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Preparation and food applications of physically modified starches
Review
Preparation and food Cold-water-soluble starches are of commercial interest for use in instant foods such as puddings and microwave-cooked foods, and small-crystallite starches have applications as fat substitutes. Various methods have been developed to produce a range of modified starch preparations with a variety of physical characteristics and applications. Study of such modi-
applications of physically modified starches*
fied starches may also aid understanding of the structure of starch granules.
Native starch has a complex granular structure l . Within the granule, amylopectin molecules are arranged radially, and the branch chains form double helical crystalline clusters 2.1 ; amylose molecules are interspersed among the amylopectin molecules'!. Because of the complex, semi-crystalline structure of the starch granules, energy is required to melt (gelatinize) the starch crystallites. Physical modifications of the starch can be applied alone or with chemical reactions to change the granular structure and convert native starch into cold-watersoluble starch or into small-crystallite starch. Cold-watersoluble starch has recently attracted significant interest, particularly for applications in microwave-cooked and instant foods, and starches with submicron (less than I /lm in diameter) crystallites have been applied for use as fat substitutes, to provide a fat-like texture and mouth-feel. This review focuses on the physical modification of starches and the use of a physical treatment to study the internal structure of starch granules.
Cold-water-soluble starch Traditionally, cold-water-soluble starch is prepared by the pre-gelatinization of native starch slurry, followed by drum drying. These technologies have been reviewed by Powell'. One drawback of such processing techniques is a loss of starch granule integrity and, conse~uently, reduced paste viscosity. In the past decade, sev~ral new technologies have been developed to produce ~ranular cold-water-soluble starches, and some have )een commercialized. Examples of the techniques are liscussed below.
njection and nozzle-spray drying process A method of treating starch by using Injection and lozzle-spray drying was developed by Pitchon et al. 6 :tarch slurries (15-50%, w/v) are atomized through an tomization aperture within a nozzle chamber. Steam is ljected into the atomized starch through a second open19 in the chamber to maintain the temperature of the ozzle chamber at -150°e. The time the starch slurry lkes to travel from the atomization opening and ournal Paper No. ).14880 oj the Iowa Agriculture and Home Economics periment Station. Ames. Iowa Project No. 2863.
Jay-lin Jane through the chamber to exit from the nozzle chamber defines the cooking time of the starch. The process produces a uniformly cooked or gelatinized starch with a minimum of shear and heat damage. The gelatinized starch is then finely atomized and dried in a spraydrying tower. The process results in 100% granular precooked starch. The granular precooked starch can be used in instant pudding mixes. Puddings made from the starch are smooth, continuous, homogeneous and non-grainy, and have a high sheen and desirable mouth-feel and viscosity characteristics; previously, the preparation of puddings with such characteristics required the use of heavily modified (crosslinked and substituted) starches.
Thermal treatment of starch in aqueous alcohol solution An alternative method of producing cold-water-soluble starches is to suspend granular normal starches 7.X or mixtures of normal and waxy starches9 in aqueous alcohol solutions (75% alcohol, w/w) and then apply high temperatures (J 50-170°C) under pressure for a short time. Simple alcohols, such as methanol, ethanol and propanol, are used for the reaction; of these, ethanol is probably the best choice. Starch thus treated is more than 90% soluble in cold water at room temperature. Characterization of the cold-water-soluble granular starch thus formed 1o showed that the starch displayed a strong V-type X-ray diffraction pattern I I (see Glossary). However, the starch showed a strong birefringence under polarized light microscopy, without exhibiting the
Glossary V-type X-ray diffraction pattern: X-ray diffraction ana lysis of native starch granules reveals one of three characteristic patterns each representing a different conformation of the amylose chains: the A pattern, which is usually found in cereal starches; the B pattern, which is found in potato starch and retrograded starch; and the C pattern, which denotes a mixture of A- and B-type forms. A V-type pattern is characteristic of starch with a single-helix conformation ; the V-conformation is stabilized by inclusion complexes. l
Amylograph: An instrument that measures the viscosity of an aqueous starch suspension as a function of temperature. The resultant plot of viscosity versus temperature is an amylogram. As the sample is heated the amylogram curve rises sharply to a maximum viscosity value after he starch gelatinizes. I
1·lin Jane is at the Department oj Food Science and Human ,Nutrition. Iowa te University, Ames, IA 50011, USA.
'nds in Food Science & Technology June 1992 IVol. 31
©1992. Elsevier Sciencc
Pllblish('r~
Ltd, I LXI
145
'Maltese cross' pattern that is characteristic of native starches (see Fig. 1), or any other discernible pattern. These characteristics demonstrate that the starch conformation and internal structure of the granule have changed. When the starch is suspended in water, the granules swell and gelatinize. Thus, dispersion of the starch in water at room temperature instantly increases the viscosity (as measured by an amylograph). After shearing in an amylograph at 25°C for about 60 minutes, the paste develops greater viscosity than the paste of a native starch prepared by the normal cooking process. The molecular size of the treated starch, as determined by a high-performance liquid chromatography (HPLC) size-exclusion column, is reduced by the treatment, suggesting that it is degraded slightly. After 13 days of treatment at 100% RH (relative humidity) at 25°C, the starch loses the V-pattern and regains the A-type pattern characteristic of native starch.
