Comprehensive evaluation of physico-chemical, antioxidant, and antimicrobial properties in commercial soaps: A study on bar soaps and liquid hand wash

Abstract

Soap is essentially used for various purposes. Therefore, it is important to determine the physico-chemical, antioxidant, and antimicrobial properties of soaps to evaluate their quality. The present research work focuses on 22 commercial soaps which include bar soaps (SN 1-SN 19) and liquid hand washes (SN 20-SN 22). The values of different parameters for commercial soaps are observed which are between 3.84–83.75 % for moisture content, 6.83–25.16 cm for foam height, 7.01–10.17 for pH, 62.47–91.39 % for total fatty matter, 0.20–1.17 % for total alkali content, zone of inhibition, 17.33–29.27 mm/100 % conc., antioxidant activity, 357.41–777.15 μg/mL. It is observed that all of the soaps have a foam height, and pH within the permitted limit (FH-above 1.5 cm, pH 7–10). All bar soap has a total fatty matter and a total alkali content within the accepted value. Moreover, 8 soaps show antimicrobial properties against Staphylococcus aureus, which were further tested for rapid efficacy and antioxidant properties. A survey report of 22 commercial branded soaps was done on 50 people in Bangladesh to get feedback from customers. Combining all the parameters, SN 3 has good results among bar soaps with mild moisturizing and cleansing properties (MC-8.30 ± 0.035 %, FH-22.33 ± 0.29 cm), is safe for the skin (pH-9.83 ± 0.02, TAC-0.99 ± 0.03), has good TFM values (72.89 %), has proficient bacteria growth-inhibiting properties (ZI-18.67 ± 0.29 mm/100 % conc.) and has the ability to remove free radicals (357.41 ± 2.07 μg/mL). Besides, SN-21 among liquid hand wash soaps possesses overall excellence as it moisturizes skin well (MC-83.75 ± 0.05 %), is almost safe for skin (pH-7.73 ± 0.02, TAC-0.98 ± 0.01 %), has TFM (11.35 %), has good cleansing capacity (FH-22.66 ± 0.58 cm) and bacteria growth inhibiting activity (29.27 ± 0.25 mm/100 % conc.), destroys selected bacteria (MIC – 1.2 × 10⁷ CFU at 1 min and 5.5 × 10⁷ CFU at 10 seconds), and wash away free radicals from skin (420.64 ± 3.62 μg/mL).

Graphical abstract

1. Introduction

The production of soap is one of ancient chemical methods which has been known from 2800 years ago and is essentially used to fulfill human need for cleanliness. Soap is prepared by heating fat or oil (animal fat such as tallow and lard-pork fat which contains fatty acids such as saturated palmitic and stearic acids and an unsaturated fatty acid such as oleic acid or vegetable fat such as olive oil, palm oil and coconut oil) with a strong base such as sodium hydroxide, potassium hydroxide and magnesium hydroxide [1,2,3]. The modern commercial method of making soap is a saponification reaction which is a base catalyzed hydrolysis reaction between an acid (oil or fat) and a base (lye) to form a salt (soap) [4,5,6]. Fats are composed of fatty acid and glycerides whereas oils are compounds of liquid glycerol esters of fatty acids. Generally, soaps produced from sodium hydroxide are applied in bar soap and potassium hydroxide in liquid soap. Divalent metals such as calcium, magnesium made soaps are insoluble in water and they have different characteristics than sodium and potassium soaps [7]. Finally, the soap is then dried and pressed into bars in which perfumes, dyes and sand can be included for scented, colored scouring soaps.

Soaps possess the ability to remove dirt from surfaces such as skin, textiles and other solids and thus remove germs, soil and contaminants and keep us healthy [2,4,8]. According to the World Health Organization (WHO), frequent and correct washing of hands and surfaces with soap and water is the cost-effective way to prevent catastrophic effect of COVID-19. Therefore, increased attention for production of soap has been given to enhance their multifunctional applicability including their classical roles. Antimicrobial soap, which contains chemical ingredients has a significant role as it has the capability to inhibit the growth of microorganisms and sometimes completely destroy bacteria and virus that give rise to its properties of being antimicrobial [9,10]. There is also an increasing consumer demand for cosmetics compromising natural ingredients as healthier, organic, and ecological and also have antimicrobial and antioxidant activities which protects skin from free radicals. Free radicals cause skin damage by breaking down skin’s collagen resulting in blemishes like wrinkling and dark spots [11]. Antioxidant properties may contribute to healthier skin by protecting against oxidative stress.

Physico-chemical analysis of soap determines ingredients, cleansing ability, impact on skin. The total fatty matter (TFM) and total alkali content (TAC) are factors impacted by saponification process and define the quality of soap. TFM is the water-insoluble fatty material including unsaponifiable matter, glycerides and any rosin acids contained in soap obtained by decomposing the soap with mineral acid under specified conditions whereas TAC is the sum of alkali bases including free alkali metal hydroxides, carbonates and silicates [12,13]. Lower TFM is associated with hardness and lower quality of soap [4]. Some skin diseases such as skin irritation, dryness, scaling and fungal attacks on skin are caused by alkaline soaps because of containing excess alkali as excess alkali can react with the fats and oils found on the skin as protective coat, to form soluble soap and therefore get washed away, and makes the skin dry. Dry skin needs soap with higher TFM content, which makes skin smooth by rehydrating. Additionally, high oil content acts as lubricant throughout the day. For this, good toilet soap should contain little total alkali content, super fat products, which have little impact on the skin, are excellent products. Moreover, the presence of high moisture causes a chemical reaction between excess water and unsaponified fat by hydrolysis of soap in its storage life. Soaps have different values of moisture content based on ingredients and shelf life and precise moisture content has a connection with stability [14]. Besides, foam height determines the acceptability of soap. Soaps with a balanced pH are gentler on the skin whereas the protective action of skin and wound healing depends on pH [15,16].

