Difference Between Fermentation and Pickling

FOOD PROCESSING AND PRESERVATION: FERMENTATION AND ACID

Figure 1: Fermentation makes its own acid slowly, while pickling adds vinegar at once. The four boxes are steps in order, so the arrows show a real sequence.

Introduction

Fermentation and pickling both preserve food with acid, but the acid comes from different places. In fermentation, friendly microbes, usually lactic acid bacteria, turn sugars in the food into lactic acid, so the pH falls slowly over days or weeks. In pickling, the acid is added directly, as vinegar, so the pH drops below 4.6 at once, in hours instead of weeks. Fermentation is slower, but it brings flavour, texture, and a living population of friendly microbes; pickling is fast and predictable, with the sharp taste of vinegar.

Quick comparison

Table 1: Fermentation compared with pickling

Scientific explanation

Definition

Fermentation is a process in which microbes turn sugars into acid, alcohol, or gas, changing the food and preserving it. Lactic acid fermentation is the most widely used food fermentation: lactic acid bacteria, a group of harmless bacteria, turn sugars such as glucose, C₆H₁₂O₆, and lactose into lactic acid. Pickling is putting vegetables in vinegar, usually with salt, sugar, and spices, so the acetic acid in the vinegar drops the pH below 4.6 at once.

Working principle

Most bacteria grow best near neutral pH and slow down as food gets more acid. Below pH 4.6, the spores of Clostridium botulinum cannot grow or make toxin, which is why that number decides canning processes too. Acid works in two ways: it makes the inside of a microbial cell harder to keep steady, forcing the cell to spend energy pumping acid out, and the undissociated form of weak acids such as acetic acid and lactic acid can pass into the cell and release acid inside it, which is more damaging than the pH outside alone. In fermentation, salt is added first, usually 2 to 3 percent for vegetables, which draws water out of the food by osmosis, lowers the water activity slightly, and guides which microbes grow: salt-tolerant lactic acid bacteria thrive, while many spoilage bacteria are held back. Then the lactic acid bacteria grow, make acid, and the pH falls from near neutral to about 4.0 or below. Once the pH is low and the acid is high, the food is stable. In pickling, the vinegar does the same job at once, and the food is then usually heated gently and sealed, so the heat and the acid work together.

Apparatus

Fermentation needs a clean jar or vat, a weight to keep the food under its brine, and a warm place for the bacteria to grow. The bacteria can come from the food itself, as in sauerkraut, where the bacteria live naturally on the cabbage leaves, or from a starter culture, a known strain added on purpose, as in yoghurt and cheese. Starter cultures give a faster, more predictable fermentation. Pickling needs jars, vinegar, salt, sugar, and spices, and usually a gentle heating step before sealing.

Measurement procedure

Both methods are proved by measuring pH, not by hoping. A fermentation that stalls above pH 4.6 is not a preserved food, it is a risk, and it is discarded or fixed by a safe route, never by hoping. A pickle that is not acid enough is not safe, whatever the recipe said. The salt level is measured, not guessed: too little salt lets the wrong microbes grow first, and too much salt stops even the lactic acid bacteria.

Factors affecting readings

Salt level, temperature, and air all change a fermentation. Too little salt lets spoilage bacteria grow first, making slime and bad smells. Too much salt stops the lactic acid bacteria, so the pH never falls. Too cold, and the bacteria grow too slowly; too hot, and they die or make off-flavours. Air on the surface lets yeasts and moulds grow, forming a white film called kahm yeast, which is usually harmless but tastes bad. A slow fermentation is dangerous because, while the pH is still high, harmful microbes have time to grow. The strength of the acid also depends on pH: at pH 4.0, 85 percent of acetic acid is in the whole, membrane-crossing form, but at pH 5.5 only 15 percent is, so the same vinegar is about six times stronger as an antimicrobial at pH 4.0 than at pH 5.5.

