Sustainable valorization of animal tallow and Aloe abyssinica gel for bar soap formulation with physicochemical properties and quality assessment

Abstract

This study investigates the sustainable production of bar soap using locally sourced Aloe abyssinica gel and animal tallow as primary ingredients. Animal tallow was rendered from local butcher waste and characterized, showing a saponification value of 181.78 mg/g NaOH, acid value of 17.32 mg/g NaOH, pH of 5.45, and specific gravity of 0.929 all within acceptable ranges for soap manufacturing. Fresh A. abyssinica gel was extracted from its leaves and characterized by exhibiting 99.23% moisture content, pH of 4.82, refractive index of 1.33, total dissolved solids of 3.350°Brix, and viscosity of 1.359 cP. Twelve soap formulations were prepared using the cold process method, incorporating varying concentrations of A. abyssinica gel (0–25%) with tallow (50–75%), supplemented with coconut oil (15%) and either olive oil or palm oil (10%). Physicochemical analysis revealed that formulations containing 15% A. abyssinica gel demonstrated optimal properties: pH values of 9.75–9.92, total fatty matter content of 73.33–76.8% (Grade I quality), enhanced foamability (4.8–7.3 cm), superior cleansing power, and reduced free caustic alkali (0.08–0.13%). Results confirm that combining A. abyssinica gel with animal tallow produces high-quality, skin-compatible soap while promoting waste valorization and sustainable manufacturing practices.

Introduction

Soap is among the oldest known chemical products, with its origins tracing back over two millennia to ancient civilizations such as Babylon and Egypt. Early formulations involved the saponification of animal fats using alkaline substances derived from plant ashes. Over the centuries, soap-making evolved into a chemically controlled process integral not only to personal hygiene but also to numerous industrial sectors including textiles, pharmaceuticals, and lubricants¹. Chemically, soap is defined as the alkali salt of a fatty acid, typically produced through the saponification reaction wherein triglycerides derived from fats or oils react with strong alkalis such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), yielding glycerol and soap.

The properties of the resulting soap are significantly influenced by the composition of fatty acids present in the raw materials. Saturated fatty acids such as palmitic and stearic acids contribute to harder, longer-lasting soaps with higher melting points, while unsaturated fatty acids like oleic and linoleic acids produce softer soaps with better moisturizing properties but lower oxidative stability^2,3. Additionally, the type of alkali used also plays a key role sodium-based soaps are ideal for solid bars due to their hardness, while potassium-based soaps are softer and more suited to liquid formulations. Specialized metallic soaps made from calcium, magnesium, or aluminum are used in industrial applications like lubricants and cosmetics, although they are not suitable for cleaning due to their low solubility in water^4,5.

The cleansing power of soap arises from its amphiphilic nature possessing both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions allowing it to emulsify oils and lift away dirt and grease in the form of micelles⁶. Several factors influence the quality of soap, including the free fatty acid content of oils, the alkali-to-fat ratio, temperature, and mixing conditions. These parameters must be carefully controlled to ensure optimal soap characteristics such as hardness, lathering ability, and low residual alkalinity⁷.

Growing consumer demand for sustainable and locally sourced materials has sparked our investigation into incorporating Aloe abyssinica gel and animal tallow as key ingredients in soap manufacturing. Choosing Aloe abyssinica because of its well-documented skin benefits the plant’s antimicrobial and anti-inflammatory properties come from bioactive compounds including lupeol, salicylic acid, cinnamic acid, phenols, and sulfur compounds⁸. These components make it particularly valuable for creating gentle, skin-soothing products.

Animal tallow presents an intriguing opportunity as a soap base. This meat industry byproduct contains high concentrations of saturated fatty acids, particularly palmitic and stearic acids, which produce the hard texture and rich, creamy lather that consumers expect from quality bar soap. Beyond its functional properties, tallow offers significant advantages: it’s considerably more affordable than many vegetable oils, demonstrates excellent chemical compatibility in saponification reactions, and transforms what would otherwise be waste into a valuable resource supporting circular economy principles⁹ This study pioneers the systematic integration of Aloe abyssinica gel with rendered animal tallow for sustainable soap production. Novel contributions include establishing optimal gel-tallow ratios (15%:60%), achieving Grade I quality standards, demonstrating Aloe abyssinica ‘s dual pH-buffering and bioactive functions, and developing waste valorization protocols converting meat industry byproducts into premium cosmetic products.

Materials and methods

Chemicals and materials

Fresh Aloe abyssinica leaves from a cultivation site located approximately 200 km south of Gondar, Ethiopia, ensuring to obtained mature plants with optimal gel content. For our chemical analyses and soap formulation used analytical-grade reagents exclusively from Merck India Pvt Ltd, including sodium hydroxide (NaOH), sodium chloride (NaCl), sodium silicate, Linear Alkyl Benzene Sulphonic Acid (LABSA), sodium sulfate, citric acid, ethyl alcohol, hydrochloric acid, nitric acid, and chloroform.

To create well-balanced soap formulations, combined animal tallow as our primary fat base with supplementary oils coconut oil, palm oil, and olive oil. All these items were procured from reliable local suppliers in Gondar’s commercial markets. This sourcing approach allowed us to maintain ingredient quality while supporting regional commerce and minimizing supply chain complexities.

