Chapter 1     Kitchen Science

You experience "science" everyday and everywhere. Your home is the best place to learn and think about "science". In the kitchen, we are familar with the cookware and the cooking methods, the food flavour, texture and its storage, etc.; however, the science behind those things are not clear in mind. You are, sometimes, puzzled about some experience in the kitchen. In this chapter, you will have a better understanding on those mind-blowing questions.

Boiled and Raw Eggs

Boiled and raw eggs cannot be distinguished by a glance, since both eggs have the same look. However, you can make a conclusion once you know the trick. Here shows the way. Place a boiled egg and a raw egg on a table. Spin the eggs one by one gently with your fingers. Then, touch them lightly in a manner of a brake with your fingers. As soon as they stop, take your finger away. The one that spins again is the raw egg.

How Things Work
A boiled egg has solid white and yolk struck firmly to the eggshell. A raw egg does not. During the spin, the contents in the raw egg have a redistribution, and thus giving a greater inertia to the egg. According to the first law of motion (law of inertia) by Sir Issac Newton (1642-1727), objects are inert to change its motion. This is illustrated in the right movie on a tablecloth puill (Credit: Wake Forest University). The vase, plate and the bowl tend to stay at their original positions and ignore the pull on the tablecloth which they are sitting on. An object which occupies a greater inertia will be more inert to the motion change.
Let's go back to the raw egg. The white and yolk (in liquid form) are loosely attached to the eggshell. The contents moves inside the egg while it is spinning, although there are contraint force due to the viscous drag. The redistribution of the contents in the raw egg gives it a greater inertia and thus it is more inert to the motion change. The egg is stopped by your disturbance on the shell, however, when your finger is away, the inertia of the white and the yolk moves the egg again, since the contents in the egg are not stopped by your disturbance. On the other hands, the boiled egg is a solid (rigid) body as a whole (including the shell, white and the yolk) which stops readily and its motion is suppressed. Generally, a boiled egg has a smaller inertia than a raw egg. The following movies (Credit: The Sociey of Women Engineerings) illustrate the trick mentioned above. The left one shows the raw egg while the right one shows the boiled egg.

Another illustration is shown by rotating a glass of water which contains a floating tea leave. The tea leave does not rotate with the glass. It stays at its position as if it was struck there.

The Hot Pizza

A hot pizza just out of the oven will burn your tongue more severe when you bite the fruit filling. The much drier crust in the pizza will burn your tongue less severe. Do you have such experience? The following paragraphs tell you the reasons about, and in addition the key concepts in "temperature" and "heat" are introduced.

How Things Work
All ingredients in a hot pizza contain heat, but the amount of heat stored in each ingredient is not the same though they are at the same temperature. A one-gram strawberry stores more heat than a piece of baked crisp of the same weight, since strawberry contains high water content which has large "specific heat capacity" (to be explained later). Or simply, water stores more heat than other ingredients of the same weight under the same temperature increment. When you eat the wet portion of a pizza, a large amount of heat flows and come to your month, the high temperature of the portion drops finally to your body temperature, say 36.5oC. You are then, of course, in a serious burn due to the large amount of heat that your mouth adsorbs in a short time. Surely, the heat scalds you, but not the high temperature!
For the same amount (you are familar with the term "weight" or more technical "mass"), of water and iron, say 1 kg, if one increases the temperature of these substances by one degree Celsius (oC), the amount of heat that is further absorbed by water is almost ten times that in iron. Technically, we say that water has a higher "specific heat capacity" than iron, and water is an efficient and common coolant as it absorbs heat from hot bodies without increasing its temperature greatly. Poor coolant may escape away as its temperature increases rapidly and fininally it boils and evaporates.
Science in Depth
There are few terms that are quite common in the discussion of heat.
  • Heat: It is the energy transferred between objects because of temperature difference. Heat always flows from hot to cold objects.
  • Thermal contact: Objects that are in thermal contact if heat can flow between them.
  • Thermal equilbrium: Objects that are in thermal contact, but have no heat exchange between them, are said to be in thermal equilibrium. Two objects are said to be in thermal equilibrium when they have the same temperature.
  • Temperature: The quantity that determines whether or not two objects will be in thermal equilibrium. Common scales are: Celsius scales (oC), Fahrenheit scale (oF) and the Kelvin scale (oK). In the Celsius scale water freezes at 0oC and boils at 100 oC. In the Fahrenheit scale water freezes at 32oF and boils at 212oF. There are totally 100 divisions and 180 divisions respectively, for the Celsius and Fahrenhenit scales, between the freezing and boiling points of water. A useful formula for exchanging one and other is given as TF = (9/5)TC + 32 or TC = (5/9)(TF − 32). The degree size in Celsius scale and Kelvin scale is the same, but the two scales have different reference zeros.
  • Specific heat capacity: It is a quantity of heat that shows the propertry of a substance. The heat needed for a unit mass of substance having a unit change in temperature is referred to as the specific heat capacity. It depends on the substance itself but it is independent to the quantity of a substance.
Substance Specific Heat, c (J/kg · K)

