McGee, H. (2004) On Food and Cooking: The Science
and Lore of the Kitchen. Scribner: New York, NY.
CHAPTER 15
THE FOUR BASIC
FOOD MOLECULES
Water
Water Clings Strongly to Itself
Water Is Good at Dissolving
Other Substances
Water and Heat: From Ice
to Steam
Water and Acidity: The pH
Scale
Fats, Oils, and Relatives: Lipids
Lipids Don't Mix with Water
The Structure of Fats
Saturated and Unsaturated Fats,
Hydrogenation, and Trans
Fatty Acids
Fats and Heat
Emulsifiers: Phospholipids,
Lecithin, Monoglycerides
Carbohydrates
Sugars
0ligosaccharides
Polysaccharides: Starch, Pectins,
Gums
Proteins
Amino Acids and Peptides
Protein Structure
Proteins in Water
Protein Denaturation
Enzymes
793
793
794
794
795
797
797
797
798
This chapter describes the characters of the
four chemical protagonists in foods and
the cooking process, the molecules referred
to constantly in the first fourteen chapters.
•
•
Water is the major component of
nearly all foods-and of ourselves! It's
also a medium in which we heat foods
in order to change their flavor, tex­
ture, and stability. One particular
property of water solutions, their
acidity or alkalinity, is a source of
flavor, and has an important influ­
ence on the behavior of the other
food molecules.
Fats, oils, and their chemical relatives
are water's antagonists. Like water,
they're a component of living things
792
•
801
802
803
803
803
804
805
806
806
808
808
809
and of foods, and they're also a cook­
ing medium. But their chemical nature
is very different-so different that they
can't mix with water. Living things
put this incompatibility to work by
using fatty materials to contain the
watery contents of cells. Cooks put
this quality to work when they fry
foods to crisp and brown them, and
when they thicken sauces with micro­
scopic but intact fat droplets. Fats also
carry aromas, and produce them.
Carbohydrates, the specialty of
plants, include sugars, starch, cellu­
lose, and pectic substances. They
generally mix freely with water. Sug­
ars give many of our foods flavor,
while starch and the cell-wall carbo-
WATER
•
hydrates provide bulk and texture.
Proteins are the sensitive food mole­
cules, and are especially characteris­
tic of foods from animals: milk and
eggs, meat and fish. Their shapes and
behavior are drastically changed by
heat, acid, salt, and even air. Cheeses,
custards, cured and cooked meats,
and raised breads all owe their tex­
tures to altered proteins.
WATER
Water is our most familiar chemical com­
panion. It's the smallest and simplest of
the basic food molecules, just three atoms:
H20, two hydrogens and an oxygen. And
its significance is hard to overstate. Leaving
aside the fact that it shapes the earth's con­
tinents and climate, all life, including our
own, exists in a water solution: a legacy of
life's origin billions of years ago in the
oceans. Our bodies are 60% water by
weight; raw meat is about 75% water, and
fruits and vegetables up to 95%.
H
0
H
793
WATER CLINGS STRONGLY
TO ITSELF
The important properties of ordinary water
can be understood as different manifesta­
tions of one fact. Each water molecule is
electrically unsymmetrical, or polar: it has
a positive end and a negative end. This is
because the oxygen atom exerts a stronger
pull than the hydrogen atoms on the elec­
trons they share, and because the hydrogen
atoms project from one side of the oxygen
to form a kind of V shape: so there's an
oxygen end and a hydrogen end to the
water molecule, and the oxygen end is more
negative than the hydrogen end. This polar­
ity means that the negative oxygen on one
water molecule feels an electrical attrac­
tion to the positive hydrogens on other
water molecules. When this attraction
brings the two molecules closer to each
other and holds them there, it's called a
hydrogen bond. The molecules in ice and
liquid water are participating in from one
to four hydrogen bonds at any given
moment. However, the motion of the mol­
ecules in the liquid is forceful enough to
overcome the strength of hydrogen bonds
and break them: so the hydrogen bonds in
liquid water are fleeting, and are constantly
being formed and broken.
+
Water molecules. Here are three different ways of representing a molecule of water, which is
formed from one oxygen and two hydrogen atoms. Because the oxygen atom exerts a stronger
pull on the electrons (small dots) it shares with the hydrogen atoms, the water molecule is elec­
trically unsymmetrical. The separation of positive and negative centers of charge leads to the
formation of weak bonds between oppositely charged centers on different molecules. These
weak bonds between molecules, shown here by dashed lines, are called hydrogen bonds.
794
THE FOUR BASIC FOOD MOLECULES
This natural tendency of water mole­
cules to form bonds with each other has a
number of effects in life and in the kitchen.
WATER Is GOOD AT DISSOLVING
OTHER SUBSTANCES
Water forms hydrogen bonds not only with
itself, but with other substances that have at
least some electrical polarity, some uneven­
ness in the distribution of positive and neg­
ative electrical charges. Of the other major
food molecules, which are much larger and
more complex than water, both carbohy­
drates and proteins have polar regions.
Water molecules are attracted to these
regions and cluster around them. When they
do this, they effectively surround the larger
molecules and separate them from each
other. If they do this more or less completely,
so that each molecule is mostly surrounded
by a cloud of water molecules, then that
substance has dissolved in the water.
WATER AND HEAT:
FROM ICE TO STEAM
The hydrogen bonds among its molecules
have a strong effect on how water absorbs
and transmits heat. At low temperatures,
water exists as solid ice, its molecules
immobilized in organized crystals. As it
warms up, it first melts to become liquid
water; and then the liquid water is vapor­
ized to form steam. Each phase is affected
by hydrogen bonding.