The purpose of the present work is to assess the quality of commercial soap by evaluating their physico-chemical characteristics of soap such as moisture content, foam height, pH, total fatty matter, and total alkali content and to investigate their antimicrobial and antioxidant activity [[17], [18], [19], [20]]. For each property, specific tests, methodologies and standards are used to evaluate and quantify these characteristics. Due to the variations of formulations of different types of soaps, the properties can differ considerably which can help to determine their efficacy and impact on the skin [14,21]. The present research provides an in-depth analysis of the quality characteristics of 22 different commercial soaps, conducted for the first time in Bangladesh, based on multiple quality and safety parameters with relevant survey. It offers the detailed comparison between bar and liquid soap formulations in terms of antimicrobial and antioxidant properties. Examining these properties will collectively provide a complete understanding of the soap’s quality, effectiveness and potential benefits for consumers.

2. Materials and methods

2.1. Study area

The total 22 selected soap samples were collected from supermarket, Jashore, Khulna, Bangladesh (Table 1).

Table 1.

Specification of commercial soaps sold in Bangladesh.

SN No.	Soap Type	Color	Mass (mL or gm)	Price (TAKA)	Origin	Standard	Ingredients
SN 1	Elder Soap	White	150	48	Chattagram, Bangladesh	BSTI-BDS 13	Sodium Soap, Aqua, Tetrasodium EDTA, Etidronic Acid, Silver Oxide, VP/VA Copolymer, Thymol, Terpineol, Perfume.
SN 2	Elder Soap	Yellow	30	10	Chattagram, Bangladesh	BSTI-BDS 13	Soap Noodles, Talc, Pine Oil, PCMX, Color.
SN 3	Elder Soap	White	75	38	Narayanganj, Bangladesh	BSTI-BDS 13	Soap Noodles, Glycerin, Silicon Fluid, Titanium dioxide, SLS, Perfume, Antioxidant, Tinopal CBSX, Colloidal, Gold, and Platinum.
SN 4	Elder Soap	Pink	125	45	Chattagram, Bangladesh	BSTI-BDS 13	Moroccan Iris and Patchouli Oil Scent, Sodium Palmate, Sodium Palm Kernelate, Aqua, Glycerin, Perfume, Sodium Chloride, Lauric Acid, Mica, Titanium dioxide, Tin dioxide, Tetrasodium Etidronate, Tetrasodium EDTA, Disodium disterilebiphynyl disulfonate, CI 12890, CI 74160
SN 5	Elder Soap	Yellow	125	40	Gazipur, Bangladesh	BSTI-BDS 13, ISO 9001:2015	Cocoa Butter, Vitamin E, Sodium Palmitate, Sodium Palm Kernelate or Cocoate, aqua, Perfume, Glycerin, HEDP, EDTA, CI 11680, CI 77891.
SN 6	Baby Soap	White	75	60	Narayanganj, Bangladesh	BSTI-BDS 1798	Sodium Palmate, Sodium Palme Kernelate, Aqua, Glycerin, Fragrance, Honey Extract, EDTA, CI 45100, CI 11680 and CI 77891.
SN 7	Elder Soap	Orange	125	44	Tejgoan, Dhaka, Bangladesh	BSTI-BDS 13	Sodium Soap, Sandal Wood Oil, Preservatives, Color, Perfume and Aqua.
SN 8	Elder Soap	White	125	45	Gazipur, Bangladesh	BSTI-BDS 13: 2006	Jasmine Extracts, Sodium Palmate, Sodium Palm Kernelate, Lauric Acid, Palmitic Acid, Stearic Acid, Sodium Chloride, Titanium Dioxide, Tetrasodium Etidronate, Tetrasodium EDTA, Lye, Glycerin, Aqua, Fragrance.
SN 9	Laundry Soap	Red	130	16	Tejgoan, Dhaka, Bangladesh	BSTI-BDS 12	Sodium Soap, Preservatives, Color, Perfume, Aqua.
SN 10	Glycerin Soap	Red	80	35	Tejgoan, Dhaka, Bangladesh		Glycerin, Coconut Oil, Castor Oil, Caustic, Stearic, Gum Rosin, Sodium Lauryl Sulfate, Perfume.
SN 11	Elder Soap	Blue	125	120	India		Water, Sorbitol, Sodium Palm Kernelate, Sodium Palmate, Propylene Glycol, Sodium Stearate, Sodium Lauryl Sulfate, PEG-4, Sodium Laurate, Sodium Castorate, Isopropyl Alcohol, Sodium Chloride, Fragrance, Sodium Sulfate, Sodium Metabisulfate, Menthol, Tetrasodium Etidronate, Lauryl Alcohol, Glycerin, Tetrasodium EDTA, BHT, Mentha viridis (spearmint) Oil, Green 3 (CI 42053)
SN 12	Elder Soap	Light Brown	–	–	England		Sodium Palmate, Aqua, Sodium Palm Kernelate, Talc, Glycerin, Sodium Laureth Sulfate, Perfume, Sodium Chloride, Sodium Carbonate, Tetrasodium EDTA, Etidronic Acid, CI 77891, CI 77220, CI 11680, CI 71105, CI 74260, CI 12490
SN 13	Elder Soap	Off White	113	155	Toronto, Canada		Not mentioned
SN 14	Baby Soap	White	75	76	India		Vitamin E, Sodium Palm Kernelate, Sodium Palmate, Water, Mineral Oil, Sodium Chloride, Fragrance, Glycerin, Titanium Dioxide, Dimethicone, C12- C15 Alkyl Benzoate, Disodium EDTA, Stearoxytrimethyl silane, stearyl alcohol Disodium Etidronate, Tocopheryl Acetate, Hydrolyzed Milk Protein.
SN 15	Elder Soap	Bottle Green	100	80	Gazipur, Bangladesh		Virgin Coconut Oil, RBD Palm Oil, Aqua, Castor Oil, Sodium Hydroxide and Neem Oil.
SN 16	Elder Soap	Green, Striped texture	75	30	Gazipur, Bangladesh	BSTI-BDS 13	Sodium Soap, Sodium Lactate, Perfume, Titanium dioxide, Glycerin, Aqua, Neem Extract, Aloe Vera Extract, Cucumber Extract, Color.
SN 17	Elder Soap	Green	100	42	Gazipur, Bangladesh	BSTI-BDS 13	Soap Noodles, Perfume, Glycerin, Titanium dioxide, Preservatives, Silicon Fluids, SLS, Neem Extract, Aloe Vera Extract, Olive Extract, Color: CI No 11680, CI No 74260, CI No 12490.
SN 18	Elder Soap	Black	100	160	Bangkok, Thailand		Blackseed oil, Combination of Herbal Extracts, Water, Sodium Laurate, Sodium Palmitate, Propylene Glycol, Cetyl Alcohol, Alcohol, Sorbitol, Sodium Stearate, Glycerin, Sodium Lactate, Nigella Sativa Seed (Black Cumin), Sodium Hydroxide, Tetrasodium EDTA, Iron Oxide Black Ct No. 77499, Fragrance.
SN 19	Elder Soap	Off white	113	180	Toronto, Canada		Not mentioned
SN 20	Liquid Hand wash	Greenish White	9	25	Mumbai, India		Neem, Aloe Vera.
SN 21	Liquid Hand Wash	White	170	65	Chattagram, Bangladesh	BSTI – BDS	Water, Myristic Acid, Lauric Acid, Potassium Hydroxide, Potassium Chloride, Pamitic Acid, Sodium Lauryl Salfate, Glycol distearate, Cocamidopropyl Betanine, Hydroxypropyl Methylcellulose, Glycerin, Tetrasodium EDTA, Helianthus Anus (Sunflower) seed Oil, PEG-40 Hydrogenated Castor Oil, Prunus Persica (Peach) Juice, Dried Cream, Xanthan Gum, Prunus Amygdalus Dulcis (Sweet Almond) Oil, Lavandula Angustifolia (Lavender) Oil, Pentasodium Pentetate, BHT , Silver Oxide, Thymol, Terpineol, Etidronic Acid, VP/VA Copolymer.
SN 22	Liquid Hand Wash	Orange	180	55	Pabna, Bangladesh	ISO 22716: 2007, CGMP Certified	Aqua, Lauric Acid, Stearic Acid, LLES, Cocamidopropyl Betaine, Glycerin, Methylchloroisothyazolinon, CI-15985, Perfume.

