Four Basic Food Molecules
Food Science
WATER
• Major component of nearly all foods
---and ourselves
• Medium in which we heat foods in order to change:
flavor
texture
stability
•
pH of water :
source of flavor
influences behavior of other food molecules
Water and its Poles
(and I am NOT referring to the North and South poles!)
Each water molecule is electrically
unsymmertical or POLAR
• Oxygen exerts a stronger pull on electrons
than hydrogen (i.e. oxygen is more
electronegative)
• The hydrogen atoms bond to the oxygen atom
at a 107.5⁰ angle due to the two hydrogen
atoms and the 2 lone pairs of electrons on the
oxygen atom
• This forms a V shape
Polarity and Hydrogen Bonding
• The negative oxygen end of a water molecule
is attracted to the positive hydrogen end of
another water molecule
• A hydrogen bond forms when this attraction
brings the two water molecules closer to each
other and holds them there.
Importance of Hydrogen Bonding
• Decreases rate of vaporization
• Increases boiling point
• Enables solutes to change the boiling and
freezing point of water (colligative properties)
• Aids solubility of polar substances (salts,
minerals, vitamins, etc.)
• Affect pH of a substance
• Makes ice less dense than water
Effects of Hydrogen Bonds in Water
on Life and in the Kitchen
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Water and Dissolving
Water and Heat
Water and pH Scale
Water and chocolate
Water and Dissolving
Water forms hydrogen bonds not only with
itself, but with other substances that have at
least some polarity (negative and positive
ends)
Water molecules are attracted to the polar ends
of other molecules.
When these other polar molecules are almost
surrounded by water molecules than that
substance has DISSOLVED in the water.
When water molecules surrounds a sugar molecule, we say that
the sugar has dissolved.
Water and Heat
• At low temperature water exists as solid ice—
its molecules immobilized in organized
crystals.
• As the molecules movement slows down, the
attraction for each other becomes stronger
and the molecules settle into a geometric
arrangement that results in a solid with more
space between molecules than the liquid
phase.
Because water expands when it
freezes that is why:
Containers of soup or sauce will shatter in the
freezer if they’re too full for the liquid to
expand freely
Raw plant and animal tissues are damaged
when they’re frozen and leak liquid when
thawed—the expanding ice crystals rupture
cell membranes and walls
Ice floats—it is less dense (same mass but
larger volume) than liquid water
Liquid Water is Slow to Heat Up
• Due to hydrogen bonds, water has a high
specific heat capacity—the amount of energy
required to raise 1 gram by 1 degree Celsius
• Water takes 10 times the energy to heat 1
gram by 1⁰C as it does to heat 1 gram of iron
by 1⁰C.
WHY?
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 the so the
molecules are free to move faster
Hard Water: Dissolved Minerals
Water is so good at dissolving other substances
its seldom found in a pure form.
 Tap water varies in composition, depending on
its source (well, lake, river) and its municipal
treatment (chlorination, fluoridation)
 Two common minerals in tap water are:
Calcium carbonate (CaCO3)
Magnesium carbonate (MgCO3)
Calcium sulfate (CaSO4)
Magnesium sulfate (MgSO4)
 Two common minerals in tap water are:
Calcium carbonate (CaCO3)
Calcium sulfate (CaSO4)
Magnesium carbonate (MgCO3)
Magnesium sulfate (MgSO4)
• So called hard water can affect the color and
texture of vegetables and bread dough
The dissolved calcium in hard tap water slows
the softening of vegetables by reinforcing or
“cementing” the cross links in the cell walls.
• Hard water has few hydrogen ions that are
free to displace the magnesium in chlorophyll.
Therefore, green vegetables will be “greener”
when cooked in “hard” water.
• COOKS can make use of these influences of
water type by “softening” the water with a
pinch of baking soda and table salt. This will
shorten the softening time and help preserve
the bright green of the chlorophyll.
Water Absorbs a Lot of Heat as It
Vaporizes into Steam
• Hydrogen bonds gives water an unusually high
“latent heat of vaporization” or the amount
of energy that water absorbs without a rise in
temperature as it changes from a liquid to a
gas.
• This is how our sweating cools us: as the water
on our skin evaporates, it absorbs large
amounts of heat and carries it away into the
air.
• Ancient cultures used this same principle to
cool their drinking water and wine by storing
them in porous clay vessels that evaporate
moisture continuously, thus taking away the
“heat” as the water evaporated.
Cooks today take advantage of latent heat of
vaporization 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.
• Evaporation removes energy from the food or
its surrounds and causes it to cook more
gently.
Condensation
• When steam condenses into water it releases
A LOT OF HEAT
• That is why steam is an effective and quick
way of cooking foods compared with plain air.
• You can put your hand into an oven at 212⁰F
(100⁰C) oven and hold it there for sometime,
but a steam will scald us in a second!
• In baking bread, an initial blast of steam
increases the dough’s expansion.
Water and Acidity: the pH Scale
• Water tends to “dissociate” ever so slightly—
thus giving us H3O+ ions and OH- ions.
• The relative number of free H+ ions (H3O+ ions )
is very small…about 2/10,000,000 of 1%.
• However….this small number of free hydrogen ions
can have drastic effects on other molecules in
solution…so drastic that we have a specialized taste
sensation to estimate it: sourness.
