The processing of foods by heat (or heat treatment) is the most important conservation technique of long duration. It aims to destroy or partially or totally inhibit enzymes and microorganisms, whose presence or proliferation could alter the food in question or make it unfit for human consumption.
The effect of heat treatment is related to the couple time / temperature. In general, the higher the temperature is high and the longer the term, the greater the effect will be important. However, we must also take into account the thermal resistance of microorganisms and enzymes and is highly variable.
According to the objective, there are several techniques of food preservation by heat treatment such as sterilization, pasteurization, thermisation, cooking and laundering.
Sterilization by heat treatment
The heat sterilization involves exposing food to a temperature generally exceeding 100 ° C for a period sufficient to inhibit enzymes and all forms of microorganisms, including bacteria spore.
Sterilization of a food is not sufficient in itself for its long-term conservation. A subsequent contamination of food by microorganisms environment might occur. To address this, we proceed to the sterilization of container (container) and content (the food), the container must be watertight and microorganisms to not recontamination after sterilization.
The sterilization of the food and its container can be achieved in two ways: The first is a simultaneous sterilization of the container and the content (appertization), while the second is to separate sterilization of the container and the content followed (aseptic packaging).
Appertization: Sterilization of both the container and contents
The appertization is a process of conservation is to be sterilized by the heat of perishable goods in containers (cans, jars, etc..) Tightly closed. Its discovery dates back to the 1790s. Nicolas Appert was the first who developed the method of sterilization by heat in a tightly closed.
The appertization is widely used today for long term food of animal or vegetable origin. The shelf life of foods appertisés is several months to several years.
Sterilization of the container and separate the content
In this case, the food product (content) is sterilized by heat treatment before being enclosed in its container. The latter is also sterilized by heat or by other processes (by ultra-violet, for example), but prior to contain the product. Then, the content is sterilized sealed in its packaging (container), and sterilized. The process of conditioning takes place in an enclosure which prevents the contamination of the product by the microorganisms in the environment: This is the aseptic packaging. This technique is generally used for the conservation of liquid products (milk, juice, etc..) In packages that can not stand the appertization as plastic bags and cartons Tetra Brik type.
When the sterilization of the product is carried out at high temperature (135 ° C to 150 ° C) for a short duration (15 sec. To 1 sec.), We talk about sterilization UHT (Ultra High Temperature). This technique has the advantage of preserving the organoleptic and nutritional quality of the product sterilized. However, it can not be used in the case of liquid products like milk.
Pasteurization by heat treatment
Pasteurization is a heat treatment sufficient to moderate and the destruction of pathogenic microorganisms and a large number of microorganisms of alteration.
The temperature of treatment is generally less than 100 ° C and lasts a few seconds to several minutes. The following table provides illustrative examples of scales of pasteurization.
|Products||Pasteurization temperature||Duration treatment|
|Fruit juices (pH <4.5)||65 ° C||30 min|
|77 ° C||1 min|
|88 ° C||15 s|
|Beer (pH <4.5)||68 ° C||20 min|
|75 ° C||4 min|
|Milk (pH> 4.5)||63 ° C||30 min|
|72 ° C||15 s|
|Ice cream (pH> 4.5)||65 ° C||30 min|
|71 ° C||10 min|
|80 ° C||15 s|
All microorganisms were not eliminated by pasteurization, the heat treatment must be followed by an abrupt cooling. Pasteurized foods are usually kept in cold (4 ° C) to slow the development of germs still present. Their shelf life is so limited.
Outside of refrigeration, other means of conservation can be used in parallel to counter the growth of microorganisms survivors, such as the addition of chemical preservatives, reducing water activity (aw), acidification, etc..
Thermisation is a heat treatment applied to raw milk. The latter should be brought to a temperature of at least 63 º C for 16 seconds.
Thermisation is a weakened form of pasteurization. Its main objective is the destruction of pathogenic bacteria that may be found in milk, all without altering its technological characteristics. However, this treatment may decrease the microbial load banal milk (lactic acid bacteria, bacteria refining, etc.).. That charge is generally used for the preparation of raw milk cheese. In this case, we must appeal to bacteria grown in the laboratory to replace those missing.
Money laundering is a heat treatment of a few minutes at 70 ° C to 100 ° C to destroy the enzymes that may affect the vegetables or fruit before further processing (freezing, drying, etc.).. In fact the destruction of enzymes is one objective among many others and the role of money as a pretreatment before drying, lyophilization, or freezing appertization, is multiple.
Role of money
Before drying or freeze-drying, bleaching is used as a pretreatment for the destruction of enzymes responsible for organoleptic changes such as changes in flavor and / or color. The treatment also allows the reduction of microbial load and facilitates rehydration.
Before appertization, bleaching is not intended to destroy the enzymes when the scales applied widely enough for their destruction. In this case, money has other roles:
- Removal of occluded gas in the tissues before casing, otherwise their presence leads to an internal overpressure, with a risk of bending or flochage boxes.
- Fill hot: It generally takes the precaution to fill the hot juice boxes so that crimping (or capping) will take place in atmosphere of steam and the air is so driven. Bleaching can follow and hot vegetables to prevent a cooling of the juice in contact with vegetables which are cold.
- Elimination of false taste: Cauliflowers not bleached retains a bitter taste intolerable.
- Elimination disorders: Money helps eliminate troubles such as those made by extracting starch.
Before freezing, the money allows, in addition to the destruction of enzymes, the removal of occluded gas in the tissues and consequently limit the phenomena of oxidation.
The effectiveness of bleaching can be controlled by testing the inactivation or the presence of two enzymes widely distributed in plants: catalase and peroxidase.
Catalase is an enzyme oxydative resulting in the formation of oxygen from hydrogen peroxide:
2 H2O2 → 2 H2O + O2
The activity of catalase is highlighted by grinding the product can be milled and mixed with calcium carbonate and hydrogen peroxide: an off-gas (O2) indicating a catalase activity.
Peroxidase is an enzyme that catalyzes the oxidation with hydrogen peroxide of various substrates:
H2O2 + 2 AH → 2H2O + 2 A
The activity of peroxidase is revealed by the appearance of a brown color instead of black when guaiacol and hydrogen peroxide in contact with the product milled.
Peroxidase is considered the most heat-and the effectiveness of laundering can be evaluated based solely on the test inactivation or presence of this enzyme.
Cooking is a heat treatment of foods to make them consumable. Its main objective is the development of the organoleptic characteristics of the product: Improving the taste, odor, color and texture. According to the scales used, the cooking may be associated with a substantial reduction or even elimination of the microbial load present on the product. Cooked products can be stored in the refrigerator for a few days, and in the freezer for a few weeks.
There are five ways of cooking basic foods can be dipped in a liquid such as water, broth (flavored or not) or wine (poaching, boiling broth or stew), immersed in animal fat or oil (frying), exposed to steam (steam cooking and to some extent, braising) or dry heat (roasting, baking, cooking on the grill) or even stoves in small quantities of hot fat (skipped).