SN-Sample Number.

2.2. Soap sample preparation

The collected soap samples were grinded with cheese grater after removing the packaging materials. The preservation process was maintained in air free bag, specimen transport airtight zipper bag and the whole sampling process was done by wearing hand gloves to avoid contamination.

2.3. Materials

All the solvents, chemicals and reagents are of analytical grade and used in this investigation without further purification (Table 2). UV Spectrophotometer (Thermo Fisher Scientific - USA, 840–210800) was used to determine antioxidant properties of soap.

Table 2.

The detailed information of the substance used in this study.

Chemicals	Origin	Purity
Sulfuric Acid	Merck Germany	98 %
Diethyl Ether	Central Drug House Ltd, India	99.5 %
Sodium Sulfate	Merck Germany	98 %
Sodium Hydroxide	Merck India	97 %
Methyl Orange	LOBA CHEMIE PVT. LTD
Methanol	Merck Germany	99.8 %
Dichloromethane	Sisco Research Laboratories Pvt. Ltd., India	99.5 %
Muller Hinton Agar and Broth	Oxoid, UK
DPPH	Sigma-Aldrich, EU

2.4. Bacterial strain

Staphylococcus aureus (ATCC 23235-American Type Culture Collection), a commonly found bacteria on skin which is responsible for skin infection and some other severe pathogenesis. It was obtained from the Department of Microbiology, Jashore University of Science and Technology (JUST), Jashore – 7408, Bangladesh. To assess whether soaps possess antibacterial properties, this pathogen was subsequently selected as the test organism [22,23]. Bacteria culture and studies of antimicrobial properties have been conducted in Climate Change Research Laboratory, Department of Microbiology, Jashore University of Science and Technology.

2.5. Moisture content determination

The value of moisture content is important to know when soap is ready to be sold by reaching a constant mass of dry matter called total solid [24]. For determination of moisture content, the soap sample was heated at a temperature 104 °C in an electrical oven for 1.5 h (Fig. 1). After being cooled, it was weighed and then repeats the experiment until the difference between the next two weights was less than 0.01 [SI Table 1] [14]. The moisture content was thereafter calculated using the expression:

$$MoistureContent(\%)=\frac{Lossinweight}{Weightofsample}\times 100$$

Fig. 1.

Soap samples in petri dishes for moisture content determination.

2.6. Foam height (FH) determination

Preparation of soap solution was carried out in a 500 mL measuring cylinder by dissolving 10 g of representative soap sample in 90 mL distilled water to examine the foam height. After that, it was carefully shaken 20 times vigorously to form foam of the soap. Foam height was measured and recorded starting at the 100 mL mark [SI Table 2] [14].

2.7. Determination of pH

The pH is the quantitative measurement of acidity or alkalinity of an aqueous or a liquid solution. To determine the pH of a specific soap, the 5 % (w/w) soap sample was prepared by dissolving soap sample in hot water which was heated on a hot plate. The prepared clear solution was allowed to reach ambient temperature (25 °C). The pH of the solution was determined by a pH meter [SI Table 3], [17,25].

