This preview shows page 1. Sign up to view the full content.
Unformatted text preview: WATER 1 WATER'S IMPORTANCE Solvent Most molecules dissolved in water Water's involvement in hydrolysis reactions Water's involvement in condensation reactions E.g. boiling, steaming, cooling Reactant Product Heat transfer medium 2 WATER'S IMPORTANCE Texture Juiciness, mouthfeel Snack foods Vegetables Meat Preservation Highly perishable foods usually have high water activity E.g. bread vs. cracker or cereal Economics More water added = more $ UNDERSTANDING THE PHYSICAL AND CHEMICAL UNDERSTANDING PROPERTIES OF WATER IS IMPORTANT IN THE STUDY OF FOOD AND PROCESSING STUDY
3 PHYSICAL & CHEMICAL PHYSICAL PROPERTIES OF WATER PROPERTIES Water has very unique properties not shared by other Water similar hydrogen compounds or compounds of similar weight weight
Compound H2O H2S NH3 Methanol Melting point 0ºC -83ºC -78ºC -98ºC Boiling point 100ºC -60ºC -33ºC 65ºC Why? – this is explained by the unique structure of H2O Why? unique
4 STRUCTURE OF WATER Tetrahedral arrangement Two free electrons of O Two act as H-bond acceptors while H acts as donor while Highly electronegative O Highly pulls electrons from H, making H behave like a bare proton Forms a dipole because of the electronegative O 5 STRUCTURE OF WATER Because of the DIPOLE Because and TETRAHEDRAL structure we can get strong H-bonding H-bonding Water capable of bonding Water to 4 other water molecules to Unique properties of water Unique from other hydrides from H-bond NOT a static H-bond phenomenon phenomenon T dependent 6 PHASE CHANGES OF WATER 7 WATER VAPOR Water is “free” and devoid of any H-bonds Large input of energy needed Large input an endothermic process Large dissipation of same energy needed to make Large dissipation water lose kinetic energy water an exothermic process Waters latent heat of vaporization is unusually Waters latent high high to change 1 L from liquid to vapor need 539.4 kcal 8 LIQUID WATER Extensively H-bonded H-bond formation dependent on T With increasing T get more mobility and increased fluidity T (ºC) 0 5 25 100 999.9 1000.0 997.1 958.4 Density (kg/m3) Viscosity (m2/s) 1.7895 1.535 0.884 0.294 9 ICE Forms when exactly 4 H-bonds Forms are formed between water molecules molecules 2.78 A vs. 2.85 A in liquid To get this order a lot of energy To needs to be adsorbed by the environment environment The strong H-bonding in ice The forms an orderly hexagonal crystal lattice crystal 6 H2O molecules Has 4X more thermal Has conductivity than water at same temperature temperature
10 Can go from ICE to GAS BASIS FOR FREEZE DRYING • SUBLIMATION 11 PROPERTIES OF ICE Crystallization Crystal growth occurs at freezing point Rate of crystal growth decreases with decreasing temperature Solutes slow ice crystal growth Slow freezing results in few nucleation sites and large, coarse Slow crystals crystals Fast freezing results in many nucleation sites and small, fine Fast crystals crystals Heterogeneous nucleation Nucleation - affects ice crystal size. usually caused by a foreign particle, such as salt, protein, fat, etc. very rare, mainly occurs in pure systems Homogeneous nucleation 12 PROPERTIES OF ICE SUPERCOOLING Water can be cooled to temperatures below its Water freezing point without crystallization freezing When an ice crystal is added to supercooled water, When temperature increases and ice formation occurs temperature 13 PROPERTIES OF ICE
Freezing induced changes Freezing in foods (examples) in Example: Effect of freezing on seafoods Destabilization of emulsions Flocculation of proteins Increased lipid oxidation Meat toughening Cellular damage Loss of water holding capacity 14 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 Water holding capacity Hydrophilic substances are able to entrap large amounts of Hydrophilic water water “Trapped” water Jellies, jams, yogurt, jello, meat Yogurt - often see loss of water holding as whey is released Yogurt at the top of the yogurt at 15 WATER SOLUTE INTERACTIONS Ionic polar solutes React readily with water and most React are usually soluble in water are Water HYDRATES the ions Charge interactions due to waters Charge high DIELECTRIC CONSTANT high Can easily neutralize charges due Can to its high dipole moment to Large ions can break water Large structure structure Have weak electric fields Small ions can induce more Small structure in water structure Have strong electric fields 16 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 can H-bond disrupters H-bond urea - disrupts water Water bridge
17 WATER SOLUTE INTERACTIONS Nonpolar Unfavorable interaction with water Water around non-polar substance Water is forced into an ordered state is Water affinity for water high Water compared to non-polar compound compared Water forms a shell Tries to minimize contact Hydrophobic interactions Caused because water interacts Caused with other water molecules while hydrophobic groups interact with other hydrophobic groups other 18 EFFECT OF SOLUTES ON WATER
Boiling point Vapor pressure is equal to Vapor atmospheric pressure Strongly influenced by water - solute Strongly interaction interaction Solutes decrease vapor pressure and Solutes thus increase boiling point thus Sucrose +0.52ºC/mol Sucrose NaCl +1.04ºC/mol NaCl ATMOSPHERIC PRESSURE VAPOR PRESSURE 19 EFFECT OF SOLUTES ON WATER
1 atm (sea level) mountains 90C 100C So does it take longer or shorter to boil an egg in the Rocky Mountains? Why?
