microwave oven e book

7/31/2019 Microwave Oven E Book

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Introduction

I thank you for reading this e-book.

This e-book is a publication of Arimifoods.com, a website owned and run by

Joshua Arimi. Arimifoods was conceived after a realising that consumers are

overloaded with complex scientific information on food and nutrition to the

extent that they get confused instead of getting enlightened.

Consumers are bombarded with TV adverts, brochures information and

newspaper articles on health and nutrition that are difficult to understand and

sometimes contradictory.

Arimifoods comes to the rescue of consumers by simplifying complex food facts

into simple understandable and applicable information. This sequentially helps

consumers to make the correct decisions and choices on food, nutrition and

lifestyle.

Academically, Arimi holds a BSc degree, 2 Masters degrees and a PhD degree in

Food Science.

This e-book addresses the mechanism employed by microwave oven and

frequently asked questions concerning the safety of microwave oven.
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Tips to remember when using the microwave oven: ........................................... 20

Do micro-waves kill nutrients? .................................................................................. 21

What leads to loss of nutrients during cooking of vegetables?........................... 21

Reasons why microwaving causes minimal loss of nutrients in vegetables..... 22Tips on cooking vegetables in a microwave oven to reduce loss of vitamins and

minerals. ...................................................................................................................... 23


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Invention of Microwave Oven

Microwave oven was discovered by Pierce Spencer in 1946. At the time, Dr.

Spencer was working for Raytheon corporation in USA developing a tube called

magnetron. Something that was to change cooking forever happened, a candy

bar in his pocket melted. Voila, he accidentally discovered that magnetron can be

used to heat food. Magnetron is the main component in a microwave oven which

generates micro-waves used to heat food.

Following the melting of the candy, he placed popcorn kernels near the tube and

they popped. He placed an egg near the magnetron and it exploded. The rest is

history.

Before Spencer's discovery, during World War II, magnetron, was used by

British to spot Germany warplanes. It is after world war II, while Spencer was

working on a new type of magnetron for American radar system he bumped on

the food heating effect.

At the time of publication , microwave ovens are now found in every kitchen in

developed world and all middle and rich class homes in developing countries.


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there is spatial variation of electric field amplitude, or hot and cold spots which

contribute to the uneven heating. To reduce the effect of these hot spots and cold

spots a stirrer is used.

Turn table

The turntable rotates the food product through the high and low intensity

electric field regions (hot and colds spots), providing a spatial averaging effect.

In ovens with a turntable, the microwaves are fed directly into the cavity from

the top or side without the use of a stirrer. Some turntables have a shaking action

for uniform heating of liquids.

Some microwave ovens have a transparent shelf that is elevated from the

metallic bottom. The shelf can be glass or ceramic and is either permanently

installed or in the form of a removable tray. The shelf allows microwave power

to reflect off the metal bottom of the cavity and impinge on the underside of food

eliminating poor bottom heating.

The cavity

The cavity (cooking chamber Figure 1) is the area in the microwave where food is

placed for heating.


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Most domestic microwave ovens have rectangular cavities, although the cavity

can be of any shape.

Door

The door is the aperture through which the food is placed in the microwave oven

cavity. The door is carefully engineered with a viewing screen designed to

eliminate or reduce MW leakage while allowing viewing of the product being

heated.

The door is also fitted with non-visible choke to prevent microwave radiation

leaking through the minute gaps that exist between the door and the cavity face

plate.

Mechanism of heating food

Microwave oven uses electromagnetic waves called micro-waves to heat food.

That is why the appliance is called microwave oven.

When food is placed in a microwave oven, the oven is switched on, the micro-

waves are either transmitted through the food, are absorbed or reflected by the

food.


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If micro-waves are transmitted through food, it means the food is transparent to

micro-waves and hence they pass through the food without heating it. Most of

microwave safe plastics, ceramic and glass that are used in a microwave oven are

transparent to micro-waves.

Reflected micro-waves bounce off the food. Metals reflect micro-waves.

Materials containing water, such as foods, fluids or tissues absorb micro-waves.

If the micro-waves are absorbed by the food constituents, they cause the food to

heat up. Microwaves heat food mainly by rotating the water molecules and

causing them to collide and rub against each other at very high speed leading to

generation of heat. The generated heat travels through food inducing the heating

effect.

Technically, when food is placed in a microwave oven, various food ingredients

(carbohydrates, fats, proteins and water) respond differently. Among these

ingredients, water is the main ingredient responsible for micro-wave heating.

Water in molecular level behaves like a magnet. Water has two oppositely

charged ends due to presence of 2 positively charged Hydrogen atoms and a

negatively charged Oxygen molecule. If a bar magnet is held above another bar


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magnet, and the held magnet is rotated, the magnet below rotates. Similarly, due

to two different poles in water, when micro-waves oscillate, the water molecules

rotate. This is because the negatively charged (Oxygen) end of water is attracted

to positively charged end of micro-waves, while the positive end (Hydrogen) of

water is attracted to the negative charged end of micro-waves.

