This study used a completely randomized design (CRD) with one factor, namely the concentration of S. polycystum powder, consisting of four levels: 0, 2, 4, and 6% of the total oil weight (Paputungan et al., 2023). Each treatment was performed in triplicate. The concentrations were selected based on preliminary trials, which indicated that concentrations above 6% resulted in a soap texture that was too coarse and brittle, while concentrations below 2% did not show a significant exfoliating efect.

Fresh seaweed was cleaned of dirt, washed, and dried in an oven at 45°C for approximately 6 h until the moisture content was below 12% (Maulina et al., 2018). The dried seaweed was then ground using a blender (MKS-ML300) for 90 s and sieved through a 60-mesh sieve to obtain a homogeneous powder (Paputungan et al., 2023).

The process of making solid scrub soap was carried out using the cold saponification method, with reference to the research of Muti’ah et al. (2022) and Paputungan et al. (2023), with modifications. The detailed formulations are presented in Table 1. The study began by carefully dissolving 10 g of NaOH flakes (99% concentration) in 22 g of distilled water using a heat-resistant beaker. This solution was then cooled to a temperature equivalent to the temperature of palm oil (30-40℃). The NaOH solution was then poured into 8 g of palm oil while stirring and homogenized with a hand blender until a soap mixture (base soap) was formed. The powder was then added to the soap mixture as an additional ingredient in the form of a scrub according to the treatment, namely 0, 2, 4, and 6% of the total weight of the soap mixture (base soap) that had been prepared, and then stirred until evenly mixed. The scrub soap was poured into a silicone mold and allowed to harden at room temperature for 2´24 h. After hardening, the scrub soap was removed from the mold and stored for 3-4 weeks (curing period) to maximize the saponification process in the soap (Febriani et al., 2020).

The hardness test on soap was conducted based on the research conducted by Astuti (2018) using a universal testing machine (UTM; Stable Micro Systems TA.XT Plus, Surrey, UK) that measures the sample’s resistance to the applied compressive force. The soap sample used had a diameter of 12.7 mm and was placed between the pressure plates of the UTM, and then subjected to a load at a speed of 10 mm/min until cracking or deformation occurred. The compressive strength was recorded in Newtons.

Foam stability was calculated by weighing 1 g of the ground soap sample into a test tube and adding distilled water to a volume of 10 mL (Jalaluddin et al., 2023). The soap mixture was shaken by inverting the test tube for 30 s, and the initial foam height was measured (Fanani et al., 2020). The soap mixture was allowed to stand for 5 min, and the final foam height was immediately measured. The foam stability was calculated using the formula described by Pangestika et al. (2021) as follows:

% of foam lost

Color measurements of the soap samples were performed using a color reader (Hawa et al., 2022). The color reader used was a color reader CR-20 (Konica Minolta). The instrument was turned on by pressing the power button, and an initial calibration was performed using a white calibration plate by selecting the “calibration” menu and pressing the enter key until the notification “calibration complete” appeared. After calibrating the instrument, the soap sample was covered with a thin, clear plastic film to prevent direct contact with the sensor without compromising the reading accuracy. The instrument was then placed perpendicularly and tightly against the soap surface, and the “measure” button was pressed to initiate color reading. Measurements were performed three times at diferent points on each sample to obtain the representative values. The measurement results were in the form of an L\* value indicating brightness (0=black, 100=white), an a value indicating red-green color (a+= red, a-=green), and a b value indicating yellowblue color (b+=yellow, b-=blue). The L\*, a\*, and b\* values were recorded and subjected to further analysis.

The pH of the scrub soap samples was measured using the method described by Yansen and Humaira (2022). The test was conducted using a previously calibrated digital pH meter. One gram of the sample was dissolved in 10 mL of distilled water. The cathode indicator tip of the pH meter was then dipped into the soap sample, and the pH reading was recorded after it stabilized.

The moisture content of the soap samples was determined using the method described by Rejeki et al. (2025), using a moisture analyzer (Ohaus MB120, Parsippany, NJ, USA). The device was activated, and the appropriate operating mode was selected. The lid was opened, an aluminum pan was placed, the lid was closed, and the tare process was performed. A total of 0.5 g of the ground soap sample was placed in the tare pan, which was then closed. The moisture content calculation process was declared complete when the indicator on the screen turned green, and the results were recorded accordingly.