The Thermostat

An electric hot pot boils and maintains water inside at a specific temperature. The working principle is not difficult but an application of basic science - thermal expansion in substances. A small device which adopts the mentioned idea is mounted to the pot such that water is heated up automatically when it is below the preset temperature. The rice cookers apply the same device to warm the rice after cooking.

How Things Work
Most substances expand when heated. Two metallic strips, with the same original lengths but not of the same kinds, increase differently in lengths when they are heated up. The thermostat is made up by a bimetallic strip which consists of two metals bonded together to form a strip. Suppose metal A expand/shrunk less than metal B when heated/cooled. The figure below shows a bimetallic strip which has metal A (e.g. iron) colored in black and metal B (e.g. copper) colored in brown.

This means that its length will change by a greater amount than metal A for the same temperature change. Hence, if this strip is cooled, the B side will shrink more than A side, resulting in the strip bending toward the B side. On the other hand, if the strip is heated, the B side expands by a greater amount than the A side. Thus, the shape of the bimetallic strip depends sensitively on temperature. The bimetallic strip deflects in one direction or the other, which either closes or breaks the electrical circuit connected to the heater, as shown in the right figure below. Click the left figure for the movie on a bimetallic strip. (Credit: NCSU Physics Education Technology Facility)

  • The domestic fruit spread (jam) is stored in a glass container which has a metal lid (cover). Sometimes, you find a hair dryer very helpful to ease the tight cover. The metal lid which has a greater expansivity, expands more than that in the glass container when hot air blows on it. The small leak so formed between them reduces the friction acting on to each other, and thus the container is opened. Alternatively, you can open the container without using the hair dryer but instead dipping the metal lid into hot water.
  • You can peel a hot boiled egg easily if you immerse it in cold water for some minutes. Apparently, the eggshell and the white shrink differently under cooling. The separation so caused between them accounts for the trick that you use it mostly.
  • Pouring hot water into a glass quickly is sometimes dangerous, since thick glass with high expansivity may crack. The inner part of the glass expands greatly due to its contact with hot water and, the inner circumference increases by a considerable amount. The outer circumference of the glass, in contrast, have such change in a latter time due to the small thermal conductivity. Usually, glass is referred to as a poor conductor. The glass cracks finally.
  • The Eiffel Tower, constructed in 1889 by Alexandre Eiffel, is an impressive latticework structure made of iron. The tower is 301 m high on a 22 oC day, but the height decreases by 7.9 cm when the temperature cools to 0 oC. A quantity to measure the linear expansion is the coefficient of linear expansion a. A greater a implies that the material has a greater fractional expands/shrinks when it is under a change (increase/decrease) in temperature.

Science in Depth
Substance Coefficient of linear expansion, a (× 10-6 K-1)

Water Droplets on a Red-hot Wok

When a few water droplets are poured onto a red-hot wok, they run to and fro vigorously. The wok seems a perfect surface and the droplets slide on it without hindering. Repeat the whole process, but this time with a wok at room temperature. You observe nothing special.

