Water Properties (PDF)

WATER Water  Next to oxygen, is the most important constituent of life.  Widely distributed compound as liquid, solid and vapour.  Fruits and vegetables contain more than 90% water.  Milk is 87% water  Meat 65-75% water  Even dried pulses, cereals and flour contain appreciable amount of...

WATER Water  Next to oxygen, is the most important constituent of life.  Widely distributed compound as liquid, solid and vapour.  Fruits and vegetables contain more than 90% water.  Milk is 87% water  Meat 65-75% water  Even dried pulses, cereals and flour contain appreciable amount of water.  Water influences the appearance, texture and flavour of food.  Water is involved in most of the changes that takes place when food is cooked.  Some of the desirable and undesirable changes that takes place when food is cooked are due to the properties of water as a food solvent. Water  Characteristic flavour of coffee and tea is due to the ability of water to dissolve the flavour materials present in them.  The loss of water soluble vitamins during cooking is due to their high solubility in water.  Water is also dispersion medium in foods.  Too much water can be danger as it favours growth of undesirable bacteria and micro organisms.  The aim of modern food technology is to keep water content of food as low as possible to increase shelf life. Physical properties of water  Physical constants are very different when compared with molecules of similar molecular weight and atomic composition such as hydrogen sulphide, hydrogen fluoride, ammonia etc.  Water has unusually large values for melting point, boiling point, surface tension, specific heat and dielectric constant.  Its heat of fusion, vapourization and sublimation are also high.  It has maximum density at 4*c and not at 0*c its freezing point.  PHYSICAL & CHEMICAL PROPERTIES OF WATER  Water has very unique properties not shared by other similar hydrogen compounds or compounds of similar weight Compound Melting point Boiling point H 2O 0ºC 100ºC H 2S -83ºC -60ºC NH3 -78ºC -33ºC Methanol -98ºC 65ºC  Why? – this is explained by the unique structure of H2O  STRUCTURE OF WATER A water molecule is made up of three atoms, 2 hydrogen atoms bonded with 1 oxygen atom.  Water H Each hydrogen has 1 valence electron Each hydrogen wants 1 more The oxygen has 6 valence electrons O The oxygen wants 2 more They share to make each other happy  Water  Put the pieces together  The first hydrogen is happy  The oxygen still wants one more HO  Water  The second hydrogen attaches  Every atom has full energy levels HO H STRUCTURE OF WATER ◼ Tetrahedral arrangement ◼ Two free electrons of O act as H-bond acceptors while H acts as donor ◼ Highly electronegative O pulls electrons from H, making H behave like a bare proton ◼ Forms a dipole because of the electronegative O  Tetrahedral EXAMPLE: H 2O Tetrahedral molecule – a Number of Bonds = 4 central atom located at center Number of Shared Pairs of Electrons = 4 with 4 substituents that are located at Number of Unshared Pairs of Electrons the corners of =0 tetrahedron Bond Angle = 109.5° Water  In water, each hydrogen nucleus is bound to the central oxygen atom by a pair of electrons that are shared between them; chemists call this shared electron pair a covalent chemical bond.  In H2O, only two of the six outer-shell electrons of oxygen are used for this purpose, leaving four electrons which are organized into two non-bonding pairs. Water  The four electron pairs surrounding the oxygen tend to arrange themselves as far from each other as possible in order to minimize repulsions between these clouds of negative charge.  This would ordinarly result in a tetrahedral geometry in which the angle between electron pairs (and therefore the H-O-H bond angle) is 109.5°.  Tetrahedral structure  However, because the two non-bonding pairs remain closer to the oxygen atom, these exert a stronger repulsion against the two covalent bonding pairs, effectively pushing the two hydrogen atoms closer together.  The result is a distorted tetrahedral arrangement in which the H—O—H angle is 104.5°. Hydrogen bonding  When hydrogen is bonded to a highly electronegative atom X (such as nitrogen, oxygen, fluorine), the bonding electron pair is pulled towards the electronegative atom X that a strong dipole exits.  Since the shared pair is removed farthest from H atom, its nucleus ( the proton) is practically exposed.  The H atom at the positive end of a polar bond ,nearly stripped of its surrounding electrons, exerts a strong electrostatic attraction on the lone pair of electrons around X in a nearby molecule. Hydrogen bonding Electrostatic attraction between an hydrogen atom covalently bonded to small, strongly electronegative atom X (N, O and F) and a lone pair of electrons on X in another electronegative molecule. Hydrogen bonding  Because of the DIPOLE and TETRAHEDRAL structure we can get strong H-bonding  Water capable of bonding to 4 other water molecules  Unique properties of water from other hydrides  H-bond NOT a static phenomenon ◦ T dependent  Water is Polar  A polar molecule has an unequal distribution of charge- each molecule has a positive end and a negative end.  