Principles of Cereal Science and Technology, Third Edition

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Students will get a firm and broad background in cereal science. Professionals utilizing grain-based ingredients in food products or those with responsibility for sourcing, testing, or maintaining quality grains will find this new edition extremely informative and useful. Those with technical background but no training in cereals as well as those with a limited technical training can quickly come up to speed by reading this book.

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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.

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. At left, intact starch granules are shown, followed by gelatinized granules with leached amylose, and finally starch gel with indications of starch granule remnants containing intergranular crystalline amylose. Gelders The viscosity increase that occurs when starch is heated in water is the result of the starch taking up water and swelling substantially.

With continued heating, the starch granule becomes distorted, and soluble starch mainly leached-out amylose is released into the solution. The soluble starch and the continued uptake of water by the remnants of the starch granules are responsible for the increase in viscosity. Solubilization of starch is continuous during pasting. Thus, in any food system, complete pasting or complete solubilization of starch would not occur.

In cooked food, the starch occurs as remnants of the granule, with a small level of soluble starch. One property of starch yet to be explained adequately is that solubilization appears to be controlled mostly by temperature and not by the interaction of time and temperature. Holding starch at a specific temperature for a period of time does not increase its solubility.

The temperature must be raised or the sample stirred or otherwise sheared to increase the solubility. The rapid viscogram in Figure 2. However, from a given moment onward, the relative viscosity of the starch system decreases markedly. The decrease in viscosity is caused by the molecules of soluble starch orienting themselves in the direction that the system is being stirred, as well as by shear-induced destruction of the not necessarily fully swollen and hence fragile granules. It is of practical relevance for many food systems.

If one wants to make a thick soup, one must not stir excessively or pump the paste through a pipe, as, in both cases, shear thinning will occur, giving a lower viscosity. Different starches vary in the amount of shear thinning that they show, and starch modification see below affects this property. Generally, the more soluble the starch, the more it will thin on shearing. Finally, it is of note that, while amylose-lipid inclusion complexes may be present in native starch already, additional quantities may be formed with free starch lipids during the starch gelatinization and pasting process.

This complexation evidently influences the properties of starch as well. With differential scanning calorimetry DSC, see below , which measures heat flow as a function of temperature, one can detect two types of amylose-lipid complexes. It has been stated that the first type corresponds to noncrystalline complexes and that the lower-melting endotherm is the result of the dissociation of the complexes, while the second, higher-melting endotherm can be ascribed to the melting of crystalline amylose-lipid complexes. Whatever may be the case, it follows from the above that at least part of the amylose-lipid complexes present in the starch granules and those complexes formed during the starch gelatinization and pasting process dissociate during the cooking phase.

Starches vary in the amount of setback they display. The phenomenon is caused by a decrease of energy in the system that allows more hydrogen bonding and entanglement between starch chains. Gelation and Retrogradation Simply stated, a gel is a liquid system that has the properties of a solid. Some common examples are gelatins, pie fillings, and puddings.

In gels, a small amount of solid material controls a large amount of water. It is amazing that water does not leak out of the gels when they are left standing. Calculations show that the distances between the starch chains in gels are very large compared to the size of the water molecule. Studies with diffusing solutes show that the water in gels has properties essentially equal to those of pure water. However, the water is held in the gels. We do not understand the forces involved in this.

However, it seems very logical that hydrogen bonds are involved. One can indeed visualize a food system gel as protein or carbohydrate chains with layers of water molecules attached by hydrogen bonding. In starch chemistry, it was first used to describe the observation that, following gelatinization, the starch would regain crystallinity. Strictly speaking, of course, only amylopectin molecules go back to a crystalline entity; hence, the term retrogradation should be used only for amylopectin crystallization.

Under certain conditions, amylose may also crystallize.

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However, since it was not crystalline to start with, this should not be referred to as retrogradation. Therefore, in this book, the terms amylopectin retrogradation and amylose crystallization are used. The starch concentration, the temperature, and the shear applied during the steps preceding the cooling phase determine the structure of the starch suspension.

The structure may vary from 1 densely packed swollen granules without a continuous amylose gel phase outside the granules, in which case the suspension forms a paste with flow properties rather than a gel, to 2 swollen granules dispersed in a continuous gel phase of leached amylose Fig. Schematic representation of changes that occur in a starch-water mixture during heating, cooling, and storage. I, Native starch granules; II, gelatinization i. Adapted from Goesaert et al In the second case Fig. Initially, double helices are formed between the amylose molecules that were solubilized during gelatinization and pasting, and a continuous network develops gelation.

