Drying & Dehydration

Why this method

Drying preserves by removing water until microbes can no longer use what remains. The governing number is water activity (aw) — the fraction of water that is chemically available, on a scale from 0 to 1 — not percent moisture. Two foods at the same moisture content can have very different aw depending on how much sugar and salt bind the water. That is why salt curing and drying pair so well: salt lowers aw before the dehydrator even starts.

The thresholds are sharp. Most bacteria stop growing below aw 0.91; Salmonella and E. coli need at least 0.95; Staphylococcus aureus has the lowest pathogen threshold at aw 0.86; and C. botulinum stops near 0.93. The practical line is aw 0.85: at or below it, no pathogenic bacterium can grow or produce toxin, which is why the FDA exempts products controlled to aw 0.85 or less from the low-acid canning rules of 21 CFR Part 114. Mold growth stops below 0.70, and nothing microbial grows below 0.60.

Parameters

Parameter	Value
Shelf-stable line (pathogens)	aw <0.85
Mold-free target	aw <0.70
Dehydrator temperature (fruit/veg)	55–60 °C / 130–140 °F
Dehydrator temperature (jerky, after a cook step)	60–70 °C / 140–160 °F
Air drying	Cool, dry, well-ventilated; control humidity
Verification	Water-activity meter on finished product

Equipment

🌶️ Dehydrator with set temperature

🧮 Gram scale for moisture tracking

💧 Water-activity meter

🫙 Airtight storage jars

Workflow

Prep for even drying

Slice to a uniform thickness so every piece reaches target together. Thin, even slices dry predictably; thick ones leave a moist core that can spoil.

Pre-treat where it helps

A salt cure, sugar, or an acid dip lowers starting aw and shortens drying. This is where curing and drying stack as hurdles.

Dry at a controlled temperature

Hold the dehydrator at its set point. Too cool and the surface stays wet long enough to spoil; too hot and you case-harden — a dry skin sealing a wet interior.

Measure aw, do not guess

“Leathery” and “snaps when bent” are texture cues, not safety data. Cool a sample to room temperature and read aw on a meter. For unrefrigerated storage you must confirm <0.85.

Condition and store

Loosely jar dried fruit for a week and shake daily to equalize residual moisture; discard any jar that shows condensation. Store finished product airtight and dark.

Caution

Texture and “dryness to the touch” are not safety tests, and oven-drying does not sterilize. A shelf-stable claim for a dried meat or a low-acid dried food requires a validated process and a measured aw <0.85 — and shelf-stable dried/cured meats (jerky, biltong) are a refer-out process. Follow the USDA Complete Guide to Home Canning and NCHFP drying guidance. When in doubt, refrigerate or freeze.

Note

No meter? You can still dry for quality and short refrigerated storage, but you cannot make a room-temperature shelf-stability claim. Without an aw reading, treat the result as a fresh food: keep it cold.

See also

• Preservation hurdles — how aw combines with salt and acid
• Salt curing — the pre-treatment that lowers starting aw
• Smoking as preservation — drying plus antimicrobial phenols
• Water and minerals — the water side of the equation
• Safety first — when to refer out