Lecture 3 - Sample Prep and Data Handling

Question 1

General flow of food analysis?

Answer 1

1. sampling
2. preprocessing
3. processing
4. testing

Question 2

why sample pre treatment and sample processing?

Answer 2

• reduce sample size
• make samples homogenous
• prevents changes in samples (contamination and deterioration)
• avoids matrix interference during analysis

Question 3

lab samples (large quantity: ie 1kg) need to be reduced to….

Answer 3

analytical sample (small quantity: ie 2-5g)

Question 4

what does the methods used for sample size reduction depend on?

Answer 4

the physical form of the sample

Question 5

what is coning and quartering for?

Answer 5

used for powder samples

in reducing sample size

Question 6

samples obtained from population is usually…

Answer 6

heterogeneous

Question 7

what are reasons that there may be sample heterogeneity?

Answer 7

variations of different units within a sample (ie milk from different cows)

Question 8

what is cryogenic grinding?

Answer 8

grinding frozen samples in liquid N using pre-cooled mortar and pestle

Question 9

reasons for sample losses as dust or particulates during preparation ?

Answer 9

• dry dust powder
• air flows generated by changes in temperate (ie opening furnace when hot)
• breathing onto dry powder sample

Question 10

how to reduce dry dust sample losses during preparation?

Answer 10

• don’t open hot furnace door
• cover ash or ground samples
• add reagents slowly to prevent losses as spray

Question 11

reasons for sample losses through volatilization?

Answer 11

• during heating of samples

- decrease in water during grinding solids b/c of localized heating

Question 12

how to reduce sample losses through volatilization

Answer 12

use properly sealed vessels for wet ashing

Question 13

chromium can be volatilized under oxidizing conditions in the presence of…?

Answer 13

chloride

Question 14

reasons for losses due to absorption?

Answer 14

absorption of molecules to plastic or glass containers

Question 15

how to avoid losses due to absorption?

Answer 15

• use pretreated glassware with a hydrated layer

- soak new glassware overnight in dilute nitric of HCl solution

Question 16

5 common types of changes that can occur in samples?

Answer 16

• enzymatic inactivation
• lipid oxidation
• microbial growth
• physical change
• contamination

Question 17

how to avoid enzymatic degradation in samples?

Answer 17

• heat denaturation (Blanching)
• storage in freezer (-20 to -30)
• pH
• adding reducing agents to prevent oxidative enzymes

Question 18

how to avoid lipid oxidation in samples?

Answer 18

• store under nitrogen or vacuum

- use antioxidant

Question 19

why does lipid oxidation occur?

Answer 19

• unsaturated lipids are sensitive to oxidative degradation

- exposure to light can accelerate lipid peroxidation

Question 20

how to avoid microbial growth?

Answer 20

• add preservatives (eg sodium azide)
• low temp strorage
• freeze drying
• storage under modified atmosphere

Question 21

why does microbial growth occur in samples?

Answer 21

• microbial contamination

- foreign microbial components introduced

Question 22

why does physical change occur?

Answer 22

caused by

• drying
• fluctuating storage temperature (eg ice cream)
• fluctuating gas pressure

Question 23

how to avoid physical changes?

Answer 23

storage in air tight humidity controlled containers

maintain temp

Question 24

sources of contamination?

Answer 24

airborne (moisture and dust)

reagents

glassware/equipment

facilities

cross contamination

Question 25

what are the 2 steps needed for reducing matrix interference?

Answer 25

1. extraction of target analytes

2. removal of interfering substances

Question 26

why do you need to reduce matrix interference?

Answer 26

matrix components can interfere with assay

eg pigments interfering with colorimetric assay for reducing sugars

Question 27

4 methods of analyte extractions?

Answer 27

digestion
solvent extraction
sorbent extraction
membrane extraction

Question 28

what type of extraction method is microwave and UV photolysis?

Answer 28

digestion

Question 29

what are examples of technologies that need no or minimal processing?

Answer 29

MRI (magnetic resonance imaging)

infralab-e series: infra red spectroscopy based instruments for measuring moisture, fat, protein, collagen content of meat

Question 30

which axis is the concentration and peak area on the standard curve?

Answer 30

independent variable: concentration = x axis

dependent variable: peak area = y axis

y = ax + b

Question 31

what is the confidence band?

Answer 31

defines statistical uncertainty of the regression line

Question 32

what is the correlation coefficient (r)

Answer 32

defines how well the data fits to a straight line

want value of r as close to +1.00 or -1.00 as possible

Question 33

what is an outlier

Answer 33

a data point that is far outside the norm for a variable or population

they:

• increase error variance
• reduce the power of statistical tests
• decreases normality

Question 34

how to determine outlier value? (equation)

Answer 34

dixon Q test

Q value = (X2-X1)/W

where X1 = outlier value
X2 = next closest vale to X1
W= total spread of all values obtained by subtracting the lowest value from the highest value

Question 35

reasons for outlier data

Answer 35

• data errors (human error, error in data collection, recording, entry)
• sampling error
• intentional misrepresentation of sample
• instrument/assay condition failure
• legitimate cases =need to be probed