lec 11 transitions in foods Flashcards
Phase transitions in foods are governed by what?
their components
(Carbohydrates, Proteins, Lipids , Water)
The components of food exist in what states?
Liquid state
Solid crystalline state
Solid amorphous state
Solution
*Carbohydrates and proteins are present in the amorphous,
crystalline state or in solution.
* Lipids are often in various crystalline polymorphs or liquid.
what is Molecular mobility important for?
- Important factor for transitions
*The ability to molecules to move within a food system. - Molecular mobility influences diffusional transport, viscosity, electrical conductivity and glass transition.
- The mobility of dissolved molecules depends strongly on the water content.
- Amorphous polymers have higher mobility than crystalline polymers.
what does The amount of water influence in food transitions?
mobility and diffusion in a food matrix
* Water content above a critical value may cause changes in structure, chemical reactions, enzymatic activity etc.
* Most structural and phase transitions of foods are affected by water.
* Adsorption of water occurs under conditions where the vapour pressure is higher than the vapour pressure within the solids.
* Desorption occurs under the inverse conditions.
what does The vapour pressure describe?
the tendency of molecules to
“escape” a liquid or solid state.
what do Sorption isotherms show?
the amount of water adsorbed
At steady state we can define the water activity as?
p/p0=aw
Where p is the vapour pressure of the food and p0 is the vapour pressure of pure water.
Sorption isotherms
Sorption isotherms are graphical representations that depict the relationship between the equilibrium moisture content of a material and the relative humidity of the surrounding environment at a constant temperature.
Monomolecular layer of water
At low relative humidity (RH), materials may adsorb a monomolecular layer of water molecules onto their surface. This layer is often tightly bound to the material’s surface and represents the first stage of moisture uptake.
A few molecular layers of water
As relative humidity increases, additional layers of water molecules may be adsorbed onto the material’s surface, forming a thicker film. This stage typically occurs at moderate levels of RH.
Condensation of water in pores and capillaries
At higher relative humidity, water vapor may condense within the pores and capillaries of the material, leading to an increase in moisture content. This stage is characterized by the formation of liquid water within the material’s internal structure.
what is Hysteresis?
Hysteresis in the context of moisture sorption refers to the difference in moisture content observed in a material between two processes: adsorption and desorption, under the same relative humidity (RH) conditions. In simpler terms, when you increase the relative humidity to a certain level and then decrease it back to that same level, the amount of moisture retained by the material during the decreasing RH (desorption) phase may be different from the amount absorbed during the increasing RH (adsorption) phase.
Hysteresis in the context of moisture sorption refers to the difference in moisture content observed in a material between two processes: adsorption and desorption, under the same relative humidity (RH) conditions. In simpler terms, when you increase the relative humidity to a certain level and then decrease it back to that same level, the amount of moisture retained by the material during the decreasing RH (desorption) phase may be different from the amount absorbed during the increasing RH (adsorption) phase.
why is there Higher water content during desorption?
During desorption, the material may retain a higher moisture content compared to adsorption at the same relative humidity. This is often attributed to the energy barrier associated with removing water molecules from the material’s internal structure.
why is Wetting and de-wetting of polar domains = energetically unfavorable
Wetting and de-wetting refer to the process of water molecules entering and leaving polar domains or regions within the material. This process may be energetically unfavorable due to interactions between water molecules and polar functional groups present in the material.
what is Supersaturation of solutes (common in sugar-containing foods)?
the presence of solutes, such as sugars, in foods can lead to supersaturation of water vapor in the material. This occurs when the concentration of water vapor exceeds the equilibrium concentration at a given temperature and RH.
when do phase Transitions happen?
A change in the physical state resulting from a change in temperature or pressure.
* Can be reflected by changes in heat capacity, viscosity, textural characteristics etc.
* Foods are often meta-stable and exhibit time-dependent changes as equilibrium is approached.
what happens during Crystallization?
- Often: nucleation -> propagation-> maturation (perfection and/or growth)
- Nucleation occurs
- supersaturation due to solvent removal.
- decrease in temperature of a solution or a melt.
- Homogeneous nucleation (less common in foods)
- Heterogeneous nucleation (common in foods)
- Presence of solid impurities or surfaces
- Secondary nucleation
- In the presences of existing crystals.
- Needs external forces
- Common in sugar crystallization.
Nucleation:
The initial formation of tiny crystal nuclei in a supersaturated solution or melt. Nucleation can be triggered by factors such as supersaturation (excess solute in solution), decrease in temperature, or the presence of solid impurities or surfaces.
Supersaturation:
A state in which the concentration of a solute in a solution exceeds its equilibrium concentration at a given temperature. Supersaturation typically occurs when solvent is removed, such as during evaporation, or when the temperature of a solution or melt decreases.
Homogeneous nucleation:
Nucleation that occurs uniformly throughout a solution or melt without the presence of foreign particles or surfaces. Homogeneous nucleation is less common in foods due to the absence of perfectly pure solutions.
Heterogeneous nucleation:
Nucleation that occurs on the surface of solid impurities or foreign particles present in the solution or melt. Heterogeneous nucleation is common in foods because solid impurities or surfaces often serve as nucleation sites.
