Whey proteins have insulinotropic effects and reduce the postprandial glycemia in healthy subjects. The mechanism is not known, but insulinogenic amino acids and the incretin hormones seem to be involved.
WHEY PROTEIN CONCENTRATES
Commercially available whey protein concentrates contain from 35 to 95% protein. If they are added to food on a solids basis, there will be large differences in functionality due to the differences in protein content. Most food formulations call for a certain protein content and thus whey protein concentrates are generally utilized on a constant protein basis. In this case the differences due to protein content as such should be eliminated. As the protein content increases, the composition of other components in the whey protein concentrate must also change and these changes in composition might be expected to have an effect on functionality. These changes will be addressed in the appropriate sections below.
The solubility profiles of native whey proteins at acid, neutral and alkaline conditions make them unusual. They have special utility in soups and beverages especially those that require solubility under acidic conditions. The solubility of commercial whey protein concentrates has been shown to be highly variable . Examination of eight different kinds of commercially available whey protein concentrates showed that protein solubility ranged from 35 to 91%. Differences in solubility might be related to protein functionality. It has been stated that protein solubility is the single most important factor governing functionality . In many cases, however, this has not been shown to be the case. Liao & Mangino, for example, examined ten commercially available acid whey protein concentrates and found the protein solubility ranged from 25.4 to 82.4 %. When models were generated to determine factors important to functionality in a number of systems (whipped topping overrun, foaming, foam stability and emulsion capacity ) solubility was found to be the third most important factor affecting whipped topping overrun and was not one of the four most important factors governing functionality in the other three systems. It has been demonstrated that solubility in the range of from 25 to about 85% is not as important as the amount of calcium or protein hydrophobicity in the determination of gel strength of acid whey protein concentrates.
Another study on the effects of heating on the functionality of whey protein concentrates found that pasteurization of the milk utilized to produce cheddar cheese or of the resulting whey had no significant effect on the solubility of the resulting whey protein concentrates. Pasteurization of the ultrafiltration retentate, on the other hand, was significantly correlated to decreased protein solubility. Solubility of the proteins in this study ranged from 76 to 90%. Solubility was not found to be significantly correlated to gel strength at pH 6.5, foaming or foam stability. Solubility was significantly related to gel strength at pH 8.0.
The free sulfhydryl content of the whey protein concentrate has been significantly related to protein solubility and to gel strength at pH 8.0, but not at pH 6.5. It has been suggested that the effect of decreased solubility was due to a decrease in soluble b-lactoglobulin which resulted in a decreased concentration of free sulfhydryl groups that were required to form the gel matrix at this pH.
A positive relationship between protein solubility and the foaming and emulsion stability properties of whey protein concentrates has been noted. It has been observed that the pH 4.6 insoluble fraction of a number of whey protein concentrates were more effective emulsifying agent than either the pH 4.6 soluble material or the unfractionated whey protein concentrates. The insoluble fraction was less functional in all other applications. It was suggested that the increased functionality in emulsion formation could be attributed to the presence of residual phospholipid material.
This data would suggest that for a number of applications, solubility is far from the most important property of whey protein concentrates. Protein most be able to interact with other molecules and thus must be able to be dispersed in the continuous phase. A material so insoluble that protein hydration is not possible would not be very functional. On the other hand, total solubility is not necessary either. For many applications it has been observed that over reasonable ranges of solubility (35 to 95%) that protein solubility was not the primary factor in the determination of protein functionality. Protein solubility is very important where clarity of the finished product is important such as in beverages.
Recent advances in analytical procedures have made it possible to routinely determine the number of different proteins present in a mixture and the amount of each. It has been reported that the distribution of soluble proteins in commercially available whey protein concentrates produced by a variety of procedures as determined by high-performance gel permeation chromatography can show considerable variation. An average b-lactoglobulin to a-lactalbumin ratio of 2.52,which is within the wide range of values reported for whole milk, has been reported. The values for the different whey protein concentrates showed considerable variation with a product produced by ion exchange chromatography having a ratio of 13.7 while an ultrafiltered acid whey protein concentrate had a value of 1.89. Such a considerable range of protein compositions would be expected to have an effect on the functionality of the whey protein concentrates.
