Lesson 7 Flashcards
cold temperature preservation (40 cards)
2 types of low temperature preservation
- refridgerated or cool storage preserve most perishable foods for days or weeks
- frozen storage (with proper packaging) preserves foods for months or even years
55% of foods consumed in North America are refridgerated or frozen
Refridgeration
i.e. cool storage
- from -2 to 16°C but usually around 4°C
- short-term shelf-life extension (days or weeks) assuming good initial quality
at 4°C or lower, most pathogens can’t grow but some psychotrophs (spoilage, pathogens) can grow
Preservation principle of refridgeration
lowering the temperature slows down the rate of microbial growth, enzymatic and chemical reactions
for every 10°C↓, rate of senescence ↓2-3 fold and microbial growth ↓3-6 fold
Factors affecting the quality of food during refridgeration
- controlled temperature
- humidity
- gas atmosphere composition
Refridgeration load
for controlled temperature
heat that must be removed from food products to bring the food from its initial temp to the desired storage temp
Factors affecting refridgeration load
- intital temp of product
- volume of product
- specific heat capacity of the food product
Specific heat capacity
amount of energy to raise or lower the temperature of each gram of product by 1°C
foods with higher water content (e.g. soup) have higher specific heat capacity (i.e. takes longer to cool down)
Possible undesireable effects of controlled temperature
apart from MO growth
- chill injury of some fruits and vegetables
- staling of bread
- loss of crispness in fruits and vegetables
- changes in color of fresh meat
- loss of flavor, nutrient value (e.g. vitamins)
- oxidation
- drip from fish
Chill injury
- fruits and vegetables are living systems with optimum temp requirements
- cold temp may result in off-color development
Effect of humidity on food quality
during refridgeration
- prevents dehydration and excess moisture
- packaging required to help maintain proper humidity within each food
2 gas atmosphere compositions
during refridgeration
- controlled atmosphere (CA) where composition is constantly monitored and maintained for long-term storage in storage rooms/chambers (e.g. apples)
- modified atmosphere (MA) for short-term preservation
MA doesn’t intentionally inhibit MO growth
Steps for modified gas atmosphere
- remove air in packaging
- back-flush (i.e. replace) with the desired gas mixture
- seal the package (e.g. fresh pasta, cured meats, cheese)
packaging material must exclude oxygen and moisture
- OR completely remove air (vacuum pack) to prevent the growth of aerobic, psychoptrophic MOs
- gas composition may change over time (packages are permeable) so must still be refridgerated
Temperatures for frozen storage
- commercial freezing at -18°C
- home freezers at -12 to -14°C
- MOs can’t grow below -9.5°C but some grow below 0°C
- longer shelf-life extension (weeks, months, or years)
Preservation principle of freezing
lower temperature (below -9.5°C) and water activity inhibits MO growth
makes water unavailable for chemical, enzymatic, microbial activity
- usually higher nutritional and sensory quality than thermally processed foods
- BUT MOs can survive and resume growth upon thawing
Sensible vs latent heat
freezing/thawing curve
- removal of sensible heat from the product (change in temp, not phase)
- release of latent heat of crystallization in freezing of water (liquid to solid or ice crystals)
- further cooling to surrounding temperature (complete crystallization)
latent heat of fusion absorbed when ice is melted!
Concentration effect in frozen storage
- until all water is frozen, constant increase in the concentration of solutes in the unfrozen phase
- amount of remaining unfrozen water depends on temp and the product
i.e. remaining water becomes more concentrated with solutes
e.g. beef: 70% unfrozen at 4°C, 3% unfrozen at 9°C, very small amounts remain unfrozen at 18°C
Changes that occur during freezing process or storage
-
high solute concentrations in unfrozen phase =
* oxidative deterioration
* enzymatic reactions -
physical damage from large ice crystals =
* dehydration or freezer burn
* formation of package ice
* textural changes or protein denaturation (e.g. textural hardness where meats become tough)
Factors affecting quality during freezing
- rate of freezing (faster, forming small and abundant ice crystals = better quality)
- final storage temperature (usually -18°C)
- stability of storage temperature (fluctuation > freeze-thaw cycles > larger ice crystals)
- rate of thawing (faster thawing > less time for damage from concentration effect or formation of large ice crystals)
What does the rate of freezing depend on?
- composition of food (e.g. fats, proteins can act as insulators, prolonging freezing)
- temp difference where colder temp = faster freezing (e.g. lowering air temp from -18°C to -30°C shortens time from 40 mins to 20 mins)
- product thickness/geometry where thin, flat package = faster heat transfer
- air velocity where greater velocity = faster freezing rate (FR)
- degree of contact between food and cooling medium where more contact = faster FR
3 freezing methods
- air freezing
- indirect contact freezing
- immersion and cryogenic
Air freezing
- still air freezers (e.g. household freezers) are slow-freezing and have no circulation
- air blast freezers (e.g. fluidized bed freezers) with a temp of -18°C to -34°C cold air at high velocity
Fluidized bed freezing
i.e. individually quick frozen (IQF)
a type of air freezing
food particles are placed on a perforated conveyor belt with a counter-current flow of air directed upward, creating a fluidized state and preventing the particles from sticking together
food items are frozen as individual pieces!
used for frozen vegetables like garden peas, corn, prawns, etc.
Advantages of air blast freezers
i.e. fluidized bed freezers (IQF)
- economical
- can freeze various sizes and shapes of food
- more efficient heat transfer and increased rate of freezing
Disadvantages of air blast freezers
i.e. fluidized bed freezers (IQF)
- possible excess dehydration (freezer burn)
- undesireable bulging of the packages (by the expansion of the product) may occur
- non-uniform cannot be fluidized easily