20 EFFECT OF SOLUTES ON WATER Let's go back to our egg, what would happen if you added salt? Raoult's Law Recommended that you add salt to water at high altitudes
21 EFFECT OF SOLUTES ON WATER
Freezing point lowering Freezing point can get extensive Freezing depression via solutes depression Alter ability of water to form crystals Alter due to H-bond disruption due Sucrose -1.86ºC/mol Sucrose NaCl -3.72ºC/mol NaCl Where “all” water is frozen - usually Where around -50ºC around Eutectic pt - temp. Eutectic In most cases small amounts of In water remains unfrozen (-20ºC) water These small patches of water can These promote chemical reactions and damage damage 22 EFFECT OF SOLUTES ON WATER
What explains all this? Raoult's law * 1 P = P /X or P*-P/P*= x/55.5M P = vapor pressure of solution; P* = vapor pressure pure solvent; X1 = mole fraction of solute; x = grams solutes in solution; 55.5M = moles of water per liter This relationship is not only important for explaining the This concepts of depressing freezing point and elevating boiling 23 point point EFFECT OF SOLUTES ON WATER
Osmotic pressure of solutions There is a tendency for a system containing water and a There solution separated with a membrane to be at equilibrium solution The pressure needed to bring the two solutions at equilibrium The is called OSMOTIC PRESSURE is The more the solution has of dissolved solutes (e.g. salt) the The higher its osmotic pressure higher Can use this in food processing and preparation E.g. Crisping salad items Increase turgor 24 EFFECT OF SOLUTES ON WATER
Surface tension Water surface behaves Water differently than bulk phase differently Like an elastic film Due to unequal inward force Resist formation of a new Resist surface thus forming surface tension tension 25 EFFECT OF SOLUTES ON WATER Water has high surface tension 72.75 dynes/cm (20ºC) Because of the high surface tension special Because considerations are needed in food processing considerations To affect it one can: Increase T (more energy) reduces surface tension Increase Add solutes NaCl and sugars increase surface tension NaCl Amphipathic molecules reduce surface tension Amphipathic 26 PhotoFrost®
27 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 water Pure water: Keq = Equilibrium (or ionization) constant Keq = [H3O]+ [OH][H eq [H2O] [H [H3O]+ [OH]- = Keq = Kw (Water dissociation constant) eq [10-7] [10-7] = [10-14] [10 28 EFFECT OF SOLUTES ON WATER Acids and bases in food systems Acid - proton donor NH3 + H2O NH4+ + OH CH3COOH + H2O CH3COO- + H3O+ Base - proton acceptor Weak acids and bases Most foods are weak acids Most These constituents are responsible for buffering of food These systems systems Acetic, citric, lactic, phosphoric, etc.
