7. Crystallization

Question 1

What is crystallization

Answer 1

the process by which solid crystals of a solute are formed from a solution

Question 2

The liquid phase remaining after the formation of crystals is called

Answer 2

mother liquor

Question 3

purpose of crystallization in the food industry

Answer 3

1. recovery of crystalline products (eg sugar, glucose, lactose, citric acid)
2. removal of undesirable components (eg oils, wax, other high melting point components)
3. modification of certain food products to obtain desirable structure
4. confectionery (crystallization of fat in chocolate and margarine, candying of fruits)

Question 4

undesirable changes in food due to crystallization

Answer 4

1. crystallization of sugar in jams and preserves
2. crystallization of lactose in ice cream, causing “sandiness”
3. recrystallization of cocoa butter in chocolate in the form of white crystals (bloom) on the surface

Question 5

what can be used to trigger crystallization

Answer 5

crystal seeds/seed blooms

Question 6

conditions for crystallization to occur

Answer 6

supersaturated solutions

Question 7

how to bring a solution to supersaturation

Answer 7

• removal of solvent by evaporation, membrane separation or freeze concentratiob
• cooling of solution
• changing pH or ionic strength
• addition of second solvent miscible with solution, to reduce solubilty of solute
• chemical complexation/precipitation

Question 8

what is the main methods of crystallization in the food industry

Answer 8

1. removal of solvent by evaporation (concentration)
2. cooling

Question 9

two mechanism of the crystallization process

Answer 9

1. nucleation
2. crystal growth

Question 10

what is nucleation

Answer 10

• formation of crystalline lattice structure
• solute molecules dispersed in solvent start to gather into clusters
• the stable clusters constitute the nuclei, which is stable if the clusters reach a critical size
• if clusters not stable, they dissolve

Question 11

what is crystal growth

Answer 11

• formation and continuous addition of crystals until equilibrium stage of solution is reached
• crystal growth is subsequent growth of nuclei that have achieved critical cluster size

Question 12

what binds crystals of organic compounds

Answer 12

weak forces such as van der waals and weak dipoles

Question 13

what is polymorphism

Answer 13

crystallization of organic comounds in a number of different forms

Question 14

characteristics of supersaturation

Answer 14

• not thermodynamically stable
• is temperature dependednt

Question 15

equation for degree of supersaturation/supersaturation ratio

Answer 15

β = C/Cs

C = actual concentration (kg solute/kg solvent)
Cs = saturation concentration

super saturated β >1
saturated β = 1
not saturated β<1

Question 16

characteristcis of meta stable zone

Answer 16

• crystallization occur if propagated (eg addition of crystal seed)
• slow growth
• limited number of crystaks

Question 17

characteristics of labile zone

Answer 17

spontaneous nucleation

Question 18

what is magma

Answer 18

mixture of crystals and mother liquor, contains crystals with different sizes

Question 19

what is ostwald ripening

Answer 19

when very small crystals are at equilibrium with supersaturated mother liquor, causing dissolution of small crystals and growth of large ones

Question 20

undesirable effect of ostwald ripening in ice cream

Answer 20

• changes in texture during storage, destruction of creamy texture
• fluctuations in storage temperature cause preferential melting of small ice crystals, followed by further growth of larger crystals

Question 21

crystallizers are also called…

Answer 21

pans

Question 22

types of pans operation mechanisms

Answer 22

1. batch
2. contunuous
3. vacuum/atmospheric

Question 23

what pans are used for large scale vs small scale crystallization

Answer 23

large: continuous
small: batch

Question 24

why is agitation important

Answer 24

sufficiently vigorous agitation helps facilitate mass transfer and good mixing

Question 25

how to avoid inversion and caramelization in sugar syrup making

Answer 25

application of vacuum to lower the boiling temperature

Question 26

principles of crystallization of sucrose

Answer 26

- final step in recovery of sugar from sugar cane or sugar beet - requires precise control, skill and experience - controlled parameters: temp, pressure, concentration (brix), purity, crysta;/liqui ratio, crystal mean size and shape and flow rates - monitored by digital image processing

Question 27

mechanism of crystallization of sucrose in large scale production

Answer 27

1. preparation of seed done in batch process 2. use both batch and continuous process (mixed process)

Question 28

how can multistage crystallization be beneficial

Answer 28

- as more sugar gets crystallized, the consistency increases; because crystal content increases mixing becomes more difficult, flow channels can get blocked, and crystal rupture due to friction can occur - decrease in purity of mother liquor interferes with crystal growth with multistage fermentation, first crystals separated and mother liquor send to next stage for further concentrating