2.8. Total fatty matter (TFM) determination

At first, 100 mL of deionized water was added to 5 g of soap sample in the conical flask and heated the mixture until the soap sample finely dissolved. Subsequently, 0.5 M H₂SO₄ was added until layer separation and next, 10 mL of excess H₂SO₄ was added. The total fatty matter layer was filtered in order to determine TFM. After that, the solution was mixed with 100 mL diethyl ether (dichloromethane for liquid soaps) in a separatory funnel. The two-time extraction was carried out using 50 mL of diethyl ether. The conical flask was then heated to 60 °C in a water bath [SI Table 4]. Total fatty matter was determined using the following expression:

$$TotalFattyMatter(\%)=\frac{WeightofTFMafterdrying}{Weightofsoapsample}\times 100$$

2.9. Total alkali content (TAC) determination

To determine TAC, experiments were conducted on the previously obtained aqueous layer [12]. In order to accomplish this, 0.5M NaOH was employed to titrate 10 mL of the aqueous layer using methyl orange indicator (SI Table 5). Total alkali content was determined as

$$TotalAlkaliContent\left(\%\right)=4.0\times \frac{\left(Volumeofacidtaken-volumeofbaseatendpoint\right)}{massofsoap}$$

2.10. Assessment of antimicrobial activity

2.10.1. Preparation of seed-plate media

An overnight fresh pure culture of Staphylococcus aureus was grown in Muller Hinton Broth at 40 °C and harvested by centrifugation at 10000g. The harvested cells were washed with autoclaved 0.9 % NaCl aqueous solution (NS) twice and re-suspended in NS. Seeded plates were prepared using Muller Hinton Agar medium. The Medium was prepared according to the instruction manual, was autoclaved and cooled down to 40 °C with constant gentle stirring in a water bath. The re-suspended bacterial inoculum was poured into the medium and mixed with gentle agitation to achieve a concentration of 0.5 McFarland standard by visual inspection. The media immediately poured into petri plates (20 mL/90 mm plate) inside a safety cabinet to prevent contamination. The plates were allowed to solidify and were stored at 20 °C until use. All the seeded plates were prepared on the same day of the test. Any unused plates were disposed of after autoclave sterilization.

2.10.2. Addition of sample and incubation

When the seeded plates were solidified enough, four wells per plate were created in the medium using a sterilized borer with an 8.00 mm diameter (Fig. 2). In this step, well diffusion method was used and 100 % soap sample was placed in the wells to get zone of inhibition. This 100 % sample was gained by grating a soap bar into small pieces using a cheese grater. The grated soap samples were added to the wells and labeled appropriately. As a negative control, one well was kept empty. The plates were then incubated at 37 °C for 24 h in an upright position.

Fig. 2.

The zone of inhibition of soaps against Staphylococcus aureus.

Calculation: The Antimicrobial activity of soap was assessed by measuring the zone of bacterial growth inhibition (mm). The measurement was taken across the zone of inhibition, from edge-to-edge, passing through the center of the well [[26], [27], [28]].

2.11. Assessment of time dependent antimicrobial effect of soap

2.11.1. Preparation of media

Muller Hinton Agar plates were prepared according to the manufacturer's guide and autoclaved and poured into petri plates (Fig. 3). A bacterial suspension was prepared as described in the previous section and re-suspended in NS to a concentration of 0.5 McFarland standards by visual inspection comparing the solution. A soap suspension was prepared by dissolving the grated soap in sterile water (1 % w/v). The bacterial suspension and soap solution were mixed in a 1:1 ratio. The soap-bacterial mixtures (100 μL) were kept for 10 seconds and then serially diluted 10 fold in 900 μL of 0.9 % NaCl saline. Ten microlitres of solution from each dilution were dropped onto the agar. Three replicative drops were applied to check accuracy. The same procedure was repeated changing the exposure time from 10 seconds to 1 min. All plates were allowed to dry in a safety cabinet and incubated at 37 °C for 24 h in an upright position. After incubation, the plates were observed and colonies were counted as colony forming units (CFU). Considering that one colony has been grown from single bacteria. Finally, the results (CFU/mL of the 1:1 soap mixture) were compared [29].

Fig. 3.

Killing effect of soaps against Staphylococcus aureus at different exposure time (for 1 min and 10 s).

2.12. Antioxidant properties determination

The DPPH (2,2-Diphenyl-1-picryl-hydrazyl) free radical scavenging assay was applied to evaluate the antioxidant properties of soap samples by following a standard protocol with minor modification [30]. In brief, 1 mg/mL stock solution was prepared for all experimental soaps in methanol and took 1 mL of methanolic soap solution in six different concentrations (10, 25, 50, 100 and 150 μg/mL) in each test tube for observing the dose dependent activity. Then, 1.5 mL of DPPH solution (0.04 mg/mL) was added in each test tube for all respective soaps and mixed it properly. After that, the solutions were incubated for 30 min in a dark area at ambient temperature to allow the proper reaction. Finally, the absorbance of the solutions was measured at 517 nm by using a UV Spectrophotometer against a blank solution. In the experiment L-Ascorbic acid (standard or positive control) was used to compare the antioxidant activity of those soaps. The percentage of scavenging was measured using the following this formula:

$$\%ofInhibition=\frac{A_0-A_s}{A_0}\times 100$$

Where A₀ represents the absorbance of the positive control and A_S represents the absorbance of the sample. Scavenging assay is an accepted mechanism for examining the antioxidant activity [31]. The 50 % inhibition concentration (IC₅₀) values were calculated from linear regression by plotting a scavenging percentage graph against the different concentrations (extract and L-ascorbic acid).