29 Some examples Some 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+ + ClpH = -log [acid] = -log [H]+ HOAC Weak Keq = [H]+ [OAC] When placed in solutions weak acidsHOAC] equilibrium [ form an pKa = -log Ka
30 acids H+ + OAC- EFFECT OF SOLUTES ON WATER Weak acids and bases One cannot relate pH to concentration for weak acids One and bases because of this equilibrium and One must understand how the acid behaves in solution Knowing the dissociation constant of the acid is Knowing important to determine the effect on the pH of the system system The relationship of pH for weak acids and bases relies The on the Henderson - Hasselback equation: on pH = pKa + log [salt] [acid]
31 EFFECT OF SOLUTES ON WATER Weak acids Graphically behave Graphically like the figure when titrated with a strong base. The reverse holds true for weak bases bases What do we call this point? 32 EFFECT OF SOLUTES ON WATER Buffering Buffers resist Buffers changes in pH when acids and bases are added bases Characteristics of a Characteristics buffer buffer What is this point and its significance to food systems? Maximum when Maximum pH = pKa or when [acid] = [salt] [salt] Rule of thumb: pH = pKa ± 1 pH 33 EFFECT OF SOLUTES ON WATER Examples of natural pH control Fruits - citric, malic, acetic, etc Microbial control Flavoring Controlled by three components Milk – pH around 6.5 Phosphate, citrate, carbonate Eggs Fresh eggs - pH = 7.6 After storage for several weeks - pH = 9-9.7 Due to loss of CO2 Problem - Loss of carbohydrate groups on Problem proteins. Loss of protein functionality, causing decreased viscosity and poor foaming properties properties 34 EFFECT OF SOLUTES ON WATER Examples of “man made” pH control Food additives - ACIDULANTS Citric acid - pectin jellies pH must be around 2.9-3.0 Also provides balance between tartness and sweetness Fermentation - glucose or lactose to lactic acid pH reduction to around 4.6 will cause the gelation pH Can add acidulants to imitate dairy yogurts - lactic, citric, Can phosphoric, HCl phosphoric, Alkaline salts of phosphoric acid to get good protein dispersion pH below 4.5 usually hinders C. botulinum growth pH C. Less severe heat treatment required for these Acidulants used to lower pH below 4.5 for some fruit and tomato Acidulants products products
35 Yogurt and cottage cheese Cheese Thermal process control EFFECT OF SOLUTES ON WATER Examples of “man made” pH control Acidulants - leavening agents Used in the baking industry to give rise (release of CO2) alternative to yeast alternative When HCO3- becomes acidic (pH < 6), CO2 forms, CO2 not very soluble so released as a gas very Overall eq: H+ + HCO3- H2O + CO2 36 EFFECT OF SOLUTES ON WATER Examples of “man made” pH control Leavening systems Bicarbonate (NaHCO3) - source of HCO3 and CO2 Leavening acids Drive bicarbonate (HCO3) to CO2 Rate of acid release varies and therefore CO2 release Phosphate - rapid release of CO2 Sulfate – slow release of CO2 Pyrophosphate - can be cleaved by phosphatases Pyrophosphate becoming more soluble - used in refrigerated doughs becoming d-Glucono-lactone - used in refrigerated doughs 37 EFFECT OF SOLUTES ON WATER Examples of “man made” pH control Acidulants - antimicrobials pH is important for two reasons: 1. Solubility and 2. Activity The salt is more soluble in aqueous systems The acid is more active in its antimicrobial efficiency Found naturally in prunes, cranberries, cinnamon and cloves Active below pH 4 (active acidic form of the salt) Highly soluble in the form of sodium salt Effective - yeasts and bacteria, less for molds Uses in acid foods - soft drinks, juices, pickles, dressings etc. Broader pH range (active at higher pH) Mainly use methyl and propyl esters Uses in baked goods, wines, pickles, jams, syrups, etc. Benzoic acid (0.05-0.1%) Parabens or r-hydroxybenzoate esters (0.05-0.1%) 38 EFFECT OF SOLUTES ON WATER Acidulants - antimicrobials Sorbic acid (Na+ and K+ salt forms) (0.02-0.3%) Max activity at pH 6.5; active at acid pH values Most effective for yeast and molds Inhibit, not inactivate Uses in cheese, juices, wines, baked goods, etc. Active up to pH 5 Uses in breads (retards Bacillus) which causes ropiness in breads Uses Bacillus Ropiness - thick yellow patches that can be formed into a rope-like Ropiness structure making the bread inedible structure Proprionic acid (proprionate) Ca2+ salt Acetic acid Nitrites and Nitrates Sulfites 39 WATER ACTIVITY What is meant by water activity? Water has different levels of binding and thus activity or Water activity availability in a food sample availability Simply put, Water activity (aw) helps to explain the helps relationship between perishability and moisture content relationship Greater moisture content faster spoilage (normally) Greater Why are there some perishable foods at the same moisture Why content that don't spoil at the same rate? content There is a correlation found between aw and various different spoilage and safety patterns spoilage 40 WATER ACTIVITY Water has different levels of binding and thus activity or availability in a food Water activity availability sample 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 deteriorate unsafe, and it also helps them set regulatory levels of aw for different foods unsafe Highly perishable foods aw > 0.9 Intermediate moist foods aw = 0.6-0.9 Shelf stable foods aw < 0.6
