Acid Milk Gels

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Acid Milk Gels
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Lucey and Singh (1998)

• Yoghurt
• Stirred
• Set
• Fermented milk beverages (ayran)

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• Glucono-?-lactone
• Hydrolyzed to Gluconic acid
• Reduce pH and simulate fermentation
• pH depends on concentration
• Bacteria
• Slow growth
• Slow pH change

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• pH 5.0-5.5
• CCP dissolve
• Caseins liberated into serum phase
• T dependent
• 30 C none, 4C 40 liberated
• Aggregation at pH 4.6
• No change in micelle size
• Swelling

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• Heating
• gt70C, whey protein denaturation
• Association with k-casein
• Hydrophobic and intermolecular disulphide bonds
• Micellar integration and increase in particle
  size
• Little effect on CCP and the release of caseins
• Micelle-like CCP-depleted casein particles
• Hydrophobic interactions important

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Study of gelation

• Rheological measurements
• Measurement of number and strength of bonds
• Dynamic testing
• Gelation properties
• Microstructure
• Electron microscopy
• Confocal laser scanning microscopy

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• Rheological measurements
• Oscillatory strain or stress applied and
  resulting stress or strain measured
• G elastic or storage modulus, energy stored per
  oscillation cycle
• G Viscous or loss modulus, energy dissipated
  as heat per cycle
• Loss tangent (tan ?) G/G, relaxation of bond

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• Can follow gelation with small deformation
  studies
• During gelation G increases rapidly
• Can compare G values of different samples
• In gels resistance against deformation is
  proportional to the number of contact points per
  cross section of the network
• G is higher in heated acid gels
• Crosslinking or bridging by whey proteins
• A high tan ? may increase susceptibility of the
  bonds and strands in the gel to break or relax
  facilitating more rearrangements

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• Can measure strength of a gel by large
  deformation tests
• Yield stress in yoghurt
• Shear stress at fracture
• increased with decreasing gelation T
• decreased in heated (75?C) milk gels
• increased with further treatment (85?C)
• decreased again above 85?C

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• Microstructure
• Coarse particulate network of casein particles
  linked together in clusters, chains and strands
  with pores containing aqueous phase
• High gelation T, low protein large pores
• Heated milk gels highly branched,
  interconnectivity of aggregates, straightening of
  strands
• Unheated milk gels large protein clusters,
  irregular, bent clusters

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• Permeability
• Can calculate experimentally with a formula
• Rearrangements in a gel cause formation of pores
• Not affected by heating, the number and size of
  the largest pores in gels are similar

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• Acid milk gels Particle gel
• Fractal scaling at level of clusters
• any of various extremely irregular curves or
  shapes for which any suitably chosen part is
  similar in shape to a given larger or smaller
  part when magnified or reduced to the same size
• Spherical aggregates form clusters
• Then these clusters can aggregate further
• During and after gelation cluster aggregations
  observed (continued fusion and rearrangement of
  bonds and strands)

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• Unheated milk gels
• particle rearrangements during gelation
• dense clusters of casein particles
• lower G compared to heated milk gels
• Heated milk gels
• higher pH at gelation
• firmer gel
• whey protein denaturation
• not only complexing with casein but also
  bridging casein particles
• pI 5.3, aggregation due to reduction in
  charge and exposure of hydrophobic groups
• branched microstructure
• casein cannot form larger clusters due to
  hinderance by whey proteins on the surface

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• Appearance
• Yoghurt made from milk heated 90C/30 min
• grainy
• Yoghurt made from milk heated 80-85C/30 min
• smooth and firm-bodied
• Rearrangement of the network after gelation
• Weaker gel
• Protein-protein junction broken and breaking of a
  strand

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• Whey separation
• Occur if gel network is damaged or structural
  rearrangements take place
• Syneresis
• shrinkage of the gel
• whey separation
• Spontaneous
• Forced (centrifugation, drainage)
• Prevention by increasing total solids or using
  stabilizers

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• Whey separation
• Rapid acidification
• High incubation temperatures
• Excessive heat treatment (gt85C, 30 min)
• Low solid content
• Agitation during gelation
• Low acid production
• Containers with sloping walls or excessive height
  to width ratio

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• Textural Defects
• Firm texture
• high solids, stabilizers, low gelation T
• Weak body
• low solids, low heat treatment, low acidity,
  high gelation T
• Lumpiness
• large protein aggregates due to excessive acid
  production at high incubation T, excessive amount
  of starter

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• Total solid content (16-18)
• SMP (1-3), WPC, Na-caseinate, stabilizers
• Concentration
• Evaporation
• UF or RO
• Homogenization
• improve consistency not as much as heat
• reduce whey separation
• inert filler as native fat globule
• Homogenized fat globules associate with casein
  and whey protein and interact with protein

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• Heat treatment
• Increase in firmness, viscosity
• Redution in gelation time and increase in
  gelation pH
• But more brittle
• UHT
• Less firm and viscous
• Low incubation T better
• But long fermentation time

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