California Citrus and the Post-Harvest Cold Chain
California produces approximately 80% of US lemon supply and significant volumes of navel oranges, Clementine mandarins, and specialty citrus varieties. The San Joaquin Valley (Tulare and Fresno counties) and the Southern California inland valleys (Ventura, San Bernardino, Riverside) are the primary production regions. Citrus is a non-climacteric fruit — it does not continue to ripen after harvest — but post-harvest handling significantly affects peel color development, juice content maintenance, and decay resistance.
The key challenge unique to citrus (compared to most other produce) is the degreening process: freshly harvested citrus often has green peel even when fully mature and sweet internally, because chlorophyll in the peel persists when daytime temperatures remain warm after fruit reaches internal maturity. For retail markets that associate green peel with immature fruit, degreening before cold storage is commercially essential for most California citrus.
Ethylene Degreening: How It Works
Ethylene degreening uses controlled ethylene gas exposure to accelerate the natural breakdown of chlorophyll in the citrus peel, revealing the underlying carotenoid pigments (orange, yellow) that define the marketable color grades. The process takes place in sealed degreening rooms before refrigerated cold storage.
Standard degreening conditions: ethylene at 1–5 ppm concentration, temperature at 60–65°F, relative humidity at 90–95%, and duration of 24–72 hours depending on the degree of green cover and variety. CO2 produced by fruit respiration must be ventilated (CO2 above 1% inhibits ethylene response and causes off-flavors) — degreening rooms require active CO2 venting.
After degreening, fruit is washed, fungicide-treated (conventional) or hot water-treated (organic), waxed, and graded before packing. Waxed and packed fruit then enters refrigerated cold storage for staging before distribution.
Refrigerated Cold Storage Requirements by Citrus Type
Navel oranges: 38–42°F with 85–90% RH. Navels are chilling-sensitive at temperatures below 38°F (they develop peel pitting and increased decay at lower temperatures). Storage life at proper conditions is 8–12 weeks. Export navels are often stored at 40°F to provide a safety margin against chilling injury during transit temperature variations.
Lemons: 50–55°F — significantly warmer than other citrus. Lemons are highly sensitive to chilling injury below 50°F, developing a condition called “membranosis” (internal membrane breakdown) when exposed to cold temperatures. Storage at 50–55°F extends shelf life to 3–4 months. Lemons are one of the few citrus crops that benefit from long cold storage, as growers deliberately store early-season lemons through the summer for late-season market delivery when prices are typically higher.
Clementine mandarins: 38–42°F with 90–95% RH. Clementines are among the most perishable citrus varieties — storage life is typically 4–6 weeks at optimal conditions. The segment-separating (puffy skin) disorder develops rapidly when Clementines are stored beyond their optimal window.
Decay Management in Cold Storage
Post-harvest decay — primarily from Penicillium digitatum (green mold) and Penicillium italicum (blue mold) — is the primary cause of postharvest loss in California citrus. Cold storage dramatically slows mold growth (rates roughly halve for every 18°F temperature reduction), but does not eliminate it — cold storage extends the window, it does not prevent decay indefinitely.
Combined management of temperature, humidity, post-harvest fungicide application (or hot water treatment for organic), and regular decay monitoring during storage are all components of an effective citrus cold storage quality program.
