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
15
Water Activity (a
w
)
• 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.
16
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
17
Ash and Mineral Analysis
18
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
19
Ash Contents in Foods
Wheat flour, whole grain 1.6%
Macaroni, dry, enriched 0.7%
Milk, whole, fluid 0.7%
Butter, with salt 2.1%
Apple, raw with skin 0.3%
Banana, raw 0.8%
Egg, whole, raw 0.9%
Hamburger, regular, plain 1.7%
20
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
21
Dry Ashing
• Principles
– High temperature (>525°C) overnight (12-18 hr)
– total mineral content
• Instrumentation
– Muffle furnace
–Crucible
•quartz
• porcelain
•steel
•nickel
• platinum
22
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
% ash (db) = × 100
wt after ashing - crucible wt
Sample wt × solid%/100
23
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
24
Ion-Selective Electrodes
• Direct measurement via chemical potential
of cations (Ca, Na, K), anions (Br, Cl, F), or
even dissolved gases (O
2
, CO
2
)
• Components:
– sensing electrode
– reference electrode
– readout device
• Types: glass membrane, polymer-body,
solid-state
25
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
26
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: CO
2
– wine: Na, K
– can vegetable: nitrate
The end!
. 1 .7%
20
Methods for Determining Ash
– Dry ashing
• high temperature
– Wet ashing
• oxidizing agent and/or acid
– Low-temperature plasma ashing
• dry ashing. MOISTURE AND ASH
ANALYSIS
Pham Van Hung, PhD
MOISTURE
DETERMINATION
Moisture Determination
• Moisture or water is by far