Exam 4 Food Analysis Flashcards
Definitions of lipids, total fats, fats, and oils
*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)
FDA definitions for: total fat content, saturated fat, polyunsaturated fat, monounsaturated fat, trans fatty acids
*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
Preparation methods for fat extraction
*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
Soxhlet extractor (semi-continuous)
*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)
Chloroform-methanol extraction
*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
Mojonnier extraction (alkaline hydrolysis method)
*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
Babcock method
*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
In what form are fatty acids during FAME analysis
fatty acid methyl esters
What is learned from GC of fatty acids
fat content
GC analysis of fatty acids process
*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
Refractive index
*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)
Iodine value
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)
Solid fat content
*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
Saponification value
*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
Why are free fatty acids analyzed
*indication of adequacy of refining
*breakdown after storage or use
What are unique characteristics of proteins that are related to protein analysis
*nitrogen content
*peptide bond
*aromatic amino acids
*dye binding capacity
*ultraviolet absorptivity