All polar substances are attracted to other polar substances.  In each water molecule, the oxygen atom attracts more than its "fair share" of electrons  The oxygen end “acts” negative  The hydrogen end “acts” positive  Causes the water to be POLAR  However, Water is neutral (equal number of e- and p+) --- Zero Net Charge Water is Polar  Although the water molecule carries no net electric charge, its eight electrons are not distributed uniformly;  there is slightly more negative charge (purple) at the oxygen end of the molecule, and a compensating positive charge (green) at the hydrogen end.  The resulting polarity is largely responsible for water's unique properties. Why is this polarity important?- Water’s high solvent power  Polar water molecules attract other water molecules as well as ions and other polar molecules.  Because of this attraction, water can dissolve many ionic compounds, such as salt and polar molecules like sugar.  Its called universal solvent which facilitates chemical reactions both outside of and within biological systems How do water molecules attract?  The slightly negative Hydrogen bonds are weak bonds that form between regions of one the positively charged molecule are attracted hydrogen atoms of one to the slightly positive water molecule and the negatively charged oxygen regions of nearby atoms of another water molecules, forming a molecule. hydrogen bond. ◦ Each water molecule can form hydrogen bonds with up to four neighbors.  HYDROGEN BONDS  Hold water molecules  Extraordinary Properties that together are a result of hydrogen bonds. ◦ Cohesive behavior  Each water molecule can ◦ Resists changes in form a maximum of 4 temperature hydrogen bonds ◦ High heat of vaporization  The hydrogen bonds ◦ Expands when it freezes joining water molecules ◦ Versatile solvent are weak, about 1/20th as strong as covalent bonds.  They form, break, and reform with great frequency  Cohesion  Attraction between particles of the same substance ( why water is attracted to itself)  Results in Surface tension (a measure of the strength of water’s surface)  Produces a surface film on water that allows insects to walk on the surface of water Cohesion … Helps insects walk across water Adhesion  Attraction between two different substances.  Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton.  Capillary action-water molecules will “tow” each other along when in a thin glass tube.  Example: transpiration process which plants and trees remove water from the soil, and paper towels soak up water. Adhesion Causes Capillary Action Which gives water the ability to “climb” structures  Adhesion Also Causes Water to … Attach to a silken spider web Form spheres & hold onto plant leaves Organisms Depend on Cohesion Hydrogen  Cohesion is responsible for bonds hold the transport of the water column in plants the substance  Cohesion among water together, a molecules plays a key role in phenomenon the transport of water called against gravity in plants cohesion  Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.  Surface tension Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. ◦ Water has a greater surface tension( 72.8) than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. ◦ Water behaves as if covered by an invisible film. ◦ Some animals can stand, walk, or run on water without breaking the surface.  High Specific Heat  Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1*C.  Water resists temperature change, both for heating and cooling.  Water can absorb or release large amounts of heat energy with little change in actual temperature.  Water has a high specific heat capacity of 1000 cal/g  High Heat of Vaporization  Amount of energy to convert 1g or a substance from a liquid to a gas  In order for water to evaporate, hydrogen bonds must be broken.  As water evaporates, it removes a lot of heat with it.  Water's heat of vaporization is 540 cal/g.  In order for water to evaporate, each gram must GAIN 540 calories (temperature doesn’t change --- 100*C).  As water evaporates, it removes a lot of heat with it (cooling effect).  Moderates Temperatures on Earth Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air. Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. Celsius Scale at Sea Level 100oC Water boils 37oC Human body temperature 23oC Room temperature 0oC Water freezes Evaporative Cooling  The cooling of a surface occurs when the liquid evaporates  This is responsible for: ◦ Moderating earth’s climate ◦ Stabilizes temperature in aquatic ecosystems ◦ Preventing organisms from overheating  Expansion on freezing  Most substances decrease in volume and hence increase in density as their temperature decreases.  