FOOD90023 · Food Microbiology
Microbial Growth
Bacteria multiply by binary fission — one cell splits into two, two into four — so a population grows exponentially, and everything in food preservation is a fight against that exponent. Read the growth curve (lag → log → stationary → death) and you can see where the danger lives: the log phase. Learn the growth equation (N = N₀·2n and N = N₀eμt) and you can prove why time and temperature matter more than how clean you started. Map the factors — temperature (cardinal temps, the 4–60 °C danger zone, Listeria growing in the fridge), pH (the pH 4.6 C. botulinum limit that defines a low-acid food), and water activity aₜ (the 0.91 / 0.86 / 0.80 / 0.60 cut-offs) — and you know which lever each preservation method pulls. The exam tests this as a guaranteed calculation plus classic factor questions, and the hurdle concept ties them together.
What this chapter covers
- 01Binary fission → exponential growth (2ⁿ)
- 02The bacterial growth curve — lag, log, stationary, death
- 03The growth maths: N = N₀·2ⁿ, N = N₀e^(μt), generation time g and μ = ln2/g
- 04Temperature — cardinal temps, classes and the 4–60 °C danger zone
- 05pH — the pH 4.6 C. botulinum limit and low-acid foods
- 06Water activity (a_w) — the 0.91 / 0.86 / 0.80 / 0.60 cut-offs
- 07Intrinsic vs extrinsic factors and the hurdle concept
Worked example: the growth calculation, mark by mark
- +1(a) Divisions. n = t / g = 300 min / 20 min = 15.
- +1(a) Final count. N = N₀ × 2n = 100 × 215 = 100 × 32,768 ≈ 3.3 × 106 cells.
- +1(b) Growth rate. μ = ln 2 / g = 0.693 / 20 = 0.0347 min−1.
- +1(b) Exponential form. N = 100 × e(0.0347 × 300) = 100 × e10.4 ≈ 100 × 3.3 × 104 = 3.3 × 106 — agrees.
- +1Show the logs. log₁₀ N ≈ 6.5; quoting the log (not just the number) is where students drop a mark.
- +1(c) Conclude. Growth is exponential in μt, so 100× more starting cells only adds 2 logs, while extra time multiplies the count without limit — time and temperature dominate the initial dose.
Key terms
- Binary fission
- The way bacteria reproduce: a cell grows, copies its single circular chromosome and divides into two. Each round doubles the count, so after n divisions one cell becomes 2n cells — the source of exponential growth.
- Generation (doubling) time, g
- The time for one round of binary fission — for one cell to become two. It is read off the log phase of the growth curve and links to the growth rate by μ = ln 2 / g. A short g (≈20 min for E. coli) means rapid, dangerous growth.
- The growth curve
- The four-phase story of a batch culture: lag (cells adapt, count flat), log/exponential (constant maximum rate μ, most heat-sensitive, g read here), stationary (division = death, plateau, spores and secondary metabolites begin) and death/decline (viable count falls). Preservation is about extending lag and never reaching log.
- Water activity (aₜ)
- The free, available water in a food on a 0–1 scale (not total moisture). It is a key growth factor and preservation hurdle: most bacteria stop below ~0.91, Staphylococcus aureus can reach ~0.86, most moulds below ~0.80, and almost nothing grows below 0.60.
- Hurdle concept
- Stacking several mild preservation factors — modest heat, lower pH, reduced aₜ, chilling, preservatives — so their combined effect stops microbes that no single mild factor would. The synergy preserves the food while keeping more of its quality than one harsh treatment.
Microbial Growth FAQ
How do I get full marks on the growth calculation?
Follow a fixed routine: find n = t/g, apply N = N₀·2n, then either confirm with N = N₀eμt (using μ = ln 2 / g) or convert to logs — and crucially show the logs (report log₁₀ N), because the marker rewards the working, not just the final number. State the assumption that the cells are in log phase throughout, so your answer is a worst-case estimate. That set-up is the recycled Final Q7.
Why does Listeria break the usual 'keep it cold' rule?
Listeria monocytogenes is a psychrotroph — it grows slowly even at 0–4 °C. So refrigeration is not a kill step for it, and ready-to-eat chilled foods (deli meats, soft cheeses) can accumulate Listeria over a long shelf life. This is a recurring trap: 'the fridge stops bacteria' is false for the psychrotrophs.
What exactly makes a food 'low-acid', and why does pH 4.6 matter?
pH 4.6 is the limit below which Clostridium botulinum cannot grow or produce toxin. A low-acid food is one with pH above 4.6 (most meats, vegetables, dairy), which is why these foods need a full botulinum cook when canned — their pH alone will not stop the organism. Acid foods (pH ≤ 4.6, such as most fruits and pickles) are intrinsically protected from C. botulinum.
What is the difference between intrinsic and extrinsic factors?
Intrinsic factors belong to the food itself — pH, water activity, nutrients, natural antimicrobials, structure. Extrinsic factors belong to the storage environment — temperature, gas atmosphere, relative humidity. Preservation usually means adjusting both at once, which is the hurdle concept: several mild intrinsic and extrinsic factors combined.
Exam move
Make the growth calculation automatic first, because it is a guaranteed SAQ: find n = t/g, apply N = N₀·2n, confirm with N = N₀eμt using μ = ln 2 / g, and always show the logs and state the log-phase assumption. Then learn the growth curve cold — the four phases, what is happening in each, and that g is read in log phase. For the factor questions, memorise the three headline numbers (the 4–60 °C danger zone, the pH 4.6 C. botulinum limit, the aₜ cut-offs 0.91/0.86/0.80/0.60) with the organism each one gates, and keep Listeria-in-the-fridge ready as the standard trap. Finish by being able to define intrinsic vs extrinsic factors and the hurdle concept, which connect this chapter to preservation.