Methods

Collection animal fat and rendering of tallow for soap

Local butchers in Bahir Dar city supplied animal fat as the primary raw material. The fat underwent rendering a controlled heating process that extracts clean tallow by separating it from unwanted tissue and impurities (Fig. 1). Visible tissues were trimmed away, and the fat was cut into small, uniform pieces for even melting. The chopped fat was placed in a large pot (filled to half capacity to prevent overflow) with enough water to submerge it. Salt was added at a ratio of 20 g per 500 g of fat to draw out impurities and prevent scorching. The mixture was boiled, then simmered just below boiling point for 30 min with frequent stirring to ensure uniform melting and prevent sticking. As the fat is liquefied, it naturally separated from solid residues like meat fibers¹⁰. The liquid fat was filtered through muslin cloth to remove solid waste, with the cloth pressed to extract maximum rendered fat. This yielded a soft, golden-brown liquid tallow that was poured into a wide bowl and refrigerated overnight. Upon cooling, the tallow solidified into a white disc on the surface while water and sediment settled below. The solid tallow was carefully removed, scraped clean of any remaining impurities, and re-melted as needed for soap formulation. This traditional method provides a sustainable, effective approach for small-scale soap production in resource-limited settings, ensuring high-quality tallow that enhances manufacturing efficiency and viability¹¹. All experiments were conducted in triplicate (n = 3).

Fig. 1.

Animal tallow rendering process.

Characterizations of animal tallow oil

(a) Saponification value determination.

The characterization of animal tallow oil included the determination of its saponification value, which indicates the amount of alkali required to saponify a given fat sample. For this, 2 g of tallow oil was placed in a volumetric flask along with 20 mL of 97% ethanol and 20 mL of 0.1 N sodium hydroxide (NaOH). The mixture was then connected to a reflux condenser and heated for 30 min to ensure complete dissolution and reaction of the fat. After cooling, 2 to 3 drops of phenolphthalein indicator were added, and the solution was titrated with 0.5 M hydrochloric acid (HCl) until the pink color just disappeared, signaling neutralization (AOCS Official Method Cd 3–25)¹². A blank test was performed under identical conditions without the tallow oil to account for any reagent consumption³. The saponification value (SV) was calculated using Eq. (1):

1

Where V₀ is the volume of the solution used for blank test, V₁ is the volume of the HCl solution used for determination; N is actual normality of the HCl, 40.02 is molecular weight of NaOH and W is weight of tallow oil.

(b) Acid values determination.

The acid value of animal tallow oil, which indicates the amount of free fatty acids present, was determined through titration. A volume of 100 mL of 97% ethanol was heated with 10 g of the tallow oil sample in a volumetric flask until the mixture began to boil. Once boiling commenced, heating was stopped, and two drops of phenolphthalein indicator were added. The hot mixture was then titrated with a standard 0.1 N sodium hydroxide (NaOH) solution while shaking continuously until a persistent pink color was observed, indicating the endpoint of the titration. The acid value (AOCS Official Method Cd 3a-63)^13,14 was calculated using the following Eq. (2):

2

(c) pH Value determination.

The pH value of the soap sample was determined to assess its alkalinity or acidity, which is a key indicator of skin compatibility and overall product safety. For the measurement, 2 g of the sample was transferred into a clean 250 mL beaker, and 13 mL of hot distilled water was added. The mixture was stirred slowly until the sample was fully dispersed. It was then cooled in a cold-water bath to a stable temperature of 25 °C. Prior to measurement, the pH meter (Hanna Instruments HI 2211, Romania) was calibrated using standard buffer solutions. Once calibrated, the electrode was immersed in the prepared sample solution, and the pH value (IS 4955:2013)^15,16 was read directly from the instrument and recorded^17,18.

(d) Specific gravity determination.

The density, or specific gravity, of the tallow oil was determined using a density (specific gravity) bottle method. A clean, dry 50 mL capacity specific gravity bottle was first weighed empty to obtain the initial weight (W_o). The bottle was filled completely with tallow oil, a stopper was inserted to eliminate air bubbles, and the combined weight of oil and bottle (W₁) was recorded. Next, the bottle was emptied and thoroughly cleaned, dried completely, then refilled with distilled water to the same volume mark. The water-filled bottle was weighed to obtain the second measurement (W₂).

This approach provides accurate determination of how the oil’s density compares to water as a fundamental property when characterizing fats for soap manufacturing applications¹⁹. The specific gravity (Sp. gr) (AOCS Official Method Cc 10a-25)²⁰ of the tallow oil was calculated using Eq. (3):

3

Aloe abyssinica leaf collection and gel extraction

Aloe abyssinica gel was extracted from its mature leaves collected in South Gondar, approximately 200 km from Bahir Dar city (Fig. 2). Traditional hand-filleting methods were employed to preserve the gel’s bioactive properties.

Fig. 2.

Aloe abyssinica gel extraction.

After washing the leaves thoroughly, serrated edges were trimmed and both green rind layers carefully removed to expose the inner gel. This clear, mucilaginous gel contains concentrated polysaccharides, vitamins, enzymes, and phenolic compounds responsible for Aloe abyssinica’s skin benefits²¹. Manual extraction using sterilized knives avoided the yellow latex layer (aloin) beneath the rind, which contains skin-irritating anthraquinones.

This extraction method successfully retained the gel’s natural moisturizing, anti-inflammatory, and antimicrobial properties, making it suitable for cosmetic soap applications²².

