Principles of Cereal Science and Technology, Third Edition

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It is also well known that wetting and drying vitreous grain in the field or in the laboratory make it lose its vitreousness. Wheat Endosperm Texture and Visual Appearance In summary, the wheat endosperm varies both in hardness and vitreousness. In general, high-protein hard wheats tend to be vitreous, and low-protein soft wheats tend to be opaque.

However, the causes of hardness and vitreousness are different, and the two do not always go together. There is no causal relationship. Hardness is caused by the genetically controlled strength of the bond between protein and starch in the endosperm. Vitreousness, on the other hand, results from lack of air spaces in the kernel. The controlling mechanism is not clear but appears to be at least partially related to the level of protein in the sample. For example, high-protein soft wheats are more vitreous than low-protein soft wheats, and low-protein hard wheats have more yellow-berry opacity than their high-protein counterparts.

It is also clear that both genetic and environmental conditions affect the vitreous properties of wheat. Maize Many types of maize Zea mays L. This discussion is limited to the dent type. Popcorn is discussed in Chapter It is by far the largest of the common cereal seeds, weighing an average of mg.

For maize, the term hull is a misnomer. It is not synonymous with the hull of barley or oats but more akin to the bran of wheat milling terminology. However, the use of the term hull is strongly ingrained in the wet-milling industry and thus will persist. The tip cap, the attachment point of the cob, may or may not stay with the kernel during removal from the cob. The botanical parts of the maize caryopsis pericarp, endosperm, and germ are the same as those found in wheat. Longitudinal and cross sections of a maize kernel.

The color of the maize kernel can be quite variable.

Principles Of Cereal Science And Technology

It may be solid or variegated and can be white, yellow, red, blue, dark brown, or purple. Yellow is the most common color, followed by white. The remaining part of the kernel is endosperm. The cells are large with very thin cell walls. Maize differs from wheat in that both translucent and opaque areas are found in the endosperm of a single kernel. In general, the translucent part is near the aleurone, and the opaque part is near the center of the kernel.

Scanning electron micrographs of maize kernels. Reprinted from Robutti et al 1. A broken kernel, showing the cellular nature of the endosperm. Cross section of the vitreous part of a maize kernel, showing the polygonal shape of the starch granules, the indentation in the starch, and the tight compact structure. Cross section of the opaque part of a kernel, showing the spherical shape of the starch granules, the protein, and the large number of air spaces. Cross section of the hard endosperm of a kernel, showing the starch hilum the point from which the starch granule grew, arrow and broken starch BS.

The starch granules are polygonal in shape and held together by a protein matrix. Protein bodies are quite noticeable in the photomicrograph. These have been identified as bodies of zein the prolamin protein fraction of maize; see Chapter 3. Also noticeable are indentations in the starch. In the opaque endosperm Fig. The many air spaces lead to opacity.

Chemical analysis of the separated opaque and translucent parts of the endosperm has shown that the two have similar protein concentrations but that the protein types are quite different in terms of protein distribution and amino acid composition. In general, maize kernels are quite hard. The large number of broken starch granules in Figure 1. The fact that water alone will not allow a good separation of protein and starch during wet milling suggests that the bond is different in maize and wheat. The starch granules in the opaque and translucent parts of the endosperm differ in shape. One possible explanation for two starch-granule shapes in a single kernel is that, during the natural drying process, the protein loses water and shrinks.

The adhesion between protein and starch is strong enough to pull the starch granules closer and closer together. At this stage, the starch granules are pliable and, as they are tightly packed, they become polygonal in shape. Further evidence of their plasticity before maturity is the fact that the zein bodies make indentations on the starch granules in the translucent endosperm. In the opaque endosperm, protein-protein bonds rupture during drying, giving intergranular air spaces and maintaining spherical starch granules. If maize is harvested before it dries, essentially all of its starch granules are spherical, showing that the differentiation of granule shape occurs during grain drying.

