1st International Conference on Environmental, Industrial and Applied Microbiology BioMicroWorld2005

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1st International Conference on Environmental,
Industrial and Applied Microbiology
(BioMicroWorld-2005) March 15-18th 2005, Badajoz,
Spain
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SCANNING ELECTRON MICROSCOPE STUDY OF FISH AND
RICE FLOUR COEXTRUDATES

• TUMULURU JAYA SHANKAR1 AND SUKUMAR BANDYOPADHYAY2
• 1Department of Process and Chemical Engineering
• University College Cork, Ireland
• 2Agricultural and Food Engineering
  Department
• Indian Institute of Technology,
  Kharagpur-721302, India

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INTRODUCTION

• Fish mince or powder, blended and coextruded with
  rice flour, produces some promising snack food
  products with excellent nutritional combinations.
  
• Extrusion cooking imparts characteristic texture
  to the product, which is a desirable functional
  property for use as cereal-based fish snacks and
  crackers.
• Texture of these products varies depending upon
  processing parameters like extruder barrel
  temperature and extruder screw speed coupled with
  moisture and protein level of the raw material.
• Food texture being a direct consequence of
  microstructure, an examination of product
  microstructure has proven a satisfactory method
  for evaluation and prediction of textural
  properties.
• Scanning electron microscope (SEM) is a
  frequently adopted technique for microscopic
  analysis of extruded products.

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OBJECTIVE

• To study the effect of the process variables
  like extruder barrel temperature (C), extruder
  screw speed (rev/min), fish content () and feed
  moisture content () on the extrudates prepared
  out of fish and rice flour blends

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Materials and Methods

• Preparation of the extrudates
• Tropical marine fish known as Bombay duck
  (Harpodon nehereus) and rice flour both made into
  powder, passing through 353 um mesh sieve, were
  used for preparing the extrudates.
• Extruder barrel temperature (100-200 C),
  extruder screw speed (70-110 rev/min), fish
  content (5-45 ) and feed moisture content (20-60
  ) were chosen as independent process variables.
• A five level central composite design was used to
  analyze the data (Table 1).
• The combinations of extruder barrel temperature,
  extruder screw speed, fish content and feed
  moisture content used for SEM study are indicated
  in Table 2.

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TABLE 1ACTUAL AND CODED LEVELS OF EXPERIMENTAL
DESIGN
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TABLE 2LEVEL COMBINATIONS FOR SEM OBSERVATIONS
Note. The SEM photographs were taken where one
process variable was at its maximum and minimum
values and the other three were at the center
point of the rotatable experimental design in
order to study the effect of each individual
process variable on microstructure.
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Extrusion Cooking

• Calculated amount of fish and rice flour was
  mixed well, adjusted to a desired level of
  moisture content of the mixture by adding water,
  and was extruded.
• The extrudate strands were dried at 60-65 C for
  2-2.5 h.
• The dry extrudate containing moisture content
  within 7-15 (w.b) were kept in sealed
  polyethylene pouches and stored in airtight
  containers for the moisture content to
  equilibrate.
• The samples were periodically checked and found
  to vary between 7 and 9 moisture content (w.b)
  and were used for further analysis.

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Scanning Electron Microscope

• Small quantity of dry extruded rods, dried under
  vacuum to moisture content 5-6 (d.b) were
  cryo-fractured by liquid nitrogen immersion
  technique.
• This method consisted of dipping the extrudate
  rods in liquid nitrogen for 2-3 minutes and the
  frozen extruded rods were broken into 2-5 mm
  sizes, and cut into sections of sharp edges.
• The sections were collected and lyophilized to
  make them free of moisture and a silver paste was
  used to mount the samples on the stud.
• The samples were then coated with gold in vacuum
  using a sputter coater and examined at 20 kV with
  JEOL JSM-5800 scanning microscope (made in
  Japan). Each SEM photograph was observed at 1000X
  magnification.

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Results and Discussions
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Effect of extruder barrel temperature (C)
Fig. Q. SEM photograph of the extruded product at
an extruder barrel temperature of 200 C,
extruder screw speed of 90 rev/min, fish content
of 25 and feed moisture content of 40
Fig. R. SEM photograph of the extruded product at
an extruder barrel temperature of 100 C,
extruder screw speed of 90 rev/min, fish content
of 25 and feed moisture content of 40
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Observations

• At 200 C of extruder barrel temperature (Fig. Q)
  the product became porous and fibrous with
  network of holes after solidification at ambient
  temperatures.
• Sudden drop of pressure and vaporization of
  superheated entrapped water resulted in porous
  and fibrous product and the starch embedded in
  the protein matrix quickly solidified retaining
  partially open-celled structure resulting in a
  low density product.
• The high extruder barrel temperature resulted in
  extensive protein denaturation and fiber
  formation and resulted in the fiber strands
  getting cemented with starch material and leading
  to cross-linking.
• Product extruded at 100 C of extruder barrel
  temperature (Fig. R) resulted in no holes and
  resulted in compact structure.

