Water Properties and Crystallization

Water's Physical States

• Water can undergo liquid-to-solid-state transitions and form ice.
• The melting point of pure water (at 1.00 atm) is 273.16 K.

Ice Properties

• Ice has an "open structure" that is less dense (91%) than the liquid state.
• Ice will float instead of sink due to its lower density.
• Ice formation can disrupt tissue structure.

Hydrogen Bonds

• When ice melts, only 15% of hydrogen bonds are broken.
• This explains why the latent heat of fusion is small compared to the latent heat of vaporization.
• Water is a highly structured liquid.

Crystal Structure

• The hexagonal pattern of water molecules in ice is shown in Figure 2.2.
• The lattice is loosely built, resulting in a high specific volume.

Melting and Crystallization

• When ice melts, some hydrogen bonds between water molecules break, and the molecules become more densely packed in the liquid state.
• Crystallization occurs when water is cooled below its melting temperature (0°C at atmospheric pressure).

Phase Diagram

• Water can exist in three phases: solid, liquid, and gas.
• The phase diagram (Figure 2.3) shows the temperatures and pressures under which each phase exists.

Nucleation

• Nucleation is the first step in crystallization, where the nuclei of ice crystals form.
• Ice crystals will grow around the nuclei.
• Pure water can be cooled below its melting temperature (0°C at atmospheric pressure), a process called supercooling or undercooling.
• Heterogeneous nucleation occurs when a small ice crystal or solute is added to the supercooled liquid, causing the temperature to rise to the melting temperature.

Crystallization Process

• Nucleation is the first step in crystallization, where nuclei of ice crystals form.
• Ice crystals grow around the nuclei.

Supercooling and Undercooling

• Pure water can be cooled below its melting temperature (0°C at atmospheric pressure).
• This phenomenon is termed supercooling or undercooling.

Types of Nucleation

• Heterogeneous nucleation occurs when a small ice crystal or solute is added to the supercooled liquid, causing the temperature to rise to the melting temperature.
• This type of nucleation involves nucleation on the surface of tiny foreign molecules.
• Homogeneous nucleation, on the other hand, involves the formation of tiny nuclei of pure water molecules.
• Most nucleation is heterogeneous rather than homogeneous.

Crystallization Process

• Nucleation is the first step in crystallization, where nuclei of ice crystals form.
• Ice crystals grow around the nuclei.

Supercooling

• Pure water can be cooled below its melting temperature (0°C at atmospheric pressure), a process known as supercooling or undercooling.
• Adding a small ice crystal or solute to supercooled liquid initiates nucleation, causing the temperature to rise to the melting temperature.

Nucleation Types

• Heterogeneous nucleation occurs on the surface of tiny foreign molecules, which is the most common type of nucleation.
• Homogeneous nucleation involves the formation of tiny nuclei of pure water molecules, which is less common than heterogeneous nucleation.

Crystallization Process

• Crystallization involves two stages: nucleation and crystal growth
• The heat of fusion is released during crystallization

Factors Affecting Crystallization Speed

• The speed of crystallization depends on the removal of heat from the area
• At low degrees of supercooling (temperatures close to freezing), crystallization is slow
• At high degrees of supercooling (very low temperatures), crystallization proceeds rapidly

Temperature and Crystallization

• At temperatures close to the freezing point, crystal growth is a predominant factor
• At temperatures much lower than the freezing point, nucleation rate takes over
• The number and size of ice crystals formed depend on the speed of crystallization

Crystallization and Crystal Formation

• The number of nuclei formed during crystallization affects the resulting crystal size and number: many nuclei lead to many small crystals, while fewer nuclei result in fewer, larger crystals.

Seeding and Crystal Structure

• Seeding, or adding nuclei to liquids prior to freezing, increases the number of initial nuclei and promotes a finer crystalline structure.

Importance of Crystalline Structure in Frozen Foods

• The crystalline structure in frozen foods is crucial to food scientists.
• The preservation effect in frozen foods comes from low temperatures, not from ice formation.
• Ice formation in cellular foods and gels can be detrimental, causing: • Freeze concentration • Cellular disruption

Freeze Concentration

• Freeze concentration involves the concentration of non-aqueous constituents in the unfrozen phase during freezing
• Water from solution is transferred into ice crystals, concentrating non-aqueous constituents in a diminished quantity of unfrozen water
• Properties of the unfrozen phase change, including pH, titratable acidity, ionic strength, viscosity, freezing point, surface and interfacial tension, and redox potential

Effects on Chemical Reactions

• Freeze concentration can increase reaction rates due to concentration of reactants
• However, low temperature decreases reaction rates, counteracting the effect of freeze concentration
• Non-enzymatic reactions, including oxidative reactions and protein insolubility, have been observed to increase in rate during freezing
• Freeze-induced rate acceleration occurs a few degrees below the initial freezing point of the sample

Effects on Enzymatic Reactions

• Some enzymatic reactions increase during freezing due to cellular disruption and de-compartmentalization
• Cellular disruption occurs because water converts to ice, increasing in volume by 9%
• This disruption allows for mixing of enzyme and substrate, catalyzing reactions

Recrystallization and Food Stability

• Recrystallization can occur in foods during frozen storage, leading to the growth of ice crystals
• Temperature fluctuations are especially damaging to frozen foods, as smaller crystals melt first and then crystallize on larger crystals when temperature decreases
• Freezing can be detrimental to foods due to ice formation, but can also improve stability by decreasing reaction rates
• Freezing should be carried out to minimize potential damage to foods

Enzymatic Reactions during Freezing

• Some enzymatic reactions increase during freezing due to cellular disruption and de-compartmentalization.
• Cellular disruption occurs because water conversion to ice results in a 9% increase in volume.

