High-Limonene Orange Peel Essential Oil as a Natural Antibacterial Agent in Hand Sanitizer Gels

Abstract

Orange peel waste has potential to be valorized from agro-industrial and food sectors to formulate products for personal hygiene and public health. This study presents the formulation of alcohol-based antibacterial gels incorporating essential oils extracted from Citrus sinensis orange peel waste and its sensory evaluation among 770 participants in a holistic approach. The orange essential oil, obtained via hydrodistillation, demonstrated a high limonene content of 96.5% by GC-MS. Antibacterial activity assessed by agar diffusion assays showed orange essential oil efficacy against Escherichia coli and Staphylococcus aureus, with inhibition zones of 25.9 mm and 23.62 mm, respectively. Two gel prototypes, GSA and GSB, were developed and sensorily evaluated. GSA was preferred for its superior appearance, spreadability, absorption, and smell, with 99% acceptability. Appearance and spread sensory parameters were the differentiators between both formulations according to user preferences. Thus, 93% of respondents are willing to use either GSA or GSB as a daily hygiene product over commercial ones. Although the gels exhibited reduced antibacterial activity relative to essential oil, with inhibition zones measuring 8.3 mm for E. coli and 9.0 mm for S. aureus, they retained satisfactory user acceptability. These findings support the use of citrus biowaste-derived essential oils in sustainable personal hygiene products.

1. Introduction

Growing concern about the rise of antimicrobial–antibacterial resistance, coupled with the need to adopt more sustainable practices in the cosmetics, pharmaceutical, and healthcare industries, has motivated the development of natural alternatives with biocidal potential and low environmental impact. Among these, essential oils extracted from agro-industrial waste have gained interest due to their antimicrobial–antibacterial activity, biodegradability, and economic viability [1,2,3,4]. Essential oils (EOs) are heterogeneous mixtures of terpenes, sesquiterpenes, acids, esters, phenols, and lactones [5,6,7]. EOs are composed mainly of terpenes but also contain other chemical compounds [8].

Citrus sinensis orange essential oils (OEOs), rich in bioactive compounds such as limonene, have shown significant activity against Gram-positive and Gram-negative bacteria, opening up possibilities for their incorporation into topical formulations such as hand sanitizers [9]. This is consistent with recent GC-MS analyses of citrus peel essential oils, which demonstrated significant antimicrobial activity of Citrus paradisi, Citrus aurantifolia, and Citrus sinensis extracts when evaluated as potential hand sanitizer agents [10]. Also, it is further supported by in vitro studies showing that peel essential oils from Citrus sinensis and Citrus limon possess antioxidant, antidiabetic, and antibacterial activities, underscoring their potential for applications in food, cosmetic, and pharmaceutical products [11].

Hand sanitizers are available in various formulations, including liquids, gels, and foams, to accommodate different user preferences and application needs. Based on their active ingredients, they are generally classified into two categories: alcohol-based and alcohol-free. Alcohol-based sanitizers—typically containing ethanol or isopropanol at concentrations ranging from 60% to 95%—are widely recommended for general use due to their broad-spectrum antimicrobial–antibacterial efficacy. In contrast, alcohol-free formulations are not consistently recommended, as their effectiveness against various pathogens is generally lower [12,13,14]. Furthermore, hand sanitizers containing less than the recommended alcohol concentration may be insufficient for effective disinfection. Rather than inactivating or killing pathogens outright, these suboptimal formulations may only reduce microbial growth, leading to decreased—but not eliminated—microbial presence. Ethanol has been shown to be effective against a variety of enveloped viruses, beginning at concentrations of 42.6% [15]. Antibacterial gel has become an essential product in the market since 2019, because of the COVID-19 pandemic, during which consumers used it as an antiseptic solution and as a replacement for soap and water, with it remaining part of their personal hygiene habits even after the emergency ended. It was recommended by the World Health Organization (WHO) that, in the absence of water, the use of alcohol-based hand sanitizers could prevent the transmission of coronavirus [16].

