Proteins: choosing the right application

Consumer demand for more natural ingredients is increasing. Whey and soy are the leading sources of protein for foods, and the range of oilseed sources is offering product developers even more alternatives. Researchers at The Netherlands' Wageningen University and Research Centre discuss alternatives and variables to keep in mind

Proteins are being used as ingredients in man-made food products because they contribute to one or more of the desired characteristics of that food product. These characteristics might be consumer related, such as texture, mouthfeel, appearance or taste, as well as technology related. The latter includes both storage — shelf life and palatability — and processing, such as mixing behaviour, foam, emulsion or gel formation. Proteins contribute to one or more of these characteristics because of their functional properties — physicochemical properties that govern the performance and behaviour of a protein in food systems during preparation, processing, storage and consumption.¹

It is the exact composition, spatial structure and size that determine how proteins 'act' in food products, thus how they function. Extrinsic factors such as pH and temperature; the presence of other constituents, such as salts, surfactants, gums, water and more; and process treatments all can alter the performance. Therefore, the functionality of a protein is largely affected by circumstances. The behaviour in model systems can be different from that in real food products. There are many reasons for these discrepancies.

Furthermore, the use of a protein in a product is governed by, on one hand, economic considerations — costs of preparation and handling, availability, and constancy in quality, for example — and on the other hand, by technological ones. Regarding the latter, the protein should contribute as much as possible to the optimal combination of functional properties required to reach the full palette of desired product characteristics. For example, egg white, a rather expensive protein preparation, is used in meringues primarily because of its excellent foaming properties. However, its good gelling properties are also of major importance for this application. The combination of functional properties should be optimal; each separate functionality need not be maximal.

Protein-flavour relationships
In addition to the effects of proteins on the texture and the mouthfeel of food products, the complex conformational structure of proteins creates an important source for interactions with flavour compounds influencing the flavour perception of food products. In general, proteins themselves have little flavour of their own. However, the complex conformational structure of proteins creates an important source for (ir)reversible interactions with flavour compounds. This ability is exploited in several ways, both with respect to enhancing and masking the flavour of food products. Flavour compounds are often deliberately added to food to enhance its smell and taste. Proteins may then serve to bind these compounds, in other words to act as a reservoir.

In this context another application of proteins can be mentioned. Proteins, for example as gelatin, are used to encapsulate flavour compounds to protect them from deterioration during processing. The latter usage may even result in controlled release of flavour compounds, for instance during chewing.³

In general, the internal region of a protein, being rather hydrophobic, is able to bind nonpolar flavour compounds, while the hydrophilic surface of the protein enables interactions with polar compounds. Small increases in the protein content can significantly decrease the perception of certain flavour compounds, ⁴ and therefore change the sensory properties of a given food to a large extent. Two prime examples that demonstrate the importance of flavour-protein interaction are whey and soy proteins.

Whey proteins are a byproduct of the cheese industry, generated in large amounts. They are added to food systems because of their good rheological and surfactant functionalities. Another important reason for the use of whey proteins is their ability to form fatlike systems. In this way protein-based fat substitutes are used to simulate the mouthfeel of fat. Whey proteins are able to bind various types of compounds. Specifically, beta-lactoglobulin is known for its interaction with a large variety of hydrophobic ligands.^5,6^,7 This protein has been used by several research groups as a model macromolecule for flavour release measurements.8^,9 It was found that the release of flavour depended on protein concentration. Also, these researchers showed that flavour compounds within the same chemical class had an increase in binding affinity with an increase in chain length. This effect was also observed for a homologous series of ethyl esters in the aqueous phase of sodium caseinate.¹⁰

During the production and storage of whey protein concentrate (WPC) the formation of an off-flavour is possible. Among others, lipid oxidation may result in formation of short-chain aldehydes that have a high affinity for long-term binding by covalent interactions with whey proteins. Flavour formation is of special importance when whey proteins are stored at a high water activity, resulting in an increase in the formation of aldehydes, ketones, furans and sulphur-containing compounds.⁵ The strongest binding affinities were observed for WPC, while the separate proteins alpha-lactalbumin and beta-lactoglobulin did not contribute that much to the flavour-binding capacity of the total whey protein, due to their low binding affinities.¹¹

In food product development the different effects mentioned above are mostly used to reduce negative sensory attributes. A common example is the usage in coffee whiteners. Sodium caseinate is often used in these products. The actual content in the recipe varies depending on the fat content because in low-fat products it is also used as a fat replacer. Due to the high content of fat or protein, these coffee additives change completely the perception of the coffee beverage. ¹²

Soy proteins from soybeans provide a high-quality protein, but the demand for this protein is hampered because soy protein preparations are often associated with an undesirable flavour.¹³ In contrast to alcohols, which do not interact with soy protein, aldehydes, especially unstaturated aldehydes, react with the protein.¹⁴ This reaction is at least partly irreversible. The binding constant increases with the chain length by three orders of magnitude.¹⁵ Specifically, the medium-chain aldehydes largely contribute to the 'beany' and 'grassy' off-flavour of soy protein.¹⁶