The micro- waves rotate at extremely high speed of 2450 times per second. This

means for every second, a micro-wave rotates, the water molecule rotates 2450

times. This fast rotation creates friction between water molecules leading to

generation of heat. The heat flows through the food either by conduction,

convection or radiation causing food to heat up.

How Microwave heating differs from conventional methods

Heating food by micro-waves differ from conventional cooking methods in that;

water is the main ingredient that heats up while in conventional heating, all

ingredients heat up.

Micro-waves heat the foods from inside out, while in conventional cooking

methods, the surface of the food particularly the bottom, sides or top that is in

contact with the source of heat, will heat up first. After the surface heats, the


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heat travels to the core of food. Conventionally cooked foods have lower core

temperature compared to surface temperature.

11 Frequently asked questions about Microwave oven

There are several frequently asked questions about micro-waves heating of food.

These include:

1. Is Microwaved food safe for human consumption?

Yes. Microwaved food is safe for human consumption. According to World

Health organisation (WHO), if microwave oven is used according to

manufacturer's instruction, the food is safe and of the same nutrient value to that

cooked by conventional methods.

The term 'nuke the food' is frequently used when warming food in a microwave

suggesting that food becomes radioactive. Nuke refers to radioactive material.

Using this reference in association to warming food in a microwave oven is

misleading. Micro-waves are not radioactive and do not cause the microwave

oven or the food to become radioactive.


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2. Do micro-waves remain in food after warming food?

No. According to FDA, WHO and respected published scientific data, micro-

waves are not embedded in food or in the cavity of the oven after warming the

food.

Micro-waves act like a light bulb, after switching off no light is left in the room.

3. Which are the safe containers to use in a microwave oven?

Always use containers labeled microwave safe. Some have inscription

'microwave safe' on them.

Using unsuitable containers to warm food in a microwave might cause toxic

compounds to migrate from those containers into the food. Those compounds

might be extremely toxic, and may actually be carcinogenic.

Fats and foods high in fat content can reach to quite very high temperatures in a

microwave oven making them candidates that favour migration of harmful

compounds from the containers.

Most plastic containers found in the kitchen such as: water bottles, and plastic

tubs or jars made to hold margarine, cooking fat, yoghurt can be used in a


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microwave but are not microwave-safe and should not be used. They are only

ideal for holding cold foods and not suitable for reheating.

Never use metallic utensils inside a microwave oven. Metals, including

aluminium foil reflect micro-waves leading to generation of sparks which can

lead to fire.

4. Microwave warmed food seems to cool faster than food cooked over open fire,

gas or electric cooker, why is this?

Generally, microwave heated food cools faster than conventionally heated food

for several reasons. Microwave oven heats food by heating water which has a

boiling point of 100 degrees Celsius. During heating water, the water starts to

evaporate, there is minimal increase in temperature once water reaches boiling

point (100 degrees Celsius) due to latent heat of vaporisation. Following

evaporation of all of the water, it is ideally difficult to heat the food further in a

microwave (sometimes it burns). Due to this phenomenon, foods heated in a

microwave oven achieve lower maximum temperatures compared to open fire,

hot plate or gas cooked foods.

In contrast, food cooked over hot plate, gas cooker or electric cooker continues to

heat up even after all the water has evaporated.


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6. Micro-waves are dangerous, can they leak through the transparent glass door?

The small holes in the door allow light waves to pass through, thus making it

possible to see the food cooking inside, because the frequency of light waves is

extremely high, which means that their wavelength very small--physically small

enough to pass through small holes. On the other hand, microwaves are lower in

frequency, thus they have a longer wavelength (at 2450 MHz, about 4 3/4 inches,

in fact). Therefore, microwaves are simply too large to pass through the small

holes in the door. To the microwaves, these small perforations actually represent

a solid metal wall that effectively blocks or reflects the microwaves back in the

opposite direction.

To increase the level of safety, the door has a mechanism to ensure that if it (the

door) is open, the microwave oven does not work.

Attempts to repair broken microwave oven or tamper with parts of a functioning

oven, particularly the outer covering metal can lead to serious consequences. A

broken microwave oven should be disposed safely. Leaking micro-waves pose

extremely a very serious health risk.


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7. Some cups and plates heat up in a microwave, can they pose a risk?.

There are types of plates and cups that absorb microwave energy and heat up.

Such utensils can heat up to high temperatures and pose a risk of burning the

holder.

It is always advisable to use microwave safe containers which are transparent to

micro-waves.

8. Why cover food during heating in a microwave oven

Covering food while in a microwave oven prevents soiling the microwave cavity

with spattering food.

During microwaving, food generates steam, the steam creates local pressure that

causes food to spatter. Manufacturers of microwavable food advise on creating

holes on the covering lid to allow to vent steam, otherwise explosion might

occur.

Partial covering of food in the microwave oven also helps to retain the steam

aiding in uniform heating of the food.


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9. Some parts of warmed food remain cold, are they safe?

The answer is Yes and No.

One disadvantage of heating food in a microwave oven is non-uniform heating;

some parts of the food heat up very well, while others remain unheated. The

heated parts are called hot spots while the unheated parts are called cold spots.

