INTRODUCTION TO FOOD
ANALYSIS
1126
Steven C Seideman
Extension Food Processing Specialist
Cooperative Extension Service
University of Arkansas
1
INTRODUCTION
• This module is a very brief overview of
common methods of food analysis used in
food processing organizations.
2
WHY ANALYZE FOOD?
• Government regulations require it for certain
products with standards of identity (e.g.% fat and
moisture in meat products).
• Nutritional Labeling regulations require it.
• Quality Control- monitor product quality for
consistency.
• Research and Development- for the development
of new products and improving existing products.
3
What Properties are Typically
Analyzed?
• Chemical Composition – water, fat,
carbohydrate, protein etc
• Physical Properties- Rheological or stability
• Sensory Properties- Flavor, mouth-feel,
color, texture etc.
4
References on Analytical
Techniques
• Official Methods;
- Association of the Official Analytical Chemists (AOAC)
- American Oil Chemists Society (AOCS)
- American Association of Cereal Chemists (AACC)
5
Criteria for Selecting an
Analytical Technique
• There are many techniques to analyze foods but
each has drawbacks or compromises.
• You must select the technique that is required or
fits into your system.
• For example, the most accurate techniques
generally take longer to perform and you may not
have the time if the food product you are making
requires “real time” results such as in the
formulation of processed meats.
6
Criteria for Selecting an
Analytical Technique
•
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•
•
•
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Precision
Accuracy
Reproducibility
Simplicity
Cost
Speed
•
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Sensitivity
Specificity
Safety
Destructive/ Nondestructive
• On-line/off-line
• Official Approval
7
SAMPLING AND SAMPLE
PREPARATION
8
What is the Purpose of the
Analysis
•
•
•
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Official Samples
Raw Materials
Process Control Samples
Finished Products
9
Sampling Plan
• A sampling plan is a predetermined procedure for
the selection, withdrawal, preservation,
transportation and preparation of the portion to be
removed from a lot as samples.
• The sampling plan should be a clearly written
document containing details such as;
- Number of samples selected
- Sample location (s).
- Method of collecting samples
10
Factors Affecting a Sampling
Plan
• Purpose of inspection
-acceptance/rejection, variability/average
• Nature of the product
-homogenous, unit, cost
• Nature of the test method
-Critical/minor, destructive, cost, time
• Nature of the population
-uniformity, sublot
11
Developing a Sampling Plan
• Number of samples selected
-Variation in properties, cost, type of analytical techniques
• Sample location
-random sampling vs systematic sampling vs judgment sampling
• Manner in which the samples are collected
-manual vs mechanical device
12
The Bottom Line in Sampling
• Depending upon the nature of the material
to be analyzed, you must determine a
method of taking small subsamples from a
large lot ( 5,000 lb blender, 20 combos on a
truck etc) that accurately reflect the overall
composition of the whole lot.
• An inaccurate sample of a large lot may
actually be worse than no sample at all.
13
Preparation of Laboratory
Samples
• You may have taken as much as 10 lbs of subsamples from a lot that now needs to be further
reduced in size;
-Make the sample homogeneous by mixing and grinding
and then more sub-sampling.
-Be aware of any changes that might occur between sampling and
analysis and take proper action ( e.g. enzymatic action, microbial
growth etc).
-Properly label the final sample with name, date/time, location, person
and other pertinent data.
14
FOOD COMPONENTS
• Food consists primarily of water( moisture),
fat (or oil), carbohydrate, protein and ash
(minerals).
• Since food consists of these 5 components,
it is important that we understand how these
components are measured.
15
COMPOSITION OF FOODS
COMPONENT
% Water
Milk
Beef
Chicken
Fish
Cheese
Cereal grains
Potatoes
Carrots
Lettuce
Apple
Melon
87.3
60.0
66.0
81.8
37.0
10-14
78.0
88.6
94.8
84.0
92.8
%Carbohydrates %Protein % Fat % Min/Vit
5.0
0
0
0
2.0
58-72
18.9
9.1
2.8
15.0
6.0
3.5
17..5
20.2
16.4
25.0
8-13
2.0
1.1
1.3
0.3
0.6
3.5
22.0
12.6
0.5
31.0
2-5
0.1
0.2
0.2
0.4
0.2
0.7
0.9
1.0
1..3
5.0
0.5-3.0
1.0
1.0
0.9
0.3
0.4
16
pH DETERMINATION
17
pH Determination
• pH refers to the relative amounts of acid and base
in a product.