How Things Work
Think about you heat up a wok and pour on it some water droplets when it is in a red-hot condition. Water on the surface of droplet evaporates and changes its state to steam readily when they meet. During the process, the water in liquid state is heated up and it expands tremendously to gas state, that is, the steam. The abrupt expansion of water ensures enough separation between the droplet and the wok. The droplet floats itself on the wok and runs freely on this perfect surface (frictionless surface). Similar observation is obtained while pouring liquid nitrogen on a table. Click the right figure to see the movie on such phenomenon. (Credit: Department of Physics and Astronomy at Georgia State University)

The thermal expansion of water in the kitchen is a common scene in the kitchen. The following example is familar to you, but it is really dangerous. Droplets of vegetable oil spill out in the kitchen when you pour small water droplets into the oil at red-hot temperature. The reasons are straighforward. The boiling point of oil is about 200 oC, which is much higher than that of water, so water is heated up and it changes to steam before the boiling of oil. As the steam at 100 oC has a volume which is 1700 times that of liquid water in room temperature. Droplets of oil spill out everywhere.
Science in Depth
Information for your reference: We quite often use 18 g of water in our calculation, since one "mole" of water contains 18 g in mass and the term "mole" is a very useful quantity (unit) in chemistry. We regard one dozen to represent 12 objects in daily life, but in science, we adopt one mole to represent 6.02 × 1023 molecules. The volume of one mole of gas is fixed at 22.4 dm3 at 0 oC. At a higher temperature, say, 100 oC, one mole of gas has a volume of 30.6 dm3 after calculations. Remind that one mole of liquid water has 18 g in mass, thus has a volume of 18 cm3. The ratio in expansion, 1700 (mentioned in the last paragraph), is obtained readily if we divide 30.6 dm3 by 18 cm3. (Note: 1 dm3 = 1000 cm3).

Pour the Ketchup

Sometimes you find the ketchup in the bottle hard to pour out from it when it is not shaked for a long time. A "genius" (it could be your mother or father when you was young) comes and tells you to do the following.

Oh, it works. The sauce comes out readily. You can get the last glops of ketchup from the bottom of the bottle.

How Things Work
Imagine you swing a small mass which is attached to one end of a string. The mass moves in a circular path and maintains a constant radial distance from your hand. Clearly, the mass is pulled and constrained by a force (tension) which points toward your hand (swing center) along the string (read more in "Science in Depth"). The figure in the right describes the motion of the mass constrainted in a horizontal circular path by a string.

Now, let us think about the force on the ketchup droplet. Suppose every ketchup droplet moves with the swinging bottle while its radial distance from the bottom of bottle unchanged, then the possible force which works similar to the tension in the string-mass system should be the viscous force among droplets. However, this force is not great enough (we have no string now) to maintain the sauce droplet at its original position in the bottle. So, sauce pours out when it is moving with angular change. Specifically, the migration of sauce towards the lid of bottle is the result of the so called 'centrifugal (pointing away from the center) force' which is referred to in the view of the swinging bottle. However, the centrifugal force is regarded as a 'fictitious force' in the view of the person who are sitting outside the rotating bottle.

You will find the above trick quite often applied in daily life.
  • A dog with wet hairs swings its body, water leaves its body by then.
  • You swing your wet umbrella before coming in a shopping mall.
  • The washing machine swings the wet clothes inside the drum in order to separate water from clothes.
  • You swing the sign pen when it is almost dry.

Science in Depth
Issac Newton (1642-1727), the greatest British scientist, announced his three primary laws of motions. The first law consider a mass on which no force acts. If the mass is at rest, it will remain at rest. If the mass is moving with constant velocity, it will continue to do so. This law is quite often described as the "law of inertia" and therefore it explains the need of a force (tension) in the string-mass system, since the mass ever changes it direction in a circular motion (see the figure above in the section "How Things Work"). The tension force in the string is specified as "centripetal (pointing towards the center) force" which points toward the swing center. The mass is pulled to change its direction while moving in a circular orbit. When such force disappears (e.g. the string is broken) the mass will no longer move in a circular path but instead a straight line.