Formulas and calculations

The Henderson-Hasselbalch equation gives the fraction of whole, active acid: pH = pKa + log₁₀([A⁻] / [HA]), and the fraction that is whole is α = 1 / (1 + 10^(pH – pKa)). Acetic acid has pKa 4.76.

Worked example 1: acetic acid at pH 4.0. pH – pKa = 4.0 – 4.76 = -0.76, so 10^(-0.76) = 0.174, and α = 1 / (1 + 0.174) = 0.85, meaning 85 percent of the acid is in the whole, membrane-crossing form.

Worked example 2: acetic acid at pH 5.5. pH – pKa = 5.5 – 4.76 = 0.74, so 10^(0.74) = 5.50, and α = 1 / (1 + 5.50) = 0.15, meaning only 15 percent is whole.

Worked example 3: fermentation stoichiometry. In homofermentative lactic fermentation, one glucose gives two lactic acids: C₆H₁₂O₆ → 2 C₃H₆O₃. By mass, 180 g of glucose gives 180 g of lactic acid, since each lactic acid weighs 90 g, so the sugar is fully converted to acid with nothing lost as gas. In heterofermentative fermentation, one glucose gives one lactic acid, one ethanol, and one CO₂: C₆H₁₂O₆ → C₃H₆O₃ + C₂H₅OH + CO₂. Half the sugar’s mass leaves as gas, which is why heterofermentative sauerkraut bubbles and why its acid yield per gram of sugar is lower.

Industrial applications

• Fermentation: yoghurt and cheese plants, sauerkraut and kimchi makers, sourdough bakeries, fermented sausage lines, and soy sauce and fish sauce makers, where salt and fermentation work together over months.
• Pickling: vinegar pickle lines for cucumbers, onions, and mixed vegetables; pickled fruit and chutney lines where a fast, predictable acidity is wanted.

Advantages and limitations

Fermentation, advantages: preserves and improves the food at the same time, adding complex flavour and live cultures; needs no added acid or heat; the friendly microbes crowd out rivals. Limitations: slow and less predictable; needs the right salt and temperature; a slow or stalled fermentation above pH 4.6 is a risk and must be discarded.

Pickling, advantages: fast and predictable, with the same acidity every time; simple to control; the sharp vinegar flavour suits many vegetables. Limitations: flavour is the sharp taste of vinegar, with less complexity; no live cultures; the pH must still be measured, because a pickle that is not acid enough is not safe.

FAQ

What is the main difference between fermentation and pickling? Vinegar pickling adds acetic acid directly and is fast and predictable, while fermentation lets lactic acid bacteria make lactic acid slowly, which also adds complex flavour and live cultures.

Why is pH 4.6 a key number? Because below pH 4.6, the spores of Clostridium botulinum cannot grow or make toxin, so acid foods below that pH are safe from botulism.

What does the salt do in sauerkraut making? It draws water out of the cabbage by osmosis, lowers water activity slightly, and guides the microbes so that salt-tolerant lactic acid bacteria grow while many spoilage bacteria are held back.

Why is a slow fermentation more dangerous than a fast one? Because while the pH is still high, harmful microbes have time to grow, so the food is not yet preserved.

What is the white film on a ferment? It is likely kahm yeast, which grows where air meets the brine. The maker should skim it off, keep the food under the brine, and check that the pH is low.

References and further reading

FTN subjects: Principles of Food Preservation, Unit 5 (Fermentation and Acid); Principles of Food Preservation, Unit 2 (Preservation by Heat), for the pH 4.6 line. Books used: Hui, Y. H., Meunier-Goddik, L., Hansen, A. S., Josephsen, J., Nip, W. K., Stanfield, P. S., and Toldra, F., Handbook of Food and Beverage Fermentation Technology; Frazier, W. C., and Westhoff, D. C., Food Microbiology; Adams, M. R., and Moss, M. O., Food Microbiology; Fellows, P. J., Food Processing Technology: Principles and Practice.