Alcoholic alkali treatments Granular cold-water-soluble starch can also be prepared by treatment with alcoholic alkali solutions IZ. Aqueous simple alcohol solutions, such as methanol, ethanol and propanol, and strong bases, such as NaOH and KOH, can be used to convert native starches into cold-water-soluble starches that retain a granular structure. Starches prepared by this method are more than 90% water soluble at room temperature. The treated starches show no molecular degradation as assessed by gel-permeation column chromatography, and produce -85% of the viscosity achieved by starches prepared by a normal cooking process, as assessed by amylography. Advantages of the alcoholic alkali treatment over other methods include its applicability to a broad variety of starches (e.g. normal, waxy and high-amylose starches). Starches prepared by this method are easily dispersed in water and develop smooth pastes without lumpslz. Treatment of starch with polyhydric alcohols at atmospheric pressure Normal starches of wheat, corn, tapioca, potato and mung bean, as well as chemically modified normal and waxy starches, can be converted to granular cold-watersoluble starches by treating them with mixtures of water and ethylene glycol, glycerol, propan-l,2-diol, butan1,3-diol or butan-l,4-dioJl3,14. The mixtures are heated to 145-155°C for -15 minutes, except those containing ethylene glycol, which are heated to 11O-120°C for 15 minutes. Absolute (95%, v/v) ethanol is then added with stirring. After multiple washes and exchanges with ethanol, the starch is dried for 24 hours in a vacuum desiccator over CaCl z, and then in an oven at 40-45°C, Yields range from 90% to 97% on the basis of starch solids content. Propan-l,2-diol has been found to be the most effective solvent among those tested. Mixtures of water and propan-l ,2-diol are particularly effective for the treatment of chemically modified starches l5 such as hydroxypropylated starches, octenylsuccinylated starches and erosslinked starches. The cold-water solubilities of the treated, chemically modified 146
starches are in the range 86-94%. The efficacy of the method varies with the variety of the native starch. The cold-water solubilities of native starches range from 2% to 92%. The final paste viscosity of the treated starch also varies with the starch variety, Treated tapioca starch has a viscosity consistency similar to that of a freshly cooked tapioca-starch paste. In contrast, treated wheat starch has only -50% of the viscosity of freshly cooked wheat-starch paste l5 ,
Small-granule starches Small-granule starch, which has a granule diameter similar to the diameter of lipid micelles (-2/lm), has been proposed as a good fat substitute (Daniel, J.R. and Whistler, R.L., unpublished). Small-granule starch is also used as filler in starch-filled biodegradable plastic films I6 ,17, as face (dusting) powder, as a stabilizer in baking powder, and as a laundry-stiffening agent. Native starches from certain grass and cereal seeds and from some roots have been reported to have small granule diameters. For example, starch from Saponaria vaccaria (known as cow cockle, cow soapwort or cow fat) has extremely small granules with diameters in the range 0.5-1.6/lm (Ref. 18). Starch from Amaranthus retroflexus has a granule diameter of 0.75-1.25/lm (Refs 19 and 20), and starch from Chenopodium quinoa has a granule diameter of 1-2.5/lm (Ref. 21). Taro starch (from Colocasia esculenta Schott) is also known to have small granules; the starch of the 'Bun-long' variety has an average diameter of 2.6/lm (Ref. 22). Small-granule starch from grass or cereal seeds is more difficult to isolate and purify than larger-granule starches. Thus, the production costs involved in the preparation of such small-granule starches make them economically unfavorable in competition with common starches such as corn or wheat starch.
Preparation of small-particle starch from native cornstarch Small-particle starch has been made from normal cornstarch by a combination of acid hydrolysis and mechanical attrition of the native starch z3 . Using this method, the particle size of the small-granule starch correlates with the degree of acid hydrolysis. Native starch is hydrolysed by strong acids (e.g. hydrochloric and sulfuric acids) in aqueous, alcoholic or aqueous alcoholic media. When the starch is hydrolysed in an aqueous solution, the temperature must be maintained below the gelatinization temperature of the starch. The partially hydrolysed starch becomes very fragile, and can be broken into small pieces by attrition with a pestle and mortar or in a ball mill. Average diameters of the small-particle starches prepared by this method, as determined by an image analyser, vary between 1.2 /lm and 1.8 /lm, depending on the acid hydrolysis conditions, Yields vary from 66% to 80%. Scanning electron micrographs of the smallparticle starches display irregularly shaped granules. Under a polarized light microscope, the small-particle starch displays a strong birefringence, without the Trends in Food Science &Technology June 1992 [Vol. 31
'Maltese cross' pattern (Fig. 1). X-ray diffraction patterns of the small-particle starches display a strong A-type pattern. A proposed mechanism"' for the conversion of native starch into small-particle starch involves acid hydrolysis of the starch in the amorphous regions of the granule. The remaining starch granule consists of small crystallites, which are no longer connected after hydrolysis'. These crystallites can then be easily separated by mild attrition. The small-particle starch thus formed is highly crystalline compared with the native starch.
