crystallization in foods
DESCRIPTION
PowerPoint presentation of my lecture on the topic of food crystallization.TRANSCRIPT
Dr
Abd
Kar
im A
lias,
200
5
1
CRYSTALLIZATION IN FOODS
PRESENTED BY:
Prof. Abd Karim Alias
Dr
Abd
Kar
im A
lias,
200
5
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What do chocolate, butter, margarine, ice cream, sugar, and hard candy have in common?
Dr
Abd
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im A
lias,
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} Bagaimana hablur terbentuk dalam sistem makanan?
} Apakah faktor-faktor yang mempengaruhi proses penghabluran?
} Apakah signifikan proses penghabluran semula terhadap kualiti makanan dan bagaimana cara untuk mengawalnya?
} Bagaimana pembentukan hablur (dan proses penghabluran semula) dapat mempengaruhi kualiti dan kestabilan makanan?
CRYSTALLIZATION IN FOODS
Dr
Abd
Kar
im A
lias,
200
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REFERENCES
} R.W. Hartel (Editor) (2001). Crystallization in Foods. Aspen Publishers,Gaithersburg, Maryland. [TP370.5 H328]
} Hartel, R.W. & Shastry, A.V. (1991). Sugar crystallization in food products. Critical Reviews in Food Science and Nutrition 1(1), 49-112.
CRYSTALLIZATION
Dr
Abd
Kar
im A
lias,
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} The structural elements in foods (air cells, crystals, fat globules) play an important role in determining quality & shelf stability. It is the structure that provides the desired rheological properties of the food (hardness, stiffness, snap, etc) & contributes to organoleptic properties (melt-down rate, cooling effect, etc.)
} Crystalline phase is one of the most important structural elements in many foods Ú it has impact on quality, texture, and shelf life.
CRYSTALLIZATION
Dr
Abd
Kar
im A
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} For example, the crystalline microstructure of cocoa butter in chocolate provide the desired snap when a molded bar is broken
} The main components that form crystalline phase are water (ice), sugars (and sugar alcohol), lipids, and starches
} Other components may crystallize: salts, organic acids (eg. citric acid), proteins, and emulsifiers
CRYSTALLIZATION
Dr
Abd
Kar
im A
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} Crystallization is a term that describes several different phenomena related to the formation of a crystalline lattice structure.
} When a crystal forms, the molecules orient themselves in a regular pattern, or lattice structure. In contrast, molecules in amorphous material are more randomly oriented & move about freely
CRYSTALLIZATION
Dr
Abd
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im A
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} Crystal lattices are composed of an ordered layering of molecules, with the ordering dependent on the nature of the forces & the interactions between individual molecules—freedom of motion is greatly restricted in a crystal lattice.
CRYSTALLIZATION
Dr
Abd
Kar
im A
lias,
200
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9
} Control of crystallization in foods is an important aspect of food quality.
Crystallization may be employed as:
} Separation process (eg. sugar refining, fat fractionation) or
} To provide a certain texture within the food itself (eg. ice cream, fondant, chocolate)
CRYSTALLIZATION
Dr
Abd
Kar
im A
lias,
200
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Arrangement of atoms in a simple cubic unit cell. The rest of this crystal can be built by stacking identical unit cells next to one another along the x, y and z axes to form a crystal lattice.
Unit Cell
CRYSTALLIZATION
Dr
Abd
Kar
im A
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Shape of lactose crystals grown at different supersaturation
Arrangement of sucrose molecules within crystal lattice
structure
CRYSTAL FORMATION
Dr
Abd
Kar
im A
lias,
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Four steps in crystallization typically include:
} generation of supersaturated state
} nucleation—the formation of crystalline lattice structure from solution or melt
} growth—subsequent growth of nuclei until equilibrium is attained
} recrystallization —a reorganization of the crystalline structure to a lower energy state, generally without any further change in the amount of crystalline phase volume
CRYSTAL FORMATION
Dr
Abd
Kar
im A
lias,
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} Any solutes (eg. sugar) will have its own solubility value (Cs) in solution; solutions with solute contents below this concentration are undersaturated & will allow further dissolution of solute crystals.
