Margot “ChemE Mom” Vigeant Profile picture
Apr 7, 2021 23 tweets 13 min read Read on X
Time for some #FoodSci tweets! There’s a new analytical device in the lab - a super cute water activity meter. Let’s take it for a spin and talk about my very favorite food science & thermodynamics topic: Water Activity (aw)! #FoodSci 1/21 Image
Water activity is molecular-basis effective concentration of water in a food (or in anything). “Molecular basis” means it’s not grams / liter or teaspoons/gallon, but it’s on the basis of number of molecules present (moles of water / total moles aka mole fraction). #FoodSci 2/21
“Effective” means it’s corrected to account for the ways certain molecules punch above their weight as it were, by interacting more or less than you’d expect based on their actual concentration. #FoodSci 3/21
We care about water activity in foods because it is an important predictor of shelf-stability. Foods with high water activity are particularly susceptible to molds and/or bacterial growth, #FoodSci 4/21
and we generally have to eat them quickly and keep them refrigerated or frozen. Pretty much any fresh fruit or vegetable or meat or fish has a high aw. #FoodSci 5/21
Foods with aw of 0.85 or below have a much better chance of being shelf-stable. In fact, one of the reasons salt & sugar are added to “processed” foods is so they will meet this metric and keep better. #FoodSci 6/21
Ex: a fresh strawberry has a aw of about 0.97, but strawberry jam has an aw around 0.85. See video here for the full details #FoodSci 7/21 :
We can estimate water activity for a food by looking at the masses of sugar/fat/etc on the nutrition label, calculating the number of moles of each component, then computing the mole fraction of water. #FoodSci 8/21
Or we can measure water activity with the appropriately named Water Activity Meter. It measures aw by measuring relative humidity of the air that’s in equilibrium with a given sample of food. It turns out, water activity is = relative humidity exactly! #FoodSci 9/21 Image
I happen to have a convenient chocolate creme sandwich cookie. Shall we test this all out? #FoodSci 10/21 Image
This cookie is crunchy, which is a hint that aw is going to be relatively low, probably 0.5 or less. It also happily keeps at room temperature without going fuzzy, another hint that aw is on the low side #FoodSci 11/21 Image
Using the nutrition label and some typical molecular weights, I’ve worked out the number of moles (a measure of the number of molecules) of each component. #FoodSci 12/21 Image
Working through the math, I calculate that I expect the aw to be about 0.20! That’s DRY #FoodSci 13/21 Image
And what does the aw meter say? Well, I made a measurement of the cookie and the filling separately. The cookie comes out at 0.24…… #FoodSci 14/21 ImageImage
And within error, the value for the filling is just about exactly the same, aw=0.25 #FoodSci 15/21 ImageImage
So wait - the crunchy cookie has the SAME water content as the creamy filling? Why do that? How is that even possible? #FoodSci 16/21
First, why: Any chemical will tend to migrate over time from regions of high activity to regions of low activity. At equilibrium, all components in contact will have the same activity. #FoodSci 17/21
SO If the cookie and the filling had different aw to start, as the cookie sat on the shelf, the water would migrate. If the creamy filling had a higher aw, over time some of that water would migrate to the cookie, and the cookie wouldn’t be as crisp anymore. #FoodSci 18/21
2nd, How: While actual whipped cream has a very high aw (it’s mostly water), “creme” fillings are almost entirely of fats & sugars. Very little water, in order to keep the cookie crisp. The filling is literally designed to have exactly the same aw as the cookie. #FoodSci 19/21 Image
(PS - this is almost certainly what’s behind that creme-filled cake having its legendary shelf-life - its components all have lower-than-you’d expect aw) #FoodSci 20/21
So there you go-we can get an estimate within ~20% of the expt value using only info from the nutrition label, avg mw, & a quasi-ideal solution assumption. & your favorite sandwich cookie should be fine without refrigeration, even though it’s got a creamy filling! #FoodSci 21/21
PS - Thank you very much to the Garman Fund at @BU_ENGR for purchasing the Water Activity Meter that makes this kind of exploration possible! Today’s activity was testing the machine for the first time. #FoodSci Image
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More from @ProfVigeant

Mar 14, 2020
Ok! I’ve been asked by @johndavidian to do a #PiDay tweet-storm on Key Lime pie and how it thickens without cooking! Strap in, #FoodSci ahead! Heads up, I'm about to disagree with Wikipedia en.wikipedia.org/wiki/Key_lime_… 1/29
Here’s a recipe. Something fun about key lime pie is that it’s kinda new, as the history of pie goes - it’s missing from my collection of midcentury cookbooks (this is Doubleday) 2/29 Image
The interesting thing about the recipe is that we are not using heat to set the filling. This is no-cook. YET we add 3 liquids together to make the filling (four if you count the food coloring). What? 3/29
Read 32 tweets
Nov 27, 2019
New thread! Now that we’re on a #FoodSci roll, let’s talk rolls! #1/16 Image
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 Image
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
Read 17 tweets
Nov 27, 2019
’Tis the day before #Thanksgiving, time for more #FoodSci tweets! Up today we have: Dinner rolls, two kinds of dip, finishing the pumpkin pie. Let’s get started! 1/14
Our lovely CSA (Johnny & Leah of Tewksbury Grace) gave us a *lot* of beets this year. We have no #Thanksgiving food traditions for beets, time to make new ones! #FoodSci 2/14 Image
Something that’s come up a lot in #FoodSci class this year is a suspicion of foods that arise from human activity - yes, processed foods, but also foods that have been created by humans such as GMOs. 3/14 Image
Read 18 tweets

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