Thread by @ProfVigeant

New thread! Now that we’re on a #FoodSci roll, let’s talk rolls! #1/16

I want to use my rolls as an excuse to talk about my favorite #FoodSci & #Thermo concept, that’s right, it’s water activity (aw) time! 2/16

Brief recap: water activity is the thermodynamic activity of water in a mixture. It’s related to (but not exactly) the molecular-level concentration of water in the mix. It’s 1.0 for pure water, then less for mixtures, 0 for things w/o water #FoodSci 3/16

Water activity, aw, is important because it tells us about food safety, self-life, and informs overall tastiness. #FoodSci 4/16

For example, the microbes that can make us sick, like salmonella, can only thrive in certain high aw environments. That’s why cured meats can be safe without “cooking” - water’s been removed and replaced with salts & nitrates 5/16

For my rolls, aw is interesting primarily for what it tells us about freshness. #FoodSci 6/16

You see, the migration (diffusion) of water throughout a closed system is from high aw towards low aw, until aw is uniform everywhere (see prior tweets about garlic flavor evening out in dip) #FoodSci 7/16

In baked goods, you often want a lovely browned surface and a moist interior crumb. #FoodSci 8/16

The browned surface is from the Maillard reactions, which like T about 300F; this is why it’s a surface-thing mostly- b/c we drive off near y’all water so the reaction can happen 9/16 #FoodSci

That’s also why crust is crusty- there’s less water there, leading to chewy &/or crunchiness - and lower aw 10/16 #FoodSci

Meanwhile, the interior of the bread is much wetter, making it softer. You can see that it’s wetter from the steam that escapes when you break a fresh roll. Higher aw. 11/16 #FoodSci

Which leads to our freshness issue. Straight from the oven, the rolls have high-ish aw inside (maybe 0.85) and lowish (maybe 0.97) outside. 12/16 #FoodSci

This is a problem. There is now a driving force for water to migrate from the high aw area to the lower aw area. And this problem isn’t fixed by tightly wrapping the rolls in plastic, because both bits are in the roll! 13/16 #FoodSci

Solution? Well, eat the rolls Right Now! Diffusion is reasonably fast, check in with Prof. Einstein to work out how long this might take through rigorous calculation (or wing it - it’s a few hours, max). 14/16 #FoodSci

Because we can’t beat #Thermodynamics, eventually aw will even out and the crust will be soggy and the interior will be drier. Commercially, we slow this down with water-binding molecules like lecithins. But at home…… 15/16 #FoodSci

We just consume our rolls as soon as we can, and maybe re-toast them a little first to re-lower the surface aw. These will still be yummy tomorrow! Happy Thanksgiving everyone! 16/16 #FoodSci

@threadreaderapp unroll please!