In 2024, global orange production reached 48.8 million tons, with Brazil leading at 16.5 million tons. Mexico produced 4.8 million tons, positioning itself as one of the world’s top producers alongside countries like China, the United States of America, and Spain [17,18]. Orange peel represents one of the main residues generated by the citrus industry, accounting for approximately 50–60% of the total fruit weight [19,20,21]. Orange peel is disposed of in open dumps, posing risks of soil, water, and air contamination due to biomass decay and leachate generation [22]. The valorization of this biowaste as a source of bioactive compounds can contribute to a circular and sustainable economy [23,24]. Several studies have reported that citrus essential oils exhibit antibacterial, antioxidant, and anti-inflammatory properties [25,26], making them promising candidates for the development of natural antibacterial gels. However, the incorporation of these compounds into dermatologically safe and sensorially acceptable products requires an integrated evaluation combining characterization, microbiological, and sensory methodologies.

Therefore, the objective of this study was to formulate and evaluate antibacterial gels based on essential oils extracted from Citrus sinensis orange peel waste, through characterization by GC-MS, antibacterial activity determination against Staphylococcus aureus and Escherichia coli by agar diffusion assays, and field-based sensory tests to assess acceptance and feasibility as a personal hygiene product, bridging the gap between technical formulation and user experience. This holistic approach was implemented to ensure not only the product’s efficacy but also its acceptability and practicality for daily use. Therefore, this study contributes to the design of sustainable solutions through the utilization of citrus waste, promoting natural alternatives for personal hygiene and public health.

2. Materials and Methods

2.1. Reagents

Fresh orange peel waste (Citrus sinensis from the Rutaceae family) was obtained as a donation from a company dedicated to the production and sale of natural juices. The oranges acquired by the company were cultivated in Redlands, CA, USA. The standards β-pinene (98%), δ-3-carene (≥98.5%), limonene (97%), β-myrcene (≥90), α-phellandrene, γ-terpinene (97%), and C₇–C₄₀ saturated alkanes standard (certified reference material) were purchase to Sigma-Aldrich (St. Louis, MO, USA); linalool (≥97%), α-pinene (≥95%), and dichloromethane were purchased to Merck (Darmstadt, Germany).

2.2. Sample Preparation, Essential Oil Extraction, and Gel Formulation

Fresh orange peel waste (Citrus sinensis from Rutaceae family) was obtained as a donation from a company dedicated to the production and sale of natural juices. The oranges acquired by the company were cultivated in Redlands, CA, USA. By prior agreement, the company donated buckets containing orange peel waste generated on the same day. The residues were collected on the same day they were discarded by the company and were transported to the laboratory, and the essential oil extraction process was carried out upon arrival, since the freshness of the plant material is important to obtain better yields of essential oils. The orange peel consisted of the rind, albedo, pulp, and seeds.

The process was performed using a flexible and modular pilot-scale essential oil extraction system through the hydrodistillation technique. The extraction process included particle size reduction, heating, a 20 L extraction vessel, LP gas (10 kg capacity) and gas piping, a counter-current concentric tube condenser, and decanter. Four batches of essential oil extraction were performed. In each one, 1 kg of orange peel waste was grinded in a Retsch GM 300 mill with a 5 L capacity container at 300 rpm for 10 s, with a water-to-waste ratio of 1.9 L/kg, using purified water. Then, the essential oil extraction process was performed for 50 min, with a gas flow of 1.29 L/h and a heat flow of 33.88 MJ/h. The cold stream inlet was at 25 °C and 113.17 mL/s [27]. At the end of the extraction and separation of the oil, the amount of essential oil obtained was measured and the extraction yield was determined. The essential oil was stored in amber tubes of 60 mL and kept refrigerated at 4 °C until further use.

Two antibacterial gel formulations, GSA (Gel Sample A) and GSB (Gel Sample B), 1 L per each, were developed based on the recommendation by the US Centers for Disease Control and Prevention for alcohol-based hand sanitizers with a concentration of 60–95% with distilled water [14]. The selection of 60% ethanol was based on the need to balance antimicrobial efficacy with skin tolerance. This concentration lies at the lower end of the recommended range and was considered appropriate to minimize potential adverse effects. At higher concentrations, ethanol is frequently associated with undesirable outcomes such as skin irritation, dryness, allergic contact dermatitis, and intense odor, which may compromise user acceptability and long-term application [28].

