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Analysis of Bulk Food Components

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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


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