Ice Damages Cells Normally, the solid
phase of a given substance is denser than
the liquid phase. As the molecules' attrac­
tion for each other becomes stronger than
their movements, the molecules settle into
a compact arrangement determined by
their geometry. In solid water, however, the
molecular packing is dictated by the
requirement for even distribution of hydro­
gen bonds. The result is a solid with more
space between molecules than the liquid
phase has, by a factor of about one­
eleventh. It's because water expands when
it freezes that water pipes burst when the
heat fails in winter; that bottles of beer put
in the freezer for a quick chill and then
forgotten will pop open; that containers
of leftover soup or sauce will shatter in the
freezer if they're too full for the liquid to
expand freely. And it's why raw plant and
animal tissues are damaged when they're
frozen and leak liquid when thawed. Dur­
ing freezing, the expanding ice crystals rup-
Hard Water: Dissolved Minerals
Water is so good at dissolving other substances that apart from distilled water, it's sel­
dom found in anything like pure form. Tap water is quite variable in compositi on,
depending on its ultimate source (well, lake, river) and its municipal treatment (chlo­
rination, fluoridation, and so on). Two common minerals in tap water are carbonate
(C03) and sulfate (504) salts of c alcium arid magnesium. Calcium and magnesium
ions are troublesome because they react with soaps t o form insol uble scums, and
because they leave crusty precipitates on showerheads and teapots. Such so-called hard
water can also affect the color and texture of vegetables, and the consistency of bread
dough (pp. 282, 535). Hard water can be softened either city wide or in the home, usu­
ally by one of two methods: precipitating the calcium and magnesium by adding
lime,. or using an ion-exchange mechanism to replace the calcium and magnesium with
sodium. Distilled water, which is produced by boiling ordinary water and collecting the
condensed steam, is fairly free ot impurities.
-
WATER
795
ture cell membranes and walls, which then
lose internal fluids when the crystals melt.
energy from the food or its surroundings
and causes it to cook more gently.
Liquid Water Is Slow to Heat Up Again
thanks to the hydrogen bonding between
water molecules, liquid water has a high
specific heat, the amount of energy required
to raise its temperature by a given amount.
That is, water absorbs a lot of energy
before its temperature rises. For example, it
takes 10 times the energy to heat an ounce
of water 1° as it does to heat an ounce of
iron 1°. In the time that it takes to get an
iron pan too hot to handle on the stove,
water will have gotten only tepid. Before
the heat energy added to the water can
cause its molecules to move faster and its
temperature to rise, some of the energy
must first break the hydrogen bonds so
that the molecules are free to move faster.
The basic consequence of this character­
istic is that a body of water--our body, or a
pot of water, or an ocean--can absorb a lot
of heat without itself quickly becoming hot.
In the kitchen, it means that a covered pan
of water will take more than twice as long
as a pan of oil to heat up to a given temper­
ature; and conversely, it will hold that tem­
perature longer after the heat is removed.
Steam Releases a Lot of Heat When It
Condenses into Water Conversely, when
Liquid Water Absorbs a Lot of Heat
as It Vaporizes into Steam Hydrogen
bonding also gives water an unusually high
"latent heat of vaporization," or the
amount of energy that water absorbs with­
out a rise in temperature as it changes from
a liquid to a gas. This is how sweating cools
us: as the water on the skin of our over­
heated body evaporates, it absorbs large
amounts of energy and carries it away into
the air. Ancient cultures used the same prin­
ciple to cool their drinking water and wine,
storing them in porous clay vessels that
evaporate moisture continuously. Cooks
take advantage of it when they bake delicate
preparations like custards gently by partly
immersing the containers in an open water
bath, or oven-roast meats slowly at low
temperatures, or simmer stock in an open
pot. In each case, evaporation removes
water vapor hits a cool surface and con­
denses into liquid water, it gives up that
same high heat of vaporization. This is why
steam is such an effective and quick way of
cooking foods compared with plain air­
also a gas-at the same temperature. We
can put a hand into an oven at 212°F/
100°C and hold it there for some time
before it gets uncomfortably warm; but a
steaming pot will scald us in a second or
two. In bread baking, an initial blast of
steam increases the dough's expansion, or
oven spring, and produces a lighter loaf.
WATER AND ACIDITY:
T HE PH SCALE
Acids and Bases Despite the fact that the
molecular formula for water is H20, even
absolutely pure water contains other com­
binations of oxygen and hydrogen. Chem­
ical bonds are continually being formed
and broken in matter, and water is no
exception. It tends to "dissociate" to a
slight extent, with a hydrogen occasion­
ally breaking off from one molecule and
rebonding to a nearby intact water mole­
cule. This leaves one negatively charged
OH combination, and a positively charged
H30. Under normal conditions, a very
small number of molecules exist in the dis­
sociated state, something on the order of
two ten-millionths of a percent. This is a
small number but a significant one, because
the presence of relatively mobile hydrogen
ions, which are the basic units of positive
charge (protons), can have drastic effects
on other molecules in solution. A struc­
ture that is stable with a few protons
around may be unstable when many pro­
tons are in the vicinity. So significant is the
proton concentration that humans have a
specialized taste sensation to estimate it:
sourness. Our term for the class of chemical
compounds that release protons into solu-
THE FOUR BASIC FOOD MOLECULES
tions, acids, derives from the Latin acere,
meaning to taste sour. We call the comple­
mentary chemical group that accepts pro­
tons and neutralizes them, bases or alkalis.
The properties of acids and bases affect
us continually in our daily life. Practically
every food we eat, from steak to coffee to
oranges, is at least slightly acidic. And the
degree of acidity of the cooking medium
can have great influence on such charac­
teristics as the color of fruits and vegetables
and the texture of meat and egg proteins.
Some measure of acidity would clearly be
quite useful. A simple scale has been
devised to provide just that.
The pH Scale The standard measure of
proton activity in solutions is pH, a term
suggested by the Danish chemist S. P. L.
S0renson in 1909. It's essentially a more con­
venient version of the minuscule percent-
" ·"
-
+
ages of molecules involved (for some details,
see box below). The pH scale runs from 0 to
14. The pH of neutral, pure water, with
equal numbers of protons and OH ions, is
set at 7. A pH lower than 7 indicates a
greater concentration of protons and so an
acidic solution, while a pH above 7 indi­
cates a greater prevalence of proton accept­
ing groups, and so a basic solution. Here's a
list of common solutions and their usual pH.
