Food Science Review 70 MC 30 Math problems bring calculator 10 15 from lab section Water Medium for chemical reactions Reactant acts as carriers in hydrolysis reactions Provides food texture Affects shelf life of food Major constituent in most food and even with dry food you have to consume water Don t need to remember the content number in the figure in the powerpoint Tetrahedral structure with two lone pairs which makes oxygen partially negative and hydrogen partially positive Hydrogen bonding is its unique characteristic Hydrogen bonding happens from the atom to other common groups Water has a high boiling point because of hydrogen bonding 100 C Specific heat is 1cal g C High latent heat for ice to become water it needs heat of fusion which is 80 cal g and heat of vaporization is 540 cal g X H X X electronegative atom H hydrogen atom OH hydroxyl NH2 amino C O carbonyl CO NH amide NH imino Know how to calculate total E that is required to heat ice to vapor he will provide specific heat for ice just remember specific heat for water is 1 Times specific heat grams of water latent phase energy 18cal g 100 Organic molecules like sucrose will be 1 dissociated species but something like sodium chloride will have two dissociated species Dissolvable inorganic salts number of cations that will dissociate Ion Products of Water 2 H2O H3O OH Keq H3O OH H2O 1 8 10 16 Very few molecules ionize concentration remains constant H2O m MV 1000 g 18 g mol 1 L 55 5 M at 25 C Kw Keq 55 5 M 1 0 10 14 H3O OH pH A measure of the H3O defined as the negative logarithmic of H3O pH meter measures the pH of a substance it is the second most widely used analytical instrument The pH meter was invented by Beckam in 1934 and is still used to measure the acidity of citrus juices pH log10 H3O Kw H3O OH 1 0 10 14 At neutrality 25 C H3O OH 1 0 10 7 M pH log10 1 0 10 7 7 log10 H3O OH log10 1 0 10 14 log10 H3O log10 OH 14 pH pOH 14 Acids Acids are proton donors Strong acids have complete dissociation Weak acids only go through partial dissociation o Exhibit incomplete dissociation o Equilibrium between the weak acid HA and its conjugate base A is given by the Ka or pKa acid dissociation constant Bases Bases are proton acceptors Strong bases like strong acids can dissociate completely Respectively weak bases only partially dissociate Which acid tastes more sour 0 1 M hydrochloric acid Strong inorganic acid pH 1 00 0 1 M citric acid Weak organic acid pH 2 08 Buffers Buffers are a mixture of weak acid and its conjugate base that resist change in pH when strong acid or base is added HA H2O H3O A Acetic acid has a pKa of 4 8 what is the approximate buffering range of sodium acetate acetic acid buffer Weak Acids Weak acids exhibit incomplete dissociation o HA H2O H3O A At a given temperature dissociation constant K is given as o K H A H2O HA Since H2O is very large 55 5 M compared to the other parameters it is treated as a constant o Ka K H2O H A HA Henderson Hasselbalch Equation HA H2O H3O A Ka H A HA To simplify the equation it is written in a log form and rearranged as o log10Ka log10 H3O log10 A log10 HA o log10 H3O log10Ka log10 HA A o log10 H3O log10Ka log10 HA A o If we define log10Ka as pKa o pH pKa log10 HA A or o pH pKa log10 A HA Similarly pOH pKb log10 BH B Molality Molarity and Normality Molality Moles of solute in 1 kg solvent c nsolute msolvent mol kg Molarity Moles of solute in 1 L solution c nsolute Vsolution mol L or M needed to produce 1 mole of H c nsolute Vsolution feq Eq L or N Normality Molarity divided by hydrogen equivalent factor feq moles of substance Don t forget to study Titration Curve of Weak Acids in the powerpoint set Free vs Bound Water Water activity Aw Partial pressure of water above the sample partial pressure of pure water at the same temperature equilibrium relative humidity 100 Aw is related to water content of food but not DIRECTLY related Aw p p ERH 100 Relative humidity the ratio of quantity of water vapor present in the atmosphere to the quantity which would saturate at that temperature In 1953 Scott showed that microorganisms have a limiting Aw level for growth Aw is more closely related to the microbial chemical and physical properties of foods than is total moisture content Water activity meters capacitance hygrometer ability of a polymer membrane dielectric to hold a charge increases when it absorbs water capacitance is measured or dew point hygrometer temperature at which dew forms on a clean surface due point is measured Dew point temperature to which a given amount of air must be cooled at constant pressure and constant water vapor content in order for saturation to occur Relative humidity at a prescribed temperature pressure of water vapor in sample pressure of water vapor at saturation Example of water activity concept o Walk into grocery store and buy a refrigerated cold egg o When you get home it s sweating o because air may have high humidity and when cold eggs cool the surrounding air sufficiently reaches dew point then moisture condensation may occur sweating o Happens on any fruits vegetables soft drinks etc Intermediate moisture foods IMF o Food products f soft texture o Subjected to one or more technological treatments o Consumable without further preparation and with a shelf stability of several months o Assured without thermal sterilization freezing refrigeration etc o Assured by adequate adjustment of their formulation Composition pH additives C mainly Aw which must be approximately between 0 6 0 84 at 25 deg o Contain 15 30 water and are shelf life stable even though their Aw is 0 70 0 85 ex Dried fruits jellies etc Advantages of IMF o Less energy intensive o Higher retention of nutrients and quality o Prolonged shelf life o Plasticity and chewability without oral sensation of dryness o Direct consumption Production of IMF o Addition of humectants most properties existing microflora o Materials that lower Aw but impart a plastic texture and allow foods to retain their o Control of pH temperature oxidation reduction potential preservatives and o Practice problem A packaged cupcake has cake with 9 moisture content and icing with 13 5 moisture content What will happen to water in the cupcake as it sits on the shelf Water moves from the cake to the icing Moisture Sorption Isotherm Zone 1 monolayer water strongly associated Zone 2 multilayer water less strongly associated Zone 3 bulk water not associated Dispersion of Matter in Foods True solutions typically the