3. Results and discussion

3.1. Analysis of moisture content

Moisture content is an important criterion to understand the shelf life of a soap sample. In the current study, moisture content was found for SN 1–10.16 ± 0.04 % and SN 4–10.27 ± 0.08 %, SN 8–10.90 ± 0.09 which fall between the limit of Encyclopedia of Industries Chemical Analysis (EICA) which is 10–15 % [14,21]. So, these 3 soaps out of 22 soaps perfectly satisfy the standard limit. The moisture contents were found for SN 2–8.25 ± 0.05 %, SN 3–8.30 ± 0.04 %, SN 5–7.06 ± 0.03 %, SN 7–7.20 ± 0.08 %, SN 13–6.65 ± 0.02 %, SN 14–5.84 ± 0.04 %, SN 16–6.75 ± 0.04 %, SN 20–3.84 ± 0.03 % are below the EICA standard (Fig. 4). As higher moisture content spoils soap on storage [14], so these 9 soaps out of 22 are considered good for shelf life. Moisture contents were found for SN 6–20.38 ± 0.05 %, SN 9–16.07 ± 0.04 %, SN 11–26.91 ± 0.05 %, SN 12–18.56 ± 0.03 %, SN 15–22.58 ± 0.09 %, SN 17–10.82 ± 0.07 %, SN 18–18.11 ± 0.08 %, SN 19–21.10 ± 0.09 % (Table 3). Most of these soaps are luxurious and costlier than others. They are advertised as moisturizing bathing bar, hydrating cream bar, beauty bar or bar pain. Although they exceed the EICA limit, their application on skin leaves the skin soft and moisturized which coincides with their package mentioned specifications and advertisement. High moisture content in soap containing excess water can lead to reaction with unsaponified fat to give free fatty acid and glycerol in the hydrolysis process of soap in storage [14,21]. Herein, moisture content in glycerin soap (SN 10–20.50 ± 0.09 %) is high as glycerin locks skin's natural moisture and prohibits over-drying which is important to maintain skin condition. They are generally formulated by dissolving the soap in heated alcohol, sometimes with glycerol and sugar, and allowing the soap to crystallize as the alcohol evaporates. Glycerin also improves the stability of soaps [32,33].

Fig. 4.

Moisture content variation in selected commercial soaps.

Table 3.

Values of parameters of soap found in the current research. Values are means of triplicate analysis.

SN	MC (%)	FH (cm)	pH	TFM (%)	TAC (%)	ZI (mm) per 100 % conc.	Rapid Effectiveness (CFU)	Antioxidant Assay (IC50, μg/mL)
SN 1	10.16 ± 0.04	9.60 ± 0.36	9.64 ± 0.02	64.29	0.33 ± 0.01	NI
SN 2	8.25 ± 0.05	14.33 ± 0.58	9.98 ± 0.02	73.32	0.69 ± 0.03	NI
SN 3	8.30 ± 0.04	22.33 ± 0.29	9.83 ± 0.02	72.89	0.99 ± 0.03	18.67 ± 0.29	TNTC	357.41 ± 2.07
SN 4	10.27 ± 0.08	24.83 ± 0.58	9.50 ± 0.02	72.82	1.17 ± 0.03	NI
SN 5	7.06 ± 0.03	6.83 ± 0.58	9.70 ± 0.01	77.06	0.94 ± 0.01	NI
SN 6	20.38 ± 0.05	14.16 ± 0.29	9.94 ± 0.02	80.02	0.29 ± 0.01	NI
SN 7	7.20 ± 0.08	20.17 ± 0.58	10.16 ± 0.01	91.39	0.77 ± 0.01	NI
SN 8	10.90 ± 0.09	20.83 ± 0.76	9.82 ± 0.01	90.49	0.92 ± 0.02	NI
SN 9	16.07 ± 0.04	19.16 ± 0.76	10.17 ± 0.02	66.01	0.58 ± 0.02	NI
SN 10	20.50 ± 0.09	13.50 ± 0.50	9.87 ± 0.01	83.67	0.95 ± 0.03	NI
SN 11	26.91 ± 0.05	16.67 ± 0.29	9.43 ± 0.08	74.82	0.88 ± 0.02	NI
SN 12	18.56 ± 0.03	15.33 ± 0.58	10.15 ± 0.02	74.56	1.15 ± 0.00	NI
SN 13	6.65 ± 0.02	16.50 ± 0.87	7.22 ± 0.01	62.47	0.34 ± 0.03	17.33 ± 0.29	TNTC	777.15 ± 2.57
SN 14	5.84 ± 0.04	23.30 ± 0.76	9.71 ± 0.01	81.77	1.11 ± 0.00	NI
SN 15	22.58 ± 0.09	21.66 ± 0.58	9.82 ± 0.01	73.32	1.10 ± 0.03	21.50 ± 0.50	TNTC	588.97 ± 1.78
SN 16	6.75 ± 0.04	22.65 ± 0.58	10.02 ± 0.02	60.03	0.51 ± 0.01	NI
SN 17	10.82 ± 0.07	24.83 ± 0.29	9.99 ± 0.03	84.23	0.58 ± 0.02	NI
SN 18	18.11 ± 0.081	21.66 ± 0.58	9.50 ± 0.02		0.20 ± 0.01	18.63 ± 0.12	TNTC	438.83 ± 1.89
SN 19	21.10 ± 0.09	21.33 ± 0.58	9.53 ± 0.02		0.69 ± 0.01	20.67 ± 0.29	TNTC	421.15 ± 3.68
SN 20	3.84 ± 0.03	14.83 ± 0.29	7.01 ± 0.01	1.438	1.00 ± 0.01	25.97 ± 0.35	TNTC	576.06 ± 2.07
SN 21	83.75 ± 0.05	22.66 ± 0.58	7.73 ± 0.02	11.35	0.98 ± 0.01	29.27 ± 0.25	1.2 × 107 (1 min), 5.5 × 107 (10 s)	420.64 ± 3.62
SN 22	76.76 ± 0.06	25.16 ± 0.29	8.75 ± 0.03	19.98	0.32 ± 0.01	19.93 ± 0.21	TNTC	517.94 ± 3.92

TNTC – (Bacteria Colony) Too Numerous to Count.

NI - Non Inhibiting (to bacteria).

