FOOD PRESERVATION
| Why Food Preservation? | Causes of Food Spoilage | Priorities of Food Preservation | Techniques of Food Preservation |
| Causes
of Food Spoilage
Senescence | Chemical Deterioration | Microbiological Decay |
| Senescence
All foods are still living organisms to some degree and they are still respiring. Hence they can no longer effectively maintain their integrity because there are no more nutrients supply. The tissues would begin to be deteriorated. Thus, senescence occurs, in which the food would soften, darken in colour and rot. |
| Chemical
Deterioration
After the food is harvested or slaughtered, the enzymes present are still active and are able to catalyze reactions, which could adversely affect the quality of food. These enzymatic reactions are often rapid and cause significant deterioration. For example, keeping corn at room temperature causes the sugar to be converted to starch, the desirable sweetness is thus lost. Handling food sometimes allow contact of some natural chemicals which results in deterioration. Rancidity is one of the examples. |
| Microbiological
Decay
Of all the decay mechanisms, microbial growth is the foremost cause of food spoilage because they multiply very quickly in room conditions. They require water, warmth, oxygen and nutrients to grow and multiply. Also, some microorganisms known as pathogens can cause diseases easily. There are millions of microorganisms living all around us. They are very tiny organisms that we can't see with our naked eye. We notice their presence only where many of them collect together. For example, the molds on bread and the slime formation on fresh meat. |
| Links:
FACTORS AFFECTING MICROBIAL GROWTH The Latest Word on Mold |
Priorities
of Food Preservation
|
| Techniques
of Food Preservation
Heat Preservation | Cold Preservation | Dehydration | Chemical Preservation | Irradiation | Packaging |
| Heat
Preservation: Canning, Pasteurization
& UHT
Heating the foodstuffs can destroy the spoilage and pathogenic organisms that may be present. It also inactivates the enzymes present. |
|
The nutrients
in canned foods are usually retained. However, vitamin
C may be destroyed at elevated temperatures during the process.
Modern tin cans composed of 98.5 % of sheet steel with a thin coating of tin. Bulged ends may be caused by microbial, chemical or physical actions. Such cans may progress through various stages of swelling and finally explode. |
| Links:
Canning Basics Canning Basics for Preserving Food The Complete Guide to Home Canning Beginner's Guide to Home Canning Home Canning - Who & Why? How long will canned food keep? |
| Pasteurization
of milk
Milk is such a rich source of nutrients that it is an ideal medium for the growth of microorganisms. To kill the microorganisms present, milk is heated to at least 72 oC for at least 15 seconds, and then rapidly cooled to below 10 oC. The microorganisms are either killed or inactivated by the processes. However, pasteurization causes a slight change in flavour and nutritional value. UHT
Sterilization of milk
|
 |
| Cold
Preservation: Refrigeration & Freezing
Storage at low temperature prolongs food life by decreasing the respiration and metabolic rates of most spoilage and pathogenic microorganisms hence they cannot grow. |
| Food | Temperature (?#060;/font>F) | Food | Temperature (?#060;/font>F) | |
| Butter
Cheese (cream) Cheese (soft) Condensed Milk (bulk) Cream (fresh) Margarine Milk (fresh) Milk (evaporated) Fish (fresh) Fish (dried) |
34
30-36 32-36 35-40 32-35 32-35 32-35 36-40 32 35-40 |
Oysters
Beer Chocolate Cider Honey Maple Syrup Molasses Olive Oil Sauerkraut Wine |
32-34
32-40 38-42 32-36 40-45 40-45 40-45 35-40 34-38 40-45 |
| Refrigeration
The life of many foods may be increased by storage at temperatures below 4 oC because activities of microorganisms and enzymes are much reduced. In general, for every 10 oC lowering of temperature, shelf life increases by 2 - 5 times. |
 |
Refrigeration cannot improve the quality of decayed foods, but only retard deterioration. It does, however, afford substantial protection against carbohydrate loss. Dehydration of foodstuffs due to moisture condensation can also be overcome through humidity control within the storage chamber and by appropriate packaging techniques. |
| Links:
REFRIGERATION AND FOOD SAFETY |
| Freezing
Freezing is a very effective mean of food preservation, as most microorganisms cannot grow at temperatures below 0 oC. Microorganisms become inactive at about -10 oC while enzymes are largely inactivated below -18 oC. It is the most efficient preservation method for many fatty foods. Freezing also destroys certain parasites in food and prevents insect infestations. The water present is frozen to ice, which is then unavailable to microorganisms and reaction. Usually, shelf life can be increased by 3 - 40 times for every 10 oC lowering. |
 |
|
Freezing of food usually occurs at temperatures between -4 oC
and 0 oC. It does not destroy nutrients
and produce little change in the nutritive value of protein.
However, repeated freezing and thawing may alter food appearance and quality.
Some oxidation and destruction of vitamins
may occur during freezing.
Some very juicy foods like strawberries and melons suffer from extensive drip and consequent loss of vitamin C during thawing. |
| Links:
Freezing Basics Refreezing Food |
Dehydration
 |
Dehydration is an effective means of inhibiting microbial growth. Very few microorganisms can grow in food with less than 5 % moisture. It is the preservation method most frequently applied to fruits. Loss of moisture content by drying resulted in the increased concentration of nutrients in the remaining food mass, and the nutritive value of most reconstituted or rehydration foods are comparable to that of fresh items. However, the vitamin level would be less satisfactory. |
|
Foodstuffs may be dried in air, superheated steam, vacuum, inert air or
by direct application of heat. Air is the most generally used drying medium,
because it is plentiful and convenient, and permits gradual drying, allowing
sufficient control to avoid overheating that might result in scorching
and discolouration.