The above discussion is basic and important in circular motion. Not surprisingly, you might struggle on its irrelevance to the case "swinging the ketchup bottle". Nevertheless, they are not isolated stories. As a illustration similar to swinging the ketchup bottle, the centrifugal motion is demonstrated in the right video on a merry-go-round. The weights leave the surface of the merry-go-round due to the rotation of the merry-go-round. Specifically, the merry-go-round is unlike the bottle which has bottle wall to govern the angular change of sauce droplet.

The Secret of "Simmer"

The Chinese healing soup is commonly prepared in families to cure minor complaints in body or to improve general health. The ginseng cooker (a special soup cooker; "ginseng" is the same as "ren shen") for "simmer" is specially designed to produce rich soup. With a ginseng cooker, which has two lids, good soup essence is not dissipated in steam. The cooking process is as follows. Place a ginseng cooker inside a larger pot along with enough water covering the ingredients. The pot is then filled with a large amount of water. During the cooking process, the flavour and essence of the ingredients are retained in the soup as water in the ginseng cooker never boils. You might be puzzled by the situation that "it never boils" despite of the stove.

How Things Work
The soup (water) in the ginseng cooker absorbs heat from the boiling water outside (i.e. the water in the pot) and reaches a temperature 100 oC. The ginseng cooker which is made of ceramic material acts as the medium to conduct heat in the cooking process. Apparently, water needs extra heat (latent heat) to change itself from liquid state to steam, and such heat is referred to as the latent heat of vaporisation. However, we notice that the water in the ginseng cooker and the pot is at the same temperature (100 oC), and it stops further thermal conduction. Thus, water in the ginseng cooker never boils and the flavour of soup never escapes from the cooker. It is obvious that the flavour will lose when water evaporates in boiling. Furthermore, the essences of soup, sometimes they are with medical uses, are not spoiled during the simmering process.

  • One should note that water boils at 100 oC, water temperature is then maintained without any increment even though you bring to the boil over high heat. Further supply of heat will just boil the water vigorously.
  • Heat flows (conducts) between objects when they have temperature difference. This is referred to as heat conduction. The rate of heat flow is directly proportional to the temperature difference. There will be no heat flow through conduction when two objects have identical temperature.

The Refrigerators

Refrigerators are heat-transfer machines (heat pumps), they move heat energy from inside the refrigerators to outside. Apparently, this machine is to against heat's natural flow, we know that in the domestic refrigerator, the flow is from a cold region to a hot region. The cold region is further cooled down. Refrigerators work by compressing a substance called a refrigerant and then letting it expands. Refrigerants are liquids with low boiling point (around 112 oF), able to change between liquid and gaseous states with ease at room temperature. A common refrigerant in early refrigerators is ammonia. Evaporisation and condensation are repeated in the refrigerators.

How Things Work
There are three states in matters, namely, the solid, liquid and gaseous states. In everyday life, we know the three states of water as solid ice, liquid water and gaseous steam respectively. The refrigerator works on the state changes of refrigerant, e.g. between liquid and gaseous states, as heat transfer takes place in such process.
  • Evaporation - changing state from a liquid to a gas - requires heat to take place, which is absorbed from the contents of the refrigerator.
  • The reverse change - condensation - gives out heat, which is released from the refrigerant to the outside of the refrigerator.
The latter is carried out by a compressor [1] which compresses the refrigerant and pumps it into the condenser (it is in the form of narrow pipe, highlighted in green in the figure), to form liquid. Heat is given out in the condenser, which contacts closely with thin metal vanes [3] on the back of the refrigerator. The vane transfers heat to the outside quicker and easier. Liquid refrigerant then flows to the evaporator (a wide pipe, highlighted in purple in the figure) after leaving the expansion valve [4], expands and evaporates due to the low pressure there, thus drawing in heat from the interior of the refrigerator. That is to say, we obtain a lower temperature (-4oF) inside the refrigerator cabinet. Warmer vapor flows back to the compressor and the whole cycle begins over again. Generally, the evaporator is installed around the icebox [2].

Convection currents carry cold air from the refrigerator, which is cooled to around 37oF. Internal temperature is controlled by a thermostat: this consists of a sealed, air-filled tube terminating in the refrigerator [5]. As air within the tube warms up, it expands, pushing out a set of bellows [6]. The expanding bellows close an electrical switch [7], which switches on the compressor. The refrigerator cabinet is made of polyurethane foam [8]: this functions as an insulator as well as giving mechanical strength.

