Exam 1 Flashcards
what is the mean and how do you calculate it
-mean = average
-sum of values/ number of values
accuracy
-how close a measurement is to the true value
-compare mean to true value
-assess by absolute error or relative error
absolute error
diff btwn experimental value and true vlaue (Eabs = x - T)
relative error
-compare diff btwn experimental value and true value, against the true value
-(Erel = (x-T)/T
precision
-how close are replicate measurements to each other
-measure of repeatability
-assess by:
stdev
CV
CI
std error of the mean
relative deviation from the mean
standard deviation
-measures precision: how close vlaues are to each other
-use n in denominator for 30 + samples, otherwise use n-1
-SD = square root (sum (xi - mean)^2 / n-1)
sources of error with evaluation of analytical data
-systematic error
-random error
-blunders
systematic error
-results consistently deviate from true value
ex: pipette not calibrated correctly
random error
-can fluctuate and are unavoidable
-ex: differences btwn balances (drafts in the room)
blunders
-big “screw ups”
-ex: pipetting 0.5 ml instead of 1 ml
what is the reliability of analysis?
sensitivity vs. limit of detection
sensitivity
-magnitude of change in measurement with change in conc. of compound
-ex: how great is the change on y-axis relative to change on x-axis
limit of detection (LOD) & values similar to LOD
-lowest possible amount that we can detect with statistical significance
-values similar:
method detection limit
limit of quantitation
what is regression analysis & how is it used
-standard curves: estimates the linear relationship btwn a scalar response and one or more explanatory variables (dep. and indep.)
-used to determine unknown concentrations
->only if conc of substance is
proportional to measurement
what is linear regression most often used to construct?
curves
what is the importance of moisture content
-preservation and stability
-quality factor
-convenience in packaging or shipping
-meeting compositional standards and Standards of Identity
-required for nutrition label calculations
-express results of other analytical determinations on a dry weight basis, for comparison
what are total solids
dry matter that remains after moisture removal
how are challenges created in moisture and water activity analysis
-struct. of water molecules and its moving to and from the environment during sample handling
-measuring water in all its common states
-water interacts with food components
what are water’s common states
solid, liquid, gas
what do you need to do prior to moisture analysis?
minimize moisture losses and gains in sample collection and sampling
how do water molecules interact with each other
-hydrogen bonding
hydrogen bond characteristics
-relatively weak and short-lived
-ex: picoseconds for forming, breaking, and reforming
-stable enough overall for good interactions
dynamic equilibrium
-exists once reversible reaction occurs
-substances transition btwn states at equal rates, no net change
-there is a steady state
steady state
conc. of reactants and products form at such a rate that neither changes
water content direct measurement methods (& how its done)
DONE BY REMOVING WATER
-forced draft oven drying
-vacuum oven drying
-microwave analyzer
-rapid moisture analyzer technology
-thermogravimetric analyzer
-lyophilization
-chemical desiccation
-Karl Fischer titration
water content indirect measurement methods (& how its done)
BASED ON A PROPERTY OF FOOD RELATED TO PRESENCE OF WATER
-dielectric capacitance
-hydrometer
-refractometer
-NIR spectroscopy
-freezing point, cryoscope
-microwave absorption
-conductivity
what do you need to consider when regarding methods?
-how should samples be handled
-what is measured?
-how is water removed/reacted/identified?
-assumptions? and sources of error?
-applications?
-official method requirements
oven drying method principle
-sample is heated to evaporate off water
-weight loss equals moisture content
oven drying method calculation
-% moisture (wt./wt.) = (wet wt. - dry wt. / wet wt.) x 100
oven drying method sources of error
-particle size
-volatile compounds present
-lipid oxidation
-sample very hygroscopic
-alteration of carbohydrates
-surface crust formation
-splattering
-improper storage in dessicator
what does the red box indicate in moisture content by drying oven graph
-indicates the break points at which there is sample break down
microwave drying analyzers characteristics
-greatly reduces analyses time
-allows for in-process analyses & process adjustment
-variables: time and power
infrared drying characteristics
-involves heat penetration directly into the sample (vs. heat conductivity & convection with conventional oven)
Karl Fischer method principle
-volumetric titration of sample with Karl Fischer reagent (KFR), involving reduction of I2 (molecular iodine) by SO2 (sulfur dioxide) in the presence of water
-reach the endpoint of titration when excess I2 present that cannot react with water (measure by visual (color), potentiometric, or conductometric endpoint)
-volume of titrant (KFR) consumed is used to calculate % moisture
Karl Fischer Coulometric method
-Karl Fischer reagent (KFR) must be titrated with a standard to determine KFR water (moisture) equivalence (i.e. how much water reacts with 1 ml KFR)
-water content can be determined immediately from the coulombs required for electrolytic oxidation
how can standardization of the Karl Fischer be done?