• It is scientifically defined as the negative log of
the hydrogen ion concentration.
• pH ranges from 0 to 14 with pH of 7 being
neutral. pH values below 7 are considered acids
and pH values above 7 are basic or alkaline.
• pH is generally determined with a pH meter
although litmus paper can also be used.
18
MOISTURE
DETERMINATION
19
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.
20
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
21
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
22
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.
23
WATER ACTIVITY
• Aw
1.0-0.95
Microorganism
Bacteria
• Foods
Meat, fish, sausage, milk
0.95-0.91
Bacteria
Cheese, cured meat (ham), fruit
juice conc
0.91-0.87
Yeasts
Fermented sausages (salami), dry
cheeses, margarine
0.87-0.80
Molds
Juice conc, syrups, flour, fruit
cakes, honey, jellies, preserves
0.30-0.20
No microorganism
proliferation
Cookies, crackers, bread crusts
24
PROTEIN ANALYSIS
25
PROTEINS
• Proteins are made up of amino acids.
• Amino acids are the building blocks of protein.
• Nitrogen the most distinguishing element versus
other food components (carbohydrates, fats etc)
• Nitrogen ranges in proteins : 13.4 - 19.1%
• Non-protein nitrogen: free amino acids, nucleic
acids, amino sugars, some vitamins, etc.
• Total organic nitrogen = protein + non-protein
nitrogen
26
Types of Protein Analysis
• Kjeldahl – measures the amount of nitrogen
in a sample.
• Lowry- measures the tyrosine/tryptophan
residues of proteins.
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Total organic nitrogen - Kjeldahl
method
• Crude protein content
• Johan Kjeldahl (1883) developed the basic
process
• Principle: total organic N released from
sample and absorbed by acid
– Digestion: sulfuric acid + catalyst
– Neutralization and distillation; Sodium hydroxide
– Titration; Hydrochloric acid
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Total organic nitrogen - Kjeldahl
method
Digestion
Sulfuric acid
Protein Heat, catalyst
(NH4)2SO4
(ammonium sulfate)
Protein N  NH4+ + H2SO4  (NH4)2SO4
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Total organic nitrogen - Kjeldahl
method
Neutralization and distillation
(NH4)2SO4 + 2NaOH  2NH3 + Na2SO4 + 2H2O
NH3 + H3BO3  NH4+ : H2BO3 + H3BO3
-
(boric acid)
(ammonium-borate complex)
excess
Color change
30
Total organic nitrogen - Kjeldahl
method
– Titration (direct titration)
H2BO3 + H+  H3BO3
(HCl)
– Calculation
moles HCl = moles NH3 = moles N in the sample
%N = N*(HCl) (mL acid sample-mL acidblank) 14g N
1000  g sample
%N = N*(HCl)  (mL acid sample-mL acid blank)
N*=Normality of HCl
g sample
100
mole
 1.4
31
Total organic nitrogen - Kjeldahl
method
• Calculation
%Protein = %N  conversion factor
Conversion factor: generally 6.25
– most protein: 16% N
egg or meat
milk
wheat
soybean
rice
Conversion factor
6.25
6.38
5.33
5.52
5.17
32
Kjeldahl Apparatus
33
Total organic nitrogen - Kjeldahl
method
• Advantages:
– applicable to any foods
– simple, inexpensive
– accurate, official method for crude protein content
• Disadvantages:
– measuring total N not just protein N
– time consuming
– corrosive reagents
34
Lowry Method
• Principle: Color formation between tyrosine and
tryptophan residues in protein and Biuret reagent
and Folin-Ciocalteau phenol reagent (750 nm or
500 nm).
• Procedure
protein solution + biuret reagent
(20-100 g)
room temp10 min
+ Folin reagent
50C 10 min
650 nm
35
Lowry Method
• Advantages
– most sensitive (20-200g)
– more specific, relatively rapid
• Disadvantages
– color development not proportional to protein
concentration
– color varying with different proteins
– interference (sugars, lipids, phosphate buffers, etc)
36
Infrared Spectroscopy
• Principle: absorption of radiation of peptide bond
at mid-infrared (MIR) and near-infrared (NIR)
bands
• Advantages
– NIR applicable to a wide range of foods
– rapid, nondestructive
– little sample preparation
• Disadvantages
– expensive instruments
– calibration for different samples
37
Crude Fat Analysis
38
Fats
• Fats refers to lipids, fats and oils.
