Analysis of Bulk Food Components

7/29/2019 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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Analysis of Bulk Food Components

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