Cold spots can favour growth of harmful bacteria while hot spots can cause

scalding.

Microwave ovens are designed with turn table to rotate food guaranteeing

uniform distribution of heat. It is also a good practice to stop microwave mid-

way heating of food, to stir the food.

Following heating, food should be left to stand in the microwave oven for a few

minutes to allow the heat to distribute evenly.

10. Is it safe to warm baby food in a microwave?

Yes. It is safe to warm baby food in a microwave.

However, great care is necessary to ensure that the hot food does not scald the

baby's mouth. Baby bottle warmed in a microwave might seem just warm on the


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surface, while the contents might be extremely hot. Mixing the warmed food or

milk thoroughly minimises the chances of hot parts (hot spots).

A baby's raw food, particularly raw meat, might not be cooked in the microwave

oven. The heating system of the microwave oven might not guarantee complete

microbial inactivation in such food.

11. Why is it not advisable to defrost food in a microwave more than once?

Thawing or defrosting food such as meat more than once creates conducive

environment for growth of harmful bacteria.

Ideally, freezing food suppresses the growth of bacteria and do not kill them.

Microwave defrosting is very fast allowing food to reach ambient temperatures

that are conducive for bacteria to multiply. Re-freezing the defrosted food

suppresses a higher number of bacteria than the initial freezing. A follow up

thawing would allow that high number of bacteria to multiply. This can pose a

serious health risk.

This applies to any method of thawing food such as use of warm water.

Defrosted food has to be consumed once it has been thawed, otherwise it should

be disposed.


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The best method to defrost food is slowly in the refrigerator. Temperatures in

the refrigerator are not conducive for proliferation of bacteria.

Tips to remember when using the microwave oven:

Never use metallic utensils in a microwave oven.

Dont operate the oven when empty (it is not dangerous, but you never

know).

Take great care when heating liquids. Sometimes they overheat and can

bump and burn the handler.

Always follow the manufacturers instruction (both on the food be heated

and the microwave oven) especially on the heating periods. Overheating

some plastics can cause them to melt.

Only use the recommended containers when heating food in a microwave

oven. This minimises risk of physical injury and dangerous compounds

migrating into your food.

Air tight containers can cause steam to be trapped inside that might lead

to explosion. This can cause injuries.

Keep out of the reach of children.

If the microwave oven is broken, or you suspect a malfunction, dispose it

immediately; do not try to repair it.


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Do micro-waves kill nutrients?

Yes, Microwave cooking causes some loss of nutrients, but when compared with

conventional cooking such as boiling and steaming, microwave cooked foods

and vegetables retain higher level of nutrients due to rapid heating and use of

little or no water.

There has been several misleading claims that microwaves zaps nutrients for

vegetables.

What leads to loss of nutrients during cooking of vegetables?

There are three key factors that contribute to the loss of nutrients during cooking

whether over open fire, by hot plate, gas cooker or microwave oven. If these

critical factors are well controlled, losses are minimized and the cooked

vegetables deliver high levels of nutrients.

Long cooking periods leads to high losses while short cooking periods are less

detrimental to nutrients. Very fast cooking methods such as microwaving and

pressure cooking offer an advantage of minimal loss of nutrients. On the other

hand, slow cooking methods such as open fire, hot plate and gas cooker that

require long periods of cooking lead to high losses of nutrients.


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During the cooking, water soluble nutrients (mostly vitamins, minerals and other

health promoting compounds) migrate from the food into water and eventually

get lost through evaporation. High amounts of water provide a sea into which

nutrients can get lost. That is why boiling and similar cooking methods that use a

lot of water lead to high losses of nutrients.

High cooking temperatures destroy nutrients. Most beneficial minerals and

Vitamins such as Vitamin C are sensitive to heat. They easily get lost at very high

heat (temperatures). Combination high heat and long periods of cooking cause

very high losses of nutrients and therefore are never recommended for cooking

vegetables.

Reasons why microwaving causes minimal loss of nutrients in vegetables.

Microwave heats the food by rotating water molecules at very high speed

causing them to collide and rub against each other. This leads to friction which

generates heat. Due to the heating mechanism, microwave cooks food very fast.

It is advisable not to add any water to vegetables during microwave cooking. If

water has to be added, it should be very little. Combination of fast heating and

water-free cooking is the reason for minimal loss of nutrients in a microwave.


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Tips on cooking vegetables in a microwave oven to reduce loss of vitamins and

minerals.

Here are suggestions on how to cook vegetables with little or no loss of vitamins

and minerals.

After cleaning, rinse the vegetables in cold water, this leaves them with sufficient

cooking water. To make it completely water-free cooking, dry the vegetables

with a kitchen towel. Place the vegetables in a covered, microwave-safe

container. Do not add water (some times it is recommended to add 1 to 2

tablespoonfuls of water-this depends with the type of vegetables). Heat at the

recommended power as indicated on the packaging instruction. For the

vegetables bought in the open market, cook for three to four minutes or until

vegetables are tender. This requires trial and error. Do not overcook.

Thank you for reading this e-book. Visit www.arimifoods.comfor more.
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