After extracting the fresh Aloe abyssinica gel, the material was homogenized using a laboratory disperser set to 700 rpm. This step helped create a smooth, consistent texture while eliminating any fibrous pieces that remained²³. The homogenized gel was then strained through muslin cloth and filtered again to remove any lingering particles. The finished gel was carefully placed into sterile, food-grade containers for storage.

To improve the gel’s shelf life, natural preservatives were incorporated at specific amounts: 6.52 g of ascorbic acid and 22.68 g of citric acid were added to every 3.78 kg of gel. This particular combination was chosen to maintain proper pH levels, prevent bacterial growth, and shield the gel’s active compounds from breaking down due to enzyme activity. The preserved gel was kept refrigerated at -2 °C until needed for soap production. This preservation method aligns with established best practices for maintaining the gel’s quality and bioactive properties²¹.

Characterization of Aloe abyssinica gel

(a) Moisture content determination.

Moisture levels were assessed using the conventional oven-drying approach within the Organic Chemistry Laboratory. This method involved transferring accurately measured samples of fresh Aloe abyssinica gel into clean, dry crucibles before heating them to 105 °C. The heating continued until no further weight loss occurred, confirming complete water elimination. Once dried, the samples underwent cooling in a desiccator to avoid moisture reabsorption from the surrounding air, after which their dried weights were documented²⁴. The moisture content (IS 4955:2013)¹⁵ percentage was then determined through the calculation shown in Eq. (4):

4

where W₁ represents the initial weight of crucible and gel, and W₂ the weight after drying. This technique delivers precise measurements of water content, which is crucial for assessing how stable the gel remains and how long it will last in cosmetic formulations.

(b) Refractive index determination.

The refractive index of the Aloe abyssinica gel was determined using a digital refractometer (RX-5000i-plus, Japan). Before testing, the instrument was calibrated with distilled water, which has a known refractive index of 1.3323 at 20 °C. For each measurement, two drops of gel were gently applied to the prism surface, and the chamber was then closed and left to stabilize thermally before taking the reading. The refractometer provided direct readings for both the refractive index and total dissolved solids, expressed in degrees °Brix²⁵. These measurements (AOAC 932.14)²⁶ offer important insights into the gel’s purity levels, concentration of dissolved substances, and overall uniformity of all critical quality parameters when developing pharmaceutical and cosmetic products²⁷.

(c) Viscosity determination.

The viscosity of the Aloe abyssinica gel was determined using a laboratory viscometer (Brookfield, USA) following the ASTM D2196 standard method²⁸. Since the gel’s flow resistance plays a crucial role in how easily it spreads, maintains stability, and incorporates into soap formulations, accurate viscosity measurements were essential. Testing was conducted under controlled laboratory conditions, allowing the instrument to capture detailed data about the gel’s specific flow behavior. This viscosity data proves vital for achieving the desired texture and consistency in the finished product²⁹ and helps optimize both the manufacturing process and quality control of the formulated soap.

(d) pH value determination.

The pH of the Aloe abyssinica gel was measured to determine its acidity. This is a crucial parameter, as it affects both how well the product works with skin and how stable the final soap formulation is. A digital pH meter (Hanna Instruments HI 2211, Romania) was used for the readings; its electrode was carefully placed into the gel until a stable value was displayed. The mildly acidic pH (IS 4955:2013)¹⁵ of Aloe abyssinica gel usually between 4.5 and 5.5 is beneficial because it helps maintain the skin’s protective barrier and restricts microbial proliferation in the soap³⁰.

Bar soap preparation

The cold process method was used for bar soap production, primarily because it consistently yields high-quality soaps with excellent skin benefits and a long shelf life³¹. For each batch, the goal was to produce 250 g of finished soap, using a formulation centered on Aloe abyssinica gel and animal tallow, along with other functional additives. All necessary raw materials and equipment were prepared in advance of processing. The specific formulations were created to meet established quality standards and to be scalable for varying production volumes. Each soap formulation was prepared in triplicate (n = 3) to ensure reproducibility.

A 30% sodium hydroxide (NaOH) solution was first prepared by slowly dissolving 24 g of NaOH flakes into 56 mL of distilled water with continuous stirring and then set aside to cool. Meanwhile, the oil blend was prepared by weighing 112 g of rendered animal tallow, 12 g of coconut oil, and 14 g of olive oil. The mixture was heated to approximately 38 °C and stirred uniformly to ensure homogeneity³².

Once the NaOH solution and oil blend reached similar temperatures (~ 38 °C), the NaOH solution was gradually poured into the melted oils while stirring continuously in one direction. Stirring continued until a light trace was achieved, characterized by slight thickening and a visible trail left on the surface. At this point, 8 mL of 33% sodium sulfate solution was added to enhance the cleansing and foaming properties³³.

Aloe abyssinica gel was incorporated into the saponifying mixture at varying concentrations (5%, 10%, 15%, 20%, and 25% by weight relative to the tallow). The stirring process continued until the mixture reached medium to heavy trace. To further improve the soap’s performance, 8 mL of sodium silicate and 4 mL of linear alkyl benzene sulfonic acid (LABSA) were added to enhance foaming and hardening properties³.

