The study, published in Journal of Agriculture and Food Chemistry, reveals that certain common food flavonoids may act as “excellent stabilizers” of oil-in-water emulsions. Researchers from the University of Leeds explained that the compounds stabilise the emulsions through their adsorption as water-insoluble particles to the surface of the oil droplets – inducing the formation of an emulsion.
“We demonstrate that many flavonoids, present as water- and oil-insoluble particles, have a tendency for accumulation at the Oil−Water interface and in some cases can provide very good stabilization of oil-in-water (O/W) emulsions,” said the authors, led by Brent Murray, Professor of Food Colloids at the University of Leeds.
Emulsifiers work by stabilizing oil suspended in water, binding the two together and creating a stable mixture.
Despite experimental results indicating that flavonoids may have very poor water solubility, under certain circumstances, such water-insoluble materials can show “a strong preference for adsorption at the oil−water (O−W) interface and indeed can act as stabilizers of oil−water emulsions.”
Such emulsions – known as Pickering emulsions – are characterized by the accumulation of particles at the oil−water interface, forming a densely packed layer around the emulsified droplet, which prevents shrinkage or coalescence.
Currently it is commonplace for surfactants to be used as stabilizing agents for emulsions and foams. However, there is a limited choice of food-grade surfactants, and there are strong links between surfactants and environmental damage.
But unlike surfactants, the particles involved in Pickering emulsions are usually much larger than individual molecules.
“In other words, there is a huge energy barrier to droplet shrinkage or coalescence, and the emulsions are very stable,” said the authors.
The use of solid particles such as proteins is a well documented alternative to the use of surfactant emulsifiers; however the use of small bioactive compounds such as flavonoids as an alternative remains relatively unexplored.
The authors screened a wide range of flavonoids for their emulsification behaviour, and noted that it is difficult to predict how good an emulsifier the flavonoid will be, based on the molecular structure of individual flavonoids, or on the partition coefficient of the flavonoid (the ratio of concentrations of a compound in the two parts of the mixture).
“The reasons for this unpredictability are probably due to the different size, shape, and surface charge properties of the flavonoid crystals or their aggregates, which will affect their contact angle and adsorption energy with the oil−water interface,” said Murray and co-workers.
The authors found that five flavonoids (K7Neo, Q3R (rutin), N7Neo(naringin), N7R, and tiliroside) provide good emulsion stability.
They noted that all of the flavonoids (with the exception of tiliroside) have partition coefficients close to zero.
“This highlights even more clearly the unique position of tiliroside, which is very insoluble in water but acts as a moderately good emulsifier,” added Murray and colleagues.
They explained that the practical significance of the findings “remains to be seen” because the emulsion droplets formed in the process “can be quite large, due to the large size of the flavonoid particles.”
They said that although the emulsions formed are very stable, there may be better, more practical food oil emulsifiers.
Source: Journal of Agriculture and Food Chemistry
Published online ahead of print, doi: 10.1021/jf1041855
“Particle-Stabilizing Effects of Flavonoids at the Oil−Water Interface”
Authors: Z. Luo, B.S. Murray, A. Yusoff, M.R. A. Morgan, M.J. W. Povey, A.J. Day