• The most distinguishing feature of fats
versus other components ( carbohydrates,
protein etc) is their solubilty. Fats are
soluble in organic solvents but insoluble in
water.
39
Solvent Extraction Methods
• Sample preparation: Best under nitrogen &
low temperature
– Particle size reduction increases extraction
efficiency
– Predrying sample to remove water is common.
40
Solvent Extraction Methods
• Solvent selection
– Ideal solvent
•
•
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•
•
•
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•
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high solvent power for lipids
low solvent for other components
easy to evaporate
low boiling point
nonflammable
nontoxic
good penetration into sample
single component
inexpensive
non-hygroscopic
41
Solvent Extraction Methods
• Common Solvents
– Ethyl ether - best solvent for fat extraction,
more expensive, explosion, fire hazard,
hygroscopic
– Petroleum ether - cheaper, more hydrophobic,
less hygroscopic
– Hexane - soybean oil extraction
42
Types of Fat Analysis
• Extraction Methods
Continuous – Goldfinch
Semi-Continuous- Soxhlet
Discontinuous- Mojonnier
• Instrumental Methods
Dielectric
Infrared
Ultrasound
43
Solvent Extraction Methods
• Continuous extraction: Goldfish method
– Principle: Solvent continuously flowing over
the sample with no build-up
– Advantages: fast, efficient.
– Disadvantages: channeling – not complete
extraction.
44
Solvent Extraction Methods
• Semicontinuous extraction:
Soxhlet method
– Principle: Solvent building up in
extraction chamber for 5-10 min
before siphoning back to boiling
flask.
– Advantages: no channeling
– Disadvantages: time consuming
45
Solvent Extraction Methods
• Discontinuous extraction: Mojonnier
method (wet method extraction)
– Principle: a mixture of ethyl ether and
petroleum ether in a Mojonnier flask
– Advantages: no prior removal of moisture
– Disadvantages: constant attention
46
Instrumental Methods
• Dielectric method
– Principle: low electric current from fat
• Infrared method
– Principle: Fat absorbs infrared energy at a
wavelength of 5.73 m
• Ultrasound method
– Principle: sound velocity increases with
increasing fat content
47
CARBOHYDRATE ANALYSIS
48
Introduction
• Next to water, carbohydrates are the most
abundant food component
• %carbohydrate=100% - (H2O + ash + fat + protein)
• Types of carbohydrates include;
–
–
–
–
monosaccharide: glucose, fructose, galactose
disaccharide: sucrose, lactose, maltose
oligosaccharids: raffinose
polysaccharide: starch, cellulose
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Ash and Mineral Analysis
50
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
51
Ash Contents in Foods
Wheat flour, whole grain
Macaroni, dry, enriched
Milk, whole, fluid
Butter, with salt
Apple, raw with skin
Banana, raw
Egg, whole, raw
Hamburger, regular, plain
1.6%
0.7%
0.7%
2.1%
0.3%
0.8%
0.9%
1.7%
52
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
53
Dry Ashing
• Principles
– High temperature (>525C) overnight (12-18 hr)
– total mineral content
• Instrumentation
– Muffle furnace
– Crucible
•
•
•
•
•
quartz
porcelain
steel
nickel
platinum
54
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
% ash (db) =
Sample wt  solid%/100
 100
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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
56
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
57
Ion-Selective Electrodes
• Activity (A) vs. Concentration (C)
A=C
=activity coefficient
A: chemical activity
C: a measure of ions in solution
 is a function of ionic strength; ionic strength is a
function of concentration and charge on all
ions
AC
58
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 (10-4M)
• Applications:
– processed meats: salt,
nitrate
– butter and cheese: salt
– milk: Ca
– low-sodium products:
sodium
– soft drink: CO2
– wine: Na, K
– can vegetable: nitrate
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Physical Properties of Foods
60
PHYSICAL PROPERTIES
• While chemical properties measures the
chemical components of food such as water,
protein, fat, carbohydrates, the physical
properties determine how the chemical
properties and processing ultimately effect
the color and texture of foods.
