Principles and Methods of Food Preservation
What this domain tests
The exam asks you to state the classical principles of preservation, place a given method under the correct principle, and explain why a combination of methods is used.
Core principles
- Prevention or delay of microbial decomposition: asepsis (clean handling, sterile packaging), removal of microorganisms (filtration, centrifugation), hindering growth (low temperature, drying, chemicals, anaerobic storage) and killing (heat, irradiation).
- Prevention or delay of self-decomposition: inactivating enzymes by blanching or heat, preventing oxidation with antioxidants or sulphites, and avoiding mechanical damage.
- Prevention of damage by insects, rodents and mechanical causes: proper packaging, storage and handling.
Hurdle technology
Leistner's hurdle concept states that several mild hurdles together give stable, safe food while keeping quality high. Each hurdle alone may be sub-lethal, but the organism cannot overcome all of them. Common hurdles are low pH, reduced aw, heat, preservatives, low temperature and competitive fermentation.
Methods
- Physical: heat, cold, drying, irradiation, filtration, high pressure, pulsed electric field.
- Chemical: salt, sugar, acid, permitted preservatives, smoke.
- Biological: fermentation with lactic acid bacteria or yeasts.
Salt and sugar act mainly by lowering aw, not by killing microbes. Refrigeration is inhibitory, not lethal. Pasteurization is not sterilization.
Worked example
A bottled mango squash keeps for six months because of combined hurdles: pasteurization at 85-90°C, pH near 2.5-3.0 from added citric acid, about 45% total soluble solids, 350 ppm sulphur dioxide and a hot fill with an airtight crown seal. No single hurdle would give that shelf life.
Traps that cost marks
Do not list a preservative as a "principle". Do not call pasteurization sterilization. Do not claim asepsis kills microbes; it excludes them.