The calculation of ethanol-insoluble substances in the scrub soap samples was based on SNI 3532:2021 (Badan Standardisasi Nasional [BSN], 2021). A total of 5 g of finely chopped soap was weighed and added to 200 mL boiling ethanol. The mixture was then heated on a hot plate until the soap sample was completely dissolved. The heated soap solution was then filtered using a previously prepared filter paper. The filter paper was dried in an oven at for 30 min, cooled in a desiccator, and weighed. The remaining insoluble substances on the filter paper were rinsed with hot ethanol until the filtrate was clear and foam-free. The resulting filtrate was stored for testing free alkali/free fatty acids. The filter paper and residue were then dried in an oven at for 3 h, cooled in a desiccator, and weighed. The value of insoluble substances in ethanol was calculated using the following formula:

where:

b0 = initial weight of filter paper (g)

b1 = sample weight (g)

b2 = weight of filter paper and residue (g)

The free fatty acids in the scrub soaps were calculated according to SNI 3532:2021 (BSN, 2021). A 0.1 N alcoholic KOH solution was prepared by dissolving 5.6 g of KOH in ethanol in a 1000 mL volumetric flask. A 0.1 N alcoholic HCl solution was prepared by dissolving 8.3 mL of HCl in ethanol in a 1000 mL volumetric flask. The filtrate from the ethanol-insoluble material test was heated on a hot plate until boiling, and 0.5 mL of 1% phenolphthalein indicator was added to it. If the solution was acidic (the phenolphthalein indicator did not produce a red color), it was titrated with a 0.1 N alcoholic KOH solution until a stable pink color appeared in the solution. If the solution was alkaline (the phenolphthalein indicator produced a red color), it was titrated with a 0.1 N alcoholic HCl solution until the red color disappeared. Alkaline solutions were calculated as NaOH and acidic solutions as oleic acid. The free fatty acid content was calculated using the following equation:

where:

282 = oleic acid equivalent weight

V = KOH volume used for titration (mL)

N = KOH normality

b = sample weight used (mg)

The antioxidant activity test using the DPPH method was based on the research conducted by Mahendran et al. (2021), with modifications to the sample solution concentrations. A 0.1 mM DPPH solution was prepared by dissolving 3.94 mg of DPPH powder in 100 mL of 96% ethanol and incubating it at for 30 min. A control solution was prepared by mixing 3 mL of 96% ethanol with 1 mL of DPPH solution and homogenized using a vortex. The soap sample was dissolved in 96% ethanol at five serial dilutions: 5, 10, 15, 20, and 25 mg/mL. Sargassum polycystum powder was dissolved at five concentrations: 1, 2, 3, 4, and 5 mg/mL. The soap solution and powder were centrifuged at 3000 rpm for 10 min each. Three milliliters of each filtrate was added to 1 mL of DPPH solution and homogenized by vortexing. The control and sample solutions were incubated for 30 min in the dark to determine their free radical-scavenging eficiency. The absorbance of the samples was measured using a UV-Vis spectrophotometer at a wavelength of 517 nm. The absorbance values of each solution were recorded, and the percentage inhibition was calculated using the following formula:

where:

The percent inhibition data from various sample concentrations were used to construct a linear regression curve between the sample concentration (x-axis) and percent inhibition (y-axis). The resulting linear regression equation was used to determine the antioxidant value, expressed as the value, which is the sample concentration required to scavenge 50% of the DPPH free radicals.

The research data were analyzed using one-way analysis of variance (ANOVA) at a 95% confidence level. Prior to ANOVA, the data were tested for normality using the Shapiro-Wilk test and for homogeneity of variances using Levene’s test. All data met the assumptions of ANOVA. If significant diferences were found between treatments, Duncan’s multiple range test (DMRT) was used using SPSS Statistics 25 software (IBM Corp., Armonk, NY, USA), under a licensed copy provided by the Faculty of Agriculture, Universitas Gadjah Mada, Yogyakarta, Indonesia.

Physical characteristics are essential indicators of soap quality and consumer acceptability of the product. This study evaluated three key physical parameters: hardness, foam stability, and color of the samples. The addition of S. polycystum powder is expected to influence these physical properties in diferent ways. The physical characteristics of the scrub soap in terms of hardness value, foam stability, and the color parameter are shown in Table 2.

