Pea-protein emulsion stability – a microfluidic analysis

The experiment showcased in this short review is originally based on the research article “Microfluidic investigation of the coalescence susceptibility of pea protein-stabilised emulsions : Effect of protein oxidation level” written by Emma B.A. Hinderink, Wael Kaade, Leonard Sagis, Karin Schroen, and Claire C. Berton-Carabin. The article was published in 2019 in the Food Hydrocolloids Journal. It explores the emulsion stability of pea-protein-stabilised droplets, when made at a range of different protein concentrations, adsorption times, and degree of protein oxidation with the precision of a pressure-driven flow controller and droplet-based microfluidics.

Abstract

Proteins are used to stabilize oil-in-water emulsions. Plant proteins are getting more and more attention as a functional ingredient due to their higher sustainability potential compared to conventional dairy proteins. So far, the emulsification properties of plant proteins are not well understood. Furthermore, the protein physicochemical status depends on their production process. 

In this study, the soluble fraction of a commercial pea protein isolate is used to stabilise O/W emulsion droplets formed in a microfluidic device. The coalescence stability is recorded after droplet formation for different protein concentrations (0.1–1 g/L). It was found that for the shortest adsorption times droplets were unstable, whereas for longer adsorption times, differences in coalescence stability were detected. In addition, metal-catalysed oxidation of pea proteins performed for up to 24 h, prior to emulsion formation and analysis, increased the coalescence stability of the droplets, compared to fresh pea proteins. It is clear from the study that the emulsifying properties of pea are strongly dependent on their chemical status, and associated structural properties at the molecular and supramolecular levels. The microfluidic device used in this experiment successfully captures such effects, at time scales that are relevant to industrial emulsification.

This microfluidic investigation highlights the dependence of the emulsifying pea proteins properties upon their chemical status and associated structural properties at the molecular level.

Introduction

Food products, for example dressings, mayonnaise, ice-creams are all oil-in-water (O/W) emulsions. Oil in water emulsions are dispersions of oil droplets in an aqueous phase. In standard procedures, large-scale homogenisers are used to make emulsions that disperse the oil phase into the continuous water phase, at the expense of considerable energy. The industrial homogenisation process generally involves a combination of droplet break-up and rapid droplet re-coalescence in-case the droplets are not sufficiently stabilised, with both phenomena taking place within a short time interval.^[1] There is a lack of  understanding of how droplet formation and re-coalescence are related because under such high-shear conditions and short time scales, it becomes difficult to make quantifiable measurements and draw conclusions.

Dairy proteins are the most widely used emulsifiers because of their ability to stabilise emulsion droplets, and add nutritional quality of the product. ^[2,3]  

Studies need to be conducted on these typical time scales to improve researchers’ understanding of the various phenomena at stake in emulsion stability. Thus, new emulsion formulations could be better developed based on various raw materials. 

Such experiments will be valuable for current change in the emulsion formulation industrial field that promotes the use of greener emulsion stabilizers such as plant proteins. In that regard, plant proteins such as pea and soy are gaining interest, although pea proteins have an advantage thanks to their low allergenic potential and their high nutritional value.^[4]

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Aim and objectives

• To analyse and determine the coalescence stability of pea protein-stabilised droplets.
• To study the effect of parameters such as pea protein concentration, protein adsorption time and degree of protein oxidation on pea protein emulsion stability.
• To relate these macroscale phenomena to the pea protein interfacial properties evaluated by atomic force microscopy (AFM).
• To better adapt the formulation of pea protein emulsion to their emulsification process.

Key findings

Materials and methods

The materials, protein samples, microfluidic chips, devices and connectors used in the experimental setup, along with specific configurations for this study have been listed below:

• PPI (6 wt%) was dispersed in a 10 mM phosphate buffer (pH 7.0) and stirred for at least 24-h at 4℃. The soluble protein concentration was established using a bicinchoninic acid.
• Oil droplets were generated in a custom-designed borosilicate T-junction glass microfluidic chip (Fig 1) and the coalescence stability was studied in the coalescence channel.
• The flow control of the various solutions was finely tuned with a pressure-driven flow controller (Elveflow OB1, Mk3) in combination with a mini-Coriflow sensor. 

Non oxidised pea proteins: coalescence stability of emulsion

The coalescence stability of emulsion droplets stabilised by the soluble fraction of fresh pea protein (PPI) was measured for the protein concentration of 0.1-1 g/L.

The initial droplet size was independent of the protein concentration used, for all levels of concentration. This result suggests that the interfacial tension at the droplet formation timescale is constant for all concentrations used during the experiment.

Taking into consideration the results obtained when using an adsorption range of 100 ms (Fig 2), the coalescence frequency was lower in the beginning of the channel compared to the end of the coalescence channel. This shows that the coalescence occured immediately for the low concentrations used during the experiment.

When meandering channels are used corresponding to adsorption times of 11-173 ms, a decrease in coalescence frequency is noted upon increase of adsorption time. (see Fig 3). Interestingly, at the adsorption times of 11 and 31 ms, droplets coalescence occurred immediately for all protein concentrations. 

A short summary of the obtained results is available in Table 1. From these values, it was showed that pea proteins require a higher concentration or longer adsorption times to form stable emulsion droplets compared to dairy proteins in a previous study.^[5]

For further insight into the effect of pea protein oxidation, please refer to the original paper available on this link.

Conclusion

Within this study, the short-term coalescence stability of emulsion droplets stabilised by soluble pea protein fractions was assessed. Higher pea protein concentrations were needed to prevent rapid re-coalescence of oil droplets compared to whey proteins. The oxidation of proteins caused an increase in re-coalescence stability, which was related to protein fragmentation, and also resulted in a structurally more homogeneous interface compared to non-oxidised proteins. This explains the improved emulsion stability to re-coalescence. From this study, a reasonable conclusion can be made that the oxidative state is very relevant for plant protein functionality, which should be an integral consideration during product design.

In this study, droplet-based microfluidics was employed as means to generate pea-proteins emulsions in a very reproducible and fast manner. The use of pressure-driven flow controlled microfluidics allowed the authors to produce droplets in a precise and systematic way.

References