} Supersaturation can be expressed as the difference between the concentration of a saturated solution (Css) and the equilibrium solubility (Ceq): ∆C = Css – Ceq
CRYSTAL FORMATION
Dr
Abd
Kar
im A
lias,
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�A
�C Labile (supersaturated — unstable)
Stable ���(Under saturated solution)
Suga
r con
centr
ation
Temperature
CRYSTAL FORMATION
Metastable (supersaturated)
�B
Solubility curve
Dr
Abd
Kar
im A
lias,
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} When solute contents > solubility concentration (Cs), ⇒ supersaturated solutions.
} Any crystals present in a supersaturated solution will not dissolve, but, rather, will grow larger as the solution attempts to approach its equilibrium condition.
} Supersaturated solutions are necessary for the crystallization processes of nucleation & growth.
CRYSTAL FORMATION
Dr
Abd
Kar
im A
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How to achieve supersaturation? } (1) for melt, cooling below the melting point of a
compound (eg. water, fats) or
} (2) for solution, producing a concentration in solution greater than some solubility concentration, Cs, (sugars, salt, etc)—can be achieved by: § heating the solvent prior to dissolving the solute so
that high concentration solutions can be made; cooling this solution below the saturation temperature results in supersaturated solution (A º B)
CRYSTAL FORMATION
Dr
Abd
Kar
im A
lias,
200
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How to achieve supersaturation?.... § evaporate solvent from a solution until the
saturation concentration has been exceeded, and (A º C)
§ adding a second solvent in which a solute is insoluble (shift the solubility curve down).
CRYSTAL FORMATION
Dr
Abd
Kar
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Solubility-Supersolubility Diagram�
A B
C
Labile (supersaturated — unstable)
Stable ���(Under saturated solution)
Metastable (supersaturated)
Suga
r con
centr
ation
Temperature
CRYSTAL FORMATION�
Dr
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} Once the solution/melt has been supersaturated, there is a thermodynamic driving force for crystallization, i.e., the molecules tend toward the crystalline state to lower the energy level of the system.
} …BUT not every supersaturated solution will crystallise during the time available for observation …(metastable solution)
CRYSTAL FORMATION
Dr
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} METASTABILITY – a region/zone where nucleation does not occur within the time scale of the process
} In metastable zone, crystal may grow (by adding seed crystal – heterogeneous nucleation) but nucleation is negligible.
} In labile zone, nucleation occurs spontaneously.
} In the stable zone, nucleation will not occur – any crystals put in this solution will dissolve.
CRYSTAL FORMATION
Dr
Abd
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} NUCLEATION – molecules in the liquid state rearrange & eventually form into a stable cluster that organizes into a crystalline lattice.
} The ordered arrangement of molecules in the lattice involves a release of latent heat as the phase change occurs.
CRYSTAL FORMATION
Dr
Abd
Kar
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NUCLEATION
PRIMARY SECONDARY
Homogeneous Heterogeneous
CRYSTAL FORMATION
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} PRIMARY NUCLEATION – formation of crystal nuclei from a solution that contained no preexisting crystals } Homogeneous nucleation is the formation of nuclei within a homogeneous fluid ─ rarely occurs in practical food processing conditions.
} Heterogeneous nucleation is initiated by contact with foreign particles and surfaces.
} SECONDARY NUCLEATION refers to the formation of crystal nuclei due to the presence of existing crystals
CRYSTAL FORMATION
Dr
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CRYSTAL FORMATION
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Crystal - WallCrystal - Crystal
Crystal - Stirrer
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Crystal - WallCrystal - Crystal
Crystal - Stirrer
Fluid shear
New nuclei
Fluid shear
New nuclei
Secondary Nucleation
Dr
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CRYSTAL FORMATION
Rate of Nucleation
Homogeneous
Heterogeneous
Driving force
Nuc
leat
ion
rate
SolubilityMelting point
Glass transition
A B C
Homogeneous
Heterogeneous
Driving force
Nuc
leat
ion
rate
SolubilityMelting point
Glass transition
A B C
Dr
Abd
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} CRSYTAL GROWTH – nuclei that form can grow to larger size based on the available supersaturation in the solution. The extent of this growth depends on the magnitude of supersaturation remaining (DC) in solution after nucleation has occurred.
} Growth continues until all of the available supersaturation has been depleted & the system approaches an equilibrium in phase volume, which depends on temperature & composition of the system.
CRYSTAL FORMATION
Dr
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} For example, if a highly concentrated sugar syrup is allowed to crystallize, the concentration of the remaining solution phase decreases until it reaches the equilibrium concentration (solubility) for that temperature & system.