-pure water, water-in-methanol standard
-sodium tartrate dehydrate (commonly used)
Dielectric method principle
-based on electrical properties of water
-measure change in capacitance or resistance to an electric current passed through sample
-can assess consistency in processing
-dielectric constant (permittivity) is measured as an index of capacitance
what is capacitance
-ability of material object/device to store electrical charge
Freezing point measurement
-use cryoscope to measure freezing point
-used to test for added water in milk
-if freezing point is above -0.507 C then there is too much water
importance of water activity
-affects important quality and safety factors
-> growth of spoilage and pathogenic
microorganisms
-can be a critical control point in HACCP plans
what does the water activity of 0 vs. 1 mean
-aw of 0 = absolutely no water
-aw of 1 = pure water
water activity methods
-dew-point measurement (chilled mirror)
-electric hygrometer sensors
-direct measurement of manometric pressure
-inc. in sorbent mass (isopiestic method)
-thermocouple psychrometry
-freezing point determination
rheology
science devoted to the deformation and flow of all materials
rheological properties
-subset of mechanical properties of food
-determined by applying and measuring forces and flows or deformations as a function of time
what do rheological method measure
force, deformation, flow
stress vs. strain definition
-stress: force per area
-strain: relative deformation
how are stress and strain linked
by constant of proportionality (moduli)
what do ideal solids and ideal fluids obey
-ideal solids: obey Hooke’s Law
-ideal fluids: obey Newtonian principle
viscosity
internal resistance to flow
stress
-fundamental measurement of force
-stress = force/ area
-expressed in Pascals (Pa) for low pressure and Bars (1 bar = 100 kPa) for high pressure
normal stress
force is perpendicular to surface
ex: chewing piece of gum
shear stress
force is parallel to surface
ex: spreading butter on toast
strain
dimensionless quantity representing relative deformation of material
normal strain
-ɛ̝ (epsilon)
-applied stress is perpendicular to surface
-ex: large deformation such as in texture testing
shear strain
-γ (gamma)
-applied stress is parallel to sample
-ex: pumping tomato paste through pipe
shear (strain) rate
-degree of deformation with respect to time
simple shear
-laminar (layered) deformation in a plane parallel with an applied force
Hooke’s Law
-when solid materials are exposed to stress, the resulting deformation or strain is proportional to the magnitude of the stress
-so, stress = modulus x strain
elastic modulus (E)
proportionality constant when normal stress is applied to sample
shear modulus (G)
proportionality constant when shear stress is applied to sample
relationship of viscosity to temp
viscosity dec as temp inc
apparent viscosity
-shear-dependent viscosity
-shear stress divided by corresponding shear rate
-what the viscosity “appears to be” under shear stress
newton’s postulate
doubling shear stress doubles shear rate within fluid
Newtonian fluids (ideal fluids)
-viscosity is constant for all shear rates; follows Newtonian principles
-ex: water, oil, honey
-there is a constant viscosity independent of stress (doesn’t matter what you do to it)
Non-Newtonian fluids
-viscosity is not constant and is function of shear rate
-xanthan gum solution, butter, cheese
-viscosity changes with force to either more liquid or more solid
shear thinning (pseudoplastic) fluids
-viscosity decreases as shear rate increases, and is independent of time
-ex: applesauce, pie fillings
shear thickening (dilatant) fluids
-viscosity increases as shear rate increases, and is independent of time
-ex: corn starch slurries
thixotropic fluids
-viscosity decreases over time with agitation
-ex: ketchup, pumpkin pie filling
-ex: shaking bottle of ketchup actually thins the contents, which then quickly return to gel-like state
antithixotropic (rheopectic) fluids
-viscosity increases with time
-ex: some clay mixtures
thinning (time dep vs. time indep)
-time dep: thixotropic
-time indep: pseudoplastic
thickening (time dep vs. time indep)
-time dep: anti-thixotropic
-time indep: dilatant
yield stress
-minimum force (stress) required to initiate flow (strain)
-fluids that do not flow at low magnitudes of stresses:
-> exhibit yield stress
-> are non-newtonian
-> ex: ketchup, yogurt, mayo, salad
dressing
rheometers
-designed to determine viscosity, elasticity, and other rheological properties of materials
-physical values (ex: pressures, flow rates) are used to determine relationships btwn stress and strain/ stress and strain rates
what do rheometers use based on if they are testing a solid vs. liquid sample
-solid samples: use compression, extension, and torsion (shear) analysis
-liquid samples: use rotational viscometry
rotational viscometry
-viscosity will determine the torque needed to rotate the bob at a constant rate
-bob: hanging ball or weight
zahn cup
-determines viscosity
-cup is dipped and completely filled with a substance, after lifting the cup out of substance, you measure time until liquid stops dripping out of it
bostwick consistometer
-measures distance sample flows in given time interval
-only measures how far the sample will flow under its own weight
what are viscoelastic materials
-materials that exhibit both viscous and elastic characteristics
-viscous (water): resist shear flow and strain linearly with time when stress is applied
-elastic: exhibit strain when stress is applied and quickly return to their original state once stress is removed
oscillatory rheometry
-characterize viscoelastic properties of a material under small stresses and strains
TA
-measure the total acid concentration
-affects flavor
-an indicator of maturity
pH
-measure of the free hydrogen ion
-measure of active acidity
-affects certain reactions in foods
acids vs. conjugate base examples
EX 1
-> acid: H+
-> conjugate base: Cl-
EX 2
-> acid: H+
-> base: CH3COO-
molarity vs normality
-molarity: moles of solute / liter of solution
-normality: number of mole equivalent of solute / liter of solution (1 M H2SO4 = 2 N H2SO4 bc 2 H)
equivalent weight
-molecular weight / number of equivalent
-ex: H2SO4
-> molar mass = 98g
-> 2 H
-> 98 / 2 = 49 g (Et wt)
percent concentration
ratio of weight or volume of solute to weight or volume of solute plus solvent x 100
when do you use ppm
when concentration is < 1%
what is the titratable acidity a measure of
-dissociated and undissociated acids in sample
what is pH only a measure of
dissociated acids
equivalence point in titration
number of acid equivalence equals the number of base equivalence & total acid neutralization is achieved
potentiometric method
-using pH meter to identify endpoint
-advantage: precise equivalence point identified
-disadvantage: slow response
colorimetric method
-using a phenolphthalein indicator to identify endpoint
-phenolphthalein endpoint = phenolphthalein changes from clear -> red in pH range of 8 - 9.6
when is it better to use a pH endpoint vs a colorimetric one?
when phosphoric or carbonic acid are present since they buffer at pH of 8.2 but food acids DO NOT