That this process occurs fast does not need to be a surprise. It is well known that crystallization comprising the steps of both crystal nucleation and propagation can occur between the glass transition see Chapter 5 temperature of a given system and the melting temperature. When applying this concept to amylose systems, one comes to the following reasoning. Crystals can thus form over a broad temperature range. As long as the starch is above the melting temperature of amylopectin, new crystals would not form. The crystal nucleation and propagation temperature range between room temperature and the crystal melting is narrower than that of amylose.

In practice, upon storage, gel stiffness slowly increases because of crystallization of amylopectin within the granule remnants, which, in the second case described above, are embedded in and reinforce the continuous amylose matrix. Therefore, amylose crystallization determines to a great extent the initial hardness of a starch gel, while amylopectin retrogradation determines the long-term development of gel structure and crystallinity in starch systems. It was stated above that, during the cooking phase, at least part of the amylose lipid complexes dissociate.

However, cooling of starch pastes leads to renewed formation and subsequent crystallization of amylose-lipid complexes. Finally, as the gel ages, the starch chains have a tendency to interact strongly with each other and thereby force water out of the system. Starch in Limited Water Systems The above systems used to study the interaction of starch, water, and temperature work only at dilute starch- in-water concentrations.

These conditions are quite different from the concentrated starch-in-water systems such as found in baked products. Little or no data suggest that results obtained in dilute systems can be used to understand what occurs in concentrated food systems. However, the changes that starch undergoes in such systems are very important.

All baked products set; that is, they reach a temperature at which the dough or batter can no longer expand under the gas pressure generated by the increasing temperature. The changes that starch undergoes are at least partially responsible for that setting. It has always been, and still is, difficult to study starch in limited water systems. Yet most, if not all, of our food systems are limited in water less than 2: DSC has been quite beneficial for studying starch in such concentrated systems.

When starch is heated in excess water water-starch, 2: The start of the peak where it deviates from the base line corresponds to the start of birefringence loss. The end point of the loss of birefringence and the end of the peak are not quite the same, as there is a considerable lag in the DSC instrument. However, in general, the two correlate well. As the amount of water in the sample is reduced, the DSC peak widens and becomes clearly bimodal Fig. At low water contents water-starch, 0. The slow loss of birefringence can also be seen directly in light photomicrographs.

For some waxy and regular cereal starches, gelatinization enthalpy correlates significantly with crystallinity, as expected because hydrogen bonding between adjacent double helices has marked effects on stability. After partial gelatinization, the residual enthalpy correlates both with crystallinity and double-helical order. Several views on the impact of moisture content on DSC enthalpic transitions have been formulated.

One of these maintains that, initially, water enters in the more-accessible, amorphous regions of the starch granules. Swelling of the amorphous regions induces stress on the crystallites, and starch chains are stripped from the surface of crystallites, thereby reducing the crystallinity and causing the loss of birefringence of the starch granules. In excess water, the degree of swelling is sufficient to completely gelatinize the granule. At lower water levels, swelling is insufficient to completely disrupt the starch granule. Differential scanning calorimeter thermograms of wheat starch heated with water-to-starch ratios of 2.

Reprinted from Ghiasi et al a Starch-Degrading Enzymes Carbohydrate-hydrolyzing enzymes have, in the recent past, been grouped into glycoside hydrolase families, based on genetic information, structural and amino acid sequence similarities, and hydrophobic cluster information.

In the particular case of starch-degrading enzymes, glycoside hydrolase families 13, 14, and 15 are of particular relevance.

Depending on the enzyme, an endoaction or an exoaction pattern can be discerned. Enzymes that display an endoaction can hydrolyze the starch chain internally, while enzymes displaying an exoaction generally hydrolyze the chain from the nonreducing end. The enzyme is optimally active at about pH 5.

However, given sufficient time, they also degrade granular starch, such as is observed during cereal germination. The level of sprouting is generally an important factor in the suitability of grain for food uses. In addition, the time needed for a standardized object to move through a starch paste i. Several direct chemical and enzymatic tests have recently been developed to measure amylase activity.

