1989, Evans and Lips, 1992); however, it is uncertain how crosslinking is achieved at the macromolecular level of the 3-D granule structure. Jane et al. (1992) reported that when granular starch was cross-linked, a greater amount of AP than AM was found cross-linked. When corn starch was treated with a cross-linking reagent (0.07% epichlorohydrin, at pH 10.5 for 24 hr), 91% of its AP and 45% of its AM became insoluble. Cross-linking of pregelatinized and dispersed starch caused less difference in the proportion of soluble AM and AP than did the cross-linking of native granular starch. After the starch had been cross-linked in the granular form, there was no increase in the size of AM as a result of cross-linking between two or more AM molecules. However, susceptibility of the AM to sequential hydrolysis by isoamylase and -amylase decreased. The relative blue values of AP peaks indicated that AM was cross-linked to AP, which was confirmed when the AP isolated from crosslinked starches was debranched with isoamylase.

10.4 Genetically modifying corn starches for use in the food industry It is important to search both inside and outside the Corn Belt in the United States for corn with desirable properties, because of the potentially beneficial properties these new corn starches can contribute to produce high-quality food products, and because of the introduction of new genes to diversify the gene pool. Pollak and White (1997) examined the range of variability of starch functional traits in Corn Belt inbred lines and exotic inbred lines from Argentina, Uruguay, and South Africa. Reciprocal hybrids of some of the lines within each set were compared with their parents. Functional traits were examined by using DSC on starch extracted from single kernels of genotypes (Table 10.3). The Corn Belt lines had a wider range of values for most traits than did the set of exotic lines. For both sets of lines, the maximum value for peak height index (a measure of the enthalpy change divided by the range) was as high as that previously reported for the wx endosperm mutant (data not shown). The exotic lines showed a wider range of values for percentage retrogradation. Hybrid values were not consistently higher, lower, midpoint, or similar with respect to the values of their parents. This finding was true regardless of germplasm type or functional trait. Reciprocal cross values showed trends suggesting reciprocal differences, although there was no trend suggesting greater

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Table 10.3 Means, maximum and minimum values and standard errors for starch thermal traits of corn belt and exotic inbred lines and their crosses (Pollak and White, 1997) Peak onset temperature (ëC)

Gelatinization range (ëC)

Change in enthalpy (J/g)

Retrogradation (%)

66.4 71.2 61.2 1.16

12.0 17.6 8.2 1.13

12.13 13.39 10.88 0.25

50.7 53.0 46.0 0.76

Corn belt crosses Mean 68.9 Maximum 70.1 Minimum 66.4 Standard error 0.26

8.0 9.3 6.0 0.19

12.60 13.80 10.88 0.21

48.1 53.0 42.0 0.62

Exotic lines Mean Maximum Minimum Standard error

65.7 68.1 62.9 0.65

10.3 13.5 7.6 0.73

11.18 12.55 8.37 0.42

52.9 63.0 42.0 2.35

Exotic crosses Mean Maximum Minimum Standard error

68.2 70.6 65.9 0.35

8.5 10.9 6.9 0.28

12.55 14.64 11.30 0.21

51.3 57.0 45.0 0.70

Corn belt lines Mean Maximum Minimum Standard error

effect of the female parent. These traits seemed to be controlled by many modifying effects in addition to major effects. Results indicated that sufficient variability exists even within the Corn Belt germplasm to conduct breeding and inheritance studies effectively and that there should be potential for breeding for functional traits. Campbell et al. (1995) examined the genetic modifiers of corn starch thermal properties by using DSC. The su2 kernels from segregating ears were identified based on textural appearance of starches following crosses between an exotic corn accession with the inbred OH43, which was homozygous for the su2 allele (OH43 su2). The two exotic corn accessions used were the PI213768 and PI451692. Germs were retained from su2 kernels and used to produce an F2 population of su2 plants containing 50% exotic germplasm. With few exceptions, F2 ears from the populations were homozygous for the su2 allele. Significant (P less than or equal to 0.05) differences were observed between the exotic populations and OH43 su2 for gelatinization onset temperature, range, enthalpy, and retrogradation. The number of DSC values with significant withinpopulation variations was greater among F2 ears within the exotic populations

