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ASH

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The students will be able to:

Define ash and examples of ash.

State various methods for ashdeterminations

Describe the principles for ash

determinations.Analyse and evaluate the compositions of

food products.

To calculate the ash content in food

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Definition

ASH

Inorganic residue remaining after ignition or

complete oxidation of organic matter in a

foodstuff.

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Three major types of ashing:

Dry ashing

majority samples

Wet ashing/oxidation high fat samples;

preparation step for elemental analysis.

Plasma ashing preparation for volatile

elemental analysis

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Importance of Ash Analysis

Indicator of nutritional valuefor some food/feedsespecially for exporting goods.

Eg: Mineral content for bran 20 times than in endosperm,indicates thoroughness separation of bran and germ

from wheat kernel.

Indicator for adulteration of some juices and beverages.

Eg: Able to distinguishes fruit vinegar from syntheticvinegar.

Index of fruit content for jellies and fruit preserves.

Eg: In fruit jellies and marmalades, ash content used toestimate fruit content in product.

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Ash content represents total mineral infoods.

Part of proximate analysis for nutritionalevaluation.

Ashing is the first step in sample preparationfor specific elemental analysis.

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Minerals composed ash are:

i) Calcium- high concentration in most dairy and dairy

containing products, cereals, nut, egg, fish and

certain vegetables.

ii) Phosphorus- dairy products, grains, nut, meat,

fish, poultry, eggs and legumes.

iii) Iron- grains and grain products, eggs, legumes,

seafood, poultry.

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iv) Sodium- poultry, meat, fruits, cereals,

fish, egg and others.

v) magnesium- nut, cereal and legumes.

vi) sulfur- in protein rich foods andvegetables.

Others are potassium, zinc, copper, cobalt.

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Ash Contents in Foods

Food % Ash

Milk and dairy products 0.5 5.1

Meat, poultry and fish 0.8 3.0

Fruits and vegetables 0.3 2.3

Cereals 0.4 1.7

Fats and oils 0.0 4.0

Nuts 0.8 3.4

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Sample Preparation

i) Most dry samples (eg. whole grain, cereals, dried

vegetables) does not require preparation.

ii) Fresh vegetables need to be dried prior to ashing.

Plant materials pre-dried first then ground. Ifmoisture

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continue

iii) High fat products, syrups and spices requiretreatments prior to ashing, where samples need tobe dried on a steam bath and fat extracted before

ashing. This is because high fat and moisture maycause spattering, swelling or foaming that mayresult in sample loss. Therefore causeunderestimation of ash content.

iv) Solid foods must be finely ground but may addmineral contents from the grinders steelconstruction. Not suitable in analysis of specific

mineral analysis.

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Crucible Selection

Quartz : resistant to acids and halogens but not alkaliat high temperatures.

Vycor brand glass : stable up to 900oC, resistant tomost chemicals and acid except bases.

Porcelain : resemble quartz properties. Usual choice,withstand 1200oC, easy to clean, cheap, susceptible toalkali, but will crack with rapid T changes. Relativelyinexpensive.

Steel : resistant to both acids and alkali, inexpensive,but possible sources of contamination (composed ofchromium and nickel).

Platinum : very inert, the best crucibles but veryexpensive for routine use. Recommended procedures

of AOAC, for cereal, dairy, meat and fish products.

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DRY ASHING

Principle

Incineration at high temperatures (500oC or higher)

water and volatile materials are vaporized and organic

substances burned in presence of O2in air, producingCO2, H2O and N2.

Most minerals converted to oxides, sulfates, phosphate,

chloride and silicates.

Crucibles are pre-dried and weighed before ashing andcooled to room temperature after ashing before

weighing.

% Ash (wet basis) = M ash x 100

M wet

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Advantages

Safe method.

Most common procedure to determine total mineralcontent of foods.

Requires no reagents.

Resultant ash can be used for other analyses e.g.water-soluble, water-insoluble, and acid-insolubleash.

Applicable also in determination of most common

metals. Requires little attention and simple.

Large numbers of samples can be handled atonce.

No blank subtraction required.

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Disadvantages

Time consuming (12-18hrs or overnight).

Loss of volatile minerals at high

temperatures (e.g. Cu, Fe, Pb, Hg, Ni, Zn,As, Cd, Cr).

