Vol. XIII · Keeping Food Plate 4 of 10drying, dehydrating, water activity, dried fruit, jerky, herbs, raisins, moisture content, 0.85, 0.6MS

Drying.

A kilogram of apples becomes about 170 g of dried apple rings. Nearly everything that left was water, and its absence is what keeps the rest.

Microbes need available water — water activity (III.9) — and each group has a floor: most bacteria stop near 0.91, Staphylococcus aureus grows to about 0.83 but makes its toxin only above about 0.86, most yeasts near 0.88, most moulds near 0.80 and the hardiest moulds and yeasts near 0.61. Drying takes food below those floors. Dried fruit at about 20 % water has a water activity near 0.6; jerky must reach 0.85 or below to be shelf-stable (USDA); crisp crackers and dried herbs sit lower still.

Drying is slow because water must travel from the inside to the surface and then evaporate, and each gram costs 2.26 kJ (VIII.2). Too much heat too soon seals the surface (case hardening) and leaves the inside moist. For meat jerky the USDA advises heating to 71 °C first or during drying, because drying alone at low temperature does not reliably kill pathogens. The workshop dries fruit.

Below: the weight left and the water activity of fruit as it dries to the water content you choose (exact mass balance; water activity from a teaching sorption curve for fruit), with the floors of the main groups of microbes marked.

What you will learn
Workshop
Mass balance: the dry matter is conserved, so final weight = dry matter ÷ (1 − final water fraction). Water activity against water content from a teaching sorption curve shaped on published isotherms for dried fruit. Floors for growth: most bacteria 0.91, S. aureus 0.83 (toxin 0.86), most yeasts 0.88, most moulds 0.80, xerophilic moulds and osmophilic yeasts 0.61 (ICMSF).

Take away the water, keep the rest

A kilogram of apples at 86 % water dried to 20 % leaves about 175 g; the rest was water, which took about 0.5 kWh of heat just to evaporate — before any losses. At 20 % water the fruit's water activity is near 0.6 in this teaching curve, below the floor of nearly every microbe, which is why raisins and dried apricots keep for months in a cupboard. Partly dried "soft" fruit at 30–40 % water is near 0.75–0.85 and needs preservatives or the fridge.

Dry at moderate heat with moving air: 50–60 °C for fruit and vegetables, slices of even thickness, turned now and then; condition dried fruit in a sealed jar for a week, shaking daily, so moisture evens out, and dry further if drops form inside. For jerky, use lean meat, heat it to 71 °C (74 °C for poultry) before or at the start of drying, then dry until it bends and cracks without snapping. Store everything airtight: dried food takes water back from humid air.

At the stove

Weigh a tray of thin apple slices, dry them in a low oven (about 60 °C, door ajar) for a few hours, and weigh again. Work out what share of the fresh weight was water.

Drying is the oldest preservation method: dried fish, meat and fruit are found at Neolithic sites, and Chinese 腊 foods, Andean chuño (freeze-dried potato) and Norwegian stockfish are all drying adapted to a climate.

Terms

Water activity
How available the water in food is to microbes and reactions (III.9).
Case hardening
A dried-out skin that traps moisture inside.
Conditioning
Holding dried food sealed so its moisture evens out.
Jerky
Thin strips of dried meat; heated first for safety.
Drying keeps food by taking water activity below microbes' floors (0.91 bacteria, 0.80 moulds, 0.61 the hardiest). Jerky needs heating to 71 °C as well.

Sources

  1. 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.
  2. USDA FSIS, "Jerky and Food Safety" and the Compliance Guideline for Meat and Poultry Jerky (2014): heat meat to 71 °C (poultry 74 °C) before or during drying; a water activity of 0.85 or below makes jerky shelf-stable.
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.