Maltogenic amylase EC 3. An important difference between these enzymes is that pullulanase debranches side chains of two or more glucose units, whereas isoamylase requires at least three glucose units. It is optimally active at a pH value of about 5. In general, the level does not increase much as a result of germination. The graphs show the percentage of residual activity noted after a preincubation for a given time. Modified Starches Granular starches can be modified by chemical reaction to change their properties and functionality to better meet desired end uses.

Starch gels can be produced with a wide range of properties by varying the source of starch as well as the degree and type of modification. One can use a mixture of unmodified, cross-linked, or substituted starches to achieve the desired viscosity, paste clarity, and freeze-thaw stability desired for a specific product. The more common modifications and how they modify starch properties are discussed in this section.

The early work with acid modification was done in the late s by Lintner and Naegeli. After treatment, the slurry is neutralized and the starch recovered by filtration. During the treatment, the acid freely penetrates the amorphous parts of the starch granule and hydrolyzes glucosidic bonds. The acid cannot attack the crystalline areas, perhaps because of their double helices, so they remain intact.

The major effect of the acid is to reduce the molecular weight of the starch molecules while leaving the crystalline structure of the granule intact. The gelatinization temperature range is increased, presumably because the amorphous chains cannot assist in the melting of the crystalline areas. Upon gelatinization, the starch becomes much more soluble than untreated starch does. As a result of acid treatment hydrolysis of chains , the paste viscosity is much less than that for the native starch.

Because the starch chains remaining after acid treatment are smaller shorter in chain length , they tend to associate with each other more easily and thus form a rigid gel upon cooling. Common examples of such gels are jelly beans and other gum candies. The linking is accomplished by forming a diester with phosphoric acid in which case phosphorus oxychloride is the most common reagent or by forming an ether bond for which epichlorohydrin is a common reagent. The linkages are illustrated in Fig. Although one generally considers the cross-link to occur between molecules, it could also occur within the large amylopectin molecule.

However, such bonds within the molecule would not have a great effect on the starch's properties. Reactions used to cross-link native starch granules with phosphorous oxychloride top or epichlorohydrin bottom. Verswyvel High levels of cross-linking increase the starch's gelatinization temperature. Highly cross-linked starches that do not develop viscosity when they are boiled in water or sterilized in an autoclave can be prepared. Such modified starches are useful as dusting starches for surgeon's gloves. The starch can be sterilized, works well to protect the surgeon's hands from sticking to the gloves, and, if accidently lost into the wound, is degraded with no ill effects.

Starch for use in food systems is generally cross-linked to a small extent. The extent of reaction is designated by the degree of substitution DS of the products obtained. A DS of 1 indicates that, on average, an anhydroglucose residue carries one substituent. Thus, the maximum DS is 3 for linear starch and slightly less for amylopectin. For food use, the DS is generally between 0. Low levels of cross-linking do not significantly change the gelatinization temperature of the starch but do materially change its pasting properties.

Cross-linked starch swells less and is less soluble than its unmodified counterpart Fig. Thus, cross-linked starch gives a lower viscosity upon pasting. One of the advantages of cross-linked starch is that it is more resistant to shear thinning than unmodified starch. Because the starch solubilizes less, it shears less and thus gives a more viscous paste after stirring or pumping.

The temperature-time profile used is also indicated. An example is thickening a cherry pie filling. Cross-linking does not stabilize the bonds in spite of the acid; however, with sufficient cross-linking, the starch swelling is greatly restricted, and, as the acid hydrolyzes the bonds, viscosity increases instead of decreasing.

Therefore, if one starts with the right degree of cross-linking and obtains a constant amount of hydrolysis during baking, one can still end up with a thick pie filling. The change in viscosity as a function of pH is shown in Fig.

Relative viscosity of starch gels made from either normal A or cross-linked B maize starch at different pH values. At neutral or slightly acidic conditions, no acid hydrolysis takes place, and constant relative viscosities are obtained for gels of both starches. As cross-linking reduces the swelling capacity of starches, the resultant gels are less viscous for cross-linked than for normal starch. When the starch gel from normal starch is produced under acidic conditions, acid hydrolysis takes place, and the resultant starch gel is less viscous than one prepared at neutral pH.

