Yam Exam Flashcards
Air by concentration of components
21% Oxygen
78% Nitrogen
.04% CO2
Factors affecting shelf life of packaged food
Food, Environment, Package
Environmental factors for food
Temperature, Relative Humidity, Properties of food affected by environment
Relative humidity and water
Vapor pressure of water at certain temperature/saturated vapor pressure at same temperature x100%
Water Activity and moisture content
Moisture content involves all water molecules and water activity involves water free for reaction
Water on the surface pushes upward
Aw Equation
Aw = Pf/Ps
Pf: water vapor pressure exerted by food
Pa: water vapor pressure exerted by air
Relative Humidity
RH: Pa/Ps x 100%
movement of water from air into food
Compare RH and Aw to see if there is a gain or loss of moisture in food
Pressure in food
Convert percentage of air components in atm
Modified Atmosphere
Modified from the normal composition of air by lowering oxygen to a concentration that best suits food product
Elevate CO2 to extend shelf life and reduce respiration rate
Steady State vs Unsteady State
Steady: No change in time and used for produce
Unsteady: change in time and applied during a warming or cooling period
Mass Balance and energy process
Mass that enters system must leave the system
Package environments
Ambient: normal ranges for gas concentration and temperature
Physical: handling and shipping
Human: consumer interactions with packaging and sensory
Modified Atmospheric Packaging System
Match respiration of fresh produce and storage temperatures to create optimum gas composition
Sugar Caking Overview
Sugar cakes because of moisture and chemical composition of sugar effects absorption rate
Factors that Influence sugar caking
Relative Humidity
High levels of RH form liquid bridges between sugar granules
Factors influenced by sugar caking
Package
Sugar
Internal environment
external environment
Process of sugar caking
Moisture absorption
Liquid Bridge formation (rising RH): Result of thin liquid films due to capillary films
Binding and aggregation: more liquid is absorbed, more liquid bridges and more lumping
Recrystallization: moisture evaporates, and sugar crystallizes which enhances binding between sugar granules
Hardening: Liquid bridges solidify after moisture evaporates
Rate of food deterioration
F(Composition factors, environmental factors)
Composition: catalyst or inhibitor, pH and Aw, microbial population
Environmental: temperature, humidity, light, headspace composition
How do determine Quality factors
Measurable, reproducible, relevant
Sensory quality factors
Taste, aroma, texture
Data is closely related to consumer acceptance
Data often has high variability
Instrumental measurements
Microbial count, pH, Aw
Data should low variability
Good indicator of consumer acceptance
Instrumental measurements pro vs con
Pro: smaller data variation, continuous operation of instruments
Cons: not directly related to consumer acceptability
Sensory measurements Pro vs Con
Pro: related to consumer acceptability
Con: labor intensive, large data variation
Interpolation vs Extrapolation
Intra: predict values inside data points
Extra: predict values outside data points
Kinetic Rate Equation
dc/dt = +/- kc^n
Rate depends on concentration and rate constant
How to compare reactions of the same rate order
Compare rate constants
Zero Order Kinetics
Plot Time vs A/Ao to yield a straight line
First Order Reactions
Plot Ln A/Ao vs T to get a straight line
Key Factors that you can measure for deterioration
Oxidation, spoilage, enzymatic reactions
Shelf life
Span of time until consumer does not accept product
Graph quality factor vs time and measure the curve
Arrhenius plot
Plot Ln k (y) vs 1/T (x)
K = Change in A/Ts
Permeation through a plastic film
Depends on type of plastic, type of gas or vapor, gas concentration gradient and temperature
Gas moves from high concentration to low concentration
3 steps of permeation through a plastic film
Molecule near wall of film diffuse onto surface
Some are absorbed and some are desorbed
Molecules move through solid film by diffusion through a solid (activated diffusion)
Diffusion governed by Fick’s Law
Adsorption and desorption governed by Henry’s Law
Physical state of polymer
Solid: crystalline or amorphous
Liquid: melt
Gas: doesn’t exist
Thermoplastic
Flexible linear chains
Polymer chain can slide past one another
Most water bottles and cling wrap
Thermoset
Rigid 3D network
Crosslinks prevent polymer chains from sliding past one another
Will not become melt plastic
Epoxy resins and unsaturated polymers
Crystalline polymer
Molecules lineup
PE has a high degree of crystallinity
Do not allow polymers enough time so it is often opaque
Semi-crystalline polymer
Uses both crystalline and amorphous
Degree of crystalline
Highly crystallinity more opaque
Various forms of Polyethylene
HDPE: High density PE with higher crystallinity, stiffer and a better gas barrier
LDPE: Lower density PE, clearer, better impact resistance and good heat seal properties
LLDPE: Linear Low density PE, density like LDPE with greater reduced branching, good clarity, heat sensitivity and strength
High Barrier Polymers PVDC and EVOH Copolymers
Both copolymers are high O2 barrier plastics
EVOH has the advantage of being a better O2 carrier than PVDC while EVOH is more expensive and its O2 barrier decreases as relative humidity increases
PVDC has advantages of being less expensive and its O2 barrier is not affected by relative humidity.
However PVDC is considered environmentally unfriendly because it generates corrosive HCL and toxic dioxins during processing and incineration
Chain entanglement
Long chains can become entangled with one another
Motion of entangled polymer chains resembles the wriggling movement of long worms
Chain entanglement is responsible for for holding polymer chains together, as it is hard to unravel long entangled chains
Temp Time History
Lower temp, slower deterioration rate and better food quality
Shorter storage time, better food quality
K value in Arrhenius Equation
Affected by temperature and activation energy
More sensitive for lower Ea values
Accelerated Shelf Life Testing
Method used to estimate shelf life of a product in a shorter time by exposing it to elevated stress conditions.
Procedures for ASLT
Conduct experiments to obtain [A] versus t for at least three temperatures
Estimate shelf life at each temperature
Use Arrhenius Plot to estimate shelf life at lower temperature
PVC Film
Used to seal fresh meats and designed to cling to smooth surfaces
Polycarbonate
Clear and tough, good impact strength at low temperatures
Used in reusable water bottles
Cellophane
Regenerated cellulose film
Used for baked goods, candies, and tobacco products
Dries out under low humidity
Polylactic Acid
Obtained by condensation of lactic acid
Physical properties and MW
As MW increases, strength increases, melt viscosity also increases
chain entanglement is an important factor contributing to physical properties of polymers
Properties of Packaging Plastics
Mechanical strength
Gas and water barrier
Salability
Printability
Adhesive property
Ease to process
Process of packaging polymers
Extrusion of plastic film: supply mechanical energy and push resin toward die to melt with shear action. Resin cools once it exists heating system
Coextrusion
Extrusion coating
Adhesive Laminating
Extrusion Laminating
Ficks Law of diffusion
Negative because molecules are moving from high concentration to low concentration
Rate of diffusion = -D * Area * (concentration difference/distance)
Henrys Law of Solubility
Concentration of gas in polymer film = solubility * partial pressure
governs adsorption and desorption
What does diffusion of molecules depend on for Hole theory?
Availability of sufficiently large holes (function of free volume, density and degree of packing of polymer chains)
Ease of hole formation (function of segmental chain mobility)
Estimation for Permeation Controlled Shelf Life
Ts = Tolerance limit of O2/OTR through package
OTR = Q = PA/L * delta p
For O2, may need to find weight and then use Ideal Gas Law to convert to volume