The components for both formulations were triethanolamine 85% purity (AZUMEX, Puebla, Mexico) carbomer 940 100% purity (NAIMA, Morelos, Mexico), ethanol absolute ACS 99.9% purity (Fermont, Monterrey, Mexico), OEOs extracted from orange peel waste, and distilled water. The mixture of the components was carried out at 400 rpm for 20 min in a VWR Hotplate/Stirrer (VWR, Radnor, PA, USA), 650 W, 120 V, 50/60 Hz 1 Phase, without any changes in smell or appearance and a pH of 7. The main difference in both formulations was the consistency, as it was related to user preferences. In

Table 1. Antibacterial gel formulations.

Component	GSA 1 (% v/v)	GSB 2 (% v/v)
Triethanolamine	0.3	0.3
Carbomer 940	1.5	1
Ethanol	60	60
Orange essential oil	1	1
Distilled water	37.2	37.70

¹ GSA (Gel Sample A). ² GSB (Gel Sample B).

, the gel formulations are revealed.

The viscosity of GSA and GSB was measured in a viscometer CAP 2000+ BROOKFIELD, cone #2, at 50 °C and 200 rpm.

2.3. Gas Chromatography–Mass Spectroscopy

Volatile and semi-volatile secondary metabolites of essential oils were determined by the gas chromatography technique (Agilent Technologies model 7890A Network GC-MS system) coupled to a model 5975 mass selective detector (Palo Alto, CA, USA) (electron ionization, 70 eV), equipped with a split/split-less injection port (250 °C, 20:1 split ratio); and flame ionization detector (FID) (280 °C). An HP-5MS capillary column (30 m × 0.25 mm id × 0.25 μm df) with 5% phenyl poly (methyl siloxane) (Sigma Aldrich, Hamburg, Germany) stationary phase was used for the separation of the mixtures. The initial oven temperature was 50 °C for 2 min and then resumed at a rate of 5 °C min up to 250 °C (5 min). The carrier gas was helium (99.995%, Cryogas, Medellín, Colombia), with a column head inlet pressure of 12.667 psi at a rate of 1.172 mL min⁻¹, at 50 °C. Compound identification was carried out with standard terpenes analyzed under the same conditions and by comparison of mass spectra with those available in the Wiley Mass Spectral Database library. The Kováts retention index (KI) was calculated through a series of homologous aliphatic saturated hydrocarbons between C7 and C40, analyzed in the same column with the same chromatographic conditions which were used for the essential oil analysis. The mass spectra and retention indices obtained were compared with those reported in the literature [7], and fragmentation patterns of mass spectra with the NIST database, Version 2.0, from 2008.

2.4. Antibacterial Gel Cost Calculation

The antibacterial gel formulation cost for GSA and GSB was calculated by considering the necessary inputs such as reagents, OEO, water, and energy. The OEO extraction cost considered was 0.0100 USD/mL [27] as it was obtained in the same flexible and modular essential oil extraction system. The inputs and electricity costs were based on commercial tariffs for electricity and water. The commercial electricity tariff was 0.20 USD/kWh, the commercial water tariff was 19.55 USD/m³, and the LP gas was 0.66 USD/L. The cost estimate of the gel was made based on the input costs presented in

Table 2. Inputs cost for gel formulation.

Concept	Cost (USD/mL)
Orange peel essential oil	0.0100
Carbomer 940	0.0360
Triethanolamine	0.0100
Distilled water	0.0008
Ethanol absolute 99% purity	0.0005
Stirring energy cost in gel preparation	0.0002

and considering the composition of the formulation.