Other reports suggest that the ratios of b-lactoglobulin to a-lactalbumin + bovine serum albumin for whey protein concentrates produced by ultrafiltration of cheddar cheese whey ranged from 2.52 to 3.11. The amount of b-lactoglobulin could be correlated to the hydrophobicity of the whey protein concentrates. The amounts of b-lactoglobulin, a-lactalbumin and bovine serum albumin have been measured by reversed phase high performance liquid chromatography ( HPLC )in 75% protein whey protein concentrates produced from ultrafiltration of cheddar cheese whey. It was reported that b-lactoglobulin to a-lactalbumin ratios of from 4.6 to 5.9 were found in these products. The amount of these three proteins was also determined by quantitative polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE). It was reported that the numbers determined by HPLC could be related to the soluble protein content of the whey protein concentrates while the values obtained by SDS-PAGE were more closely correlated to the total protein content. The content of b-lactoglobulin as determined by HPLC was significantly correlated to whipped topping overrun, gel strength at pH 8.0, emulsion capacity, solubility, free sulfhydryl content and the hydrophobicity of the whey protein concentrates. Only the emulsion capacity was significantly correlated with protein solubility. Thus it would appear that for at least some functions of whey protein concentrates the type of soluble protein present is at least as important as the amount of total soluble protein present. More work in this area is warranted.
Whey protein concentrates contain residual lipid despite attempts by producers to remove as much lipid as is possible from the whey. Removal of residual lipid from whey has been shown to increase ultrafiltration flux and to improve whey protein concentrate functionality. It has been shown that the lipid content of whey protein concentrates tends to increase as the protein content also increases. Residual lipid has long been recognized as being detrimental to the quality of whey protein concentrates with particular attention to the foaming and flavor qualities of the product.
In 1930 reported that small amounts of fat in whey protein solutions caused the rapid collapse of foams that were otherwise stable. More recently it has been demonstrated that the removal of residual lipids from whey protein solutions by ultracentrifugation resulted in a three fold increase in overrun. When phospholipid was added back to these solutions the overrun increased while the foam stability decreased. The addition of small amount of triglyceride material resulted in a dramatic decrease in foam stability and led the author to conclude that the decrease in foaming was due to the triglyceride fraction. Others have also reported that residual lipid is detrimental to the foaming properties of whey protein concentrates. It has also been reported that residual lipid also inhibits the gel forming properties of whey protein concentrates. The lipid found in whey protein concentrates does not have the same composition as the bulk lipid of milk, but is greatly enriched in phospholipids and milk fat globule membrane material. Special treatments to remove this residual lipid greatly enhanced the foaming characteristics of whey protein concentrates.
It has been recently demonstrated that there is milk fat globule membrane material associated with the residual phospholipid fraction of whey protein concentrates. These extremely hydrophobic proteins are potent inhibitors of overrun in both egg white and whey protein systems. They reported that removal of lipid from these lipoproteins actually increases their foam suppressing properties. A decrease in the content of these lipoproteins will have a marked effect on the foaming properties of whey protein concentrates. Membrane proteins also were shown to have a marked effect on the strength of gels produced from 10% solutions of b-lactoglobulin. The addition of added milk fat globule membrane protein increased the strength of the gels up to a point. Further increases in membrane protein caused a decrease in observed gel strength. These data suggest that small changes in the content of fat globule membrane material can have rather large impacts on the functionality of whey protein concentrates. In the case of foaming the effect is uniformly negative while for gelation there appears to be an optimal membrane protein concentration for maximal gel strength. Methods designed to remove residual lipids from whey protein concentrates probably also remove residual membrane material to varying extents. More data on the role of membrane proteins on whey protein concentrate functionality and the effects of processing on the content of membrane protein in whey protein concentrates would probably be of considerable use to producers.
It has long been recognized that many of the interactions of proteins with other food constituents involved hydrophobic interactions. Early attempts to measure protein hydrophobicity involved calculations based on the solubilities of individual amino acids in polar and non-polar solvents. These calculated values have rarely related to protein functionality. A major problem with these procedures is that the calculations are based on all hydrophobic residues present. In proteins many of these residues ma be buried and unavailable to react with food constituents. Thus, calculated values are generally too high for proteins that are not able to completely unfold under the conditions present in the food product.
The use of the fluorescent molecule, cis-paranaric acid, to probe the interactions of membrane associated proteins with lipids has been investigated. This conjugated tetraenoic acid will fluoresce when it is placed in an hydrophobic environment. When added to proteins the resulting increase in fluorescence is taken as an indicator of protein hydrophobicity. Kato & Nakai applied this procedure to the study of food protein functionality. Under the conditions of their test they obtained a value, So, that they called surface hydrophobicity. In a number of studies cis-paranaric acid surface hydrophobicity has been related to protein functionality in emulsions, foaming and gelation properties of various food proteins.
