This document reviews physical characterization methods for food extrudates. It finds that while extrusion research has been conducted for over 50 years, methods for evaluating properties vary greatly. It calls for standardizing terminology, measurement techniques, sample sizes and other parameters for assessing properties like particle size, density, expansion, water absorption, texture and color. This will allow more consistent analysis and comparisons of extruded food physical attributes across studies.

1. Physical Characteristics of
Food Extrudates - A Review
Ramabhau Patil1, Jose De J Berrios2
Juming Tang1 , James Pan2 and BG
Swanson1
1- Washington State University, Pullman-WA
2-USDA-WRRC-Albany-CA

2. The physical characteristics of the extrudate reflect the
effectiveness of the extrusion process and suitability of
ingredients.
Food extruders make possible many desirable
functional changes in the final extrudate including:
– Protein denaturation/texturization
– Starch gelatinization
– Fat globules modification
– Inactivation of undesirable factors
Importance

3. Though, food extrusion research is being
conducted over 50 years, one finds lot of variation
in:
– Methods of evaluation of extrudate physical
properties
– Variation in sample size for determination of
extrudate physical properties
Limitations

4. Single Screw Extruder

5. Twin Screw Extruder

6. Particle Size Characterization
Geometric Mean Diameter Geometric Standard Deviation
Size Distribution Coefficient (Cm)
Particle Size Index (PSI)
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7. Density of Extrudate
• Method 1: Volumetric method (cylindrical container of
known volume)
• Method 2: Bulk density method (Volume displacement)
• Method 3. Dimensional measurements
• Method 4. Tapping method (powdered material)
• Apparent Solid Density: Density of rod measured with
air/gas comparison pycnometer
• True Solid Density: Density of rod measured with air/gas
comparison pycnometer
• Closed and Open Pores: Calculated from Apparent and
True solid density

8. Expansion Indices
• Diameter ratio
• Ratio of areas
• Sectional Expansion Index:
• Longitudinal Expansion Index:
• Volumetric Expansion Index:
• Overall Expansion Index:
2
2
die)of(Diameter
extrudate)of(Diameter
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ee
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.S).M-(1
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BeadsGlassByDensityBulk
PycnometerByDensityTrue

9. Water Absorption Properties
• Simplified Method: Decanting supernatant
• Swelling Index/Water Holding Capacity: Over-
night swell index
• Centrifuge method: Parameters; Particle size,
Sample size, g, Water for dispersion, Centrifugal
force and time
• Various centrifugation forces and times used:
• 3000xg for 10 min.
• 9000 rpm for 20 min.
• 1000xg for 15 min.
• 15000xg for 15 min.
• 1500xg for 15 min.
• 18000xg for 30 min.

10. Oil Absorption Capacity
• 0.5 g of the ground extrudate added to 10 mL of
soybean oil in a 15 mL conical graduated
centrifuge tube
• Mixing/vortex for 3 min
• Holding for 30 min
• Centrifugation: 3000xg/25 min
• Oil removal from mix after tube invert for 25 min
Oil absorption capacity was expressed as grams of
soybean oil bound per gram of the extrudate on a
dry weight basis.

11. Textural Evaluation
• Hardness/Breaking strength/Shear stress
• Puncture Test
• Textural Profile Analysis (TPA)
– Hardness,
– Springiness,
– Gumminess,
– Fracturability,
– Cohesiveness,
– Chewiness and
– Modulus properties of foods.
Parameters to be standardized : Cross head speed. % compression,
single bite Vs double bite, Waiting time between bites

12. Color Measurement
• Particle size
• Observations
• Color Indices
– The whiteness index =
– Change in color =
   50222 .
L-100-100W ba 
       50222 .
LbaE 

13. Specific Mechanical Energy
(Kg/h)FRate,FeedTotal
(kW)WAbsrption,PowerNet
SME 
Q
P
x
Torque
xSME r
r 100
%
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14. Residence Time Distribution
• Mean Residence Time
• Relative Residence Time (RRT)
• Vessel Dispersion Number
• Mass Flow Rate

15. Conclusions
• Particle size characteristics of powder materials are widely
determined by sieve analysis methods. Particle Size Index (PSI) can
be an appropriate single index describing the fineness as well as
spread of the grind.
• Density of the extrudate has been determined by different methods
and various terms have been used. Hence, the terminologies as well
as the methods need to be standardized.
• Expansion indices are defined differently by various researchers.
Criteria as well as number of observations use to describe
expansion property of extrudates need standardization.
• Water absorption and solubility index methods reflected variation in
sample particle size, sample size and centrifugation speed. All these
parameters need standardization.
• Textural evaluation methods also showed lot of variation. The cross
head speed, sample size as well criteria of compression (single or
double) need to be standardized.