Lecture 5 - Moisture pt 2

Question 1

how is H2O removed via distillation method?

Answer 1

• with distillation solvents (toluene or mineral oil)

- vapor is cooled and water collected and measured

Question 2

distillation solvents should be chemically (stable/unstable) at distillation temps.

Answer 2

stable

Question 3

what are the 2 versions of distillation solvents?

Answer 3

immiscible

• volatile (toluene)
• nonvolatile (mineral oil)

Question 4

distillation is a (direct/indirect) method of moisture determination

Answer 4

direct

Question 5

is the distillation technique AOAC approved?

Answer 5

yep

Question 6

distillation is more effective with (volatile/non-volatile liquid)

Answer 6

volatile

Question 7

which distillation solvent is more effective?

Answer 7

toluene

Question 8

when distilling with toluene, which layer does the water go to?

Answer 8

bottom

Question 9

disadvantages of distillation methods

Answer 9

• 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

Question 10

what is the karl fischer method?

Answer 10

titration method - adaptable to products that show erratic results when heated or subjected to vacuum

Question 11

can the KF method determine bound water?

Answer 11

yeah

Question 12

when is the KF method used?

Answer 12

for low and very low moisture foods (dried veggies, spices, chocolate)

Question 13

reagents in KF titration method?

Answer 13

• iodine
• sulfur dioxide
• pyridine
• methanol
  supplied as 2 solutions

Question 14

describe the KF titration

Answer 14

• 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

Question 15

what’s the role of iodine in the KF titration?

Answer 15

reacts with water. it gets converted to iodide.

Question 16

what’s the role of sulfur dioxide in KF titration?

Answer 16

produces sulfuric acid

Question 17

what’s the role of pyridine in KF titration?

Answer 17

acts as base

Question 18

what’s the role of methanol in KF titration?

Answer 18

solvent

Question 19

what do you do if your food sample is solid but you wanna use KF titration?

Answer 19

• moisture must be made accessible by extracting with an appropriate solvent (methanol) after powdering the sample
• methanolic extract is used for titration

Question 20

what compound is used as a primary standard during KF titration? why?

Answer 20

sodium tartrate dihydrate

it’s very stable and contains 15.66% water under all lab conditions

Question 21

true or false: particle size of sample is not an issue with KF titration

Answer 21

false - fineness of grind is important for penetration of solvent

Question 22

true or false: atmospheric moisture is problematic during KF titration

Answer 22

true

Question 23

true or false: moisture can adhere to glassware

Answer 23

true - equipment should be carefully dried prior to analysis

Question 24

what food constituents can potentially interfere with KF titration?

Answer 24

• ascorbic acid: overestimate
• acetal formation releases water: overestimation
• unsaturated fatty acids react with iodine: overestimation

Question 25

describe coulometric KF titration

Answer 25

- 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

Question 26

which is more sensitive: volumetric or coulometric KF titration?

Answer 26

coulometric

Question 27

what are the physical methods of determining moisture?

Answer 27

- electrical method (measures conductance and capacitance) - hydrometer - pycnometer

Question 28

what is conductance?

Answer 28

reciprocal of resistance (1/R)

Question 29

what is capacitance

Answer 29

related to dielectric constant

Question 30

electrical methods require _____ against samples of known moisture content as determined by standard methods

Answer 30

calibration

Question 31

the dielectric method is widely used for what kind of food commodity?

Answer 31

cereal grains

Question 32

what does IR analysis do?

Answer 32

measures abs of IR radiation by molecules in food

Question 33

what's the method of choice for the grain research laboratories for testing canadian wheat?

Answer 33

IR analysis

Question 34

are IR analysis, NMR, and chromatography techniques official methods?

Answer 34

no, they must be calibrated against a reference method

Question 35

rate of water loss is related to ______

Answer 35

RH

Question 36

how much moisture is lost at 50% RH

Answer 36

0.01% in 5 s

Question 37

how much moisture is lost at 70% RH?

Answer 37

0.01% in 10 s

Question 38

what aw is the cutoff point for pathogenic microbial growth in canned food?

Answer 38

0.85

Question 39

true or false: the relationship between moisture content and aw is linear

Answer 39

false

Question 40

how do you measure water activity?

Answer 40

measure vapor pressure of headspace after a closed system attains equilibrium (aw and RH will be equal)

Question 41

dry and dense samples require (longer/shorter) time to attain equilibrium in a closed system

Answer 41

longer

Question 42

what are some detectors for measuring vapor pressure of headspace?

Answer 42

- dewpoint - electric hygrometer sensors - direct measurement of manometric pressure - tunable diode laser sensor - freezing point determination

Question 43

what components does a dew point analyzer have?

Answer 43

- fan for air circulation in headspace - IR thermometer to measure sample temp - temp-controlled mirror - sensor that detects condensation on mirror

Question 44

dew point measures RH in the (matrix/headspace)

Answer 44

headspace

Question 45

what data is recorded in the dew point analyzer

Answer 45

- 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

Question 46

higher temps lead to (higher/lower) aw readings

Answer 46

higher

Question 47

temperature differences between sample and containers could cause ______

Answer 47

condensation

Question 48

at what temp are Aws mostly measured?

Answer 48

25C