Vol. III · Water Plate 9 of 11water activity, aw, honey, jam, dried foods, microbes, preservation, sugar, saltMS

Water Activity.

Microbes need water they can use. Salt and sugar hold water so tightly that a food can be wet and still be a desert to bacteria.

Honey is about 17 % water, cooked rice about 70 %. Rice left out for a day can make you ill (XII.9); honey keeps for years. The difference is not the amount of water but its water activity: how free the water is to evaporate, to react, and to be used by microbes. Pure water has an activity of 1; a food whose water is tied up by dissolved sugar or salt has a lower one, and below certain thresholds microbes cannot grow at all.

Most bacteria need a water activity above about 0.91; Staphylococcus aureus can grow down to about 0.86; moulds and yeasts can manage 0.80 or lower; almost nothing grows below 0.60. The U.S. Food Code treats foods below 0.85 as not needing temperature control for safety on that ground alone. The workshop mixes sugar and salt into water and places the result on that scale.

Below: the water activity of a solution with the sugar and salt you choose (Norrish's equation for sucrose, Robinson–Stokes data for salt, multiplied as Ross's rule allows), against the limits for microbes.

What you will learn
Workshop
Water activity of the solution, and the approximate lowest water activity at which each group grows (ICMSF 1996, rounded). The Food Code's 0.85 line is marked.

Wet, but a desert

A syrup with 65 % sugar — about the sugar content of a traditional jam — has a water activity of about 0.865, near the 0.85 line that the Food Code uses; honey, at about 82 % sugar, comes to 0.633, low enough that only a few specialised yeasts can grow, which is why it keeps. A 20 % salt brine sits at 0.839. These are the numbers behind the oldest ways of keeping food: salting fish and meat, candying fruit, boiling jam, drying (XIII.4).

Water activity also explains texture. Water moves from a food with high activity to one with low: crackers go soft beside a moist cake, the crust of a loaf softens as water migrates from the crumb, and a sugar-dusted doughnut weeps. And it is not the whole story of safety: a low-sugar jam relies on its acidity and on being sealed hot (XIII.6), and some pathogens survive (though they do not grow) at low water activity, so dried foods are not automatically safe once rehydrated.

At the stove

Put a plain cracker and a slice of moist cake in a closed box overnight. In the morning the cracker is soft and the cake slightly drier at its surface: water moved from high water activity to low.

Medieval and early modern Europe preserved fruit as "suckets" and marmalades boiled stiff with sugar; China preserved vegetables in salt and soy and fruit in honey and sugar long before anyone could measure water activity.

Terms

Water activity (aw)
The vapour pressure of water in a food divided by that of pure water: 1 for pure water, lower when water is bound.
Bound water
Water held by dissolved substances or food structures so that it cannot freely evaporate or support microbes.
Humectant
A substance such as sugar, salt or glycerol that lowers water activity.
Moisture migration
Water moving from parts of a food with high water activity to parts with low.
Microbes need free water. Sugar and salt bind it, so a food can be wet and still too dry for most bacteria.

Sources

  1. R. S. Norrish, "An equation for the activity coefficients and equilibrium relative humidities of water in confectionery syrups", J. Food Technol. 1, 1966 (K = −6.47 for sucrose).
  2. U.S. Food and Drug Administration, Food Code 2022 (sections 3-401.11, 3-403.11, 3-501.14, 3-501.16, 3-402.11 and the definition of time/temperature control for safety food).
  3. ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens, Blackie, 1996 — growth limits (minimum temperature, pH and water activity) and typical heat resistance; the course rounds them.