Lecture 5 - Moisture pt 2 Flashcards
how is H2O removed via distillation method?
- with distillation solvents (toluene or mineral oil)
- vapor is cooled and water collected and measured
distillation solvents should be chemically (stable/unstable) at distillation temps.
stable
what are the 2 versions of distillation solvents?
immiscible
- volatile (toluene)
- nonvolatile (mineral oil)
distillation is a (direct/indirect) method of moisture determination
direct
is the distillation technique AOAC approved?
yep
distillation is more effective with (volatile/non-volatile liquid)
volatile
which distillation solvent is more effective?
toluene
when distilling with toluene, which layer does the water go to?
bottom
disadvantages of distillation methods
- difficult to read meniscus
- toluene can be a health/env hazard
- toluene is flammable
- not adaptable to routine, fast testing
- possibility of decomposing CHO or inducing Maillard rxns
what is the karl fischer method?
titration method - adaptable to products that show erratic results when heated or subjected to vacuum
can the KF method determine bound water?
yeah
when is the KF method used?
for low and very low moisture foods (dried veggies, spices, chocolate)
reagents in KF titration method?
- iodine
- sulfur dioxide
- pyridine
- methanol
supplied as 2 solutions
describe the KF titration
- Iodine and SO2 are added by a buret to the sample in a closed chamber
- excess I2 not reacted with water produces dark red-brown color
what’s the role of iodine in the KF titration?
reacts with water. it gets converted to iodide.
what’s the role of sulfur dioxide in KF titration?
produces sulfuric acid
what’s the role of pyridine in KF titration?
acts as base
what’s the role of methanol in KF titration?
solvent
what do you do if your food sample is solid but you wanna use KF titration?
- moisture must be made accessible by extracting with an appropriate solvent (methanol) after powdering the sample
- methanolic extract is used for titration
what compound is used as a primary standard during KF titration? why?
sodium tartrate dihydrate
it’s very stable and contains 15.66% water under all lab conditions
true or false: particle size of sample is not an issue with KF titration
false - fineness of grind is important for penetration of solvent
true or false: atmospheric moisture is problematic during KF titration
true
true or false: moisture can adhere to glassware
true - equipment should be carefully dried prior to analysis
what food constituents can potentially interfere with KF titration?
- ascorbic acid: overestimate
- acetal formation releases water: overestimation
- unsaturated fatty acids react with iodine: overestimation
describe coulometric KF titration
- iodine is generated electrochemically during titration to titrate water in sample
- amount of I required to titrate is determined by current needed to generate iodine
which is more sensitive: volumetric or coulometric KF titration?
coulometric
what are the physical methods of determining moisture?
- electrical method (measures conductance and capacitance)
- hydrometer
- pycnometer
what is conductance?
reciprocal of resistance (1/R)
what is capacitance
related to dielectric constant
electrical methods require _____ against samples of known moisture content as determined by standard methods
calibration
the dielectric method is widely used for what kind of food commodity?
cereal grains
what does IR analysis do?
measures abs of IR radiation by molecules in food
what’s the method of choice for the grain research laboratories for testing canadian wheat?
IR analysis
are IR analysis, NMR, and chromatography techniques official methods?
no, they must be calibrated against a reference method
rate of water loss is related to ______
RH
how much moisture is lost at 50% RH
0.01% in 5 s
how much moisture is lost at 70% RH?
0.01% in 10 s
what aw is the cutoff point for pathogenic microbial growth in canned food?
0.85
true or false: the relationship between moisture content and aw is linear
false
how do you measure water activity?
measure vapor pressure of headspace after a closed system attains equilibrium (aw and RH will be equal)
dry and dense samples require (longer/shorter) time to attain equilibrium in a closed system
longer
what are some detectors for measuring vapor pressure of headspace?
- dewpoint
- electric hygrometer sensors
- direct measurement of manometric pressure
- tunable diode laser sensor
- freezing point determination
what components does a dew point analyzer have?
- fan for air circulation in headspace
- IR thermometer to measure sample temp
- temp-controlled mirror
- sensor that detects condensation on mirror
dew point measures RH in the (matrix/headspace)
headspace
what data is recorded in the dew point analyzer
- temp of sample and mirror
- temp of sample is used to determine waver vapor pressure (po)
- temp of mirror is used to determine vapor pressure of headspace (p)
aw = p/po
higher temps lead to (higher/lower) aw readings
higher
temperature differences between sample and containers could cause ______
condensation
at what temp are Aws mostly measured?
25C
