Protein based emulsifiers play an important role in food colloids. Modified proteins derived from animal sources, formed by covalent bonding with polysaccharide via Maillard reaction, have been reported in the literature to have excellent emulsifying and stabilizing abilities under harsh environmental stresses (e.g. high ionic strengths, freeze-thaw cycles, acidic pH conditions). On the contrary, conjugates based on plant derived proteins, have presented an incomplete, and often confusing picture of their colloidal stabilizing behaviour.
In the current study, milk whey protein isolate (WPI) and commercial soy protein isolate (SPI) were used as respective typical representatives of animal and plant sourced proteins. Careful comparisons were made between these two materials, undergoing exactly the same modification process (i.e. hydrolysis of proteins followed by conjugation with maltodextrin). The aim is to explore the possibility of and the difficulties in obtaining suitable conjugated plant proteins which have comparable emulsifying efficiency to their animal derived counterparts in producing stable and fine submicron sized oil-in-water (O/W) emulsion systems.
Two enzymes (i.e. trypsin and alcalase) were used to digest protein, particularly in order to improve the poor solubility and emulsifying property of SPI. Various degrees of hydrolysis (i.e. DH = 2.5%, 5.5% and 8.0%) were attempted. It is seen that for both WPI and SPI, trypsin, which has a higher level of selectivity at cleaving peptide bonds than alcalase, is more beneficial in producing polypeptides with improved emulsifying and colloidal stabilizing performance. The optimal DH was found to be roughly 2.5% and 8.0% for WPI and SPI, respectively.
Furthermore, by using an uncharged, linear and relatively small maltodextrin with no special surface functionalities (e.g. emulsifying, gelling or stabilizing properties) on its own, the impacts of conjugation with this polysaccharide on the colloidal stabilizing capacities of proteins and their hydrolysates under various pH conditions were investigated. Consistent with the literature, conjugated whey protein materials offered excellent flocculation stabilization to emulsion droplets in the absence of sufficient electrostatic repulsion. The situation was slightly different with regard to modified soy protein materials. The emulsion droplets coated by the conjugated biopolymers, based on fragmented soy protein, only exhibited limitedly enhanced flocculation stability. This is attributed to the inefficient level of Maillard reaction between protein/hydrolysates and maltodextrin. For soy protein (and probably most plant derived proteins), the major issue of synthesizing Maillard reaction products (MPRs) is the presence of particulate proteins in the sample, which is not desirable for achieving a molecular-scale intimate mixing of protein materials and polysaccharide, thus not facilitating the covalent bonding between those two species.
Last but not least, theoretical calculations were also performed, evaluating the impact of the size of a protein fragment and polysaccharide on the colloidal stabilizing capacity of emulsifiers made from these two components. The predicted theoretical results, together with experimental results, demonstrated that short peptides (and conjugated polymers derived from them) fail to deliver proper emulsifying and stabilising functionalities, as they are not able to adsorb sufficiently at the droplet surface (even though they may have a large proportion of hydrophobic amino acid residues). The critical size of a polypeptide to fulfil the role of strongly anchoring the composite polypeptide + maltodextrin biopolymer at the O/W interface was found to be roughly 10 kDa from the experiments. For conjugated polymer that can adsorb substantially, the size of its polysaccharide attachment now becomes predominant in controlling the colloidal stabilizing ability of this hybrid polymer species.
This study highlights the benefits of using highly selective enzymes, such as trypsin, in producing plant protein fragments with good colloidal performances. It also demonstrates that the major obstacle for obtaining suitable plant based conjugated emulsifier is the aggregated state of the protein material. Thus, an important prerequisite is a reasonable solubility of plant protein, which allows for a uniform mixture of protein materials with polysaccharide, prior to their Maillard reaction via heating process.