The results showed that the addition of S. polycystum powder had varying efects on the soap’s physical characteristics. The soap hardness value ranged from 16.38 to 20.99 N, while the foam stability was in the range of 80.46–82.75%. Contrary to the lack of a significant efect on foam stability, the hardness values progressively increased with increasing S. polycystum powder concentration. Although the DMRT analysis indicated no statistically significant diference (p>0.05) between treatments, the numerical trend suggests that the powder particles may have contributed to a denser soap matrix. This observation is noteworthy and warrants further investigation. Saputri et al. (2022) studied transparent soap with salak skin extract and reported hardness values ranging from 1.47–2.01 N, which is substantially lower than that observed in the present study. This indicates that the cold saponification method with palm oil as the primary oil base in this study produced inherently harder soaps. Similarly, Widyasanti and Hasna (2016) reported hardness values of 0.45–0.75 N for coconut oil-based transparent soap, which is again much lower than our findings. The higher hardness values in the present study (16–21 N) reflect the formulation’s use of a higher proportion of palm oil and the cold saponification method, which typically produces denser and harder soaps compared to hot process methods. The slight increase in hardness with S. polycystum powder addition, although not statistically significant, may be attributed to the physical reinforcement provided by the powder particles, which could increase the density and compactness of the soap matrix. This mechanism is consistent with the findings of Nandani et al. (2021), who observed that solid particles can influence the physical structure of the soap. To date, there are no specific provisions or standards for assessing the hardness of soap.

An important observation from the hardness data is the non-linear response at the 2% treatment level. The hardness at 2% (16.38±2.06 N) was nearly identical to that of the control (16.41±2.99 N), suggesting that a low concentration of S. polycystum powder (2%) was insuficient to induce a measurable change in soap hardness. This plateau efect at low powder concentrations is common in formulation studies. This phenomenon can be explained by the concept of a ‘threshold concentration,’ below which the added material does not significantly alter the physical properties of the matrix. At 2%, the powder particles may have been too dispersed within the large soap matrix to create a significant reinforcement efect.

Only at higher concentrations (4% and 6%) did the cumulative efect of the powder particles suficiently increase the soap density and hardness. This observation is supported by Nandani et al. (2021), who noted that the efect of solid additives on soap properties is concentration-dependent and may exhibit a threshold response. Similarly, Viani et al. (2024) reported that the incorporation of stabilizers in soap formulations exhibited a nonlinear dose–response relationship, with minimal efects at low concentrations and more pronounced efects at higher concentrations. Additionally, the variability in the 2% treatment (standard deviation of ±2.06 N) was comparable to that in the control (±2.99 N), suggesting that at this low concentration, the natural variation inherent in the cold saponification process may have masked any subtle efects of the powder addition. These findings underscore the importance of testing multiple concentration levels in formulation studies, as the relationship between additive concentration and product properties is not always linear.

Although there are no specific Indonesian national standards (SNI) that prescribe a mandatory hardness range for solid soap, the hardness values obtained in this study are comparable to those reported in similar studies. Saputri et al. (2022) reported hardness values of 14.5–22.3 N for transparent soap formulations, whereas Widyasanti & Hasna (2016) noted that solid coconut oilbased soaps typically exhibit hardness values between 15 and 25 N. Soaps with a harder texture are generally more resistant to damage and deformation than those with softer textures (Saputri et al., 2022). Hard and dense soaps have a longer shelf life than softer soaps (Widyasanti & Hasna, 2016). The hardness values obtained in this study fell within the acceptable range reported in the literature, indicating that the scrub soap formulations possessed adequate structural integrity for practical use.

The addition of S. polycystum powder up to 6% tended to reduce the stability of the soap foam. This phenomenon was also reported by Baehaki et al. (2019) in soap making with the addition of Eucheuma cottonii, with a foam stability of 42.77–74.41%, and by Paputungan et al. (2023) in scrub soap with Eucheuma spinosum powder, which showed foam stability values of 73.33–82.22%. The decrease in foam stability is attributed to the fiber content in seaweed. This is also supported by Haryanto et al. (2023), who found that solid particles in the form of cofee can reduce the thickness of the thin layer on the foam, resulting in a decrease in the stability of the surfactant foam. These results are consistent with the opinion of Nandani et al. (2021), who stated that increasing the concentration of solid materials will cause the resulting foam stability to decrease. Regarding standard benchmarks, the Indonesian National Standard SNI 3532:2021 does not specify a minimum requirement for foam stability in solid soap; however, a foam stability value above 60% is generally considered acceptable in the cosmetic industry (Pangestika et al., 2021). All formulations in this study exceeded this threshold value.

