Brassica vegetables are also known as crucifers and include broccoli, cauliflower, cabbage, Chinese cabbage and Brussels sprouts.
To reduce the rate of respiration and transpiration of vegetables, care should be taken to ensure temperature, humidity and air movement are optimised for storage. Good practice for vegetable storage starts immediately after harvest.
Temperature
Freshly harvested high quality brassica vegetables should be cooled rapidly to below 2°C and stored between 0 and 2°C.
Pre-cooling and storage at low temperatures slow down the physiological and biochemical processes associated with deterioration and decay. Low temperatures also reduce water loss through transpiration and delay the growth of micro-organisms which cause rot. An increase in the temperature of 10°C can increase the rate of deterioration and decay by two to three times.
When and how to pre-cool?
Start cooling to remove field heat to less than 2°C within four hours after harvesting.
The most common and effective pre-cooling method for Brassica vegetables is forced-air cooling to cool vegetables within two to six hours. This draws cool air through the product in ventilated containers, bins, crates or packages — usually on pallets — and rapidly lowers the temperature of the produce. The main advantages of forced-air cooling is that it is simple to use and can be easily accommodated into grading/packaging/storage systems.
Hydro-cooling is another effective method. This cools the produce with chilled, chlorinated water (temperature 0.5°C; chlorine 100 to 300ppm; pH 7.3 to 7.6) but is more difficult than forced-air cooling as more sophisticated equipment is needed.
With broccoli, crushed or flaked ice is sometimes used to remove field heat quickly but there is a danger the ice may injure the florets, causing an entry point for soft rot micro-organisms such as Erwinia and Pseudomonas.
Relative humidity
Brassica vegetables should be stored at relative humidity higher than 90%.
Although relative humidity of air increases when air is cooled, it is still necessary to check that the relative in a coolroom is satisfactory. In low temperature coolrooms (1°C), water is continually lost during defrost cycles. This is because the water which comes from the air, the vegetables and which freezes the evaporator coil, is melted and drained out of the coolroom.
Minimise water loss from a coolroom and the vegetables by using a large evaporator coil to keep defrosting to a minimum. A large coil allows a low temperature differential (TD) of around 3°C to be maintained between the coolroom air returning to the evaporator and the refrigerant within the evaporator coil, thereby minimising icing up.
Adding moisture
Moisture can be added to coolroom air in a variety of ways including wetting down the floor and hanging wet hessian ‘curtains’. Water can also be added by using humidifiers, wetting containers and sprinkling products with water.
Controlling air movement
Rapid air movement over exposed perishables can sweep water molecules away from the vegetables resulting in higher rates of water loss, dehydrating the vegetables. Stop this problem by controlling air circulation in the coolroom and using protective covers over the products.
Other storage life factors
Harvesting and maturity
Harvest vegetables early in the morning when temperatures are cool. The higher the temperature of the products, the greater the need for refrigeration for cooling and the greater the cost. High temperatures increase the rate of respiration and deterioration. Once harvested, exposure of vegetables to the sun causes shrivelling and rapid quality deterioration.
Before transport to a packing shed, ensure vegetables are kept in the shade in a shed or at least under a tree in a paddock. They should be transported with a cover to the packing shed for pre-cooling no more than four hours after harvesting.
It is very important to harvest vegetables at the correct maturity and size for marketing. The requirements for local and overseas markets often differ.
Environmental factors and nutrition
Keeping quality depends on temperature, light and mineral nutrition during growth, and balanced use of irrigation and pesticides. Calcium is an important nutrient for long storage life.
Product quality
Products should be as free as possible from breaks, bruises, decay and other damage which increases moisture loss and provides entry points for bacteria and fungi.
Ethylene
Storage areas must be free of ethylene. A natural ripening hormone, ethylene leads to deterioration in vegetables. It is produced by fruit and vegetables as part of their physiological processes. Any damage, bruise, mechanical injury or other kind of stress sustained during harvesting, storage and transportation process will promote ethylene production.
Do not store vegetables with apples, pears, avocados, kiwifruit, stone fruits, tomatoes and melons. More information on Storage of fresh fruit and vegetables and Mixed storage of fruits and vegetables is available.
Packaging
Suitable packaging can facilitate handling, protect the produce, extend storage/shelf life and help to maintain good quality. Vegetables should be tightly packed with a plastic liner to maintain moisture content in storage, reducing water loss and shrivelling helping to maintain quality.
Active packaging, for example, modified atmosphere packaging using sealed polyethylene bags, creates an atmosphere inside the package which slows the ageing/deterioration process. Active packaging material is a plastic film which can be used as a wrap or liner for produce inside a carton or a pallet.
Additionally, sachets of an ethylene absorber can be added to the packaging. This limits the availability of oxygen and builds up the level of carbon dioxide while absorbing the ethylene. The active packaging film partially blocks the escape of carbon dioxide, but does not allow oxygen to remain above certain limits.