The high levels of lignin and silica make the rice hull of rather low value both nutritionally and commercially. Longitudinal section of a rice kernel. Scanning electron micrographs of cross sections of a rice kernel. The outer surface of the rice hull. Compound starch granules and protein bodies arrows near the aleurone layer.

Compound starch granules near the center of the kernel, with certain granules broken, showing the individual granules arrows. However, the caryopsis does not have a crease. It varies from 5 to 8 mm in length and weighs about 25 mg. As with the other cereals, the aleurone is the outermost layer of the endosperm, but it is removed with the pericarp and seed coat during abrasive milling to produce white rice.

The thin-walled endosperm cells are tightly packed, with polygonal starch granules, protein matrix, and protein bodies. Comparison of Figures 1. The polygonal starch granules may be formed by compression of the starch granules during grain development. Rice and oats are the only two cereals with compound starch granules i. The compound granules appear to result from many small individual granules being synthesized in a single amyloplast, i.

Barley Barley Hordeum vulgare L. The tightly adhering hull consists of the lemma and palea. Unlike rice and oats, in which the hull is relative loose and can be separated, the hull of barley is cemented to the pericarp and difficult to separate. However, hull-less barleys, which lose the hull during threshing, have been developed. The caryopsis is composed of pericarp, seed coat, nucellar epidermis, germ, and endosperm Fig. The aleurone cells in barley are composed of two to three layers of cells Fig.

Medium-sized kernels weigh about 35 mg. The aleurone of some cultivars is blue, whereas, in others, it is colorless. The endosperm cells are packed with starch embedded in a protein matrix Fig. Like wheat and rye starch, barley starch has both large lenticular granules and small spherical granules. Longitudinal section of a barley kernel top and outer layers bottom. Van den Ende Bottom: Adapted from Palmer and Bathgate Figs. Scanning electron micrographs of cross sections of a barley kernel.

Hull H , pericarp P , and multilayered aleurone cells A. Contents of an endosperm cell. Rye The rye Secale cereale L. The kernel threshes free of glumes, has no hull, and, like wheat, possesses a ventral crease. Its color is grayish yellow.

Principles of Cereal Science and Technology, Third Edition

Like the other cereals, rye has a caryopsis consisting of pericarp, seed coat, nucellar epidermis, germ, and endosperm. The endosperm is surrounded by a single layer of aleurone cells. Scanning electron micrographs of the outer areas of the grain Fig. The starch in the endosperm cells is embedded in a protein matrix. Like wheat and barley starches, rye starch has large lenticular and small spherical granules.

Rye flour generally has much higher contents of arabinoxylan cell wall constituents than wheat flour. Scanning electron micrographs of cross sections of a rye kernel. Outer part of the kernel. Triticale Triticale Triticale hexaploide Lart. In general morphology, the grain closely resembles its parent species. The caryopsis threshes free of glumes, is generally larger than the wheat caryopsis 10—12 mm in length and 3 mm in width , and weighs about 40 mg. It consists of a germ attached to an endosperm, which has aleurone as the outer layer.

Outside the aleurone are the seed coat, a pericarp, and the remains of the nucellar epidermis. Thus, triticale closely resembles the other cereal grains in structure. The kernel has a crease that extends its full length. The yellowish brown grain is characterized by folds or ripples on the outer pericarp, apparently caused by shriveling of the grain. Grain shriveling is a major problem with triticale.

It leads to low test weight, poor appearance, and unsatisfactory milling performance. The aleurone layer in triticale is more irregular in shape than is that in wheat. The cells vary in size, and the cell walls tend to vary in thickness. In shriveled grain, the aleurone cells are badly distorted, and lesions have been noted in which complete sections of aleurone and associated endosperm cells are missing.

Oats Oats Avena sativa L. The germ extends about one-third the length of the groat, being larger and narrower than the germ of wheat. The oat groat consists of pericarp, seed coat, nucellar epidermis, germ, and endosperm. As with all cereals, the aleurone makes up the outer layer of the endosperm.