Interaction between mineral componentsand crucibles.

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II. Wet Ashing / Oxidation

Use for samples with high fat content (meats andmeat products), for determining trace elements andmetallic poisons.

Procedure involves oxidizing organic substancesusing acids and oxidizing agents (eg nitric acid witheither perchloric and/or sulfuric acids) and heatedto about 350oC.

Minerals are solubilized without volatilization.

Rapid decomposition of many organic compoundsthat difficult to oxidize.

Adding salt (e.g. potassium sulfate) raises theboiling point of the acid and accelerates

decomposition.

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Principle

Oxidation of organic substances by strong acid,

nitric acid (HNO3) and oxidizing agents perchloric

acid (HCIO4). Heat slowly up to 350oC until organic matter

completely digested and HNO3evaporated.

Boiling continues until sample becomes colourlessor light in colour.

Cool, add HCl then dilute with distilled deionized

water. Pre-dry sample then incinerate at 525oC for

1-2hrs.

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Advantages

Use low temperature therefore little

or no loss from volatilization.

Short oxidation time.

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Disadvantages

Very hazardous. Requires a hood, hot plate, long tongs,and safety equipment.

Require large amounts of corrosive reagents and correctionfor reagents.

Small numbers of samples can be handled at one time.

Need constant operator's attention.

Single acid does not give complete and rapid oxidation oforganic compounds. Nitric acid with sulfuric or perchloric

acids and potassium chlorate or sulfate are used in varyingcombination.

Special perchloric acid hood required (with wash-downcapabilities to protect from explosion).

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Principle

A sample is placed into a glass chamber which isevacuated using a vacuum pump

A small amount of oxygen is pumped into thechamber and broken down to nascent oxygen byapplication of an electromagnetic radio frequencyfield

The organic matter in the sample is rapidlyoxidized by the nascent oxygen and the moisture isevaporated because of the elevated temperatures

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Advantages & Disadvantages

Minimal chances of traceelements volatilizationcompared to classical dryashing

Equipment of choice forvolatile salts analysis.

Low temperatures used(

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IV. Soluble & Insoluble Ash in

Water

These measurements are an index of the

fruit content of preserves and jellies. A lowerash in the water-soluble fraction is an

indication that extra fruit is added to fruit and

sugar products.

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Procedure

Weigh total ash obtained and add the water

Heat near to boiling, filter on ashless filter paper

Rinse with hot water several times.

Dry and re-ash filter paper for 30mins.

Weigh and calculate as % H2O-insoluble ash.

Calculate soluble ash by subtracting insoluble ash from total

ash or dry the filtrate, re-ash and weigh

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V. Ash Insoluble in Acid

Measure surface contamination of fruits and

vegetables, wheat and rice coatings such as dirt or

sand. E.g. surface contamination of fruits and

vegetables, wheat washing prior to milling and ricecoatings. These contaminants are silicates that

remain insoluble in acid.

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Procedure:

Add HCl 10% to total ash or H2O-insoluble ash.

Boil for 5mins then filter on ashless filter paper

and wash several times with hot water

Re-ash filter paper and residue for 30mins.

Weigh and calculate as a percentage (%).

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Alkalinity of Ash

Useful measurement to determine the acid-base balance of foods and quality index offruits and fruit juices.

Ash from fruits and vegetables is alkaline(Ca, Mg, K, Na).

Ash from meat and some cereals is acid (P,S, Cl).

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Procedures:

Place ash (total or H2O-insoluble ash) in

platinum dish.

Add 0.1N HCl and warm on a steam bath.

Cool and transfer to Erlenmeyer flask.

Titrate the excess HCl with 0.1N NaOH

using methyl orange as indicator.

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Summary

Ashing conducted by any one of 3 methodologies - dryashing, wet ashing/oxidation and plasma ashing.

Dry ashing based on incineration at high temperature inmuffle furnace.

Wet ashing uses corrosive reagents that require constantsoperator's attention.

Dry ashing result in lost of volatile elements. Wet ashingand plasma ashing cause minimal volatilization.

Wet ashing and low temperature plasma ashing are

expensive; require operator time and limited to smallnumber of samples.

3 post-ashing procedures (soluble and insoluble ash inwater, ash insoluble in acid and ash alkalinity) are specialmeasurements for certain foods.