Exam 4 Food Analysis Flashcards

1
Q

Definitions of lipids, total fats, fats, and oils

A

*LIPIDS: in general, soluble in ether, chlorform, and other organic solvents, but are sparingly soluble in water
*OILS: liquid triacylglycerols at room temp
*FATS: solid triacylglycerols at room temp
*TOTAL FATS: sum of fatty acids from C4 to C24, calculated as triacylglycerls; however doesn’t include C2 & C3 (acetate and propionate)

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2
Q

FDA definitions for: total fat content, saturated fat, polyunsaturated fat, monounsaturated fat, trans fatty acids

A

*Total fat content: total lipid fatty acids expressed as triacylglycerides
*Saturated fat: sum of all fatty acids without double bonds
*Polyunsaturated fat: cis, cis-methylene-interrupted polyunsaturated fatty acids
*Monounsaturated fat: cis-monounsaturated fatty acids
*Trans fatty acids: trans isomers, not conjugated

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3
Q

Preparation methods for fat extraction

A

*Predrying sample: makes sample easier to grind, breaks fat-water emulsions to more easily dissolve fats in organic solvents, and helps free fat from food tissue (ex. vacuum oven)
*Particle size reduction: inc extraction efficiency (ex. grinding in liquid nitrogen)
*Acid and/or alkaline hydrolysis: breaks btoh covalenty and ionically bound lipids into easily extractable lipid forms

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4
Q

Soxhlet extractor (semi-continuous)

A

*dry sample if contains more than 10% H2O
*if comparing many food samples: dry them all
*weigh sample into thimble & place in Soxhlet
*extract with petroleum ether or hexane
*remove solvent (dry) & calc fat content
*solvent builds up in the extraction chamber for 5-10 min and completely surrounds the sample & then siphons back to boiling flask (ex. soxhlet method)

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5
Q

Chloroform-methanol extraction

A

*chloroform: methanol is added to create a monophasic system
*the addition of aqueous salt solution helps to partition the sample into a byphasic system
*methanol layer (at top): contains salts, carbohydrates, some proteins
*in the middle: cellular debris
*chloroform layer (bottom): contains lipids

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6
Q

Mojonnier extraction (alkaline hydrolysis method)

A

*use mojonnier flask
*treat sample, in series, with ammonium hydroxide, 95% ethanol, ethyl ether, and petroleum ether (i.e. use combination of solvents)
*repeat extration series; evaporate solvent; weight fat
*moisture removal from sample not necessary
*similar to roese-gottlieb method

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7
Q

Babcock method

A

*sulfuric acid digests protein, generates heat, and releases fat
*after centrifugation of samples, fat content is measured volumetrically
*you don’t need to dry the sample

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8
Q

In what form are fatty acids during FAME analysis

A

fatty acid methyl esters

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9
Q

What is learned from GC of fatty acids

A

fat content

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10
Q

GC analysis of fatty acids process

A

*after adding an internal standard and an antioxidant, sample is treated by acid and/or alkaline hydrolysis, then fat is extracted with ether
*fatty acids are converted to fatty acid methyl esters (FAMEs= palmitic acid/hexadecanoic acid) then separated by GC, and quanitfied with reference to the internal standard
*sum of fatty acids equals total fat

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11
Q

Refractive index

A

*degree of unsaturation (RI decreases linearly as unsaturation dec)
*a measure of purity (contaminants change RI)
*also a means of identification (each lipid matrix will have a characteristic RI response)

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12
Q

Iodine value

A

measure of degree of unsaturation
*defined as grams of iodine absorbed by 100g of sample
*measures: iodine required for absorption by double bonds
*application: used for fat characterization, to follow hydrogenation process, and as a measure of oxidation (a dec in unsaturation occurs during oxidation)

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13
Q

Solid fat content

A

*determines amt of solids in fat (vs. liquid)
*SFC preffered over SFI bc: its an actual measurement, less subject to error, & faster
*measured by: percent solid fat, by continuous wave or pulsed NMR

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14
Q

Saponification value

A

*an index for avg molecular wt of triacylglycerides in the sample
*defined as the mg of KOH required to saponify 1g of sample
*application: determines the avg fatty acid chain length of an oil or fat
*measures: alkali required to saponify fat/oil

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15
Q

Why are free fatty acids analyzed

A

*indication of adequacy of refining
*breakdown after storage or use

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16
Q

What are unique characteristics of proteins that are related to protein analysis

A

*nitrogen content
*peptide bond
*aromatic amino acids
*dye binding capacity
*ultraviolet absorptivity

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17
Q

What is measured with the Kjeldahl & Dumas protein analysis methods

A

*Kjeldahl:
-measure the presence of N by method involving digestion, neutralization, distillation, and titration
-measures total organic N (including ammonia)
*Dumas:
-N is measured by gas chromatography using thermal conductivity detector
-measures total N, including inorganic

