Food preservation: Refrigeration

• Low-temperature processing extends the shelf life of foods

Refrigeration

• Temperatures range from 0-5°C.
• Refrigeration is an ancient food storage method
• Techniques used include:
• Cold caves
• Underground storage
• Hanging on trees
• Ice boxes
• Ammonia refrigeration systems
• Mechanism:
• Evaporated liquids absorb heat
• Steaming occurs
• Vapor condensation releases heat, creating a cooling effect
• Liquids evaporate with refrigerants like hydrofluorocarbons to create cold temperatures
• Benefits:
• Minimal effect on taste, texture, and nutritional value
• Reduces the rate of food deterioration, but does not affect cold-tolerant microbes
• Slows down chemical and enzymatic reactions
• Inhibits the growth of non-psychrophilic microbes, but does not destroy microbes
• Psychrophiles can grow in extremely cold environments
• Drawbacks:
• Spoilage occurs if temperature and humidity are not optimized
• Cross-contamination can occur
• Temperature management varies between food selections
• Too cold can cause chill injury to fruits
• Too warm can cause microbial growth
• Odor transfer between foods is possible
• Effectiveness:
• Usually combined with other preservation methods
• Works best with freshly produced produce and meats
• Allows rapid heat removal through hydro cooling and vacuum cooling
• Requires control of temperature, air circulation, and humidity (80%-90%)

Freezing

• Food temperature is reduced below the freezing point.
• Water changes from liquid to solid crystal lattice structures.
• Clarence Birdseye founded it and opened a freezing plant in 1920 after researching food freezing
• Frozen produce sales increase by 4% per year because of:
• Increased consumption of plant-based foods
• Convenience
• Prepackaged produce like berry mixes and veggie mixes
• Suitability for families
• Frozen food sales skyrocketed during COVID-19 due to:
• Demand for popular restaurant foods with restaurants closed
• Increased freezer sales
• 86% of American consumers purchased frozen food
• 70% purchased more frozen food than usual
• 68% purchased new products
• Misconceptions:
• Most foods do not freeze at 0°C
• Dissolved substances freeze faster
• Ice Crystals and Product Quality
• Slow freezing creates large crystals; fast freezing creates small crystals
• Fast freezing results in better quality foods.
• Slow freezing results in poor quality.
• Texture changes can occur
• Can damage plant and animal cells
• Results in loss of moisture and firmness
• Non-uniform temperatures cause variation in crystal size
• It gives access to year-round quality fruits and vegetables
• They are convenient like frozen meals
• Increases storage time
• Inhibits microbial growth, but does not destroy it
• Reduces the rate of food deterioration
• Increases shelf life
• Freeze concentration:
• Used as an alternative evaporation process
• Used to retain aroma for high-quality foods like ice wine and fruit juice
• Freeze concentration can unintentionally reduce food quality
• It occurs when water moves and leaves behind concentrated solutes
• Solutes undergo deleterious reactions
• Fluctuating temperatures result in:
• Shorter shelf life
• Large crystals
• Freezer burn can damage food due to dehydration of the surface
• Chemical changes include:
• Oxidative enzyme activity in produce
• Prevented by blanching vegetables or adding acid to fruits
• Pigment, off flavors and odor, browning, and vitamin loss are preserved
• Rancidity (oxidation of fats) results in off-flavors and odors
• Common with fatty products
• Prevented by airtight packaging

Commercial Freezing Methods

• Rapid freezing is achieved by:
• Modifying food products to increase surface area.
• Increasing air movement over the product.
• Increasing air exposure to the product surface.
• Modifying the freezing process to speed up freezing time and maximize product quality.
• Methods of rapid freeze:
• Blast freezing: cold air is pushed across foods at high speeds
• Batch: stacked on food trays
• Continuous: conveyor belt style
• Spiral freezer: similar to conveyor belt blast freezing, but in a spiral
• Fluidized bed freezer (individually quick frozen foods or IQF):
• Cold air is forced under a tray
• Maximizes food quality and minimizes dehydration
• Foods are frozen individually like berries
• Crust freezing prevents items from sticking together and keeps ice crystals small
• Contact plate freezer: plates are compressed while refrigerated and pushed through
• Products freeze by contact
• Used for flat products like fish.
• Cryogenic freezing: foods are exposed to -60°C by direct contact with liquefied gases for rapid freezing like liquid nitrogen.
• Used on high-value products like seafood and fruits.