Food Science

Four Basic Food Molecules

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

• Water and Dissolving• Water and Heat• Water and pH Scale• Water and chocolate

Water and DissolvingWater 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) Calcium sulfate (CaSO4)

Magnesium carbonate (MgCO3) 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 1.3—3.0Lemon juice 2.1Orange juice 3.0Strawberries 3.0—3.9Tomatoes (fresh) 4.3—4.9Yogurt 4.5Black coffee 5.0Milk 6.9Egg white 7.6—9.5Baking soda in water 8.4Household ammonia 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 212984 2102,000 2083,000 2065,000 203 (Denver: 5,281 ft)7,500 19810,000 19420,000 17826,000 168 (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

• • 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:

• 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