gelatin - college of health and human sciences - purdue … · · 2013-08-19gelatin structure...
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Gelatin
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Gelatin source• Collagen (hides, bones, tendons)• In mammals, collagen comprises
one-third of total body protein– In the US, principally from pigskins
• Sub-unit is called tropocollagen (Type III collagen)– 2800Å x 15Å, it is a rigid rod.– MW approximately 300,000 daltons.
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Gelatin structure
TropocollagenEach individualprotein is a lefthanded helix
Three left-handed helices combine into a right-handed super triple helix (each of these is a gelatin molecule)
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Collagen--the source of gelatin
Image courtesy of http://ep.llnl.gov/msds/TechPrep/Activity-IV.html
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False colored SEM of connective tissue-individual collagen fibers can be seen
London Research Institute
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Amino acids in gelatin• Gelatin contains some 18 distinct amino
acids• Of these 1/4 are basic or acidic
– Lys, arg, his, asp, glu
• 1/3 is glycine or (occasionally) alanine• 1/4 is proline or hydroxyproline• Not a nutritionally complete protein.
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Amino acids in gelatin
Image courtesy of www.gelatin.com/structure.htm
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Basic repeat unit in gelatin• Gly-pro—X or Gly-X-Hyp
• where X is acidic or basic; often hydroxyproline (Hyp)– Gelatin is the only protein, except for
elastin, known to contain hydroxyproline.
– Generally, low in S-containing amino acids
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Role of proline and hydroxyproline
• These cyclic imino acids cause kinks or breaks in what would otherwise be a helical structure.
NO
IN OUT
This promotescollagen helix stability
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Gelatin manufacture• Acid extracted (type A) gelatin,or• Alkaline extracted (type B) gelatin
– In US, mostly alkaline extracted• The extracted gelatin is gelled in
thin layers• Dried• Ground into gelatin powder (MW
about 100,000)
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Gelatin gelation
• Gelatin is dispersed in water that is not too warm (35-40o C, to prevent lumping)
• Then cooled slowly and allowed to form a gel
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Gelatin gelation
• One part gelatin can trap 99 parts of water
• Gel texture is related to gel structure and can be measured in a variety of ways.
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Gel structureDenaturation
H e a t
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Gel structureAssociation and formation of junction zones
This is a gel!
trapped water
trapped water
trapped water
trapped water
trapped water
trapped water
Junction zones
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Gel structure
• Note that forming gels is something only LARGE(macromolecules) molecules can do!
• Anything that reduces molecular size, reduces the ability of that molecule to form a gel.
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Conditions for gelation
• Concentration– 0.6-0.7% minimum– Typically, gelatin with desirable
textural properties has 1.0-1.5% gelatin
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Conditions for gelation
• Temperature– Must get to 35o C for dispersion– Usually, pure gelatin is dispersed in
cold water, then boiling water is added
– Need to cool to 35o C for gelation
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Conditions for gelation
• Effect of rate of cooling
– Faster rate, thermally less stable gel• Weaker junction zones in gel
– Slower rate, thermally more stable gel• Stronger junction zones in gel
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Conditions for gelation• Gelatin molecular weight (MW)
– The lower the MW, the less strong the gel will be. This is true for virtually any gel forming polymer.
• pH– Little effect of pH on gel strength in the
range 4-8 (typical food range)– Will gel best at isoelectric point (pH 4.8-5.0
for Type B, alkaline extracted gelatin)
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Conditions for gelation-Effect of other ingredients
• Sugar– Increasing concentration of sugar
(sucrose) will generally decrease gel strength
– This is believed to be due to competition of the sugar for available water as well as a general plasticizing effect of sugar on food gels
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Conditions for gelation-Effect of other ingredients
• Certain proteolytic enzymes will hydrolyze gelatin and destroy the gel structure
• Some of the enzymes and their sources are– Papain--papaya– Ficin--figs– Bromelin--pineapple
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Proteolytic enzyme action
Enzyme
Enzyme
Enzyme
Enzyme
High molecular weight~100,000 daltons
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Proteolytic enzyme action A collection oflow molecularweight pieces -no good for agel
Avg MW 2000-5000 daltons