Low Temperature Preservation of Foods

Refrigeration (cool storage)

• short-term shelf-life extension (days, weeks)

Most pathogens can’t grow after what temperature?

4ºC

Psychrotrophs

Spoilage & Pathogens that can grow under 4ºC

Preservation principle of Refrigeration

Lowering the temperature slows down the rate of microbial growth, enzymatic & chemical reaction

Factors Affecting the quality of refrigeration (3)

1. Controlled Temperature

2. Humidity

3. Gas Atmosphere Composition

Controlled Temperature

• The target goal of refrigeration load, where u manage how much heat needs to be removed from a food to drop it to its cold storage temperature

1. initial temperature of the product

2. volume (mass) of the product

3. specific heat capacity

initial temperature of the product

the hotter the food is when entering the fridge, the more heat energy must be extracted

volume (mass) of the product

how much physical space and quantity of food you are trying to cool down at one time, cooling down larger amounts takes more energy

specific heat capacity

• how stubborn a food is when ur trying to change temperature, as some foods (like higher water content) take longer to cool down

• amount of energy to raise or lower the temperature of each gram of product by 1 °C

Chill injury

cold temp may result in off-colour development of fruits bc they are living systems

Humidity

• prevent dehydration as well as excess moisture

• packaging maintains proper humidity within each food

• don’t want food to absorb humidity from environment or lose humidity

Gas atmosphere composition

1. Controlled Atmosphere (CA)

2. Modified Atmosphere (MA)

Controlled Atmosphere

composition is constantly monitored and maintained to slow down aging and respiration of fresh produce

Modified Atmosphere

• micro level packaging food such as fresh pasta, cured meats and cheese

• Process:

  1. remove air

  2. back-flush w desired gas mixture

  3. seal package (MAP)

• air is completely remvoed to prevent growth of aerobic, psychotropic microorganisms

Freezing

• longer storage life up to years

• MOs cannot grow below -9.5ºC

Preservation Principle of Freezing

Lower temperature + lower water activity inhibits the growth of MOS

Can MOs survive freezing?

Yes, they resume growth after thawing

Freezing / Thawing Curve

1. Sensible Heat: removal of heat from the product

2. Latent of fusion/crystallization: Freezing of water (liquid → ice crystals)

3. Complete crystallization: Further cooling to the surrounding temperature

Concentration effect

• food doesn’t immediately turn into ice when being frozen

• only pure water molecules become ice crystals and the solutes are left behind

• if u have 100ml liquid water and 1g remaining solute being frozen, at 20ml water there could still be 1g solute, making the solution concentrated

Why does concentration destroy food quality?

• The remaining unfrozen liquid attacks food proteins, destroys vitamins, changes the food’s pH and oxidizes fats

• freeze food right away instead of slowly

Freezer Burn

• “surface dehydration” and loss of moisture

• crystals of ice evaporate into thin air and “burn” the texture

Changes during the process of frozen storage: high solute concentrations in the unfrozen phase

1. oxidative deterioration

2. enzymatic reactions

oxidative deterioration

highly concentrated salts and minerals accelerate make the fat oxidize, resulting in rancid and off-flavours

enzymatic reactions

extreme concentration keeps active enzymes in tight contact with food molecules, breaking down colour pigments and browning

Changes during the process of frozen storage: physical damage from large ice crystals

1. dehydration

2. Formation of “package ice”

3. textural changes (protein denaturation)

dehydration

freezer burn bc surface of food becomes dried out after ice crystals vaporize

Formation of “package ice”

temperature fluctuations inside a freezer evaporate water and condenses inside the plastic packaging, forming ice chunks and stripping food of natural moisture

textural changes (protein denaturation)

Meats become tough from losing their ability to hold water as a result of ice crystals growing and physically destroying the texture

Factors affecting the quality of Freezing (4)

1. Rate of Freezing

2. Final Storage Temperature

3. Stability of Storage Temperature

4. Rate of Thawing

Rate of Freezing

• faster → many small ice crystals are better than large

• composition of food: proteins and fats are insulators

• temp difference: colder temp = faster freezing

• product thickness/geometry: thin, flat package = faster heat transfer

• air velocity: higher velocity = faster Freezing Rate

• degree of contact between food and cooling medium: more contact = faster freezing rate

Final storage temperature

usually -18ºC but lower is better

Stability of storage temperature

• fluctuations cause freeze-thaw cycles that create larger ice crystals

• freeze-thaw cycles are not safe and results of product mishandling

• large pores on close up of muscle tissue means that large ice crystals formed and left cavities on thawing

rate of thawing

faster → less time for damage due to concentration effects or large ice crystals to form

4 Freezing methods

1. air freezing

2. indirect contact freezing

3. immersion

4. cryogenic

air freezing

1. still air freezers

2. air blast freezers

still air freezers

no circulation, slow freezing (household freezers)

air blast freezers (-18 to -34ºC, slow freezing)

fluidized bed freezers (IQF)

IQF

• Fluidized bed freezer where cold air is blown upward through a conveyor and helps individually freeze each piece

• avoids individual ice block. package ice and is fast

Indirect Contact Freezing

1. Plate Freezers

2. Scraped surface, slush freezers

Plate Freezers

• Packaging in uniform shape for proper contact w metal plates

• contact w/ metal surface cooled by refrigerant

Scraped surface, slush freezers

when liquid food mix touches cold wall, the high speed rotating shaft continuously scrapes forming ice crystals and pushes them into centre of liquid to form slush

Immersion (dipping)

food is plunged through a liquid bath, such as propionic acid, sugar solutions, or brines

cryogenic freezing

food travels on a conveyor and is sprayed with gases (nitrogen or carbon dioxide

packing requirement for frozen foods

1. resistant to transfer of water vapour

2. not shatter in cold temps

3. resist formation of pinholes

4. barrier properties towards light and oxygen