Oat groats have higher fat and protein contents than do other cereals. They are also a good source of several enzymes. The most troublesome of these is lipase, which is very active. Unless the lipase is denatured, milled products have a very short shelf life because of the production and subsequent oxidation of fatty acids. Scanning electron micrograph of the outer surface of an oat groat, showing the hairlike protuberances, or trichomes. The starch is present as large compound granules that are smooth and irregular in shape Fig.

Each compound granule is made up of many small individual granules. Scanning electron micrographs of oat starch. A partially intact compound granule. Isolated starch granules resulting from the disintegration of compound granules during oat starch isolation. Moench thresh free of hulls or glumes.

They are generally spherical, range in weight from 20 to 30 mg, and may be bronze, white, red, yellow, or brown. A hand-dissected kernel was found to consist of 7. However, other samples would be expected to vary in composition. Scanning electron micrographs of the outer layers of sorghum kernels reveal a thick pericarp, in most varieties, consisting of three layers: Unlike other cereals, some sorghum varieties contain starch granules in the pericarp.

Longitudinal section of a sorghum kernel top and outer layers bottom. Adapted from Earp et al Figs. Scanning electron micrographs of cross sections of a sorghum kernel. Note the small starch granules in the mesocarp. The outer edge, showing the presence of a thick, pigmented inner integument I. A sorghum kernel containing no inner integument. The seed coat SC , or testa, is shown. The vitreous part of the kernel, showing the content of an endosperm cell. Note the lack of air spaces, the polygonal starch granules, and protein bodies P.

While all mature sorghum seeds have a testa seed coat , certain cultivars lack a pigmented inner integument. However, if no other feed is available, they will eat the bird-resistant types. As in other cereals, the aleurone cells are the outer layer of the endosperm. In the starchy endosperm, cells containing high concentrations of protein and few starch granules are found just beneath the aleurone layer. The opaque endosperm has large intergranular air spaces Fig. In general, grain sorghum has not been selected or bred as extensively as other cereals. Thus, it is not surprising that there is much diversity in the size, texture, and shape of sorghum kernels.

Scanning electron micrograph of a cross section of the opaque part of a sorghum kernel. Note the air spaces and more-or-less spherical starch granules. Reprinted from Hoseney et al The terms hard and soft have been used to designate the vitreous and opaque areas of sorghum endosperm as well as the general appearance of kernels. However, as discussed previously for wheat, the factors determining vitreousness and physical hardness are different.

Therefore, some kernels may appear vitreous but be classified soft by objective measurements. Visual determination of hardness or softness in sorghum kernels is based on the assumption that hardness and vitreousness are the same. This appears to be an unwarranted assumption. They vary in color, with slate gray being most common, although yellow, white, and brown varieties are also known.

The caryopsis is similar to those of the other cereals. Millet pericarp does not contain starch, as the pericarp of sorghum does, nor does pearl millet contain a pigmented inner integument. Its endosperm has both translucent and opaque endosperm, as do those of sorghum and maize. The opaque endosperm contains many air spaces and spherical starch granules Fig. The translucent vitreous endosperm Fig. The matrix also contains protein bodies ranging in size from 0. The protein bodies have a well-defined internal structure, as shown by the pattern of electron scattering in transmission electron micrographs.

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Longitudinal section of a pearl millet kernel top and outer layers bottom. Also indicated are protein bodies and starch granules within one endosperm cell. Scanning electron micrograph of a cross section of a pearl millet kernel. Reprinted from Badi et al 1. Note the air spaces and the spherical starch granules.

Note the lack of air space, the polygonal starch granules, and the protein bodies P. Starch Starches from Cereals In cereals and other higher plants, starch granules are formed in amyloplasts. In the cereals with simple starch granules wheat, maize, rye, barley, sorghum, and millets , each amyloplast contains one granule.