But in case of water, there is a temperature at which its density exceeds that at higher or lower temperature.  The temperature is 4*c  Water just above the freezing point is heavier than water at the freezing point.  Therefore it moves towards the bottom, freezing begins at the surface and the bottom is last to freeze.  Water thus has minimum volume and maximum density of 1.00 at 4*c. Volume of water at 4*c increases either on heating or cooling it  Water is Less Dense as a Solid  Ice is less dense as a solid than as a liquid (ice floats)  Liquid water has hydrogen bonds that are constantly being broken and reformed.  Frozen water forms a crystal-like lattice whereby molecules are set at fixed distances.  Water is Less Dense as a Solid Which is ice and which is water? Water is Less Dense as a Solid Water Ice Density of Water  Most dense at 4oC  Contracts until 4oC  Expands from 4oC to 0oC The density of water: 1. Prevents water from freezing from the bottom up. 2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation. 3. Makes transition between season less abrupt. ◦ When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to the maximum of four partners. ◦ As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state. ◦ Ice is about 10% less dense than water at 4oC. Heat of fusion The specific latent heat of fusion (melting) of ice at 0 ºC, for example, is 334000 J.kg-1. This means that to convert 1 kg of ice at 0 ºC to 1 kg of water at 0 ºC, 334000 J of heat must be absorbed by the ice. Heat of fusion of water is 79.7 cal/g All at 0°C 1 kg 1 kg 334000 J absorbed Thermal conductivity  The thermal conductivity of water is large compared to those of other liquids  The thermal conductivity of ice is moderately large compared to those of other non metallic solids.  Of greater interest is the fact that the thermal conductivity of ice at 0°C is approximately four times that of water at the same temperature, indicating that ice will conduct heat energy at a much greater rate than will immobilized water (e.g., in tissue).  Thermal diffusivities  The thermal diffusivities of water and ice are of even greater interest since these values indicate the rate at which the solid and liquid forms of HOH will undergo changes in temperature.  Ice has a thermal diffusivity approximately nine times greater than that of water, indicating that ice, in a given environment, will undergo a temperature change at a much greater rate than will water. Thermal diffusivities  These sizable differences in thermal conductivity and thermal diffusivity values of water and ice provide a sound basis for explaining why tissues freeze more rapidly than they thaw, when equal but reversed temperature differentials are employed. PHASE CHANGES OF WATER Pressure Temperature Six Phase Changes GAS B.P. C.P. Temp LIQUI (°C) D M.P. F.P. SOLID Graphing a Phase Change Melting & Boiling Points  Melting Point: The temperature at which a solid changes into a liquid.  Boiling Point: The temperature at which a liquid changes into a gas.  What is a Freezing point? Compare the freezing and melting points of water.  Freezing Point  The temperature at which a pure liquid changes to a crystalline solid, or freezes, is called the freezing point. – The melting point is identical to the freezing point and is defined as the temperature at which a solid becomes a liquid. – Unlike boiling points, melting points are affected significantly by only large pressure changes. Summary  Heating curve for water. Phase diagram for water (not to scale).  Phase Diagrams  A phase diagram is a graphical way to summarize the conditions under which the different states of a substance are stable. – The diagram is divided into three areas representing each state of the substance. – The curves separating each area represent the boundaries of phase changes. Phase Diagrams  Below is a typical phase diagram. It consists of three curves that divide the diagram into regions labeled “solid, liquid, and gas”. B. C solid liquid pressure D. A gas temperature Phase Diagrams  Curve AB, dividing the solid region from the liquid region, represents the conditions under which the solid and liquid are in equilibrium. Melting or Fusion curve B. C solid liquid pressure. A gas D temperature Phase Diagrams  Usually, the melting point is only slightly affected by pressure. For this reason, the melting point curve, AB, is nearly vertical. B. C solid liquid pressure. A gas D temperature Phase Diagrams  If a liquid is more dense than its solid, the curve leans slightly to the left, causing the melting point to decrease with pressure. B. C