Small-crystallite starch as a fat substitute Fat substitutes have been developed through combined physical and chemical modifications of native starches. Battista24 patented a method to prepare stable gels or dispersions of amylose or of starch materials consisting of at least 85% amylose. The material is first hydrolysed by acid under reflux, and is then broken into small particles using a mill, a high-speed cutting action or high pressure. The mechanical disintegration is carried out in the presence of water; the finely divided product is characterized by its ability to form a stable suspension in the liquid medium. The suspension contains sufficient submicron particles to prevent settling, resulting in a suspension that is stable for weeks or even months. The product can be dried by freeze drying, spray drying, drum drying, liquid displacement or oven drying. The dried material is readily redispersed in water using a blender. Normal or, preferably, waxy starch can also be subjected to controlled acid hydrolysis to produce carbohydrate crystallites for use as a fat substitute 25 • The acid hydrolysis is conducted at a temperature between 55.5°C and the gelatinization temperature of the starch (or the boiling temperature of the slurry, for starches with a gelatinization temperature above the boiling temperature). The product is reported to provide a stable, cream-like gel in a 25% (w/v) suspension following a high-speed shearing process (e.g. homogenization). The cream-like gel consists of aggregated starch crystallites in a continuous water phase, resembling the fat crystals of shortening in a continuous oil phase. The submicron crystallites impart a smooth, creamy, fat-like mouth-feel and texture. Microcrystalline cellulose can be mixed with a galactomannan gum and used as a fat-like bulking agent"6. The microcrystalline cellulose is prepared from hydrolysed wood pulp and is ground to a particle size of 5-70 /lm.
Internal structure of starch granules, as revealed by physical treatments Physical treatments of starch can also be used to reveal information about the internal structure of the granules (Jane, 1. and Shen, 1., unpublished). Fractionated potato starch with a uniform granule size has been used for such studies. The starch is subjected to 'chemical erosion' in a 4 M CaCI" solution. Upon contact, the solution gelatinizes starch, starting at the
Trends in Food Science & Technology June 1992 [Vol. 31
Fig. 1 Polarized light micrographs. Above, native cornstarch; below, small-particle starch, prepared by acid hydrolysis and mechanical attrition oi native cornstarch at the same magniiication. Note the 'Maltese cross' pattern in the upper micrograph but absent in the lower one. Field oi view: 170 11m. Taken with permission irom Rei. 23.
periphery of the granules. The extent of the surface gelatinization is proportional to the period of exposure. The central portion of the granule remains intact and retains the 'Maltese cross' pattern under polarized light microscopy. The treated starch is then removed from solution, and the outer layers of gelatinized starch are mechanically sheared from the intact starch granules using a blender. The amylose contents of the outer layer of starch (the first fraction that is sheared off, representing 20% of the total starch) and the starch that remains after 80% of the material has been sheared off are 21.98 ± 0.10% and 18.78 ± 0.06% (w/w), respectively. Amylose from the outer starch layer is (as revealed by gel-permeation column chromatography) of uniform, intermediate molecular size, whereas amylose in the remaining starch is of intermediate to large molecular size. The phosphorus content of the remaining starch is greater (0.077 ± 0.001 %) than that of the gelatinized starch (0.065 ± 0.001 %). Amylopectin was purified from the starch fractions by gel permeation chromatography, treated 147
with isoamylase (EC 3.2.1.9), a debranching enzymc that hydrolyses 1,6-cx-D-glucosidic branch linkages, then analysed by 'Bio-gel P-6' gel-permeation chromatography. Amylopectin from the outer starch layer had peak chain lengths of degree of polymerization (DP) 32.0 ± 0.8 and 13.1 ± 0.6 for long- and shortbranch chains, respectively. Amylopectin from the remaining starch (after 800/c erosion) had DP values of 42.5 ± 1.8 and 13.1 ± 0.1 for long and short branches, respectively. The results indicate that amylopectin at the core (hilum) of the granule, which develops earlier, tends to have longer long-branch side chains than amylopectin that develops later, at the periphery. The amylopectin fraction of intermediate molecular size from the outer starch layer appears to have the same type of branch structure as the core amylopectin, but its smaller molecular size suggests that its biosynthesis is prematurely terminated.
