Classification of preservation methods: bactericidal and bacteriostatic
Bactericidal methods kill microbes (heat, irradiation); bacteriostatic methods only stop their growth (cold, drying, sugar, salt, chemicals), so microbes revive when conditions improve.
Bactericidal vs bacteriostatic
Some methods can be either, depending on dose: heat, chemicals and irradiation kill at high levels but only hold microbes back at low levels.
Classification of preservation methods
| Group | Methods | Kills or stops growth? |
|---|---|---|
| Asepsis | Natural covers, aseptic packaging | Keeps microbes out |
| Removal | Washing, filtration, centrifuging | Removes microbes |
| High temperature | Blanching, pasteurisation, sterilisation, UHT, canning | Kills (pasteurisation kills disease germs, not all microbes) |
| Low temperature | Refrigeration, freezing | Stops growth |
| Removal of water | Sun drying, dehydration, freeze drying, concentration | Stops growth |
| High osmotic pressure | Sugar (jam, jelly), salt (pickles, dry fish) | Stops growth |
| Chemicals | Class I (salt, sugar, vinegar, spices, oil) and Class II (benzoates, sulphites) preservatives | Mostly stops growth at permitted levels |
| Fermentation | Lactic (curd, idli batter), acetic (vinegar), alcoholic | Stops growth (acid or alcohol formed) |
| Smoking | Cold and hot smoking | Stops growth; hot smoking also kills some |
| Irradiation | Gamma rays, electron beams | Kills at medium and high doses |
Combining methods: hurdle technology
Most traditional and modern foods use several mild 'hurdles' together (heat, low water activity, acid, preservative, packaging) instead of one harsh treatment. This idea, called hurdle technology (Leistner), gives safe food with better taste. Lime pickle combines salt, acid, spices and oil; squash combines sugar, acid, heating and a preservative.
Bacteriostatic foods are safe only while the hurdle lasts: thawed fish, opened milk powder or diluted squash must be treated like fresh food. When teaching home food safety, stress that the fridge does not kill germs.
2 marks: “Differentiate bactericidal and bacteriostatic methods.” Bactericidal methods kill microbes (canning, sterilisation); bacteriostatic methods only stop their growth (refrigeration, drying), so microbes revive later.
5 marks: “Classify food preservation methods with examples.” Groups: asepsis, removal, heat, cold, drying, sugar and salt, chemicals, fermentation, smoking, irradiation; mark each as killing or stopping growth; mention hurdle technology.
10 marks: “Classify the methods of food preservation. Explain bactericidal and bacteriostatic methods.” Chart of methods with examples, then the two terms with definitions and examples, the dose-dependent methods, and hurdle technology.
-cidal kills, -static stalls. Heat and rays kill; cold, dry, sweet, salty and sour stall.