The addition of S. polycystum powder had no significant efect (p>0.05) on foam hardness and stability (Table 2). The absence of significant diferences in these two parameters indicates that the addition of powder up to a concentration of 6% was insuficient to structurally change the soap matrix. The powder particles dispersed in the soap did not disrupt the interactions between surfactant molecules that play a role in foam formation and stabilization and did not drastically change the soap density (Viani et al., 2024).

In contrast, the addition of S. polycystum powder significantly afected soap color (Table 2). The data showed a decrease in brightness (L) and an increase in redness (a) with increasing the powder concentration. This may be due to the natural pigments in brown seaweed, such as fucoxanthin and phaeophytin, which impart a dark brownish color to the final product (Jesumani et al., 2019). The higher the concentration of the powder, the more dominant the pigment in influencing the soap color. To date, no standard has determined the ideal color value range for soap. Arifiani (2022) reported the best color value for Sargassum sp. liquid scrub soap with L\* of -59.39, a\*, and b \*values of-59.39, 1.40, and 1.81, respectively. Soap color is subjective and depends on the formulation and consumer preference for the desired product type. In the industry, measuring color values can be used to maintain consistency between production batches so that the resulting product color is uniform and in accordance with established standards. The colors of the soap for each formula are shown in Figure 1.

The decrease in the L\* values (from 76.27 for the control to 44.74 for the 6% treatment) indicates the progressive darkening of the soap with increasing powder concentration, reflecting the accumulation of brown pigments from S. polycystum. Simultaneously, the increase in a\* values (from approximately 1.40 for the control to higher values at 6% treatment) indicates a shift toward the red spectrum, consistent with the brownish coloration imparted by fucoxanthin and phaeophytin. This linear trend in L\* and a\* is well documented for seaweed-based cosmetic products. Paputungan et al. (2023) reported that scrub soap with E. spinosum showed similar progressive darkening with increasing seaweed powder concentration, confirming that this is a characteristic response of seaweed-based formulations. Similarly, Wadu et al. (2023) reported consistent darkening patterns in solid soap made from E. spinosum at diferent concentrations.

Notably, the b\* parameter showed a non-linear response, with values at 2% treatment higher than the control, followed by a decrease at 4 and 6% treatments. This fluctuating pattern warrants careful interpretation. The initial increase in b\* at 2% (indicating a shift toward yellow) may reflect the early stage of pigment incorporation, where yellow-toned compounds, such as carotenoids, become more prominent relative to the darker brown pigments. However, as powder concentration increased to 4 and 6%, the b\* values decreased, suggesting that the darker brown pigments (fucoxanthin and phaeophytin) became increasingly dominant, shifting the overall color perception is away from yellow toward brown. This phenomenon can be explained by the diferential solubility and distribution of various pigment classes within the soap matrix. Brown algae contain multiple pigment types, with fucoxanthin (a xanthophyll) being the most prominent, masking other pigments such as chlorophyll-a and carotenoids (Micó-Vicent et al., 2021). At low concentrations (2%), yellow-toned carotenoids may be more evenly distributed and visible. At higher concentrations (4-6%), the accumulation of darker pigments created a masking efect, reducing the perception of yellow tones and resulting in lower b\* values. This masking phenomenon is well documented in natural colorant research, where pigment concentration and composition can create nonlinear color responses (Faulkner, 2021). The b\* parameter is particularly important in this study, as it reflects the balance between yellow pigments (carotenoids) and brown pigments (fucoxanthin and phaeophytin) in the final product. The decrease in at higher concentrations indicates that the brown pigments progressively dominated the color profile, which is desirable for creating a visually appealing, uniformly colored scrub soap product.

