Plant Proteins and Mayonnaise as a Model
Texture and stability of emulsions are product characteristics in which proteins play an important role. Therefore, mayonnaise was chosen as a test product for the application of commercial plant proteins, as this product depends on both protein functions. Mayonnaise is an oil-in-water emulsion, with a fat content of approximately 75% (whole-fat mayonnaise). The structure consists of fat globules pressed closely together in a water suspension. The proteins and phospholipids from the egg normally stabilize the fat globules and keep the emulsion intact. The proteins form a layer around the fat globules and also form bridges to multiple fat globules, reinforcing the structure. This is sometimes called “bridging.” The phospholipids, such as lecithin, stabilize the emulsion through “Pickering” stabilization (particle stabilization), in which small particles (such as lecithin) adhere to the fat globules, forming a barrier that prevents them from combining or coalescing. [Editor’s note: A Pickering emulsion is one in which a solid particle, such as lecithin, colloidal silica and so on, adsorbs onto the interface between two phases.]
Commercially available plant proteins are obtained from various sources. Examples include soya beans, lupine, potato, peas and wheat. The proteins are sold in forms such as isolates (90% or more protein) and concentrates (generally 50-70% protein). The source of the protein has an effect on the functionalities and sensory characteristics of the protein preparations, as well as on the manner in which the protein is handled and extracted. Extensive research has already been conducted into the functionalities of native plant proteins, but, as yet, little is known about commercially available preparations and their possible applications. Plant proteins are not yet commonly used, in part due to limitations in solubility and other functionalities, the presence of an additional plant-like flavor and their coarse texture.
Despite the challenges and limitations posed by plant proteins, there is an enormous potential for creating high-quality products based on plant proteins. As an optimal starting point for this study, experiments were performed to determine the solubility and emulsion functionality of commercial plant proteins, in order to select the proteins that may have the most potential for emulsification in selected applications.
The solubility of a protein is an important property, because it has an impact on the other functionalities. Research has shown the proteins extracted from lupine, potato and soya beans are more soluble than those from peas and wheat. However, large differences were found in the solubility profiles of protein preparations obtained from the same source, which demonstrates the impact of the production process on the functionality of the protein. The solubility of the proteins was also highly dependent on the pH of the solvent.
For proteins that were to some degree soluble in low-pH solutions and in water, the emulsion functionality was tested. These proteins originated from potato, soya beans, lupine and wheat. None of the proteins had an emulsifying effect in low-pH solutions, which means there are no emulsion application possibilities for these proteins in emulsions with a low pH. When the proteins were dissolved in water, however, it was possible to form stable emulsions with a number of proteins derived from soya beans, lupine and potato. The globule size of the plant-based emulsions (an important indicator for emulsion stability) was greater than that of emulsions made with whey proteins. Among the plant-based emulsions, the smallest globule size was found in emulsions made with potato protein, which indicates a more stable emulsion. The other soya bean proteins, lupine proteins and the proteins obtained from wheat did not demonstrate any emulsifying effect.
Sensory Solutions and Possibilities
As a test, NIZO used the proteins from lupine, soya beans and potato that did demonstrate good emulsion stability to develop plant-based mayonnaises. A clear difference in the viscosity was found between the plant-based and egg-based mayonnaises: the plant-based mayonnaises were much more viscous than the egg version. To correct this, the fat content was reduced. To investigate other differences between the plant-based and egg yolk mayonnaises, a sensory investigation was conducted. In this investigation (n=12), various attributes in the areas of appearance, taste and texture of the plant-based mayonnaises were compared to those of mayonnaises made with egg. Areas in which the plant-based mayonnaises differed from the egg-based ones included an additional plant-like flavor, a greyer tint, the presence of an astringent mouthfeel, a coarse or granular structure and the presence of residue after swallowing the mayonnaise. (See chart “Mayo Texture: Plant- vs. Egg-based.”)
One of the plant-based mayonnaises developed by NIZO, made with lupine protein, demonstrated few significant differences compared to egg mayonnaise. Accordingly, the assessment of this mayonnaise for general mayonnaise character did not differ significantly from a commercial egg-based version. The lupine mayonnaise was also found to be yellower, glossier, less astringent and less coarse than the other plant-based mayonnaises.
When consuming the lupine mayonnaise, residue remained in the mouth after swallowing. To improve the mayonnaise, the source of this residue was investigated. There are various reasons that emulsions form a residue in the mouth. The investigation showed, for instance, that flocculation of emulsions can be caused by an interaction with saliva in the mouth. This can lead to a dry, coarse or astringent mouth feeling (again, see the chart “Mayo Texture: Plant- vs. Egg-based”). Another possibility that can lead to particle formation is the acidification of the mayonnaise emulsion during preparation through the addition of vinegar. The pH reduction can cause aggregation of the proteins, leading to particle formation that can be detected on the tongue. NIZO found, however, the residue was not attributable to either of these causes, but rather to insoluble particles in the protein preparation.
This was proven by separating the insoluble particles from the protein solution and using only the soluble portion for preparation of the mayonnaise. By using only the soluble portion, much smoother products could be made, with less residue and astringency. Images were made of the original and improved mayonnaises, using a confocal laser scanning microscopy--CLSM (see chart “Less Viscosity with Insoluble Protein Removal”). These show great differences in the quantity of undissolved particles in the mayonnaises. In these figures, the protein is shown in green and the fat in red. The large light-green particles are undissolved particles that are large enough to detect in the mouth.
By eliminating the insoluble portion of the protein preparation before making the mayonnaise, the resulting mayonnaise was much less viscous. This can be corrected by restoring the fat content to the original level used in whole-fat egg mayonnaises. This new plant-based mayonnaise was found to be creamier and less astringent than the original lupine mayonnaise, and it had more “body” (n=4). As a result, the improved lupine mayonnaise was more similar to a whole-fat egg mayonnaise in terms of sensory characteristics.
Plant proteins can also be employed as replacements in a wide range of other products where dairy or animal proteins are used. Many plant proteins still require improvement, before they can serve as a suitable replacement for animal proteins. With knowledge of protein behavior in food applications and proper treatment of proteins, issues--such as limited solubility, plant-like flavor, an astringent mouthfeel and undesirable colors--can be resolved. pf