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Fat characterization - Coggle Diagram
Fat characterization
Oxidation: measuring present status
Lipolysis: (measure by test for FFAs)
Hydrolysis of fatty acids from glycerol molecule
Peroxide Value
Measures peroxides, which are the primary products of oxidation
Defined as mEq of peroxides per 1 kg of sample
A freshly refined oil or fat will have a peroxide value of
zero
Rancidity: off odors & flavors resulting from lipolysis (hydrolytic rancidity) or lipid oxidation (oxidative rancidity)
Lipid oxidation (or autooxidation)
Self-sustaining free radical chain reaction
Produces hydroperoxides
p- Anisidine Value and Totox Value
p- Anisidine Value
Estimates the amount of unsaturated aldehydes,
which are the secondary oxidation products
Aldehydes react with p- Anisidine to give a chromogen measured at 350 nm spectrophotometrically
Totox value
Indicates total oxidation using both peroxide and
p- Anisidine values
Totox = p- Anisidine value + (2 × peroxide value)
Hexanal– Volatile Organic Compounds (VOC)
Secondary oxidation products, responsible for off-flavors & odors
Indications for flavor and oxidation
Measuring headspace hexanal is a means of monitoring oxidation
Correlates well with sensory evaluation
Lipid extraction is not required
Thiobarbituric Acid (TBA) Test
Measures a secondary oxidation product, malonaldehyde, and malonaldehyde-type products (all are transient products)
Malonaldehyde reacts with TBA to give a colored compound measured at 530 nm
Could use HPLC to measure actual
malonaldehyde content of distillate
Requires larger sample size
Frequently used in meat products
Conjugated Dienes and Trienes
Double bonds in lipids are changed from nonconjugated to conjugated bonds upon oxidation
Conjugated dienes absorb at 232 nm
Conjugated trienes absorb at 270 nm
Methods for lipid fractions
GC method
Fatty acid composition & fatty acid methyl esters (FAMEs)
quantifying the kind and amount of fatty acids that are present
Triacylglycerols & phospholipids are saponified and esterified to form FAMEs
GC analysis of the extracts
Trans isomer fatty acids
GC method
Infrared spectroscopy
Concentration of trans fatty acids is measurable in lipids at 966 cm-1 in IR spectrum
Only measure isolated trans isomers
Restricted to a sample contains at least 5% trans isomers
Mono-, di-, & triacylglycerols
Determined by Chromatographic methods
HPLC
GC
TLC
Short column and high temperature are needed for analysis of triacylglycerols
Cholesterol
The lipid extracted from the food is saponified
Cholesterol is extracted and derivatized to form trimethylsilyl (TMS) ethers
Quantization is done by GC
Separation of lipid fractions
TLC
Silica gel G as the adsorbent
Hexane-diethyl ether-formic acid (80:20:2
v/v/v) as the eluting solvent system
Plates are sprayed with 2’,7’- dichlorofluorescein in methanol
Observed under UV light against dark background
Permit rapid analysis and small scale preparation
Spots can be removed for further analysis
Oxidation: evaluating oxidative stability
Subject sample to oxygen and/or heat
Monitor oxidation over time
Oven Storage Test
For accelerated shelf life test
Oil Stability Index & Active Oxygen Method
The relative resistance of fat and oil to oxidation
It is faster than Oven storage test, but later is more accurate to real life
gives predictive value, which can be used to compare various oils to predict their respective shelf life
can determine how much longer a frying oil can be used before it goes bad
Oxygen Bomb
Lipid oxidation results in the uptake of oxygen from the environment
Measuring the time required for the onset of rapid disappearance of oxygen in a closed system provide a means of determining oxidative stability
Test results better correlate with shelf life & without the need to extract lipids
Determine induction period (time before detectable rancidity, or time before rapid acceleration of lipid oxidation)
Induction Period
The length of time before detectable rancidity or before rapid acceleration of lipid oxidation
Can be determined by such methods as calculating the maximum of the second derivative with respect to time or manually drawing tangents to the lines
Indication of the effectiveness of various antioxidants in preventing lipid oxidation
Used in accelerated shelf-life studies
Oxidative Stability (OS)
The resistance of lipids to oxidation
Methods for OS determination
Actual Shelf Life
At ambient conditions of storage
Usually requires months to even years
Accelerated Test (AT)
Artificially hasten lipid oxidation by exposing samples to heat, oxygen, metal catalysts, light or enzymes
Assuming reactions occur at AT are the same as normal reactions at actual shelf life
Assuming lipid oxidation is the factor that limits shelf life
Actual Shelf Life test should accompany and validate the results of AT.
