Foaming properties and colloidal stability of wheat (Triticum aestivum L.) gliadin and maize (Zea mays L.) zein based nanoparticle suspensions

Promovendus/a: Katarzyna Kaczyńska

Promotor(en): Prof. dr. ir. Jan Delcour, Prof. dr. Arno Wouters

The United Nations Population Prospects foresee that by 2050 the world population will reach approximately 10 billion. This population growth will increasingly put pressure on the global food system. One strategy to accommodate these concerns is to (partially) shift from animal to plant protein based foods. Production of the latter is more efficient in terms of i.a., cost, land use, and water use, and is overall more sustainable than production of the former. Good examples of plant protein sources are wheat and maize, which, along with rice, are the most cultivated grains worldwide. Prolamin fraction (gliadin and zein in wheat and maize, respectively) accounts for the majority of the total protein in these crops. These prolamins, however, possess low solubility in aqueous systems, which notably reduces their applicability in foods. One way to overcome this issue is to produce prolamin based nanoparticles (NPs) with a degree of colloidal stability in water. NPs based on gliadin and zein (GNPs and ZNPs, respectively) have potential as novel food additives, with possible applications e.g., stabilizing dispersions such as food emulsions and foams, traditionally stabilized by animal-sourced proteins.

Thus far, the ability of GNPs and ZNPs to stabilize foams has been investigated to a limited extent. However, it is known that foams sustained by GNP (constituents) have good stability in a narrow pH range. ZNPs, in contrast, have very limited foaming properties irrespective of the pH. Mechanisms underpinning the foaming properties of such NPs are still poorly understood. Finally, such NPs have only limited colloidal stability under food system-relevant conditions. This also limits their potential application in food systems.

Against this background, this doctoral dissertation had a two-fold aim. The first was to optimize the production of highly functional cereal prolamin NPs (based on gliadin, zein, or their blends) with good foaming properties and high colloidal stability under food system-relevant conditions. The second was to unravel the mechanisms underpinning the foaming properties of such NPs and thus, their structure-functionality relationship.

To improve the foaming properties of GNPs, their modification strategy based on the use of a crosslinking enzyme (microbial transglutaminase, TG) was optimized. This resulted in the enhanced functionality of such modified GNPs under conditions where non-treated GNPs stabilized foams poorly (pH 4.5). This was explained by the altered ability of TG-treated GNPs to mutually interact at the air-water (A-W) interface i.e., the boundary where the liquid phase meets the air phase (air bubble) in a foam. To ensure good foam stability, a coherent film must be formed at the A-W interface, which prevents, e.g., air diffusion from the bubble to the liquid. At pH 4.5, GNPs adsorb at the interface but have only limited ability to interact mutually to form this film. TG treatment induced the formation of strong links between the different GNP constituents. Consequently, a stronger interfacial film was formed by the modified GNPs, resulting in improved foam stability. TG treatment had, however, a (slightly) negative impact on GNP colloidal stability during prolonged storage and in the presence of salt. This was explained by the incorporation of chargeable groups from gliadin into the newly formed crosslinks during the enzymatic treatment. Another approach of GNP modification was based on gliadin co-precipitation with chitosan, a linear polysaccharide comprised of glucosamine units containing chargeable groups. This resulted in the formation of hybrid gliadin - chitosan (GCNPs), which possessed higher surface charges (at pH 4.5) than unmodified GNPs. This resulted in (i) worse foaming properties and (ii) notably improved colloidal stability of GCNPs during prolonged storage and in the presence of salt than those of GNPs at pH 4.5. Both effects were linked to the higher surface charge of GCNPs, resulting in their reduced mutual interactions at the A-W interface and in the suspension, respectively.

The previously reported poor foaming properties of ZNPs were hypothesized to result from structural changes in commercially available zein (CS-zein) occurring during its extraction and purification. To verify this, zein was extracted on a laboratory scale from maize flour (LS-zein). Such in-house extracted protein preparation was characterized by a relatively high zein peptide and maize lipid content compared to the highly pure CS-zein powder. LS-zein and CS-zein were then used for NP production (LS-ZNPs and CS-ZNPs, respectively). CS-ZNPs, similarly to what was reported earlier, possessed very poor foaming properties. The functionality of LS-ZNPs, however, was notably better but also pH-dependent. At pH 8.0 & 10.0, LS-ZNPs stabilized foams effectively, even if the formed interfacial films displayed only limited coherency at these pH values. This suggested that the formation of interfacial films was not the main mechanism underpinning the foam stability brought about by LS-ZNPs and other phenomena e.g., charge effects and synergistic effect of zein and lipid, resulting from the composition and structural properties of LS-zein. At pH 4.0, foam stability brought about by LS-ZNPs was lower despite the formation of a strong interfacial film. It was hypothesized that the lower functionality under such conditions was related to an anti-foaming effect of LS-ZNPs, the bridging de-wetting effect, which induced air bubbles merging and foam destabilization as a consequence.

Moreover, the foaming properties of NPs based on both gliadin and zein (GZNPs) were found to be intermediate to those of GNPs and ZNPs, and the mechanisms underpinning their functionality depended on the gliadin-to-zein ratio.

In conclusion, the work in this doctoral dissertation led to a better understanding of the mechanisms underpinning the foaming properties of cereal prolamin NPs. Moreover, a modification strategy of GNPs involving the use of TG was developed. The use of the enzyme notably improved their foaming properties at pH 4.5. In follow-up work, the foaming properties of such (TG-modified) GNPs in more complex matrices reflecting real food systems should be investigated. GCNPs, despite their poor foaming properties, hold great potential in applications for which high colloidal stability is necessary e.g., in the encapsulation of bioactive compounds. However, further investigation in this context would be, of course, required.