However, when a gel from cross-linked starch is produced at acidic pH, the impact of cross-linking is partly overcome by the hydrolysis reactions, and a gel of higher relative viscosity is obtained. This principle finds applications in the production of, for instance, acidic pie fillings pH about 3. The texture of a starch paste is also affected by cross-linking. Starch pastes are classified as being either long or short. A short paste spoons well cleanly , whereas a long paste tends to be stringy and does not spoon well.

A short paste gives a narrow deflection, and a long paste gives a broad deflection. Cross-linking of a starch makes its paste much shorter than the paste of the unmodified starch. Even low levels of cross-linking decrease the swelling and solubility of the starch.

The more soluble the starch, the more entanglement and interactions occur, giving the paste its long character. In the cross-linked starch, more of the polymers remain in the remnants of the granules. This produces the short paste. Test for short and long pastes. Starch pastes are poured and allowed to cool. An ink line is drawn across the gel, and a knife is used to cut the gel at right angles to the ink line.

Principles of Cereal Science and Technology, Third Edition

With a long paste, the line is distorted further from the knife cut, showing the interaction of the molecules. In a short paste, the knife does not cause much distortion. Many starch gels, particularly when stored under cool conditions, become opaque with time. The phenomenon is caused by starch crystallization. Crystallization is faster if the chains are smaller and more mobile.

Therefore, cross-linking delays crystallization and retrogradation and thereby delays the time when the starch gel becomes opaque. During freezing, the starch chains are rendered immobile, and ice crystals are formed. During storage, even at subfreezing temperatures, water migrates from small crystals to large ones. As a result, crystals are fewer but larger. Because the water is now concentrated into certain areas, the product also has voids.

Principles Of Cereal Science And Technology

As the ice thaws, the first water produced supplies the starch chains with solvent and thus mobility. In unmodified starch, the chains rapidly interact with each other. The water released during additional melting cannot penetrate the interacting chains, and the product loses its gel consistency. The gel becomes opaque and watery and has a tough rubbery texture. Cross-linking holds the starch chains fixed in space so they cannot interact strongly.

Therefore, the water produced during melting can again hydrate the starch chains, and the starch gel retains its properties. Cross-linked starch can withstand several freeze-thaw cycles. It then carries a bulky as well as a charged group. Both of these facts make the starch chains repel each other.

Thus, the granule tends to swell and solubilize more during gelatinization. This gives a starch paste with higher viscosity but with poorer resistance to shear thinning. Because the chains tend to repel each other, they do not interact or crystallize as easily as those in native starch. Thus, substitution stops retrogradation and opaqueness of starch gels and is helpful in giving improved freeze-thaw stability. High levels of substitution about 0. This type of modified starch is useful in making instant pie fillings and puddings. Several chemical groups can be grafted onto starch to give a substituted starch.

The role of fat in foods is complex and not well understood. Several of the important roles are to act as a plasticizer, to help incorporate air, to soften the mouthfeel of foods often referred to as adding moistness , and to trap flavors. The above roles, except for trapping flavors and incorporation of air, can be compensated for by higher levels of water in the product.

Thus, we can visualize the role of the starch-based fat mimics as producing products with much higher water contents. To do this and still produce a reasonable product requires that the water be stabilized in some way. There are essentially three types of starch-based mimics. The first type comprises long-chain starch molecules. They control water in a way similar to that of the many gums that are used as fat mimics.

The major problem with this type is that they interact with themselves to form strong gels or to crystallize. At higher concentrations, a second type i. When the viscosity is not high enough, these also may crystallize. The third type, consisting of the microcrystalline particles produced from sheared, acid-hydrolyzed starch, also controls water and gives the desired product. None of the starch-based mimics, by themselves, incorporate air or trap flavors.

However, they have interesting properties and find many uses in foods, some of which have nothing to do with replacing fat. They are used, principally, in nonfood applications, i. However, they are also used in breading formulations and are reported to give improved adhesion to meat products. Although not strictly a modified starch no covalent bond is formed , starch clathrates are also useful in modifying the properties of starch gels.

The clathrate forms, presumably, with amylose and retards both the swelling and the solubility of starch. The reduced solubility results in a much shorter gel. An example is the use of monoacylglycerols to form a clathrate that gives a much fluffier instant mashed potato product.