  • In the 1930s, Chlorofluorocarbons (CFCs) were developed to replace ammonia as refrigerant because of its less toxicity. However, CFCs were found to catalyze the depletion of ozone in the Earth's upper atmosphere. Hydroflurocarbons (HFCs) is the refrigerants used in today's to reduce the damage to Earth's ozone layer due to CFCs.
  • Refrigeration is perhaps the most widely used preservation methods because it retains the taste, quality, and nutritive value of the fresh food. Refrigerating food to between 32 and 40 oF retards the activity of autolytic enzymes and slows the growth of microorganisms. These effects are greater if the food is frozen (at between 0 and −30oF), partly because water is converted into ice and becomes unavailable to microorganisms. Freezing also kills certain parasites outright.
  • Most foods are quick-frozen: cooled from 32 to 25 oF in less than 30 minutes. This allows only tiny ice crystals to form within the food: these are too small to affect its texture or appearance. Slow freezing forms large ice crystals which cause cells in food to rupture.
  • Foods with a high moisture content supports the growth of molds and bacteria, so dehydrating a food to a low level in moisture is an effective preservation. In fact, sun-drying, salting, and smoking are dehydration methods that have been practiced for centuries.

The Microwave Oven

Since humans learned how to create fire, heat has been used to process food. In conventional stoves, heat is generated either by a controllable gas flame or by an electric heating element and is transferred to the food by conduction, As over half the heat energy is lost to the environment by conventional appliance, stoves have been developed that transfer energy more directly to the food. The most revolutionary of these new cooking technologies is the microwave oven, which was patented in 1953, but early models were too cumbersome for use in the home. Smaller and more efficient microwave ovens were developed in the 1970s, and since then they have become increasingly popular both in homes and restaurants. The oven is said to be unconvenional in cooking, as it cooks foods from within.

How Things Work
  • Background knowledge:
    • The microwaves in the oven have frequencies of around 2450 MHz (1M = 1 million = 1,000,000 = 106). Recall that visible light has much higher frequencies which range from 430,000,000 MHz for red to 750,000,000 MHz for violet.
    • There are many types of electromagnetic waves categoried by their frequencies (or wavelengths, since they are closely related. In general, longer is the wavelength, shorter is the frequency). For your reference, different types of electromagnetic waves are listed in the following figure. The microwaves that we are using in the oven are around 10 cm in wavelength.

  • Mechanism:

    A microwave oven [A] cooks rapidly by using high-frequency electromagnetic waves (microwaves) to agitate molecules within food. Inside the oven, transformers produce a voltage high enough to supply a magnetron [B] - a type of cathode ray tube (electron emission tube) that can generate microwaves. The coiled central filament of the magnetron (i.e. the cathode) emits electrons. Magnetic and electric fields within the magnetron make the free electrons bunch up into a "packet" and move quickly around a circular path, passing by a series of metal plates. As the electron packet nears a plate, it induces in it an opposite (positive) charge, and a negative charge is produced in its neighors. Because the electron packet moves rapidly, the charge on each plate oscillates between positive and negative billions of times per second. A short antenna connected to one of the plates converts this oscillation of electrons into microwaves with a frequency of 2450 MHz. The waves are "guided" by a hollow metal tube to a series of rotating metal paddles, which spread the radiation evenly over the food.

    The microwaves - which can be thought of as oscillating electric fields - then penetrate the food. Water molecules in the food [C] have a slight positive charge at one end and a negative charge at the other. Exposed to microwaves, they flip over billions of times a second to realign themselves with the oscillating electric field. This movement generates heat, which cooks the food.