Another procedure that has been used to examine the hydrophobicity of proteins and used extensively with whey proteins is the alkane binding procedure originally reported by Mohammadzadeh-K et al. In this procedure various alkanes are allowed to react with proteins and then the amount bound to the protein is determined and is utilized as a measure of protein hydrophobicity. It has been reported that heptane binding was significantly correlated with emulsion capacity, gel strength creaming rates for both a model coffee whitener and salad dressing produced with a variety of whey protein concentrates. It was suggested that when measured under the conditions described, that heptane binding was an indicator of the effective hydrophobicity of protein molecules. A number of subsequent studies have confirmed the relationship of hydrophobicity as determined by heptane binding to the functionality of both acid and sweet whey protein concentrates. Recent papers have indicated that milk fat globule membrane material is very hydrophobic and a relationship between the membrane content of whey protein concentrates and their hydrophobicity as determine by the heptane binding procedure has been shown. The content of membrane material in the whey protein concentrates could be correlated with processing conditions. It has been suggested that a measure of hydrophobicity might be useful to detect subtle changes in protein structure and functionality that occur during processing.
The mineral content of whey is altered as the whey is concentrated to form whey protein concentrates. The method of processing can have a large effect on both the total ash content and the mineral present. The ash contents for commercially available whey protein concentrates produced by ultrafiltration, electrodialysis, and metaphosphate complex formation have been reported and ranged from 0.98 to 12.18%. Calcium content ranged from 13.9 to 2180 mg/100 g sample while the phosphorous content ranged from 0.26 to 3.53%. The samples precipitated as metaphosphate complexes, as expected, contained the largest amount of phosphate. The ash content was highly correlated with the foaming properties of the whey protein concentrates and in agreement with the observation that samples with high ash contents were more functional in applications requiring diffusion of protein to an interface. Other workers have reported that high ash content may inhibit the emulsion and foaming characteristics of whey protein concentrates.
Whey protein concentrates have been shown to function better in a number of applications where minerals have either been removed or their content modified. Such applications include uses in ice cream , infant formulas , bakery products and dietetic foods. The mineral that has received most attention regarding its effect on functionality is calcium. The effects of replacing calcium ions with sodium on the functionality of whey protein concentrates has been studied. The solubility of whey protein concentrates was improved by calcium replacement with the largest improvement occurring at the isoelectric point. The textural parameters of heat induced gels also increased as sodium replaced calcium as did the time to form a coagulum at 70?C. Overrun was decreased by calcium replacement while foam stability and solution viscosity were affected in a non-linear manner.
It has long been known the the calcium concentration has a large effect on the heat stability of both b-lactoglobulin and a-lactalbumin . The dependence of calcium content on heat denaturation is probably responsible for the effects of heat treatment on gel strength. The strength of whey protein concentrate gels is dependent on the calcium content of the whey protein concentrate. At very low concentrations, the addition of calcium increases gel strength. As the calcium concentration increases past a certain maximum, further increases cause decreases in gel strength. At low concentrations calcium can increase gel strength by aiding in the formation of crosslinks that are necessary for proper gel formation. At higher concentrations of calcium, protein precipitation occurs at a faster rate than does crosslink formation and gel strength is weakened. Commercially available whey protein concentrates, both acid and sweet contain calcium at a level where it weakens gel strength and thus, gel strength generally increases with reductions of calcium.
As the concentration of protein in whey protein concentrates increases, the lactose concentration decreases. Values for the lactose content of commercially available whey protein concentrates range from 0.1 to 46%. Values below 5% are derived from products produced by an ion exchange procedure while the value of 46% was for a 35% protein whey protein concentrate. Generally lactose is considered a filler that has little effect on protein functionality. Lactose is a reducing sugar and can react with proteins via non enzymatic browning to produce less nutritious and lower functional products. This should not be a problem for whey protein concentrates stored at reasonable moisture levels . Lactose can increase the heat stability of proteins and lactose concentration has been related to the solubility of whey proteins following heat treatment. This should not be a significant factor in the solubility characteristics of most whey protein concentrates. Care is certainly warranted in the processing of products that contain very low concentrations of residual lactose and this may explain the observation that whey protein concentrates produced by ion exchange are difficult to manufacture with high solubility.
