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What happens molecularly at the freezing point of water?
At the freezing point the kinetic energy of a flickering cluster is low enough to stailize and nucleate to form a minute crystal
What happens below the freezing point of water?
The temperature is <0°C the nucleus can grow in size as more water molecules fit into the three dimensional crystal lattice
What happens as ultraputre water reaches the freezxing point?
In ultrapure water, nucleation is a truely random process and does not readily take place- rather super cooling can occur (the water can stay liquid to well below 0°C if not disrupted)
What happens if super cooled water is disturbed?
Super cooled water will freeze almost instantly if disturbed or if an ice crystal is thrown in
What do we need to start the freezing process?
Some site of organization or nucleation
In normal water what starts the freezing process?
Impurities such as dust or the container wall surface
What hinders the freezing process?
The presence of solutes as they limit the ability of flickering cluster to form into nuclei
What is the nucleation process a function of?
The rate of what cooling
What is the net result of freezing slowly?
The formation of a few large ice crystals
Why are large ice crystals bad?
- Burst the cell walls causing physical damage
- Reduce the water holding capacity
- Place enzymes previously contained in contact with their substrates leading to undesired reactions
What is a consequence of water only freezing in its pure form?
Once nucleation has taken place, water diffuses to the surface of the cyrstal, solutes are concentrated as the freezing process takes place
Why do solutes depress the freezing point?
- Interfere with the nucleation process
- Slow down the rate of diffusion
- Therefor a lower temperature or greater driving force is required for the ice crystals to form and grow
What is the eutectic point?
- A low enough temperature so that any remaining water will co-crystalize with the solute
- Usually lies between -55-->-70C
- All the water in the system is immobilized
- Water activity=0
Does deterioration take place in a home freezer?
- Yes, enzymatic reactions can still take place
- Ionic strength and pH changes can denature proteins and affect water holding capacity of proteins
- water activity isnt 0, so water can still act as a reactant and serve as a reactant medium
Why must we blanch fruits and vegetables which are to be frozen?
Because enzymatic reactions are catalytic and can thus still be carried out under conditions of frozen storage
Is most of the food quality lost during freezing or thawing?
Thawing, because all reactions speed up when the thawing is started
How do you assess the overall mobility of water?
Measure its water activity
What is the equation of water activity?
= Water Activity
- P=partial pressure of the solution which contains solute
- P0(T)= Partial pressure of water with no solute where T=temp
What is relative humidity?
- P=Partial pressure of water in the atmosphere
- Ps=partial pressure of water saturated air in an enclosed chamber
What would adding salt to pure water in a chamber lead to?
A continual reduction in partial pressure or RH until the solution was saturated with salt
How do solutes reduce water activity?
Ties up some of the water restricting its freedom of movement or mobility by hindering the formation of flickering clusters
How many categories of water are there according to water activity ranges
What is type IV water?
- Where Aw=1
- This is pure water and doesnt exist in food systems
What is type III water?
- Solutions, dispersions, and tissues in tissue- water physically entrapped in the tissue matrix and interstitial spaces ie water present in macrocapillaries which are >1microM
- Dissolved solutes are not enough to moilize solutes in any major way or depress freezing point significantly
- In this water activity range many microorganisms are capable of growth with some bacteria and yeasts inhibited at the lower end
- Tissues- most hydrolitic oxidativ and enzymatic reactions proceed readily in this water activity range
What is type II water?
- Aw= 0.25-0.8
- Very broad transitory range water icluding the outer layers of bound water to that present in microcapillaries found in tissues
- Microbial growth cannot be sustained at most of this range as water is tied up significantly, with the exception of some molds at the upper end
- At lower end, freezing point is significnalty depressed and most enzymatic reactions requiring water are slowed down substantially
- Non-enzymatic browning takes place readily with heat at the upper and and without heat over time at the lower end
What is type I water?
- Down at the molecular level and looking at the bound monolayer of water directly hydrogen bonded or associated with molecules or macromolecules
- This water is essentially mobile and very strongly hydrogen bonded to the molecules and is basically an integral part of the molecular structure
- If hyrdophobic groups are present, water may exist as clathrate hydrate structures
- Bound water is very difficult to remove and cannot be frozen (it is effectively locked into place already)
- Most reactions requiring moisture do not occur
- Autoxidation, which does not require water, speeds up
- Low moisture non-enzymatic browning reactions speed up
What does Water Activity Profile (WAP) of a food product tell us?
Tells us a lot about its potential stability, wether it would lose or gain moisture as a function of RH
How can the WAP be assessed?
- By deriving a product moisture sorption isotherm
- Use saturated salt solutions to generate specific RH
How is equilibrium RH (ERH) of a product measured?
- A food product can be dried completely (a relative term) and then placed into an enclosed chamber above a series of saturated salt solutions
- Equilibrium Aw is directly related to ERH of the chamber
- Aw= P/P0(T)=ERH/100 at a constant temperature
- By placing a sample in a series of hermeticall sealed chamber of known ERH at constant T, the product will absorb or desorb moisture the amount of moisture can be measured by weighing the sample after it has come to equilibrium
- A plot of change in weight vs ERH or Aw provides the moisture sorption isotherm
What is directional hysteresis?
Curved changes depending on werther the product starts dry or moist
What are absorption or desorption differences generally attributable to?
- Permanent structural or chemical changes in a product which have taken place due to changes in moisture content
- ex. If a steak has been dried, numerous irreversible chemical and physical changes take place which permanently change its moisture sorption behavior
What is an example of a very simple system whose moisture sorption curves wont show significant hysteresis?
What is moisture sorption information useful for?
- Defining how a product picks up or loses moisture under specified RH and temp conditions
- Used to determine the final moisture content that can be achieved in a drying operation based on temperature and RH of the drying air
- Provides information about the textural changes that could occur (good or bad) and possibly their rate (ex staling)
- Helps in selection of appropriate packaging materials in terms of its desired moisture permeability characteristic
- Provides important information about the relative stability of a product in regard to different variables
What are the different variables that determine moisture sorption information about the relative stability of a product?
- The type of microbial growth likely or possible
- The types of reactions which may predominate in a food system
- Evaluation of effects of compounds which can be used to modify the Aw profile of a product (ex. addition of sorbitol as a humectant)
- Ex. Formulation of intermediate moisture foods like marshmallows, pet foods, pound cake
What is Aw an important variable for?
chemical reactivity of a food system and the relative ability for microorganisms to grow