The soap mixture was then poured into pre-cleaned molds, covered, and left to cure undisturbed for 24 h (Fig. 4). After 24 h curing, bars were air-dried for 4 weeks (28 days curing period) at room temperature (25 ± 2 °C, 50–60% RH). The procedure was repeated with different concentrations of Aloe abyssinica and variations in the oil blend, such as substituting olive oil with palm oil, while keeping all other parameters constant. Physicochemical analyses were then conducted to assess (triplicate) the quality of each formulation, including hardness, lathering ability, and moisturizing properties^3,34.

Fig. 4.

pH value determination.

In this preliminary study, we employed a systematic one-factor-at-a-time (OFAT) approach to screen the effects of various formulation parameters. While this methodology successfully identified the optimal formulation range, future work could employ factorial design of experiments to comprehensively analyze factor interactions and further optimize the formulation. The bar sopa production process shown in Fig. 3.

Fig. 3.

Bar soap production process.

Characterization of the prepared soap

(a) pH value determination.

Soaps typically undergo hydrolysis when dissolved in water, resulting in alkaline solutions. The pH of the prepared soap samples was measured using a calibrated pH meter. To prepare the sample solution, 5 g of the soap was weighed accurately and dissolved in 50 mL of distilled water in a clean beaker. To ensure complete dissolution of the soap, the water was slightly heated before adding the soap. Once dissolved, the pH electrode was immersed in the solution, and the pH (IS 4955:2013)¹⁵ reading was recorded. This procedure was repeated for all soap formulations to assess the alkalinity of the soap solutions¹⁷.

(b) Moisture content determination.

About 3.0 g of the bar soap sample was weighed into a pre-dried, tared moisture crucible. The sample underwent drying in an oven set to 105 °C for 2 h. This drying cycle was repeated until no further weight change occurred, confirming complete moisture removal³⁵. The moisture content (IS 4955:2013)¹⁵ was then determined using Eq. (5):

5

Where W₁​ is the weight of the sample after drying and W₂​ is the weight of the sample before drying.

(c) Total fatty matter determination.

The total fatty matter content was assessed by placing a 5 g soap sample into a beaker and dissolving it completely with 100 mL of hot water. The resulting solution was acidified by adding 40 mL of 0.5 N nitric acid (HNO₃). When the mixture was heated over a water bath, the fatty acids separated out and formed a distinct floating layer on the solution’s surface⁷. The mixture was subsequently cooled in an ice water bath, which solidified the fatty acids and made them easier to separate.

Next, the solidified fatty acids were combined with 50 mL of chloroform and transferred to a separating funnel. After shaking, the solution was allowed to separate into two layers, at which point the bottom chloroform layer containing the dissolved fatty acids was drained. This extraction process was repeated two more times using 50 mL portions of chloroform. The combined chloroform extracts containing fatty acids were subsequently evaporated to dryness in a pre-weighed crucible. The final weight difference was used to calculate the percentage of total fatty matter (IS 4955:2013)¹⁵ in the soap sample, as shown in Eq. (6).

6

Where W₁​ is the weight of the empty crucible, and W₂​ is the weight of the crucible plus dried fatty matter³⁶.

(d) Foam ability taste determination.

Foaming capacity was assessed by placing about 1.00 g of a soap sample into a 50 mL graduated cylinder that contained 25 mL of distilled water. The mixture was then shaken vigorously for 2 min to produce foam and allowed to settle for approximately 10 min. The foam layer’s height was subsequently measured and recorded. This process was repeated for every soap formulation (IS 4955:2013)¹⁵, each of which was made with a different ratio of animal tallow oil to Aloe abyssinica gel^32,34.

(e) Determination of free caustic alkalinity.

The finished soap sample’s presence of free caustic alkali was determined by first accurately weighing out 5 g and dissolving it in 100 mL of ethanol. A few drops of phenolphthalein indicator were then added. Next, 20 mL of a 10% barium chloride solution was introduced to precipitate any potential impurities, such as carbonates and silicates. A pink coloration signified the presence of free alkali. When this color developed, the solution was immediately titrated with 0.1 N hydrochloric acid (HCl) until the color faded completely^6,7,33. The free caustic alkalinity (IS 4955:2013)¹⁵ was calculated using Eq. (7).

7

Where V is the volume of 0.1 N HCl used (mL) and W is the weight of the soap sample.

(f) Test for cleansing power.

The cleaning performance of the soaps was evaluated through a standardized oil-removal test. This process began by creating consistent oil stains on individual strips of muslin cloth. For testing, each stained strip was placed into a separate test tube containing a freshly prepared soap solution. The solution was made by dissolving 2 g of soap shavings into 100 mL of distilled water. To simulate the washing process, the tubes were shaken vigorously for precisely 2 min to help the soap solution interact with the oil. Afterward, the muslin strips were carefully taken out and rinsed thoroughly with distilled water to remove any residual soap.

The cleaned samples were subsequently examined visually to evaluate the degree of oil removal. This comparative procedure enabled determination of the relative cleansing power (Modified ASTM D4488)³⁷ and washing performance of the different soap formulations⁶.