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Effect of extruder screw speed (rev/min)
Fig. S. SEM photograph of the extruded product at
an extruder barrel temperature of 150 C,
extruder screw speed of 110 rev/min, fish content
of 25 and feed moisture content of 40
Fig. T. SEM photograph of the extruded product
at an extruder barrel temperature of 150 C,
extruder screw speed of 70 rev/min, fish content
of 25 and feed moisture content of 40
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Observations

• In case of Fig. S where the extruder screw speed
  was maximum at 110 rev/min, there was quicker
  movement of the material inside the extruder
  giving less time for the material to gelatinize
  and expand more, resulting in minimum holes and
  smooth structure.
• In Fig. T where the extruder screw speed was 70
  rev/min the material had been in the barrel for
  longer time resulting in more fragmentations.

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Effect of fish content ()
Fig. U. SEM photograph of the extruded product at
an extruder barrel temperature of 150 C,
extruder screw speed of 90 rev/min, fish content
of 45 and feed moisture content of 40
Fig. V. SEM photograph of the extruded product at
an extruder barrel temperature of 150 C,
extruder screw speed of 90 rev/min, fish content
of 5 and feed moisture content of 40
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Observations

• In case of Fig. U the maximum fish content of 45
  resulted in fibrous compact matrix with minimum
  holes and reasonable expansion and greater
  homogeneity.
• The higher amount of protein present in extruder
  feed favored in formation of unbroken fibers,
  better organization and orientation
• Incase of Fig. V where the fish content was 5
  resulted in cracks due to less compaction,
  insufficient binding and cross linking at the
  time of extrusion.

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Effect of feed moisture content ()
Fig. W. SEM photograph of the extruded product at
an extruder barrel temperature of 150 C,
extruder screw speed of 110rev/min, fish content
of 25 and feed moisture content of 60
Fig. X. SEM photograph of the extruded product at
an extruder barrel temperature of 150 C,
extruder screw speed of 90 rev/min, fish content
of 25 and feed moisture content of 20
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Observations

• Figures W and X shows the effect of feed moisture
  content at 60 and 20 respectively.
• A moisture content of 60 resulted in more holes
  than that at 20 . The increase in moisture
  content contributed to more holes. Low fat
  content and increased water content lowered the
  protein concentration, where the involvement of
  protein in the matrix formation got reduced. This
  resulted in the matrix of low protein content and
  less dense matrix.
• At feed moisture content of 60 and 20 cracks
  were observed, but cracks were more pronounced at
  lower moisture content indicating less cross
  linking and binding.
• Surface structure appeared almost the same except
  at higher moisture content the expansion ratio
  was more compared to lower moisture content,
  which showed a compact matrix.
• At higher moisture content the product appeared
  to be more multi layered and less compacted.

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Conclusions

• SEM photographs revealed that a high extruder
  barrel temperature of 200 C effected the
  formation of porous and fibrous product due to
  starch gelatinization with network of holes
  whereas the product extruded at 100C was more
  compact.
• A high barrel temperature of 200C during
  extrusion cooking resulted in extensive protein
  denaturation and fiber formation and the fiber
  strands got cemented with starch material leading
  to cross-linking.
• At high barrel temperature there was sudden drop
  of pressure and vaporization of superheated
  entrapped water and when the extrudate leaves the
  die, the starch embedded in the protein matrix
  quickly solidifies and retains the partially
  open-celled structure and produced a low density
  product.
• Extruder screw speed of 110 rev/min helped in
  formation of smooth texture, whereas low extruder
  screw speed of 70 rev/min resulted in more
  fragmentations.
• High protein levels due to maximum fish content
  of 45 favored greater fiber formation, better
  organization and orientation than that with low
  protein level.
• Feed moisture content of 60 and 20 induced
  cracks in the texture, but cracks were more
  pronounced at lower moisture content indicating
  less cross linking and binding.
• Low fat content and increased water content
  lowered the protein concentration, where the
  involvement of protein in the matrix formation
  got reduced and resulted in the matrix of low
  density and reduced hardness.

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• Thank you