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What is the temperature of pure water at 1.00 atm?
What is the temperature of pure water at 1.00 atm?
Why does ice float in liquid water?
Why does ice float in liquid water?
What is the percentage of hydrogen bonds broken when ice melts?
What is the percentage of hydrogen bonds broken when ice melts?
Why is water considered a highly structured liquid?
Why is water considered a highly structured liquid?
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What is the term for the process of cooling pure water below its melting temperature?
What is the term for the process of cooling pure water below its melting temperature?
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What is the term for the first step in crystallization, where ice crystal nuclei form?
What is the term for the first step in crystallization, where ice crystal nuclei form?
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What is the term for the type of nucleation that occurs on the surface of tiny foreign molecules?
What is the term for the type of nucleation that occurs on the surface of tiny foreign molecules?
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What is the shape of the lattice in ice, as shown in Figure 2.2?
What is the shape of the lattice in ice, as shown in Figure 2.2?
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why does ice have a specific volume?
why does ice have a specific volume?
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What is required to initiate nucleation in super cooled liquid water?
What is required to initiate nucleation in super cooled liquid water?
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What happens to the temperature of the super cooled liquid when nucleation begins?
What happens to the temperature of the super cooled liquid when nucleation begins?
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What is the most common type of nucleation that occurs in pure water?
What is the most common type of nucleation that occurs in pure water?
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What is the role of the nuclei in the crystallization process?
What is the role of the nuclei in the crystallization process?
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Why can pure water be cooled below its melting temperature?
Why can pure water be cooled below its melting temperature?
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What is the primary factor that determines the speed of crystallization?
What is the primary factor that determines the speed of crystallization?
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At what temperature does crystal growth occur rapidly?
At what temperature does crystal growth occur rapidly?
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What is the result of rapid crystallization?
What is the result of rapid crystallization?
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What happens to the rate of crystallization at low degrees of super cooling?
What happens to the rate of crystallization at low degrees of super cooling?
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At what temperature does nucleation rate take over?
At what temperature does nucleation rate take over?
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What is the primary reason for the preservation effect in frozen foods?
What is the primary reason for the preservation effect in frozen foods?
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What happens to the size of crystals in a system with many nuclei formed during crystallization?
What happens to the size of crystals in a system with many nuclei formed during crystallization?
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What is the purpose of seeding in crystallization?
What is the purpose of seeding in crystallization?
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What is the effect of ice formation in cellular foods and gels?
What is the effect of ice formation in cellular foods and gels?
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What determines the number of crystals formed in a system?
What determines the number of crystals formed in a system?
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What happens to the non-aqueous constituents during freezing?
What happens to the non-aqueous constituents during freezing?
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What is the effect of freeze concentration on reaction rates?
What is the effect of freeze concentration on reaction rates?
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What happens to the unfrozen phase during freezing?
What happens to the unfrozen phase during freezing?
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What is the result of cellular disruption during freezing?
What is the result of cellular disruption during freezing?
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Why do ice crystals tend to enlarge during frozen storage?
Why do ice crystals tend to enlarge during frozen storage?
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What happens to O2 and CO2 during freezing?
What happens to O2 and CO2 during freezing?
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What is the result of freeze induced rate acceleration?
What is the result of freeze induced rate acceleration?
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What is the cause of de-compartmentalization during freezing?
What is the cause of de-compartmentalization during freezing?
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What is the primary cause of cellular disruption during freezing?
What is the primary cause of cellular disruption during freezing?
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What happens to the reaction rates during freezing?
What happens to the reaction rates during freezing?
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What is the result of recrystallization during frozen storage?
What is the result of recrystallization during frozen storage?
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What is the outcome of de-compartmentalization during freezing?
What is the outcome of de-compartmentalization during freezing?
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Why are temperature fluctuations especially damaging to frozen foods?
Why are temperature fluctuations especially damaging to frozen foods?
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Why should freezing be carried out carefully?
Why should freezing be carried out carefully?
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What happens to the smallest ice crystals when there is an increase in temperature during frozen storage?
What happens to the smallest ice crystals when there is an increase in temperature during frozen storage?
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What is the effect of low temperatures on reaction rates during freezing?
What is the effect of low temperatures on reaction rates during freezing?
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Why is ice crystal formation detrimental to foods during freezing?
Why is ice crystal formation detrimental to foods during freezing?
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What happens to the water from the melted smallest crystals when the temperature is decreased during frozen storage?
What happens to the water from the melted smallest crystals when the temperature is decreased during frozen storage?
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Why are temperature fluctuations especially damaging to frozen foods?
Why are temperature fluctuations especially damaging to frozen foods?
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What is the main difference between ice and glass?
What is the main difference between ice and glass?
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When does glass formation occur in foods?
When does glass formation occur in foods?
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What is the characteristic of the glass transition temperature?
What is the characteristic of the glass transition temperature?
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What is the relationship between the glass transition temperature and the melting temperature?
What is the relationship between the glass transition temperature and the melting temperature?
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What is the role of moisture content in glass transitions?
What is the role of moisture content in glass transitions?
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What is the main difference between glass formation and ice formation?
What is the main difference between glass formation and ice formation?
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What is the characteristic of the glassy state?
What is the characteristic of the glassy state?
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What happens to the molten liquid when it reaches the glass transition temperature?
What happens to the molten liquid when it reaches the glass transition temperature?
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What happens to the remaining water in a food when it reaches the glass transition temperature?
What happens to the remaining water in a food when it reaches the glass transition temperature?
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What is the purpose of cryoprotection in food freezing?
What is the purpose of cryoprotection in food freezing?
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Why does the addition of small amounts of water to some foods destabilize the product?
Why does the addition of small amounts of water to some foods destabilize the product?
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What is plasticization in the context of food chemistry?
What is plasticization in the context of food chemistry?
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What is the effect of increasing the water content of a food on its glass transition temperature?
What is the effect of increasing the water content of a food on its glass transition temperature?
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What is the purpose of combining sorption isotherms and state diagrams in food chemistry?
What is the purpose of combining sorption isotherms and state diagrams in food chemistry?
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What is the result of freeze concentration in foods?
What is the result of freeze concentration in foods?
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What is the effect of water on the glass transition temperature of a material?
What is the effect of water on the glass transition temperature of a material?
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What is the result of a glass forming in a partially frozen food?
What is the result of a glass forming in a partially frozen food?
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What is the effect of cryoprotectants on the glass transition temperature?
What is the effect of cryoprotectants on the glass transition temperature?
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Study Notes
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.
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Description
This quiz covers the properties of water, including its transition from liquid to solid state, melting point, and crystalline structure. It also discusses the characteristics of ice and its effects on tissue structure.