2.5. Antibacterial Capacity of Orange Peel Essential Oil and Antibacterial Gels

Pathogenic bacteria associated with human infections and contaminated hands and implicated in secondary infections of post-COVID-19 were selected for antibacterial activity assays [29,30]. For the study, Escherichia coli (ATCC, Manassas, VA, USA) was chosen as the representative Gram-negative model, while Staphylococcus aureus (ATCC, Manassas, VA, USA) served as the Gram-positive counterpart. To prepare the working microbial cultures, freshly grown bacterial cells were individually inoculated into Tryptic Soy Broth (TSB; Becton Dickinson, Sparks, MD, USA) and incubated aerobically at 37 °C for 24 h. Subsequently, each inoculum was transferred into freshly prepared warm culture broth and diluted to a final concentration of approximately 1 × 10⁷ CFU/mL, ensuring standardized conditions for the antibacterial assays.

The antibacterial activity of the OEO and the formulated antibacterial gels was assessed using the agar diffusion assay immediately following their fabrication [29]. First, 0.1 mL of each freshly prepared working microorganism was cultured on tryptic soy agar (prepared with 25 mL of liquid per 100 mm Petri dish). After a 30 min incubation period, four uniform holes (each measuring 7 mm in diameter and 4 mm in depth) were aseptically created per plate using a sterile metallic cylinder. Next, 50 µL of the corresponding freshly produced OEO and antibacterial gel was dispensed into each bacterial culture. The plates were subsequently incubated at 37 °C for 24 h, and the resulting inhibition zones were measured with an electronic digital caliper and digitalized using a dark field colony counter (Reichert, Depew, NY, USA). For the statistical analysis, each sample was measured in quadruplicate, and the average results were reported with their respective standard deviation.

2.6. Sensory Evaluation of Orange Peel Essential Oil Antibacterial Gels

A sensory evaluation was developed to assess the acceptance and validation of the formulated antibacterial gels among the population of Mexicali, Baja California, Mexico. To achieve this, a survey instrument was designed that comprised seven questions to evaluate key sensory parameters: appearance, color, spread, absorption, and smell of each formulated antibacterial gel (GSA and GSB). The test used a 9-point hedonic scale, where 1 was extremely unpleasant, 5 neither pleasant nor unpleasant, and 9 extremely pleasant. Additionally, yes/no questions were incorporated to determine the perceived importance of antibacterial gel usage and the likelihood of adopting such products. Two further questions addressed the frequency of gel use and the residual hand consistency after application. The relevant items, parameters, and senses considered in the survey are highlighted in

Table 3. Data collected in the sensory evaluation process of the antibacterial gels.

Item	Parameter	Sense	Scale
Do you think it is important to use antibacterial gel?	Importance	N/A	Yes or No
How often do you use antibacterial gel?	Frequency	N/A	Always, Almost Always, Sometimes, Never
How do you consider the appearance of the gel sample compared to other commercial gels?	Appearance	Sight	9-star hedonic scale ☆☆☆☆☆☆☆☆☆ 1. Extremely unpleasant, 5. Neither pleasant nor unpleasant, 9. Extremely pleasant
How do you consider the color of the gel sample?	Color
How do you assess the spread of the gel sample in your hands?	Spread	Sight/Touch
How do you feel about the absorption of the gel sample in your skin?	Absorption	Sight/Touch
How do you feel about your skin after the gel application?	Smoothness	Touch
How moisturized do you consider your skin after the gel application?		Touch
Do you feel lumps in your hands?		Touch	Yes or No
How do you perceive the smell of the gel sample during its application to your skin?	Smell	Smell	9-star hedonic scale ☆☆☆☆☆☆☆☆☆ 1. Extremely unpleasant, 5. Neither pleasant nor unpleasant, 9. Extremely pleasant
How do you perceive the smell after applying the gel to your skin?	Smell	Smell
Would you use this antibacterial gel?	Satisfaction	N/A	Yes or No

.

The calculated sample size was 770 surveys, based on a target population of 1,049,792 inhabitants in Mexicali [31], with a 95% confidence level and a 5% margin of error, based on the finite population sample equation [32] and using a simple random sampling method. The survey was applied across various settings, including universities, shopping centers, offices, businesses, supermarkets, gyms, and residential areas. To streamline data management and analysis, the online tool Survio was utilized. This platform not only facilitated the processing of a large volume of data but also ensured a user-friendly experience for both respondents and survey administrators, thereby optimizing resources, time, materials, and personnel. Participants accessed the survey using an electronic device, such as a smartphone, tablet, or computer.