general considerations
minimization of lipid oxidation
nitrogen flushing
low temperature
adding antioxidants
avoid exposure to heat and light
methods for bulk oils and fat
physical characteristics methods
Smoke, falsh, & fire point
Application
Reflect the volatile organic material in oils/fats, especially FFAs & residue extraction solvents
Procedures
Smoke point: the T at which a thin continuous stream of bluish smoke is given off
Flash point/fire point: a test flame is paased over sample 5°C intervals when sample under continous heating
A cup is filled with oil or melted fat, heating in a well-lighted container.
Principles
Flash point: the temperature at which a flash appears at any point on the surface of the sample
Fire point: the temperature at which evolution of volatiles proceeds with enough speed to surpport continuous combustion
Smoke point: the temperature at which the sample begins to smoke when tested under specified conditions
Cold test
Procedure: oil is stored in an ice bath(0°C) for 5.5 hr and oberved for crystallisation
Application: a measure of succuess of the winterizing process, it ensures that oils remain clear even when stored at refrigerated temperature.
Principle: a measurement of the resistance of an oil to crystallization. Absence of crytals or turbidity indicates proper winterizing.
Melting point
Droping melting point is predominant in US
Wiley melting point is used in europe
Capillary tube method is less useful since no sharp melting point
Slip melting point is not favored cause it needs 16-hr stabilisation time
refractive index (RI)
the ratio of the speed of light in air to the speed of light in the oil (measured by a refractometer)
related to the amount of saturation, used as a measure of purity and identification
disadvantages: influenced by FFA content, oxidation, and heating
Cloud point
Procedure
Then cooled with agitation
Temperature of first crystallization is taken to be the point at which a thermometer in the fat is no longer visible
The sample is heated to 130°C
Principle: the temperature at which a cloud is formed in a liquid fat due to the beginning of crystallisation
Color
Lovibond method
Spectrophotometer
Solid fat index & solid fat content
Solid fat content
Determines actual percent fat in sample
Measured using continous wave or plused Nuclear Magnetic Resonance (NMR)
Solid fat index
Determines change in volume of fat with change in temperature
Fat increase in volume as it melts
Measured using dilatomery
Plot volume vs. Temperature
Consistency & spreadability
Consistency
Useful for measuring the consistency of plastic fats and solid fat emulsions
Dependent on the fat type, its history, and the temperature during measurement
Mesure the distance a cone-shaped weight will penetrate a fat in a given time period
Spreadability
Applicable to liquid-containing solid suspensions, emulsion, and pastes that can maintain their shape at the temperaure used for analysis
The force needed to compress a sample
Chemical characteristics methods
Saponification value
Principle: defined as the amount of alkali necessary to saponify a given quantity of fat or oil (expressed as mg of KOH reuqired to saponify 1g of the sample
Calculation
Free fatty acids & acid value
FFA: percentage (w/w) of a specified fatty acid
Acid value: mg of KOH necessary to neutralise the free acids present in 1g of fat or oil
Measures the fatty acids liberated upon hydrolysis of triglycerides
%FFA (as oleic)=(V
N
282)/W*100
In crude fat, FFA or acid value estimates the amount of oil that will be lost during refining steps. In refined fats, a higher acidity level means a poorly refined fat or fat breakdown after storage or use.
Iodine value
Calculation
Principle
The # of c=c bonds in relation to the amount of fat or oil
Defined as the grams of iodine absorbed per 100g sample
Application
Follow the hydrogenation process in refining
Used as indicator for lipid oxidation
To characterise oils
introduction
definitions and classification
total fat: total lipid fatty acids expressed as triglycerides
trans fatty acids
saturated fat: sum of all fatty acids without double bonds
monounsaturated fat: cis-monounsaturated fatty acids
polyunsaturated fat: cis, cis-methlyene-interrupted polyunsaturatedfatty acids
importance
nutrition labelling
non-bioavailable lipids
health concern (malonaldehyde)
palatability and sensory effects