Applications in foodstuffs
Whey protein concentrates can perform a number of functions in food products. Generally a knowledge of the characteristics of the food product will allow an assessment of the role of whey protein concentrate in each of these foods. In many cases they are serving more than one function. The remainder of this section will discuss the factors that are important for proteins to exhibit these functionalities and to describe the nature of the interactions that occur with other food components.
For a protein to dissolve in a solvent, the protein - solvent interactions must be at a lower free energy than are the sum of the protein - protein and solvent - solvent interactions. Proteins contain amino acids that can interact with water molecules through dipole induced - dipole interaction (hydrogen bonds) and through ion - dipole interactions. The latter are generally the most important. The number of these interactions changes as the pH of the solution is varied. At the isoelectric point of the protein, the number of positive and negative charges are equal and protein - protein interactions are most favored. for this reason, many proteins are insoluble at their isoelectric points. A large number of food proteins have isoelectric points near pH 4.5 and it is difficult to use them in solutions around this pH . Whey proteins, however, are soluble at their isoelectric points and can be utilized at these pH values. This makes whey protein concentrates uniquely applicable for addition to acidic beverages. There has been considerable work regarding the fortification of fruit juices and soft drinks with whey. The level of protein fortification has generally been low because of the presence of lactose and salts. Utilization of whey protein concentrates would allow for fortification at a higher level. While protein fortified soft drinks have not caught on with consumers they provide a potentially lucrative market for whey protein concentrate utilization. It has been estimated that if 1% of the soft drinks sold in the United States were fortified at the 3% protein level with 35% whey protein concentrate, there would be an increased demand for 90 million pounds of whey protein concentrate per year.
Whey protein concentrates may also be utilized in a number of beverages produced at pH values more near neutrality. In these products they can be utilized for a variety of reasons including to function as emulsifiers, nutritional supplements, to add viscosity or to provide turbidity. In these applications it is especially important that the whey protein concentrates have a high solubility and a bland flavor.
A model system for the evaluation of the functionality of whey protein concentrates in soft drinks has been described. The drinks were fortified with 2% whey protein concentrate containing 75% protein. Significant problems with protein rising in the neck of the bottle with ring formation were reported. The amount of ring formation could be related to the thermal processing the samples had received. A model infant formula has also been described that can be utilized with whey proteins or whey protein/ casein blends. This system allows for an assessment of the ability of the protein to form an emulsion and also gives a measure of its heat stability. While infant formula is not a typical beverage, more whey protein concentrates are being utilized in their manufacture and a method of evaluating heat stability of whey proteins is useful.
Protein gels can be formed by the addition of salts, the action of enzymes, changes in pH or by the application of heat. Whey protein gels are obtained by heating therefore this discussion will be limited to the mechanisms of heat induced protein gelation.
The effects of composition on the strength and texture of whey protein concentrate gels have been studied. At low calcium concentrations weak gels were formed. Gel strength increased as calcium concentration increased up to about 11 mM. At calcium concentrations above this value gel strength decreased. It has been suggested that when too much calcium was present, heating caused protein aggregation to occur before protein unfolding and a three dimensional network could not be formed. Most studies of commercially available whey protein concentrates have reported that their calcium contents where such that excess calcium was present for optimal gel strength. For these samples, chelation of calcium would increase gel strength.
The sulfhydryl content of whey protein concentrates has also been related to the strength and textural characteristics of the gels formed. As with calcium, an optimum concentration for maximum gel strength has been reported. The content of b-lactoglobulin has been shown to be correlated with the sulfhydryl content of whey protein concentrates and thus can be correlated with the strength of the gels formed. It has been noted that the appearance and strength of the gels formed from whey protein concentrates was dependent on the pH of gelation. At low pH values, gels were rather soft and opaque and were described as coagula. At higher pH values the gels became more elastic, transparent and had greater gel strengths. These gels have quite different characteristics and would be utilized in different food products. A number of workers have also described the relationship of protein hydrophobicity to the gelation characteristics of whey protein concentrates As with calcium and sulfhydryl content, there appears to be an optimal level of hydrophobicity beyond which gel strength is weakened. In commercially available whey protein concentrates this level has not been reached and there is a direct positive correlation between gel strength and protein hydrophobicity.
