July 22, 2013
gelée d’huile d’olive
(olive oil gelatins)
The professor flashed an equation on the screen. The audience erupted into applause. Not wild applause, but applause nonetheless. It happened every week. Sometimes the audience would applaud spontaneously, and sometimes they required a little prompting. The occasion on November 26th, 2012, was part of the Science and Cooking Public Lecture Series
that was just completing its third year at the Harvard School of Engineering and Applied Sciences. The equation was for the elastic constant, and was given in reference to polymer networks, in this case, gelatin.
What caught my interest was when Professor Weitz showed an animated loop of a clear, solid oil‑in‑water emulsion. Common oil‑in‑water emulsions like salad dressings and mayonnaise are neither clear nor solid. Why was this one clear? It turned out that two things were going on. To make the emulsion a solid, gelatin was added to the water. Gelatin will dissolve in water, but not in oil. The water solidified and trapped the oil droplets. That part was simple to grasp.
To make the emulsion clear, it was necessary to stop the oil droplets from scattering the light passing through the emulsion. That’s why we can’t see through mayonnaise. The light is scattered by the oil‑water interface. The scattering takes place because the refractive index
of the oil is different than that of the water. If the indices of the two materials were the same, the mayonnaise would become clear. To increase the refractive index for the water, sugar was added.
The recipe that Professor Weitz displayed was from the website Comme un lait fraise
(Like a Strawberry Milk). I’ve found a few others on the Internet, but they all seem much more complicated. Here’s the recipe with my clarifications:
cold, filtered water
finely granulated sugar
gelatin, 160 bloom strength, bloomed in cold water
extra virgin olive oil
1. Place the water, sugar, and glucose syrup in a saucepan over medium heat. When the sugar syrup starts to feel warm, drain the gelatin and add it to the saucepan. Gently stir until the gelatin is dissolved.
2. When the syrup comes to a boil, remove the saucepan from the heat. Whisk in the oil until an emulsion forms.
3. Pour into a container, and set in your refrigerator to gel.
If only life was so easy. When I went to cut the gel into cubes, it was like trying to work with hardened rubber cement: tough and sticky. When I did get a few pieces cut out in an almost orderly manner, they were too sticky to handle. Also, the gel was more translucent than clear. But all was not bad, it did have a nice flavor.
Out of frustration, I scraped as much of the gelled mixture out of the pan I had gelled it in, and transferred it to a rectangular plate whose edges were slightly higher than the center. I set the plate in my microwave and blasted it until it was bubbly. I then put the plate in my refrigerator and left the kitchen. (It was nap time!)
When I next checked the gelatin, it was set. It was still stiff and sticky. I took a small knife and made a couple of cuts in the gelatin. I peeled up a thin, 1‑cm (3⁄8‑in) wide by 5‑cm (2‑in) long strip. The strip was tapered at one end because of the way the gelatin filled the plate. I rolled the strip into a cylinder ending with the tapered end. The surface of strip was sticky enough so the cylinder easily held together.
The portion size was nice and the shape was pleasing, but the piece was still too sticky to handle without leaving fingerprints. These olive oil gelatins needed to be wrapped in something. They couldn’t exist naked.
Thinking back to the Botan Rice Candies I used to buy my kid, I decided to wrap these “candies” in a piece of obrato
(オブラート). This is the potato‑starch paper that I used for the “disappearing raviolis”
as few months back. I went back to the same store as before and bought a small packet of 200 round, flat sheets of the obrato.
I wrapped one of the cylinders once around with the obrato
and trimmed off the excess. I found out through trial and error that twisting the ends took patience. The ends would tear or easily untwist. Once the batch was wrapped, the gelatins seemed to be happy stacked together.
I watched the gelatins for a couple of weeks in the refrigerator. They seemed unchanged and showed no sign of syneresis. The flavor seemed to hold find. The translucence gave the impression of some clarity, but the effect of the surfaces was similar to glass used in bathroom windows. A lot of light seemed to pass through, but you couldn’t read a newspaper through them. I calculated that the concentration of the sugar was about 64%, and that the refractive index was about 1.45. It turns out that virgin olive oil has an index in the range of 1.4677 and 1.4705. Thus, the sugar solution used didn’t have the exact refractive index of the oil, but it was a lot closer than water, where the index is 1.33.
Addendum: I have since made the recipe with the sugar increased to produce a 72% solution. This should be enough to produce a refractive index in the middle of the oil refractive index range. The resulting mixture didn’t gel solid and was even more sticky than before. It was somewhat clear. I also used the emulsifying tip on my Bamex immersion blender so I could emulsify the water and oil without introducing any air, which may have been a problem with my earlier attempt.