MICROWAVE PROCESSING

MICROWAVE PROCESSING

Introduction

Microwave are  electromagnetic  waves of  radiant  energy, differing from such  other electromagnetic radiation as light waves only in wavelength and frequencies .Microwave are  fall between  radio waves and infrared radiations, with wavelength in the range of about 25 million to 0.75 billion nanometers, which is equivalent to about 0.025 to 0.75m. Frequency of microwave is about 20000-400 MHz For food application the approved and most commonly used microwave frequencies are 2450 MHz to 915 MHz.

During World War II, scientist found that birds collide with radar most would adapt to ground, become sizzling and well cooked. From then the idea of cooking food with microwave emerged. Shortly after the war, microwave oven was introduced to public. Microwave are travel in straight lines. Microwave can pass through materials like glass, paper, plastic and ceramic and be absorbed by foods by foods and water, but they are reflected by metals. They heat the material which absorbed it. In heating the material they lose electromagnetic energy. The term loss factor and loss tangent are used to indicate the microwave energy “lost” in passing through or being entirely absorbed by, various materials under defined conditions. Materials that are highly absorbent of microwaves are said to be highly “loosy.” Highly loosy materials are rapidly absorbed by microwaves.

The loss factor is also a measure of the degree of penetration of microwaves into materials. The greater the loss factor, and the more heat that is produced, the shorter is the distance they can penetrate before out of their consumed. It has been determined that 900 MHz microwave loss more energy than 2450 MHz microwave in certain materials, whereas the reverse is in other materials, in some materials, loss is the same at both frequencies.

• Equipment

    A microwave system typically consists of a generator to produce the microwaves, a waveguide to transport the microwaves and an applicator (usually a cavity) to manipulate microwaves for a specific purpose and a control system (tuning, temperature, power, etc.). Until recently, only fixed frequency single-mode or multimode systems (home microwave ovens) were readily available.

Magnetrons, klystrons, gyrotrons and traveling wave tubes (TWTs) are used to generate microwaves. Each has its advantages. For example, klystrons offer precise control in amplitude, frequency and phase. Gyrotrons offer the possibility of providing much higher power output (megawatts) and beam focusing. The TWTs can variable and controlled frequencies of microwave energy. Magnetrons are by far the most widely used microwave source for home microwave ovens and industrial microwave systems, due to their availability and low cost. 

v Structure of a microwave oven:

Microwave oven generally consists of the following basic component

Power supply and control: it controls the power to be fed to the magnetron as well as the cooking time;

Magnetron: it is a vacuum tube in which electrical energy is converted to an oscillating electromagnetic field. Frequency of 2450 MHz has been set aside for microwave oven for home use;

Waveguide: it is a rectangular metal tube which directs the microwaves generated from the magnetron to the cooking cavity. It helps prevent direct exposure of the magnetron to any spattered food which would interfere with function of the magnetron;

 Stirrer: it is commonly used to distribute microwaves from the waveguide and allow more uniform heating of food;

Turntable: it rotates the food products through the fixed hot and cold spots inside the cooking cavity and allows the food products to be evenly exposed to microwaves;

Cooking cavity: it is a space inside which the food is heated when exposed to microwaves; and

Door and choke: it allows the access of food to the cooking cavity. The door and choke are specially engineered that they prevent microwaves from leaking through the gap between the door and the cooking cavity

MECHANISM OF MICROWAVE HEATING:

The two major mechanisms namely dipolar and ionic interactions, explain how heat generated inside food.

1)     Dipolar Interaction: -

        Food and other certain material contains molecules that act as dipoles. They exhibit positive and negative charge at opposite end of molecules, such molecules also said to polar. Water molecules are polar with negative centered near the oxygen atom and positive charge nearer the hydrogen atom. When microwave pass into food, water molecules and other polar molecules tends to align themselves with the electric field. But the electric field reverses 915 or 2450 million times per second. The molecules attempting to oscillate at such frequencies generate intermolecular friction which quickly cause the food to heat.