In case of bar soap, moisture content found values which ranges between 3.84 % and 26.91 % was comparatively similar to other studied literature 5.40–15.12 % [14], 9.4–16.9 % [34], 1.8–92.27 % [35], 18.8–22.5 % [36]. Moreover, moisture content of liquid soaps (SN 21–83.75 ± 0.05 % and SN 22–76.76 ± 0.06 %) are high for their being liquid. Variation of the preparation methods may contribute to the difference of the results [14]. Another fact is that SN 1 & SN 21 are both the same branded soap’s bar and liquid version (MC -10.16 ± 0.04 % and 83.75 ± 0.05 % respectively) and the liquid version has approximately 8 times higher moisture content than the bar version.

3.2. Analysis of foam height

Foam relates to the dirt dispersion as the quantity of dirt is proportional to the foam concentration up to a certain level and then decreases again. The maximum amount of dirt concentration connected to the foam corresponds to the region of micelle formation [37]. Standard requirement grade of soap’s foam height is 1.5 cm for 100 mL [14]. Foam height of all soaps (SN1 to SN 22) are beyond the acceptance limit (1.5 cm and above) and they are good for cleansing purposes (Fig. 5). Foam Height ranges between 6.83–25.16 cm varies greatly than 2.30–8.50 cm [14].

Fig. 5.

The variation of foam height level in selected soaps.

3.3. Analysis of pH

The pH range of all 22 soaps tested was found to be 7.01–10.17, while the permitted limits set by the Regulating Agency in Nigeria (RAN) are 7.00–10.00, and by the Standards Organization of Nigeria, 6.5–8.5 (Fig. 6) [15,16,21]. Soap employed on skin affect the health of skin [38,35], [35,38]. Naturally derived fatty acid salts (soaps) possess much lower toxicity than synthetic detergents [39]. Among these analyzed soaps bar soap SN 13–7.22 ± 0.01 and two liquid soaps SN 20–7.01 ± 0.01, SN 21–7.73 ± 0.02 are slightly alkaline which indicates the high degree of saponification. The pH of healthy human skin is 5.40–5.90 [21]. Higher the degree of saponification, lesser the alkalinity and more is the soap skin friendly. The rest of the 18 bar soaps SN 1 to SN 12 and SN 14 to SN 19 have pH values within the range of 9.43–10.17 and the other liquid soap SN 22 within the range of 8.00–9.00 (Table 3). The results obtained from the current research work, 7.01–10.17 is comparable to the reported studies 7.29–11.53 [14], 10.50–10.80 [18], 9.6–10.4 [35], 8.6 [25]. The alkalinity of soap solution works as a barrier against pathogens like bacteria and viruses by neutralizing skin's protective acid mantle. Although too high and too low pH is corrosive to skin and contributes to hardness of hands and skin after washing causing dehydration, irritation and destruction which can be reduced by addition of excess fat or oil while saponification [14,15]. The quantity of sodium hydroxide and potassium hydroxide incorporated affects the pH, often leading to a more alkaline pH level (generally between 10 and 11) [40]. The variety of fatty acids in the oils used for saponification can also alter the soap’s final pH. Saturated fatty acids (such as lauric acid from coconut oil) may produce a different pH compared to unsaturated fatty acids (such as oleic acid from olive oil). The present study indicates that the pH values of all of these 22 commercial branded soaps had almost no adverse effect on skin. However, some recent studies suggested that some photochemical reaction such as the photodegradation and the photocatalytic reaction of the organic compound takes place in the pH range of 3–9, and by turn free radicals are produced which are very detrimental to human health and the environment as well [41,42]. In this study, the sample of bar soap SN 13 and liquid soaps SN 20, SN 21, and SN 22 has pH (7–9) values in the range of 3–9, thereby, this product may photodegrade in sunlight, can cause a minor health risk and may hamper the ecological balances of environment, although another 18 samples are in the safe zone.

Fig. 6.

The pH values of commercial branded soaps.

3.4. Analysis of total fatty matter

TFM results in the current study are shown in Fig. 7. Total Fatty Matter were obtained for bar soap SN 5–77.06 %, SN SN 6–80.02 %, SN 7–91.39 %, SN 8–90.49 % and SN 10–83.67 % fall between the limit of grade i soaps (77 – above) % according to Bureau of Indian Standards (BIS). Moreover, the International Organization of Standardization (ISO) referred good quality soaps that must have TFM above 76 % and standard TFM should not be less than 63 % [4]. Consequently, SN 5, SN 6, SN 7, SN 8 and SN 10 are good for dry skin which can make skin smooth by rehydrating and the high oil content within the soap acts as a lubricant throughout the day. Besides, TFM of bar soap SN 1–4, SN 9, SN 11, SN 12, SN 13 and SN 15 fall between the limit of grade ii (61–76) % according to BIS. As standard TFM should not be less than 63 %, the above 9 soaps are of standard quality. TFM was found for SN 16–60.03 % which is in the limit of grade iii soaps (51–60) % according to BIS [4,29]. Herein, SN 16 soap can be used for the production of bathing and toilet soap [25].

Fig. 7.

The total fatty matter in commercial branded soaps.

Moreover, three liquid soaps are exceptional with SN 20–1.438 %, SN 21–11.35 % and SN 22–19.98% as they have high moisture content (Table 3). The presence of unreacted NaOH may be responsible for lower TFM value. Total Fatty Matter ranges between 1.438–91.39 % is comparable to 8.5–80.5 % [35], 59.00–91.00 % [25]. Also, the obtained results are relatively better than 29.6–79.2 % [4] and 54–68 % [18], 60.20 % [25]. Mainly fatty acids including saturated fatty acids such myristic acid, lauric acid, palmitic acid, and stearic acid and unsaturated fatty acids such as ricinoleic acid, oleic acid and linoleic acid are responsible for the TFM in soaps [43]. High moisture content, types and number of fatty materials used, and differences in saponification methods contribute to the different values of TFM [44]. Total fatty matter also has an adverse effect on the environment, causing a reduction of water quality and production of greenhouse gases [45,46]. Freshwater bodies with high detergent concentrations have the potential to produce huge foams on the top of the water, which can slow down the pace at which oxygen enters the water. Aquatic organisms are unable to adequately absorb dissolved oxygen due to this circumstance. Acidity, turbidity, salinity, temperature, and other chemical and physical characteristics of water are changed by these compounds in the aquatic ecosystem [44]. Fish suffer biochemical and physiological deficits when the quality of the water decreases. According to this report, the products having high TFM are good for cleansing but detrimental to the environment.