Brassica vegetables
Broccoli
(Brassica oleracea L. italica group)
Minimum requirements
- Broccoli should be sound, clean and free of any off smell.
- It should also be free from caterpillars, aphids, thrips and other pests.
- The head should be full-bodied, firm, fresh and green without budding yellow flowers.
- The heads should be free of disease such as white blister and soft rots.
- The attached leaves, if any, should be green and fresh.
- Broccoli for export is trimmed to remove all leaves, weighing 300 to 400g.
Shelf life
Air storage without plastic liner:
0°C, 90–95% RH - 1–2 weeks
Polystyrene box with ice:
0°C, 90–95% RH - 3–4 weeks
Waxed carton with plastic liner:
0°C, 90–95% RH - 4 weeks
4°C, 80–90% RH - 4–7 days
20°C, 60–70% RH - 1–2 days
The maximum shelf life of broccoli is usually determined by yellowing which detracts from consumer acceptability. High temperature, low humidity and the presence of ethylene gas accelerate yellowing.
Putting ice on top of broccoli in a polystyrene box will be beneficial if temperature and humidity control are poor. Ice is frequently used for local markets and for export by air freight. Polystyrene boxes with ice topping are more expensive than waxed cartons with plastic liners. When temperature can be maintained effectively there is no need to use this method which has disadvantages. When ice melts during storage and transportation the melted water will aid the development of microbial infections which can rot the produce.
Preferred storage conditions
Pre-cooling and storage at 0°C, 95–100% RH.
Recommended storage and transit temperature: 0–2°C.
Brussels sprouts
(Brassica oleracea L. gemmifera group)
Minimum requirements:
Brussels sprouts should be intact, firm, bright green, fresh, sound, free from soil and off smell.
They should be free from signs of attack by insects, disease, mould, rot or mechanical damage.
Shelf life
0°C, 90–95% RH - 2–3 weeks
4°C, 80–90% RH - about 1 week
20°C, 60–70% RH - 2–3 days
Lining of storage containers with perforated polyethylene reduces wilting loss. Perforation allows better air movement during cooling.
Preferred storage conditions
Pre-cooling and storage at 0°C, 95–100% RH.
Recommended storage and transit temperature: 0–2°C.
Cabbage
(Brassica oleracea L. capitata group)
Minimum requirements
- Cabbage should be intact, free of cracks, rot, mould and adhering soil and insects.
- No off smell.
- The head should be firm, compact and without discolouration of the inner or outer leaves.
- The stem should be cut off directly below the lowest leaf.
Shelf life
0°C, 90–95% RH – 4–16 weeks
4°C, 80–90% RH – 2–6 weeks
20°C, 60–70% RH – about 1 week
Lining of storage containers with perforated polyethylene reduces wilting and trimming loss. Perforation allows better air movement during cooling. The storage/shelf life of cabbage is greatly influenced by cultivars/varieties. Controlled atmosphere (CA) storage of cabbage at 2–3% oxygen and 3–6% carbon dioxide can achieve a shelf life of about nine months.
Preferred storage conditions
Pre-cooling and storage at 0°C, 90–95% RH.
Recommended storage and transit temperature: 0–4°C.
Cauliflower
(Brassica oleracea L. botrytis group)
Minimum requirements
- Cauliflowers should be intact, firm, clean, sound and fresh, free of any foreign matter and pests and not affected by disease.
- No off smell.
- The curd should have no discolouration.
- Curds for export are trimmed to remove all leaves, ideal weight 0.8–1.2kg
- Curds in the same carton must have uniform size and weight.
Shelf life
0°C, 90–95% RH – 3 weeks
4°C, 80–90% RH – 2 weeks
20°C, 60–70% RH – 2–3 days
Preferred storage conditions
Pre-cooling and storage at 0°C, 95–100% RH.
Recommended storage and transit temperature: 0–2°C.
Chinese cabbage
(Brassica campestris L. subsp. pekinensis)
Minimum requirements
- Chinese cabbage should be intact, fresh, sound, clean and full-bodied with no signs of disease or mechanical damage.
- There should be no dark brown vascular bundles or any form of spots or freckles.
- It should be free of fleckles (or Gomasho) on the midribs as this usually becomes more severe after storage.
- No signs of shoot growth or visible bolting.
- No off smell.
Shelf life
0°C, 90–95% RH – 4–6 weeks
4°C, 80–90% RH – 2–3 weeks
20°C, 60–70% RH – 2–4 days
Lining of storage containers with perforated polyethylene reduces wilting and trimming losses.
Preferred storage conditions
Pre-cooling and storage at 0–2°C, 90–95% RH.
Recommended storage and transit temperature: 0–4°C.
Acknowledgement
Dennis Phillips and Rachel Lancaster were the original authors of this material.