solid liquid pressure. A gas D temperature Phase Diagrams  If a liquid is less dense than its solid, the curve leans slightly to the right, causing the melting point to increase with pressure. B. C solid liquid pressure. A gas D temperature Phase Diagrams  Curve AC, which divides the liquid region from the gaseous region, represents the boiling points of the liquid for various pressures. AC :Vapour pressure curve ( BP increase with increase in pressure up to critical point ) B. C solid liquid pressure. A gas D temperature Phase Diagrams  Curve AD, which divides the solid region from the gaseous region, represents the vapor pressures of the solid at various temperatures. AD: Sublimation Curve B. C solid liquid pressure. A gas D temperature Phase Diagrams  The curves intersect at A, the triple point, which is the temperature and pressure where three phases of a substance exist in equilibrium. B. C solid liquid pressure. A gas D temperature Phase Diagrams  The curves intersect at A, the triple point, which is the temperature and pressure where three phases of a substance exist in equilibrium. B. C solid liquid pressure. A gas D temperature Phase Diagrams  The temperature above which the liquid state of a substance no longer exists regardless of pressure is called the critical temperature. B. C solid liquid pressure. A gas D Tcrit temperature  Phase Diagrams  The vapor pressure at the critical temperature is called the critical pressure. Note that curve AC ends at the critical point, C. Pcrit B.C solid liquid pressure. A gas D Tcrit temperature WATER VAPOR  Water is “free” and devoid of any H-bonds ◦ Large input of energy needed  endothermic process ◦ Large dissipation of same energy needed to make water lose kinetic energy  exothermic process  Waters latent heat of vaporization is unusually high ◦ to change 1 L from liquid to vapor need 539.4 kcal  LIQUID WATER  Extensive H-bonding  H-bond formation dependent on T ◦ With increasing T get more mobility and increased fluidity T (ºC) Density (kg/m3) Viscosity (m2/s) 0 999.9 1.7895 5 1000.0 1.535 25 997.1 0.884 100 958.4 0.294   Forms when exactly 4 H- bonds are formed between ICE water molecules ◦ To get this order a lot of energy needs to be adsorbed by the environment  The strong H-bonding in ice forms an orderly hexagonal crystal lattice ◦ 6 H2O molecules  The lattice is loosely built and has relatively large hollow spaces; this results in a high specific volume.  Has 4X more thermal conductivity than water at same temperature  PROPERTIES OF ICE  Upon freezing, HOH molecules associate in an orderly manner to form a rigid structure that is more open (less dense) than the liquid form.  There still remains considerable movement of individual atoms and molecules in ice, particularly just below the freezing point.  At 1O*C an HOH molecule vibrates with an amplitude of approximately 0.044 nm, nearly one- sixth the distance between adjacent HOH molecules.  Hydrogen atoms may wander from one oxygen atom to another.  When ice is heated at pressures below 4.58 mm Hg, it changes directly into the vapor form. This is the basis of freeze drying Basis for Freeze Drying Sublimation Pressure Temperature PROPERTIES OF ICE  SUPERCOOLING ◦ Water can be cooled to temperatures below its freezing point without crystallization ◦ When an ice crystal is added to supercooled water, temperature increases to 0*C and ice formation occurs  PROPERTIES OF ICE Nucleation- Process where the molecules in liquid start to gather in to tiny clusters, arranging in a way that will define crystal structure of solid  Normally the presence of a nucleus is required.  Generally, nuclei form around foreign particles (heterogeneous nucleation). ◦ Heterogeneous nucleation  usually caused by a foreign particle, such as salt, protein, fat, etc. ◦ Homogeneous nucleation  very rare, mainly occurs in pure system  PROPERTIES OF ICE  Crystallization ◦ Crystal growth occurs at freezing point ◦ The speed of crystallization—that is, the progress of the ice front in centimeters per second—is determined by the removal of the heat of fusion from the area of crystallization.  The speed of crystallization is low at a high degree of supercooling  This is important because it affects the size of crystals in the ice.  Crystallization  When large water masses are cooled slowly, there is sufficient time for heterogeneous nucleation in the area of the ice point.  At that point the crystallization speed is very large so that a few nuclei grow to a large size, resulting in a coarse crystalline structure.  At greater cooling speed, high supercooling occurs; this results in high nuclei formation and smaller growth rate and, therefore, a fine crystal structure  Ice crystal size at the completion of freezing is related directly to the number of nuclei.  