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.
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.
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.
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.