Effects of Ice Formation on Cellular Tissue

• Increases in volume during ice formation lead to de-compartmentalization, disrupting cellular tissue.
• De-compartmentalization allows for mixing of enzyme and substrate, catalyzing reactions.

Recrystallization in Frozen Foods

• Recrystallization often occurs in foods during frozen storage.
• Ice crystals tend to enlarge due to temperature fluctuations.
• Smaller ice crystals melt first during temperature increases, and then crystallize on the surface of larger crystals when the temperature decreases.

Temperature Fluctuations and Food Damage

• Temperature fluctuations are especially damaging to frozen foods.
• Freezing can be detrimental to foods due to associated ice formation.

Importance of Proper Freezing Techniques

• Freezing should be carried out to minimize potential damage to foods.
• Low temperatures employed during freezing can improve stability by decreasing reaction rates.

Enzymatic Reactions during Freezing

• Some enzymatic reactions increase during freezing due to cellular disruption and de-compartmentalization.
• Cellular disruption occurs because water conversion to ice results in a 9% increase in volume.

Effects of Ice Formation on Cellular Tissue

• Increases in volume during ice formation lead to de-compartmentalization, disrupting cellular tissue.
• De-compartmentalization allows for mixing of enzyme and substrate, catalyzing reactions.

Recrystallization in Frozen Foods

• Recrystallization often occurs in foods during frozen storage.
• Ice crystals tend to enlarge due to temperature fluctuations.
• Smaller ice crystals melt first during temperature increases, and then crystallize on the surface of larger crystals when the temperature decreases.

Temperature Fluctuations and Food Damage

• Temperature fluctuations are especially damaging to frozen foods.
• Freezing can be detrimental to foods due to associated ice formation.

Importance of Proper Freezing Techniques

• Freezing should be carried out to minimize potential damage to foods.
• Low temperatures employed during freezing can improve stability by decreasing reaction rates.

Enzymatic Reactions during Freezing

• Some enzymatic reactions increase during freezing due to cellular disruption and de-compartmentalization.
• Cellular disruption occurs because water conversion to ice results in a 9% increase in volume.

Effects of Ice Formation on Cellular Tissue

• Increases in volume during ice formation lead to de-compartmentalization, disrupting cellular tissue.
• De-compartmentalization allows for mixing of enzyme and substrate, catalyzing reactions.

Recrystallization in Frozen Foods

• Recrystallization often occurs in foods during frozen storage.
• Ice crystals tend to enlarge due to temperature fluctuations.
• Smaller ice crystals melt first during temperature increases, and then crystallize on the surface of larger crystals when the temperature decreases.

Temperature Fluctuations and Food Damage

• Temperature fluctuations are especially damaging to frozen foods.
• Freezing can be detrimental to foods due to associated ice formation.

Importance of Proper Freezing Techniques

• Freezing should be carried out to minimize potential damage to foods.
• Low temperatures employed during freezing can improve stability by decreasing reaction rates.

Recrystallization in Frozen Foods

• Recrystallization occurs in foods during frozen storage due to ice crystal growth.
• Ice crystals enlarge over time, particularly when temperature fluctuations occur.
• When temperature increases, smallest ice crystals melt first, and water from these crystals crystallizes on larger crystals when temperature decreases.
• Temperature fluctuations are detrimental to frozen foods, causing damage.

Effects of Freezing on Food Stability

• Freezing can improve food stability by decreasing reaction rates at low temperatures.
• However, freezing can also be detrimental to foods due to associated ice formation.
• Freezing should be carried out in a way that minimizes potential damage to foods.

Glass Transition

• Glass transition is a critical concept in food science, improving stability by forming a glassy state in foods.
• Glass formation is distinct from freezing, and glasses can be formed in frozen foods, but it's a separate process from ice formation.

Characteristics of Glass

• Glasses are amorphous materials, unlike crystalline solids like ice.
• Glasses are very viscous liquids, with a disordered structure of the liquid state immobilized into a solid-like, disordered glassy state.
• Glasses can flow, as seen in old cathedral stained glass windows where the panes are thicker at the bottom.

Glass Transition Temperature (Tg)

• The glass transition temperature (Tg) is the temperature at which a liquid or aqueous system forms a glass.
• Tg is characterized by very high apparent viscosities (> 10^5 N/m^2).
• Tg is lower than the melting temperature (Tm) and crystallization temperature.

Role of Moisture Content

• Moisture content plays a crucial role in glass transitions.
• At high moisture contents, the system can go through a rubbery state before reaching the glassy state, which can be detrimental to food stability.
• Glasses can form in partially frozen foods, where the remaining water transforms into a glass when the Tg is reached.

Cryoprotection and Glass Formation

• The presence of substances like polymeric substances or low molecular weight molecules (e.g., sugars) can help protect cellular integrity during freezing, known as cryoprotection.
• Glasses can form at subfreezing temperatures, as well as during drying or extrusion, where water is removed quickly from foods.

Effect of Water on Glass Transition

• Increases in water content decrease the glass transition temperature in foods, related to the plasticizing effect of water.
• Water plasticizers weaken intermolecular forces, increasing the plasticity and flexibility of food polymers.

Plasticization

• Plasticization decreases the glass transition temperature Tg, making a glassy material "rubbery" or "leathery".
• Plasticization can be achieved through various means, including water, lower molecular weight sugars, and temperature changes.
• Plasticization can destabilize a product, forcing it into the rubbery state, leading to stickiness, structural collapse, and increased diffusion rates.