Effect of Pectin Extracted from Lemon Peels on the Stability of Buffalo Milk Liqueurs

Abstract

This study aimed to explore innovative process technologies for producing milk liqueurs with balanced and stable formulations. Milk liqueurs are known to pose significant technological challenges due to phase separation, which compromises product stability and reduces shelf-life. Interactions between milk proteins, alcohol, carbohydrates, temperature, and ionic strength play a crucial role in such destabilization. Pectin, known for its stabilizing effect, can mitigate phase separation, enhancing both shelf-life and sensory quality. This research focused on developing stable formulations of liqueur milk based on fresh buffalo milk by incorporating the pectin extracted from lemon peels. Rheological properties, particularly viscosity, were assessed in formulations containing varying percentages of pectin. The most stable formulation was identified as the one containing 0.10% pectin. Accelerated shelf-life testing, modelled using the Arrhenius equation, predicted a shelf-life of 15 months at 25 °C under standard lighting. The findings demonstrate that lemon peel-derived pectin, obtained from agri-food waste, sustainably improves product stability. Further studies are needed to characterize the pectin structure and optimize extraction methods for industrial-scale applications.

1. Introduction

The development of stable milk liqueurs presents a significant challenge in the food industry, primarily due to phase separation that compromises product stability and shortens shelf-life [1]. This problem is especially critical when using milk or dairy cream and alcoholic components, as differences in density and solubility between the phases often lead to destabilization [2]. To address these difficulties, this study explored the use of innovative blends composed of buffalo milk and flavored liqueurs such as limoncello, coffee, and chocolate. In this work, we focused on liqueurs based on buffalo milk, characterized by high fat and protein contents, features that give a particularly rich and creamy base. Buffalo Milk Liqueur is a creamy and distinctive spirit crafted with Campanian buffalo milk sourced from the same region as the renowned Mozzarella di Bufala Campana DOP. Enriched with 3-year-aged brandy, this liqueur offers a smooth texture and bold character. The ageing process of 3-year-aged brandy facilitates the development of complex volatile compounds, including esters, aldehydes, and lactones, which interact harmoniously with the rich dairy base [3]. This interaction results in a sophisticated aromatic profile featuring nuanced notes of vanilla, caramel, and dried fruit that complement the natural sweetness of the milk. A distinctive feature of the project was the incorporation of agri-food waste, in particular lemon peels, as raw material for the extraction of pectin. This approach also has positive implications in terms of sustainability, contributing to the valorization of food waste and waste reduction. The use of by-products as functional ingredients offers innovative and eco-friendly solutions for food production [4].

Pectin, a natural polysaccharide found in the cell wall of plants, is increasingly popular as a functional additive in the food industry due to its thickening, gelling, and emulsifying properties [5,6]. It is primarily extracted from citrus fruits and apples and is widely used in products such as jams, sauces, desserts, and dairy formulations to enhance texture, structural stability, and shelf-life [7,8,9]. One of the distinguishing features of pectin is its ability to form gel-like structures through interactions with water and calcium ions [10,11,12]. This gelling ability is particularly important in food emulsions, where pectin acts as a stabilizer, reducing the risk of phase separation and improving product texture [6,13]. Furthermore, pectin’s water-binding ability increases the viscosity of formulations, ensuring consistent texture and sensory quality throughout the product’s shelf-life [14,15]. The stabilizing effect of pectin represents significant added value in dairy systems, where it interacts with milk proteins, particularly caseins, and calcium ions to form a stable and cohesive network, preventing the coalescence of lipid droplets and reducing the risk of syneresis. These interactions enhance creaminess, viscosity, and the mouthfeel of the product [6,12,13,14,15,16,17].

As a plant-derived additive, pectin is particularly suitable for vegetarian and vegan formulations. The growing demand for natural, sustainable ingredients further promotes the use of pectin over synthetic alternatives. In particular, its extraction from agro-industrial waste, such as citrus peels or apple residues, represents an environmentally sustainable and cost-effective solution that contributes to reducing food waste and improving production efficiency [18,19].

The milk liqueurs are particularly sensitive to variable temperature conditions and complex interactions between alcohols, lipids and proteins, which can cause destabilization or phase separation [20]. The addition of pectin stabilizes the oil/water interfaces, limiting aggregation and improving product stability [2,21,22]. Pectin contributes to the optimization of sensory properties, such as smooth texture and visual homogeneity [23], and improves resistance to freezing–thawing cycles [24].