-
pH
Liquid
Human gastric juice
Lemon juice
Orange juice
Yogurt
Black coffee
Milk
Egg white
Baking soda in water
Household ammonia
a.ta.
,,.
..
1.3-3.0
2.1
3.0
4.5
5.0
6.9
7.6-9.5
8.4
11.9
•
,,..+
Acids. Acids are molecules that release reactive hydrogen ions, or protons, in water, where
neutral water molecules pick them up and become positively charged. The acids themselves
become negatively charged. Left: Water itself is a weak acid. Right: Acetic acid.
The Definition of pH
The pH of a solution is defined as "the negative logarithm of the hydrogen ion con­
centration expressed in moles per liter." The logarithm ofa number is the exp<:1�ent, or
power, to which 10 must be raised in order to obtain the number. For example� the
hydrogen ion co ncentration in pure water is 10-7 moles per liter, so the pH ofpure
water is 7. Larger concentrations are described by smaller n�ative exponents, so a
more acidic solution will have a pH lower than 7, and a less acidic, more. bask sOlution
will have a pH higher than 7. Each increment of 1 in pH signifies an increase or
decrease in proton concentration by a factor oflO; sothere are 1,000 times the num
ber of hydrogen ions in a solution of pH 5 as there are in a solution ofpH �·
­
FATS, OILS, AND RELATIVES
FATS, OILS, AND
RELATIVES: LIPIDS
LIPIDS DON'T MIX WITH WATER
Fats and oils are members of a large chemi­
cal family called the lipids, a term that comes
from the Greek for "fat." Fats and oils are
invaluable in the kitchen: they provide flavor
and a pleasurable and persistent smooth­
ness; they tenderize many foods by perme­
ating and weakening their structure; they're
a cooking medium that allows us to heat
foods well above the boiling point of water,
thus drying out the food surface to produce
a crisp texture and rich flavor. Many of
these qualities reflect a basic property of the
lipids: they are chemically unlike water, and
largely incompatible with it. And thanks to
this quality, they have played an essential
role in the function of all living cells from the
very beginnings of life. Because they don't
mix with water, lipids are well suited to the
job of forming boundaries-membranes­
between watery cells. This function is per­
formed mainly by phospholipids similar to
lecithin (p. 802), molecules that cooks also
use to form membranes around tiny oil
droplets. Fats and oils themselves are created
and stored by animals and plants as a con­
centrated, compact form of chemical energy,
packing twice the calories as the same
weight of either sugar or starch.
In addition to fats, oils, and phospho­
li pids, the lipid family includes beta­
carotene and similar plant pigments,
vitamin E, cholesterol, and waxes. These
are all molecules made by living things that
consist mainly of chains of carbon atoms,
with hydrogen atoms projecting from the
chain. Each carbon atom can form four
bonds with other atoms, so a given car­
bon atom in the chain is usually bonded to
two carbon atoms, one on each side, and
two hydrogens.
This carbon-chain structure has one
overriding consequence: lipids can't dissolve
in water. They are "hydrophobic" or
"water-fearing" substances. The reason for
797
this is that carbon and hydrogen atoms pull
with a similar force on their shared elec­
trons. So unlike the oxygen-hydrogen bond,
the carbon-hydrogen bond is not polar, and
the hydrocarbon chain as a whole is non­
polar. W hen polar water and nonpolar
lipids are mixed together, the polar water
molecules form hydrogen bonds with each
other, the long lipid chains form a weaker
kind of bond with each other (van der
Waals bonds, p. 814), and the two sub­
stances segregate themselves. Oils minimize
the surface at which they contact water by
coalescing into large blobs, and resist being
divided into smaller droplets.
Thanks to their chemical relatedness,
different lipids can dissolve in each other.
This is why the carotenoid pigments-the
beta-carotene in carrots, the lycopene in
tomatoes-and intact chlorophyll, whose
molecule has a lipid tail, color cooking fats
much more intensely than they do cooking
water.
Lipids share two other characteristics.
One is their clingy, viscous, oily consis­
tency, which results from the many weak
bonds formed between their long carbon­
hydrogen molecules. And those same mol­
ecules are so bulky that all natural fats,
solid or liquid, float on water. Water is a
denser substance due to its extensive hydro­
gen bonding, which packs its small mole­
cules more tightly together.
T HE STRUCTURE OF FATS
Fats and oils are members of the same
class of chemical compounds, the triglyc­
erides. They differ from each other only in
their melting points: oils are liquid at
room temperature, fats solid. Rather than
use the technical triglyceride to denote
these compounds, I'll use fats as the
generic term. Oils are liquid fats. These
are invaluable ingredients in cooking.
Their clingy viscosity provides a moist,
rich quality to many foods, and their high
boiling point makes them an ideal cooking
medium for the production of intense
browning-reaction flavors (p. 778).
THE FOUR BASIC FOOD MOLECULES
Glycerol and Fatty Acids Though they
contain traces of other lipids, natural fats
and oils are triglycerides, a combination
of three fatty acid molecules with one
molecule of glycerol. Glycerol is a short 3carbon chain that acts as a common frame
to which three fatty acids can attach them­
selves. The fatty acids are so named because
they consist of a long hydrocarbon chain
with one end that has an oxygen-hydrogen
group and that can release the hydrogen as
a proton. It's the acidic group of the fatty
acid that binds to the glycerol frame to
construct a glyceride: glycerol plus one fatty
acid makes a monoglyceride, glycerol plus
two fatty acids makes a diglyceride, and
glycerol plus three fatty acids makes a
triglyceride. Before it bonds to the glycerol
frame, the acidic end of the fatty acid is
polar, like water, and so it gives the free
fatty acid a partial ability to form hydrogen
bonds with water.
Fatty acid chains can be from 4 to about
HO
glycerol
E
35 carbons long, though the most com­
mon in foods are from 14 to 20. The prop­
erties of a given triglyceride molecule
depend on the structure of its three fatty
acids and their relative positions on the
glycerol frame. And the properties of a fat
depend on the particular mixture of triglyc­
erides it contains.
