🥫 Food preservation science chemistry

What you’ll learn

1. The Enemy Is AliveUnderstand that spoilage is active consumption by microbes and enzymes, and that all preservation removes something they need.Food doesn't decay on its own; it is eaten by microbes (bacteria, yeasts, moulds) and digested by its own enzymes. Every preservation method denies these eaters a necessity rather than adding a magic ingredient.
2. Take Away the WaterExplain water activity and the thresholds below which microbes cannot grow.Microbes need free (available) water, measured as water activity (aw). Most bacteria stop below ~0.91, yeasts ~0.88, moulds ~0.80, and essentially nothing grows below ~0.60. Total water content is less relevant than how much is free.
3. Salt, Sugar, and the Physics of ThirstExplain how salt and sugar preserve food through osmosis.Salt and sugar lower water activity by osmosis, pulling water out of the food and out of microbial cells until they shrivel (plasmolysis). They are not sinister chemicals but physics-based preservatives.
4. Slow the Clock with ColdUnderstand that cold slows but does not kill microbes, and how freezing differs.Chilling slows microbial and enzymatic reactions (about halving per 10°C drop) without killing microbes, hence the 'danger zone.' Freezing both stalls chemistry and locks water into unusable ice, but ice crystals damage texture.
5. Appert's Jar and Pasteur's ProofTrace how canning and pasteurisation use heat to kill and a seal to protect.Nicolas Appert invented canning in 1810 by trial and error, before microbes were known. Canning kills microbes with heat and seals out new ones; Pasteur later showed gentle heating (pasteurisation) could make milk and wine safe.
6. Draw the Acid LineUnderstand acidity as a preservative and the significance of the pH-4.6 safety line.Acid stops most microbes' enzymes from working. The logarithmic pH scale hides ten-fold steps. Below pH 4.6, Clostridium botulinum cannot grow, which is why high-acid foods can be water-bath canned and low-acid foods must be pressure-canned.
7. Preserving by Controlled RotExplain fermentation as controlled spoilage that protects food with self-made acid.Fermentation favours helpful lactic-acid bacteria that eat the food's sugars and produce acid, souring it below the safety line and excluding dangerous microbes. Salt starts the process; acid finishes it.
8. The Whole ToolkitSynthesise all methods as different ways of removing a microbial necessity, and reframe 'chemical' preservatives.Smoking (drying + smoke chemistry + salt) and modern vacuum/modified-atmosphere packaging attack water and oxygen. Every method removes a necessity; every preservative, natural or modern, is a chemical doing one of the same handful of jobs.

Questions this course answers

Grounded in trusted sources

• FDA, 'Water Activity (aw) in Foods' (Inspection Technical Guide)
• Food Safety Institute, 'Water Activity: A Key Factor in Food Preservation'
• USDA / UC Master Food Preserver Program, 'Water Activity and its Role in Food Preservation' (2025)
• K-State Research & Extension, 'pH Values of Common Foods and Ingredients'
• FDA, 21 CFR Parts 113 & 114 (low-acid and acidified canned foods)
• Nicolas Appert, 'L'art de conserver pendant plusieurs années toutes les substances animales et végétales' (1810)

Every Wunder lesson is built from real, reputable sources — never invented.

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