Secondary nucleation:
The formation of new crystal nuclei in the presence of existing crystals. Secondary nucleation is induced by external forces. Common in sugar crystallization
The amorphous state
a disordered arrangement of molecules in a solid or highly viscous material, where there is no long-range order or regular crystalline structure.
Properties of the amorphous state:
enhanced solubility, increased glass transition temperature, and improved stability against moisture absorption and chemical degradation.
how is the Rubbery state (viscous flow) transitioned to the glassy state (solid-like properties)
At higher temperatures, amorphous materials may exist in a rubbery state where they exhibit viscous flow behavior, resembling a soft, rubber-like consistency. As the temperature is lowered, the material undergoes a phase transition known as the glass transition, where it transitions from a rubbery state to a glassy state with solid-like properties. During the glass transition, the mobility of molecules decreases significantly, resulting in an increase in viscosity and a transition to a rigid, glassy state.
Glass transition temperature (Tg):
The glass transition temperature is the temperature at which an amorphous material undergoes a transition from a rubbery or viscous state to a glassy or solid-like state. Tg is a critical parameter that influences the mechanical, thermal, and barrier properties of amorphous materials. It is reversible and has time-dependent properties, meaning that the material’s behavior near Tg can change depending on factors such as heating rate, cooling rate, and aging.
what are glassy materials like?
- Glassy materials are solid-like but lack the molecular order of solids and are not at equilibrium.
- A common property of the glassy state is brittleness.
- The glassy state is of great importance to foods like potato crisps, cookies, extruded snacks etc.
Molecular mobility in glassy materials is highly restricted. The glassy state has a viscosity of approx. 1012 Pa s due to very slow structural relaxation times and mechanical changes.
If T is increased above the glass transition temperature (Tg ) a dramatic decrease in viscosity occurs.
what Effect does water have on Tg?
Water has a plasticizing effect and lowers Tg.
This is because water molecules act as plasticizers, increasing molecular mobility and reducing intermolecular forces within the material, thereby lowering the temperature at which the material transitions from a glassy to a rubbery state.
Some monosaccharides have low Tg -> foods with high sugar content are very sensitive to water at room T
Upon freezing, why are solutes excluded from the solid water
During the process of freezing, water molecules arrange themselves into a crystalline structure to form ice. As water freezes, the solutes in the solution are typically excluded from the ice crystals and remain in the unfrozen liquid phase, leading to an increase in solute concentration.
explain how The unfrozen phase can transition to a glassy state
The unfrozen liquid phase, which contains a higher concentration of solutes due to freezing, can undergo a phase transition to a glassy state under certain conditions. This glassy state occurs when the mobility of molecules within the liquid becomes restricted, typically at temperatures below the glass transition temperature (Tg). In this glassy state, the material exhibits solid-like properties, such as rigidity and lack of flow, despite being in a non-crystalline state.
what happens when Super-saturation occurs during freezing
Super-saturation refers to a state where the concentration of solutes in the solution exceeds their solubility limit at a given temperature. During freezing, the concentration of solutes in the unfrozen liquid phase may increase, leading to super-saturation. Under certain conditions, this can result in the spontaneous crystallization of solutes, forming crystals within the solution.
why can Rapid cooling diminish the amount of ice formed when mobility is reduced below Tg?
When a solution is rapidly cooled, the mobility of molecules within the liquid is reduced, potentially below the glass transition temperature (Tg). As a result, the formation of ice crystals may be hindered or diminished, as the molecules are unable to rearrange themselves into a crystalline structure. Instead, the unfrozen liquid phase may transition directly to a glassy state, leading to the formation of an amorphous solid with reduced crystallization.
when is Crystallization from the amorphous
state most rapid?
Most rapid for polymers and sugars with a high Tm/Tg
how does Crystallization from the amorphous
state Affects food quality and kinetics
- Sandiness in ice cream
- Caking in milk powders
- Staling of bread
what happens Below Tg
mobility is typically too low to allow for crystal formation
what is the difference between Glassy (i.e. amorphous) sugar and crystalline sugars
Glassy (i.e. amorphous) sugars are much more hygroscopic already at low relative humidity compared to crystalline sugars.
describe Glass formation
Rapid cooling from melt
Common for carbohydrates
Cooling faster than crystallization
Cooling of sugar syrups (melt) - example: hard sugar candies
Cooking extrusion - rapid cooling and dehydration of starch - example: crispy snacks, candy sprinkles
Water content important to ensure a glassy state!
Solvent removal ->Spray drying
example: dried dairy powders
what forms do Most triglycerides exist in?
in at least three crystalline forms–>α, β’
and β (with increasing stability)
Transitions between crystalline states are in most cases monotropic i.e. transitions occur in one direction.
where does Initial crystallization preferably occur in lipids?
in the α-form (which is a rather ”disordered crystal”.
Temperatures slightly above the melting temperture (annealing) may cause recrystallization to more stable forms.
what may influence the crystalline form of lipids?
The presence of polar lipids:
example:
mono- and diglycerides
phospholipids (lecithin)
which types of lipids in food have the β’-form as the preferred crystalline state?
Palm
rapeseed oil
milk fat
Solid-liquid ratio – 10 % solid phase results in?
a plastic fat
Fat crystals can form networks in a liquid fat mixture