Statistical analysis

All experiments were conducted in triplicate (n = 3), and results are expressed as mean ± standard deviation (SD). Statistical analysis was performed using IBM SPSS Statistics software (Version 26.0, IBM Corp., Armonk, NY, USA). One-way Analysis of Variance (ANOVA) was applied to determine statistically significant differences among the twelve soap formulations for each physicochemical parameter. Where ANOVA indicated significant differences (p < 0.05), pairwise comparisons were performed using Tukey’s Honest Significant Difference (HSD) post-hoc test³⁸. Means sharing the same lowercase superscript letter within each parameter column of Table 3 are not significantly different at the 5% significance level (p ≥ 0.05). Statistical significance was set at p < 0.05 for all analyses.

Table 3.

Physico-chemical characterization of the prepared bar soap.

No	Percentage (%)					Physico-chemical characterization
	Tallow	Aloe abyssinica gel	Olive oil	Coconut oil	Palm oil	pH value	Moisture (%)	Foam ability (cm)	Total fatty matter (%)	Free caustic alkali (%)	Cleaning power
1	75	0	10	15	0	10.3 ± 0.26b	12.42 ± 0.18a	1.15 ± 0.22a	56.75 ± 0.17b	1.16 ± 0.02f	Poor
2	70	5	10	15	0	10.13 ± 0.27ab	12.63 ± 0.24a	3.4 ± 0.26c	62.42 ± 0.21d	0.98 ± 0.03e	Good
3	65	10	10	15	0	10.1 ± 0.32ab	14.2 ± 0.26b	5.4 ± 0.14e	73.66 ± 0.18g	0.58 ± 0.07c	Good
4	60	15	10	15	0	9.75 ± 0.25a	14.85 ± 0.31b	7.3 ± 0.17f	76.8 ± 0.24h	0.08 ± 0.01a	Very Good
5	55	20	10	15	0	9.79 ± 0.31a	21.5 ± 0.41d	6.8 ± 0.31f	72.78 ± 0.19g	0.14 ± 0.03a	Very Good
6	50	25	10	15	0	10.12 ± 0.32ab	23.7 ± 0.27e	7.1 ± 0.26f	69.56 ± 0.21ef	0.18 ± 0.02b	Good
7	75	0	0	15	10	10.43 ± 0.17c	11.5 ± 0.23a	1.07 ± 0.23a	53.47 ± 0.17a	1.23 ± 0.01g	Poor
8	70	5	0	15	10	10.31 ± 0.26b	12.3 ± 0.23a	2.2 ± 0.26b	58.2 ± 0.23c	1.15 ± 0.04f	Poor
9	65	10	0	15	10	9.81 ± 0.18a	13.6 ± 0.21b	3.5 ± 0.13c	67.57 ± 0.21e	0.78 ± 0.02d	Good
10	60	15	0	15	10	9.92 ± 0.12a	15 ± 0.32b	4.8 ± 0.14d	73.33 ± 0.28g	0.13 ± 0.01a	Very good
11	55	20	0	15	10	10.11 ± 0.31ab	18.3 ± 0.42c	4.2 ± 0.17d	70.09 ± 0.13f	0.15 ± 0.01a	Good
12	50	25	0	15	10	10.19 ± 0.29ab	21 ± 0.18d	3.9 ± 0.18cd	71.33 ± 0.17fg	0.14 ± 0.02a	Good

Values are expressed as mean ± SD (n = 3). Different superscript lowercase letters within each column indicate statistically significant differences among formulations (p < 0.05, one-way ANOVA followed by Tukey’s HSD post-hoc test). Means sharing the same superscript letter are not significantly different.

Result and discussion

Characterization of tallow oil

The tallow oil showed a saponification value of 181.78 mg/g NaOH, slightly below the standard tallow range (190–202 mg/g NaOH) but adequate for soap production. This value falls between olive oil (192 mg/g NaOH) and beeswax (93 mg/g NaOH), reflecting tallow’s medium-chain triglyceride profile that produces firm, durable soap bars³⁹.

The marginally lower saponification value likely results from variables in animal diet or rendering methods but doesn’t compromise the oil’s suitability for soap manufacturing⁴⁰.

The tallow oil demonstrated an acid value of 17.32 mg/g NaOH, indicating free fatty acids including palmitic, stearic, and oleic acids (Table 1). While elevated, this level remains acceptable for soap production and enhances cleansing properties without promoting rancidity⁴². These fatty acids contribute to effective cleaning while maintaining formulation balance.

Table 1.

Characterization of animal tallow oil⁴¹.

Test	Result	Standard/comparison
Saponification value	181.78 mg/g NaOH	Standard: 190–202 mg/g NaOH
Acid value	17.32 mg/g NaOH	Comparable to Albizia seed oil (13.66 mg/g KOH)
pH value	5.45 ± 0.26	Typical animal fats (acidic range pH 5.0–6.5)
Specific gravity	0.929	Within typical edible tallow range (0.92–0.94)

The oil’s pH of 5.45 reflects its mildly acidic nature from free fatty acid content typical for animal fats and beneficial for saponification. This acidity produces gentle soaps suitable for skincare applications⁴³.

The measured specific gravity of 0.929 reflects a high level of purity with negligible moisture and impurities, aligning with the quality benchmarks established by the Codex Alimentarius Commission (1999) and ISO (1988) for edible-grade tallow. Such a low specific gravity is also indicative of good storage stability, which contributes to an extended shelf life during soap manufacturing⁴⁴.

Overall, the physicochemical profile demonstrates that locally sourced tallow oil is a suitable raw material for bar soap production. Although minor deviations were observed specifically, a slightly lower saponification value and a relatively higher acid value the results remain within acceptable ranges for effective soap formulation. These findings highlight the potential of tallow oil as a cost-effective and reliable feedstock for producing durable, high-quality soap bars.