During the survey process, each participant was provided with two 50 mL plastic bottles with the antibacterial gel formulations GSA and GSB. Each bottle was tagged with a unique QR code that, when scanned, directed the respondent to the corresponding Survio survey to record their evaluation for that specific sample. Consequently, every participant completed two surveys—one for each gel formulation. To prevent bias, the order in which the gels were evaluated was randomized, and disinfectant wipes were supplied to eliminate any potential cross-sample interference. A three-minute rest interval was observed between evaluations to ensure accurate sensory assessments.

From the survey results, questions 3, which evaluates appearance; 4, which evaluates color; 5, which evaluates spread; 6, which evaluates absorption; and 8, which evaluates smell, were selected as representative elements. Using the survey data, a bar chart was created to show participant preferences for each characteristic of the antibacterial gel.

For the statistical analysis, 770 responses for each evaluated parameter were used. The mean and standard deviation were calculated, and a comparative analysis of means was performed using a t-test with a 95% confidence interval. The purpose of this analysis was to determine statistically whether there were differences among the means of the antibacterial gels.

2.7. Ethical Statement

The Ethics Committee of the Instituto de Ingeniería, Universidad Autónoma de Baja California (UABC) approved the research protocol on 5 November 2025 (approval code 84/2025-1). A sensory evaluation was conducted among 770 participants, all of whom provided informed consent prior to participation. The research was carried out in accordance with institutional ethical guidelines and international standards for studies involving human participants.

3. Results and Discussion

3.1. Gas Chromatography–Mass Spectroscopy Results

The OEO yield extraction was 17 mL/kg. The chemical composition of the EOs of Citrus sinensis is shown in

Table 4. OEO composition by GC-MS.

Peak Number	RT (min)	Compound	CAS	IK 1	IK 2	Relative Area, %
1	7.1485	α-pinene *	007785-26-4	930	939	0.63
2	8.633	Sabinene	003387-41-5	968	975	0.20
3	8.7083	β-pinene *	018172-67-3	972	979	0.10
4	9.4167	β-Myrcene *	000123-35-3	986	990	1.67
5	9.8537	α-Phellandrene *	000099-83-2	1005	1002	0.10
6	10.0722	3-Carene *	013466-78-9	1015	1011	0.14
7	10.9313	Limonene *	000138-86-3	1032	1029	96.5
8	11.2779	γ-Terpinene *	99-85-4	1060	1059	0.10
9	14.1865	Linalool *	000078-70-6	1088	1096	0.30

¹ References to experimental Kováts retention index were calculated relative to C7–C40 n-alkanes on the HP-5 capillary column. ² Theorical Kováts retention index reported in the literature (Adams, 2017). * Peak identity was also confirmed using a standard compound.

, according to their order of elution on an HP-5 capillary column. Only components with a relative percentage ≥ 0.1 are indicated. In the EOs of C. sinensis, nine compounds were identified; the major components were limonene (96.50%), β-myrcene (1.67%), α-pinene (0.63%), and linalool (0.30%). Discrete percentages of Sabinene with 0.1751% and 3-carene with 0.1417% were found. This essential oil content falls within the range of the published literature, as it ranges from 70% to above 97% [33,34,35,36,37]; therefore, it can be considered a high-content limonene essential oil. The highest amount (up to 97%) is found in sweet orange (Citrus sinensis syn. Citrus aurantium var. sinensis) essential oil [38]. Limonene is the main constituent of orange essential oil. It is a terpene responsible for the characteristic odor of orange and lemon [39]. The composition, quality, and biological properties of the essential oil may vary considerably, depending on factors such as geographical origin, plant variety, extraction method, and storage [40].