Another modified starch that contains no new covalent bonds is cold-water-swelling starch. Such starch is produced by heating the native starch granules in an alcohol-water mixture. The starch crystallites are melted, but there is insufficient water to swell the starch granules. The solvent is then removed and the dried starch is stable.

Addition of these granules to cold water results in their swelling. Thus, with these products, foods can be thickened without the use of heat except for that contained in the cold water. However, it is now clear that some of the starch that we consume escapes digestion in the intestinal tract of healthy individuals. It has some beneficial physiological effects that make it somewhat comparable to dietary fiber constituents. Four types can be discerned, i. RS I resists digestion because it is physically entrapped in its storage cell. Examples include those foods with whole or only partially milled cereal grains such as muesli.

RS II is native crystalline and hence ungelatinized starch. Potato starch is a typical example. Its crystalline polymers are only poorly degradable by human amylolytic enzymes. RS III can, in a first approximation, be considered to be crystalline amylose. In practice, it has, by far, received most of the attention in the literature. Its production involves 1 starch gelatinization, 2 crystallization of the gelatinized starch molecules, and, if a concentrated form is desired, 3 amylolysis of the nonresistant starch residue.

RS IV is starch that has been chemically modified. Conversion of Starch to Sweeteners Because starch is composed essentially of glucose, hydrolysis produces glucose syrup. Large amounts of starch, especially maize and wheat starch, are commercially converted to syrups.

It would appear straightforward to cook starch with acid to produce such syrup. However, numerous side reactions can and do occur. From a practical standpoint, acid hydrolysis is effective only to thin the starch reduce its viscosity as it is being gelatinized. Before continuing with a discussion of hydrolyzing starch to syrup, it is useful to discuss sweeteners and sweetness.

The relative sweetness of several sugars is given in Table 2. Great variation exists in the way different people perceive sweetness. What is very sweet to one person may only be slightly sweet to the next one. Also, the relationship between the concentration of sugar and the perceived sweetness is not linear. Sweetness perception is also affected by pH, as well as by other materials in the food. Thus, the values in the table are useful but not absolute.

Sucrose is taken as the reference and given a value of Maltotriose and larger glucose oligomers are not sweet. Therefore, to produce sweet syrup from starch, much of the starch needs to be converted to glucose. It is also of interest to note the high relative sweetness of fructose. Another concept important in understanding conversion of starch to syrups is that of dextrose equivalent DE. Dextrose is the trivial name for glucose. The term is used extensively in the wet-milling industry. Glucose is a reducing sugar i.

Thus, free glucose is a reducing sugar, whereas in maltose, one of the glucose molecules is reducing and the second one is not. In starch, there is only one reducing group in each starch molecule; all of the other glucose molecules are linked at the C-1 position. Dextrose equivalent is a measure of the percentage of glucosidic bonds that are hydrolyzed. Complete hydrolysis produces glucose. Thus, if one measures the reducing power of a solution obtained by complete hydrolysis of starch, expresses the reducing power as glucose, and divides the thus-estimated glucose content by the total weight of carbohydrate in the sample, that value is DE.

Hydrolysis of one additional bond at any location in every chain doubles the reducing power while the amount of total carbohydrate stays the same. The DE is then Thus, DE tells what percentage of the bonds is broken, but it does not tell the chemical composition of the resultant syrup. To produce starch syrup, the starch is gelatinized in the presence of acid. This reduces the viscosity of the starch paste so that large amounts of water are not necessary to make it pumpable.

Syrups can be made with acid hydrolysis alone; however, at about 40 DE, side reactions start to be important, and dark-colored undesirable syrups are obtained. After acid thinning, several enzymes can be used to hydrolyze further, depending on the syrup desired. Solids having DE values varying from 10 to 35 are sold commercially.

They are also useful as flavor diluents. This gives a higher percentage of maltose and a slightly higher DE. Note that a pure maltose solution has a DE of only To produce high-DE syrups, glucoamylase must be used. Thus, it can theoretically produce a syrup of DE. In commercial practice, values of 92—95 DE are more common. High-DE syrups do contain high levels of glucose and thus are relatively sweet. They are nearly completely fermentable by yeast and give solutions of high osmotic pressure. To obtain higher sweetness, part of the glucose must be converted to fructose. This is accomplished with the enzyme glucose isomerase EC 5.