In rice and oats, which have compound starch granules, many granules are found in each amyloplast. The starches from different cereals vary widely in size and shape Table 2. Wheat, barley, and rye all have two types and sizes of starch granules, as shown inFigure 2. In barley, the lenticular granules are formed during the first 15 days after pollination. Amyloplasts in barley and wheat each initially form a large, lenticular starch granule. Later, they form evaginations outgrowths in which small granules are formed. These much smaller amyloplasts separate from the mother plastid by constriction.

Light photomicrograph of wheat starch granules, showing large lenticular and small spherical granules. Scanning electron photomicrograph of isolated wheat starch granules. The starch granules of maize and sorghum are similar to each other in size and shape. Starch granules in cells near the outside of the kernel i. As far as we know, the properties of the differently shaped granules are the same. Light photomicrograph of isolated maize starch granules.

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Scanning electron photomicrograph of isolated maize starch granules. Scanning electron photomicrograph of isolated pearl millet starch granules. Reprinted from Hoseney et al Fig. Light photomicrograph of isolated rice starch granules. The compound granules see Figs. While those of oats are large and spherical, those of rice are smaller and polygonal. The reducing power of many carbohydrate molecules is a basis for their quantitative estimation.

The monomers are glycosidically linked to either position 4 or position 6 of the neighboring glucose Fig. Verswyvel Two polymers can be discerned, i. Its molecular weight varies between about 80, and about 1,, —6, anhydroglucose units. The molecular weight varies not only among species of plants but also within a species and depends upon the stage of grain maturity.

Although the polymer is generally assumed to be linear, this appears to be true for only part of the amylose. When, in excess water, amylose is leached from starch by being heated slightly above the starch's gelatinization temperature see below , the amylose solubilized is essentially linear. As the leaching temperature is increased, amylose of higher molecular weight and more branching is extracted. Both enzyme and viscosity studies have indicated that the branches are of long-chain length, with the side chains containing hundreds of glucose residues Fig.

However, the branches on amylose are so long and so few that, in many ways, the molecule acts as an unbranched entity. Adapted from Ghiasi et al b The long, linear nature of amylose gives it unique properties, such as its ability to form complexes with iodine, organic alcohols, or fatty acids Fig.

Nuclear magnetic resonance studies have revealed that amylose-lipid complexes also occur in native cereal starches, and free lipids occurring in the cereal starch may form additional amylose-lipid complexes during starch gelatinization see below. Depiction of an organic acid forming a clathrate, i.

This level of branching means that the average unit chain in amylopectin is only 20—25 glucose units long. The molecular weight of amylopectin has been reported to be as high as 10 8. It is truly a huge molecule, one of the largest found in nature. The amylopectin molecule has three types of chains Fig. Thus, A chains do not carry branches, and B chains do.

The C chain is branched and also has the only reducing group unbound C-1 in the molecule. Based on the mutual organization of A and B chains, a laminated, a herringbone, and a randomly branched structural model further referred to as Meyer's model were originally proposed for amylopectin. Over time, more insight into the exact structure of amylopectin has been gained by use of a series of enzymes that partially degrade the molecule in very specific ways.

Cluster structure of amylopectin, with indication of A, B, and C chains and illustration of crystalline and amorphous lamellae. The external chains are A chains. In agreement with the above, it has short A chains with either two or three glucose units and still has a very large molecular weight about Maltose molecules released by the enzyme action are not shown. Bijttebier Another enzyme that has been helpful in determining the structure of amylopectin is the debranching enzyme pullulanase EC 3. Thus, treating amylopectin with this enzyme releases all linear A and B chains, each of which now has a reducing end and hence possesses reducing power.

By determining the reducing power per unit weight, the average chain length can be calculated. Amylopectin has an average chain length of about The size distribution of the chains can be studied by gel permeation chromatography. The distribution is bimodal, with most by weight of the chains being about 19 glucose units in length and the others about 60 glucose units. The sum of the maltose and maltotriose molecules gives the number of A chains.