} Once this solubility concentration is attained, crystallization ceases; the system has attained a stable phase volume of crystalline material.
CRYSTAL FORMATION
Dr
Abd
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CRYSTAL FORMATION
Kinetics of Growth Rate
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force
Melting pointSolubility Glass
transition
Gro
wth
rate
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force Glass transition
Gro
wth
rate
A B C
Nucleation
Growth
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force
Melting pointSolubility
Melting pointSolubility Glass
transition
Gro
wth
rate
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force Glass transition
Gro
wth
rate
A B C
Nucleation
Growth
Dr
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} RECRYSTALLIZATION – Once equilibrium in phase volume has been attained, changes still may take place in the crystalline structure during long-term storage, especially where temperature & RH are likely to vary over time —this changes within the crystalline structure is to minimize the energy of the system further (ie approach global equilibrium).
} Two examples of recrsytallization: coarsening of ice crystals in frozen food during storage & formation of fat bloom in chocolate.
CRYSTAL FORMATION
Dr
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The complexity of the molecular structure may lead to differences in the crystal lattice structure. Such cases may be due to:
ü crystallization of different types of isomers of a molecule present in solution; example: enantiomers (D-glucose & L-glucose) and anomers (a-lactose & b-lactose);
TYPES OF CRYSTAL LATTICE STRUCTURE
Dr
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ü crystallization of hydrates, where one or more molecules of water are incorporated into the crystal lattice along with the molecule of the crystallizing species;
ü polymorphism, where complex molecules crystallize in more than one molecular arrangement depending on conditions.
TYPES OF CRYSTAL LATTICE STRUCTURE
Dr
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} Conversion between enantiomeric & hydrated forms may occur, depending on the conditions. Example: a lactose solution can crystallize into either a-lactose monohydrate at T <93.5 ºC or b-lactose (anhydrous) at T>93.5 ºC.
} If a lactose solution is concentrated & cooled rapidly, an amorphous lactose glass can be formed. In addition, an anhydrous a-lactose crystal can be formed, either by drying a-lactose monohydrate crystals or by direct crystallization from nonaqueous solvent.
TYPES OF CRYSTAL LATTICE STRUCTURE
Dr
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33 Different anomeric forms of α- and β-lactose crystals
TYPES OF CRYSTAL LATTICE STRUCTURE
Dr
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34 Different forms of lactose crystals
TYPES OF CRYSTAL LATTICE STRUCTURE
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} POLYMORPHISM – some materials exhibit the ability to crystallize into multiple crystal structures, depending on the arrangement of the molecules, eg. Sorbitol forms 4 polymorphs (α, Δ, β and γ).
} Polymorphism in lipids — Lipids (triglycerides or fats in particular) forms 3 main polymorphs, ie. α, β’ and β, although in some cases there may be additional polymorph (a very low stability γ form) & subclasses within a main polymorphic type (β g β1 and β 2).
TYPES OF CRYSTAL LATTICE STRUCTURE
Dr
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} The arrangement of the lipid molecules (angle of orientation & packing arrangement) in the crystal lattice structure is different for each polymorph.
} Differences in physical properties between lipid polymorphs arise due to differences in molecular arrangement. The least stable forms have lower density, lower melting point, and lower heat of fusion than the more stable forms, since the molecules in the less stable forms are not arranged in their lowest energy conformation.
TYPES OF CRYSTAL LATTICE STRUCTURE
Dr
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Free energy change during nucleation for different lipids polymorphs
Nucleation rate of lipid polymorphs
TYPES OF CRYSTAL LATTICE STRUCTURE
Dr
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Transformation of polymorphic form of lipid crystals
Monotropic polymorphism of lipids
α βʹ′
β
LIQUID
Tem
pera
ture
Stab
ility
, mel
ting
poin
t
Fat Oil Solidification heat rapid cooling
SOLID
Dr
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} Lipids exhibit monotropic polymorphism, where the less stable polymorphs form first and then transform to the more stable polymorphs
} Rapid cooling generally results in formation of less stable polymorphs since the more stable polymorphs require longer times to allow proper orientation of the molecules into the most stable lattice structure.