41 WATER ACTIVITY Thermodynamic definition of aw The tendency of water molecules to escape the food The product from liquid to vapor defines the aw product aw = p/pO=%RH/100 Water activity is a measure of relative vapor pressure Water of water molecules in the head space above a food vs. vapor pressure above pure water vapor Scale is from 0 (no water) to 1 (pure water)
42 WATER ACTIVITY Sorption isotherms Help relate moisture content to Help aw Each food has their own Each sorption isotherm sorption It is interesting that when It water is added to a dry product, the adsorption is not identical to desorption identical Some reasons Temp. dependent Metastable local domains Diffusion barriers Capillary phenomena Time dependent equilibrium 43 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 problems This is one reason why it is important to know the aw of a food product or ingredient product Examples: Caramel, marshmallows and mints – all similar %moisture but very Caramel, different aw different Fudge (aw = 0.65-0.75) covered with caramel (aw = 0.4-0.5) – what happens? 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? 0.2-0.35) 44 WATER ACTIVITY So, knowing the aw of a food component one can select the proper ingredients for a particular food product particular For example, it is For possible to create a multipossible textured food product if textured components are added at the same aw the 45 WATER ACTIVITY Temperature dependency of the sorption isotherm can Temperature be a major problem and often overlooked be Example: Crackers that experience a temperature rise during transportation At the same moisture content which would spoil faster? 46 WATER ACTIVITY Sorption isotherms also explain the level Sorption of water binding in a food (i.e. types of water) Type I: Tightly “bound” water Type (monolayer) (monolayer) Unavailable/Unfreezable (at -40C) Water - ion; water - dipole interactions Slightly more mobility Some solvent capacity Type II: additional water layer (Vicinal Type water) True monolayer Monolayer Type III: Water condensating in Type capillaries and pores (multilayer bulkphase water) More available (like dilute salt solution) Can be entrapped in gels Supports biological and chemical rections Freezable 47 WATER ACTIVITY Importance of aw in foods Food stability directly Food related to aw related Influences storage, Influences microbial growth, chemical & enzymatic deteriorations, etc. etc. Vit C loss 48 WATER ACTIVITY
A) Microbial stability Foods with aw > 0.9 require refrigeration because of bacteria spoilage because
Exception: Very low pH Foods Food with low aw to prevent microbial spoilage at room temp. But which can be eaten w/o hydration eaten Aw = 0.7 - 0.9 (20 -50% water) - achieved Aw by drying or using solutes (sugar, salt) by Can control by making intermediate Can moisture foods (IMF) moisture 1.00 0.80 0.60 0.40 0.20 aw is a major HURDLE for microorganisms but not the only one May need mold inhibitor Lipid oxidation - may need antioxidant or Lipid inert packaging inert Important in grains to prevent mold growth Important & possibly mycotoxin development possibly Must be below 0.8
49 BACTERIA Minimal processing however preferred over Minimal IMF IMF Special problems OSMOPHILIC YEAST YEAST & MOLDS dried fruits, jelly and jam, pet foods, fruity dried cakes, dry sausage, marshmallow, bread, country style hams country 0.00 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 Increases reactants become more mobile Reduced in type III - dilution or water is an inhibitor Depends on food product (aw 0.53-0.55 in apple juice vs. 0.93 in anchovy) anchovy) 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 Above helps to dissolve the catalysts for the reaction helps 50 WATER ACTIVITY 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 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 Exceptions: Frozen foods Lipases (work in a lipid environment) Lipases 51 ...
View Full Document
- Spring '08