18
Q

Dye binding

A

*anionic dye binding: dye binds to protein (basic AA: His, Arg, Lys & free terminal amino group of protein) to percipitate protein
*bradford: dye changes color when bound to protein (due to amphoteric nature of proteins)

19
Q

Copper binding

A

*biuret: peptide bonds
*lowry: peptide bond + Tyr & Trp
*BCA: peptide bonds + four amino acids (Cys, Cystine, Trp, Tyr)
proteins reduce cupric ions to cuprous ions under alkaline conditions; cuprous ions react with BCA reagent to give purple color
*based on peptide bonds and the presence of four amino acids: Cys, Cystine, Trp, & Tyr

20
Q

UV absorbance

A

*UV absorption:
-280 nm (Tyr & Trp)
-220 nm (peptide bond)
*non-destructive
*rapid
*has to be a pure & clear solution

21
Q

Salting out

A

proteins are precipitated from solution as ionic strength is inc

22
Q

Isoelectric precipitation

A

proteins aggregate & precipitate at their pI bc there is no electrostatic repulsion

23
Q

Solvent fractionation

A

addition of ethanol or acetone dec dielectric constant of aqeous solution and dec solubility of most proteins

24
Q

Dialysis

A

*used to separate molecules in solution by the use of semipermeable membranes that permit passage of small molecules but not large molecules
*protein solution is placed into dialysis tubing that has been clamped at one end & the other end is sealed, the bag is placed in large volume of water or buffer (usually have to switch water out ~3 times)

25
Q

What is electrophoresis

A

defined as the migration of charged molecules in a solution through an electrical field

26
Q

what is PAGE & SDS-PAGE

A

*polyacrylamide gel electrophoresis
*PAGE with anionic detergent, sodium dodecyl sulfate (SDS), is used to separate protein subunits by size

27
Q

Isoelectric focusing

A

proteins separated by charge in an electric field on a gel matrix in which a pH gradient has been generated using ampholytes, under constant voltage, proteins migrate to the location on the gradient at which pH equals the pI of the protein, size is not a factor

28
Q

How are two systems combined for 2D electrophoresis of proteins

A

proteins separated in tube gels by isoelectric focusing, the tube gel containing the separated proteins is then placed on top of an SDS-PAGE slab gel, & proteins are separated

29
Q

What is a good method for AA analysis

A

*cation-exchange chromatography (elution by stepwise inc in pH & ionic strength), postcolumn derivatization with ninhydrin or o-phthalaldehyde, then quantify spectrophotometrically
*precolumn derivatization using phenylthiocarbamyl, separation by reversed-phase liquid chromatography, then quantify by UV spectroscopy
*with either chromatography method: use internal std (ex. norleucine) & std curves

30
Q

How are total carbohydrates calc for nutrition labeling

A

*total carbohydrate= 100% - [% Moisture + % Ash + %Lipid + %Protein]

31
Q

FDA definitions of carbohydrates for nutrition labeling purposes

A

a statement of the grams of total carbohydrate in a serving expressed to the nearest gram, except that if a serving contains less than 1 gram

32
Q

What information is determined from phenol-sulfuric assay

A

*often used for research purposes to determine “total carbohydrate”
*principle: carbohydrates are heated in the presence of acid to produce furan derivatives, that condence with phenol, to form compounds that can be quantified spectrophotometrically

33
Q

What information is determined from a reducing end assay

A

*principle: based on reduction of cupric ions in alkaline solution to cuprous ions by reducing sugars: amt of cuprous ions generated is measired to quanitfy reducing sugars

34
Q

What is the method for identification of mono & oligo saccharides

A

pulsed electrochemical detector (ECD)

35
Q

How must sugars be treated by GC analysis

A

*convert sugars to volatile derivatives
*use flame ionization detector

36
Q

What is gelatinized starch

A

*converted totally into D-glucose by enzymes specific for starch & D-glucose is quantified enzymatically

37
Q

Dietary fiber

A

*the non-digestible soluble & insoluble carbohydrates (w/ 3 + monomeric units) & lignin that are intrinsic & intact in plants; isolated or synthetic non-digestible carbohydrates (w/ 3+ monomeric units) determined by FDA to have physiological effects that are beneficial to human health

38
Q

What are the basic steps in dietary fiber determination

A

*defeat sample if >10% lipid
*treat mixture w/ alkaline protease
*treat hot mixture w/ thermostable a-amylase
*treat mixture w/ glucoamylase
*add 4 volumes of 95% ethanol
*collect residue + precipitate by filtration
*wash w/ ethanol & acetone
*air dry then oven dry & weigh
*substract weights of protein & ash determined on duplicate samples

39
Q

List several components of dietary fiber

A

*insoluble: cellulose, lignin, resistant starch
*soluble: native pectin, hydrocolloids, non-digestible oligosaccharides

40
Q
A