SATURATED AND UNSATURATED
FATS, HYDROGENATION,
AND TRANS FATTY Acrns
The Meaning of Saturation The terms
"saturated" and "unsaturated" fats are
familiar from nutrition labels and ongoing
discussions of diet and health, but their
meaning is seldom explained. A saturated
lipid is one whose carbon chain is satu­
rated-filled to capacity-with hydrogen
atoms: there are no double bonds between
carbon atoms, so each carbon within the
fatty acids
0
HO
o��
HO
triglyceride
Fats and fatty acids. Fatty acids are mainly chains of carbon atoms, shown here as black dots.
(Each carbon atom has two hydrogen atoms projecting from it; the hydrogen atoms are not
shown. ) A fat molecule is a triglyceride, which is formed from one molecule of glycerol and
three fatty acids. The acidic heads of the fatty acids are capped and neutralized by the glyc­
erol, so the triglyceride as a whole no longer has a polar, water-compatible end. The fatty­
acid chains can rotate around the glycerol head to form chair-like arrangements (bottom).
FATS, OILS, AND RELATIVES
chain is bonded to two hydrogen atoms.
An unsaturated lipid has one or more dou­
ble bonds between carbon atoms along its
backbone. The double-bonded carbons
therefore have only one bond left for a
hydrogen atom. A fat molecule with more
than one double bond is called polyunsat­
urated.
Fat Saturation and Consistency Satura­
tion matters in the behavior of fats because
double bonds significantly alter the geome­
try and the regularity of the fatty-acid chain,
and so its chemical and physical properties.
A saturated fatty acid is very regular and
can stretch out completely straight. But
because a double bond between carbon
atoms distorts the usual bonding angles, it
has the effect of adding a kink to the chain.
Two or more kinks can make it curl.
A group of identical and regular mole­
cules fits more neatly and closely together
than different and irregular molecules. Fats
HO
799
composed of straight-chain saturated fatty
acids fall into an ordered solid structure­
the process has been described as "zipper­
ing"-more readily than do kinked
unsaturated fats. Animal fats are about half
saturated and half unsaturated, and solid at
room temperature, while vegetable fats are
about 85% unsaturated, and are liquid oils
in the kitchen. Even among the animal fats,
beef and lamb fats are noticeably harder
than pork or poultry fats, because more of
their triglycerides are saturated.
Double bonds are not the only factor in
determining the melting point of fats. Short­
chain fatty acids are not as readily "zip­
pered" together as the longer chains, and so
tend to lower the melting point of fats.
And the more variety in the structures of
their fatty acids, the more likely the mixture
of triglycerides will be an oil.
Fat Saturation and Rancidity Saturated
fats are also more stable, slower to become
saturated
0
HO
HO
0
0
cis poly-unsaturated
HO
0
trans poly-unsaturated
Saturated and unsaturated fatty acids. An unsaturated fatty acid has one or more double
bonds along its carbon chain, and a rigid kink at that point in the chain. The structural irreg­
ularity caused by the double bond makes it more difficult for these molecules to solidify into
compact crystals, so at a given temperature, unsaturated fats are softer than saturated fats.
In the hydrogenation ofvegetable oils to make them harder, some cis-unsaturated fatty acids
are converted to trans-unsaturated fatty acids, which are less kinked and behave more like
a saturated fatty acid, both in cooking and in the body.
800
THE FOUR BASIC FOOD MOLECULES
rancid than unsaturated fats. The double
bond of an unsaturated fat opens a space
unprotected by hydrogen atoms on one
side of the chain. This exposes the carbon
atoms to reactive molecules that can break
the chain and produce small volatile frag­
ments. Atmospheric oxygen is just such a
reactive molecule, and is one of the major
causes of flavor deterioration in foods con­
taining fats. Water and metal atoms from
other food ingredients also help fragment
fats and cause rancidity. The more unsatu-
rated the fat, the more prone it is to deteri­
oration. Beef has a longer shelf life than
chicken, pork, or lamb because its fat is
more saturated and so more stable.
Some small volatile fragments of unsat­
urated lipids actually have desirable and
distinctive aromas. The typical aroma of
crushed green leaves and of cucumber both
come from fragments of membrane phos­
pholipids generated not just by oxygen,
but by special plant enzymes. And the char­
acteristic aroma of deep-fried foods comes
Saturated and Unsaturated Fatty Acids in Foods and Cooking Fats
Proportions of fatty acids are given as a percentage of the total fatty-acid content.
Fat or Oil
Saturated
Fatty Acids
Monounsaturated
Fatty Acids
Polyunsaturated
Fatty Acids
62
29
4
50
42
4
47
42
4
40
45
11
30
45
21
6
2
86
11
2
37
9
35
2
51
14
26
18
50
19
59
18
17
47
31
17
46
32
14
23
58
13
74
8
13
24
59
13
24
59
11
16
68
7
55
33
9
12
75
9
16
70
FATS, OILS,
AND
in part from particular fatty-acid fragments
created at high temperatures.
Hydrogenation: Altering Fat Satura­
tion For more than a century now, manu­
facturers have been making solid, fat-like
shortenings and margarines from liquid
seed oils to obtain both the desired tex­
ture and improved keeping qualities. There
are several ways to do this, the simplest
and most common being to saturate the
unsaturated fatty acids artificially. This
process is called hydrogenation, because it
adds hydrogen atoms to the unsaturated
chains. A small amount of nickel is added
to the oil as a catalyst, and the mixture is
then exposed to hydrogen gas at high tem­
perature and pressure. After the fat has
absorbed the desired amount of hydrogen,
the nickel is filtered out.
Trans Fatty Acids It turns out that the
hydrogenation process straightens a cer­
tain proportion of the kinks in unsaturated
fatty acids not by adding hydrogen atoms
to them, but by rearranging the double
bond, twisting it so that its bend is less
extreme. These molecules remain chemi­
cally unsaturated-the double bond
between two carbons remains-but they
have been transformed from an acutely
RELATIVES
801
irregular cis geometry to a more regular
trans structure (see illustration, p. 799).
