analysis of bulk food components
TRANSCRIPT
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Analysis of Bulk Food
Components (Proximate
Analysis)
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Bibliography
Kirk,RS & Sawyer,R (1991) Pearsons composition &analysis of Foods, 9th edn, Longman, ISBN 0-582409101
Nielsen S.S. (2003) Food analysis 3rd ed. Kluwer
Academic/Plenum Publishers, 2003. ISBN: 0306474956 Coultate,TP (2009) Food The chemistry of its
components, 5th edn, Royal Society of Chemistry, ISBN9780854041114
Near IR spectroscopy:http://www.spectroscopyeurope.com/NIR_18_3.pdf
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Definition of Proximate
Analysis Analysis of foods fornitrogen (for
protein), ether extract (for fat), crude
fibre and ash (mineral salts), togetherwith soluble carbohydrate calculated by
subtracting these values from the total
(carbohydrate by difference).
http://www.encyclopedia.com/doc/1O39-nitrogen.htmlhttp://www.encyclopedia.com/doc/1O39-fibrecrude.htmlhttp://www.encyclopedia.com/doc/1O39-ash.htmlhttp://www.encyclopedia.com/doc/1O39-carbohydrate.htmlhttp://www.encyclopedia.com/doc/1O39-carbohydrate.htmlhttp://www.encyclopedia.com/doc/1O39-ash.htmlhttp://www.encyclopedia.com/doc/1O39-fibrecrude.htmlhttp://www.encyclopedia.com/doc/1O39-nitrogen.html -
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Why Analyse Food?
Legislation e.g. fat in chocolate, additives
Nutrition
- a) macronutrients; b) micronutrients
Nutritional labeling eg fat, protein
Toxicology (a) Natural toxins e.g. alkaloids (b)
fungal toxin (mycotoxin) (c) contaminant
Control of Trade Food manufacture & Quality
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Accuracy and Precision
Accuracy - closeness of observed result totrue result
Precision - Agreement between replicate
determinations
Accurate
Precise (no bias)
Reproducible
Inaccurate
Precise or imprecise?
(reproducible)
Bias or no bias
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Composition of Foods
Water
Protein
Fat Carbohydrates (soluble)
Dietary fibre (non-starch polysaccharides
plus lignin) Minor components eg vitamins, pigments,
organic acids, flavonoids etc.
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Strategy for Food Analysis
Obtain representative sample
http://www.encyclopedia.com/video/0j8DpLjImeA-proximate-analysis-sample-
preparation.aspx Pretreatment e.g. heating to inactivate
enzymes, drying etc
Extraction/ matrix reduction/ particle sizereduction often required
Separation/ purification
Identification (minimise errors at all stages)
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WATER IN FOOD
1.FREE WATER- water that is free to act as
a solvent for molecular solutions or colloids
2.ADSORBED WATER - Water is adsorbedonto internal or external surfaces of solids
(hydrogen bonding)
3.CONSTITUTIONAL WATER- Present inchemical combination as water of hydration
NB - Boiling point 3>2>1
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Moisture Content of Food
FRUIT
Apples 79-91%
Oranges 83-89
VEGETABLES
Carrots 83-91
Tomatoes 91-97 MILK PRODUCTS
Milk 87-90
Cheese (hard) ~37
MEAT PRODUCTS
Beef cuts 55-70 %
Ham 53
FISH
Cod 83
Salmon 63 DRIED FOODS
Soup Powder 7
Cocoa 5
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Determination of water-
Oven Drying Sample preparation- grinding, dispersion
Select weight of sample and container size.
