NUTRITIONAL AND PHYTOCHEMICAL COMPOSITION OF PROCESSED HAWAIIAN SEAWEED

Abstract

Limu, referring to various types of Hawaiian seaweed, like limu ogo (red algae) and limu manauea (green algae), is known for its rich profile of minerals, carotenoids, and vitamin E. However, certain limu species may contain potentially harmful heavy metals such as arsenic and copper. This research seeks to understand how various cooking techniques, namely boiling and soaking, influence the retention of beneficial compounds like carotenoids. Drawingfrom existing literature, cooking methods can enhance the bioaccessibility of carotenoids and reduce harmful heavy metal content but may degrade essential nutrients. While extensive research exists on various seafood, studies on edible seaweed, in particular those from Hawaii, remain limited. Thus, the objectives of this work were to: 1) quantify the primary carotenoids in seaweeds grown in Hawaii and 2) estimate the effects of cooking (soaking and boiling) on carotenoid and chlorophyll retention. Various red and green seaweeds (Gracilaria parvispora, Graciliara tikvahiae, Halymenia formosa, Ulva expansa, and Ulva ohnoi) were generously donated from aquaculture facilities on the island of Hawaii and Oahu. Fresh or cooked (soaked for 24 hrs, boiled for 10 minutes, or boiled for 20 minutes) were freeze-dried, ground into a powder, and stored at -20°C prior to further analysis. Proximate analysis was conducted to estimate protein, crude fat, ash, and moisture content. Total carbohydrates were calculated by taking the difference of protein, crude fat, ash, and moisture from 100%. Lipophilic bioactive compounds were extracted from the samples using a mixture of hexane, acetone, and ethanol (2:1:1 v/v/v) assisted with ultrasonication. Specific carotenoids were quantified using reverse-phase high-performance liquid chromatography (RP-HPLC) equipped with a diode array detector (DAD) and a C-30 column. Specific carotenoids were identified and quantified by comparison with external calibration curves of analytical standards. Total carotenoid, chlorophyll a, and chlorophyll b content was quantified spectrophotometrically. Digestive stability of fresh and cooked seaweed were measured using a three-stage in vitro digestion model. Digestive stability was calculated as the relative % retention of the bioactive compound (total carotenoid or chlorophyll) in the digested fraction compared against a non-digested sample. Statistical analysis was conducted using analysis of variance (ANOVA) and the Tukey-Kramer method for post-hoc comparisons. Statistical significance was established at P<0.05. Proximate analysis indicated that Gracilaria tikvahiae was significantly higher in protein (~17 % of the freeze-dried weight) compared to others (~12.3-13.6%). Ash was the highest in Halymenia formosa (~39% of the freeze-dried weight) compared to all other samples (~12-34%). Additionally, total carbohydrates were statistically highest in Ulva expansa (~70% of the freeze-dried weight) compared to others (~42-51%). Across the seaweeds, provitamin A carotenoids (beta-carotene and beta-cryptoxanthin) were prominent in red seaweeds (Gracilaria parvispora, Gracilaria tikvahiae, and Halymenia formosa). Non-provitamin A carotenoids lutein and zeaxanthin were notable in fresh Ulva ohnoi, but were reduced in all cooked samples (soaked 24hr, boiled 10 min, boiled 20min). Beta-carotene was prominent in raw Gracilaria tikvahiae, but substantially decreased in all cooked samples. In digestive stability tests, total carotenoids were not significantly different in raw vs. boiled (20 and 30 min) samples, indicating that although cooking may reduce specific carotenoids, total carotenoids remained relatively unchanged. However, samples that were boiled for 30 min had a statistically higher carotenoid digestive stability compared to raw samples and those boiled for 20 min, suggesting that there may be some matrix-specific effects with cooking. Chlorophyll a was significantly higher in samples that were boiled (20 and 30 min) compared to raw samples, likely due to enhanced release and extraction efficiency, but chlorophyll b was not significantly different across raw and cooked samples. Although differences were observed in digestive stability of chlorophylls, there were no significant differences across raw and boiled samples. Overall, these findings indicate that red and green seaweed in Hawaii can be sources of provitamin A carotenoids and non-provitamin A carotenoids, respectively. In most cases, soaking and boiling appeared to reduce the content of specific carotenoids in seaweeds, although total carotenoid content appeared relatively unchanged. Digestive stability tests suggest that carotenoid content is affected by cooking-induced matrix changes but chlorophyll content is not significantly affected. Overall, these findings guide culinary practices to maximize limu's nutritional benefits and demonstrate the value of both fresh and cooked edible seaweeds in Hawaii.