Food Composition Analysis
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Moisture and Total Solids – Part 1
Ash – Part 1
Protein Analysis – Part 1
Vitamin Analysis (Discussed separately in
Vitamins and coenzymes slides)
• Lipid (Fat) Analysis – Part 2
• Carbohydrate Analysis – Part 3
• Secondary Metabolites and Nutraceuticals
(Analysis methods to be discussed as we go)
From: Nielsen, “Food Analysis”, 3rd edition, Kluwer, 2003
Chapters 8 and 14.
Lipids
• Mainly present in foods as triacylglycerols (fats
and oils), other fatty acid esters, phospholipids
(e.g. lecithin), sterols and other isoprenoids, fatsoluble vitamins (A, D, E, K)
• Lipid composition varies greatly between animal
and plant-based foods and within plant-based
foods (table 8-2)
• Most lipids are soluble in nonpolar solvents (e.g.
ether, chloroform) but some also have limited
water-solubility
From: Nielsen, “Food Analysis”, 3rd edition, Kluwer, 2003
Chapters 8 and 14.
Lipids – quantitative analysis
• Accurate lipid analysis requires effective
extraction from the food matrix
• extraction methods tailored to particular food
• nonpolar solvents utilized – hexane, ethyl ether,
petroleum ether
• extract, dry and weigh
• hydrolysis may be needed with complex lipids to
release lipid from carbohydrate/protein moieties
• steps may be taken to minimize oxidation
– addition of BHT or other antioxidant
Extraction of lipids
• Most effective if samples pre-dried
• Samples are ground to reduce particle size, maximize
surface area
• Blending helps saturate particles
• Hydrolysis (1 hr reflux w 3 M HCl or saponification with
ethanolic KOH) needed with dairy, grains, animal
products
• Sequential solvent extractions of plant materials begin
with least polar solvent and increase in polarity so that
lipids are removed first, other constituents follow
• Reverse-phase separation methods isolate the polar
constituents first, lipids last
Lipid extraction methods
• Continuous solvent extraction – sample exposed to
boiling solvent for several hrs
• Semicontinuous (Soxhlet extraction) – same idea but
periodic solvent exposure
• Mojonnier flask or Babcock methods (dairy) use a
digestion step prior to allowing fat to separate
• Microwave-assisted extraction often used w dairy, meats
• Ultrasound-assisted extraction
• Supercritical fluid extraction – “green” method uses
pressurized CO2
– hybrid gas-liquid state at 80oC, 10,000 psi
– fluid pumped through sample cell, collected at ambient pressure,
60oC and rotary evaporated
Diagram of SFE
From Brazilian J. of Chem. Eng. at http://www.scielo.br/scielo.php?
Instrumental lipid quantification
• NMR – low resolution is effective because one
monitors broader regions
– CW NMR can determine degree of unsaturation
– pulsed NMR – oil vs. solid fat content can be
determined - protons relax faster in solids
– signal amplitude is proportional to quantity
• IR - fat absorbs strongly at C=O stretching
region (1745 cm-1)
– meat, dairy, oilseeds
• x-ray absorption – higher in lean meat than fat
• ultrasound – fat content in meats
Lipid characterization
Ch. 14 (Nielsen)
• Iodine value – degree of unsaturation
• Saponification equivalents
• Free fatty acids
Lipid oxidation status & oxidation products
• TBARS test for malondialdehyde
• conjugated dienes & trienes by UV
• volatile organics by GC
• fatty acid composition by GC (14.6.2)
• trans fatty acids by IR (14.6.3) – total trans fatty acids
quantified based on trans C=C absorption at 967 cm-1
Chemistry of TBARS test
Colored adduct is measured at 530 nm
GC analysis for FA composition
• Fatty acids can be derivatized to their volatile methyl
esters (FAME)
• acid/MeOH or NaOMe can be used
• short chain FA are volatile, some water solubility; can
prepare iPr esters instead
• derivatization as DMOX (4,4-dimethyloxazolines) often
used
• GC-MS employed if standards are inadequate
• Capillary GC on moderately polar PEG or carbowax,
medium length column (25 m x 0.25 mm) separates
most FAME
• Longer (100 m) cyanopropyl columns needed to
separate trans-FA from cis-isomers
• complex mixtures of FAME including w-3/w-6 fatty acids
can be separated by cap-GC in < 30 min (Fig. 2.3).
From Methods of Analysis for Functional Foods and Nutraceuticals, Chapter 2”
Cap GC analysis of fish/sunflower oil mixture
Conjugated linoleic acid (CLA)
• Discussed in detail in Hurst, Ch. 2
• Analysis poses a challenge because of the
mixture of isomers
• Cap GC on 100 m cyanopropyl column
requires long runtime (80 min)
• Silver-ion HPLC: Ag+ forms polar pi-bonded
complexes with unsat. FA
– Si column packings impregnated with silver
developed for lipid analysis (normal-phase LC)
– http://lipidlibrary.aocs.org/topics/silver/index.htm