Lecture 6 - Ash Analysis

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WEEK 7

ASHANALYSIS

FST 3141


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INTRODUCTION

Ash refers to the inorganic residue remaining after

either ignition or complete oxidation of organic matter

in a foodstuff

A basic knowledge of the characteristics of various

ashing procedures and types of equipment is

essential to ensure reliable results

Two major types of ashing are used:

Dry ashing, primarily for proximate composition and for

some types of specific mineral analysis

Wet ashing (oxidation), as a preparation for the analysis

of certain minerals

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INTRODUCTION

Microwave systems are now available for both dry

and wet ashing, to speed the process

Most dry samples (i.e. whole grain, cereals, dried

vegetables) need no preparation, while fresh

vegetables need to be dried prior to ashing

High fat products such as meats may need to be

dried and fat extracted before ashing

The ash content of food can be expressed on either

a wet weight (as is) or on a dry weight basis

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DEFINITIONS

Dry Ashing: Refers to the use of a muffle furnace

capable of maintaining temperature of 500-600C

Water and volatiles are vaporized, and organic

substances are burned in the presence of oxygen in

air to CO2and oxides of N2

Most minerals are converted to oxides, sulfates,

phosphates, chlorides, and silicates

Elements such as Fe, Pb, Se, and Hg may partially

volatilize with this procedure, so other methods

must be used if ashing is a preliminary step for

specific elemental analysis4


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DEFINITIONS

Wet Ashing: Is a procedure for oxidizing organic

substances by using acids and oxidizing agents or

their combinations

Minerals are solubilised without volatilization

Wet ashing often is preferable to dry ashing as a

preparation step for specific elemental analysis

Wet ashing often uses a combination of acids and

requires a special perchloric acid hood if that acid is

used

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IMPORTANCEOFASHINFOODS

Ash content represents the total mineral content in

foods

Determining the ash content may be important for

several reasons:

It is part of proximate analysis for nutritional evaluation

Ashing is the first step in preparing a food sample for

specific elemental analysis

Because certain foods are high in particular minerals,

ash content becomes important One can usually expect a constant elemental content

from the ash of animal products, but that from plant

sources is variable6


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ASHCONTENTINFOODS

The ash content of most fresh foods is rarely

greater than 5%

Pure oils and fats generally contain little or no ash

Products such as cured bacon may contain 6% ash Dried beef may be as high as 11.6%

Fats, oils, and shortenings vary from 0.0 to 4.1%

ash

Dairy products vary from 0.5 to 5.1% Fruits, fruit juice, and melons contain 0.2-0.6% ash,

while dried fruits are higher (2.4-3.5%)

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ASHCONTENTINFOODS

Flours and meals vary from 0.3 to 1.4% ash

Pure starch contains 0.3% and weather germ 4.3%

ash

It would be expected that grain and grain productswith bran would tend to be higher in ash content

than products without bran

Nuts and nut products contain 0.8-3.4% ash

Meat, poultry, and seafood contain 0.7-1.3% ash

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METHODS

Sample Preparation:

It cannot be overemphasized that the smallsample used for ash, or other determinations,needs to be very carefully chosen so that it

represents the original materials A 2-10g sample generally is used for ash

determination

Solid foods are finely ground and then carefullymixed to facilitate the choice of a representativesample.

Before carrying out an ash analysis, samplesthat are high in moisture are often dried toprevent spattering during ashing. 9


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METHODS

Sample Preparation (continued):

High fat samples are usually defatted by solvent

extraction, as this facilitates the release of the

moisture and prevents spattering.

Other possible problems include contamination

of samples by minerals in grinders, glassware or

crucibles which come into contact with the

sample during the analysis.

For the same reason, it is recommended to use

deionized water when preparing samples.

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DRYASHING

Incineration at high temperature (525C or higher)

Incineration is accomplished with a muffle furnace

Several models of muffle furnaces are availableranging from large-capacity units to small benchtop

units Crucible selection is critical in ashing because the

type depends upon the specific use.

