Hi Yula,

Zeta-potential is a property of a particulate species rather than a dissolved molecule. Since anthocyanins are generally dissolved species I would noet expect them to have a measurable zeta-potential. Perhaps you mean the overall electrostatic charge of the molecule? If this is so I would recommend using the pKa and the Henderson-Hasselbalch equation as a first estimator of their charge. Anthocyanins are tricky though as there are hundreds of known species that have slight differences in structure adn properties and if derived from plant sources they are generally mixtures of compounds in which the ratio of the different species ranges not only depending on their source but also the stage at which the e.g. fruit was harvested.

The other difficulty with anthocyanins is that they undergo a number of chemical cahnges when varying the pH from low to high or vice versa; hydration and dehydration is seen (please see Article The Effect of pH on Color Behavior of Brassica oleracea Anthocyanin

I recommend determining the pKa of your sample to estimate the charge present. How this can be done is shown here: http://mucc.mahidol.ac.th/~scess/2011_scch109/pKa.pdf

Hope this helps, kind regards, Leo

Leo Vleugels thanks so much for your answer! I am looking specifically at the change in zeta potential of an anthocyanin solution (specifically 3-deoxathocyanins) with the addition of hydrocolloids. The 3-deoxys precipitate within an hour of addition to an aqueous solution so would they still be considered dissolved? Further, is ZP not applicable to dissolved hydrocolloid solutions?

Thanks so much for your help.

The presence of a hydrocolloid is important to understanding what might be happening.

If you measure zeta potential via light scattering (Zetasizer, ZetaPALS etc) then it requires substance(s) that scatter the laser light. A true solution won't do this but a colloid will. As Leo Vleugels explained, zeta potential is a concept related to colloids and not solutions.

What is your hydrocolloid? Is it at a sufficient concentration to form a gel?

If the hydrocolloid is negatively charged then decreasing the pH could neutralize it and cause precipitation. It could be that the anthocyanin just happens to be there and is acting as an indicator of the change in the nature of the hydrocolloid!

What you would need to do is measure the "zeta potential" of just the hydrocolloid at different pHs. I put the term in quotes because a gel won't move in the same way as a dispersed particle but the instrument may still detect vibration due to the electric field. Typically, this would require using the so-called phase analysis mode (also known as fast field reversal and monomodal). Not seeing a difference will not rule out the mechanism but a difference will support it.

In addition, you can simply observe the visual appearance of the hydrocolloid as a function of pH. Shine a laser pointer through the sample and see if there is an obvious change. This will help determine if it is simply the hydrocolloid changing its physical properties.

Another possibility is that the anthocyanins may become protonated at low pH leading to a positively-charged polyions. These may act as electrolytes that - if at sufficient concentration - could destabilize the hydrocolloid. What is the molar concentration of the anthocyanin? I would guess that if it is above a few tens of millimolar then this could happen.

I don't known enough (okay - anything) about the solution behavior of anthocyanins. My suggestions above are based on general concepts and may be irrelevant in your case.

Thanks so much for your answer, John Francis Miller ! You've given me a lot to think about and investigate.

For more background, the objective of my research is to show how pectin is stabilizing (reducing precipitation) of the 3-deoxanthocyanins in an aqueous solution. I have been using a Zetasizer with pectin concentrations of 0.1, 0.5, and 1 g/L at pH 3 and 5. The solutions do not gel at these concentrations.

With the laser, are you suggesting measuring refractive index?

I appreciate your answer but need to think through things more before continuing the conversation!

Hi Jula, next to John Francis Miller 's excellent explanation, I would like to point out that pectin, being a polyelectrolyte, does not have a fixed pKa it has an apparent pKa . In recent work I did, where I investigated the use of a dye (TBO, ortho-toluidine-blue) to determine the pKa of hydrocolloids, it showed that pectin at low concentration and low ionic strength has an apparent pKa of 4.6. Which means that at pH= 3 it is uncharged and may indeed precipitate, at pH=5 it bears a bit more than half of its charges and may still be dissolved. Raising the ionic strength in the pectin solution lowers the apparent pKa but not below 3.6 which is the literature value for high ionic strength. So, in any case at pH=3, the more favorable pH for Anthocyanin stability, pectin is always mostly uncharged and thus badly soluble. Since TBO resembles anthocyanins (only is far more stable) I advice you to read the included publication. It shows the concept of pKa and also looks at interactions of dyes with hydrocolloids. If you want to stabilize anthocyanins by the use of their interaction with hydrocolloids, I would recommend using kappa-carrageenan which has an apparent pKa of 1.2 and is fully charged at pH= 3 and 5.

Kind regards, Leo