Distribution of sample important
Drying conditions:
70-130C / 25-760 mm mercury pressure
Dry for Constant time or to constant weight
Air stream if infrared heating used
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Problems in Oven Drying
Mainly free water
Reactions of carbohydrates may
produce water
Loss of other volatiles
Oxidation of unsaturated fatty acids,
phenols in some foods
Practical problems:- case hardening, re-
absorption of moisture during weighing
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Karl-Fischer Titration to
Determine Water Content Titration between food containing water
extracted into, or dissolved in, methanol
and Karl Fischer reagent ( iodine, sulfurdioxide, pyridine in methanol)
Determine end point of titration by
electrical conductivityI2 + SO2 + H2O + CH3OH + 3 C5H5N =
2C5H5N.HI + C5H5NH. SO4.CH3
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Dean and Stark Volumetric
method for water
Solvent (toluene)
food
Collected
water
Heat
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Lipid Analysis
All lipids are soluble in non-polar organic
solvents and have low solubility in water
Lipid class mainly comprises edible fats,
which contain triglycerides (>90%),
diglycerides, monoglycerides,
phospholipids (
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Fat content of foods
Butter 80%
Margarine 80%
Reduced / Low-fat spreads 20-60% Chocolate 30%
Milk 3 - 4%
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solvent
Soxhlet extractor
food
Water
in
Water
out
for lipids in solid
foods
http://www.rsc-teacher-
fellows.net/labTechniques/SoxhletExtractionAnimation.htm
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Fat extraction (contd)
Soxtec method:
http://www.encyclopedia.com/video/ylyjm8
iY23Q-proximate-analysis-percent-fat.aspx
Treat food with acid or alkali to release
lipid from interfering componentseg Rose-Gottlieb method
Treat food with conc. ammonia. Then
extract fat with solvents (eg milk products)
http://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspxhttp://www.encyclopedia.com/video/ylyjm8iY23Q-proximate-analysis-percent-fat.aspx -
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Gerber method for fat in milk
products Volumetric method
Treat milk in milk butyrometer with
Macdonalds reagent ( emulsifier,butanol, methylated spirits solution) or
concentrated sulfuric acid. Heat at 65C
for 5 min. Centrifuge. Heat at 65C for 5min.
DIRECT READING IN % FAT ON
CALIBRATED SCALE
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Milk butyrometer for the Gerber method
stopper
fat
Add milk,
Macdonalds
reagent/ sulfuricacid
Aqueous phase of
milk, Macdonalds
reagent
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Johan Kjeldahl
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Development of Kjeldahl
method
Johan Kjeldahl
employed by Carlsberg brewery to develop
assay for grain protein
less protein in grain = more beer
published 1883
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Protein Content of Food
% Dry wt % Fresh
Flour 11.9 10.5
milk 26.9 3.5
cheese (swiss) 45.1 27.5
eggs 49.3 12.8
orange juice 6.3 0.8
beef (roast) 62.5 25.0
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KJELDAHL PROTEIN ANALYSIS
Weigh food; add conc.sulphuric acid,selenium and potassium sulphate; heat
under reflux until sample is fully
dissolved Dilute to volume. Introduce into
distillation apparatus. Add sodium
hydroxide Distill and collect in boric acid solution
Add screened methyl red indicator and
titrate with sulphuric acid till grey colour
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Kjeldahl distillation unit
Food
digest
Sodium
hydroxide
Boric acid
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Chemical Changes in the
Kjeldahl Analysis of Protein Digestion
Protein N + H2 SO4 ------> (NH4)2 SO4
Distillation
(NH4)2 SO4 + 2NaOH -----> NH3 + Na2 SO4
NH3+ H
3BO
3--------> NH
4.H
2BO
3 Titration (screened methyl red)
2NH4 .H2 BO3 + H2 SO4 -----> (NH4)2 SO4 +
2H3 BO3
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Nitrogen to Protein
Conversion factors Wheat, barley, oats 5.83
Rice 5.95
Soybean 5.71
Milk 6.38
Meat, eggs 6.25
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Other methods of Nitrogen
determination Dumas-Pregl
Burn sample to CO2, H2O and nitrogen
oxides (NO, N2O, NO2) at 1100oC.Reduce nitrogen oxides to nitrogen by
heating with copper (at 600oC); removeCO
2and H
2O, and determine nitrogen
by gas chromatography
http://www.encyclopedia.com/video/K87FQf_yYYE-proximate-analysis-percent-
protein.aspx
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Carbohydrate analysis
Sugars - sweet, crystalline, water-soluble
e.g.glucose, sucrose, lactose, fructose Polysaccharides - polymers which
contain between 10 and several thousand
sugar units eg starch
Polysaccharides other than starch are
non-digestible so are classified as dietary
fibre (together with lignin)
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Reducing sugars
Molecules with free aldehyde or ketone
group are reducing sugars e.g. glucose
fructose, lactose Sucrose is not a reducing sugar, since it
does not contain a free aldehyde or
ketone group; sucrose can readily behydrolysed to glucose + fructose (acid
catalysed)
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Lane and Eynon method for
Reducing Sugars Reagent: Fehlings solution - alkaline
copper tartrate
Principle: a volumetric procedure basedon a titration in which reducing sugars
reduce Cu(II) to Cu(I). A redox indicator
(methylene blue) is used. This is blue inpresence of Cu(II), and becomes when
all Cu (II) is reduced.