Cis is Latin for "on this side of," and trans
for "across from"; the terms describe the
positions of neighboring hydrogen atoms
on the double bond between carbon
atoms. Because the trans fatty acids are
less kinked, more like a saturated fat chain
in structure, they make it easier for the fat
to crystallize and so make it firmer. They
also make the fatty acid less prone to
attack by oxygen, so it's more stable.
Unfortunately, trans fatty acids also resem­
ble saturated fats in raising blood choles­
terol levels, which can contribute to the
development of heart disease (p. 38).
Manufacturers will soon be required to list
the trans fatty acid content of their foods,
and they're beginning to implement other
processing techniques that harden fat con­
sistency without creating trans fatty acids.
FATS AND HEAT
Most fats do not have sharply defined
melting points. Instead, they soften gradu­
ally over a broad temperature range. As
the temperature rises, the different kinds of
fat molecules melt at different points and
slowly weaken the whole structure. (An
interesting exception to this rule is cocoa
omega-3 fatty acids
HO
0
linolenic acid
eicosapentaenoic acid
Omega-3 fatty acids. Omega-3 fatty acids are unsaturated fatty acids whose first double
bond begins at the third carbon atom from the end. (The most common unsaturated fatty
acids are omega-6 fatty acids.) They are essential in our diet for, among other things, the
proper function of the immune and cardiovascular systems. Linolenic acid has 3 double
bonds among its 18 carbon atoms, and is found in green leaves and in some seed oils. Eico­
sapentaenoic acid has 20 carbons and 5 double bonds, and is found almost exclusively in
seafood (p. 183).
802
THE FOUR BASIC FOOD MOLECULES
butter, p. 705). This behavior is especially
important in making pastries and cakes,
and it's what makes butter spreadable at
room temperature.
Melted fats do eventually change from a
liquid to a gas: but only at very high tem­
peratures, from 500° to 750°F/2604000C. This high boiling point, far above
water's, is the indirect result of the fats'
large molecular size. While they can't form
hydrogen bonds, the carbon chains of fats
do form weaker bonds with each other
(p. 814). Because fat molecules are capable
of forming so many bonds along their
lengthy hydrocarbon chains, the individu­
ally weak interactions have a large net
effect: it takes a lot of heat energy to knock
the molecules apart from each other.
The Smoke Point Most fats begin to
decompose at temperatures well below
their boiling points, and may even sponta­
neously ignite on the stovetop if their fumes
come into contact with the gas flame. These
facts limit the maximum useful temperature
of cooking fats. The characteristic temper­
ature at which a fat breaks down into vis­
ible gaseous products is called the smoke
point. Not only are the smoky fumes
obnoxious, but the other materials that
remain in the liquid, including chemically
active free fatty acids, tend to ruin the fla­
vor of the food being cooked.
The smoke point depends on the initial
free fatty acid content of the fat: the lower
the free fatty acid content, the more stable
the fat, and the higher the smoke point.
Free fatty acid levels are generally lower in
vegetable oils than in animal fats, lower in
refined oils than unrefined ones, and lower
in fresh fats and oils than in old ones. Fresh
refined vegetable oils begin to smoke
around 450°F/230°C, animal fats around
375°F/190°C. Fats that contain other sub­
stances, such as emulsifiers, preservatives,
and in the case of butter, proteins and car­
bohydrates, will smoke at lower tempera­
tures than pure fats. Fat breakdown during
deep frying can be slowed by using a tall,
narrow pan and so reducing the area of
contact between fat and atmosphere. The
smoke point of a deep-frying fat is low­
ered every time it's used, since some break­
down is inevitable even at moderate
temperatures, and trouble-making parti­
cles of food are always left behind.
EMULSIFIERS: PHOSPHOLIPIDS,
LECITHIN, MONOGLYCERIDES
Some very useful chemical relatives of the
true fats, the triglycerides, are the diglyc­
erides and monoglycerides. These mole­
cules act as emulsifiers to make fine,
cream-like mixtures of fat and water-such
sauces as mayonnaise and hollandaise­
even though fat and water don't normally
mix with each other. The most prominent
natural emulsifiers are the diglyceride phos­
pholipids in egg yolks, the most abundant
of which is lecithin (it makes up about a
third of the yolk lipids). Diglycerides have
only two fatty-acid chains attached to the
glycerol frame, and monoglycerides just
one, with the remaining positions on the
frame being occupied by small polar
groups of atoms. These molecules are thus
water-soluble at the head, and fat-soluble
at the tail. In cell membranes, the phos­
pholipids assemble themselves in two lay­
ers, with one set of polar heads facing the
watery interior, the other set the watery
exterior, and the tails of both sets mingling
in between. When the cook whisks some
fat into a water-based liquid that contains
emulsifiers-oil into egg yolks, for exam­
ple-the fat forms tiny droplets that would
normally coalesce and separate again. But
the emulsifier tails become dissolved in the
droplets, and the electrically charged heads
project from the droplets and shield the
droplets from each other. The emulsion of
fat droplets is now stable.
These "surface-active" molecules have
many other applications as well. For exam­
ple, monoglycerides have been used for
decades in the baking business because they
help retard staling, apparently by com­
plexing with amylase and blocking starch
retrogradation.
CARBOHYDRATES
CARBOHYDRATES
The name for this large group of molecules
comes from the early idea that they were
made up of carbon and water. They are
indeed made up of carbon, hydrogen, and
oxygen atoms, though the oxygen and
hydrogen are not found as intact water
complexes within the molecules. Carbohy­
drates are produced by all plants and ani­
mals for the purpose of storing chemical
energy, and by plants to make a supporting
skeleton for its cells. Simple sugars and
starch are energy stores, while pectins, cel­
lulose, and other cell-wall carbohydrates
are the plant's structural materials.
tant to all life because two of them, ribose
and deoxyribose, form the backbones of
ribonucleic acid (RNA) and deoxyribonu­
cleic acid (DNA), the carriers of the genetic
code. And the 6-carbon sugar glucose is
the molecule from which most living things
obtain the energy to run the biochemical
machinery of their cells. Sugars are such an
important nutrient that we have a special
sense designed specifically to detect them.
