Postharvest handling and storage of the edible red seaweed Gracilaria

Red macro-algae (Gracilaria spp.) are used as a fresh food in Hawaii. Species commonly marketed include G. coronopifolia, G. parvispora, G. salicornia and G. tikvahiae, however, these seaweeds have a short postharvest life of about 4d. This research was undertaken to determine the changes that occur postharvest and how the postharvest life of G. salicornia can be extended. The changes in color, phycobilins, respiration rate, ethylene production, protein content, leakage and ion thalli content were determined. Postharvest effects of storage temperature, light/dark storage, modified atmospheres, calcium and nitrogen nutrient treatments, chlorine and heat treatments were evaluated to extend storage life and minimize microbial growth. When stored overnight at 2°C, thalli became limp and entirely pinkish-red indicating possible chilling injury. At 10°C, the color change and decline in phycobilin content occurred in 2d and by the 4th day more than 90% of the thalli were either pink or red, and the phycobilin content had declined by 50%. At 12.5°C and higher storage temperatures, the rate of color change was similar and the optimal storage temperature was between 15 and 17°C in the dark. This storage temperature maintained quality but did not extend overall postharvest life. A steady rise in respiration rate occurred when stored in the dark or in the light. Light seemed to stimulate respiration rate when stored at 16°C, but not at 21°C. No relationship was found between ethylene production and respiration rate either with or without light exposure during storage. Ethylene production did not appear to be related to any physiological changes. Light did increase the rate at which thalli turned pink. Seaweed submerged in seawater in the dark had an extended postharvest life of about 30d. Treatment with chlorine (50mgL⁻¹) to minimize microbial growth was phytotoxic. Neither a 2 nor a 60min postharvest dip in artificial seawater supplemented with 1 or 10mM calcium, potassium, sodium, ammonium as the nitrate salt extend postharvest life. Treating G. parvispora and G. tikvahiae with hot seawater at 42°C for 5min was beneficial in maintaining appearance and extended postharvest life 40-60%. Storage at 15°C and submerged in seawater or treated at 42°C for 5min depending upon species, showed potential at increasing postharvest life of red seaweed.