Industrial microwave

Microwave ovens are mainly used for heating and reheating of foods; however, trends show that they are used increasingly also for cooking and defrosting.

Industrial scale microwave processing units have been developed for drying, precooking of meat, pasteurization of ready meals, and tempering of meat and fish. Microwave processing is generally characterized by uniform heating on a macroscopic scale and rapid heating rates, as opposed to conventional processing.

Microwave heating talks place throughout the volume of the product. This volumetric heat delivery leads to a much higher rate of heating than conventional methods, limited by the heat penetration from the heated surface to the bulk of the material.

Microwave processing is used in various industrial application:
*Pasteurization of liquid and viscous in the food industry
*Sterilization of liquid products on food/pharmaceutical sector
*Drying of casings in sausage production
*Heating of minced meat mixtures after the extruder for flashing
*Preheating of viscous products before spray dryers or vacuum dryers
Industrial microwave

Mechanism of ohmic heating process

Both irradiation and microwave heating employ radiant energies which effect foods when their energy is absorbed, whereas ohmic heating raises the temperature of foods by passing an electrical current through the food.

When electrical current flows through a conductor, the motion of charges within the material results in agitation of molecules therein. This results in increased temperature.

Within metallic conductor, the moving charges are electrons; however, within food materials, the charges are usually ions or other charged molecules such as protein, which migrate to the electrode of opposite polarity.

Because heating occurs by internal energy generation within the conductor, the method results in a remarkably even distribution of temperatures within the material. Ohmic heating is alternatively called resistance heating or direct resistance heating. This due to food system serves as an electrical resistance.
Mechanism of ohmic heating process

Food processing of Ohmic heating

Ohmic heating is one of the newest methods of heating foods. Ohmic heating’s major advantage is that is simultaneously heats solid pieces and liquids in a food with minimal destruction.

It has been shown that ohmic heaters can provide an interesting alternative to heat exchanger for thermal processing applications.

In this method the food is placed between two electrodes serving as an electrical resistor and an alternating electric current is passed through the circuit.

Due to the electrical resistance, heat is generated volumetrically in the form of internal energy throughout the food. The electrical energy is directly converted into the heat causing a temperature rise.

Hence, ohmic heating is sometimes also referred to as Joule heating, electrical resistance heating, direct electrical resistance heating, electro-heating or electro-conductive heating.

Ohmic heating has been practiced since the nineteenth century, when a number of patents were filed for heating of flowable materials.

In the twentieth century ohmic heating was practice in and off: first in the 1930s for electric pasteurization of milk, and later in the 1980s and 1990s for continuous flow sterilization and aseptic packaging for solid–liquid food mixtures.
Food processing of Ohmic heating

Effect of canning on vegetables

From the textural standpoint, vegetables may be divided into those that are eaten raw and valued primarily for their crispness, those that are eaten cooked and valued for their softness and those that can be consumed in either form such as cauliflower and carrots.

Vegetables like peas, beans, greens are sometimes canned. The retention of original color is of great importance effecting the marketability and consumer response.

In the process of canning, vegetables are heated to destroy spoilage disease causing microorganisms.

Such heat treatments also produce a number of of undesirable chemical and textural changes in the vegetables. The textural changes are due to partial destruction of the cell wall and cell membrane.

Blanching, plus the strong heat treatments applied to nonacid vegetables, appears to be responsible for the large vitamin losses in canning.

Heat treatments also cause chemical alteration of the green pigment chlorophyll, thus resulting in a processed vegetable with less green color.

During canning chlorophyll gets converted to pheophytin due to the high temperature used. Sometimes to retain the color and to neutralize the acid, alkali is added.
Effect of canning on vegetables

Destruction of Salmonella by heat

Heating process are widely used in the food industry to enhance product quality and safety. Destruction of Salmonella by heat, it should be explained that as they are heated and a temperature is reached at which they are destroyed, they are not all destroyed at once.

Large numbers are destroyed when the heat is first applied, but the death rate quickly drops.

For instance , if at 48.9 degree C, 90% of the organisms would be destroyed in a period of 5 minutes, it would take 10 minutes to kill 99% of the organisms, 15 minutes to kill 99.9% of the organisms and so on.

Salmonella is not a spore forming organism. It is not, therefore, a heating resistant organism; pasteurization and equivalent heat treatments will destroy the organism under normal circumstances.

It should be noted that if some of these Salmonella organisms in foods survive whatever heating they receive during coking, and the food is thereafter held at temperatures at which they will grow 6.7-43.3 degree C, especially at room temperature, the organisms may grow again to large numbers.

When heating foods, it is important that all parts of the foods satisfy this temperature time requirements.

Some types of cooking is not sufficient to destroy all Salmonella bacteria that may be present in foods. Examples of cooked foods in which these organisms may survive are scrambled, boiled, or fried eggs, meringue, turkey stiffing , oyster stew, steamed clams, and some meat dishes.

Salmonella are more heat resistant in yolk than in whole egg due to the lower pH an higher total solids content in the yolk.
Destruction of Salmonella by heat