1)     Ionic Interaction

In addition to the dipolar water molecules, ionic compounds (i.e. dissolved salts) in food can also be accelerated by electromagnetic field and collide with other molecules to produce heat. Hence the composition of food will affect how it will be heated up inside the microwave oven. Food with higher moisture content will be heated up faster because of dipolar interactions.

As the concentration of ions, dissolved salt increases the rate of heating also because of ions interaction of with microwaves. Even though oil molecules are much less polar than water molecules and are non-ionic, food products with high oil content has a fast heating rate. Because of the specific heat of oil is about less than half that of water.

FOOD CONTACT METERIALS FOR MICROWAVE COOKING:

Materials like plastics, paper, glass and ceramics are generally transparent to microwaves. Nevertheless, some of them may absorb certain amount of microwave energy and hence reduce the amount of energy to be absorbed by food.

1.      Plastics

Plastic containers are commonly used for microwave cooking and re-heating food. Not all types of plastic materials are suitable for microwave cooking. Even though high density polyethylene can be used for foods with high water content, it cannot be used for foods with high fat or high sugar content as these foods may reach temperature above 100^oC during microwave cooking. Among plastic materials, the most commonly used ones for microwave cooking are polypropylene and crystalline polyethylene terephthalate (CPET), which have melting points of 210-230 ^oC.  For plastic wraps, commonly used materials are poly-vinyl chloride (PVC) and polyethylene.

2.      Paper

Paper and board can also absorb some microwave energy. However, it is not ideal for microwave food because the strength of the paper would be affected when wet and not all types of paper are suitable for microwave cooking. A study found that food wrapped with waxed papers or wax bags may be contaminated with waxed hydrocarbons after microwave cooking.

3.      Glass

When food is microwave, heat is also retained in the glass. The degree of energy absorption depends on the types of glass.

4.      Ceramics

Ceramics itself is suitable for microwave cooking. However, it has been observed that sparks caused by electric arcing occurred when ceramic container with a metal gilded rim was used in a microwave oven. The arcing effect was resulted from reflection or bouncing-off microwaves from the metallic components.

5.      Metals :

Microwave energy would be reflected by metals and not be able to penetrate it.

  NUTRIENT LOSSES ASSOCIATED WITH MICROWAVE COOKING:

v Proteins:

Proteins would be denatured with the modification in molecular structure upon heating. The degradation rates depend on the heating time and temperature. It has been shown that the nutritive value of proteins in foods treated by conventional and microwave heating are comparable.

v Lipids:

Heating of food would lead to various decomposition reactions (i.e. thermolytic and oxidative reactions) of its lipid components, including triglycerides, saturated and unsaturated fatty acids, as well as cholesterol in the presence of oxygen. The subsequent increase in fat oxidation products is of particular health concern. Various studies have been conducted to investigate the stability of lipids upon microwave cooking, including studying the hydrolysis of triglycerides in soya, egg yolk and meats; fatty acid profiles in chicken and beef patties, chicken fat, beef tallow, bacon fat, rainbow trout and peanut oil; peroxidation of polyunsaturated fatty acids in meat, egg yolk and chicken. Available evidence suggested that microwave cooking did not result in significantly more chemical modifications.

v Vitamins:

Many studies have been conducted to compare the retention of vitamins in different types of meat and vegetables subject to conventional and microwave cooking. Generally speaking, water soluble vitamins such as vitamin B and C are more susceptible to heat treatment. The retention of vitamins varies with size and shape of the food, cooking time, internal temperature, etc. Review of available literature showed that vitamin retention in microwaves foods is equal or better than conventionally prepared foods because of the shorter heating time of microwave cooking.

v Minerals:

Minerals are generally not destroyed during cooking including microwave cooking. However, they might be lost in cooking water or meat drippings. Nevertheless, a study comparing microwave and conventional braised beef found that significantly more phosphorus and potassium were retained in microwave cooking.

