Ekstraksi Ulvan dari Ulva lactuca L. sebagai Krioprotektan dan Aplikasinya pada Surimi Beku

Abstract

Ulvan is a sulfated polysaccharide extracted from green seaweeds, mainly from the genera Ulva and Enteromorpha, and serves as a major structural component of the cell wall, accounting for approximately 8–40% of the biomass. Ulvan is located outside the cell lumen and associates with cellulose, proteins, and other polysaccharides through hydrogen bonding, ionic interactions, and metal bridges, forming a strong yet flexible cell wall network. Chemically, ulvan is a complex heteropolysaccharide composed predominantly of L-rhamnose as the main monomer, along with glucuronic acid and/or iduronic acid, and smaller amounts of xylose and glucose. These components are arranged in repeating disaccharide units known as ulvanobiuronic acids, in which rhamnose is sulfated. The presence of hydroxyl and sulfate groups renders ulvan highly hydrophilic, endows it with strong water-binding capacity, and imparts distinctive rheological properties. These characteristics support the potential of ulvan as a functional ingredient, particularly as a cryoprotectant, due to its ability to retain water, form viscoelastic gel systems, and reduce damage during freezing and frozen storage, such as protein denaturation and ice crystal formation. This study aimed to obtain ulvan from Ulva lactuca L. as an alternative cryoprotectant that meets the criteria of water holding capacity (WHC) > 80% and gel strength > 700 g/cm², in accordance with SNI 2694:2021. The study was conducted in three stages: (i) a systematic literature review (SLR) to evaluate the effects of different extraction methods on ulvan characteristics, particularly its water holding capacity and viscoelastic properties relevant to cryoprotective activity; (ii) determination of the extraction method that produces ulvan with structural, physicochemical, and rheological characteristics supporting cryoprotective activity, and identification of the parameters contributing to these properties; and (iii) evaluation of the cryoprotective effectiveness of the selected ulvan extract in frozen surimi during freezing and storage. The SLR results indicated that hot water extraction (HWE), enzymatic extraction (EAE), and ultrasonic-assisted extraction (UAE) were able to preserve the main monomer rhamnose, molecular weight, and relatively high sulfate content, resulting in ulvan with high WHC and viscoelastic properties. In contrast, microwave-assisted extraction (MAE) and chemical-assisted extraction (CAE) tended to produce ulvan with lower molecular weight and sulfate content, which consequently reduced WHC and rheological properties, particularly viscosity. Ulvan with high WHC and favorable rheological characteristics shows strong potential for application as a cryoprotectant in protein-based frozen products. The results of the second research phase demonstrated that increasing temperature and extraction time during ultrasonic-assisted extraction (40 kHz, 120 W) significantly enhanced yield, purity, sulfate and rhamnose content, water holding capacity (WHC), molecular weight (Mw), viscosity, hysteresis area, and viscoelastic parameters of ulvan. Rheologically, ulvan exhibited pseudoplastic behavior with shear-thinning characteristics, as indicated by a decrease in viscosity with increasing shear rate. Ulvan extracted at 80 °C for 90 minutes yielded 33.61%, with sulfate content of 17.93 g/100 g, rhamnose 3.63 g/100 g, xylose 5.28 g/100 g, WHC of 7.92 g water/g sample, Mw of 3,580 kDa, viscosity of 12.48 mPa·s, and elastic modulus (G’) of 13.27 Pa, although it exhibited lower lightness (L* = 45.60) and low protein content (0.05 g/100 g). The structural and physicochemical characteristics of the resulting ulvan were defined by high sulfate content, molecular weight, and WHC, as well as viscoelastic rheological behavior with pseudoplastic and thixotropic properties, a small hysteresis area, and shear-thinning behavior. These characteristics support ulvan’s ability to bind and retain water, facilitate homogenization during mixing, and form a stable semi-network structure after incorporation into surimi. Based on decision analysis using the Bayesian method, the ulvan produced met the criteria for use as a cryoprotectant. The results of the third research phase showed that ulvan concentration, frozen storage duration, and their interaction significantly affected the structural and functional characteristics of surimi. Increasing ulvan concentration was correlated with improved stability of protein secondary structure, particularly the a-helix fraction, as well as enhanced functional properties of surimi, indicated by increased WHC, gel strength, and textural characteristics, supported by microstructural observations using SEM. Conversely, prolonged frozen storage tended to reduce all observed parameters. Application of ulvan at concentrations of 2–3% demonstrated superior cryoprotective effectiveness in frozen surimi stored for up to 60 days compared to commercial cryoprotectants (sucrose + STPP). This was indicated by a-helix content of 81.87–87.30%, WHC of 75.50–82.54%, gel strength of 1,014.47–1,217.20 g/cm², and improved textural parameters. The successful application of ulvan as a cryoprotectant was confirmed by WHC values exceeding 80% and gel strength meeting the requirements of SNI 2694:2021. Ultrasonic-assisted extraction using a bath system (40 kHz, 120 W) produced ulvan with structural configurations and rheological behavior that support cryoprotective activity. The combination of structural, physicochemical, and rheological properties enhanced ulvan’s interaction with myofibrillar proteins, enabling it to maintain the structural stability and functional properties of surimi during freezing and frozen storage. This study successfully developed ulvan from Ulva lactuca L. as an effective alternative cryoprotectant for protein-based frozen food products.