food chem 22

At the freezing point the kinetic energy of a flickering cluster is low enough to stailize and nucleate to form a minute crystal

The temperature is <0°C the nucleus can grow in size as more water molecules fit into the three dimensional crystal lattice

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)

Super cooled water will freeze almost instantly if disturbed or if an ice crystal is thrown in

Some site of organization or nucleation

Impurities such as dust or the container wall surface

The presence of solutes as they limit the ability of flickering cluster to form into nuclei

The rate of what cooling

The formation of a few large ice crystals

• 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

Once nucleation has taken place, water diffuses to the surface of the cyrstal, solutes are concentrated as the freezing process takes place

• 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

• 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

• 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

Because enzymatic reactions are catalytic and can thus still be carried out under conditions of frozen storage

Thawing, because all reactions speed up when the thawing is started

Measure its water activity

A_W=P/P0(T)

• A_w= Water Activity
• P=partial pressure of the solution which contains solute
• P_0(T)= Partial pressure of water with no solute where T=temp

• %RH=(P/P_s(T))*100
• P=Partial pressure of water in the atmosphere
• P_s=partial pressure of water saturated air in an enclosed chamber

A continual reduction in partial pressure or RH until the solution was saturated with salt

Ties up some of the water restricting its freedom of movement or mobility by hindering the formation of flickering clusters

4

• Where A_w=1
• This is pure water and doesnt exist in food systems

• A_w=0.8-0.99
• 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

• A_w= 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

• A_w=~0-0.25
• 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

Tells us a lot about its potential stability, wether it would lose or gain moisture as a function of RH

• By deriving a product moisture sorption isotherm
• Use saturated salt solutions to generate specific RH

• 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
• A_w= P/P_0(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

Curved changes depending on werther the product starts dry or moist

• 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

Starch powder

• 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

• 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

chemical reactivity of a food system and the relative ability for microorganisms to grow