Most of these compounds including limonene are responsible for antioxidant, anti-inflammatory, antifungal, antimicrobial–antibacterial, and wound-healing activities [41,42]. The antibacterial activity of OEO could be related to the lipophilic properties and the synergy between the different bioactive compounds [33]. The synergistic effects of these constituents make the essential oil a promising natural alternative for antimicrobial-antibacterial applications. Limonene has been studied for its antibacterial activity against Gram-positive and Gram-negative pathogenic bacteria [43]. Limonene, a-pinene, and β-pinene exhibit a wide spectrum of antimicrobial–antibacterial activity, as shown by many studies in other plants [44,45,46,47]. Linalool and β-Myrcene have an antibacterial effect on E. coli and S. aureus [48]. In vitro studies have demonstrated that sabinene has antibacterial activity and shows concentration-dependent antioxidant activity [49,50]. 3-Carene exhibits strong antibacterial activity, particularly against Gram-positive and Gram-negative bacteria [51].

3.2. Antibacterial Gel Cost Calculation and Viscosity Results

The production cost of the formulated antibacterial gels was estimated at USD 1.45 per liter for GSA, while for GSB, it was USD 1.27 per liter. This cost advantage is primarily due to orange peel waste valorization, as a source of OEO. In addition, the use of a modular and flexible extraction system contributes to cost-effectiveness through optimized yields and efficient resource management. It should be noted that the cost estimate encompasses only operating and production expenses; capital expenditures, logistics, packaging, and other ancillary costs are not included. For context, commercial antibacterial gels are generally priced between USD 5 and USD 10 per liter. Viscosity analysis revealed that GSA was 13 cP, while GSB measured 11 cP, indicating minor differences in rheological behavior between the two formulations.

3.3. Antibacterial Findings

The inhibition zones observed on a Petri dish are a qualitative indicator of the antimicrobial activity. According to the Kirby–Bauer method manual published by the American Society of Microbiology [52], inhibition zones can be interpreted to classify bacteria as resistant, intermediate, or susceptible. According to the National Committee for Clinical Laboratory Standards, they are classified as sensitive, moderately sensitive, or resistant [53].

Some authors use the sensitivity of essential oil assessment based on the size of the inhibition zone. EOs with an inhibition zone diameter (IZ) ≤ 8 mm can be considered non-efficient, whereas EOs with an IZ ≥ 15 mm are considered very efficient, and ≥20 mm is considered extremely efficient [54,55,56,57]. On the other hand, another research article used the limits of IZ < 12 mm (non-inhibitory EO) and ≥20 mm for strongly inhibitory EO [58]. Azghar et al. [33] and Ambrosio et al. [9] mentioned the considerations and classification regarding the IZ as follows: <8 mm, considered resistant bacteria; 8 mm < IZ < 14 mm, moderately sensitive bacteria; 14 mm < IZ < 20 mm, sensitive bacteria; and IZ > 20 mm, highly or extremely sensitive bacteria. Figure 1 illustrates the results of antibacterial tests of OEO and gel samples against E. coli and S. aureus by the agar diffusion assay.

For E. coli, the threshold for susceptibility starts at 17 mm, while for S. aureus, inhibition larger than 19 mm is considered indicative of susceptibility. The OEO presented antibacterial activity, considering that some types of EO could show difficulty for an effective diffusion among the agars. Both pathogenic bacteria showed similar susceptibility to OEO with average inhibition zone diameters of 25.9 mm for E. coli and 23.62 mm for S. aureus. Based on inhibition zones ≥ 20 mm, it can be stated that the OEO is sensitive, strongly inhibitory, or extremely efficient against both bacteria tested. The antibacterial activity of the OEO revealed by the inhibition zone diameters is similar as those of known sensitive antibiotics such as cefuroxime, cloxacillin, erythromycin, azithromycin, ciprofloxacin, and amoxicillin, based purely on the inhibition zone diameters exhibited by those antibiotics [53].

The EO could diffuse through the bacterial cell membrane, leading to an increase in its permeability. The accumulation of EO in the cytoplasm leads to cell lysis and leakage of intracellular components. This accumulation, especially considering potential cell membrane alteration, disrupts the mechanisms of energy production, nutrition, structural molecules, and growth regulators [59,60,61]. For example, Limonene has shown effective antibacterial activity and can rupture the cell membranes of bacteria, leading to a change in morphology and growing functionality [62]. Therefore,

Table 5. Comparison of orange essential oil antibacterial activity.