Sugars taste sweet, and sweetness is a
nearly universal source of pleasure. It's the
essence of the dishes we serve at the end of
the meal, as well as of candies and confec­
tions. Sugars and their properties are
described in detail in chapter 12.
0LIGOSACCHARIDES
SUGARS
Sugars are the simplest carbohydrates.
There are many different kinds of sugar
molecules, each distinguished by the num­
ber of carbon atoms it contains, and then
by the particular arrangement it assumes.
Five-carbon sugars are especially impor-
�
� oil
The oligosaccharides ("several-unit sug­
ars") raffinose, stachyose, and verbascose
are 3-, 4-, and 5-ring sugars, respectively, all
too large to trigger our sweet detectors, so
they're tasteless. They're commonly found
in the seeds and other organs of plants,
where they make up part of the energy sup-
~
phospholipid
Phospholipid emulsifiers. Phospholipids are diglycerides, and are excellent emulsifiers, mol­
ecules that make possible a stable mixture of oil and water. Unlike the triglycerides of fat and
oil, they have a polar, water-compatible head. Such emulsifiers bury their fatty-acid tails in
oil droplets, while their water-compatible, electrically charged heads project from the surface
and block the droplets from contacting each other and coalescing.
THE FOUR BASIC FOOD MOLECULES
ply. These sugars all affect our digestive
system, thanks to the fact that we don't
have digestive enzymes capable of breaking
them down into single sugars that can be
absorbed by the intestine. As a result, the
oligosaccharides are not digested and pass
intact into the colon, where various bacte­
ria do digest them, producing large quanti­
ties of carbon dioxide and other gases in the
process (p. 486).
POLYSACCHARIDES: STARCH,
PECTINS, GUMS
Polysaccharides, which include starch and
cellulose, are sugar polymers, or molecules
composed of numerous individual sugar
units, as many as several thousand. Usually
only one or a very few kinds of sugars are
found in a given polysaccharide. Polysac­
charides are classified according to the over­
all characteristics of the large molecules: a
general size range, an average composition,
and a common set of properties. Like the
sugars of which they're composed, poly­
saccharides contain many exposed oxygen
and hydrogen atoms, so they can form
hydrogen bonds and absorb water. How­
ever, they may or may not dissolve in water,
QOH H
H OH
glucose
depending on the attractive forces among
the polymers themselves.
Starch By far the most important polysac­
charide for the cook is starch, the com­
pact, unreactive polymer in which plants
store their supply of sugar. Starch is simply
a chain of glucose sugars. Plants produce
starch in two different configurations: a
completely linear chain called amylase, and
a highly branched form called amylopectin,
each of which may contain thousands of
glucose units. Starch molecules are
deposited together in a series of concen­
tric layers to form solid microscopic gran­
ules. When starchy plant tissue is cooked in
water, the granules absorb water, swell,
and release starch molecules; when cooled
again, the starch molecules rebond to each
other and can form a moist but solid gel.
Various aspects of starch-the way it deter­
mines the texture of cooked rice, its for­
mation into pure starch noodles, its role in
breads, pastries, and sauces-are described
in detail in chapters 9-11.
Glycogen Glycogen, or "animal starch,"
is an animal carbohydrate similar to amy­
lopectin, though more highly branched. It's
QOHH H9H0HO HtrHOH H
OHH
OHH
0
H
OHOHH
HOH HOH H H
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amylose
A sugar, glucose, and a polysaccharide, starch, which is a chain of glucose molecules.
Plants produce two broadly different forms of starch: simple long chains called amylase, and
highly branched chains called amylopectin.
805
PROTEINS
a fairly minor component of animal tissue
and so of meats, although its concentration
at the time of slaughter will affect the ulti­
mate pH of the meat, and thereby its tex­
ture {p. 142).
bers of the onion and lettuce families,
notably garlic and the sunchoke. Like the
oligosaccharides, inulin is not digestible,
and so feeds bacteria in our large intestine
and generates gas.
Cellulose Cellulose is, like amylose, a lin­
ear plant polysaccharide made up solely
of glucose sugars. Yet thanks to a minor
difference in the way the sugars are linked
to each other, the two compounds have
very different properties: cooking dissolves
starch granules but leaves cellulose fibers
intact; most animals can digest starch, but
not cellulose. Cellulose is a structural sup­
port that's laid down in cell walls in the
form of tiny fibers analogous to steel rein­
forcing bars, and it's made to be durable.
Few animals can digest cellulose, and hay­
eating cattle and wood-eating termites can
do so only because their guts are popu­
lated by cellulose-digesting bacteria. To
other animals, including ourselves, cellu­
lose is indigestible fiber (which has its own
value; see p. 258).
Plant Gums There are a number of other
plant carbohydrates that cooks and manu­
facturers have found useful for thickening
and gelling liquid foods, helping to stabi­
lize emulsions, and producing smoother
consistencies in frozen goods and candies.
Like the cell-wall cements, they're generally
complex polymers of several different sug­
ars or related carbohydrates. They include:
•
•
•
•
Agarose, alginates, and carrageenans,
cell-wall polymers from various sea­
weeds
Gum arabic, which exudes from cuts
in various species of Acacia trees
Gum tragacanth, an exudate from
various species of Astralagus shrubs
Guar gum, from seeds of a shrub in
the bean family ( Cyamopsis
tetragonobola)
Hemicelluloses and Pectic Substances
These polysaccharides (made from a vari­
ety of sugars, including galactose, xylose,
arabinose) are found together with cellu­
lose in the plant cell walls. If the cellulose
fibrils are the reinforcing bars in the cell
walls, the amorphous hemicelluloses and
pectic substances are a sort of jelly-like
cement in which the bars are embedded.
Their significance for the cook is that,
unlike cellulose, they are partly soluble in
water, and therefore contribute to the soft­
ening of cooked vegetables and fruits.
