Properties of Water

FS Unit 5HUM-FS-6 Explain why water and acidity are

important factors in food preparation and preservation.

Chemical Description of Water- One atom of oxygen bound to two

atoms of hydrogen make up water.

Polarity – uneven distribution of charge Because the hydrogen atoms are attached

to one side (positive charge) and the oxygen atom is on the other side (negative charge)

Properties of Water

Adhesion – the attraction of the water molecules to other materials. (Water’s attraction to materials other than itself )

LET’S TRY IT

Properties of Water

Cohesion – attraction between water molecules. (Water’s attraction to itself ) This is why water is kind of “sticky”

With the molecules attracting to each other they tend to clump together. That’s why they are drops.

If it wasn’t for earth’s forces like gravity a drop of water would be a perfect SPHERE.

LET’S TRY IT

Properties of Water

Capillary Action – water molecules being attracted to molecules of other materials as well as themselves. Adhesion and Cohesion are working

together

WATCH IT IN ACTION

Properties of Water

Surface Tension – a tension is created at the surface as the water molecules at the surface are pulled into the body of water. Acts as the water’s skins Is strong and can support a paper clip or

needle

LET’S TRY IT

Properties of Water

In the cleaning process you have to reduce surface tension so that water can spread and wet the surface.

Chemicals called surface active agents or Surfactants - make a surface “wetter”. Surfactants functions can also loosen, disperse

water and hold soil in suspension until it can be rinsed.

Surfactants can provide alkalinity which can remove acidic soils.

Properties of Water

Water is the “Universal Solvent”. It dissolves more substance than any other

liquid

Wherever water goes (through the ground or our body) it takes along valuable chemicals, minerals and nutrients.

Properties of Water

Water is the only natural substance that is found in all states Liquid Solid (ice) Gas (steam)

Water Freezes at 32 degrees Fahrenheit

Water Boils at 212 degrees Fahrenheit

Properties of Water

Force of the weight of gases in the air pressing down on a surface.

Measured by the average pressure on an object at sea level. Similar to the pressure that water puts on your

body when you dive into the pool Gases in the air are pressing against your

body all the time.

Atmospheric Pressure

The boiling point of a substance is the temperature at which its vapor pressure equals the air pressure above the liquid. When water reaches its boiling point, water molecules in the liquid

are escaping into the air as steam.

As the atmospheric pressure changes, the boiling point of water changes. Less AP on the water, molecules need less energy to

escape. More AP on the water, molecules need more energy to

escape. Temperature is a measure of energy. Heat of Vaporization is the amount of heat needed to

change 1.0g of a substance from a liquid to a gas.

Boiling Point

Two main functions in the preparation of foods...1. Water is an important medium for transferring heat.2. Water is a necessary ingredient in forming many food mixtures.

Function of water in Food Preparation

Water in both liquid and gaseous stages is used to transfer heat into foods.

When water is heated, energy is transferred from the heat source by both conduction and convection. The slower/lower temp you cook the food on the

more even the heat distribution will be. Boiling certain food too high can affect the

texture. Ex: pasta needs a higher temp to boil and

potatoes need a lower temp (they can get mushy from banging around in the pan)

Heat Medium

The melting point is the temperature at which a substance changes from a solid to a liquid. Melting occurs as heat energy enables

molecules to breakout of their solid structure. For pure substances melting and freezing

occur at the same temperature.

Melting Point

Solvent – the substance that dissolves another substance.

Solute – a substance that can be dissolved into another substance.

As a polar covalent compound water can dissolve many substances. Remember it’s the universal solvent.

Water as a Solvent

Examples… When sodium chloride (table salt) is mixed with

water the negative oxygen ends of water molecules attract the positive sodium ions, while the positive hydrogen ends attract negative chloride ions. (Food Science, chapter 9, figure 9-8)

Sugar dissolves in the free water of strawberries to form sweet syrup.

Water dissolves the flavor molecules in tea leaves and coffee beans to create our beverages.

Water as a Solvent

Colloidal Dispersion (kuh-LOYD-ul dis-PUR-zhun)-

Homogeneous mixture that is not a true solution. Contain relatively large solute particles. Particles are called Colliods (KAHL-oyds) (Food

Science, Ch 9, pg 139)

Not large enough to settle out of the colloidal Dispersion.

Scattered or dispersed in a solvent.

Water as a Disperser

Emulsion– a mixture of two liquids containing droplets that don’t normally blend with each other. Ex: Oil and water

Immiscible (ih-MISS-suh-fy-ur) – Liquids that don’t blend or mix.

Emulsifier – a substance composed of large molecules that are polar at one end of nonpolar at the other. (Simply “glues” the two types of molecules together)

Water as a Disperser

Water Content – the amount of water that is in a food. Ex: Many fruits and vegetables are made up of

over 70% water Water Activity – the measure of a the

particular water pressure over a food. Takes into account the water available to

support activity of enzymes and growth of microorganisms.

Water in Food

Free Water – readily separates from foods that are sliced, diced and dried.

Ex: Bread to toast heat evaporates the free water. Grapefruit squirts when you cut it open.

Bound Water – water that cannot be easily separated in food.

Ex: Proteins and starches It can’t react with other substances as free water

can. Freezes only at VERY low temperatures & doesn’t

evaporate. Ex: adding dry gelatin to water. Within a few minutes

the water is unable to flow and is bound.

Water Content

Water content alone will not predict a food perishability.

Both carrots and milk are 88% water… A carrot is safe in a lunch bag all day because

it has starch molecules that attach to the water.

Milk doesn’t have the same insulation and will go bad if let in a lunch bag.

Water & Food Preservation