4.1. Characterization of the Sinami Seed
The results of the proximal analysis, humidity, and total fiber of the sinami seed powder are summarized in
Table 3. The sinami seed had more than triple (25.1%) the moisture content of the açaí seed; however, both seeds contain less moisture than other tropical fruits such as aguaje [
25]. This finding holds implications for using sinami as an exfoliating agent. Linares-Devia et al. [
24] found that some samples with a high water content retained their structure, potentially withstanding high compression forces. Therefore, sinami’s exfoliating effects will depend on the user’s force. Furthermore, hardness is important considering the necessary characteristics of an abrasive particle [
24,
26].
The lipid levels did not exceed 3% in either seed. The
O. mapora variety had lipid levels <1%. Although the different varieties of
Oenocarpus have higher lipid contents, they are mainly distributed in the mesocarp. The pulp of
O. bataua can reach values between 41.8% and 51.6% of its dry weight [
27,
28] and the lipid content reaches 35.6% in
O. bacaba [
29].
The protein value of sinami (3.43%) was lower than the reported average value for açaí at 4.89% [
18], while values of aguaje are reportedly 8.6%–12.4% [
30]. Given its low protein content, sinami is not considered a potential ingredient for a farm animal diet. According to Surayah Osman et al. [
31], pets such as hamsters and rabbits require a protein demand of around 12%–25%, far higher than what sinami is able to provide.
The sinami seed features an ash content (1.0%) similar to açaí (1.3%) and aguaje (1.2–1.3%) [
18,
30]. Linares-Devia et al. [
24] recommend caution with these values because they can lead to problems with solution stability. If the ashes are extracted in the aqueous phase it would affect their ionic nature. We report our formulation and stability results in an upcoming section.
The total fiber content of sinami seeds was 9.94%, markedly lower than other Amazonian fruits such as açaí (77.20%) and arazá (36.59%,
Eugenia stipitata) [
32,
33]. However, Melo et al. [
18] reported that the total fiber values may vary due to the conditioning of the seed or the analysis method used. According to Muñoz et al. [
11], sinami contains fibrous tissue that adheres to its external walls making extraction difficult. This fibrous tissue is similar to coconut coir, hairs surrounding the endosperm. Unfortunately, no comparable research was found in the
Oenocarpus seed research literature.
4.2. Characterization of Total Phenolic Content and Antioxidant Activity
Phenolic compounds are a group of micronutrients in the plant kingdom and are an important part of the human and animal diet. They constitute a wide group of chemical substances considered secondary metabolites of plants [
34]. These compounds are a focus of increasing interest due to their various biological activities that benefit human health by preventing cancer, cardiovascular disease, and other inflammatory pathologies [
35,
36].
Table 6 shows the total phenolic content of the sinami seed and other Amazonian fruit seeds found in the literature. These results vary from 12.3 to 452.8 mg GAE/g across different extracts. Sinami seeds demonstrated the lowest levels of total phenolic compounds (12.3 mg/g). The açaí fruit, often used as a reference due to its similarities, had triple the phenol content of sinami (65 mg/g) [
18].
The total phenol content in
O. bataua reached 452.8 mg GAE/g in one study [
37]. These results are markedly different from
O. mapora. Fidelis et al. [
41] explained that external factors such as the growing region, climatic conditions, type of soil, and extraction solvent can affect the endogenous synthesis of polyphenols in fruits. Our sample was taken from the Madre de Dios region (Perú), while the
O. bataua variety was harvested in Pucallpa (Perú). Future work could focus on sinami seed phenols relative to the area where they were harvested.
Antioxidants are natural or synthetic substances that can prevent or reduce the action of reactive species by inhibiting free radicals and metal complexation [
42]. Different assays must be evaluated because each antioxidant substance acts at different stages of the oxidative process [
40].
The DPPH assay involves the transfer of electrons and hydrogen atoms and antioxidant activity is measured colorimetrically [
43]. The DPPH reagent is deep violet in solution and turns colorless to pale yellow when neutralized by radical scavengers [
44]. As reported by Amrani et al. [
45], the lower an extract’s IC
50 value, the greater its antioxidant power.