Pectin is abundant enough to be extracted
from citrus fruits and apples and used to
thicken fruit syrups into jams and jellies.
These carbohydrates are described in detail
in chapter 5.
Inulin Inulin is a polymer of fructose sug­
ars, from a handful to hundreds per mole­
cule. lnulin is a form of energy storage and
a source of antifreeze (sugars lower the
freezing point of a water solution) in mem-
•
•
Locust-bean gum, from seeds of the
carob tree, Ceratonia siliqua
Xanthan gum and gellan, polysac­
charides produced by certain bacteria
in industrial fermentation
PROTEINS
Of all the major food molecules, proteins
are the most challenging and mercurial.
The others, water and fats and carbohy­
drates, are pretty stable and staid. But
expose proteins to a little heat, or acid, or
salt, or air, and their behavior changes dras­
tically. This changeability reflects their bio­
logical mission. Carbohydrates and fats are
mainly passive forms of stored energy, or
structural materials. But proteins are the
active machinery of life. They assemble all
the molecules that make a cell, themselves
included, and tear them down as well; they
move molecules from one place in the cell
to another; in the form of muscle fibers,
806
THE FOUR BASIC FOOD MOLECULES
they move whole animals. They're at the
heart of all organic activity, growth, and
movement. So it's the nature of proteins
to be active and sensitive. When we cook
foods that contain them, we take advantage
of their dynamic nature to make new struc­
tures and consistencies.
AMINO Acrns AND PEPTIDES
Like starch and cellulose, proteins are large
polymers of smaller molecular units. The
smaller units are called amino acids. They
consist of between 10 and 4 0 atoms,
mainly carbon, hydrogen, and oxygen, with
at least one nitrogen atom in an amine
group-NH2-that gives the amino acids
their family name. A couple of amino acids
include sulfur atoms. There are about 20
different kinds of amino acids that occur in
significant quantities in food. Particular
protein molecules are dozens to hundreds
of amino acids long, and often contain
many of the 20 different kinds. Short
chains of amino acids are called peptides.
Amino Acids and Peptides Contribute
Flavor Three aspects of amino acids are
especially important to the cook. First,
amino acids participate in the browning
reactions that generate flavor at high cook­
ing temperatures (p. 778). Second, many
single amino acids and short peptides have
tastes of their own, and in foods where
proteins have been partly broken down­
aged cheeses, cured hams, soy sauce-these
tastes can contribute to the overall flavor.
Most tasty amino acids are either sweet or
bitter to some degree, and a number of
peptides are also bitter. But glutamic acid,
better known in its concentrated commer­
cial form MSG (monosodium glutamate),
and some peptides have a unique taste that
is designated by such words as savory,
brothy, and umami (Japanese for "deli­
cious"). They lend an added dimension of
flavor to foods that are rich in them, includ­
ing tomatoes and certain seaweeds as well
as salt-cured and fermented products.
W hen heated, sulfur-containing amino
acids break down and contribute eggy,
meaty aroma notes.
Amino Acids Influence Protein Behav­
ior The third important characteristic of
amino acids is that they have a variety of
chemical natures, and these influence the
structure and behavior of the protein
they're a part of. Some amino acids have
portions resembling water and can form
hydrogen bonds with other molecules,
including water. Some have short carbon
chains or carbon rings that resemble fats,
and can form van der Waals bonds with
other similar molecules. And some, espe­
cially those that include a sulfur atom, are
especially reactive, and can form strong
covalent bonds with other molecules,
including other sulfur-containing amino
acids. This means that a single protein has
many different chemical environments
along its chain: parts that attract water mol­
ecules, parts that avoid water molecules,
and parts that are ready to form strong
bonds with similar parts on other proteins,
or on other parts of the same protein.
PROTEIN STRUCTURE
Proteins are formed by linking the amine
nitrogen of one amino acid with a carbon
atom on another amino acid, and then
repeating this "peptide bond" to make a
chain dozens or hundreds of amino acids
long. The carbon-nitrogen backbone of the
protein molecule forms a sort of zigzag
pattern, with the "side groups"-the other
atoms on each amino acid-sticking out
to the sides.
The Protein Helix One effect of the pep­
tide bond is a certain kind of regularity
that causes the molecule as a whole to
twist and form a spiral, or helix. Very few
proteins exist as a simple regular helix,
but those that do tend to join together in
strong fibers. These include connective-tis­
sue collagen in meat, an important factor
in its tenderness, and the source of gelatin
(pp. 130, 597).
807
PROTEINS
Protein Folds The other influence on pro­
including via hydrogen bonds, van der
Waals bonds, ionic bonds (p. 813 ), and
strong covalent bonds (especially between
sulfur atoms). This bonding is what gives a
particular protein molecule the character­
istic shape that allows it to carry out its
particular job. The weak, temporary nature
of the hydrogen and hydrophobic bonds
tein structure is the side groups of its amino
acids. Because the protein chain is so long,
it can bend back on itself and bring
together amino acids that are some dis­
tance along the chain from each other.
Amino acids with similar side groups can
then bond to each other in various ways,
o
� °'Lt}) �
o
N�
glutamic acid
'
o
N�
NH
n
N�
tryptophan
cysteine
�
'
Amino acids and proteins, denaturation and coagulation. Top: Three of the 20-odd amino
acids important in food. Each amino acid has a common end including an amino (NH;J
group, by which amino acids bond to each other into long chains called proteins, and a vari­
able end or "side group" that can form different kinds of bonds with other amino acids.
Center: A chain of amino acids shown schematically, with some of the side groups project­
ing from the chain. The amino acid chain can fold back on itself, and some of the side groups
form bonds with each other to hold the chain in a folded shape. Bottom: Heating and other
cooking processes can break the fold-stabilizing bonds and cause the long chains to unfold,
or denature (left, center). Eventually the exposed side groups form new bonds between dif­
ferent protein chains, and the proteins coagulate, or form a permanently bonded solid
mass (right).
808
THE FOUR BASIC FOOD MOLECULES
allows it to change its shape as it works.
