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Food preservation |
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Food preservation |
Food = eatable organic matter: living, freshly killed or preserved. Fresh food is classified as vegetable or animal.
Vegetal food (vegetables and fruits): once cropped, it continues living (f.i. breathing) so it cannot be enclosed. When rotting (putrefaction) it gives off ethylene gas (bad smell). Typical metabolic rate is 0.1 W/kg.
Animal food (fish or meat): once killed, it starts to decompose (outgas, bad smell). Typical metabolic rate (only alive) is 1 W/kg.
Biological evolution: internal cause due to its own enzymes (catalytic proteins). Fermentation of sugars, rancidity of fats. Bad smell.
Decomposition by attack: external cause due to the growth of microrgaanisms that secrete enzymes. Bad smell.
Chemical attack: oxidation in air. Bad colour.
Physical attack: dehydration in dry air, overheating by sun-beam, axphyxiating in enclosures. Bad texture.
Spoiled food must be disposed of as soon as possible (neither store not transport).
Bacteria: unicellular plants that divide (reproduce) every 1/2 h or so, secreting enzymes that dissolve organic matter.
Fungi: anaerobic plants:
Yeast: unicellular plants that convert sugars to alcohol+CO2 (fermentation).
Moulds (moho in Spanish): pluricellular plants, chain-like (cilliae ® penicillium).
Drying (physical) and Dehydration (chemical): dried fruits.
Smoking: dries and protects with an antiseptic (creosote).
Salting: dries and prevents reproduction.
Pickling: with brine, vinegar and species that dry and hydrolyse.
Canning of boiled food: T>70 ºC pateurizes and sterilises (kills bacteria and anabolic enzymes, but also kills vitamins). It was massively used by Napoleon troops, soon after bacteria were seen under the microscope. T>120 ºC kills even spores.
Refrigeration: low T reduces metabolism but does not kill f.i. vitamins. Best preservation is frozen at -20 ºC for fish, meat and some vegetables (-25 ºC for icecream) and from -0.5 ºC to 5 ºC for vegetables, fruits and dairy (tropical fruits at 15 ºC).
UV radiation: may directly kill living organisms, or decompose O2 to atomic oxygen and that decompose microorganisms.
Ozone: reacts with organic matter and produce atomic oxygen that oxidises (burns) living organisms (the same as for H2O2).
High voltage electric pulses (some 1000 kV/m).
High pressure (some 100..1000 MPa). It has been found that high pressure inactivates microorganisms, without altering nutritional and sensory food quality; food (ready-to-eat food, oysters, etc.) is put on a small water or oil bath that is hydraulically pressed. High-pressure food freezing or thawing, by removing or applying pressure, respectively, gives a much faster and homogeneous transition than freezing or thawing by heat transfer.
PROBLEMATICS of food-preservation by refrigeration
Requires continuous expense.
The cold chain (production, transport, store, consumption) should not be broken. Problems of interruption of cooling machine power and/or shortage of cooling agent (ice, eutectic mixtures).
Reduce enzymatic activity but do not kill, implies T<6 ºC for a few days but T<-18 ºC for a few months).
Cooling increases relative humidity and increases surface fungi. Packed enclosure prevents it, but it asphyxiates vegetables. (Ambient humidity not relevant for frozen food.)
In mixed spaces some food give odours (potatoes) and others take (dairy). Some stores require ventilation for fresh air or air treatment for controlled atmosphere.
Freezing may damage food (f.i. tomatoes).
Freezing requires rapid cooling (chilling) before refrigerated storage.
Freezing requires slow thermalisation after refrigerated storage.