Lecture 2 Water

Food Chemistry

Lecture 2: Water

Introduction Water is the most abundant molecule on Earth's surface,

composing of about 70% of the Earth's surface as liquid and solid state in addition to being found in the atmosphere as a vapor

Water is the chemical substance with chemical formula H2O: one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom

At room temperature, it is a nearly colorless, tasteless, and odorless liquid, with a hint of blue

Many substances dissolve in water and it is commonly referred to as the universal solvent

Water is the only pure substance found naturally in all three common states of matter – solid, liquid and gas

Water in three states: liquid water, solid water is ice and

clouds are condensated water vapor

Why Water is Somewhat Blue? Common misconception: large bodies,

such as the oceans, the water's color is blue due to the reflections from the sky on its surface

The blue color is caused by: Selective absorption and scattering of the light

spectrum Impurities dissolved or suspended in water

may give water different colored appearances – silica and lime

Large bodies of water such as oceans manifest water's inherent slightly blue color,

not a reflection of the blue sky, as was once believed

High concentrations of dissolved lime make the water of Havasu Falls appear turquoise

Introduction Water is an essential constituent of many foods:

May occur as intracellular or extracellular in vegetable and animal products

As the dispersed phase in some emulsified products such as butter and margarine

As a minor constituent in other foods Because of the importance of water as a food

constituent, an understanding of this properties and behavior is necessary

Water presence influences the chemical and microbiological deterioration of foods

Removal (drying) or freezing of water is a method of food preservation

Water Structure Water composed of one oxygen atom and two

hydrogen atoms Each hydrogen atom is covalently bonded to the

oxygen via a shared pair of electrons Oxygen also has two unshared pairs of electrons Thus there are 4 pairs of electrons surrounding

the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom

Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen

Water Structure Water is a "polar" molecule, meaning that there is

an uneven distribution of electron density Water has a partial negative charge (–) near the

oxygen atom due the unshared pairs of electrons, and partial positive charges (+) near the hydrogen atoms

An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond (as shown in the illustration)

Model of hydrogen bonds between molecules of water

Water Structure The ability of ions and other molecules to

dissolve in water is due to polarity For example (in the illustration) sodium

chloride is shown in its crystalline form and dissolved in water

Water Structure Many other unique properties of water are due to

the hydrogen bonds: For example, ice floats because hydrogen bonds hold

water molecules further apart in a solid than in a liquid, where there is one less hydrogen bond per molecule

High heat of vaporization, strong surface tension, high specific heat, and nearly universal solvent properties of water are also due to hydrogen bonding

The hydrophobic effect, or the exclusion of compounds containing carbon and hydrogen (nonpolar compounds) is another unique property of water caused by the hydrogen bonds

Chemical and Physical Properties Water is a tasteless, odorless liquid at ambient temperature

and pressure The color of water and ice are, essentially, a very light blue

hue, although water appears colorless in small quantities. Ice also appears colorless, and water vapor is essentially invisible as a gas

Water is transparent, and thus aquatic plants can live within the water because sunlight can reach them

Water is primarily a liquid under standard conditions Water has a polar molecule

Since oxygen has a higher electronegativity than hydrogen, a charge difference is called a dipole. The charge differences cause water molecules to be attracted to each other by hydrogen bonds.

Chemical and Physical Properties The boiling point of water is directly related to the

barometric pressure For example, on the top of Mt. Everest water boils at about

68 °C (154 °F), compared to 100 °C (212 °F) at sea level. Conversely, water deep in the ocean near geothermal vents can reach temperatures of hundreds of degrees and remain liquid.

Water sticks to itself Water has a high surface tension caused by the strong

cohesion between water molecules because it is polar. The apparent elasticity caused by surface tension drives the capillary waves

Water also has high adhesion properties because of its polar nature

Capillary action refers to the process of water moving up a narrow tube against the force of gravity

Capillary action of water compared to mercury

Dew drops adhering to a spider web

Impact from a water drop causes an upward "rebound" jet surrounded by circular capillary waves

This daisy is under the water level, has risen gently and smoothly. Surface tension prevents the water from submerging the flower

Chemical and Physical Properties Water is a very strong solvent, referred to as the

universal solvent, dissolving many types of substances Substances that will mix well and dissolve in water, e.g.

salts, sugars, acids, alkalis, and some gases: especially oxygen, carbon dioxide (carbonation), are known as "hydrophilic" (water-loving) substances, while those that do not mix well with water (e.g. fats and oils), are known as "hydrophobic" (water-fearing) substances

Pure water has a low electrical conductivity, but this increases significantly upon addition of a small amount of ionic material water such as hydrogen chloride

Chemical and Physical Properties Water has the second highest specific heat

capacity of any known chemical compound, after ammonia, as well as a high heat of vaporization (40.65 kJ mol−1) Result of the extensive hydrogen bonding between its

molecules. These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature

The maximum density of water is at 3.98 °C (39.16 °F) This causes an unusual phenomenon: ice floats upon

water, and so water organisms can live inside a frozen pond because the water on the bottom has a temperature of around 4 °C (39 °F).