3.5. Analysis of total alkali content

Total alkali content is one of the most important characteristics that indicate the corrosivity of soap [18]. In the current research, 22 soaps were analyzed for TAC and the experimented values found for SN 1 to SN 22 were all within 1 % (Table 3). All of the values are within range of the International Organization of Standardization (ISO) limit which is below 2 % (Fig. 8). Also, according to BIS, good quality soap must have TAC below 5 % and the BIS limit is 1 % [4,25]. TAC found in the experiment ranges between 0.20 % and 1.16 % is comparative to other research work 0.76 % [25] and 0.13–1.60 % [17]. The results obtained are relatively better than the previously reported study 1.61–2.96 % [4]. Presence of excess alkali indicates incomplete saponification that can even cause burning effects on the users [18]. Accordingly, as all of these 22 soaps tested TAC value is within 1 %, they can be said to be good quality soap based on TAC and consequently, all of the experimented soaps are non-corrosive to skin. The total alkali content is primarily influenced by the lye used (sodium or potassium hydroxide), and the specific oils and fats in the soap formulation. Unsaturated oil such as corn or olive, tend to produce softer soap compared to saturated oils or fats such as tallow or coconut oil create soaps. Sodium soaps are more solid and less soluble in water than potassium soaps made from the same oils. However, a hard bar of soap may frequently contain some potassium soap [32].

Fig. 8.

Total alkali content in commercial branded soaps.

3.6. Assessment of soaps’ antimicrobial effects

All of the 22 soaps were tested for antimicrobial properties, among which eight soaps have shown a zone of bacterial growth inhibition (Fig. 9) [38]. Table 3 shows the zone of inhibition in the experiment ranges between 17.33–29.27 mm/100 % conc. which is comparative to 28 mm/100 % conc. against Staphylococcus aureus for medicated soap [47], but higher than 13–14 mm/100 % conc against S. aureus [48]. According to package-mentioned specifications, among these 8 soaps, SN 3 is antiseptic with zone diameter of 18.67 ± 0.29 mm/100 % conc. [49], SN 13 is beauty and bar pain soap (Bar pain soap is for very dry sensitive skin to cause pain or inflicts to be recovered) with zone diameter of 17.33 ± 0.29 mm/100 % conc., SN 15 is neem bar with zone diameter of 21.50 ± 0.50/100 % conc., SN 18 is enriched with black cumin oil with zone diameter of 18.63 ± 0.12/100 % conc., SN 19 is bar soap with zone diameter of 20.67 ± 0.29 mm/100 % conc., SN 20 is enriched with neem and aloe vera with zone diameter of 25.97 ± 0.35 mm/100 % conc., SN 21 is enriched with sunflower seed oil, castor oil and sweet almond oil with zone diameter of 29.27 ± 0.25 mm/100 % conc. and SN 22 is marigold enriched with zone diameter of 29.27 ± 0.25 mm/100 % conc. (SI Table 6).

Fig. 9.

Zone of bacterial growth inhibition of commercial branded soaps.

Antiseptic soap slows and stops the growth of microorganisms used in hospitals and other medical settings to reduce risk of infections which is responsible for the antimicrobial property in SN 3 [21]. Besides, SN 13 and SN 19 possess antimicrobial properties. Thymoquinone of black cumin oil in SN 18 [50], methanolic extracts of marigold plants in SN 22 [51], azadirachtin of neem oil in SN 15 and SN 20 [52], anthraquinone of aloe vera in SN 20 [53], phytochemicals of sunflower oil in SN 21 [54], ricinoleic acid of castor oil in SN 21 [55] and oleic acid of sweet almond oil in SN 21 [56] mainly give rise to the antimicrobial properties of these soaps. Zone diameter of neem oil containing SN 15 is 21.50 ± 0.50 mm/100 % conc. and addition of aloe vera to neem may be uplift the value of zone diameter of SN 20 to 25.97 ± 0.35 mm/100 % conc. The highest value of zone diameter of SN 21 is 29.27 ± 0.25 mm/100 % conc. which is the combination of sunflower seed oil, castor oil and sweet almond oil [57]. Herein, it can be hypothesized that soaps can have different values of zone of inhibition due to the antimicrobial power of these compounds according to their combination.

3.7. Analysis of time dependent efficacy

Eight soaps out of 22 exhibited zone of inhibition and were examined for a further test. Time dependent effectiveness was measured among the soap samples. SN 21 with the observed highest zone of inhibition (29.27 ± 0.25 mm/100 % conc.) in the previous test gave prominent time dependent efficiency (Table 3). The number of Staphylococcus aureus was reduced from 5.5 × 10⁷ CFU after 10 seconds of exposure to 1.2 × 10⁷ CFU after 1 min of exposure. From this study it has been observed that, 1 min application of soap will be more effective than 10 seconds, resulting in a five times reduction in the viable cells of Staphylococcus aureus.