The number of larger molecules gives the number of B chains. The ratio of the reducing power of the outer small chains maltose and maltotriose to that of the large chains larger than maltotriose is the A to B chain ratio each reducing group comes from one chain. Approaches such as outlined here have shown that the ratio of A to B chains ranges from 1. However, complete debranching of amylopectin yielded chains with a polymodal chain length distribution, suggesting that the branching pattern is not completely random.

Extensive amylolysis of waxy rice and maize amylopectins furthermore showed that significant numbers of amylopectin branch points were retained in single-limit dextrins. In accordance with these observations, the Meyer model was abandoned in favor of the cluster model Fig. Its name indicates that the branching points occur in clusters.

With the exception of rice starches, the varietal variation of amylose content in regular starches of a single plant species is quite limited. Mutants that have starches with unusually high levels of amylose are also known. However, that is not necessarily the case, as shown by the following reasoning. Assuming that each of the amylose molecules in the hypothetical starch consists of 3, glucose units, while those of amylopectin consist of , glucose units, one sees that the molar ratio of amylose to amylopectin in the starch is Thus, on a molar basis there are roughly 33 times as many relatively mobile amylose molecules as relatively immobile amylopectin molecules.

Even though these minor constituents are present only in small concentrations in the starch, they can and do affect the starch properties. Generally, the level of lipids in cereal starch is between 0. The lipids associated with starch are generally polar and hence are best extracted with polar solvents such as methanol-water. In some cases, even more polar solvents are necessary to extract all the lipids.

In cereal starches, lipids are prevalent inside the starch granules. They consist mainly of lysophospholipids and free fatty acids and are present in proportions positively correlated with amylose content. Irrespective of the lipid class, the most abundant fatty acids in cereal starches are linoleic and palmitic acids.

In wheat, barley, rye, and triticale starches, lysophospholipids are present almost exclusively, while maize and rice starches contain significant proportions of free fatty acids.

The lipids in starch can occur either free or as amylose-lipid complexes see below. In contrast, noncereal starches contain essentially no lipids. Phosphorus is the most abundant mineral in starch. Normal starch contains lysophospholipids see above and variable levels of esterified phosphate. The monoesterified phosphate groups are found exclusively on amylopectin, with preference for the longer amylopectin branches. Potato amylopectin contains —1, ppm of monoesterified phosphorus, compared to 40— ppm for root starches and 0—20 ppm for cereal starches.

Part of this is from the lipids, and the remainder is mostly proteinaceous. A considerable part of the nitrogen stems from granule-bound starch synthase, a starch- synthesizing enzyme associated with amylose. The absence of this enzyme in waxy starches results in intrinsically lower protein contents. In the first, amylose is selectively leached from granules heated to slightly above the gelatinization temperature see below.

A higher leaching temperature solubilizes amylopectin in addition to amylose, and thus further purification is required. The fractionation achieved by leaching is not quantitative. In the second approach, granular starch is completely dispersed before the amylose and amylopectin are separated. Cereal starches are particularly difficult to disperse completely. Preventive measures include starch defatting, buffering, and protection from oxygen.

Several pretreatments such as the use of liquid ammonia, dimethylsulfoxide, or dilute base have been used to help completely disperse the starch.

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These components form an inclusion complex with amylose, the nature of which is similar to that shown in Figure 2. Several reprecipitations are necessary to obtain pure amylose; the amylopectin can be recovered by lyophilization or precipitation with alcohol. Organization of the Starch Granule Several levels of organization of the starch granule can be distinguished.

We here cover what is known based on microscopic and X-ray diffraction studies as well as from more advanced work. Starch biosynthesis is initiated at the hilum, a site easily recognizable in the mature granule. A granule grows from the hilum by apposition of carbohydrate material at its expanding surface.

