[Enthalpy]

Hello dear friends!

I make a sauce with strong Dijon mustard (water, mustard seeds, vinegar, salt, citric acid, potassium metabisulfite) and Italian olive oil (oil of olives). Usually it goes very easily, and despite the concentrated mustard needs an important dilution, adding the oil in two steps suffices easily.

I tried to add all the oil in a single step and it failed. Long stirring with the fork couldn't get that little mustard in the oil. Adding more and more mustard (up to a first-step proportion I'd say) didn't help.

Do you have an explanation? Inverted emulsion, viscosity too low, needs air bubbles, others...?
Thank you!

[Borek]

No idea, but I do remember being not able to make a mayonnaise using a specific brand of oil (not olive oil though). Even blender didn't help.

[Irlanur]

was it out of the fridge? I remember reading a book ("Die Genussformel") where it was pointed out that mayonnaise can fail terribly with cold ingredients....

still this of course not an explanation...

[Furanone]

By adding the oil in slowly you are aiding in the homogenization of the oil droplets to smaller oil droplets with the high-shear mixing. The oil droplets get smaller in the mixing vortex by cavitation, and this maximizes surface area for the emulsifying proteins from the mustard to effectively coat the smaller oil droplets and stabilize them as an emulsion. Based on Stokes equation the smaller the oil droplets can be the slower they will separate to surface (also-- increasing viscosity of continuous phase (add xanthan), decreasing density difference of two phases (use 3% brominated vegetable oils), and decreasing effect of gravity (no centrifugation!) will all aid in stabilizing the emulsion).

By adding in oil all at once, the high-shear mixing is not as effective in breaking up the oil to smaller droplets (ideally you want 1-10 microns diameter) and now the proteins bind some oil but instead prefer forming aggregates with each other, no longer effective in coating the oil. Also the type of emulsifier used makes a big difference. A high shear-propeller system likely makes the least stable emulsions, a tissue homogenizer (Turrax blender) is better, next a high pressure piston-pump cavitation homogenizer is very good (most common in industry) while a microfluidizer would be considered the best emulsions with the smallest oil droplet sizes but these are more for small-scale lab applications and tend to get clogged often. It is useful to mention that your failed (separated) emulsion may be able to be saved by switching to a better homogenizer/emulsifier system where it would be inputting much more shear energy into system then your current mixer is.

Typically good emulsifying proteins have flexible conformations with an amphiphilic nature (charged hydrophilic amino acids on exterior with hydrophobic amino acids in core and with some properly placed cysteine residues but not too many!). This is why another trick in industry is to pre-mix at high shears using a Turrax homogenizer (at >10,000 rpm) the water system with mustard emulsifying proteins and soluble ingredients (no oil added yet) to help open up the protein’s conformation to aid in rapidity that these will be able to coat surface of oil droplets. Then oil is added slowly until all fully emulsified, and then for added stability this emulsion mix would be added to a piston-pump homogenizer and passed through several times. Measuring oil droplets with a Malvern Mastersizer would show oil droplet size distribution decreasing with each additional step but with greatest decrease going from Turrax to high pressure piston homogenizer.

This also explains why when making a salad dressing, the vinegar is typically the last ingredient added after the oil is effectively emulsified, since vinegar decreases pH and adds protons to negatively-charged amino acids making them more hydrophobic. By adding vinegar in before oil emulsification step, you have a more likely chance for same effect described above where emulsifying proteins become more protonated and more hydrophobic and prefer protein-protein interactions forming aggregates and thus when oil is added (even if added slowly) it may not be as effective in forming a nice stable film over the decreasing sized oil droplets with overall larger surface area, and your emulsion will be unstable and separate.

[Irlanur]

awesome:)

[Furanone]

Glad there are some questions on this forum that I can contribute to. Most questions are about organic chemistry, and unfortunately for me 'organic' means growing vegetables without pesticides!

[Enthalpy]

Very nice, thank you all!

The mustard comes directly from the fridge but the olive oil is at room temperature, both for the successful and failed attempts.

So with my fork at 60rpm, the fluid olive oil has too little viscosity to begin the emulsion with much oil. Adding oil in two steps is ze solution.

[Arkcon]

Or drop by drop is another method.  The very best such emulsions are made with a blender, the circular motion is what makes a stable mayonnaise.  Beating, by hand or by electric mixer, gives a sort of back and forth motion, which is going to produce less than perfect results.  I realize you're making a sauce, and not a mayonnaise, but there's no point in making it more difficult than it has to be.

[Corribus]

Very nice, thank you all!

The mustard comes directly from the fridge but the olive oil is at room temperature, both for the successful and failed attempts.

So with my fork at 60rpm, the fluid olive oil has too little viscosity to begin the emulsion with much oil. Adding oil in two steps is ze solution.

I make vinaigrette frequently using olive oil, vinegar (or citrus juice) and a dollop of mustard (ratio 1:1:0.25 or thereabouts), no complicated addition methods, just stir with a whisk for a few seconds. The emulsion usually holds pretty well, at least until the salad is gone. Even poured over hot vegetables, the emulsion usually doesn't break. Mayonnaise works better, though, no surprise.

[Enthalpy]

And what's the nature of the emulsion:
Oil droplets surrounded by water?
Or water droplets surrounded by oil?