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The Effect of Addition Glycerol Against Nori Characterization from Gracilaria sp and Ulva sp Seaweeds

Nori is an edible seaweed product and is known to form a sheet. This study involves the production of nori, from a mixture of Gracilaria and Ulva Lactuca seaweeds, at a ratio of 1:1. Besides, glycerol was adopted as a plasticizing biopolymer for food packaging materials. This study, therefore, aims to evaluate the characteristics of nori, following the addition of glycerol concentration (0.4; 0.6; 0.8; 1.0 percent) as a plasticizer on the nori/edible seaweed. The parameters evaluated include elongation percentage, tensile strength, WVTR, thickness, and water content. Moreover, treatments with 0.8 percent glycerol concentration generated edible seaweed films with the following characteristics: elongation percentage of 12.15%, while the tensile strength, WVTR, thickness, and water content were 2,67 M.Pa, 2268 gr/m2 day, 328.93 μm, and 12,75 %, respectively.


Introduction
Seaweeds are macromolecules that are used in the food as thickener, glazing agents, and gelling agent [1]. Seaweed as form sheet (nori) using for wrapping sushi [2]. Commercially roasted nori sheets are produced at packaging factories [3]. Nori is prepared from dried Porphyra sp. seaweed with high nutrients. Porphyra sp. is cultivated and harvested from a sea-farm, cleaned with water, cut into small pieces, formed into a sheet using a nori mat, and dried [4]. Porphyra sp. is grown in subtropics such as in Japan, Korea, and China, where all the cultured algal products are produced [5]. The added demand for nori is caused principally in the food market segment every year.
Porphyra growth and the cultivation of the most successful, but only on the development of the East Asian subtropical climates [6]. Because of the high market demand of nori, an alternative source of nori from other seaweed outside Porphyra which has similar properties and functions as edible packaging such as Gracilaria sp, Eucheuma cottonii, Ulva sp. [7].
Researchers have been studying biopolymers as one of the ways to conventional (petroleum) food packaging regarding their film-formation properties to produce biodegradable and edible films and coatings for food packaging [8]. Edible films and coatings usually use polysaccharides, proteins, and lipids [9]. These hydrocolloids provide the structural backbone of the edible films [10]. They also provide food-grade plasticizers such as glycerol, sorbitol, or polyethylene glycol are added to improve film flexibility. This research aimed to evaluate the characteristic of nori from mixture Gracilaria sp. and Ulva lactuca seaweeds with the addition of glycerol concentration (0.4; 0.6; 0.8; 1.0%) as a plasticizer on the nori/edible seaweed.

Preparation of Seaweeds
Fresh Gracilaria sp. was collected from Brebes, Center Java, Indonesia. While Ulva lactuca was collected from Binuangeun, Banten. Fresh seaweed was under running seawater, then rinsed and gently cleaned again with distilled water then kept in a cool room before using. The ingredients seasons were salt, sesame oil, oil, oyster sauce, white rice vinegar, commercial nori was purchased from a local market.

Methods of making nori
The procedure for making Nori from Gracilaria sp. and Ulva lactuca was modified in Teddy 2009 [11], as follows in Figure 1.

Characterization of nori
The moisture content of nori is determined by gravimetry. Then the moisture content was calculated by subtracting the final weight from the initial weight of the sample [12]. The WVTR of three bag materials were determined using the wet cup method described by ASTM E96/E96M-16 [13]. Mechanical properties of nori are tensile strength and percentage of elongation at the break were measured by a tensile test using a Gotech Al-7000 -2kN testing machine 95T. The standard used for tensile testing was ASTM D882-12 (ASTM D882 -12, 2012). All samples were conditioned speed 5mm/min at 60 ± 5%  [14]. Preparation and measurement of nori film were conducted according to Yadav et al. (2018) [15].

Moisture content
Moisture content is very important for product processing and handling in the food industry. The moisture content of nori commercial (9.73%) was found to be lower than the nori analog containing glycerol 0.4%, 0.6%, and 0.8% (Figure 2). The increase of glycerol concentration contributes to the addition of moisture content of nori. The increase in the moisture content of nori could be attributed to the hydrophilic nature of glycerol.

Water vapor transmission rate (WVTR)
Controlling moisture migration is vital for retaining packaging quality. The initial and critical moisture contents of food and the humidity gradient between the inside and the outside of the package, the shelf life of the product could be predicted to a fair degree by conducting the WVTR sample [16]. The WVTR of the packaging material is usually determined by the course method. The sheet nori containing glycerol (2,643.65 g/m2 /day) showed the highest water vapor transmission for all treatments including commercial nori (Figure 3). This indication can be explained by the upgrade of intermolecular forces of attraction between the polymer chains because of the increase of plasticizer concentration. The effect was observed in our nori analog at a glycerol concentration of 0.8%. Process of making nori analog with a press manual maybe the intermolecular, not homogenous. The permeability of the tested bag materials is affected by environmental conditions and treatments. The results showed that the increase in glycerol will increase the value of WVTR. This is due to the tendency of glycerol to modify the polymer structure which causes its density to decrease, then causes water molecules to diffuse more easily and gives a higher WVTR [17].

Tensile strength (TS) and Percentage elongation
Edible films must have high TS to be used as food packaging. This parameter is important to protect food during handling, transportation, and marketing [18]. TS and percent elongation increased with the addition of plasticizer glycerol concentration (Figure 4), but after additional concentration, 1.0% glycerol tended to decrease. The result in this study was also observed by Venugopal (2011) that the addition of glycerol in edible film production can reduce TS due to a decrease in the interaction between water molecules and agar [19]. The decrease in TS with the increase in plasticizer concentration has also been reported in other studies [18] [20]. The percent is the ability of the nori to extend before breaking. It describes the nature of the nori plasticity. To maintain the ability of the material to protect food products, high elasticity is required [21]. The increase in elongation and flexibility of nori was due to reduced interactions between polymer chain molecules, this occurred from increasing the plasticizer up to glycerol concentration 10% [22] [23]. The same phenomenon percent elongation with TS, after addition glycerol 1.0% the percentage of elongation to decrease.

Thickness
Thickness describes the feasibility of edible films as packaging materials for meal products. It affects other film characteristics, such as percent elongation, gas transmission rate (GTR), tensile strength, and vapour transmission. The GTR is reverse to characteristic of the thickness. The thickness of the sample is dependent on both the material of the sample and step processing conditions [24]. The commercial nori sheet had 163.2 µm thick, the thinnest compared to all treatments. The addition of glycerol increased the thickness of the film. Similar results are also observed in other studies [25]. Krochta (1994) reported that the film thickness correlated to the number of three-dimensional networks that are formed [26].

Conclusion
Glycerol concentration affected the physical and mechanical properties of nori. Increasing concentrations of tensile strength, percentage elongation, and thickness, however after addition glycerol were 1% to decrease. The optimum glycerol concentration in this study was 0.8% to get the best TS, percent elongation, and thickness.