Handling of Fresh Culinary Herbs
Marita Cantwell, Postharvest Specialist, Vegetable Crops Dept., UC Davis
All the postharvest principles that apply to leafy green tissues apply to the handling of fresh herbs. Temperature is the single most important factor in maintaining quality after harvest. Optimum postharvest temperature for fresh thyme, oregano, rosemary, mints, sage, parsley, cilantro, savory, marjoram, dill, and tarragon is 32°F. A shelf life of 3 to 4 weeks can be achieved at this temperature. With a temperature of 41°F, a minimum shelf life of 2 to 3 weeks can be expected. If herbs are harvested early in the morning, the need for cooling is minimized. If harvested later, the appropriate cooling method depends on the type of herb. Most respond favorably to room and forced air cooling. Herbs have also been successfully vacuum-cooled.
Prevention of excess moisture loss is the second most important postharvest factor. Most herbs respond favorably with very high humidity (>95%). Some herbs can be held successfully in water (basil, mints, tarragon), but water loss in most can best be controlled by packaging and maintaining high humidity. Lowering the holding temperature to the recommended levels also greatly reduces water loss.
Herbs can be packaged in bags designed to minimize water loss. When herbs are packaged this way, it is particularly important to maintain constant temperatures, to reduce condensation inside the bag and the consequent risk of fungal or bacterial growth. The bags may be partially ventilated with perforations, or may be constructed of a polymer that is partially permeable to water vapor.
Ethylene gas is another factor which limits the shelf life of leafy tissues. Ethylene causes yellowing of leaves, and an increased rate of deterioration. It is possible to routinely find ethylene in the environment surrounding fruits and vegetables during commercial handling.
Careful handling to avoid physical injury to the leafy tissue of the fresh herbs is also important. Rigid clear plastic containers such as those sometimes used for sprouts may be used for soft herbs. "Pillow packs" (plastic bags which are partially inflated when sealed) may be an alternative packaging technique. Growth of microorganisms can also be reduced by proper temperature management and good hygienic practices in the field and packing station. Chlorinated water can reduce microbial load if water is used during handling.