Quartzcruciblesare resistant to acids and

halogens, but not bases, at high temperatures Porcelain cruciblesresemble quartz crucibles in

their properties, but will crack with rapidtemperature changes. However, they areinexpensive and usually the crucible of choice

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DRYASHING

Steel cruciblesare resistant to both acids and alkaliesand are inexpensive, but they are composed ofchromium and nickel, which are possible sources ofcontamination

Platinum cruciblesare very inert and are probably thebest crucibles, but they are far too expensive for routineuse for large numbers of samples

Quartz fiber cruciblesare disposable, unbreakable,and can withstand temperatures up to 1000C. They areporous allowing air to circulate around the sample andspeed combustion. This reduces ashing timessignificantly and makes them ideal for solids and viscousliquids. Quartz fiber cools in seconds, eliminating therisk of burns

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DRYASHING

All crucibles should be marked for identification

Marks on crucibles with felt-tip marking pen will

disappear during ashing in a muffle furnace

Laboratory inks scribed with a steel pin areavailable commercial

Crucibles also may be etched with a diamond point

and marked with a 0.5 M solution of FeCl3in 20%

HCl

Crucibles should be fired and cleaned prior to use

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DRYASHING

The advantagesof dry ashing are that:

It is a safe method

Requires no added reagents or blank substrate

Little attention is needed once ignition begins A large number of crucibles can be handled at

once

The resultant ash can be used additionally in

other analyses for most individual elements

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DRYASHING

The disadvantagesof dry ashing are that:

The length of time required (12-18 hours)

Expensive equipment

Loss of volatile elements Risk of interactions between mineral

components and crucibles

Volatile elements at risk of being lost include: As,

B, Cd, Cr, Cu, Fe, Pb, Hg, Ni, P, V, and Zn

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DRYASHING

AOAC has several dry ashing procedures such as

AOAC Methods 900.2 A or B, 920.117, and 923.03

The general procedure includes the following:

1. Weigh a 5-10-g sample in a tared crucible. Predry the

sample if very moist

2. Place crucibles in a cool muffle furnace

3. Ignite 12-18 hours (or overnight) at about 550C

4. Transfer crucibles to a desicator with a porcelain plant

and desiccant

The ash content is calculated as follows:% ash (dry basis)

= wt after ashing / original sample wt x dry matter coefficient

Dry matter coefficient = % solids / 100 16


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DRYASHING

For example, if corn meal is 87% dry matter, the dry

matter coefficient would be 0.87

If moisture was determined in the same crucible

prior to ashing, the denominator becomes:

(dry sample wt tared crucible wt)

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WETASHING

Wet ashingis sometimes called wet oxidation or

wet digestion

Its primary use is preparing for specific mineral

analysis and metallic poisons

There are several advantagesto using the wet

ashing procedure:

Minerals will usually stay in solution

There is little or no loss from volatilization because of

the lower temperature

The oxidation time is short and requires a hood, hot

plate, and long tongs, plus safety equipment

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WETASHING

The disadvantagesof wet ashing are that:

It takes constant operator attention

Corrosive and dangerous reagents are

necessary Only a small number of samples can be handled

at one time

If wet digestion utilizes perchloric acid, all work

needs to be carried out in an expensive specialfume hood called a perchloric acid hood

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WETASHING

While wet ashing with perchloric acid is an AOAC

procedure (i.e. AOAC 975.03), many analytical

laboratories avoid if possible the use of perchloric

acid in wet ashing

Instead the use of a combination of nitric acid with

either sulfuric acid, hydrogen peroxide, or

hydrochloric acid is preferred

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MICROWAVEASHING

Both wet ashing and dry ashing can be performedusing microwave instrumentation, rather than theconventional dry ashing in a muffle furnace and wetashing in a flask or beaker on a hot plate

While ashing procedures by conventional meanscan take many hours, the use of microwaveinstruments can reduce sample preparation time tominutes, allowing laboratories to increase samplethroughput significantly

This advantage has led to widespread use ofmicrowave ashing, especially for wet ashing, bothwithin analytical laboratories and quality controllaboratories within food companies 21


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COMPARISONOFMETHODS

The conventional dry ashing procedure is simple to

carry out, is not labor intensive, requires no

expensive chemicals and can be used to analyze

many samples simultaneously.

However the procedure is time-consuming and

volatile minerals may be lost at the high

temperatures used.

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COMPARISONOFMETHODS

Microwave instruments are capable of speeding up

the process of dry ashing.

Wet ashing is more rapid and cause less loss of

volatile minerals because samples are heated to

lower temperatures.

However, the wet ashing procedure requires the

use of hazardous chemicals and is labor intensive

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REFERENCES

Textbook: Neilson et. al. (2010). Food Analysis,

4thEdition. Springer Publishing

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Lecture 6 - Ash Analysis

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