7
Eastman, IE. and Moore, e.0. (1984) EP Patent 0110549 A2
8
Eastman, I.E. and Moore, e.0. (1984) US Patent .:I 465 702 Eastman, I.E 11987) US Patent -l 634 596
9 10
lane, I., Craig, S.A.S., Seib, PA and Hoseney, R.e. (1986) Starch/Stjrke
11
lane, I., Craig, S.A.S., Seib, P.A. and Hoseney, R.e. (1986) Carbohydr.
12
lane, I and Seib, P.A. (1991) US Patent 5 057 1)7
13
Rajagopalan, S. and Seib, P.A. (1991) US Patent 5 037 929
38, 258-263
Res. 150, C5-CCO
14
Rajagopalan, S. and Seib, P.A.]. Cereal Sci. (in press)
15
Rajagopalan, S. and Seib, P.A. J. Cereal Sci. (in press)
16
Griffin, G.I.L.11990) in Wheat is Unique (Pomeranz, Y., ed.!.
17
Lim,S., lane,
pp. 695-706, Association of American Cereal Chemists
18
Rajagopalan, S. and Scib, P.A. (1992) Biotechnol. Prog.
Goering, KJ and Brelsford, D.L. (1966) Cereal Chem. 43, 127-136
19
Goering, K.I (19671 Ceredl Chem. 44, 245-252
20
Subba Rao, P.V. and Goering, K.I. (1970) Cereal Chem. 47,
21
Atwell, W.A., Patrick, B.M., lohnson, L.A. and Glass, RW. 11982)
22
lane, I., Shen, L., Chen, I., Lim,S., Kasemsuwan, T. and Nip, W.K. Cereal Chem. (in press)
23
lane, J.. Shen, L., Wang, L. and Maningat. e.c:. (1992) Cereal Chem. 69, 280-283
655-661
Cereal Chem. 60, 9-11
References French, D. (19841 in Starch: Chemistry and Technology (2nd edn)
J..
8,51-57
(Whistler, R.L., ed.l, pp. 184-247, Academic Press 2
Kainuma, K. and French, D. (1971) Biopolymersl 0, 1673-1680
24
Battista, O.A. (1967) US Patent 3 351 489
3
Kainuma, K. and French, D. (1972) Biopolymers 11, 2241-2250
25
Chiou, R.G., Brown, e.c., Little, lA, Young, A.H., Schanefilt. RV.,
4
lane, I., Xu, A., Radosavljevic, M. and Seib, P.A. Cereal Chem. (in press)
Harris, D.W., Stanley, K.D., Coontz, H.D., Hamdan, e.1.,
5
Powell, E.L. (1967) in Starch: Chemistry and Technology. Vol. 1/
Wolf-Rueff, I.A., Slowinski, L.A., Anderson, K.R., Lehnhardt. WJ. and
(Whistler, R.L. and Paschall, EJ., eds!. pp. 523-536, Academic Press 6
Pitchon, E., O'Rourke, I.D. and loseph, T.H. (1981) US Patent 4280851
Witczak, Z.I. (19911 EP Patent 0 443 844 A1 26
McGinley, E.I. and Tuason, D.e., Ir (19901 WO Patent 9 014 017 A1
Software Review
Easy Plot Spiral Software, MA, USA/Cherwell Scientific, UK, 1991. $349.00/£249.00 (educational and multi-user discounts available) Easy Plot is a simple, easy-to-use graphics package. The pull-down menus and optional mouse support make for quick and easy graph handling. It can plot in a variety of different styles (lines, curves, scatter plots, bar graphs and histograms)' though it cannot handle staircase plots. Three-dimensional plots can also be generated. We found that the zoom and scroll features allowed for good visual ization of calorimetry data. Another welcome feature is the plotting of error bars for both axes. Dual y-axes can be plotted, in linear or logarithmic format. The graphs can be customized to different styles for data marks or correcting lines, and annotations can be positioned within or outside the graph. Lines or arrows from annotations can also be drawn anywhere on the graph.
148
The statistics options offered are adequate, allowing data transformation as a function of x and y, the calculation of standard deviations, means and variance of the data set, curve-fitting of polynomials, as well as iterative curvefitting of nonlinear equations. Visual removal of outlying data points allows for immediate remodelling of curves. However, it is not possible to obtain the correlation coefficient directly, and residuals are not automatically generated. This makes it difficult to assess best-fit curves quickly. Other features include the integration of areas under curves, differentiation of curve derivatives and data smoothing. A useful feature is the facility to run the program in the batch mode. This means creating a batch file, which is referred to when starting the program,
such that global commands can be used. It is therefore possible to customize the program to specific requirements (e.g. specific data styles and data inputs). This removes the need to use the pull-down menus, but it takes some time to learn the commands; for a single graph, it is easier to use the pulldown menus. The package can support expanded memory, needs 400K of memory and can be used within a network, though this leaves less memory free for current use. Easy Plot drives most common printers and plotters, and plots can be imported into other applications. ASCII and Lotus-compatible data files can be imported, and it is possible to split input files - a useful feature. Overall, we found Easy Plot easy to use, and it compares well (in both performance and price) with other packages for simple needs.