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Lane and Eynon method for
Reducing Sugars (II) Procedure - heat Fehlings solution to
boiling in a beaker. Add methylene blue
indicator.Add sugar solution from a burette
keeping sample boiling until blue colour
disappears leaving an orange-red liquid(NB standardise the Fehlings reagent
with a known sugar solution)
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Dietary fibre
Definition: Polysaccharides (and other
molecules e.g. lignin) that are not
hydrolysed by the endogenous secretions
of the mammalian digestive tract.
Components include cellulose,
hemicellulose, lignin and pectins from the
walls of cells; resistant starch; and severalother compounds
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Sources of dietary fibre
vegetables, wheat and most other grains.
Foods rich in soluble fibre include fruits,
oats, barley and beans.
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Analysis of Dietary Fibre
Methods can be classified as:
Nonenzymatic-gravimetric
Enzymatic-gravimetric Enzymatic-chemical methods, which
include
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Dietary fibre classified
according to solubility in water Total Dietary Fibre (TDF) - sum of
soluble and insoluble polysaccharides and
lignin
Insoluble dietary fibre (IDF) which includes
celluloses, some hemicelluloses and
lignin;
Soluble dietary fibre (SDF) which includes-glucans, pectins, gums, mucilages and
some hemicelluloses.
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Dietary fibre in foods
Product AOAC method Englyst method
(g/100g)* (NSP) (g/100g)
Apples (with skin) 2 1.6
Bananas 1.9 1.1Carrots (boiled) 3.1 2.5
Baked beans 4.2 3.7
White bread 2 1.5Brown bread 4.5 3.5
Wholemeal bread 7.4 5.8
*non-starch polysaccharides + resistant starch + lignin
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AOAC method
gelatinize with a thermo-stable -amylase
treat with protease and amyloglucosidase
(to remove protein and starch)
separate SDF and IDF by filtration
Dry residue after filtration (IDF) and
correct for protein and ash content. Weigh
filtrate, containing SDF, precipitated with
ethanol, recovered by filtration and dried.
TDF = SDF + IDF.
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Englyst method
isolation of DF with -amylase, followed by
treatment with a mixture of pancreatin and
pullulanase.
Hydrolysis with sulfuric acid at 100C/ 2 h.
Add NaOH and 3,5-dinitrosalicylate to
develop colour; 100C for 10 min .
absorbance was measured at 530 nm
Prepare calibration plot with standard
glucose samples.
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New methods
Near infrared spectoscopy for fat, protein,
carbohydrate and water but needs
calibrating for each food.
Nuclear Magnetic Resonance for fat
HPLC for carbohydrates
N IR t f b
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Near IR spectra of soybean
powder and defatted soybean
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Uses radiation in range 12500 - 4000 cm-1
Use for analysis of protein, fat, water, starch,
sugars, fibre, alcohol etc
Used widely by food industry for quality control
eg grains (wheat, rice), soybeans, meat, wine near IR peaks are broad, overlapping peaks
Calibrate method using standards with
measured parameter determined by another
method
Use multiple linear regression to relate
concentration to absorbance values at several
Near Infrared analysis