FOOD 2400 Study Guide - Final Guide: Adsorption, Ice Crystals, Lipid Peroxidation

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Published on 14 Apr 2013
School
University of Guelph
Department
Food Science
Course
FOOD 2400
Professor
FOOD 2400 Summer 2012
Water
List some unique chemical and physical properties of water and ice
Describe some interaction of water with other molecules
Define what is meant by „water activity'
Explain the 3 regions represented in a sorption isotherm
Understand the importance of sorption isotherms to food stability
Define what is meant by „hysteresis'
Explain the differences between bound water and free water in food systems
Explain the process of ice crystallization
Explain the significance of nucleation and „seeding' during ice crystal formation
Explain the effect of cooling rate on ice crystal formation
Describe the changes which occur in foods during frozen storage; specifically freeze
concentration and ice crystal damage.
Explain the concept of “glass formation” in foods and how this impacts on food stability
Explain the significance of water activity in relation to microbial growth/inhibition, chemical
reactions, enzyme activity, and packaging requirements.
Chemical and physical properties
-high melting and boiling temperatures
-large values for surface tension, dielectric constant, heat capacity and heat of phase
transitions (fusion, vaporization, sublimation)
-ice will conduct heat at a much faster rate than immobilized water
-thermal diffusivities of water and ice indicate the rate solid and liquid forms of
water undergo changes in temperature
-ice's is 9 times greater than water
-explains why tissue freezes 9 times quicker then they thaw
Interaction of water with other molecules
-able to influence noncovalent bonds (hydrogen, ionic, and apolar) that stabilize
large molecules
-hydrophilic substances interact strongly with water by ion-dipole and dipole-dipole
interactions, causing changes in water structure and mobility
-bound water = water which is very tightly associated with molecular structures
-reduced molecular mobility
-can't act as a solvent
-structure is disrupted by the addition of dissociable solutes
-water binding (hydration), tendency of water to associate with varying degree to
hydrophilic substances; dependent on nature of non-aqueous constituents, salt,
composition, pH and temperature
-water holding capacity, ability of a matrix of molecules to entrap large amount of
water to prevent exudation
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Describe water activity
-better indicator of food stability than water content
-aw = ERH = P/Po
-P = partial pressure of water above the sample
-Po = vapor pressure of pure water at the same temperature
-ERH equilibrium relative humidity
-its is an indication of how much water is available to participate in reactions for
microbial growth
3 regions of sorption isotherm
-plots of water activity vs water content in a food at a constant temperature
-useful in predicting stability in foods with moisture content below 50%
-can be gathered from dehydration (desorption) or rehydrating (adsorption)
-adsorption used to observe hygroscopic products
-desorption used to observe drying process
-sigmoidal in shape and divided into 3 areas corresponding to different types or
conditions of water in the food
A - absorption of a monomolecular layer of water
B - absorption of additional layers of water
C - condensation of water in capillaries and pores of the material
ZONE A
-most strongly absorbed
-most immobile in food
-at -40oC this water is unfreezable
-boundary between zones A and B is a monolayer of water (Langmuir water)
-amount of water needed to form a monolayer over the accessible highly
polar groups of dry matter
ZONE B
-known as multilayer water
-water bounded to itself or to the solute water or through H-bonding
-has solvent properties and acts as a plasticizing agent
ZONE C
-least strongly bound and most mobile
-bulk phase water found in gels and cellular systems, and is physically entrapped
-freezable, can act as a solvent, will allow chemical and microbial growth
-most widely used isotherm is the BET, the monolayer coverage of water and water
surface area can be calculated
Importance of sorption isotherms to food stability
Hysteresis
-absorption and desorption curves are not always superimposable = hysteresis
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-depends on the nature of the food, physical changes (addition or removal of water,
temperature, rate of desorption)
-related to water condensing in capillaries of the food
-when filling rate is controlled by the bulb of the capillary, when leaving flow is
controlled by the capillary
Ice crystallization
-first step is nucleation
nuclei crystals of ice crystals form
-ice crystals will grow around the nuclei
-supercooling = pure water cooled below its melting temperature, no nucleation.
when a small ice crystal is added to the supercooled liquid, nucleation beings and
temperature rises to the melting temperature
-low degrees of supercooling = crystallization is slow
-when lower temperatures are used process is quicker
-speed of crystallization determines the number and size of ice crystals
-fast cooling = high rate of nucleation, slow crystal growth rate = fine crystal
structure
-slow cooling = low rate of nucleation, fast crystal growth rate = fewer but larger ice
crystals
-seeding (addition of nuclei to liquids prior to freezing) can be used to increase the
number of initial nuclei present and to encourage a finer crystalline structure
Changes which occur in frozen foods during storage (Freeze concentration and ice
crystal damage)
-freeze concentration
-concentration of non-aqueous constituents in the unfrozen phase
-during freezing water from solution is transferred into ice crystals and non
aqueous constituents become more concentrated in a diminished quantity of
unfrozen water
-unfrozen phase can have changes in pH, ionic strength, viscosity, freezing
point, surface and interfacial tension, redox potential
-low temperature decreases reaction rates, but freeze concentration can
increase non-enzymatic reactions (oxidative reactions and protein
insolubility)
-ice crystal damage
-due to decompartmentalization, enzymes and substrates may come
together, thus catalyzing a reaction. thus some enzymatic reaction may
increase
-recrystallization, melting and refreezing of ice crystals with slight
temperature fluctuations can cause damage to the food
Glass Formation and impact on food stability
-can be formed in frozen foods, but it is not ice formation
-glass is a very viscous liquid
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Document Summary

Large values for surface tension, dielectric constant, heat capacity and heat of phase transitions (fusion, vaporization, sublimation) Ice will conduct heat at a much faster rate than immobilized water. Thermal diffusivities of water and ice indicate the rate solid and liquid forms of water undergo changes in temperature. Explains why tissue freezes 9 times quicker then they thaw. Able to influence noncovalent bonds (hydrogen, ionic, and apolar) that stabilize large molecules. Hydrophilic substances interact strongly with water by ion-dipole and dipole-dipole interactions, causing changes in water structure and mobility. Bound water = water which is very tightly associated with molecular structures. Structure is disrupted by the addition of dissociable solutes. Water binding (hydration), tendency of water to associate with varying degree to hydrophilic substances; dependent on nature of non-aqueous constituents, salt, composition, ph and temperature. Water holding capacity, ability of a matrix of molecules to entrap large amount of water to prevent exudation.

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