The overall shape of a protein can range
from a long, extended, mostly helical mol­
ecule with a few kinks or loops, to com­
pact, elaborately folded molecules that are
called "globular" proteins. Collagen is an
example of a helical protein, and the vari­
ous proteins in eggs are mainly globular.
PROTEINS IN WATER
In living systems and in most foods, protein
molecules are surrounded by water. Because
all proteins are capable to some extent of
hydrogen bonding, they absorb and hold at
least some water, although the amounts
vary greatly according to the kinds of side
groups present and the overall structure of
the molecule. Water molecules can be held
"inside" the protein, along the backbone,
and "outside," on polar side groups.
Whether or not a protein is soluble in
water depends on the strength of the bonds
between molecules, and on whether water
can separate the molecules from each other
by hydrogen bonding. The wheat proteins
that form gluten when flour is mixed with
water are a kind of protein that absorbs
considerable amounts of water but doesn't
dissolve, because many fat-like groups
along their molecules bond with each other,
hold the proteins together, and exclude
water. Similarly, the proteins that make up
the contracting muscle fibers in meat are
held together by ionic and other bonds.
On the other hand, many of the proteins in
milk and eggs are quite soluble.
PROTEIN DENATURATION
A very important characteristic of proteins
is their susceptibility to denaturation, or
the undoing of their natural structure by
chemical or physical means. This change
involves breaking the bonds that maintain
the molecule's folded shape. (The strong
backbone bonds are broken only in
extreme conditions or with the help of
enzymes.) Denaturation is not a change in
composition, only a change in structure.
But structure determines behavior, and
denatured proteins behave very differently
from their originals.
Proteins can be denatured in many ways:
by exposing them to heat-usually to
somewhere between 140-180°F/60-80°C­
or to high acidity, or to air bubbles, or to a
combination of these. In each case, the
unusual chemical or physical conditions­
increased molecular agitation, or lots of
reactive protons, or the drastic difference
between the air bubble and the liquid wall
that surrounds it-breaks many of the
bonds between amino acid side groups that
hold the protein molecule in its specific
folded shape. The long proteins therefore
unfold, exposing many more of their reac­
tive side groups to the watery environment.
Protein Coagulation There are several
general consequences of denaturation that
follow for most food proteins. Because the
molecules have been extended in length,
they're more likely to bump into each other.
And because their side groups are now
exposed and available for forming bonds,
denatured proteins begin to bond with each
other, or coagulate. This happens through­
out the food, and results in the development
of a continuous network of proteins, with
water held in the pockets between protein
strands. The food therefore develops a kind
of thickness or density that can be delicate
and delightful, as in a barely set custard or
perfectly cooked piece of fish. However, if
cooking or other denaturing conditions
continue, given the extreme physical or
chemical environment that caused the pro­
teins to denature in the first place, only the
stronger bonds can form and survive, which
means that the proteins bond together more
and more tightly, densely, and irreversibly.
And as they do so, they squeeze the pockets
of water out from between them. The cus­
tard gets dense and a watery fluid sepa­
rates from the solid portion; the fish gets
tough and dry.
The details of protein denaturation and
coagulation in any given food are intricate
and fascinating. For example, acidity and
PROTEINS
salts can cause egg proteins to cluster
together even before they begin to unfold,
and thus affect the consistency of scrambled
eggs and custards. Such details are noted in
the descriptions of particular foods.
ENZYMES
There's a particular group of proteins that
are important to the cook not so much for
their direct contribution to food texture
and consistency, but for the way they
change the other components of the food
they're in. These proteins are the enzymes.
Enzymes are biological catalysts: that is,
they increase the rate of specific chemical
reactions that otherwise would occur only
very slowly, if at all. Enzymes thus cause
chemical change. Some enzymes build mol­
ecules up, or modify them; some break
molecules down. Human digestive
enzymes, for example, break proteins into
individual amino acids, and starch into
individual glucose units. A singe enzyme
molecule can catalyze as many as a million
reactions per second.
Enzymes matter to the cook because
foods contain enzymes that once did impor­
tant work for the plant or animal when it
was alive, but that can now harm the food
by changing its color, texture, taste, or
nutritiousness. Enzymes help turn green
chlorophyll in vegetables dull olive, cause
cut fruits to turn brown and oxidize their
vitamin C, and turn fish flesh mushy. And
bacterial spoilage is largely a matter of bac­
terial enzymes breaking the food down for
the bacteria's own use. With a few excep­
tions-the tenderizing of meat by its own
internal enzymes, the firming of some veg­
etables before further cooking, and fer­
mentations in general-the cook wants to
prevent enzymatic activity in food. Storing
foods at low temperatures delays spoilage
in part because it slows the growth of
spoilage microbes, but also because it slows
the activity of the food's own enzymes.
Cooking Accelerates Enzyme Action
Before Stopping It Because the activity of
an enzyme depends on its structure, any
change in that structure will destroy its
effectiveness. So cooking foods sufficiently
will denature and inactivate any enzymes
they may contain. One vivid example of
this principle is the behavior of raw and
cooked pineapple in gelatin. Pineapples and
certain other fruits contain an enzyme that
breaks proteins down into small fragments.
If raw pineapple is combined with gelatin to
make a jelly, the enzyme digests the gelatin
molecules and liquifies the jelly. But canned
pineapple has been heated enough to dena­
ture the enzyme, and makes a firm gelatin
jelly.
There's a complication, though. The
reactivity of most chemicals increases with
increasing temperature. The rule of thumb
is that reactivity doubles with each rise of
20°F/10°C. The same tendency goes for
enzymes, up to a range in which they begin
to denature, become less effective, and
finally become completely inactive. This
means that cooking gives enzymes a chance
to do their damage more and more quickly
as the temperature rises, and only stops
them once they reach their denaturation
temperature. In general, the best rule is to
heat foods as rapidly as possible, thereby
minimizing the period during which the
enzymes are at their optimum tempera­
tures, and to get them all the way to the
boiling point. Conversely, desirable enzyme
action-meat tenderizing, for example­
can be maximized by slow, gradual heating
to denaturing temperatures.