Polymorphism in lipids ….cont’
Dr
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} Therefore, the less stable polymorphs (γ, α, β’) generally crystallize before the most stable β-polymorph. After crystallization has taken place, the less stable polymorphs then transform to the more stable forms. Eventually, the most stable polymorph will form, although this may take many months in food products.
} Some fats, eg milk fat & palm oil does not crystallize into the β form (due to the molecular complexity of the fatty acids); the β’-polymorph is the most stable form found in these fats.
Polymorphism in lipids ….cont’
Dr
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} control of the correct number, size (and distribution), shape, and polymorph of crsytals is required to provide the desired processing characteristics, quality (texture, flavor, etc), appearance, and shelf stability of the product
Several circumstances for controlling crystallization in foods: (1) Control to produce desired crystalline structure
CONTROLLING CRYSTALLIZATION
Dr
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} A certain crystal size distribution is desired in sugar products (sugar-frosted cereals, fondants, panned candies, caramels, etc), frozen foods (ice cream, frozen desserts, etc), and lipid-based products (tempering of chocolate, butter, margerine, shortening, etc)—in these products, numerous small crystals are desired to give the desired smooth texture.
(1) Control to produce desired crystalline structure …cont’
CONTROLLING CRYSTALLIZATION
Dr
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} In other processes, fewer but larger crystals are desired, eg, for efficient separation during refining of sugars, fractionation of fats & freeze concentration
} Controlling crystal size distribution (by controlling the kinetics of nucleation & growth) is vitally important—through proper choice of formulation & processing conditions
(1) Control to produce desired crystalline structure …cont’
CONTROLLING CRYSTALLIZATION
Dr
Abd
Kar
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Schematic diagram of nucleation and growth rate curve
Crystal size distributions generated at different points on the crystallization rate curves (at point A, B and C)
CONTROLLING CRYSTALLIZATION
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force
Melting pointSolubility Glass
transition
Gro
wth
rate
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force Glass transition
Gro
wth
rate
A B C
Nucleation
Growth
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force
Melting pointSolubility
Melting pointSolubility Glass
transition
Gro
wth
rate
Homogeneous
Heterogeneous
Driving force Glass transitionDriving force Glass transition
Gro
wth
rate
A B C
Nucleation
Growth
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Few nuclei formed at points A and C
Many nuclei formed at points B
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Few nuclei formed at points A and C
Many nuclei formed at points B
Dr
Abd
Kar
im A
lias,
200
5
45
} In some products, crystallization is undesired even though, thermodynamically, there is a driving force (supersaturation or subcooling) for crystallization to occur.
(2) Control to prevent crystallization
CONTROLLING CRYSTALLIZATION
Dr
Abd
Kar
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Some examples: § The unfrozen phase of ice cream, which is
supersaturated in lactose, may crystallize during storage, causing a sandy defect in the ice cream
§ In caramel, the aqueous phase contains sucrose & lactose, both of which may be supersaturated during storage at room temperature & develop crystalline structure Ú graining defect
(2) Control to prevent crystallization CONTROLLING CRYSTALLIZATION
Dr
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CONTROLLING CRYSTALLIZATION
Two ways to prevent crystallization: (i) the system must have sufficiently low mobility
(high viscosity), or (ii) the crystallizing species must be inhibited by
addition of other molecules, eg, by addition of corn syrup to prevent graining in caramels
(2) Control to prevent crystallization
Dr
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(3) Control of change in crystalline structure (recrystallization)
§ Ice crystals in ice cream & frozen foods gradually increase in size during storage, leading to coarse/sandy texture & loss of structural integrity upon thawing
§ Changes in lipid crystals (due to polymorphic transformation)—bloom formation in chocolate, grainy texture in margerine)
CONTROLLING CRYSTALLIZATION
Some examples:
Dr
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} The overall process is called recrystallization — typically driven by reequilibrium to either a lower surface energy or lower internal energy without a change in the amount of crystalline mass
(3) Control of change in crystalline structure (recrystallization)
CONTROLLING CRYSTALLIZATION ….Cont’
Dr
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Mechanisms of recrystallization
§ Ostwald ripening —conditions may exists where small crystals are in undersaturated conditions at the same time that large crystals are in supersaturated conditions ⇒ small crystals dissolve/melt & large crystals grow
§ Accretion —crystals that are very close to each other tend to grow together to form one large crystal; this mechanism is very important in some foods, particularly those with high numbers of very small crystals in close proximity (eg. ripening of ice crystals in ice cream)
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§ Polymorphic transformation —lipids undergo a polymorphic change as less stable crystal forms change to a more stable forms. The subsequent release of additional latent heat during the transformation may cause substantial problems in the product
Mechanisms of recrystallization ….Cont’
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§ Melt/refreeze —A change in the phase volume of crystalline material occurs as temperatures fluctuate during storage, accompanied by a change in crystalline structure (size, number, shape, etc); the smallest crystals are most sensitive to temperature increases & may eventually disappear or dissolve/melt away.
