Vol. XIII · Keeping Food Plate 1 of 10spoilage, microbes, enzymes, oxidation, moisture, shelf life, preservation, mould, staling, browningMS

Why Food Spoils.

Milk sours, bread stales, apples brown, oil turns rancid and crackers go soft. Four different processes, and four different cures.

Food is perishable for four main reasons. Microbes — bacteria, yeasts and moulds — feed on it, multiply and produce acids, gases, slime and off-smells; some are pathogens, most just spoil (XII.2). Enzymes left from the living plant or animal keep working: they soften fruit, brown cut surfaces (VIII.6) and turn meat and fish soft and fishy. Oxidation by the air turns fats rancid (IX.4) and fades colours. And moisture moves: crisp foods take up water and go soft, moist foods dry out, and bread stales as its starch recrystallises (VII.3).

Every preservation method in this volume works by removing something one of these processes needs. Cold slows all of them. Heat kills microbes and enzymes. Removing water (drying, salting, sugaring) stops microbes. Acid stops many microbes. Excluding air slows oxidation and stops moulds. The workshop compares how temperature changes the four for a few familiar foods.

Below: estimated keeping time of four foods, each limited by a different process, against storage temperature (teaching rates scaled from typical storage guidance by the temperature sensitivity of each process).

What you will learn

Name the four routes of spoilage.
Match a food to the process that limits it.
Explain why cold slows most spoilage.
Explain why bread is an exception.

Workshop

Teaching model: milk (bacterial souring, 10 days at 4 °C, scaled by the growth rate of a typical cold-tolerant spoiler, Q10 about 3); cut apple (enzymatic browning, an hour at 20 °C, activation energy 50 kJ/mol); cooking oil (oxidation, 12 months at 20 °C, 80 kJ/mol); bread (staling by retrogradation, course model: fastest near 4 °C).

Four clocks, four cures

In this model milk keeps about 10 days at 4 °C but 5 at 10 °C: microbial spoilage is very sensitive to a few degrees. Bread is the exception: in this model it stales about 3.0 times faster in the fridge than on the counter, because starch recrystallises fastest just above freezing; in the freezer it barely stales at all. So bread goes in a bin for a day or two and in the freezer for longer — never in the fridge (VII.3).

Read every keeping method as an attack on one of the four. The fridge slows microbes, enzymes and oxidation together. Blanching before freezing kills enzymes (XIII.3). Airtight tins keep crackers crisp and oils fresh. Salt, sugar and drying take away the water microbes need (XIII.4–XIII.6); vinegar and fermentation add acid (XIII.7, Vol. XIV). Canning kills everything inside a sealed container and keeps new microbes out (XIII.8). And all of them have limits: none makes spoiled food good again.

At the stove

Put three slices of the same loaf in the fridge, on the counter in a bag, and in the freezer. After two days toast none of them and compare: the fridge slice is the stalest.

Nicolas Appert won Napoleon's 12 000-franc prize in 1810 for preserving food in sealed, boiled bottles — fifty years before Pasteur explained why it worked.

Terms

Spoilage: Changes that make food unacceptable to eat.
Shelf life: How long a food keeps its quality and safety under stated conditions.
Enzymatic spoilage: Breakdown by the food's own enzymes.
Oxidative spoilage: Damage by oxygen: rancidity, fading.

Food spoils by microbes, enzymes, oxidation and moisture change. Each keeping method takes away what one of them needs.

Sources

A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.