A modern development of dehydration is freeze-drying in which frozen foods is dried under high vacuum. It maintains the food at the best level for successful dehydration and satisfactory rehydration, with substantial retention of nutrients, colour, flavour and texture characteristics. |
| Links:
Drying Food Food Drying Quality for Keeps - Drying Foods DRYING FOOD AT HOME WHEN IS FOOD DRY? Freeze drying |
| Chemical
Preservation: Pickling, Smoking
&
Sugaring
Chemical food preservatives are substances which, under certain conditions, either delay the growth of microorganisms or prevent deterioration of quality. Chemical preservatives can be natural food constituents, or any food additives like antioxidants, antibiotics, blanching agents, acidulants, neutralizers, stabilizers, firming agents and humectants. |
| Permitted Preservative | Examples of Use |
| E200 sorbic acid | soft drinks, fruit yogurt, processed cheese |
| E201 sodium sorbate
E202 potassium sorbate E203 calcium sorbate |
frozen pizza, flour, confectionery |
| E210 benzoic acid
E211 sodium bezoate E212 potassium benzoate E213 calcium bezoate E214 ethyl 4-hydroxybenzoate E215 sodium ethyl 4-hydroxybenzoate E216 propyl 4-hydroxybenzoate E217 sodium propyl 4-hydroxybenzoate E218 methyl 4-hydroxybenzoate E219 sodium methyl 4-hydroxybenzoate |
beer, jam, salad cream, soft drinks, fruit pulp, fruit-based pie fillings, marinated herring and mackerel |
| E220 sulphur dioxide
E221 sodium sulphite E222 sodium hydrogen sulphite E223 sodium metabisulphite E224 potassium metabisulphite E226 calcium sulphite E227 calcium hydrogen sulphite |
dried fruit, dehydrated vegetables, fruit juices and syrups, sausages, fruit-based dairy products, cider, beer and wine, to prevent browning of raw peeled potatoes and to condition biscuit dough |
| E230 diphenyl
E231 2-hydroxydiphenyl E232 sodium 2-phenylphenate |
surface treatment of citrus fruits |
| E233 thiabendazole | surface treatment of bananas |
| E234 nisin | cheese, clotted cream |
| E239 hexamine | marinated herring and mackerel |
| E249 potassium
nitrite
E250 sodium nitrite E251 potassium nitrate E252 sodium nitrate |
bacon, ham, cured meats, corned beef and some cheeses |
| E280 propionic
acid
E281 sodium propionate E282 calcium propionate E293 potassium propionate |
bread and flour, confectionery, christmas pudding |
| Links:
FOOD PRESERVATION WITH CHEMICALS Uses and effects of main food additives Fresh Look at Food Preservatives Antioxidants Blanching |
| Pickling
Most fresh fruits and green vegetables can be preserved by pickling. Salt is added to suppress undesirable microbial activity, creating a favourable environment for the desired fermentation. Pickles can also be prepared by adding vinegar, sweat spice, herb and spices. Pathogens generally do not grow below pH 4.6. Meat and fish may also be preserved by curing or pickling. The ingredients used are sodium nitrite, sodium chloride, sugar, citric acid or vinegar. |
 |
| Links:
Quick Process Pickles Equipment for Pickling Vinegar for Pickling SALTING FISH |
| Smoking
Smoke acts as a dehydrating agent and coats the meat surfaces with various chemicals. Bacon, ham and corned beef are examples of smoked foods. However, it has been identified that some organic compounds in smoke are known to be carcinogenic. |
| Sugaring
Concentrated solution of sugar is added, to a point where microbial spoilage cannot occur. Water available for enzymatic action and microbial growth is removed by osmosis. It can be used in making jams, jellies, marmalades and candying fruits. |
| Links:
Jams & Jellies Osmosis |
| Irradiation
Certain radioactive isotopes emit electromagnetic radiation called gamma-rays, which are extremely effective in killing microorganisms. Gamma-rays are able to penetrate food to a considerable depth. Consequently, food can be processed in bulk without fearing that the innermost parts will not be properly processed. Radiation prolongs storage lives and destroys parasites, food-poisoning organisms and insects. Sterilization of foods by irradiation requires only one-fiftieth of the energy needed for thermal sterilization. It permits sterilization of substances with a negligible temperature rise of only about 3 oC. However, it produces widespread chemical changes in irradiated materials. Enzymes are resistant to irradiation, and they must be inactivated by other means in complement to the irradiation action. Nutrients are somewhat reduced and development of protective methods requires further study. In the early 1980s, the safety of foods treated by irradiation was still the subject of testing and evaluation. |
| Links:
Food Irradiation Radiation Pasteurization of Food PRESERVATION BY RADIATION Advantages and Disadvantages of Using Food Irradiation |
| Opaque packaging prevents light from reaching the food, which accelerate lipid oxidation and destroy vitamins. |
| Written
By
Joanna Chung (12) L6Sc 7 - 2000 |
| Glossary |
References
|