The greater the number of nuclei, the smaller the size of the crystals  If the PROPERTIES OF ICE temperature is lowered to below the FP , crystal growth is the predominant factor at first but, at increasing rate of supercooling, nucleation takes over. PROPERTIES OF ICE  Solutes depress the nucleation temperature to the same extent that they depress the freezing point  Slow freezing results in large ice crystals  Rapid freezing results in tiny ice crystals  When water freezes, it expands nearly 9 percent.  PROPERTIES OF ICE Freezing induced changes in foods (examples) Example: Effect of freezing on seafoods  Destabilization of emulsions  Flocculation of proteins  Increased lipid oxidation  Meat toughening  Cellular damage  Loss of water holding capacity  Water and its features Property Chemistry Result Universal solvent Polarity Facilitates chemical reactions Adheres and is Polarity, Hydrogen Serves as transport cohesive bonding medium Resists changes in Hydrogen bonding Helps keep body temperature temperature constant Resists change of Hydrogen bonding Moderates earths state( from liquid to temperature ice and from liquid to steam) Less dense as ice than Hydrogen bonding Ice floats on water as liquid water changes Types of water in food  Free and Bound Water  Water that can be extracted easily from foods by squeezing or cutting or pressing is known as free water,  Whereas water that cannot be extracted easily is termed as bound water. Bound water  Many food constituents can bind or hold water molecules such that they cannot be easily removed and they do not behave like liquid water. Some characteristics of bound water include: It is not free to act as a solvent for salts and sugars Bound water has more structural bonding than liquid or free water thus it is unable to act as a solvent  It can be frozen only at very low temperatures (below the freezing point of water).  It exhibits essentially no vapor pressure  Its density is greater than that of free water  As the vapor pressure is negligible the molecules cannot escape as vapor; and the molecules in bound water are more closely packed than in the liquid state. So the density is greater.  An example of bound water is the water present in cacti or pine tree needles – the water cannot be squeezed or pressed out; extreme dessert heat or a winter freeze does not negatively affect bound water and the vegetation remains alive. Even upon dehydration food contains bound water. WATER SOLUTE INTERACTIONS Association of water to hydrophilic substances ◦ Bound water - occurs in vicinity of solutes  Water with highly reduced mobility  Water that usually won't freeze even at -40ºC  Water that is unavailable as a solvent ◦ “Trapped” water  Water holding capacity  Hydrophilic substances are able to entrap large amounts of water  Jellies, jams, yogurt, jello, meat  Yogurt - often see loss of water holding as whey is released at the top of the yogurt  WATER SOLUTE INTERACTIONS  Ionic polar solutes ◦ React readily with water and most are usually soluble in water ◦ Water HYDRATES the ions ◦ Charge interactions due to waters high DIELECTRIC CONSTANT  Can easily neutralize charges due to its high dipole moment  Large ions can break water structure ◦ Have weak electric fields  Small ions can induce more structure in water ◦ Have strong electric fields  WATER SOLUTE INTERACTIONS  Nonionic polar solutes ◦ Weaker than water-ion bonds ◦ Major factor here is H-bonding to the polar site ◦ Example: SUCROSE  4-6 H2O per sucrose  Concentration dependent  >30-40% sucrose all H2O is bound  T dependent solubility ◦ C=O, OH, NH2 can also interact with each other and therefore water can compete with these groups ◦ H-bond disrupters  urea - disrupts water Water bridge  WATER SOLUTE INTERACTIONS  Nonpolar ◦ Unfavorable interaction with water ◦ Water around non-polar substance is forced into an ordered state  Water affinity for water:- high compared to non-polar compound  Water forms a shell  Tries to minimize contact ◦ Hydrophobic interactions  Caused because water interacts with other water molecules while hydrophobic groups interact with other hydrophobic groups  EFFECT OF SOLUTES ON WATER Boiling point  Vapor pressure is equal to atmospheric pressure  Strongly influenced by water - solute interaction ◦ Solutes decrease vapor pressure and thus increase boiling point ATMOSPHERIC PRESSURE  Sucrose → +0.52ºC/mol  NaCl → +1.04ºC/mol VAPOR PRESSURE  EFFECT OF SOLUTES ON WATER Freezing point lowering  Freezing point can get extensive depression via solutes  Alter ability of water to form crystals due to H-bond disruption  Sucrose → -1.86ºC/mol  NaCl → -3.72ºC/mol ◦ Eutectic pt - temp.  Where “all” water is frozen - usually around -50ºC ◦ In most cases small amounts of water remains unfrozen (-20ºC)  These small patches of water can promote chemical reactions and damage  EFFECT OF SOLUTES ON WATER What explains all this?  