Food Science Review70% MC30% Math problems – bring calculator10-15% from lab sectionWater:- Major constituent in most food and even with dry food you have to consume water- Medium for chemical reactions- Reactant (acts as carriers in hydrolysis reactions)- Provides food texture- Affects shelf life of food- 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 C⁰ ) - Specific heat is 1cal/g/ C⁰- 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/gX-H…...XX: electronegative atomH: 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 Water2 H2O ⇌ H3O+ + OH-Keq = [H3O+][OH-]/[H2O] = 1.8 × 10-16Very few molecules ionize, concentration remains constant[H2O] = m/MV = 1000 g / (18 g/mol × 1 L) = 55.5 M at 25 °CKw = 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 juicespH = -log10[H3O+]Kw = [H3O+][OH-] = 1.0 × 10-14At neutrality & 25 °C, [H3O+] = [OH-] = 1.0 × 10-7 MpH = -log10(1.0 × 10-7)= 7-log10 {[H3O+][OH-]} = -log10(1.0 × 10-14)-log10[H3O+] - log10[OH-] = 14pH + pOH = 14Acids• Acids are proton donors • Strong acids have complete dissociation• Weak acids only go through partial dissociationo Exhibit incomplete dissociationo Equilibrium between the weak acid (HA) and its conjugate base (A-) is given by the Ka or pKa acid dissociation constantBases• Bases are proton acceptors• Strong bases, like strong acids, can dissociate completely• Respectively, weak bases only partially dissociateWhich acid tastes more sour?• 0.1 M hydrochloric acid– Strong inorganic acid– pH = 1.00• 0.1 M citric acid– Weak organic acid– pH = 2.08Buffers• 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 dissociationo 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-], oro 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)• Normality: Molarity divided by hydrogen equivalent factor (feq, moles of substance needed to produce 1 mole of H+).c = nsolute/(Vsolution*feq) (Eq/L or N)***Don’t forget to study Titration Curve of Weak Acids in the powerpoint set***Free vs. Bound WaterWater activity (Aw) = Partial pressure of water above the sample/partial pressure of pure water at the same temperature = equilibrium relative humidity/100Aw is related to water content of food but not DIRECTLY relatedAw = p/p = ERH/100ₒRelative humidity: the ratio of quantity of water vapor present in the atmosphere to the quantity which would saturate at that temperatureIn 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 contentWater 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) eggo 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 textureo Subjected to one or more technological treatmentso Consumable without further preparation and with a shelf stability of several monthso Assured without thermal sterilization, freezing, refrigeration, etc.o Assured by adequate adjustment of their formulation: Composition pH additives mainly Aw which must be approximately between 0.6 – 0.84 (at 25 deg. C)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 intensiveo Higher retention of nutrients and qualityo Prolonged shelf-lifeo Plasticity and chewability without oral sensation of drynesso Direct consumption• Production of IMFo Addition of humectantso Materials that lower Aw but impart a plastic texture and allow foods to retain their most propertieso Control
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