lecture 7 moisture & ash

MOISTURE AND ASH ANALYSIS Pham Van Hung, PhD MOISTURE DETERMINATION Moisture Determination • Moisture or water is by far the most common component in foods ranging in content from 60 –

MOISTURE AND ASH

ANALYSIS

Pham Van Hung, PhD

MOISTURE DETERMINATION

Moisture Determination

• Moisture or water is by far the most

common component in foods ranging in

content from 60 – 95%

• The two most common moisture

considerations in foods is that of total

moisture content and water activity.

Moisture Content

• The total moisture content of foods is generally determined by some form of drying method whereby all the moisture is removed by heat and moisture is determined

as the weight lost

• % water = wet weight of sample-dry weight of sample

wet weight of sample

Methods of Moisture Loss

Measurement

• Convection or forced draft ovens (AOAC)

-Very simple; Most common

• Vacuum Oven

-Sample is placed in oven under reduced pressure thereby

reducing the boiling point of water

• Microwave Oven

-Uses microwave as a heat source; Very fast method

• Infrared Drying

-Uses infrared lamp as a heat source; Very fast

Forced draft ovens

• Objective

Determine the moisture content of cereal flour using

a forced draft oven method

• Principle of Method

The sample is heated under specified conditions and the loss of weight is used to calculate the moisture content of the sample

• Equipment

– Forced draft oven – Analytical balance, 0.1 mg sensitivity

Forced draft ovens

• Application

– Syrup (corn, starch, etc.)

– Flour (wheat, rice, corn, etc.)

– Liquid Milk

– Nonfat Dry Milk

– Fresh Basil

Forced draft ovens

• Procedure

– Weigh accurately dried pan with lid (Note identifier number on pan and lid.) – Place 2–3 g of sample in the pan and weigh accurately

– Place in a forced draft oven at 130°C for 1 h Be sure metal covers are ajar, to allow water loss – Remove from oven, realign covers to close, cool, and store in desiccator until samples are weighed – Calculate percentage moisture (wt/wt) as described below

VACUUM OVEN

• Objective

– Determine the moisture content of corn syrup by the

vacuum oven method, with and without the addition of

sand to the sample

• Principle

– The sample is heated under conditions of reduced

pressure to remove water and the loss of weight is used

to calculate the moisture content of the sample

• Equipment

– Vacuum oven (capable of pulling vacuum

to <100 mm of mercury)

– Analytical balance, 0.1 mg sensitivity

VACUUM OVEN

• Procedure

– Label weighing pans (i.e., etch identifier into tab of disposable aluminum pan) and weigh accurately – Place 5 g of sample in the weighing pan and weigh accurately

– Dry at 70°C and a vacuum of at least 26 in for 24 h, but pull and release the vacuum slowly (Note that samples without drying sand will bubble up and mix with adjoining samples if pans are too close together.) Bleed dried air into the oven as vacuum is released – Store in a desiccator until samples are cooled to ambient temperature Weigh

MICROWAVE DRYING OVEN

• Objective

– Determine the moisture content of corn syrup and milk

(liquid) using a microwave drying oven

• Principle

– The sample is heated using microwave energy, and the

loss of weight is used to calculate the moisture content of

the sample

• Equipment

– Microwave drying oven

(e.g., from CEM Corporation,

Matthew, NC).

MICROWAVE DRYING OVEN

• Procedure

– Follow instructions from manufacturer for use

of the microwave drying oven, regarding the following:

• Turning on instrument and warming up

• Loading method for specific application (i.e., sets time, power, etc.)

• Taring instrument

• Testing sample

• Obtaining results

NEAR INFRARED ANALYZER

• Objective

– Determine the moisture content of corn flour using a near infrared

analyzer.

• Principle

– Specific frequencies of infrared radiation are absorbed by the

functional groups characteristic of water (i.e., the –OH stretch of the

water molecule) The concentration of moisture in the sample is

determined by measuring the energy that is reflected or transmitted by

the sample, which is inversely proportional to the energy absorbed.

• Equipment

– Near infrared analyzer

NEAR INFRARED ANALYZER

• Procedure

– Follow instructions from manufacturer for use of the near infrared analyzer, regarding the following:

• Turning on instrument and warming up

• Calibrating instrument

• Testing sample

• Obtaining results

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Water Activity ( aw)

• Water Activity (Aw) is the amount of free

water in a sample that is not bond and

therefore free for microbial growth, enzyme

and vitamin decomposition and can reduce

color, taste and flavor stability.

• Two general types of sensors:

– Capacitance sensor: electrical signal

– Chilled-mirror dew point method (AquaLab):

dew point temperature change due to ERH

change

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WATER ACTIVITY

• Aw Microorganism 1.0-0.95 Bacteria

0.95-0.91 Bacteria

0.91-0.87 Yeasts

0.87-0.80 Molds

0.30-0.20 No microorganism

proliferation

• Foods Meat, fish, sausage, milk

Cheese, cured meat (ham), fruit juice conc

Fermented sausages (salami), dry cheeses, margarine

Juice conc, syrups, flour, fruit cakes, honey, jellies, preserves

Cookies, crackers, bread crusts

Ash and Mineral Analysis

Definitions

• Ash: total mineral content; inorganic residue remaining after ignition or complete oxidation of organic matter

• Minerals:

– Macro minerals (>100 mg/day)

• Ca, P, Na ,K, Mg, Cl, S – Trace minerals (mg/day)

• Fe, I, Zn, Cu, Cr, Mn, Mo, F, Se, Si – Ultra trace minerals

• Va, Tn, Ni, Sn, B – Toxic mineral

• lead, mercury, cadmium, aluminum

Ash Contents in Foods

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Methods for Determining Ash – Dry ashing

• high temperature – Wet ashing

• oxidizing agent and/or acid – Low-temperature plasma ashing

• dry ashing in partial vacuum at low temperature

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Dry Ashing

• Principles

– High temperature (>525°C) overnight (12-18 hr)

– total mineral content

• Instrumentation

– Muffle furnace

– Crucible

• quartz

• porcelain

• steel

• nickel

• platinum

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General Procedure for Dry

Ashing

1 5-10g pretreated sample into a crucible

2 Ignite crucible to constant weight at

~550°C for 12-18 hr

3 Cool in desiccator

4 Weigh cooled crucible

wt after ashing - crucible wt Sample wt × solid%/100

Dry Ashing

• Advantages

– safe and easy

– no chemical

– many samples handled at one time

– resultant ash for further mineral analysis

• Disadvantages

– loss of volatiles

– interaction

– long time and expensive equipment

Ion-Selective Electrodes

• Direct measurement via chemical potential

of cations (Ca, Na, K), anions (Br, Cl, F), or even dissolved gases (O2, CO2)

• Components:

– sensing electrode – reference electrode – readout device

• Types: glass membrane, polymer-body, solid-state

Ion-Selective Electrodes

• Activity (A) vs Concentration (C)

A=γC γ=activity coefficient

A: chemical activity

C: a measure of ions in solution

γ is a function of ionic strength; ionic strength is a

function of concentration and charge on all

ions

A ≤ C

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Ion-Selective Electrodes

• Advantages – more precise, rapid, practical – direct measurement of a wide range of ions – inexpensive and simple

• Disadvantages – inability to measure below 2-3 ppm – unreliable at low concentration (10 -4 M)

• Applications: – processed meats: salt, nitrate

– butter and cheese: salt – milk: Ca

– low-sodium products: sodium

– soft drink: CO2 – wine: Na, K – can vegetable: nitrate

The end!