New corn starches 313 Table 10.4 Differential scanning calorimetry values for starches from exotic and inbred corn lines crossed with the corn mutant, sugary-2 (Campbell et al., 1995) Su2 F2 population

MeanÔSD

(Exotic X OH42su2)

Peak onset temperature (ëC)

Gelatinization range (ëC)

Change in enthalpy of gelatinization (J/g)

Retrogradation (%)

P1213768su2 P145169su2 OH43su2

52.8Ô1.6 53.5Ô1.3 54.6Ô0.8

13.4Ô1.7 12.2Ô1.3 10.6Ô0.7

6.3Ô0.67 5.4 0.88 5.4Ô0.21

34.5Ô4.8 32.9Ô5.1 29.0Ô4.1

than among ears within the inbred line OH43 su2. Standard deviations for DSC values were consistently greater for exotic su2 populations than for those of OH43 su2 (Table 10.4). Also, starch from the population PI213768 su2 had mean values by DSC that were significantly different from those of from starch of OH43 su. It was concluded that examining the texture of starches from single kernels can be useful in identifying and developing populations homozygous for the su2 allele. In addition, the increased variability for DSC values within populations containing 50% exotic germplasm suggests that genetic modifiers might be used to alter thermal properties and, possibly, functional properties of su2 starch. Other variations in corn starch structure and function may arise from the introduction of ancient relatives of corn into modern corn lines. Teosinte, a variable wild grass, has some resemblance to maize in that it has tassels (male flowers) at the end of stalks, and broad, flat, pointed leaves. Reeves (1950) determined that crossing teosinte with corn resulted in greater resistance of the corn to heat and drought damage. In a recent study, the structural and chemical properties were compared of starches isolated from modern corn (maize), Chalco teosinte, and a Chalco teonsinte-maize cross (Keppel, 2001). The Chalco teosinte was developed for growth in the US Corn Belt, with adaptation for photo sensitivity of the region. Teosinte contained only 25% starch compared with 70% starch in the corn. The starch content of the cross was similar to that of the corn. Teosinte starch had a gelatinization onset temperature of 61 ëC, a gelatinization enthalpy change of 11.4 J/g, and a retrogradation of 56.2%, compared with values for the corn of 66.1 ëC, 15.2 J/g and 41%, respectively. The cross had intermediate values of 64.0 ëC, 13.9 J/g and 49.7%, respectively, suggesting some modification of corn starch with the introduction of the teosinte genes. Starch granules from the teosinte were similar in size and shape to corn starch granules, but had the most irregularly shaped, broken or hollow granules, as noted by scanning electron microscopy. All granule types were similar in x-ray diffraction patterns (A-type) and amylose contents, but teosinte starch had the greatest proportion of short chains (18.5%) and shortest average chain-lengths

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(23.5 DP), with corn starch having values of 16.6% and 25.1 DP, respectively. Once again, the cross had values that were intermediate between those of corn and teosinte starches. Pasting properties by Rapid Visco Analysis showed that teosinte starch had the lowest peak viscosity, final viscosity, and setback values, but the highest pasting temperature among the starch types, again suggesting promise for introducing variability into Corn Belt lines. Trypsacum, another ancient maize-like plant, may provide variance in corn starch structure and function when crossed with modern corn lines to create unusual corn properties. Duvick et al. (2003) have a patent pending on the use of such crosses to create corn with altered starch, oil, and protein quality and quantity.

10.5

Future trends

As noted in this chapter, there are many available choices for selecting a specific corn starch with properties desirable for the food industry, including normal corn from different genetic backgrounds, naturally occurring endosperm mutants, as well as chemically modified corn starches. Another potential source of corn starches with altered properties is genetically modified (GM) crops, but the future of GM materials is uncertain, pending government and consumer acceptance in the field and market. GM corn can enhance the successful growing of corn in the field with the absence of pesticides and other chemicals, thus leading to a more profitable product for the growers. But the benefits and risks of these crops has not yet been fully evaluated to the satisfaction of some countries.

10.6

Sources of further information and advice

Valuable information regarding the latest issues, as accessed June 2003, can be found on the web at: American Association of Cereal Chemists http://www.aaccnet.org Iowa Corn Promotion Board: http://www.iowacorn.org National Corn Growers Association: http://www.ncga.com

10.7

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