Sample	Inhibition Zone (mm)		Limonene	Reference
	E. coli	S. aureus
Orange peel waste essential oil	25.90 ± 3.64	23.62 ± 4.82	96.50%	Current work
Gel Sample with OEO from waste 1	9.00 ± 2.15	8.38 ± 2.20	-	Current work
Citrus sinensis	8.33 ± 0.58	8.00 ± 1.00	86.59%	[41]
Sweet orange essential oil	11.33 ± 2.73	-	78.30%	[35]
Orange oil phase essence	27.10 ± 2.30	28.4 ± 1.90	87.22%	[9]
Citrus terpens	17.90 ± 0.40	19.3 ± 1.00	28.67%	[9]

¹ Referrers to Gel Sample B (GSB).

shows a comparison of IZ from antibacterial activity of OEO against E. coli and S. aureus, and the limonene content is presented.

It can be observed that the antibacterial activity of OEO in the current work with a limonene content of 96.50% is competitive and comparable to the best results depicted by the orange oil phase essence with a limonene content of 87.22%. Compared to the other results, the inhibition zones of OEO in the current work increased from 43% to 270% against E. coli, while for S. aureus, it increased from 22.38 to 195.25%. There is a trend that the higher the limonene content is in the essential oil composition, the higher the antibacterial activity, also leading to the most preferred odor products for commercial applications.

Regarding the antibacterial activity of the gel sample on S. aureus, it depicted an inhibition zone of 9 mm, while for E. coli, it was 8.3 mm. These results are slightly greater than 8 mm; therefore, they can be classified as resistant, not inhibitory, or, depending on the reference classification, can be labeled as non-efficient or moderately sensitive. The antibacterial gel sample presented smaller inhibition zones than those of the OEO. This could be attributed to the consistency of the gel sample, which makes its diffusion, propagation, and action against bacteria difficult in the assayed agar. This is not the case for the OEO because its liquid nature facilitates its diffusion, propagation, and action against the bacteria, allowing inhibition zone creation. Another reason may be that in the gel formulation, the OEO could be encapsulated in the gel, making it difficult for the OEO to easily act and be exposed to the tested bacteria.

Since ethanol and OEO were used as active ingredients in the formulations, the expected effect was to enhance the antibacterial activity, but this did not occur. Therefore, it would be recommended to increase the proportion of one or both, while maintaining the formulation’s organoleptic properties. The OEO improved the sensory and organoleptic properties of the antibacterial gel.

3.4. Sensory Evaluation of Antibacterial Gels

A total of 56.36% of respondents indicated that they always use hand sanitizer, 34.93% almost always, and 8.71% sometimes; none indicated that they never use hand sanitizer. Ninety-nine percent of all respondents concluded that using hand sanitizer is important in their daily lives. Figure 2 shows the results for the sensory evaluation of hand sanitizers in terms of appearance, color, spread, absorption, and smell.

It can be observed that the trend in the results of the GSA sample is higher, more favorable, more stable, and with higher scores for the five parameters, reflecting greater acceptance in the sensory evaluation by the respondents. Meanwhile, the results of the GSB sample show greater variability and lower scores, demonstrating lower acceptance than the GSA sample. The GSB was evaluated mainly with lower scores regarding the appearance and spread sensory parameters.

When classifying the results into five age subgroups: 8–23, 24–32, 33–41, 42–53, and 54–72, a trend was detected where the 8–23, 24–32, and 33–41 age ranges indicated that the GSA sample was more pleasant than the GSB sample for the parameters of appearance, spread, color, absorption, and smell. In total, 78% of respondents fell into these age ranges. However, in the age range of 42–55 and 56–72, it represents 22% of respondents, and only 7% of them preferred the GSB sample rather than the GSA sample.