Characteristics of Aloe abyssinica gel extracted

To determine how suitable the Aloe abyssinica gel was for soap formulation and related applications, a series of its critical physicochemical properties were evaluated. The analysis covered moisture content, pH, refractive index, total dissolved solids (TDS), and viscosity. These measurements created a comprehensive profile of the gel’s performance characteristics. All the results are summarized in Table 2.

Table 2.

Characterization of Aloe abyssinica gel extracted.

No	Test	Value
1	Moisture content	99.23 ± 0.02%
2	pH	4.82 ± 0.06
3	Refractive index	1.3378 ± 0.07
4	TDS	3.350 ± 0.04 °Brix
5	Viscosity	1.359 ± 0.025 cP

The Aloe abyssinica gel tested contained 99.23% moisture, which falls comfortably within the globally documented range of 97–99.98%⁸. This exceptionally high-water content is what gives the gel its excellent moisturizing properties in cosmetic applications. Several factors can influence moisture levels, including the aloe species used, harvest timing, and growing conditions⁴⁵.

pH measurements revealed a value of 4.82 (Table 2), confirming the gel’s mildly acidic character. This finding aligns well with established literature values of 4.5–5.5⁴⁶. Natural acidity stems from organic acids like linoleic and oleic acids, which also contribute to anti-inflammatory properties. This slightly acidic pH helps maintain the skin’s natural protective barrier, making it particularly valuable when incorporated into alkaline soap formulations⁴⁷.

The gel showed a refractive index of 1.3378, which is marginally higher than pure water at 20 °C (1.33). This elevated reading indicates the presence of dissolved substances including polysaccharides, glycoproteins, and various minerals ⁴⁸. These measurements suggest good purity and transparency, both essential qualities for cosmetic applications.

Total dissolved solids measured 3.350 °Brix, representing a moderate concentration of sugars, minerals, and phenolic compounds⁴⁹. This TDS level strikes a good balance by supporting the gel’s biological activity while maintaining processing stability and reasonable shelf life.

Viscosity testing yielded 1.359 cP, indicating a fluid consistency that works well for soap incorporation⁵⁰. This flowing behavior results primarily from mucilaginous polysaccharides like acemannan, which enhance both the gel’s therapeutic and moisturizing capabilities while improving spread ability in finished cosmetic products⁵¹.

When considered together, these physicochemical characteristics include high moisture content, mildly acidic pH, appropriate refractive index, moderate dissolved solids, and optimal viscosity all meet international quality standards. The results confirm that Aloe abyssinica gel serves as an outstanding natural ingredient for boosting soap’s moisturizing, soothing, and antimicrobial properties.

Physico-chemical characterization of the produced bar soap

The quality of any soap depends largely on its physicochemical properties, since these factors determine how well it cleanses, how easy it is to use, and its overall performance characteristics. Evaluating these properties becomes crucial for understanding the soap’s actual functionality and effectiveness in practice. This chapter examines the essential physicochemical attributes of the bar soaps produced, focusing on moisture content, total fatty matter (TFM), free caustic alkali (FCA), foaming capacity, cleansing ability, and pH levels.

All testing followed well-established, standardized analytical methods to ensure reliability and accuracy. The results obtained were then compared against internationally accepted quality benchmarks for commercial bar soaps. This comparative approach proves valuable because it reveals whether the formulated soaps meet industry requirements while also indicating their potential market suitability for everyday consumers. Furthermore, this evaluation provides insights into how specific formulation ingredients, especially the Aloe abyssinica gel addition, affect the final product’s properties and functional behavior.

pH value determination

The pH is a measure of the hydrogen ion concentration in an aqueous solution and serves as a key indicator of a substance’s alkalinity or acidity. In soap formulations, pH plays a crucial role in determining mildness and compatibility with the skin. The pH values of the produced bar soaps ranged from 9.75 to 10.43, falling within the standard range of 9.0 to 11.5, as specified by the National Agency for Food and Drug Administration and Control (NAFDAC)52. As shown in Fig. 4, the lowest pH value of 9.75 was recorded for the formulation consisting of 60% animal tallow oil, 15% Aloe abyssinica gel, 10% olive oil, and 15% coconut oil. A slightly higher pH of 9.81 was observed when 10% palm oil was used in place of olive oil, with the other ingredients kept constant.

The gradual decrease in pH from 10.3 to 9.75 as the Aloe abyssinica gel content increased to 15% is attributed to the acidic nature of Aloe abyssinica, which helps neutralize the excess alkali (sodium hydroxide) used in the saponification process. This buffering effect enhances the soap’s mildness, making it more suitable for skin application. In contrast, a higher pH value, such as 10.43, indicates a strongly basic soap, which can be harsh, especially for sensitive skin types. According to NAFDAC guidelines, skin-friendly soaps should ideally have a pH between 8.5 and 10. High pH occurs at both elevated tallow levels (75%) due to incomplete saponification, and at high Aloe abyssinica content (25%) due to excess moisture concentrating residual alkali. The optimal formulation (60% tallow, 15% Aloe abyssinica) achieves balanced pH (9.75) through complete saponification and optimal moisture content. The citric acid present in the Aloe abyssinica gel formulation (added as preservative) also helps neutralize minor alkali excess. This multi-factorial pH behavior demonstrates the importance of optimizing both fat composition and moisture content for skin-compatible soap production.