Effect of pH
• A class of chemicals compounds that release
protons (commonly H+ ions) as called acids.
• We call the complimentary groups of
compounds that accept these protons, bases
or alkalis.
• The degree of acidity of the cooking medium
(water, sauce, etc.) can have a great influence
on color and texture of fruits, vegetables,
meat and egg proteins.
pH of Common substances
Human gastric juice
Lemon juice
Orange juice
Strawberries
Tomatoes (fresh)
Yogurt
Black coffee
Milk
Egg white
Baking soda in water
Household ammonia
1.3—3.0
2.1
3.0
3.0—3.9
4.3—4.9
4.5
5.0
6.9
7.6—9.5
8.4
11.9
Water and the Seizing of Chocolate
• Chocolate is very dry and thus vulnerable to moisture
• Chocolate is made up of many tiny sugar and cacao
particles whose surfaces attract moisture
• Thus…when water is added to molten chocolate the
chocolate with seize up into a stiff paste.
• The small amount of water acts like a “glue” that
binds the sugar and cacao together and separates
them from the cacao butter.
How to Boil Water
• Boiling water is very easy to do, but it is crucial to many
meals.
• Choose a pot that's large enough to hold the amount of
water you want to boil, and has a lid that fits. You might
be tempted to use water that's already warm or hot from
the tap, but this water has been sitting in your pipes for
some time, getting stale. Use cold water if you're going to
drink it or cook with it.
• Don’t fill the pot all the way up. Keep in mind that
anything you add to the boiling water will increase the
volume, and plus, you’ll need to allow room for those
bubbles to do their thing. Without enough room in the
pot, for example, rice or pasta will boil over.
• Place the pot on the stove and turn the heat to high. If
you want to speed up the process, put a cover on it
• Check for steam escaping from under the lid, then lift
the lid carefully to see how the water is doing.
• Look at the water. If large bubbles are rising from the
bottom of the pot to the surface, the water is boiling.
NOTE: Small bubbles that stay at the bottom or sides of
the pot are air bubbles present in the water; they don't
necessarily indicate that boiling is imminent. Wait for
bubbles that rise to the top of the pot.
Poaching 160—180⁰F
Simmering 185—200⁰F
Boiling 212⁰C (note the
bubbles rising from the
bottom of the pan)
What Affects the Boiling Point of
Water?
• Salt: Salt, sugar, and practically any other
substance elevates the boiling point and
therefore shortens cooking time. The
difference in temperature between unsalted
and salted water (one teaspoon of salt per
quart of water) is about 1° to 2° F, a difference
that can be critical in cooking situations
demanding exactness.
• Hard Water: Hard water defines water with a
high level of dissolved mineral salts. Therefore,
hard water boils at a higher temperature. The
difference in the boiling point between typical
supplies of hard and soft water is about a
degree or two.
• Alcohol: Alcohol has a lower boiling point that water
(about 175° F as compared with 212° F). If you dilute
water with alcohol, the mixture will have a lower
boiling point up until the alcohol completely
evaporates. Should you decide to alter an existing
recipe by substituting a fair portion of wine for some
of the water, remember to extend the cooking time
by 5 to 10 percent depending on the alcohol strength
of the wine and the heaviness of your touch.
• Weather: The boiling point of water is a
degree or two lower on stormy, as opposed to
fair, weather days. Consequently, boiled food
will take longer to cook on a stormy day.
• Different Size Pans: Will a given volume of water boil
at a higher temperature in a tall, narrow pot than in a
short, wide one? Yes. since the tall, narrow pot has a
great depth, its bottom-lying water is under greater
pressure from the water above it than is the water at
the bottom of the short, wide pot. The greater the
pressure, the high the boiling point. The difference is
approximately 1° F.
• Altitude: The higher the altitude, the lower the
atmospheric pressure. The less atmospheric pressure
that bears down on the surface of the liquid, the
easier it is for water molecules to escape into the air.
Thus, the water comes to its full rapid boil at a lower
temperature in the mile-high city of Denver than it
can in coastal Miami. For each thousand feet above
sea level, the boiling point of water drops almost
2° F.
Altitude
Boiling Point (⁰F)
(Feet above sea level)
Sea level
984
2,000
3,000
5,000
7,500
10,000
20,000
26,000
212
210
208
206
203
198
194
178
168
(Denver: 5,281 ft)
(Nepel: 29,000 ft)
Water Activity in Food
• Water activity is a measure of the “free water”
in a sample and should NOT be compared to
water content (g of water/g of substance) of
food.
• Water activity is given as aw
• Value and range is 0 (absolute dryness) to
1 (condensed humidity)
Influence of Water
Activity in Food
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• Chemical compound
• Temperature
• Water content
• Storage environment (T / RH)
• Absolute pressure
• Packing
Free” water in products is jointly responsible for the growth
of undesirable organisms such as bacteria or fungi, which
produce “toxins” or other harmful substances. But also
chemical/biochemical reactions (e.g. the Maillard reaction)
increasingly take place and possibly change the following
factors of a product:
also chemical/biochemical reactions (e.g. the Maillard
reaction) increasingly take place and possibly change
the following factors of a product:
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Microbiological stability (growth)
• Chemical stability (see table)
• Content of proteins and vitamins
• Colour, taste and nutritional value
• Stability of the compound and durability
• Storage and packing
• Solubility and texture