The antioxidant activity of the sinami sample (IC
50) was 0.34 μg/mL and this is a more effective value when compared to that of other Amazonian fruits such as the piquiá seed (
Caryocar villosum), pequí pulp (
Caroycar brasilense), or araçá-boi fruit (
Eugenia stipitata); here, the DPPH method showed IC
50 values of 41.07 μg/mL, 9.4 μg/mL, and 0.69 μg/mL, respectively [
46,
47,
48].
The ABTS assay is also used to measure antioxidant activity. As with the DPPH method, it involves the transfer of electrons and hydrogen atoms; however, ABTS acts in both lipophilic
and hydrophilic environments and is thus more suitable for quantifying antioxidant activity [
40,
49].
The antioxidant activity observed with the ABTS method in the seed (IC
50 = 0.34 μg/mL) surpassed that of the pulp (191.03 μg/mL) and skin (IC
50 = 43.67 μg/mL) of
Ambelania duckei, another tropical fruit [
40]. Once again, the sinami sample produced high levels of antioxidant activities. Likewise, our sample showed better results than other fruits of tropical plants, such as pitomba (77.3 mg/mL,
Talisia esculenta) and tucumã-do-Amazonas (
Astrocaryum aculeatum) [
50,
51].
Sinami seed extracts have a significant level of antioxidant activity. Wani et al. [
52] reported that the lower the IC
50 value, the greater the antioxidant capacity to reduce free radicals, and powerful antioxidants have IC
50 values of 0.05–0.10 mg/mL. Therefore, vitamin C, whose strong antioxidant power is reflected in an IC
50 of 0.009 mg/mL, can be used as a standard referent. Thus, the sinami seed could be repurposed for use in the food or cosmetic industry, as Ribeiro et al. [
53] suggested for camu-camu (
Myrciaria dubia) seeds. However, food by-products that are non-reusable—including residues from other tropical fruits such as acerola (
Malpighia emarginata), pineapple (
Ananas comosus), or passion fruit (
Passiflora edulis)—remain an issue. Moreover, it is difficult to compare published DPPH and ABTS results because of a dearth of scientific investigations into
Oenocarpus seeds.
4.3. Exfoliant Prototype Characterization
The sizes of the most widely used abrasive agents are 250–500 µm, according to Azconia [
20]. This size would be ideal for scrubs because it does not cause hypersensitivity reactions or skin irritation. However, the sinami seed is a grainy particle of natural origin; consequently, its size is non-uniform and it has many edges on its borders (
Figure 4). Considering these factors, we settled on a particle size of 250 µm.
During the preliminary test, we found the sinami seeds caused the media to change color. Initially, the mixtures were colorless but gradually assumed various shades of reddish-brown (
Figure 5). This does not occur with commercial exfoliants because they are pretreated to remove any trace of organic matter. Therefore, it appears that certain dry pulp compounds are attached to the seed within retained fibrous tissue.
Linares-Devia et al. [
24] found that dry pulp remnants that were attached to the seed could serve as a chemical exfoliant or antioxidant. However, the darkening could be perceived negatively within the final cosmetic product. Therefore, the authors added dyes to darken the product’s tint.
With this information, sodium cocoyl glycinate (Hostapon Sci-85p) is the only surfactant in prototype 1. The prototype was unaffected by the presence of foam during processing. However, the detergent action was unexpected, and the abrasive particles could not be suspended. This type of cosmetic requires a gel-type viscosity (
Figure 6a). Our results suggested that the concentration of gelling agent (acrylates) was inadequate since the abrasive particles could not be suspended.
To complement the cleaning action, we added sodium laureth sulfate (Alkopon)—a surfactant widely used in cosmetics. Unfortunately, some users find sodium laureth sulfate irritating to the skin. In such cases, a milder surfactant such as sodium lauryl sulfoacetate (Lathanol) is typically added. Both ingredients are excellent foaming agents and detergents. Both of these qualities enhance the distribution of the cosmetic following skin application. However, two issues arose: bubbles and solution color changes.