A.V. Kurpad and M.L. Sheela Nutrition Research Centre, SI John's Medical College, Bangalore 560034, India.
Trends in Food Science & Technology June 1992 IVol. 3J
Preparation and food Cold-water-soluble starches are of commercial interest for use in instant foods such as puddings and microwave-cooked foods, and small-crystallite starches have applications as fat substitutes. Various methods have been developed to produce a range of modified starch preparations with a variety of physical characteristics and applications. Study of such modi-
applications of physically modified starches*
fied starches may also aid understanding of the structure of starch granules.
Native starch has a complex granular structure l . Within the granule, amylopectin molecules are arranged radially, and the branch chains form double helical crystalline clusters 2.1 ; amylose molecules are interspersed among the amylopectin molecules'!. Because of the complex, semi-crystalline structure of the starch granules, energy is required to melt (gelatinize) the starch crystallites. Physical modifications of the starch can be applied alone or with chemical reactions to change the granular structure and convert native starch into cold-watersoluble starch or into small-crystallite starch. Cold-watersoluble starch has recently attracted significant interest, particularly for applications in microwave-cooked and instant foods, and starches with submicron (less than I /lm in diameter) crystallites have been applied for use as fat substitutes, to provide a fat-like texture and mouth-feel. This review focuses on the physical modification of starches and the use of a physical treatment to study the internal structure of starch granules.
Cold-water-soluble starch Traditionally, cold-water-soluble starch is prepared by the pre-gelatinization of native starch slurry, followed by drum drying. These technologies have been reviewed by Powell'. One drawback of such processing techniques is a loss of starch granule integrity and, conse~uently, reduced paste viscosity. In the past decade, sev~ral new technologies have been developed to produce ~ranular cold-water-soluble starches, and some have )een commercialized. Examples of the techniques are liscussed below.
njection and nozzle-spray drying process A method of treating starch by using Injection and lozzle-spray drying was developed by Pitchon et al. 6 :tarch slurries (15-50%, w/v) are atomized through an tomization aperture within a nozzle chamber. Steam is ljected into the atomized starch through a second open19 in the chamber to maintain the temperature of the ozzle chamber at -150°e. The time the starch slurry lkes to travel from the atomization opening and ournal Paper No. ).14880 oj the Iowa Agriculture and Home Economics periment Station. Ames. Iowa Project No. 2863.
Jay-lin Jane through the chamber to exit from the nozzle chamber defines the cooking time of the starch. The process produces a uniformly cooked or gelatinized starch with a minimum of shear and heat damage. The gelatinized starch is then finely atomized and dried in a spraydrying tower. The process results in 100% granular precooked starch. The granular precooked starch can be used in instant pudding mixes. Puddings made from the starch are smooth, continuous, homogeneous and non-grainy, and have a high sheen and desirable mouth-feel and viscosity characteristics; previously, the preparation of puddings with such characteristics required the use of heavily modified (crosslinked and substituted) starches.
Thermal treatment of starch in aqueous alcohol solution An alternative method of producing cold-water-soluble starches is to suspend granular normal starches 7.X or mixtures of normal and waxy starches9 in aqueous alcohol solutions (75% alcohol, w/w) and then apply high temperatures (J 50-170°C) under pressure for a short time. Simple alcohols, such as methanol, ethanol and propanol, are used for the reaction; of these, ethanol is probably the best choice. Starch thus treated is more than 90% soluble in cold water at room temperature. Characterization of the cold-water-soluble granular starch thus formed 1o showed that the starch displayed a strong V-type X-ray diffraction pattern I I (see Glossary). However, the starch showed a strong birefringence under polarized light microscopy, without exhibiting the
Glossary V-type X-ray diffraction pattern: X-ray diffraction ana lysis of native starch granules reveals one of three characteristic patterns each representing a different conformation of the amylose chains: the A pattern, which is usually found in cereal starches; the B pattern, which is found in potato starch and retrograded starch; and the C pattern, which denotes a mixture of A- and B-type forms. A V-type pattern is characteristic of starch with a single-helix conformation ; the V-conformation is stabilized by inclusion complexes. l
Amylograph: An instrument that measures the viscosity of an aqueous starch suspension as a function of temperature. The resultant plot of viscosity versus temperature is an amylogram. As the sample is heated the amylogram curve rises sharply to a maximum viscosity value after he starch gelatinizes. I
1·lin Jane is at the Department oj Food Science and Human ,Nutrition. Iowa te University, Ames, IA 50011, USA.
