Preservation by Low Temperature
What this domain tests
Questions cover refrigeration temperatures, chilling injury, controlled atmosphere storage, freezing rate and the changes that occur during frozen storage.
Refrigeration and cold storage
Refrigeration generally means 0-7°C; cold storage for apples is often 0-2°C. Bananas are held near 12-14°C because below about 12°C they suffer chilling injury, shown by pitting and failure to ripen. Leafy vegetables need near 0°C and high humidity. Low temperature slows enzyme activity and microbial growth but does not kill microorganisms; psychrophiles can still grow.
Controlled and modified atmosphere
CA storage uses low oxygen (about 2-5%), raised carbon dioxide (about 3-10%) and controlled temperature, with ethylene removal for climacteric fruits such as apples. MA storage relies on the respiration of the produce or on packaging films to modify gases; it is less precisely controlled than CA.
Freezing
Blanching before freezing inactivates peroxidase and lipoxygenase. Quick freezing at about -30 to -40°C gives many small ice crystals and less tissue damage; slow freezing gives large crystals that rupture cells and cause drip loss. Frozen storage is normally at -18°C or lower. Freezer burn is surface dehydration by sublimation, prevented by airtight packaging. Thaw in a refrigerator and never refreeze thawed food.
Worked example
Peas frozen without blanching develop a hay-like off-flavour within three months because lipoxygenase remains active. Blanching at 90-95°C for about 2 minutes destroys the enzyme. If the same peas are stored at -12°C instead of -18°C, ice recrystallization and quality loss accelerate.
Traps that cost marks
Freezing does not sterilize food. Chilling injury is not microbial spoilage. CA and MA are not identical, and slow freezing is not better than quick freezing.