3.8. Analysis of antioxidant assay

The results indicate that the soaps tested (SN 3, SN 13, SN 15, SN 18-SN 22) showed a dose-dependent increase in antioxidant activity, which suggests that higher concentrations of the soaps lead to greater inhibition of free radicals (Fig. 10A). The calculated IC₅₀ values indicate the concentration required to inhibit 50 % of the free radicals in the DPPH assay. All tested soaps showed IC₅₀ values greater than 250 μg/mL, categorizing their antioxidant activity as "very mild" [31]. Among the soaps, SN 3 exhibited the highest antioxidant activity, with the lowest IC₅₀ value of 357.411 μg/mL, indicating it is the most effective at scavenging free radicals among the samples tested. In comparison, the standard antioxidant, L-ascorbic acid, showed an IC₅₀ of 26.339 ± 1.168 mg/mL, which suggests that the soaps possess antioxidant properties (Fig. 10B). However, they are not as potent as L-ascorbic acid. The findings presented in SI Table 7 and Fig. 10C provide a visual representation of the antioxidant capacity of the soaps compared to the standard.

Fig. 10.

Determination of antioxidant activity of soaps. A. DPPH scavenging assay. B. L-Ascorbic Acid Standard Curve C. IC₅₀ comparison of soaps with L-ascorbic acid.

3.9. Survey

A survey was run among 50 people including doctors, pathologist, nurse, general patients, dermatology patients, medical representatives, health workers and common people. It was based on seven different questions including name, age, gender, occupation, frequency of use, type of preferable soap, and effectiveness and satisfaction about soap. This report will provide comprehensive insights into consumer behaviors, preferences and expectations about commercial soap.

They are aged between 18 and 74 years. The doctors and pathologist demand commercial soaps that are usually aligned with promoting skin health, ensuring safety, destroying germs and leaving skin fresh, clean and moisturized. Consumers also expect a soap to be cost friendly, safe, widely available, require less time to clean and have no side effects. Around 80 % consumers avoid 2nd or 3rd graded soaps and tend to use soap which has TFM value (77 – above) %. They use different soap for general cleansing and facial purposes. According to the survey report, SN-7 is used by most people (22 %) with highest total fatty matter content (91.3892 %). Consequently, it possesses excellent hydrating, lathering and lubricating properties. Also, it has moderate moisturizing property (MC-7.20 ± 0.08 %), good cleansing property (FH-20.17 ± 0.58 cm) and it is safe to skin (pH-10.16 ± 0.01, TAC-0.77 ± 0.01 %). This soap is widely advertised and available. Surprisingly, 18 % consumer use SN 13 for facial application only as it is a luxury moisturizing and beauty bar. Again, 16 % of consumers use SN 21 and they think this soap is good for all purposes. Another 16 % use SN 3 which is a mild antiseptic soap. The data collected from such a survey can be invaluable to tailor their products to consumer needs and preferences.

4. Conclusion

Selecting soap that strikes a balance between different characteristic parameters is essential. Soaps with average moisture content, accepted foam height, ranged pH, high total fatty matter content, less total alkali content, high zone of inhibition, high rapid efficacy, and high antioxidant activity indicate high quality. SN 7 and SN 13 have moisturizing, rehydrating, lubricating, and moisture-locking capacities with highest TFM and high moisture content. Herein, 7 soaps among 22 soaps are of grade I (TFM - 77 and above). The TAC of all 22 tested soaps is observed within the standard limit. According to antimicrobial parameters, SN 20 and SN 21 have exhibited the highest growth inhibition against selected bacteria staphylococcus aureus and SN 21 is completely proved as bactericidal. Based on the antioxidant assay, SN 3 and SN 21 have the lowest IC₅₀ values which indicate their ability to capture and wash away free radicals on the skin making these commercial soaps antioxidant. It has been found that SN 3 and SN 21 are of the best quality among bar soap and liquid soap, respectively. Herein, SN 3 stands out as an effective bar soap with gentle moisturizing and cleansing capacity (MC-8.30 ± 0.04 %, FH-22.33 ± 0.29 cm). It proves to be skin-friendly (pH-9.83 ± 0.02, TAC-0.99 ± 0.03 %), exhibits good TFM value (72.89 %), demonstrates proficient inhibition of bacteria growth, and possesses the capability to eliminate free radicals (357.41 μg/mL). Moreover, SN 21 has overall excellent quality with well-moisturizing properties, high cleansing capacity, and almost safe pH (7.73 ± 0.02) values although may cause minor health risks for photodegradation reaction and low TFM as a liquid soap. It also has the highest bacteria growth inhibiting property and can destroy selected bacteria at the rate of 1.2 × 10⁷ CFU at 1 min and 5.5 × 10⁷ CFU at 10 seconds and can remove free radicals. In the present study, none of these experimented soaps are found harmful and they are suitable for cleansing, bathing, and clinical purposes. Assessing the properties ensures the quality and safety of soap and also underscores the importance of these properties in enhancing consumer safety and encouraging innovation in soap formulations.

5. Statistical analysis

All the experiments were replicated 3 times. The statistical analysis was performed using GraphPad Prism v8.0.2 and a test was conducted at a 5 % level of significance to evaluate the validity of the calculated mean.

CRediT authorship contribution statement

Jannatul Ferdous Nova: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Susmita Zaman Smrity: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Mahadi Hasan: Writing – original draft, Formal analysis. Md. Tariquzzaman: Methodology, Formal analysis, Data curation. Md Al Amin Hossain: Writing – review & editing, Writing – original draft, Formal analysis, Data curation. Md. Tanvirul Islam: Writing – review & editing, Writing – original draft, Methodology. Md. Raysul Islam: Methodology, Formal analysis. Selina Akter: Writing – review & editing, Writing – original draft, Formal analysis, Data curation. Md. Sifat Rahi: Writing – review & editing, Writing – original draft, Formal analysis, Data curation. Md. Tuhinur R. Joy: Writing – review & editing, Writing – original draft, Formal analysis, Data curation. Zannatul Kowser: Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

Data availability statement

Data included in article/supp. material/referenced in article.

Funding statement

The current study did not receive any specific grants from funding organizations in the public, commercial, or non-profit sectors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the supplementary data to this article: