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DEPARTMENT OF FOOD ENGINEERING
UNIVERSITY OF AGRICULTURE, FAISALABAD
Processing and food structure
2
Freezing of foods
o Water is the medium causing most deleterious reactions
o Removal or immobilization of water, when possible, is highly desirable
o It is also well known that rate of deleterious reactions decreases exponentially
by decreasing temperature
o Freezing involves simultaneous immobilization of water and decrease in
temperature
o Seems to be the perfect preservation technique for foods
o Ice crystallization causes extensive microstructural changes inside organized
tissues
o When freezing occurs at moderately fast rate, water molecules arrange
themselves into hexagonal crystallization unitsUNIVERSITY OF AGRICULTURE FAISALABAD – UAF 3
Freezing of foods
o Hexagonal crystallization units is the only form of ice of importance in foods
o Other possible forms are irregular dendrites and coarse and evanescent
spherulites
o Slow freezing (at a rate of 1 ºC/min) causes ice crystals to grow in extracellular
locations
o This causes the formation of large crystals, dislocation of water, shrunken
appearance of cells and reduced quality
o Rapid cooling produces small crystals, uniform crystallization and
comparatively superior quality
o Recrystallization or “Ostwald ripening” during frozen storage is the most
important quality loss in frozen foods 4
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 5
Hexagonal
Dendrites Spherulites
Freezing of foods
o Recrystallization involves enlargement of large crystals at the expense of
smaller ones
o At above freezing temperatures, proportion of frozen water decreases and
smaller crystals melt
o Conversely, as the temperature decreases, water being frozen is deposited on
the surface of large crystals having more stable energy state
o Hence, total number of crystals decreases and mean crystal size increases
o Such recrystallization in meat leads to physical disruption of the tissues and
denaturation of proteins
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 6
Structuring water at subfreezing temperature (Ice cream)
o Ice cream is a good example of a restructured food
o Processing of ice-cream is aimed at producing an edible texture
o Ice cream can be categorized as partly frozen foam containing
o 40-50% air by volume
o 10-14% fat by weight
o 12-15% sucrose
o 10-14% milk SNF
o 0.5% stabilizers and emulsifiers
o 45-55% water
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 7
Structuring water at subfreezing temperature (Ice cream)
o The continuous phase of the foam is composed of a highly concentrated
unfrozen aqueous solution
o This is actually a solid amorphous glass of soluble milk salts, lactose and
added sugars
o Continuous phase also incorporates dispersed colloidal solids (casein, salts,
stabilizers) and a lipid phase
o Ice crystals (another coarsely dispersed phase) occupy a major portion of
space between air cells
o Two main determinants of structure and texture are
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 8
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 9
Structuring water at subfreezing temperature (Ice cream)
o Freezing process particularly the rate of freezing
o Addition of two additives i.e. stabilizers and emulsifiers
o Freezing can be done by three methods
o Traditional process of employing ice and salt as refrigerant
o Batch freezing process employing an industrial refrigerant; takes 10-30 min to
reach a satisfactory state for packaging and hardening
o Continuous freezing process employing an industrial refrigerant; completes the
freezing process in approximately 30 seconds
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 10
Structuring water at subfreezing temperature (Ice cream)
o Nucleation and growth of ice crystals is a time dependent phenomena
o Rapid freezing promotes the creation of many ice crystal nuclei of small
dimension
o This has great impact on organoleptic evaluation of ice cream
o Traditional and batch processes produce ice crystals of large sizes 60-70 µm
or even greater
o Continuous process produces crystals in the range of 25-30 µm
o Thresh hold for organoleptic detection of ice crystals in ice cream is between
65-70 µm
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 11
Structuring water at subfreezing temperature (Ice cream)
o Crystals continue to grow throughout storage and distribution of ice cream
o Especially when it suffers significant freeze-thaw or heat shock cycles
o Many high molecular weight polysaccharide gums are used in ice cream
formulation
o These include locust bean gum, guar gum, carboxymethyl cellulose,
carrageenan etc.
o Primary purpose of these stabilizers is to promote and maintain smooth
texture produced by rapid freezing
o They make it possible by controlling ice crystallization during storage
o Their ability to hydrate and hold large quantities of water protects the ice
cream from heat shock
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 12
Structuring water at subfreezing temperature (Ice cream)
o Structural preservation of frozen systems can also be related to storage
temperature and mobility of water
o During freezing of sugar solution, a glass transition temperature is reached at
which solution becomes maximally freeze concentrated (𝑇𝑔
′) below which no
more water will freeze in time frame of storage
o In glassy region, serum phase exists as a high viscosity amorphous solid; no
ice crystal growth can occur due to kinetic constraints
o In rubbery state above glass transition line, serum phase becomes mobile and
reactive; ice crystals growth can occur in time frames of food storage
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 13
Structuring water at subfreezing temperature (Ice cream)
o Emulsifiers contribute to structure in a different way compared to stabilizers
o Reduce the amount of protein adsorbed per surface area of fat at the time of
homogenization
o Reduce the fat serum interfacial tension faster than proteins
o When mixture is subjected to high rate of shear in dynamic freezing; fat
globules have less physical protection from coalescence; emulsion partially
destabilizes
o Internal matrix of fat is produced offering the product many beneficial
attributes similar to whipping of heavy cream and formation of stable foam
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 14
Structuring water at subfreezing temperature (Ice cream)
o Fat destabilization needs to be optimized; if process proceeds too far
o Depress overrun
o Gives the product a greasy mouthfeel
o Becomes visibly evident in the form of butter particles
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 15
Structuring water at subfreezing temperature (Ice cream)
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 16
Milling and crushing
o Flour milling is an example of controlled destruction of the microstructure and
exploitation of the mechanical properties of food material for technological
applications
o Milling attempts to achieve as completely as possible a separation of
endosperm, bran and germ
o The process is difficult due to presence of a crease in the wheat grain and
strong adhesion between aleuron layer in bran and endosperm
o Milling involves many grinding steps that combine shearing, scraping and
compression
o Fluted rolls are used to break open the grain and scrap the endosperm from
the bran through shearing
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 17
Milling and crushing
o Smooth rolls compress the endosperm reducing the particle size
o Increases in moisture contents (15-17 %) during conditioning or tempering
causes two effects
o The bran becomes tougher and more ductile
o Endosperm becomes mellower and more brittle
o The germ is a separate structure that is easily segregated almost intact from
the rest of the grain
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 18
Dough formation and baking
o Dough formation and baking is a polymer based restructuring operation
o It involves participating polymers from native structure and modification of them
o Dough formation starts by mixing basic ingredients including wheat flour, sugar, salt
and water
o It is then fermented at 30 ºC and then inserted into baking pans for proofing
o Proofing provides carbon dioxide for leavening, modifies dough proteins and imparts
desirable rheological properties
o Heating in oven produces gelatinization of starch, coagulation of proteins and
desirable permanent structure of crust and crumb
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 19
Dough formation and baking
o When water is added to the flour, water insoluble proteins hydrate and form
gluten; an elastic and cohesive mixture of two types of proteins i.e. glutenins
and gliadin
o The glutenin fraction is tougher, less easily stretched and behaves as a
cohesive elastic solid
o Gliadin fraction has less cohesiveness and elasticity; performs like a viscous
liquid and gives extensible properties
o A liquid aqueous phase formed by albumins, globulins, soluble starch and
pentosans separates from gluten phase
o The rate of gas production during fermentation or decomposition of baking
powder affects the growth and size of gas cells
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 20
Dough formation and baking
o Heating in oven leads to following phenomena and microstructural changes
o Vaporization and transfer of water vapor from interior to outside
o Denaturation of proteins and starch gelatinization
o Extension of elastic starch-gluten matrix , expansion of air bubbles and
vaporization, all generating porous inner microstructure
o Melting of fat crystals resulting in lubrication
o Formation of crust
o The structure of fresh bread is altered after baking by staling, a process
dominated by starch
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 21
Dough formation and baking
o Initially it is the amylose fraction and amylose/lipid complexes that tend to
crystallize
o The later stages of staling and aging of bread result from the crystallization of
amylopectin; moisture migration from amorphous to crystalline regions
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 22
Crystallization and tempering of chocolate
o Crystallization of fats has significant technological importance
o These complex molecules can exist in more than one crystalline forms stable
over a certain range of temperature; called polymorphism
o Several factors determine the polymorphic form assumed after crystallization;
purity, temperature, rate of cooling, presence of nuclei and type of solvent
o Transformation of one polymorphic form to another can take place even in
solid state without melting
o Natural fats having long chains interact with each other to form crystals
o Three basic crystal forms can be distinguished
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 23
Crystallization and tempering of chocolate
o 𝛼: has lowest molecular packing, has fastest growth rate, is least stable and has
lowest melting point
o 𝛽′
: more densely packed than 𝛼, is more stable, has higher melting point
o 𝛽: has highest molecular packing, is most stable, has highest melting point
o Chocolate is formed by continuous phase of cocoa butter in which crystalline sugar
and cocoa solids are dispersed in presence of emulsifier lecithin
o Fat phase is made of liquid fat having fat crystals inclusion
o Cocoa butter is 94-96% triglycerides, formed mainly by palmitic, stearic and oleic
acids
o Cocoa butter forms six crystal types numbering from i to vi according to increasing
melting points 24
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 25
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 26
Crystallization and tempering of chocolate
o In chocolate, sugar has to be ground to particle size smaller than 25 um so
that it does not feel gritty in the mouth
o If it is too fine, more fat is needed to coat individual particles and can
transform to amorphous phase
o Amorphous sugar is more hygroscopic than crystalline and picks up moisture,
accompanying flavors and odors
o During storage, amorphous converts back to crystalline and releases moisture
o Tempering of chocolate is a process that aims at formation of large number of
smallest possible fat crystals and high melting point polymorphic form
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 27
Crystallization and tempering of chocolate
o Properly tempered chocolate posses good flow properties, sets rapidly on
cooling and presents a high gloss
o Proper tempering involves warming a partially crystallized blend to about 32 ºC followed
by rapid chilling and storage at 16 ºC
o Blooms can occur when tempering is not done properly
o Fat bloom, a well known defect, imparts a white or gray appearance to the
surface
o Bloom corresponds to VI polymorphic form and is characterized by large
needle like fat crystals
o Improper tempering causes contraction and formation of crevices
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 28
Crystallization and tempering of chocolate
o This scatters the incident light giving rise to whitish appearance
o Bloom can also develop in perfectly tempered chocolate by two mechanisms
o Continuous temperature fluctuations
o Partial melting during storage and rapid crystallization into VI form
UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 29

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Effects of processing on food microstructure

  • 1. DEPARTMENT OF FOOD ENGINEERING UNIVERSITY OF AGRICULTURE, FAISALABAD
  • 2. Processing and food structure 2
  • 3. Freezing of foods o Water is the medium causing most deleterious reactions o Removal or immobilization of water, when possible, is highly desirable o It is also well known that rate of deleterious reactions decreases exponentially by decreasing temperature o Freezing involves simultaneous immobilization of water and decrease in temperature o Seems to be the perfect preservation technique for foods o Ice crystallization causes extensive microstructural changes inside organized tissues o When freezing occurs at moderately fast rate, water molecules arrange themselves into hexagonal crystallization unitsUNIVERSITY OF AGRICULTURE FAISALABAD – UAF 3
  • 4. Freezing of foods o Hexagonal crystallization units is the only form of ice of importance in foods o Other possible forms are irregular dendrites and coarse and evanescent spherulites o Slow freezing (at a rate of 1 ºC/min) causes ice crystals to grow in extracellular locations o This causes the formation of large crystals, dislocation of water, shrunken appearance of cells and reduced quality o Rapid cooling produces small crystals, uniform crystallization and comparatively superior quality o Recrystallization or “Ostwald ripening” during frozen storage is the most important quality loss in frozen foods 4
  • 5. UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 5 Hexagonal Dendrites Spherulites
  • 6. Freezing of foods o Recrystallization involves enlargement of large crystals at the expense of smaller ones o At above freezing temperatures, proportion of frozen water decreases and smaller crystals melt o Conversely, as the temperature decreases, water being frozen is deposited on the surface of large crystals having more stable energy state o Hence, total number of crystals decreases and mean crystal size increases o Such recrystallization in meat leads to physical disruption of the tissues and denaturation of proteins UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 6
  • 7. Structuring water at subfreezing temperature (Ice cream) o Ice cream is a good example of a restructured food o Processing of ice-cream is aimed at producing an edible texture o Ice cream can be categorized as partly frozen foam containing o 40-50% air by volume o 10-14% fat by weight o 12-15% sucrose o 10-14% milk SNF o 0.5% stabilizers and emulsifiers o 45-55% water UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 7
  • 8. Structuring water at subfreezing temperature (Ice cream) o The continuous phase of the foam is composed of a highly concentrated unfrozen aqueous solution o This is actually a solid amorphous glass of soluble milk salts, lactose and added sugars o Continuous phase also incorporates dispersed colloidal solids (casein, salts, stabilizers) and a lipid phase o Ice crystals (another coarsely dispersed phase) occupy a major portion of space between air cells o Two main determinants of structure and texture are UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 8
  • 9. UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 9
  • 10. Structuring water at subfreezing temperature (Ice cream) o Freezing process particularly the rate of freezing o Addition of two additives i.e. stabilizers and emulsifiers o Freezing can be done by three methods o Traditional process of employing ice and salt as refrigerant o Batch freezing process employing an industrial refrigerant; takes 10-30 min to reach a satisfactory state for packaging and hardening o Continuous freezing process employing an industrial refrigerant; completes the freezing process in approximately 30 seconds UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 10
  • 11. Structuring water at subfreezing temperature (Ice cream) o Nucleation and growth of ice crystals is a time dependent phenomena o Rapid freezing promotes the creation of many ice crystal nuclei of small dimension o This has great impact on organoleptic evaluation of ice cream o Traditional and batch processes produce ice crystals of large sizes 60-70 µm or even greater o Continuous process produces crystals in the range of 25-30 µm o Thresh hold for organoleptic detection of ice crystals in ice cream is between 65-70 µm UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 11
  • 12. Structuring water at subfreezing temperature (Ice cream) o Crystals continue to grow throughout storage and distribution of ice cream o Especially when it suffers significant freeze-thaw or heat shock cycles o Many high molecular weight polysaccharide gums are used in ice cream formulation o These include locust bean gum, guar gum, carboxymethyl cellulose, carrageenan etc. o Primary purpose of these stabilizers is to promote and maintain smooth texture produced by rapid freezing o They make it possible by controlling ice crystallization during storage o Their ability to hydrate and hold large quantities of water protects the ice cream from heat shock UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 12
  • 13. Structuring water at subfreezing temperature (Ice cream) o Structural preservation of frozen systems can also be related to storage temperature and mobility of water o During freezing of sugar solution, a glass transition temperature is reached at which solution becomes maximally freeze concentrated (𝑇𝑔 ′) below which no more water will freeze in time frame of storage o In glassy region, serum phase exists as a high viscosity amorphous solid; no ice crystal growth can occur due to kinetic constraints o In rubbery state above glass transition line, serum phase becomes mobile and reactive; ice crystals growth can occur in time frames of food storage UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 13
  • 14. Structuring water at subfreezing temperature (Ice cream) o Emulsifiers contribute to structure in a different way compared to stabilizers o Reduce the amount of protein adsorbed per surface area of fat at the time of homogenization o Reduce the fat serum interfacial tension faster than proteins o When mixture is subjected to high rate of shear in dynamic freezing; fat globules have less physical protection from coalescence; emulsion partially destabilizes o Internal matrix of fat is produced offering the product many beneficial attributes similar to whipping of heavy cream and formation of stable foam UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 14
  • 15. Structuring water at subfreezing temperature (Ice cream) o Fat destabilization needs to be optimized; if process proceeds too far o Depress overrun o Gives the product a greasy mouthfeel o Becomes visibly evident in the form of butter particles UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 15
  • 16. Structuring water at subfreezing temperature (Ice cream) UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 16
  • 17. Milling and crushing o Flour milling is an example of controlled destruction of the microstructure and exploitation of the mechanical properties of food material for technological applications o Milling attempts to achieve as completely as possible a separation of endosperm, bran and germ o The process is difficult due to presence of a crease in the wheat grain and strong adhesion between aleuron layer in bran and endosperm o Milling involves many grinding steps that combine shearing, scraping and compression o Fluted rolls are used to break open the grain and scrap the endosperm from the bran through shearing UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 17
  • 18. Milling and crushing o Smooth rolls compress the endosperm reducing the particle size o Increases in moisture contents (15-17 %) during conditioning or tempering causes two effects o The bran becomes tougher and more ductile o Endosperm becomes mellower and more brittle o The germ is a separate structure that is easily segregated almost intact from the rest of the grain UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 18
  • 19. Dough formation and baking o Dough formation and baking is a polymer based restructuring operation o It involves participating polymers from native structure and modification of them o Dough formation starts by mixing basic ingredients including wheat flour, sugar, salt and water o It is then fermented at 30 ºC and then inserted into baking pans for proofing o Proofing provides carbon dioxide for leavening, modifies dough proteins and imparts desirable rheological properties o Heating in oven produces gelatinization of starch, coagulation of proteins and desirable permanent structure of crust and crumb UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 19
  • 20. Dough formation and baking o When water is added to the flour, water insoluble proteins hydrate and form gluten; an elastic and cohesive mixture of two types of proteins i.e. glutenins and gliadin o The glutenin fraction is tougher, less easily stretched and behaves as a cohesive elastic solid o Gliadin fraction has less cohesiveness and elasticity; performs like a viscous liquid and gives extensible properties o A liquid aqueous phase formed by albumins, globulins, soluble starch and pentosans separates from gluten phase o The rate of gas production during fermentation or decomposition of baking powder affects the growth and size of gas cells UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 20
  • 21. Dough formation and baking o Heating in oven leads to following phenomena and microstructural changes o Vaporization and transfer of water vapor from interior to outside o Denaturation of proteins and starch gelatinization o Extension of elastic starch-gluten matrix , expansion of air bubbles and vaporization, all generating porous inner microstructure o Melting of fat crystals resulting in lubrication o Formation of crust o The structure of fresh bread is altered after baking by staling, a process dominated by starch UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 21
  • 22. Dough formation and baking o Initially it is the amylose fraction and amylose/lipid complexes that tend to crystallize o The later stages of staling and aging of bread result from the crystallization of amylopectin; moisture migration from amorphous to crystalline regions UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 22
  • 23. Crystallization and tempering of chocolate o Crystallization of fats has significant technological importance o These complex molecules can exist in more than one crystalline forms stable over a certain range of temperature; called polymorphism o Several factors determine the polymorphic form assumed after crystallization; purity, temperature, rate of cooling, presence of nuclei and type of solvent o Transformation of one polymorphic form to another can take place even in solid state without melting o Natural fats having long chains interact with each other to form crystals o Three basic crystal forms can be distinguished UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 23
  • 24. Crystallization and tempering of chocolate o 𝛼: has lowest molecular packing, has fastest growth rate, is least stable and has lowest melting point o 𝛽′ : more densely packed than 𝛼, is more stable, has higher melting point o 𝛽: has highest molecular packing, is most stable, has highest melting point o Chocolate is formed by continuous phase of cocoa butter in which crystalline sugar and cocoa solids are dispersed in presence of emulsifier lecithin o Fat phase is made of liquid fat having fat crystals inclusion o Cocoa butter is 94-96% triglycerides, formed mainly by palmitic, stearic and oleic acids o Cocoa butter forms six crystal types numbering from i to vi according to increasing melting points 24
  • 25. UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 25
  • 26. UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 26
  • 27. Crystallization and tempering of chocolate o In chocolate, sugar has to be ground to particle size smaller than 25 um so that it does not feel gritty in the mouth o If it is too fine, more fat is needed to coat individual particles and can transform to amorphous phase o Amorphous sugar is more hygroscopic than crystalline and picks up moisture, accompanying flavors and odors o During storage, amorphous converts back to crystalline and releases moisture o Tempering of chocolate is a process that aims at formation of large number of smallest possible fat crystals and high melting point polymorphic form UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 27
  • 28. Crystallization and tempering of chocolate o Properly tempered chocolate posses good flow properties, sets rapidly on cooling and presents a high gloss o Proper tempering involves warming a partially crystallized blend to about 32 ºC followed by rapid chilling and storage at 16 ºC o Blooms can occur when tempering is not done properly o Fat bloom, a well known defect, imparts a white or gray appearance to the surface o Bloom corresponds to VI polymorphic form and is characterized by large needle like fat crystals o Improper tempering causes contraction and formation of crevices UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 28
  • 29. Crystallization and tempering of chocolate o This scatters the incident light giving rise to whitish appearance o Bloom can also develop in perfectly tempered chocolate by two mechanisms o Continuous temperature fluctuations o Partial melting during storage and rapid crystallization into VI form UNIVERSITY OF AGRICULTURE FAISALABAD – UAF 29

Editor's Notes

  1. Nucleation, the initial process that occurs in the formation of a crystal from a solution, a liquid, or a vapour, in which a small number of ions, atoms, or molecules become arranged in a pattern characteristic of a crystalline solid, forming a site upon which additional particles are deposited as the crystal grows.
  2. The gas holding capacity of the foam was assessed by overrun