A new deckhand on galley duty asks you why the sous vide chicken, held for two hours at 60°C, is fine, while the chicken mayonnaise sandwich left out for two hours is not. If you can answer that from the growth curve rather than from a rule sheet, you can design controls instead of merely enforcing them.
The bacterial growth curve
Bacteria multiply by binary fission. Given ideal conditions a single cell can become millions within hours.
| Phase | What is happening | What the supervisor wants |
|---|---|---|
| Lag | Cells adapting; no multiplication yet | Keep them here. Every control extends lag |
| Log | Exponential doubling — the dangerous phase | Never let food sit here |
| Stationary | Growth balanced by death; toxins may already be present | Too late — spoilage or toxin |
| Decline | Cells dying | Irrelevant: the toxin may remain |
Bacteria need FATTOM: Food, Acidity, Time, Temperature, Oxygen, Moisture. Remove or restrict any one and you interrupt the curve.
The levers, and what each one actually does
| Lever | Mechanism | Applied standard (widely taught — confirm against your FSMS) |
|---|---|---|
| Temperature — hot | Denatures proteins; kills vegetative cells | Core 75°C, or an equivalent time/temperature combination; hot-hold at 63°C or above |
| Temperature — cold | Slows or stops multiplication; does not kill | Chill at 5°C or below (an 8°C legal ceiling applies in some jurisdictions); freeze at -18°C |
| Cooling speed | Denies C. perfringens and B. cereus spores time to germinate and grow | Cool from hot to chilled within about 90 minutes; blast-chill, portion small, use ice baths |
| Time | Growth is a function of time in the danger zone | Limit total exposure; the danger zone is roughly 5°C to 63°C |
| pH / acidity | Below about pH 4.5 most pathogens cannot grow | Marinades, vinaigrettes, pickles, ceviche — but note acid does not reliably kill parasites or viruses |
| Water activity (aw) | Below about 0.85 growth stops | Cured, dried and salted foods; sugar and salt bind water |
| Atmosphere | Restricting oxygen selects for organisms; vacuum favours anaerobes | Vacuum and MAP extend shelf life but increase the C. botulinum risk — sous vide requires strict temperature and chill discipline |
| Competitive flora | Benign organisms out-compete pathogens | Fermented foods; not a control you should rely on |
Sous vide answers the deckhand's question: 60°C is above the danger zone and pathogens are being progressively destroyed. The sandwich sat inside the danger zone, and Staph aureus from hands may already have produced heat-stable toxin.
Combining hurdles
No single lever has to do everything. A cured, chilled, vacuum-packed product uses aw + temperature + atmosphere together. This is hurdle technology, and it is how you justify a critical limit: state the hazard, state the lever, state the number.
Onboard Notes
- If a control fails, ask which lever failed and why — not who to blame.
- Cooling is the lever yachts most often break, because a hot 8-litre pot in a small galley has nowhere to go. Solve it with equipment and portioning, not willpower.
- Write the number and the reason on the procedure. Crew follow rules they understand.
Scope note: xplor is not an approved centre or an accredited training provider. This lesson is preparatory study only - no certificate and no regulated qualification is awarded here, and a Level 3 supervising food safety award must be sat and assessed with an approved centre of a regulated awarding organisation.