61
Physical Properties
• Physical properties include;
Color
Texture
Viscosity (liquids)
Texture analysis machines
Sensory panels
Trained
Consumer
62
COLOR
• Color can be described in terms of hue, value and chroma;
Hue is the aspect of color which we
describe by words like green, blue,
yellow and red
Value or lightness describes the relationship between
reflected and absorbed light, without regard to specific
wavelength.
Chroma describes reflection at a given wavelength and
shows how much a color differs from gray.
63
HUNTER L,a,b
• The Hunter L,a,b
system describes the
color of a food in
terms of L
(100=white; 0= black),
a (green- red) and b
(blue to yellow).
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COLOR
• More subjective color determination
systems include;
- Paint color match pages
-The Pantone Matching System.
- Actual photos of finished food products
65
TEXTURE
• The methods of measuring the texture of
foods can be roughing divided into those
used for liquids (viscosity) versus those
used for more solid foods.
66
Fluid Viscosity
• Viscosity: a key property of liquids and a measure of the
resistance to flow.
• More energy required to make a viscous fluid flow than a
non-viscous fluid.
• The viscosity of a solution increases non-linearly with
polymer concentration.
• The properties of the solution are conventionally split into
three regions:
67
• Dilute Regime
• The polymers act as isolated "particles" too dilute
to interact with each other. They can be
approximated as spheres of radius rg (the Stokes
radius - the smallest sphere that can contain the
polymer).
• Semi-Dilute Regime
• The "particles" start to interact significantly
because their total excluded volume approaches
close packing. Further increase in concentration
leads to much greater overlap of polymer coils and
rapid increase in viscosity.
• Concentrated Regime
• The individual polymer molecules overlap in a
tangled mass. The viscosity of concentrated
polymer solutions is very high and as the
concentration increases further starts to show some
solid-like behavior.
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Brookfield (Rotational) Viscometer
• Viscosity measurement by sensing the torque
required to rotate a spindle at constant speed while
immersed in the sample fluid.
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Brabender Viscoamylograph and Rapid
Visco Analyzer
Scale - linked to
printer
llllllll
Torsion
device
Spindle
Brabender Cup
(rotates)
Heat-at 1.5oC per Minute
70
Brabender Profile
71
Brabender and RVA Applications
• Starch, flours, baking products, noodle
quality, extrusion, sprouting and enzyme
activity, malting and brewing, storage,
Effect of amount of water added
during extrusion on RVA pasting
curves of corn based extrudates.
Lower water addition causes a
higher degree of cook in the
extrudate and this is reflected in
a progressive change in the RVA
72
pasting curve.
Bostwick Consistometer
• A simple, dependable instrument which determines
sample consistency by measuring the distance which a
sample of material flows under its own weight
• The unit is constructed of stainless steel and is
equipped with two leveling screws and a level.
The gate is spring operated and held
by a positive release mechanism,
permitting instantaneous flow of
sample. The trough is graduated in
0.5cm divisions.
• Used extensively in the food industry
for jams, jellies, tomato paste, ketchup,
condensed soup and other highly
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viscous products.
Bostwick Consistometer
30 sec reading
74
Instron Universal Testing Machine
• A highly accurate and versatile material testing
instrument for the precise measurement of the
properties and behavior of materials in tension,
compression, flexure and torsion.
• The instrument weighing system employs strain
gauge load cells for measuring the load applied to
the specimen under test.
• The output from the load cell is applied to a solid
state load cell signal conditioning amplifier which
provides a wide range of full scale load ranges for
each type of load cell used. The controls provide
for adjustment and calibration of the load
weighing system to obtain accurate and reliable
test data. The load cell amplifier output is in a
signal form suitable for controlling the pen servo
system of the chart recorder.
75
Texture Analyzer
76
Sensory Properties
• Trained Sensory Panels – a few well trained
people that characterize flavor, texture and odor
versus like/dislike,
• Consumer Panels- usually consist of 200 plus
people who determine like/dislike, desirability etc.
• Additional detailed information on sensory panels
can be found in the module “Sensory Evaluation
of Foods; 1213”
77
SUMMARY
• This module has presented the topic of Food
Analysis by discussing why we analyze
food, sampling and preparation, the
components of food generally analyzed for
(water, protein, fat, carbohydrates) and
some general methods of analyzing the
physical properties of food (color, viscosity
and texture).
78