This is the classical high-fructose syrup of commerce, which, on a solids basis, is just as sweet as sucrose. This syrup has been very successful in competing with sucrose in many applications. Although it is as sweet as sucrose, it has a higher osmotic pressure and gives a lower water activity than does a sucrose solution. Both glucose and fructose are reducing sugars and therefore more subject to browning than is sucrose, which is not a reducing sugar. Proteins of Cereals Cereal grains, like all biological materials, vary widely in their chemical composition. The variation is quite noticeable in the protein content.

The proteins in cereals are important from a nutritional point of view. In the case of wheat, the protein also has a drastic impact on functionality. This is undoubtedly the reason for the wheat storage protein, gluten, being the most-studied cereal protein. Gluten is believed to be mostly responsible for the breadmaking capabilities of wheat flour.

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We discuss this protein in more detail in this chapter. The structures of the common amino acids are given in Figure 3. Amino acids are commonly grouped according to their type of R group. Verswyvel Proteins vary in molecular weight from a few thousand to several million. A protein of with a molecular weight of , would typically contain amino acid residues. The acidic and amino groups of each amino acid are involved in the peptide bonds and form the backbone of the protein. The backbone structure of all proteins is essentially the same.

The primary structure, i. However, for most of the functional differences in proteins, one must look to the secondary and tertiary structures. The peptide bonds that make up the backbone of the protein are flexible to a limited extent and can twist or curl the polypeptide into different forms. The sulfhydryl group SH- on the amino acid cysteine is an active group. It can react with another cysteine residue as a result of oxidation to form a disulfide bond -S-S-. Such linkage is one factor that gives the protein its secondary structure. The two cysteine residues can be on the same protein chain intramolecular bonding , forming a loop in the protein, or they can be on different protein chains intermolecular bonding , linking two polypeptide chains together Fig.

Disulfide bonding between polypeptide chains A and within a single polypeptide chain B. Verswyvel Several different types of noncovalent bonds are responsible for the tertiary structure of proteins. In most cases, the individual bonds are relatively weak, but their large number creates overall strength and stabilizes the structure. Two examples of such bonds are ionic bonds salt formation between an acidic and a basic group and hydrogen bonds, which are very prevalent, with side chains containing uncharged oxygen, nitrogen, and hydrogen Fig.

Another type of noncovalent bonding is hydrophobic bonding. Ionic bonds A , hydrogen bonds B , and hydrophobic bonds C between amino acid reesidues in protein chains. Verswyvel When a protein is placed in solution, several forces are active. For example, positive charges repel other positive charges and attract negative charges. Hydrophobic amino acid residues, for entropy reasons, tend to minimize their contact with water and hence associate together. The sum of all of this activity determines the tertiary structure of the protein.

It is the three-dimensional structure of the protein that determines its properties. Whether or not it is soluble in water depends on several factors, including its molecular weight larger molecules are generally less soluble and whether charges and hydrophilic groups are on the outside of the molecule where they can interact with water or are buried in the interior of the molecule. Whether or not the protein has enzymatic i. Denaturation may be reversible or irreversible.

An example of a reversible denaturation is the loss of solubility of a protein when high concentrations of salt e. The protein precipitates because the large amount of salt competes with the protein for the available water. However, when the salt is removed, the protein may regain its initial conformation and solubility; thus, the salt-induced denaturation process is reversible. A classic example of irreversible denaturation occurs when egg white protein is heated in water.

The system's kinetic energy, which increases as the temperature is increased, breaks hydrogen bonds, and the protein goes from its original conformation to a more random one. In the particular case of egg white, the process is accompanied by disulfide bond formation, and the denaturation is irreversible. Needless to say, denaturation changes the physical properties of the protein. In many instances, it becomes less soluble, and, in the particular case of enzymes, denaturation often results in loss of activity.

Heat-induced protein denaturation in general can be monitored by differential scanning calorimetry DSC , as it gives rise to a heat denaturation peak. Interestingly, DSC thermograms for wheat gluten fail to show a definite denaturation peak Fig. The small peaks seen are apparently the result of contaminating starch granules or contaminating soluble proteins.

The lack of a denaturation peak would suggest that the gluten is in a random conformation. A, commercial wheat gluten Reprinted from Hoseney et al Proteins in solution are affected to a large extent by both the pH and the ionic strength of the medium. The effects are brought about by the charge on the protein molecule and by how much the charge is shielded.

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