Mechanisms of recrystallization ….Cont’
Dr
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Increase in mean size and decrease in total crystal numbers with time of
crystallization
Typical changes in crystal size distribution in ice cream during storage
Mechanisms of recrystallization
Crystal sizeFr
eque
ncy
(%)
Storage time
Crystal sizeFr
eque
ncy
(%)
Storage time
Time
Mea
n cr
ysta
l siz
e
Num
ber o
f cry
stal
s
N0
r0
Time
Mea
n cr
ysta
l siz
e
Num
ber o
f cry
stal
s
N0
r0
Dr
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Factors affecting control of crystallization
§ The rates of heating, cooling, holding times & temperatures, evaporation of waters and agitation all can impact crystallization
§ Example: In production of grained caramel and fudge, the rate of cooling to a desired crystallization temp determines the extent & type of nucleation.
§ If crystallization is not desired,cooling must be sufficiently rapid that the material passes through the crystallization zone before nucleation occurs.
(1) Heat & mass transfer rates
Dr
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§ For lipids, suitable time-temperature protocol (tempering) must be chosen to maximize formation of the desired polymorphic form
§ If large crystals are desired for efficient separation, then nucleation must occur at a higher temperature (or lower supersaturation) so that fewer nuclei are formed.
Factors affecting control of crystallization
(1) Heat & mass transfer rates ….Cont’
Dr
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§ In products where massive nucleation is desired, it is important to provide sufficient agitation to promote maximal nucleation.
§ For example: during processing of fondant, the sugar mass is cooled to the desired crystallization temperature & then worked extensively in a beater to promote massive crystallization ⇒ results in production of many small crystals & produces a smooth fondant
Factors affecting control of crystallization
(1) Heat & mass transfer rates ….Cont’
Dr
Abd
Kar
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§ Addition of various soluble components in a formulation typically inhibits crystallization, eg. The use of glucose syrups in hard candy inhibits graining of the sugar mass during processing & storage
§ Addition of gums & proteins to ice cream reduce the rates of crystallization & the subsequent coarsening of the ice crystals
Factors affecting control of crystallization
(2) Product formulation
Dr
Abd
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Changes in crystalline structure may take place after processing or during storage. This could be due to: } Incomplete crystallization has not been
achieved during processing, ie, maximum phase volume of the crystalline phase may not be attained prior packaging
} Thermodynamic ripening effect (recrystallization) that can cause significant changes in crystalline structure & size distribution
(3) Post-processing effects Factors affecting control of crystallization
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} Equilibration in moisture content (during storage) due to moisture migration within different components of the same product (eg, between the sauce and crust of a frozen pizza) or through equilibration with the storage air conditions
(3) Post-processing effects
Factors affecting control of crystallization ….Cont’
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Control of crystallization during processing
} Tempering process involves holding chocolate at the appropriate temperature for the proper time to form the desired crystalline structure.
} In commercial operations, the chocolate mass is pumped through a heat exchanger where the temperature is controlled in different sections to ensure proper crystallization.
Example: Tempering of chocolate
CONTROLLING CRYSTALLIZATION
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} Molten chocolate at about 50°C is cooled rapidly in the first section to ~26ºC to initiate crystallization of the β’ polymorph. The rate of nuclei formation at these conditions is rapid so that many crystals are formed, which results in an increase in viscosity of chocolate mass. } Since the β-polymorph is desired, the temperature of the chocolate mass is increase to cause melting of the unstable form & transformation on the more stable β-polymorph.
CONTROLLING CRYSTALLIZATION
Example: Tempering of chocolate Control of crystallization during processing
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62 Typical time-temperature protocol during tempering of
chocolate
CONTROLLING CRYSTALLIZATION
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Chocolate bloom
CONTROLLING CRYSTALLIZATION