The gelation properties of whey protein concentrates can be important to their inclusion in a number of food products. In some cases the proteins are present to help in water binding and in others they actually add to the gel matrix. Gelation can also be important in other functional properties such as foam stability. As will be discussed later, whey protein concentrates can form excellent foams, but the stability of these foams is generally poor. In products like cakes and meringues, if the protein does not denature at a low enough temperature the foam will rupture during heating and collapse.
Often times the gelation properties of proteins are studied in rather simple systems that do not reflect the conditions that occur during the manufacture of food products. In some cases this approach has been justified. For surimi manufacture a simple test has been devised that is a good indicator of protein functionality in this type of product In another case the strength of simple whey protein solutions has been related to the final height of white cakes produced from the whey protein concentrates (Marushige & Mangino ( 1987 ). In some cases more complex model systems have been utilized. Because there are so many different types of products that have gel formation as an important property, it is not possible to devise a simple system to test whey protein functionality in every product. The effects of processing on the acceptance of a fruit flavored product produced from heat induced gels of whey protein concentrates formed at pH values below 4.6 have been studied. At these values a coagula was formed. In these products the panelists preferred products that were not elastic and more closely resembled yogurt in texture. Several textural attributes and thus acceptability of the products were correlated with the heat treatment received by the whey proteins during processing.
In many gel systems the proteins are added not only to contribute to matrix formation, but also to bind other molecules such as water and/or fat. Thus in many systems gelation is combined with the need to absorb fat or to act as an adhesive and aid in the cohesiveness of the product. In these cases functionality must be assayed in systems that closely resemble the final food product.
Whey protein concentrates can be produced that will form heat induced gels under a number of different conditions and possessing a wide range of textural characteristics. The gelation properties of whey protein concentrates will probably be one of their most important and marketable functional characteristics.
The lack of reliable standardized methods to study the emulsification properties of proteins has resulted in a body of literature that is confusing and often contradictory. A number of different approaches have been utilized to attempt to study the emulsifying properties of proteins. These generally attempt to measure the capacity of the protein to emulsify fat or the stability of the emulsion once it has been formed.
Emulsion capacity is the amount of oil that can be made into an emulsion by a given quantity of protein. Emulsion capacity is a property of not only the protein under study but also of the emulsion system, the equipment being utilized and the method employed in emulsion formation. This method does not address the stability of the emulsion system and tends to measure at protein to lipid ratios that are far removed from those found in food systems.
Emulsion capacity has not been shown to vary much in whey protein concentrates. In a study of acid whey protein concentrates that ranged in solubility from 25 to 82% the emulsion capacity ranged from 38 to 52 ml of oil per gram of protein. It was reported that the contents of potassium, phosphorus and magnesium were the factors most closely related with the emulsion capacity of these samples. They were unable to explain this data from a theoretical standpoint and suggested that further work was necessary. The emulsion capacity of eight sweet whey protein concentrates ranged between 52 and 53 ml oil per gram of protein. It was reported that although the range of values was limited a significant correlation between emulsion capacity and the soluble b-lactoglobulin content existed. In general results of emulsion capacity measurements do not correlate well with performance in food systems .
Other methods to study emulsions attempt to measure emulsion stability. A number of procedures are based on an estimate of the average fat globule diameter on the assumption that the more efficient emulsion formation the smaller the average diameter of the fat globules. It is also assumed that fat globules with a smaller average diameter will be more resistant to creaming than will particles with large diameters. One of the most popular of these procedures is the so called emulsion activity index. In this procedure light scattering by an emulsion is related to total surface area of the emulsion. It has been reported that purified b-lactoglobulin was more efficient at emulsion formation than other whey proteins and that better emulsions were formed with b-lactoglobulin than with whey protein concentrates. It was hypothesized that a-lactalbumin actually inhibited emulsion formation.
Reliable methods for the study of emulsion stability are limited. A promising method involves the modification of a products formulation to make it less stable. than the commercial product. With proper formulation, the stability of the model system can be correlated with that of the commercial product . This method was utilized to demonstrate the protein hydrophobicity could be related to the stability towards creaming of acid and sweet whey protein concentrates in coffee whitener and salad dressing systems. Considerable work will be required before a systematic study of the factors important to whey protein concentrate functionality in foods is possible.
CONTRAINDICATIONS, PRECAUTIONS, ADVERSE REACTIONS
Whey proteins are contraindicated in those who are hypersensitive to milk proteins.
No reports. However, those who are hypersensitive to milk products are expected to experience allergy symptoms, including possible serious ones, if they use whey protein products.
No known interactions with drugs, nutritional supplements, food or herbs.
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