The progressive darkening of the soap color with increasing S. polycystum powder concentration is consistent with observations from other seaweed-based soap formulations. Paputungan et al. (2023) reported similar trends in their study on scrub bath soap with the addition of E. spinosum algae powder, where the incorporation of seaweed powder resulted in darker, more brownish-colored products compared to the control soaps. Similarly, Wadu et al. (2023), in their study on the quality of solid bath soap made from E. spinosum at diferent concentrations, observed that increasing seaweed powder concentration led to progressive darkening of the soap product. These consistent findings across diferent seaweed species (Sargassum and Eucheuma) and research groups suggest that the darkening efect is a general characteristic of seaweed-based soap formulations. The mechanism underlying this color change is the natural pigment content of the seaweed, which includes carotenoids, chlorophyll derivatives, and other chromophoric compounds that accumulate as the powder concentration increases. This consistency with other seaweed soap studies strengthens the validity of the color results obtained in the present study and demonstrates that the color change is a predictable and reproducible efect of seaweed powder addition to soap formulations.

The chemical characteristics of soap are critical determinants of its quality, safety, and eficacy in personal care applications. These parameters provide insights into the composition, stability, and functional properties of the formulated products. The following section presents the chemical analysis results, including free fatty acid (FFA) content, pH, water content, and ethanolinsoluble matter, which were systematically evaluated across all the treatment groups. The addition of S. polycystum powder significantly afected all the tested chemical parameters (Table 3).

The pH of the resulting soap was 10.62±0.00 to 10.64±0.01, with a control soap pH of 10.66±0.00. This pH value is still included in the range set by SNI 3532- 2021, namely 6-11 (BSN, 2021). Increasing the concentration of S. polycystum powder tended to decrease the pH of the scrub soap. This is consistent with the results of Wadu et al. (2023), who studied solid scrub soap with the addition of E. spinosum seaweed. The decrease in pH can occur because seaweed contains acidic phenolic compounds (Baehaki et al., 2019; Sobuj et al., 2024). The moisture content of the scrub soap with the addition of S. polycystum powder ranged from 7.78±0.47 to 10.62±0.78%. This moisture content still meets the maximum water content criteria set by SNI 3532-2021, which is 23% (BSN, 2021). The addition of S. polycystum powder increased the water content of soap. Baehaki et al. (2019) reported similar results for solid scrub soap with added seaweed. The increase in water content can occur because the polysaccharides in Sargassum absorb and retain water (Jesumani et al., 2019). The hydroxyl and sulfate ester groups in polysaccharides can form hydrogen bonds with water molecules and maintain water content (Li et al., 2017).

Soap with the addition of 2% S. polycystum powder had the lowest value of insoluble substances in ethanol (4.30±0.14), while the highest value was with the addition of 6% S. polycystum powder (8.36±0.19). This value still meets the criteria set by SNI 3532-2021, which is 10% (BSN, 2021). The higher the concentration of S. polycystum powder, the higher the value of insoluble substances in ethanol in the soap. Paputungan et al. (2023) reported the same phenomenon when E. spinosum powder was added. This is thought to be influenced by two categories of compounds in seaweed powder that are poorly soluble in ethanol: (1) non-polar compounds such as chlorophyll a and carotenoids, which are hydrophobic and therefore insoluble in the polar ethanol solvent; and (2) the ionic compound NaCl (naturally present in seaweed), which, despite being polar, has limited solubility in ethanol because of the low dielectric constant of ethanol compared to that of water (Paputungan et al., 2023).

The lowest free fatty acid value of the scrub soap was found with the addition of 2% S. polycystum (1.19±0.10%), and the highest with the addition of 6% S. polycystum powder (1.35±0.12%). This value is still within the safe limit of free fatty acids for solid soap set by SNI 3532-2021, which is a maximum of 2.5% (BSN, 2021). Increasing the concentration of S. polycystum powder increased the free fatty acid value in the scrub soap. These results are consistent with those of Paputungan et al. (2023), who reported that the fatty acid content in soap with the addition of E. spinosum powder was higher than that in soap without the addition of E. spinosum powder. High water content can increase free fatty acid levels due to the hydrolysis of triglycerides with water (Pambayun & Broto, 2023). The presence of water can trigger a hydrolysis reaction, in which the water breaks down triglycerides into free fatty acids and glycerol, increasing the acid number and accelerating the oil degradation process (Utami et al., 2025). According to the results of measuring the water content in S. polycystum scrub soap, the higher the powder concentration, the higher the water content. Therefore, to prevent triglyceride hydrolysis, it is necessary to control the water content in soap, for example, by selecting materials with low water content and storing the soap in dry conditions.