As shown in
Figure 6b, prototypes 2 and 3 produced lots of foam during testing. As expected, the aqueous surfactant solutions were prone to foaming. Kelleppan et al. [
54] explain that the molecules of the surfactants are used to orient their hydrophilic head groups toward the solution with their hydrophobic tail groups pointing toward the air, thus allowing appreciable volumes of foam to develop and remain stable over time. Even though foam is a desirable property in a cleaning product—because it drags the hydrophobic particles from the substrate and improves the consumer’s perception of the product’s efficacy [
55]—it is aesthetically unfavorable since the foam conceals the mixture’s sinami particles. Furthermore, use of both surfactants changed the mixture’s color. The mixture remained crystal clear during the dissolution of water with Alkopon; however, the subsequent inclusion of Lathanol changed the color of the medium to a milky one, making it even more difficult to see the exfoliating agent.
Because the inclusion of two surfactants was unsuccessful, we decided to continue with sodium laureth sulfate (Alkopon) due to its lower cost in prototype 4 [
56]. However, we included cocamidopropyl betaine to counteract potential irritation. This cosmetic ingredient is a surfactant, which is mild and skin-friendly, used in conditioners, body washes, and other personal care products. Many prefer cocamidopropyl betaine over other surfactants due to its synergistic effects with other cosmetic ingredients, thus favoring the formulation of products [
57,
58]. We adjusted the proportion of thickening agents to prepare prototype 4, noting the point where the prototype’s abrasives were suspended (
Figure 6c). Finally, a commercial exfoliant gel was used to adjust the proportion of the abrasive agent.
Figure 7 shows our prototype compared to the commercial gel after the centrifugation test.
4.4. Storage Stability Test
The stability of a cosmetic product involves physical, chemical, and microbiological analyses. Physical stability is measured by the absence of phase separation and the modification of the product’s rheological characteristics [
59]. We evaluated the stability of our prototype sinami exfoliant gel by comparing its pH and viscosity at room temperature and under stress conditions.
The stress condition accelerates the expected changes due to normal storage and use conditions. Despite the heat treatment, no significant differences in color or particle suspension were observed in the prototype; however, a thin layer of water was observed on the exfoliant gel samples during the stability tests. According to Zięba et al. [
60], this sheet of water occurs when water evaporates from the product due to increased heat; however, when the solution was homogenized with a palette, it dispersed without major complications.
Viscosity refers to how a cosmetic product spreads across human skin [
61]. The viscosity readings recorded up to day 84 (3 months) ranged between 700 and 800 mPa·s at room temperature (20 °C) and between 500 and 600 mPa·s under stress conditions (40 °C). Notably, our results differ from Zięba et al. [
60] who examined a shower gel formulation and found the dynamic viscosity to increase by 5% after three months of storage. In contrast, in our study, the dynamic viscosity was reduced by 28.4%. Although the data obtained showed the viscosity below that of commercial gel exfoliants (3600–6000 mPa·s), future sensory evaluation tests could improve this formulation.
The variation in pH in a cosmetic product can represent instability, direct or indirect contamination during formulation, or possible chemical reactions between raw materials [
62,
63]. Human skin’s pH ranges from 4.1 to 5.8, depending on location. Thus, products that are applied to skin should be formulated in this range.
According to Berthele et al. [
64], these changes in pH are probably due to yeast and mold growth. The initial pH value of the exfoliant gel prototype was 5.25, consistent with Blaak and Staib’s recommendations [
22], and at the end of the test, the pH value increased by 0.57%. The prototype’s pH varied minimally and remained within an acceptable limit at 0.06% after a preservative (Prodice CG) was added to the base formula. These results are encouraging because they could indicate that this formulation may be adequate for cosmetic use. Importantly, cosmetic products must maintain physical stability throughout their useful life so that the user does not perceive changes as the product is repeatedly applied over time.