MICROWAVE FOOD APPLICATIONS:-

v Baking: - Internal heating quickly achieves desired final temperature throughout the products. Microwave can be combined with internal heating of air or infrared to obtain crust. It is used to bake breads, biscuits, cakes, pastries etc.

v Concentrating: - Permits concentration of heat sensitive solutions and slurries at relatively low temperature in relatively short time.

v Blanching: - Microwave are especially adoptable to blanching of fruits and vegetables without leaving losses associated with hot water or steam. Also, does not overcook the outside before core enzymes are inactivated.

v Freeze drying: - The ability of microwave to relatively heat ice crystal in matter makes it attractive for accelerating the final stage of freeze drying.

v Puffing and Foaming: - Rapid internal heating by microwave causes puffing or foaming when the rate of heat transfer is made greater than the rate of vapor transfer out of the product interior. May be applied to the puffing of snack foods and other materials.

v Thawing: - Controlled rapid thawing of bulk item is possible due to substantial penetration of microwave into frozen materials.

v Food tempering: - Meat, fish, fruit, butter and other food stuffs can be tampered for cold store temperature to around -3^oc for ease of further processing such as grinding the meat in the production of burgers or blending and a portion of butter packs.

v Drying:-

 Atmospheric pressure: - The drying of pasta is an established application compressing three stage involving microwave and hot air in various combinations to give improved sanitation and better control as well as quality. Other examples include drying of onions, parsnips, and snack food.

Vacuum drying: - Some materials are heat sensitive and can not be dried at atmospheric pressure. In it necessary to reduce pressure to reduce the boiling point and effect of drying at a reduced temperature. A modest vacuum around 100-200 nm Hg is necessary where the formation of a microwave plasma or are can still be avoided. Notable examples are the drying of fruit juices, beverages, drugs and pharmaceutical pellets.

v Heating and cooking

Many foodstuffs have been cooked by microwaves for various stages of processing. Examples include bacon cooking in a combination system, meat coagulation to upgrade scrap and doughnut cooking for frying.

v Pasteurization and sterilization

Food products, such as bread, precooked foods and animal feedstuffs have been processed using microwaves for pasteurization or include the sterilization of bone meal and the processing of barley to achieve starch to gelatin conversion. Food pasteurization of sealed sterilization or simply to improve their digestibility. Specific examples packs under pressure can be effected by microwave energy, however, as with most pasteurization processes the product after treatment needs rapid cooling to avoid infestation.

v Microwave Sterilization

Microwave sterilization is a thermal process. It delivers energy to the food package under pressure and controlled temperature to achieve inactivation of bacteria harmful for humans. Microwaves interact with polar water molecules and charged ions. The friction resulting from molecules aligning in rapidly alternating electromagnetic field generates the heat within food. Since the heat is produced directly in the food, the thermal processing time is sharply reduced. The color, texture and other sensory attributes of foods processed by microwave sterilization are often better compared with those of conventionally retorted foods while meeting microbial safety requirements. Compared with conventional sterilized food microwave sterilized food have good nutritional value.

Microwave sterilization can achieve the same reduction of bacterial population as conventional retorting. The microwave sterilization process awaits regulatory acceptance that will make sure that sterilization of food is complete. Products intended for microwave sterilization are usually packaged in plastic trays or pouches. The ability of plastics to withstand oxygen permeation will affect the organoleptic or sensory acceptance of the product during storage. Normal shelf life expectancy of microwave-sterilized products prepackaged in plastic Containers or pouches is 2-3 years or longer. Microwave sterilized foods can be stored at ambient temperatures and re-heated in the common household microwave prior to consumption. 

Advantages of Microwave Processing:-

§  Microwave cooking generally requires shorter times and may sometimes result in lower   temperature at food surface.

§  Microwave penetrate food piece up to several centimeters of thickness uniformly setting all water molecule and polar molecule in motion at the same time.

§  It is cost saving, time and energy saving. It reduces floor spaces.

§  Precise and controlled heating (instantaneous on/off heating)

§  Selective heating

§  Volumetric and uniform heating (due to deep energy penetration)

§  Short processing times

§  Improved quality and properties

§  Synthesis of new materials

§  Processing not possible with conventional means

§  Reduction of hazardous emissions

§  Increased product yields

§  Environmentally friendly (clean and quiet)

§  Self-limiting heating in some materials

§  Power supply can be remote

§  Clean power and process conditions