Kinetic stabilization's effect on yogurt

Yogurt is a neat interplay of biology, chemistry, and physics. Physics is usually disregarded but I’ve found it to be a big contributor to the resulting texture of the yogurt.

We moved a couple miles and in the process I dusted off my sous vide water heater. How could I use this thing? I had tried to make yogurt before using an Instant Pot but the results were grainy and weepy. I wondered if I could achieve something more like Straus’s velvety yogurt:

The yogurt is slowly cultured and vat-set. Unlike other yogurts, which are incubated inside their plastic cups, we incubate our yogurt in stainless-steel vats and fill our recyclable plastic containers with cooled yogurt.

Presumably they use glycol-jacketed vats to heat, cool, and ferment the milk in bulk. My intuition is that by fermenting (and cooling!) with thermal envelopes you effectively eliminate kinetic disruptions (sloshing, convection, shearing). In my research, no yogurt recipe mentioned kinetic stability but I believe it’s a huge contributor to the resulting mouthfeel of the yogurt.

I don’t understand all the forces at play, but in brief, we’re transforming liquid (milk) into a gel by neutralizing the electrostatic charge of the proteins so they can bind into a three-dimensional mesh. As the milk transitions to a gel, those delicate connections can be irreparably ruptured by movement.

This graph below shows the general approach, where time and temperature is not as critical as minimizing agitation.

To create that environment at home, you need something like sous vide to uniformly heat and cool the gel with zero physical disturbances. In all of the subsequent batches I inoculate with 5% by weight. The only variance is time and temperature.

Batch 1

• 20 minute denature at 185°F
• 8 hours at 110°F

Beginner’s luck brought me a custard-like mass with very low whey separation (syneresis).

Batch 2

• 5 minute denature at 195°F
• 12 hours at 89°F

The goal of this batch was to encourage gel strength through aggressive denaturing and a more gradual drop in pH, allowing the proteins plenty of time to organize. Unfortunately at that low temperature the cultures threw off a bunch of long-chain sugars (exopolysaccharides) that resulted in an off-putting ropey texture.