Aqueous solution of soy protein isolates showed in their headspace besides the aldehydes also sulphur-containing odour active compounds like dimethyl trisulfide. Also, these constituents contribute to the well-known unpleasant odour of this protein.¹⁷

Because the soy-protein fractions 11S and 7S showed different binding affinities, with a different temperature dependency, it is possible to remove certain off-flavours by reversibly altering the quaternary structure.¹⁸ This process could be exploited for soy

protein, food processing and the formulation of soy-protein flavourings. Knowledge of the exact interactions and binding processes allows food technologists to increase the usage possibilities for soy proteins. In food processing, the different flavour binding abilities are also important for the right choice of encapsulation material. Researchers showed that soy protein isolate was most effective while whey protein isolate was least effective for retaining orange oils during spray-drying of the liquid orange-oil emulsions. ^19,20

In the future, novel and functional foods will play important roles in our daily lives. Therefore, ingredients with new sensory attributes and/or improved nutritional value will be introduced to the market. For example, proteins such as thaumatin, monellin and miraculin have an exceptional position because of their sweet taste attributes. Traditionally, they have been used by West Africans for flavour improvement and bitterness suppression. Because consumer demand for more natural ingredients is increasing, taste-modifying proteins might be an alternative as long as they have similar properties to food products. ²¹

Furthermore, certain protein preparations could be used not only for optimizing sensoric properties but also the nutritional value of food products.²² Researchers showed that whey powder that was used to replace milk powder in yoghurt was preferred by sensory panelists. In addition to this preference whey powder is an important source for lactose, calcium and soluble vitamins — the nutritional value of this food product was increased.

Proteins from oil-producing plants
With a few exceptions, oil-producing plants have not generally been regarded as prime sources of protein for human consumption. Yet many of the oil-producing plants contain an appreciable level of protein, which has great potential for use in the human diet. Included in this group of oil-producing plants are soybean, canola (rapeseed), sunflower, safflower, peanut, corn, cottonseed, sesame, flax and even hemp.

The levels of protein in these seeds range from 13-17 per cent for safflower,^23,24 to as high as 37 per cent for soybean.²⁵ Despite these differences in protein levels, there are many similarities between these oilseed proteins. These proteins have similar molecular weights, subunits, amino-acid profiles and secondary structure.²⁶ This has resulted in similar hydrophobicity values and similar association-dissociation behaviour in response to pH. These oilseed proteins, however, exhibit distinct differences in terms of tertiary structure and surface properties.²⁶

A unique characteristic of canola protein, for instance, is the isoelectric point, which is around pH 7,²⁷ compared to pH 4.5 to pH 5 as is the case for other oilseed proteins.²³ The effect of this difference can be seen in the protein solubility where minimal solubility has been reported at both pH 4.0 and pH 8.0 compared to a single minimum around pH 4.5 for other oilseed proteins.

The range of food products that can potentially include proteins from oil-producing plants is extensive. However, with respect to proteins for oil-producing plants, the term 'potential' is significant in that demonstrated uses are primarily at an experimental level and these proteins have yet to receive that commercial recognition seen by soy protein. Nevertheless, for these proteins to become commercially viable, these experimental applications must be researched. Baked goods comprise one area where protein isolates can be used to improve nutritional quality. Adding 18 per cent sesame protein isolate to bread was possible without significantly altering the sensory properties.²⁸ With succinylated sesame proteins, the maximum level of addition was only 10 per cent and required changes to the bread preparation protocol.²⁹ Peanut proteins, as components in peanut flour, have also been incorporated into baked goods as well as a variety of other products.³⁰ Sunflower protein was included in an extruded product where 10-20 per cent protein isolate was included with corn starch.³¹

Other possible uses for these proteins that have been reported include incorporation of peanut protein into cheese,³² use of flax proteins as additives in ice cream and fish sauce,³³ use of sunflower protein hydrolysates in high-energy beverages,³⁴ and the use of corn protein that had been treated with citric acid to increase metal-binding capacity use as a scavenger in waste-water treatments.³⁵

The use of these proteins and products associated with these proteins may also find a place in the nutraceutical/functional-food arena. Peptides from the hydrolysis of these proteins may be bioactive and have a role in disease prevention. Minor components such as phytic acid and phenolic compounds, which have been considered antinutritional and detrimental to protein functionality, may be recovered for use in this area, as both have been shown to have properties that may have health benefits.³⁶

As the production of oilseed will continue to supply the demand for food-grade oil, there will be a great deal of protein available from the oilseeds. While more research is required, there will be a place for these proteins in both food and nonfood applications.

(Excerpted from Proteins?in Food Processing, RY Yada, editor. ISBN 0-8493-2536-6, Published by Woodhead Publishing Ltd, England. www.woodheadpublishing.com)

Dr H Luyten, Dr J Vereijken and Dr M Buecking are at Wageningen University and Research Centre, Agrotechnology &; Food Innovations, The Netherlands.

Respond: [email protected]

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