Lecture 2 Water
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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
Water sticks to itself
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 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
ic e
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, Cheese, fruit juice Lactobacillus, some molds 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
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
Water sticks to itself
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 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
ic e
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, Cheese, fruit juice Lactobacillus, some molds 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
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