In total, 86% of respondents felt that their hands were soft after the application of GSA and GSB. However, 13% felt that their hands were neither soft nor irritated, and 1% felt skin irritation. A total of 71% of respondents indicated that the smell of GSA and GSB was extremely pleasant.

The GSA was found to be more acceptable with 99% than GSB with 87%; this does not mean that GSA is better than GSB, only that the study population preferred GSA in terms of the parameters of appearance, spread, absorption, and color. In total, 93% of respondents are willing to use either GSA or GSB as a daily hygiene product over those found on the market.

In the literature, there are two previous studies that evaluate perceptions of hand sanitizer formulations. In one sensory evaluation test, the volunteers preferred the cosmetic properties and skin perception of the C. satuvis L-derived disinfectants [63]. While in the other study, the consumers placed more importance on the aroma followed by the thickness and stickiness of the hand sanitizer [64]. The differences can occur because of cultural differences, consumer preferences, population, or age groups, among other factors. None of the publications mentioned report the costs of the formulations.

The statistical results for the perception of the antibacterial gel are presented in

Table 6. Statistical results of antibacterial gel samples.

Parameter	GSA		GSB		Statistical Analysis
	Mean	Stdv	Mean	Stdv	t-Value	p-Value
Appearance	8.423	0.800	7.403	1.514	11.690	0.000
Color	8.577	0.832	8.506	0.899	1.120	0.262
Spread	8.457	0.940	6.836	1.942	14.740	0.000
Absorption	8.548	0.828	8.371	0.984	2.690	0.007
Smell	8.603	0.801	8.455	1.103	2.130	0.033

. Each rating for every parameter was averaged, and its standard deviation was calculated. The means for the GSA parameters are very similar to one another, with the lowest corresponding to appearance and the highest to odor. All standard deviations are below 1, indicating a small variation in participants perception of the gel. Most respondents rated the five GSA parameters with scores of 7, 8, or 9.

However, this trend does not hold for GSB, as the lowest mean is 6.836 for spread, which increases to 8.506 for color. Statistically, these values indicate greater variability among the results, with standard deviations reaching up to 1.514. In the t-test for mean comparison, the p-values reveal an important finding: all parameters of the antibacterial gel are considered significantly different, except for color, which has a p-value greater than 0.05. Figure 3 shows the distribution of responses for each parameter along with their corresponding means.

It is important to emphasize that the only difference between the two antibacterial gels was the amount of Carbomer 940 used to achieve different gel viscosities. However, according to the surveyed people’s perceptions, this small variation caused all the evaluated parameters to differ, except for the gel color.

4. Conclusions

This study successfully demonstrated the valorization of Citrus sinensis peel waste into a high-limonene (96.5%) essential oil with potent antibacterial activity. The pure orange essential oil (OEO) exhibited strong inhibition against Escherichia coli (25.9 ± 3.64 mm) and Staphylococcus aureus (23.62 ± 4.82 mm), outperforming many literature-reported citrus oils and placing it in the “extremely efficient” category (IZ ≥ 20 mm). Its efficacy is attributed primarily to the exceptionally high limonene content and synergistic effects of minor terpenes.

Two alcohol-based (60% ethanol) hand sanitizer gels (GSA and GSB) incorporating this OEO were formulated at production costs of only 1.27–1.45 USD/L—markedly lower than commercial products (5–10 USD/L). Although the gels showed reduced antibacterial activity (8.38 ± 2.20–9.0 ± 2.15 mm inhibition zones) due to limited diffusion of active compounds in the gel matrix, they retained antiseptic properties suitable for topical use.

Sensory evaluation with 770 participants revealed excellent consumer acceptance, with GSA achieving 99% overall acceptability and superior scores in appearance, color, spread, absorption, and smell. The t-test demonstrated that GSA and GSB were statistically different in their parameters, except for color. Notably, 93% of respondents preferred these natural gels over commercial alternatives for daily use.

These results confirm that high-limonene orange peel essential oil can be effectively upcycled into sustainable, cost-competitive, and user-preferred alcohol-based hand sanitizers, offering an environmentally friendly contribution to personal hygiene and circular economy practices in the citrus industry.