Lecture 3 - Sample Prep Flashcards
describe the general flow of food analysis
sampling, pre-processing, processing, testing
what’s the purpose of pre-treatment and sample processing?
- reduce sample size
- homogenize
- prevent changes (contamination, deterioration)
- avoid matrix interference
name some methods of reducing sample size
- blenders
- coning and quartering (spice)
sample obtained from populations are usually (homogeneous/heterogeneous)
heterogenous
variations in properties of different units within the sample causes what?
sample heterogeneity
name some methods for homogenizing samples
- grinders
- mixers
- slicers
- blenders
mechanical homogenization often results in the undesirable production of _____
heat
grinding frozen samples in liquid nitrogen using a pre-cooled mortar and pestle is known as:
cryogenic grinding
what are 3 causes of sample loss?
- dust/particulates
- volatilization
- adsorption
list sources of loss of dust/particulates
- dry dust powder
- air flows generated by changes in temp
- breathing
how do you avoid loss of dust/particulates?
- never open the door of a hot furnace
- use a plug of glass or quartz wool to collect particulates when combustion is aided by stream of gas
- ash or finely ground samples should be covered before they are moved
- add reagents slowly to prevent losses as spray
list sources of loss via volatilization
- during heating of samples
- heat generated in grinding (lose water)
- volatile metals: As, Sb, Sn, Po, Pb, Se, Hg, Ge, B
- Cr volitized in oxidizing chloride media
- C, P, Si volitized as hydrates
how do you mitigate loss from volatilization?
use properly sealed vessels for wet ashing
how do you get loss from adsorption?
molecules adsorb to plastic or glass containers
how do you mitigate adsorption losses?
- use pre-treated glassware with an established hydrated layer
- soak new glassware overnight in a dilute nitric or hydrochloric acid solution
when sampling, we want to (prevent/enable) change
prevent, duh
what are some causes of changes in samples? (5)
- enzymes
- lipid oxidation
- microbial growth
- physical change
- contamination
how do you prevent enzymatic changes?
- heat denaturation
- storage in freezer
- pH
- adding reducing agents to prevent oxidative enz
(saturated/unsaturated) lipids are sensitive to oxidative degredation
unsaturated
exposure to _____ can accelerate lipid peroxidation
light
how do you mitigate lipid oxidation of samples?
- store under nitrogen or vacuum
- use antioxidant if not interfering with analysis
what badness do microbes bring to your samples?
- degradation of food components
- introduction of microbial components
how do you mitigate microbial changes to your sample?
- addition of preservative (sodium azide)
- low temp storage
- freeze drying
- storage under modified atmosphere
what kind of conditions cause physical changes?
- drying
- fluctuating storage temp
- fluctuating gas pressure
how d you mitigate physical changes?
- storage in airtight humidity controlled containers
- maintaining temperature
what are sources of contamination?
- airborne
- reagents
- glassware/equipment
- facilities
- cross-contamination
how do you reduce matrix interference?
- extraction of target analytes
- removal of interfering substances
methods of extracting analytes (4)
- digestion
- solvent extraction
- sorbent extraction
- membrane extraction
read slide 18 and 19
ok
what’s a confidence band?
defines statistical uncertainty of regression line
what’s r?
correlation coefficient. tells u how well the data fits a straight line
what are outliers?
point outside the norm for a variable or population
what do outliers do?
- increase error variance
- reduce power of statistical tests
- decrease normality
how do you test for outliers?
Dixon’s Q test!
what causes outliers?
- errors in data (human)
- inherent variability of the data
- sampling error
- intentional misrepresentation of sample
- instrument/assay condition failure
- legit cases
how do we measure accuracy?
simple t test; determines whether the sample mean is statistically different from a known or hypothesized pop mean.
write the formula for a t value
aight
write the formula for standard error of the mean
ok