Why Do We Heat Our Food?
  • Cooking foods lead to a wider range of foods that we can eat. Foods which would otherwise be indigestible can be made edible. For example we cannot digest raw potatoes, since the starch is in a form our stomach is not capable of processing; however, by heating to a high enough temperature the starch is altered, and it becomes edible.
  • Cooking food can lead to a reduced risk of food poisoning. Some foods may contain toxins (e.g. pork), these toxins can often be destroyed by the application of heat.
  • Cooking can also change the texture of foods; for example the "tenderising" of some meats can make otherwise unappetising food more appealing, and increase the available food supply.
  • Cooking often leads to chemical reactions that change the flavour of foods by breaking down large molecules (which we cannot taste) into smaller molecules that we can taste.
  • At high temperatures, the autolytic enzymes present in foods are deactivated and most microorganisms are killed. Cooking can therefore be considered to be a form of short-term preservation.

The Superheat of Water in Microwave Oven

You heat a cup of water using a microwave oven. When you remove the cup from the oven, the water is very hot, but does not seem to be boiling. But as soon as you add a spoonful of instant coffee powder, the liquid immediately seems to begin boiling again, and may even overflow the cup. It is very dangerous. What seemed to happen was that the water started to boil when the powder was added.

You may guess that the coffee powder lowers the boiling point of the water and hence the water temperature would instantly be above the boiling point, and the water would begin to boil. No! Absolutely not!! It is the result of "superheating". Superheating means the heating of a liquid to a temperature above its normal boiling point. The superheated state is unstable, the liquid stays in its liquid state at or above the boiling point. However, it rapidly turns into a substantial quantity of vapor when the unstable state collapses.

How Things Work
  • Background knowledge:
    • The water in a microwave oven is heated up from itself by microwave. However, when water is heated on a stove, heat flows into the water from the hot vessel containing it.
    • When a liquid boils, molecules of the liquid rapidly pass from the bulk liquid into vapor bubbles. The vapor/liquid interface is higher in energy than either the vapor phase or the liquid phase. To make a bubble in the first place, a lot of liquid has to be moved out of the way, because vapor is much less dense than liquid. The new vapor must push outward against both the internal pressure of the liquid and the vapor/liquid interface, which provides additional force to collapse the bubble. So, it takes extra energy to form a bubble that can act as a nucleus for boiling. When water is heated in a pot on the stove, the hottest region of the liquid is that right next to the pot wall.
    • In addition, the pot walls usually contain many small scratches that hold small bubbles of air, which can act as starters for the boiling process. The heated water doesn't need to create bubbles from nothing. With the hottest liquid right next to the bubbles that start the boiling, conditions are ideal for boiling to start when water is heated on a stove.
  • Heating water in a glass, mug with microwaves makes boiling more difficult for two reasons.
    • The glasses and glazed ceramic contain fewer surface scratches than metal pots. This provides fewer starter bubbles. In addition, the containers used in microwave oven are very smooth and they sometimes give superheating.
    • Water is so commonly superheated in microwaves is that the hottest portion of the liquid is not right next to the pot surface and its little air bubbles. When water in the middle of the cup is heated to above its boiling point, it is very hard for it to create the bubble necessary for boiling.
  • In the movie you just saw, the fork and the coffee powder simply provide "nuclei" for the water to boil in the form of small bubbles of air.

When does it happen? The following conditions promote these potentially dangerous events:
  • Using a container with a very smooth surface, such as an unscratched glass or glazed container.
  • Heating for too long.
  • Quickly adding a powder, such as instant coffee (or sometimes even an object to stir it).
  • Standing with one's face above the container makes injury more likely.
Why does it occur to a greater degree in microwave ovens than in saucepans or kettles?
  • In a microwave oven, the water is usually hotter than the container, whereas parts of the kettle are usually hotter than the water. These hotter parts provide extra energy to form bubble which has the bubble surface, that is, the liquid/vapor phase.
  • Further, the surfaces of some containers used in microwave ovens may be very smooth, almost at a molecular scale, whereas this is not true for kettles.
  • If water is heated on a stove in a smooth stainless steel pot or a glass (Pyrex) pot, it can sometimes superheat. Putting something solid into the water, such as pasta, will cause the water to boil furiously. This is because the solid surface carries with it lots of little air bubbles that can act as nuclei for boiling.
  • Microwave ovens heat the water directly: the microwaves pass through the container and the water, and the water itself absorbs energy from them.
  • In a kettle, the container itself is hotter than the water. The hottest points cause a small amount of local superheating, boiling is initiated here by small bubbles, and this then stirs the water.
How to avoid it?
  • Before putting the water into the oven, insert a non-metal object with a surface that is not smooth. (e.g. a wooden stirrer. A wooden skewer or icecream stick will do.)
  • Use a container whose surface is at least a little scratched.
  • Do not heat for longer than the recommended time for the quantity of water used.
  • Tap the outside of the container a few times with a solid object while it is still in the oven. Use a long object so that your hand remains outside the oven. Alternatively, and still keeping your hand outside the oven, insert a stirrer while the container is still in the oven. (Thus, if vigorous boiling occurs, most of the boiling water will strike the inside of the oven.)
  • Keep your face well away from the open oven door and from the container.