The new layer deposited on the outside of the growing granule varies in thickness. The layers become apparent after treatment of the starch with dilute acid or enzymes Fig.

The more-resistant shells are more crystalline see below than the rapidly degradable more-amorphous shells, which, in spite of their name, contain some crystallites. Note the rings in the broken starch granules. In potato starch, the layers are obvious and easily seen in the intact starch under a light microscope Fig. The nature and cause of the layers in starch are still uncertain, but they presumably represent growth rings, as early observations on wheat and barley starch suggest one growth ring for each day after granule initiation.

Furthermore, growth rings are not visible in granules from most plants grown at constant light and temperature. For potato, however, ring formation is not abolished under such conditions. Light photomicrograph of potato starch granules. Note the concentric rings. The birefringence indicates that the starch granule has a high degree of molecular order but not necessarily that the granules are crystalline; things can be very ordered and yet not be crystalline. If they are highly ordered, they will be birefringent.

On the other hand, the cellulose in a piece of paper is semicrystalline and, although the crystallites themselves are birefringent because they are ordered, the paper as a whole is not birefringent because the crystallites are randomly oriented. Light photomicrograph of wheat starch granules taken with crossed nichol prisms showing the Maltese cross. In their work, they observed that intact starch granules of different botanical origins gave three types of X-ray patterns designated A, B, and C.

The crystallinity of starch granules can be destroyed by mechanical means. Ball milling starch at room temperature eventually completely destroys both the birefringence and the X-ray pattern. In general, we do not have a good explanation for this phenomenon, although it is clear that, when the ball strikes the starch, friction heat is generated.

Most cereal starches give A patterns Fig. Potato, other root starches, and retrograded see below starch give B patterns, while smooth pea and bean starches give C patterns. The C pattern is an intermediate form, probably a mixture of the A and B types. The A-type unit cell has 12 glucose residues and four water molecules, which are buried deep in the crystalline structure and cannot be removed without complete destruction of that structure.

Small dextrins 12—15 glucose units can yield any of the three patterns, depending on their crystallization conditions. The fact that selective leaching of amylose from granular starch increases its crystallinity implies that amylopectin structural elements have a predominant role in imparting crystallinity to starch.

Moreover, mild acid hydrolysis has been used to relate the starch crystalline properties and the molecular structure of starch polymers i. Combined chromatographic and enzymatic analysis of waxy maize Naegeli amylodextrins, regular cereal and potato lintners, and lintners of regular legume starches revealed two main populations of chains: It is now accepted that, for starches with regular or low amylose contents, short external amylopectin chains intertwine to form double helices.

It is equally accepted that double helices belonging to the same cluster are arranged such that either the A or B crystalline unit cell is formed. Hence, it is accepted that the amylopectin fraction of starch accounts for most of the granular crystallinity. In line with the above, variations in chain length distribution among amylopectins are accompanied by differences in the prevailing crystalline polymorph of the starch A-type or B-type polymorph, see above.

Moreover, it has recently been established that, on top of the differences in chain length distribution, starches bearing A- and B-type crystallites may also show a different branching pattern. X-ray diffraction of crystalline amylose helical inclusion compounds see above results in the V pattern Fig. Such patterns can be found after gelatinization see below and complexing with lipid or related compounds. X-ray diffraction patterns of starch, showing the A, B, and V patterns. The amorphous shells are less dense and contain amylose and probably less-ordered amorphous amylopectin, while the semicrystalline shells are composed of alternating amorphous and crystalline lamellae of about 9—10 nm in length.

The latter are made up of amylopectin double helices packed in a parallel fashion, while the former consist of the amylopectin branching regions and possibly some amylose. Schematic representation of the starch granule, showing amorphous and semicrystalline growth rings in a starch granule A , blocklet B , amorphous and crystalline lamellae in a stack and part of an amorphous growth ring C , and aligned double helices from amylopectin side chains within a crystalline lamella and amylopectin branch points within an amorphous lamella D.

