Water Activity is the first answer to every food science frustration...
...followed closely by "Maillard Browning". As a food scientist and especially a "candy scientist" it is critical to have a solid understanding of both the starting water activity of each of our ingredients and the end target; along with a decent prediction of how our product whether an ingredient or finished good will change when these expected values drift. Water activity (often abbreviated as “aW”) only dictates a portion of the overall situation.
Low water activity confectionery components such as crisp wafers or glassy sugar hard candy will typically pull water from high humidity environments or neighboring components. The opposite scenario is also expected where higher moisture components will lose water in environments with lower relative humidity. We see this reaction in our home kitchens, but the end result is often dependent on the region, weather, and time of year. The rate of change is follows the potential for water molecules to transfer to lower energy state in low water activity regions.
Looking at this map, it would appear that the space between water activity 0.2 – 0.3 as the sweet spot of minimal activity. In this range, lies the most stable conditions for long term storage. Enzymatic, oxidative, and browning reactions are at their slowest and the water activity is much too low for microbial growth. Texturally this region is a barren desert, inhabited by few--hard boiled candy, crispy cookies, and maybe 30 year canned food storage.
In preparation for writing this I was able to correct a few of my own inaccuracies in trying to illustrate these concepts. For instance, it is easy to condense all “water activity” talk into a single number—water activity 0.3, check!—when in the next sentence we should remember that at various temperatures moisture pickup or release follow different rates of change. A practical application observes a peanut brittle readily uptake environmental moisture, now sticky peanut taffy will not yield that moisture as readily when relocated to a drier environment. Enter water activity to center stage.
For a deep dive into the mechanics of generating a personalized moisture isotherm, specifically calculating the hyperbolic and sigmoidal curves of water activity at various temperatures, I would recommend looking to previous Candy Technologists presentations by Dr. Anthony Fontana or Wendy Ortman. Find the proceedings library at http://aactcandy.org/frame_search.asp.
If you have a moisture isotherm (correlating water activity to temperature) of each component of a low or intermediate moisture food such as the interior and exterior of a product then you should be able to predict the direction the moisture will transition at every potential (warm versus cool) temperature. I see this as the distinguishing difference between a pastry chef making a delicate dessert that will be served within a few hours or days, and the technical confectioner’s role to extend brand distinguishing textural nuances over months and years. As a technical confectioner, our situational awareness of water activity is decent and eventually becomes second nature. For instance, depressing the relative humidity in a chocolate factory to prevent sugar bloom is second nature; so much so, that when confronted with a bleaching dark chocolate I admit having been deceived first assuming the chocolate was poorly tempered. Totally not the case at all!
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Reprinted from presentation by Brandon Jahner, CFS given at American Association of Candy Technologist 2017 National Seminar - Oak Brook, IL
Image Credit: Water Activity Stability Diagram - Adapted from Labuza, T.P. 1970