The Smell of Fish

We are all familiar with the smell of fish, however, freshly caught fish have no odour at all. The "fishy" smell develops as chemical reations take place in the flesh of the fish sometime after they have been caught. The "fishy" smell cannot be eliminated completely even though the fish are preserved under low temperatures. Do you know why?

How Things Work
  • Fish are "cold-blooded" - they do not control their body temperatures but rather operate at the temperature of their surroundings. Well, many fish live in cold seas, so that the enzymes they use to metabolise food need to be active at low temperatures (sometimes as low as 4oC). On the other hands, the enzymes in meat work at body temperature (usually 36 to 38oC) and are only very slightly reactive at low temperatures. The end products of some enzymatic reactions will start to accumulate once the fish is dead and its circulatory system is no longer functioning. It is these waste products that give fish its characteristic smell - the "fishy" smell.
  • Next, the action of bacteria introduces such flavours and smells. The bacterial reactions generally begin only after the end of "rigor mortis" (stiffness of death, that is the rigidity of muscles occurring after death) which is usually after about 6 hours. Generally, when the fish is stored in ice, the end of rigor mortis can be delayed up to as long as a week, so keeping the fish fresher longer.

When you are buying fish it can be useful to remember that the fresher a fish, the less strong will be its smell. You should always eat fish as soon after it has been caught as is possible. Remember that the enzymatic reactions that can themselves consume the flesh cannot be delayed by refrigeration as they can for meats, so you need to eat the fish before its own enzymes eat it!

The Color of Meats

You may think that the red color of meats comes from the haemoglobin in the blood. However, there is not enough haemoglobin that is absorbed into the muscles making the meat as red as it is. So, what makes the meat red? On the other hand, the flesh of fish is white in color. Do you know why?

How Things Work
  • Haemoglobin (a special molecule) is used to carry oxygen around our bodies in our blood. If the muscles is to do a lot of work, it may not be able to get all the oxygen it needs from the blood. Instead some oxygen is stored in special proteins (myoglobin) in the muscles. These myoglobin molecules are similar to the haemoglobin in the blood and they also have a red color when oxygenated, and adopt a purple hue when they have given up their oxygen.
  • Heavily used muscles need lots of myoglobin, and hence are dark. Infrequently used muscles need little myoglobin and the meat from these muscles is light in color. As an example in turkey, it stands around a lot, but hardly ever fly, the leg meat is dark, while the breast is white.
  • Muscule has two types of fibres which use different chemical routes to provide their energy.
    • The "slow" fibres burn fats to provide the energy; these muscles need oxygen to operate. For example, we all use our leg and back muscles all the time, just to stay upright, so these muscules need to contain almost entirly "slow" muscles.
    • The "fast" fibres burn glycogen (a complex carbohydrate), and do no need to use any oxygen. Accordingly, muscles made from "fast" fibres do not need any myoglobin and are always white.

  • Fish are supported by water in which they swim and do not need to use their muscles to stay still, so fish is generally white in color. As we know that fish have muscles that are predominantly made from "fast" fibres. The figure below shows the pacific halibut with white flesh.

Some fish, notably, sharks and wild salmon are denser than the water in which they live; they literally have to keep on swimming all the time to stay up. These fish need some slow fibres and in consequence have darker meat than most other fish. The figure below shows the wild pacific chum salmon with red flesh.


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