Chemical and Physical Properties Water is miscible with many liquids, for

example ethanol in all proportions, forming a single homogeneous liquid

On the other hand water and most oils are immiscible usually forming layers according to increasing density from the top. As a gas, water vapor is completely miscible with air

Some substances (sodium, calcium, potassium) emit a flammable gas when wet, or react violently with water

Liquid and Solid Water Ice, like all solids, has a well-defined structure

Each water molecule is surrounded by four neighboring H2Os

Two of these are hydrogen-bonded to the oxygen atom on the central H2O molecule, and each of the two hydrogen atoms is similarly bonded to another neighboring H2O

The four bonds from each O atom point toward the four corners of a tetrahedron centered on the O atom

This basic assembly repeats itself in three dimensions to build the ice crystal

Liquid and Solid Water When ice melts to form liquid water, the

uniform three-dimensional tetrahedral organization of the solid breaks down as thermal motions disrupt, distort, and occasionally break hydrogen bonds

The methods used to determine the positions of molecules in a solid do not work with liquids, so there is no clear way of determining the detailed structure of water

2D illustration of solid water 2D illustration of liquid water

ice

The stable arrangement of hydrogen-bonded water molecules in ice gives rise to the beautiful hexagonal symmetry

that reveals itself in every snowflake

Why Ice Floats? Theoretically, solid form of most substance is

more dense than the liquid phase But, by contrast, a block of common ice will float

in a tub of water because solid water is less dense than liquid water

This is an extremely important characteristic property of water. At room temperature, liquid water becomes denser with lowering temperature, just like other substances

BUT at 4 °C water reaches its maximum density, and as water cools further toward its freezing point, the liquid water, under standard conditions, expands to become less dense

Why Ice Floats? The physical reason for this is related to the crystal

structure of ordinary ice. Water, lead, uranium, neon and silicon are some of the few materials which expand when they freeze; most other materials contract

Generally, water expands when it freezes because of its molecular structure, the unusual elasticity of the hydrogen bond

That is, when water cools, it tries to stack in a crystalline lattice configuration that stretches the rotational and vibrational components of the bond, so that the effect is that each molecule of water is pushed further from each of its neighboring molecules

This effectively reduces the density of water when ice is formed under standard conditions

Water Activity Water activity refers to the availability of water in

a food or beverage and represents the amount of water that is available to microorganisms

The term water activity (aw) refers to this unbound water. Pure water has an aw of 1.00

The water activity of a food is not the same thing as its moisture content

Although moist foods are likely to have greater water activity than are dry foods, this is not always so; in fact a variety of foods may have exactly the same moisture content and yet have quite different water activities

Water activity values are obtained by either a capacitance or a dew point hygrometer

Typical Water Activity of Some Foodstuff

Type of Product Water Activity (AW)

Fresh Meat and Fish 0.99

Bread 0.95

Aged Cheddar 0.85

Jams and Jellies 0.8

Plums puddings 0.8

Dried fruits 0.6

Biscuits 0.3

Milk powder 0.2

Instant coffee 0.2

Importance of Water Activity Water activity (aw) is one of the most critical

factors in determining quality and safety of the goods you consume every day

Water activity affects the shelf life, safety, texture, flavor, and smell of foods, pharmaceuticals and cosmetics

While temperature, pH and several other factors can influence if and how fast organisms will grow in a product, water activity may be the most important factor in controlling spoilage

Importance of Water Activity Most bacteria do not grow at water activities

below 0.91, and most molds cease to grow at water activities below 0.80

By measuring water activity, it is possible to predict which microorganisms will and will not be potential sources of spoilage

Water activity (not water content) determines the lower limit of available water for microbial growth

In addition to influencing microbial spoilage, water activity can play a significant role in determining the activity of enzymes and vitamins in foods and can have a major impact their color, taste, and aroma

AW Microorganisms grow at this aw and above Food Examples

0.95 Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens,

fresh and canned fruits, vegetables, meat, fish, milk, cooked sausages, foods with 7% NaCl

0.91 Salmonella, Vibrio parabaemolyticus, C. botulinum, Lactobacillus, some molds

Cheese, fruit juice concentrates with 55% sucrose or 12% NaCl

0.87 Many yeasts, Candida, Torulopsis, Hansenula micrococcus

sponge cakes, dry cheese, margarine, foods with 65% sucrose or 15% NaCl

0.80 Most molds, most Saccharomyces spp., Debaryomyces, Staphylococcus aureus

fruit juice concentrates, condensed milk, syrup, flour, high-sugar cakes

0.75 Most halophilic bacteria, Mycotoxigenic aspergilli Jam, marmalade, glace fruits, marzipan, marshmallows

0.65 Xerophilic molds, Saccharomyces bisporus Rolled oats with 10% moisture, jelly, nuts

0.60 Osmophilic yeasts, few molds caramel, toffee, honey

0.30 No microbial proliferation Cookies, crackers, bread crusts with 3-5% moisture