'nds in Food Science & Technology June 1992 IVol. 31
©1992. Elsevier Sciencc
Pllblish('r~
Ltd, I LXI
145
'Maltese cross' pattern that is characteristic of native starches (see Fig. 1), or any other discernible pattern. These characteristics demonstrate that the starch conformation and internal structure of the granule have changed. When the starch is suspended in water, the granules swell and gelatinize. Thus, dispersion of the starch in water at room temperature instantly increases the viscosity (as measured by an amylograph). After shearing in an amylograph at 25°C for about 60 minutes, the paste develops greater viscosity than the paste of a native starch prepared by the normal cooking process. The molecular size of the treated starch, as determined by a high-performance liquid chromatography (HPLC) size-exclusion column, is reduced by the treatment, suggesting that it is degraded slightly. After 13 days of treatment at 100% RH (relative humidity) at 25°C, the starch loses the V-pattern and regains the A-type pattern characteristic of native starch.
Alcoholic alkali treatments Granular cold-water-soluble starch can also be prepared by treatment with alcoholic alkali solutions IZ. Aqueous simple alcohol solutions, such as methanol, ethanol and propanol, and strong bases, such as NaOH and KOH, can be used to convert native starches into cold-water-soluble starches that retain a granular structure. Starches prepared by this method are more than 90% water soluble at room temperature. The treated starches show no molecular degradation as assessed by gel-permeation column chromatography, and produce -85% of the viscosity achieved by starches prepared by a normal cooking process, as assessed by amylography. Advantages of the alcoholic alkali treatment over other methods include its applicability to a broad variety of starches (e.g. normal, waxy and high-amylose starches). Starches prepared by this method are easily dispersed in water and develop smooth pastes without lumpslz. Treatment of starch with polyhydric alcohols at atmospheric pressure Normal starches of wheat, corn, tapioca, potato and mung bean, as well as chemically modified normal and waxy starches, can be converted to granular cold-watersoluble starches by treating them with mixtures of water and ethylene glycol, glycerol, propan-l,2-diol, butan1,3-diol or butan-l,4-dioJl3,14. The mixtures are heated to 145-155°C for -15 minutes, except those containing ethylene glycol, which are heated to 11O-120°C for 15 minutes. Absolute (95%, v/v) ethanol is then added with stirring. After multiple washes and exchanges with ethanol, the starch is dried for 24 hours in a vacuum desiccator over CaCl z, and then in an oven at 40-45°C, Yields range from 90% to 97% on the basis of starch solids content. Propan-l,2-diol has been found to be the most effective solvent among those tested. Mixtures of water and propan-l ,2-diol are particularly effective for the treatment of chemically modified starches l5 such as hydroxypropylated starches, octenylsuccinylated starches and erosslinked starches. The cold-water solubilities of the treated, chemically modified 146
starches are in the range 86-94%. The efficacy of the method varies with the variety of the native starch. The cold-water solubilities of native starches range from 2% to 92%. The final paste viscosity of the treated starch also varies with the starch variety, Treated tapioca starch has a viscosity consistency similar to that of a freshly cooked tapioca-starch paste. In contrast, treated wheat starch has only -50% of the viscosity of freshly cooked wheat-starch paste l5 ,
Small-granule starches Small-granule starch, which has a granule diameter similar to the diameter of lipid micelles (-2/lm), has been proposed as a good fat substitute (Daniel, J.R. and Whistler, R.L., unpublished). Small-granule starch is also used as filler in starch-filled biodegradable plastic films I6 ,17, as face (dusting) powder, as a stabilizer in baking powder, and as a laundry-stiffening agent. Native starches from certain grass and cereal seeds and from some roots have been reported to have small granule diameters. For example, starch from Saponaria vaccaria (known as cow cockle, cow soapwort or cow fat) has extremely small granules with diameters in the range 0.5-1.6/lm (Ref. 18). Starch from Amaranthus retroflexus has a granule diameter of 0.75-1.25/lm (Refs 19 and 20), and starch from Chenopodium quinoa has a granule diameter of 1-2.5/lm (Ref. 21). Taro starch (from Colocasia esculenta Schott) is also known to have small granules; the starch of the 'Bun-long' variety has an average diameter of 2.6/lm (Ref. 22). Small-granule starch from grass or cereal seeds is more difficult to isolate and purify than larger-granule starches. Thus, the production costs involved in the preparation of such small-granule starches make them economically unfavorable in competition with common starches such as corn or wheat starch.
Preparation of small-particle starch from native cornstarch Small-particle starch has been made from normal cornstarch by a combination of acid hydrolysis and mechanical attrition of the native starch z3 . Using this method, the particle size of the small-granule starch correlates with the degree of acid hydrolysis. Native starch is hydrolysed by strong acids (e.g. hydrochloric and sulfuric acids) in aqueous, alcoholic or aqueous alcoholic media. When the starch is hydrolysed in an aqueous solution, the temperature must be maintained below the gelatinization temperature of the starch. The partially hydrolysed starch becomes very fragile, and can be broken into small pieces by attrition with a pestle and mortar or in a ball mill. Average diameters of the small-particle starches prepared by this method, as determined by an image analyser, vary between 1.2 /lm and 1.8 /lm, depending on the acid hydrolysis conditions, Yields vary from 66% to 80%. Scanning electron micrographs of the smallparticle starches display irregularly shaped granules. Under a polarized light microscope, the small-particle starch displays a strong birefringence, without the Trends in Food Science &Technology June 1992 [Vol. 31
'Maltese cross' pattern (Fig. 1). X-ray diffraction patterns of the small-particle starches display a strong A-type pattern. A proposed mechanism"' for the conversion of native starch into small-particle starch involves acid hydrolysis of the starch in the amorphous regions of the granule. The remaining starch granule consists of small crystallites, which are no longer connected after hydrolysis'. These crystallites can then be easily separated by mild attrition. The small-particle starch thus formed is highly crystalline compared with the native starch.