Elevated pH¹⁵ in some formulations may also suggest the presence of unsaponified matter due to incomplete saponification, particularly in formulations with higher tallow oil content. This issue can be addressed through a process known as super fatting, which involves adding 4–6% extra neutral oils or glycerin during formulation. Super fatting not only reduces the soap’s harshness but also results in higher quality bars that are less prone to cracking and non-corrosive to both skin and fabric 43. Therefore, proper formulation and careful pH control are essential to ensure that the soap is both effective in cleansing and gentle on the skin.

Moisture content determination

Moisture refers to the presence of liquid, particularly water, typically in small amounts. One of the key factors in producing high-quality bar soaps and soap flakes is maintaining optimal moisture content. Inadequate moisture control during production can result in substandard soap quality, increased waste, and reduced profitability. According to the data presented in Table 3, the moisture content of the soap samples ranged of 11.5–23.7%. Most of the formulations fell within the recommended standard moisture range of 10–15%. A clear relationship was observed between Aloe abyssinica gel content and moisture level: as the proportion of Aloe abyssinica gel increased, so did the moisture content of the soap. This trend is attributed to the naturally high-water content present in Aloe abyssinica.

The lowest moisture content (11.5%) was recorded in the formulation that did not include Aloe abyssinica gel, whereas the highest value (23.7%) was found in the formulation containing 25% Aloe abyssinica gel. When it was used to replace up to 15% of tallow oil, the resulting moisture content remained within the acceptable standard limits (10–15%), indicating a suitable balance between hydration and bar firmness. Moisture content¹⁵ also affects the lathering and cleansing properties of soap. Over time, soap bars lose moisture through evaporation, which can improve their ability to lift and remove dirt from the skin and fabric during washing. The type of oil used also influences moisture-related properties. For instance, palm oil contributes to producing a harder soap bar due to its low solubility in water, unlike olive oil, which is more hydrating. However, palm oil contains very little natural glycerin, and while it supports lather formation, it is not considered moisturizing oil in soap production.

Total fatty matter determination

Total Fatty Matter (TFM) is one of the most important parameters used to evaluate the quality of soap and is a standard specification in commercial transactions. TFM represents the percentage of fatty substances, both saponified and unsaponified, in the finished soap. It is defined as the ratio of the mass of fatty matter to the total mass of the soap. According to the Bureau of Indian Standards (BIS), soaps are classified into three grades based on the soaps with TFM above 76% are classified as Grade I (high quality), those with TFM above 60% fall into Grade II, and soaps with TFM above 50% are categorized as Grade III. The International Organization for Standardization (ISO) similarly specifies that good-quality soaps should contain more than 76% TFM.

As shown in Fig. 5, the highest TFM value observed was 76.8%. This value was recorded for the formulation containing 15% Aloe abyssinica gel, 60% tallow oil, 10% olive oil, and 15% coconut oil, along with other additives. This formulation meets international standards for high-quality soap. In contrast, the lowest TFM value was 53.47%, observed in the soap made from 75% tallow oil and 10% palm oil without the inclusion of Aloe abyssinica gel. This formulation falls under Grade III, indicating lower quality. The reduced TFM content in this case is attributed to the presence of unreacted sodium hydroxide (NaOH), which affects the saponification efficiency. Soaps with high TFM content (above 75%) are particularly beneficial for individuals with dry skin, as they help to rehydrate the skin and make it smoother. The highest TFM (76.8%) at 15% Aloe abyssinica reflects optimal saponification efficiency, where sufficient moisture enables complete NaOH dissolution and uniform fatty acid reaction without excessive dilution. Lower TFM at 0–10% Aloe abyssinica results from insufficient moisture causing incomplete saponification, while 20–25% introduces excess water that dilutes fatty matter content. The 15% concentration achieves optimal balance, meeting international standards (> 76% per IS:4955). The bioactive compounds in Aloe abyssinica gel, particularly polysaccharides and organic acids, may facilitate soap formation by improving the emulsification of fats during the saponification process. The higher oil content in such soaps also acts as a lubricant, providing a moisturizing effect that lasts throughout the day Bureau of Indian Standards (BIS) IS 4351:2009¹⁵ and International Organization for Standardization (ISO) standard ISO 3305:2015.

Fig. 5.

Total fatty matter determination.

Foam ability test determination

Foam height is closely linked to the type of oil used in soap production, particularly oils rich in lauric acid such as coconut, palm, and olive oil. Lauric acid is known for its strong foaming properties. The efficiency of a washing soap is often assessed based on the amount and persistence of foam it generates during use. As shown in Fig. 6, the maximum foam height recorded was 7.3 cm in the bar soap formulation containing 10% olive oil. This foam persisted for approximately 10 min, outperforming the 4.8 cm foam height produced by a formulation containing 10% palm oil. Both formulations included 60% tallow oil, 15% Aloe abyssinica gel, 15% coconut oil, and other additives. Notably, the observed foam height of 7.3 cm is significantly higher than the 2.5 cm reported in previous studies⁵³.

Fig. 6.

Foam ability test.