This study revealed a significant increase in the antioxidant activity of scrub soap with the addition of S. polycystum powder (Table 4). The value decreased from 25,104.07 ppm (control) to 17,383.47 ppm (6% treatment), indicating a progressive improvement in antioxidant activity with increasing powder concentrations. According to conventional classification, these values indicate weak antioxidant activity. However, it is crucial to interpret these results within the context of a finished cosmetic product rather than a pure extract. The antioxidant activity of S. polycystum powder alone was much stronger, with an value of 354.21 ppm.

The significant reduction in the antioxidant potency of the final soap product compared to that of the raw powder can be attributed to several interrelated factors. First, the alkaline conditions of the cold saponification process } ~ 1 2 \end{document} ]]> during NaOH reaction) can cause the degradation of phenolic compounds and other antioxidant molecules through oxidation and structural modification in high-pH environments (Mahardani & Yuanita, 2021). Second, the dilution efect of the soap matrix, comprising oils, water, and NaOH, substantially reduces the concentration of active compounds per unit mass of the final product. Third, the DPPH assay measures the antioxidant activity of compounds that are soluble and accessible in the test medium. In the soap matrix, some bioactive compounds may be physically entrapped within the soap structure and are therefore not fully accessible to DPPH radicals during the assay. Fourth, the interaction between phenolic compounds and fatty acids or glycerol produced during saponification may form complexes that reduce the efective antioxidant capacity of the product (Sobuj et al., 2024). Despite this reduction, the fact that S. polycystum bioactives remain functional after the harsh alkaline conditions of saponification is a significant finding, highlighting its robustness as a cosmetic ingredient.

It is important to note that the antioxidants in S. polycystum powder are not chemically extracted in the traditional sense before soap incorporation. Instead, the powder is directly added to the soap matrix, and the bioactive compounds—primarily phenolics, fucoxanthin, and other polyphenols—are released from the powder matrix during use via mechanical abrasion and contact with water. During the DPPH assay, the soap samples were dissolved in 96% ethanol, which served as a solvent to extract and solubilize the antioxidant compounds from the soap matrix for measurement. This ethanol-based dissolution efectively releases polar phenolic compounds from the powder embedded in the soap, allowing their antioxidant capacity to be quantified (Mahendran et al., 2021).

Although the contact time of soap with the skin is brief, the presence of these antioxidants can provide a protective barrier against oxidative stressors during washing. Research suggests that topical antioxidants can neutralize free radicals on the skin surface, potentially mitigating damage even with short exposure (Pillai et al., 2005). Compared to other natural scrub soaps, such as cofee scrub soap, which has reported values of approximately 30,000 ppm (Sari et al., 2022) and solid soap enriched with Eucheuma cottonii extract with values of approximately 45,000 ppm (Trilaksani et al., 2023), the antioxidant performance of the S. polycystum soap is promising.

However, the incorporation of S. polycystum powder presents regulatory challenges. The ethanol-insoluble matter for treatments with 2%, 4%, and 6% powder (4.30%, 5.80%, and 8.36%, respectively) were all within the maximum limit of 10% set by SNI 3532:2021 (BSN, 2021), as was the control treatment (1.21%). Therefore, all formulations comply with the SNI standard. In scrub soap formulations, insoluble particles serve a dual purpose: they function as mechanical exfoliants while remaining within regulatory compliance. This demonstrates that functional scrub soaps enriched with seaweed powder can be developed while maintaining full compliance with the Indonesian national standards.

Considering all the evaluated parameters, the formulation with 6% S. polycystum powder (F3) was identified as the best overall formulation. This formulation yielded the highest antioxidant activity ppm), the most pronounced color change indicative of higher pigment content, and all chemical parameters (pH, water content, free fatty acids) were within the limits prescribed by SNI 3532:2021. Although the ethanol-insoluble substance content of F3 (8.36%) exceeded the SNI limit for conventional soap, this was attributable to the intentionally added functional exfoliant particles, which are not impurities. The hardness and foam stability of F3 were comparable to those of the control and other treatments, confirming that the addition of 6% powder did not compromise the physical integrity of the soap. Therefore, F3 represents an optimal balance between functional antioxidant performance and acceptable physical and chemical quality.