Adapted from Donald et al and Myers et al Starch in Excess Water Systems In the preparation of most cereal-based food systems, the starch-containing cereal or fractions thereof are, at some point in time, heated in the presence of water and cooled to a variable degree before consumption.

The unique character of many of our foods results from the changes that starch undergoes, especially when it is heated and subsequently cooled. Some obvious examples are the viscosity and mouthfeel of gravies and puddings and the texture of gum drops and pie filling. It is therefore important to understand what happens to starch under conditions relevant for various food systems. The granule swells slightly.

The phenomena that occur when starch is heated in excess water can be studied using a variety of experimental techniques, including optical microscopy, amylography, and rapid viscoanalysis, as well as differential scanning calorimetry and time-resolved X-ray diffraction analysis. The present discussion focuses on optical microscopy, as it allows introduction of the definition of gelatinization. It further deals with amylography and rapid viscoanalysis, as both techniques are widely used to predict starch functionality. The granule is said to gelatinize, and the process it undergoes is defined as gelatinization.

The gelatinization temperature of a starch granule is therefore the temperature at which the birefringence is lost. The starches from different cereals vary widely in their gelatinization properties Table 2. Wheat, barley, and rye all have similar gelatinization properties: Triticale starch appears to be very similar to these starches. Taken together, the starch granules of maize, sorghum, and pearl millet are very similar to each other in gelatinization properties: The starches of oats and rice differ quite widely in their gelatinization properties.

In essence, gelatinization is an irreversible process, which destroys the molecular order of the starch, but it clearly affects all structural levels. Along with the disruption of molecular order, the loss of birefringence is accompanied by the melting of crystallites, absorption of water, swelling of granules, and the progressive solubilization of the starch. Amylography or Rapid Viscoanalysis As outlined above, heating of starch granules above their gelatinization temperature results in irreversible changes. These can be shown by either an amylograph or a Rapid Visco Analyser.

Both instruments measure the relative viscosity of a starch and water buffer system as it is subjected to shear during a controlled heating, a holding period at a constant temperature, and then a controlled cooling.

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The relative viscosities measured with these instruments are a function of the temperature profile and the shear used. In addition, they drastically change with the concentration of the starch used in the analysis. In essence, we can distinguish three concentration regimes. The first is a very dilute one and is defined as that in which the starch concentration is insufficient to give rise to a signal in either the amylograph or the Rapid Visco Analyser.

The second regime is a dilute-concentration regime. Under such conditions, the fully swollen granules do not occupy the full space available to them, and their swelling is, accordingly, not limited by space constraints. In both the very dilute and the dilute regime, the rheology of the starch-water system is said to be governed by the swelling power of the granules as well as by the amylose fraction that is solubilized. The third and last regime is the concentrated regime.

Under these conditions, the starch concentration is such that the granules cannot swell to their full extent. It follows that the rigidity of the granules drastically impacts the rheology of the system. However, while the concept of separate concentration regimes is useful for our understanding, it should be kept in mind that the concentration effects are a continuum from one regime to another.

As outlined above, in the very dilute concentration regime, the instruments are not sensitive enough to measure small changes at low relative viscosity; therefore, a material such as carboxymethyl cellulose is often added to the buffer often a phosphate buffer, pH 6. When wheat starch is added to water, changes occur as the temperature is increased Fig. This coincides with the loss of granular birefringence; i.

Conceptualized rapid viscogram of a starch suspension, showing the different phases of swelling, pasting, and setback gel formation as a function of the heating, constant temperature, and cooling profile used. Customers inside the U. You will receive the book along with an invoice. If you choose to keep the book, but not adopt it for your class, simply pay the invoice. PRESS when you write "Complimentary Desk Copy" on your invoice and return it with a photocopy of your bookstore order or bookstore requisition form for 10 or more copies of the textbook.

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