introduction to food analysis ii[1]

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INTRODUCTION TO FOOD ANALYSIS

1126

Steven C Seideman

Extension Food Processing Specialist

Cooperative Extension Service

University of Arkansas

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INTRODUCTION

• This module is a very brief overview of common methods of food analysis used in food processing organizations.

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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.

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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.

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References on Analytical Techniques

• Official Methods;

- Association of the Official Analytical Chemists (AOAC)

- American Oil Chemists Society (AOCS)

- American Association of Cereal Chemists (AACC)

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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.

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Criteria for Selecting an Analytical Technique

• Precision• Accuracy• Reproducibility• Simplicity• Cost• Speed

• Sensitivity• Specificity• Safety• Destructive/ Non-

destructive• On-line/off-line• Official Approval

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SAMPLING AND SAMPLE PREPARATION

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What is the Purpose of the Analysis

• Official Samples

• Raw Materials

• Process Control Samples

• Finished Products

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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

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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

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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

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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.

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Preparation of Laboratory Samples

• You may have taken as much as 10 lbs of sub-samples 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.

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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.

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COMPOSITION OF FOODS

COMPONENT

MilkBeefChickenFishCheeseCereal grains

PotatoesCarrotsLettuceAppleMelon

% Water %Carbohydrates %Protein % Fat % Min/Vit

87.3 5.0 3.5 3.5 0.7 60.0 0 17..5 22.0 0.9 66.0 0 20.2 12.6 1.0 81.8 0 16.4 0.5 1..3 37.0 2.0 25.0 31.0 5.0 10-14 58-72 8-13 2-5 0.5-3.0 78.0 18.9 2.0 0.1 1.0 88.6 9.1 1.1 0.2 1.0 94.8 2.8 1.3 0.2 0.9 84.0 15.0 0.3 0.4 0.3 92.8 6.0 0.6 0.2 0.4

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pH DETERMINATION

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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.

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MOISTURE DETERMINATION

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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.

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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

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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

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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 Microorganism1.0-0.95 Bacteria

0.95-0.91 Bacteria

0.91-0.87 Yeasts

0.87-0.80 Molds

0.30-0.20 No microorganism proliferation

• FoodsMeat, 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

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PROTEIN ANALYSIS

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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

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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

Protein (NH4)2SO4

(ammonium sulfate)

Protein N NH4+ + H2SO4 (NH4)2SO4

Sulfuric acid

Heat, catalyst

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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

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Total organic nitrogen - Kjeldahl method

– Titration (direct titration)

H2BO3

- + H+ H3BO3

– Calculationmoles HCl = moles NH3 = moles N in the sample

%N = N*(HCl)

%N = N*(HCl) N*=Normality of HCl

(HCl)

(mL acid sample-mL acid blank) 14g N

g sample mole1000 100

(mL acid sample-mL acid blank)

g sample 1.4

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Total organic nitrogen - Kjeldahl method• Calculation

%Protein = %N conversion factorConversion factor: generally 6.25 – most protein: 16% N

Conversion factoregg or meat 6.25milk 6.38wheat 5.33soybean 5.52rice 5.17

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Kjeldahl Apparatus

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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

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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).

• Procedureprotein solution + biuret reagent

room temp10 min

+ Folin reagent 50C 10 min

650 nm

(20-100 g)

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Lowry Method

• Advantages– most sensitive (20-200g)

– more specific, relatively rapid

• Disadvantages – color development not proportional to protein

concentration

– color varying with different proteins

– interference (sugars, lipids, phosphate buffers, etc)

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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

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Crude Fat Analysis

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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.

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Solvent Extraction Methods• Sample preparation: Best under nitrogen &

low temperature– Particle size reduction increases extraction

efficiency– Predrying sample to remove water is common.

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Solvent Extraction Methods• Solvent selection

– Ideal solvent• 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

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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

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Types of Fat Analysis

• Extraction Methods

Continuous – Goldfinch

Semi-Continuous- Soxhlet

Discontinuous- Mojonnier • Instrumental Methods

Dielectric

Infrared

Ultrasound

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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.

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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

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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

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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

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CARBOHYDRATE ANALYSIS

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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

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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

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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%

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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 (>525C) 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 ~550C for 12-18 hr

3. Cool in desiccator

4. Weigh cooled crucible

% ash (db) = 100

wt after ashing - crucible wt

Sample wt solid%/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

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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

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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

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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 (10-4M)

• 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

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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.

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Physical Properties

• Physical properties include; Color Texture Viscosity (liquids) Texture analysis machines Sensory panels Trained Consumer

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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.

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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

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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.

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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:

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• 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

llllllll

^

^

Scale - linked to printer

Torsion device

Spindle

Brabender Cup(rotates)

Heat-at 1.5oC per Minute

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Brabender Profile

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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 pasting curve.

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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 viscous products.

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Bostwick Consistometer

30 sec reading

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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.

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Texture Analyzer

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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”

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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).