Raoult's law P = P*/X1 P = vapor pressure of solution; P* = vapor pressure pure solvent; X1 = mole fraction of solute; RAOULT’S LAW  The relative lowering of vapour pressure of a dilute solution is equal to the mole fraction of the solute present in dilute solution.  This relationship is not only important for explaining the concepts of depressing freezing point and elevating boiling point ◦ Also explains the concept of water activity  EFFECT OF SOLUTES ON WATER Osmotic pressure of solutions  There is a tendency for a system containing water and a solution separated with a membrane to be at equilibrium  The pressure needed to bring the two solutions at equilibrium is called OSMOTIC PRESSURE  The more the solution has of dissolved solutes (e.g. salt) the higher its osmotic pressure  Can use this in food processing and preparation ◦ E.g. Crisping salad items  Increase turgor  EFFECT OF SOLUTES ON WATER Surface tension  Water surface behaves differently than bulk phase ◦ Like an elastic film ◦ Due to unequal inward force ◦ Resist formation of a new surface thus forming surface tension EFFECT OF SOLUTES ON WATER  Water has high surface tension ◦ 72.75 dynes/cm (20ºC)  Because of the high surface tension special considerations are needed in food processing  To affect it one can: ◦ Increase T (more energy) → reduces surface tension ◦ Add solutes  NaCl and sugars → increase surface tension  Amphipathic molecules → reduce surface tension EFFECT OF SOLUTES ON WATER Ionization of water  Water can ionize into hydronium (H3O+) and hydroxyl (OH-) ions ◦ Transfer of one proton to the unshared sp3 orbital of another water molecule  Pure water: Keq = Equilibrium (or ionization) constant Keq = [H3O]+ [OH]- [H2O] [H3O]+ [OH]- = Keq = Kw (Water dissociation constant) [10-7] [10-7] = [10-14] EFFECT OF SOLUTES ON WATER  Acids and bases in food systems ◦ Acid - proton donor  NH3 + H2O → NH4+ + OH- ◦ Base - proton acceptor  CH3COOH + H2O → CH3COO- + H3O+ ◦ Weak acids and bases  Most foods are weak acids  These constituents are responsible for buffering of food systems ◦ Some examples  Acetic, citric, lactic, phosphoric, etc. EFFECT OF SOLUTES ON WATER  Acids and bases in food systems ◦ Is there a difference between weak and strong acids?  Strong acids  When placed in solution, 100% ionized HCl = H+ + Cl- pH = -log [acid] = -log [H]+  Weak acids  When placed in solutions weak acids form an equilibrium HOAC H+ + OAC- Keq = [H]+ [OAC]- [HOAC] pKa = -log Ka WATER ACTIVITY  What is meant by water activity? ◦ Water has different levels of binding and thus activity or availability in a food sample ◦ Simply put, Water activity (aw) helps to explain the relationship between perishability and moisture content  Greater moisture content → faster spoilage (normally)  Why are there some perishable foods at the same moisture content that don't spoil at the same rate?  There is a correlation found between aw and various different spoilage and safety patterns WATER ACTIVITY  Water has different levels of binding and thus activity or availability in a food sample  Food companies and regulatory agencies (e.g. FDA) rely on aw as an indicator of how fast and in what fashion a food product will deteriorate or become unsafe, and it also helps them set regulatory levels of aw for different foods  WATER ACTIVITY Highly perishable foods aw > 0.9 Intermediate moist foods aw = 0.6-0.9 Shelf stable foods aw < 0.6 WATER ACTIVITY  Thermodynamic definition of aw ◦ The tendency of water molecules to escape the food product from liquid to vapor defines the aw aw = p/pO=%RH/100 ◦ Water activity is a measure of relative vapor pressure of water molecules in the head space above a food vs. vapor pressure above pure water ◦ Scale is from 0 (no water) to 1 (pure water) Moisture sorption isotherm (MSI).  A plot of water content (expressed as mass of water per unit mass of dry material) of a food versus water activity at constant temperature is known as a moisture sorption isotherm (MSI).  WATER ACTIVITY  Sorption isotherms ◦ Help relate moisture Temp. dependent content to aw graphically represented by a curve called sorption isotherm. ◦ Each food has their own sorption isotherm ◦ It is interesting that when water is added to a dry product, the adsorption is not identical to desorption ◦ Some reasons  Metastable local domains  Diffusion barriers  Capillary phenomena  Time dependent equilibrium  Sorption isotherms  Sorption Isotherms: presents the relationship between water content and relative humidity  Adsorption isotherm → hygroscopic products (steep slope in graph – small increase in relative humidity causes large increase in moisture content, e.g. foods with high salts or sugar contents)  Desorption isotherm → drying process  The moisture sorption isotherm may be measured by  addition of water to a dry sample (adsorption)  by removal of water from a wet sample (desorption).  