Small-crystallite starch as a fat substitute Fat substitutes have been developed through combined physical and chemical modifications of native starches. Battista24 patented a method to prepare stable gels or dispersions of amylose or of starch materials consisting of at least 85% amylose. The material is first hydrolysed by acid under reflux, and is then broken into small particles using a mill, a high-speed cutting action or high pressure. The mechanical disintegration is carried out in the presence of water; the finely divided product is characterized by its ability to form a stable suspension in the liquid medium. The suspension contains sufficient submicron particles to prevent settling, resulting in a suspension that is stable for weeks or even months. The product can be dried by freeze drying, spray drying, drum drying, liquid displacement or oven drying. The dried material is readily redispersed in water using a blender. Normal or, preferably, waxy starch can also be subjected to controlled acid hydrolysis to produce carbohydrate crystallites for use as a fat substitute 25 • The acid hydrolysis is conducted at a temperature between 55.5°C and the gelatinization temperature of the starch (or the boiling temperature of the slurry, for starches with a gelatinization temperature above the boiling temperature). The product is reported to provide a stable, cream-like gel in a 25% (w/v) suspension following a high-speed shearing process (e.g. homogenization). The cream-like gel consists of aggregated starch crystallites in a continuous water phase, resembling the fat crystals of shortening in a continuous oil phase. The submicron crystallites impart a smooth, creamy, fat-like mouth-feel and texture. Microcrystalline cellulose can be mixed with a galactomannan gum and used as a fat-like bulking agent"6. The microcrystalline cellulose is prepared from hydrolysed wood pulp and is ground to a particle size of 5-70 /lm.
Internal structure of starch granules, as revealed by physical treatments Physical treatments of starch can also be used to reveal information about the internal structure of the granules (Jane, 1. and Shen, 1., unpublished). Fractionated potato starch with a uniform granule size has been used for such studies. The starch is subjected to 'chemical erosion' in a 4 M CaCI" solution. Upon contact, the solution gelatinizes starch, starting at the
Trends in Food Science & Technology June 1992 [Vol. 31
Fig. 1 Polarized light micrographs. Above, native cornstarch; below, small-particle starch, prepared by acid hydrolysis and mechanical attrition oi native cornstarch at the same magniiication. Note the 'Maltese cross' pattern in the upper micrograph but absent in the lower one. Field oi view: 170 11m. Taken with permission irom Rei. 23.
periphery of the granules. The extent of the surface gelatinization is proportional to the period of exposure. The central portion of the granule remains intact and retains the 'Maltese cross' pattern under polarized light microscopy. The treated starch is then removed from solution, and the outer layers of gelatinized starch are mechanically sheared from the intact starch granules using a blender. The amylose contents of the outer layer of starch (the first fraction that is sheared off, representing 20% of the total starch) and the starch that remains after 80% of the material has been sheared off are 21.98 ± 0.10% and 18.78 ± 0.06% (w/w), respectively. Amylose from the outer starch layer is (as revealed by gel-permeation column chromatography) of uniform, intermediate molecular size, whereas amylose in the remaining starch is of intermediate to large molecular size. The phosphorus content of the remaining starch is greater (0.077 ± 0.001 %) than that of the gelatinized starch (0.065 ± 0.001 %). Amylopectin was purified from the starch fractions by gel permeation chromatography, treated 147
with isoamylase (EC 3.2.1.9), a debranching enzymc that hydrolyses 1,6-cx-D-glucosidic branch linkages, then analysed by 'Bio-gel P-6' gel-permeation chromatography. Amylopectin from the outer starch layer had peak chain lengths of degree of polymerization (DP) 32.0 ± 0.8 and 13.1 ± 0.6 for long- and shortbranch chains, respectively. Amylopectin from the remaining starch (after 800/c erosion) had DP values of 42.5 ± 1.8 and 13.1 ± 0.1 for long and short branches, respectively. The results indicate that amylopectin at the core (hilum) of the granule, which develops earlier, tends to have longer long-branch side chains than amylopectin that develops later, at the periphery. The amylopectin fraction of intermediate molecular size from the outer starch layer appears to have the same type of branch structure as the core amylopectin, but its smaller molecular size suggests that its biosynthesis is prematurely terminated.