The data further indicated that foam ability increased with the addition of Aloe abyssinica gel. This is attributed to Aloe abyssinica ‘s role as a natural foam-boosting agent. The inclusion of linear alkylbenzene sulfonic acid (LABSA) and sodium silicate also enhanced foamability and cleansing performance due to their surfactant properties. The optimal performance at 15% Aloe abyssinica results from: maximum foam height (7.3 cm) due to mucopolysaccharides and optimal surfactant structure; “Very Good” cleansing power from maximum TFM (76.8%) and bioactive compounds; and minimum free caustic alkali (0.08%) from complete saponification and citric acid neutralization, meeting IS:4955 standards¹⁵.

Foam ability is influenced by several factors, including the concentration of hardness ions in the formulation. A high concentration of animal tallow oil (75%) resulted in a reduced foam height of only 1.15 cm, whereas decreasing the tallow content to 50% increased the foam height to 7.1 cm. This reduction in foam can be attributed to the high hardness associated with tallow oil. Additionally, due to the similar fatty acid profiles of palm oil and animal tallow, the foamability of soaps made with palm oil was lower compared to those made with olive oil, even when Aloe abyssinica gel was used to substitute a portion of the tallow.

Cleansing power test

The cleansing ability of various soaps to remove dirt was evaluated by comparing the levels of dirt before and after the washing process. Since most dirt is oily and oil does not dissolve in water, soap molecules play a critical role in removing it. Soap molecules consist of sodium or potassium salts of long-chain carboxylic acids. The carbon chain end of the molecule dissolves in oil, while the ionic end dissolves in water. This dual affinity allows soap molecules to form structures called micelles, where the hydrophobic ends surround the oil droplet, and the hydrophilic ionic ends face outward into the water. This micelle formation emulsifies the oil in water, thereby aiding in the removal of dirt during washing.

The cleansing power³⁷ of soaps depends largely on the fatty acid composition of the oils used in their formulation. Saturated fatty acids such as lauric acid and myristic acid are known to produce soaps with excellent cleansing properties and fluffy lather³³. Variations in cleansing power between soap formulations can also arise from differences in preparation methods and the nature of the fatty acids in the fats or oils used. As observed from the data, soaps containing olive oil exhibit higher cleansing power than those formulated with palm oil when blended with coconut oil and Aloe abyssinica gel in equal proportions. The best cleansing power was recorded in soaps where tallow oil was partially substituted with 15% Aloe abyssinica gel, combined with 10% olive oil and 15% coconut oil. Conversely, soaps with a high percentage of animal tallow oil (70–75%), 15% coconut oil, 5% Aloe abyssinica gel, and 10% olive or palm oil showed poor cleansing performance.

Total free caustic alkali determination

Free caustic alkali refers to the amount of unreacted alkali present in soap, which is not chemically bound during the soap-making process. The presence of free caustic alkali is critical to control because excessive amounts can cause the soap to become harsh, leading to skin irritation and damage to fabrics. According to Ghana Standards⁵⁴, soaps should contain no more than 0.09% free caustic alkali. In this study, the minimum free caustic alkali content observed in the prepared bar soaps was 0.08%, which is below the standard limit and indicates a satisfactory level. Furthermore, the detected free caustic alkali content (0.06%) aligns with the standards set by Egyptian and Malaysian authorities, which recommend values less than 1.14%, largely influenced by the amount of caustic potash used during production (Fig. 7).

Fig. 7.

Total free caustic alkali determination.

Excess free caustic alkali can cause skin itching and degrade clothing. As shown in Table 3, the highest free caustic alkali content recorded was 1.23%, observed in a soap formulation containing 75% tallow oil, 15% coconut oil, and 10% palm oil without Aloe abyssinica gel. Conversely, the lowest free caustic alkali content was found in a formulation where 15% tallow oil was replaced with Aloe abyssinica gel, combined with 10% olive oil and 15% coconut oil. This formulation is considered safe for use on human skin and fabrics due to its low free caustic alkali level.

Conclusion

This study successfully demonstrates the sustainable valorization of animal tallow and Aloe abyssinica gel for high-quality bar soap production. The optimal formulation comprising 60% animal tallow, 15% Aloe abyssinica gel, 15% coconut oil, and 10% olive oil exhibited excellent physicochemical properties: skin-compatible pH (9.75), superior foam ability (7.3 cm), high total fatty matter content (76.8%), minimal free caustic alkali (0.08%), and very good cleansing performance. All parameters met or exceeded international standards (IS:4955, ES 1631:2005, ASTM D460).

The systematic optimization revealed that 15% Aloe abyssinica gel concentration provides the optimal balance between saponification efficiency, moisture content, and product quality. This formulation transforms meat industry waste (tallow) and locally available Aloe abyssinica into a value-added product, supporting circular economic principles and sustainable development.

The economic analysis indicates potential for commercial viability, with reduced raw material costs compared to conventional vegetable oil-based soaps. Future work should explore scaling up production, conducting consumer acceptance studies, and investigating long-term stability and shelf-life characteristics.

Author contributions

Meketaye A. Edenshaw, Sahlu M. Gela, Ermias A. Tesemad: Conceived and designed the experiments; performed the experiments; Tebelay L. Andualem and Reddy Prasad D.M: analyzed and interpreted the data, software analysis and prepared the manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

AI usage declaration

AI tools were used only for language editing. All scientific content and conclusions are the authors’ own.

Additional information

No additional information is available for this paper.