Sorption isotherms have a sigmoid shape – 3 areas (monolayer, additional layers, condensation on capillaries)  correspond to different conditions of water present in the food  WATER ACTIVITY  Sorption isotherms also explain the level of water binding in a food (i.e. types of water) ◦ Type I: Tightly “bound” water (monolayer)  Unavailable/Unfreezable (at -40C) True monolayer  Water - ion; water - dipole interactions ◦ Type II: additional water layer (Vicinal Monolayer water)  Slightly more mobility  Some solvent capacity ◦ Type III: Water condensating in capillaries and pores (Multilayer → Bulk-phase water)  More available (like dilute salt solution)  Can be entrapped in gels  Supports biological and chemical rections  Freezable  Information derived from MSIs are useful (a) for concentration and dehydration processes, because the ease or difficulty of water removal is related to RVP (b) for formulating food mixtures so as to avoid moisture transfer among the ingredients (c) to determine the moisture barrier properties needed in a packaging material (d) to determine what moisture content will curtail growth of microorganisms of interest, and (e) to predict the chemical and physical stability of food as a function of water content Hysteresis  Hysteresis in foods is the phenomenon by which at constant water activity (Aw) and temperature, a food adsorbs a smaller amount of water during adsorption than during a subsequent desorption process. Hysteresis  It has been established that in hygroscopic high pectin and sugar-containing materials the sorption-desorption isotherms are sigmoidal in shape and show marked hysteresis.  Typically, this means that for values of aw below 0.40 the water content of the food will be greater during desorption than during adsorption.  WATER ACTIVITY  Water sorption of a mixture ◦ A mixture of two different food components with different aw leads to moisture migration from one food to another which can create problems ◦ This is one reason why it is important to know the aw of a food product or ingredient ◦ Examples:  Caramel, marshmallows and mints – all similar %moisture but very different aw  Fudge (aw = 0.65-0.75) covered with caramel (aw = 0.4-0.5) – what happens?  Granola bar with soft chewy matrix (aw = 0.6) and sugar coat (aw = 0.3)?  Hard candy (aw = 0.2-0.35) on a humid day?  WATER ACTIVITY  So, knowing the aw of a food component one can select the proper ingredients for a particular food product  For example, it is possible to create a multi-textured food product if components are added at the same aw  WATER ACTIVITY- factors controlling it  Temperature dependency of the sorption isotherm can be a major problem and often overlooked  Product dependent  Changes water binding,  dissociation of solutes, solubility At the same moisture content which would spoil faster? WATER ACTIVITY- factors controlling it  Colligative effects of dissolved species( salt and sugar) interact with water  Surface interactions – with chemical groups, undissolved ingredients  Capillary effect- due to which vapour pressure changes than that of pure water- because of changes in hydrogen bonding  WATER ACTIVITY IN FOOD INDUSTRY  Importance of aw in foods and food industry ◦ Food stability directly related to aw ◦ Influences storage, microbial growth, chemical & enzymatic deteriorations ◦ Changes in colour, taste, aroma,  WATER ACTIVITY A. Microbial stability ◦ Foods with aw > 0.9 require refrigeration because of bacteria spoilage  Exception:Very low pH Foods ◦ Can control by making intermediate moisture foods (IMF)  Food with low aw to prevent microbial spoilage at room temp. But which can be eaten w/o hydration  Aw = 0.7 - 0.9 (20 -50% water) - achieved by drying or using solutes (sugar, salt)  dried fruits, jelly and jam, pet foods, fruity cakes, dry sausage, marshmallow, bread, country style hams  Minimal processing however preferred over IMF  Special problems  May need mold inhibitor  Lipid oxidation - may need antioxidant or inert packaging ◦ Important in grains to prevent mold growth & possibly mycotoxin development  Must be below 0.8 By measuring the Aw – can determine which organism will not be able to grow A w Range Growth of micro organisms 0.91- 0.95 Most bacteria 0.88 Most yeast 0.80 Most mild dew 0.75 Halophile bacteria 0.70 Osmophile bacteria 0.65 Xerophile bacteria WATER ACTIVITY B. Chemical stability ◦ Maillard browning  Doesn't occur below type II water  Increases in type II water - water becomes a better solvent while reactants become more mobile (aw = 0.6-0.7 Peak reaction)  Reduced in type III - dilution or water is an inhibitor  Depends on food product WATER ACTIVITY B. Chemical stability ◦ Lipid oxidation  Low aw, lipid oxidation high - due to