7
Eastman, IE. and Moore, e.0. (1984) EP Patent 0110549 A2
8
Eastman, I.E. and Moore, e.0. (1984) US Patent .:I 465 702 Eastman, I.E 11987) US Patent -l 634 596
9 10
lane, I., Craig, S.A.S., Seib, PA and Hoseney, R.e. (1986) Starch/Stjrke
11
lane, I., Craig, S.A.S., Seib, P.A. and Hoseney, R.e. (1986) Carbohydr.
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lane, I and Seib, P.A. (1991) US Patent 5 057 1)7
13
Rajagopalan, S. and Seib, P.A. (1991) US Patent 5 037 929
38, 258-263
Res. 150, C5-CCO
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Rajagopalan, S. and Seib, P.A.]. Cereal Sci. (in press)
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Rajagopalan, S. and Seib, P.A. J. Cereal Sci. (in press)
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Griffin, G.I.L.11990) in Wheat is Unique (Pomeranz, Y., ed.!.
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Lim,S., lane,
pp. 695-706, Association of American Cereal Chemists
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Rajagopalan, S. and Scib, P.A. (1992) Biotechnol. Prog.
Goering, KJ and Brelsford, D.L. (1966) Cereal Chem. 43, 127-136
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Goering, K.I (19671 Ceredl Chem. 44, 245-252
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Subba Rao, P.V. and Goering, K.I. (1970) Cereal Chem. 47,
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Atwell, W.A., Patrick, B.M., lohnson, L.A. and Glass, RW. 11982)
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lane, J.. Shen, L., Wang, L. and Maningat. e.c:. (1992) Cereal Chem. 69, 280-283
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Cereal Chem. 60, 9-11
References French, D. (19841 in Starch: Chemistry and Technology (2nd edn)
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(Whistler, R.L., ed.l, pp. 184-247, Academic Press 2
Kainuma, K. and French, D. (1971) Biopolymersl 0, 1673-1680
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Battista, O.A. (1967) US Patent 3 351 489
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Kainuma, K. and French, D. (1972) Biopolymers 11, 2241-2250
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Chiou, R.G., Brown, e.c., Little, lA, Young, A.H., Schanefilt. RV.,
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Harris, D.W., Stanley, K.D., Coontz, H.D., Hamdan, e.1.,
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Powell, E.L. (1967) in Starch: Chemistry and Technology. Vol. 1/
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(Whistler, R.L. and Paschall, EJ., eds!. pp. 523-536, Academic Press 6
Pitchon, E., O'Rourke, I.D. and loseph, T.H. (1981) US Patent 4280851
Witczak, Z.I. (19911 EP Patent 0 443 844 A1 26
McGinley, E.I. and Tuason, D.e., Ir (19901 WO Patent 9 014 017 A1
Software Review
Easy Plot Spiral Software, MA, USA/Cherwell Scientific, UK, 1991. $349.00/£249.00 (educational and multi-user discounts available) Easy Plot is a simple, easy-to-use graphics package. The pull-down menus and optional mouse support make for quick and easy graph handling. It can plot in a variety of different styles (lines, curves, scatter plots, bar graphs and histograms)' though it cannot handle staircase plots. Three-dimensional plots can also be generated. We found that the zoom and scroll features allowed for good visual ization of calorimetry data. Another welcome feature is the plotting of error bars for both axes. Dual y-axes can be plotted, in linear or logarithmic format. The graphs can be customized to different styles for data marks or correcting lines, and annotations can be positioned within or outside the graph. Lines or arrows from annotations can also be drawn anywhere on the graph.
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The statistics options offered are adequate, allowing data transformation as a function of x and y, the calculation of standard deviations, means and variance of the data set, curve-fitting of polynomials, as well as iterative curvefitting of nonlinear equations. Visual removal of outlying data points allows for immediate remodelling of curves. However, it is not possible to obtain the correlation coefficient directly, and residuals are not automatically generated. This makes it difficult to assess best-fit curves quickly. Other features include the integration of areas under curves, differentiation of curve derivatives and data smoothing. A useful feature is the facility to run the program in the batch mode. This means creating a batch file, which is referred to when starting the program,
such that global commands can be used. It is therefore possible to customize the program to specific requirements (e.g. specific data styles and data inputs). This removes the need to use the pull-down menus, but it takes some time to learn the commands; for a single graph, it is easier to use the pulldown menus. The package can support expanded memory, needs 400K of memory and can be used within a network, though this leaves less memory free for current use. Easy Plot drives most common printers and plotters, and plots can be imported into other applications. ASCII and Lotus-compatible data files can be imported, and it is possible to split input files - a useful feature. Overall, we found Easy Plot easy to use, and it compares well (in both performance and price) with other packages for simple needs.
A.V. Kurpad and M.L. Sheela Nutrition Research Centre, SI John's Medical College, Bangalore 560034, India.
Trends in Food Science & Technology June 1992 IVol. 3J