instability of hydroperoxides (HP) - unstable w/o water, no H-bonding  Slightly more addition of water stabilizes the HP and catalysts  Above type II water, water promotes the lipid oxidation rate because it helps to dissolve the catalysts for the reaction WATER ACTIVITY B. Chemical stability ◦ Vitamin and pigment stability  Ascorbic acid very unstable at high aw  Stability best in dehydrated foods - type II water  Problem with intermediate to high moisture foods  Must consider packaging for these foods WATER ACTIVITY C. Enzyme stability ◦ Hydration of enzyme ◦ Diffusion of substrate (solubility) ◦ Not significant in dehydrated foods ◦ Little enzyme activity below type II water ◦ Exceptions: in some cases we get activity at ↓aw  Frozen foods  Lipases (work in a lipid environment) WATER ACTIVITY CONTROL  Like pH, every microorganism has a minimum, optimum, and maximum water activity for growth  Yeasts and molds can grow at a low water activity, however 0.85 is considered the safe cutoff level for pathogen growth.  A water activity of 0.85 is based on the minimum water activity needed for S. aureus toxin production. Water activity and foods Water Activity Classification Requirements for Control Above 0.85 Moist Foods Requires refrigeration or another barrier to control the growth of pathogens 0.60 and 0.85 Intermediate Moisture Does not require Foods refrigeration to control pathogens. Limited shelf-life because of spoilage, primarily by yeast and mold Below 0.60 Low moisture foods Extended shelf-life, even without refrigeration Examples of moist foods(those with water activities above 0.85) Moist Food Water activity Fresh salmon 0.99 Apples 0.99 Milk 0.98 Cured ham 0.87 Bread 0.95 Examples of intermediate moisture foods (water activity between 0.60 and 0.85) are: Intermediate Moisture Water activity Food Molasses 0.76 Heaveily salted fish, such 0.70 as cod Flour 0.70 Jams 0.80 Dried fruit 0.70 Soy sauce 0.80  Some unique foods like soy sauce appear to be a high moisture product, but because salt, sugars or other ingredients bind the moisture, their water activities are quite low.  Soy sauce has a water activity of about 0.80.  Because jams and jellies have a water activity that will support the growth of yeast and mold, they are mildly heat-treated immediately before packaging, to prevent spoilage. Examples of low moisture foods (water activity below 0.60) are: Low Moisture Water activity Food Dried noodles 0.50 Crackers 0.10  The water activity of some of the intermediate and low moisture foods is naturally low, such as for molasses and flour.  Other intermediate and low water activity foods, like dried fruit, salted fish, strawberry jam, crackers, soy sauce and noodles, begin with a high water activity food and through processing, the water activity is reduced. Control of Water Activity  Two primary ways to reduce water activity in foods including drying or adding salt or sugar to bind the water molecules.  Drying is one of the oldest methods of food preservation. While open air-drying is still practiced in many parts of the world, there are four primary methods of drying  Hot air drying -- used for solid foods like vegetables, fruit, and fish  Spray drying -- used for liquids and semi-liquids like milk  Vacuum drying -- used for liquids like juice  Freeze-drying -- used for a variety of foods.  The other method of reducing water activity in foods is adding salt or sugar.  Some examples of these types of foods include soy sauce, jams, and salted fish.  Special equipment is not needed to do this.  For liquid or semi-liquid foods, like the soy sauce or jam, the process involves formulation control.  For solid foods like fish or cured ham, salt can be applied dry, in a brine solution or brine injected. CHEMICAL INHIBITORS  Sometimes the chosen food preservation method does not provide protection against the growth of all microorganisms.  In these cases, additional protection might be provided by the addition of chemicals.  Chemical preservatives include benzoates, sorbates, sulfites, nitrites, and antibiotics. Examples include:  Hummus that uses sodium benzoate to inhibit yeast and mold.  Bread that uses calcium propionate to inhibit mold.  Smoked fish that uses sodium nitrite and some of the ingredients in the wood smoke to inhibit C. botulinum. PACKAGING  Packaging is different from the other methods of control.  Although packaging is sometimes used to control microbiological growth, it is limited to the control of spoilage microorganisms.  From a food safety standpoint, packaging serves two functions: (1) it prevents contamination of the food or (2) it extends the effectiveness of food preservation methods.  For example, packaging maintains the atmosphere in a controlled or modified atmosphere package or a vacuum package or it prevents rehydration of a dried food.