Proteins also play a crucial role as emulsifying agents in the food, beverage, and pharmaceutical industries [25,26]. Their efficiency depends on their ability to adsorb rapidly at the oil/water interface, reducing surface tension and stabilizing the dispersion of lipid droplets. This function is influenced by the delicate balance of interactions with other ingredients, such as lipids, carbohydrates, minerals and environmental factors such as pH, temperature, and ionic strength [2]. In recent years, advances in emulsion technology have highlighted the value of natural protein emulsifiers, such as whey protein and soy protein. In particular, these are valued for their functional properties, including high emulsifying capacity, stable foam formation, and the creation of protective films [27,28]. These proteins offer a sustainable alternative to synthetic surfactants and meet consumer demands for clean-label ingredients. However, despite technological advances, significant challenges remain. A common problem is competitive adsorption negatively affecting emulsion stability. In addition, environmental stress factors, such as temperature variations or ionic strength, can compromise the emulsifying properties of proteins.

In milk liqueurs, a popular emulsion-based alcoholic beverage and milk proteins play crucial roles in stabilizing the oil/water interface. Caseins, mainly introduced through dairy cream, represent a highly functional natural system for stabilizing emulsions [29]. Cream is itself a complex emulsion, stabilized by proteins and fats, the composition of which varies according to milk characteristics and processing conditions [30,31]. One of the main destabilizing factors in milk liqueurs is the interaction between casein micelles and alcohol. In ethanol-rich environments, the presence of calcium in the system can induce the aggregation of casein micelles, leading to destabilization, flocculation, and fat droplet coalescence [32,33,34]. These effects are amplified by changes in pH, ionic strength, and temperature [35]. To mitigate these problems, stabilizing additives, such as trisodium citrate, are commonly used. This compound acts by sequestering free calcium and thus preventing the aggregation of casein micelles. In addition, trisodium citrate helps maintain an optimal pH, promoting protein solubility and improving emulsion stability [36,37,38].

The crystallization of lipid phases is a crucial aspect of the stability of complex emulsions, such as milk liqueurs. This process can have opposing effects: on the one hand, fat crystals can strengthen the structure of the emulsion by providing mechanical support; on the other hand, they can destabilize the emulsion, favoring phenomena such as coalescence or phase separation. In particular, destabilization occurs when fat crystals perforate the interfaces of near droplets [39,40]. In the case of milk liqueurs, these issues are further exacerbated by the presence of ethanol, which can alter the emulsifying properties of proteins, compromising interfacial stability [29]. Studies showed that with the increasing alcohol content of the milk or cream liqueurs, the emulsion becomes more sensitive to the interaction between ingredients and production conditions. In particular, the alcohol content means that the aqueous phase becomes a poorer solvent for proteins [20]. Above 5% v/v alcohol, the solvent quality for proteins diminishes, hampering aroma dispersion and stability [41].

Addressing such complexities through protein-based stabilizers and optimized formulations is essential for developing robust and consistent emulsions.

The project consisted of two major goals:

• Improving product stability by creating innovative mixtures using fresh buffalo milk, alcohol, and lemon peel waste-derived pectin.
• The study of the rheological properties of the liquor creams to ensure the proper balance of ingredients and stability.

Experimental work included industrial trials, laboratory research, and formulation optimization.

2. Materials and Methods

2.1. Reagents

The reagents utilized in this study were of analytical grade or higher. High-performance liquid chromatography (HPLC) solvents were supplied by Merck (Whitehouse Station, NJ, USA). Trypsin, dithiothreitol (DTT), iodoacetamide (IAA), guanidine chloride, ammonium bicarbonate (AMBIC), and trifluoroacetic acid (TFA) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Electrophoresis reagents were sourced from Bio-Rad (Milan, Italy).

2.2. Materials and Pectin Extraction

Lemon peel waste from limoncello production was used as the primary raw material for pectin extraction. The Citrus limon Femminello cultivar was provided by the market in Sicily. Samples were collected at the yellow stage of ripeness in November. The maturity of lemons was determined based on colour. The lemons were harvested when the colour of the peel was slightly yellow. For the Femminello variety, the optimal time for harvesting was when the optimal performance occurred in terms of fruit quality [42]. All lemons were grown under equal agronomic and environmental conditions. After the collection of samples, they were stored at +4 °C for one night. The peels’ waste was lyophilized, ground into powder at 200 mesh, and then subjected to a pectin extraction process using an aqueous ethanol solution (2:1 ratio) in an ultrasonic bath at 60 °C for 2 h. The resulting pectin was filtered, cooled to 4 °C, and precipitated for further use. Approximately 20 kg of lemons was processed during the project (October 2021–March 2023) in laboratory-scale conditions, yielding 1 kg of pectin.

The water-binding capacity (WBC) of lemon peel powders was assessed on 1 g of powdered sample. To each sample, 250 mL of distilled water was added. The mixture was incubated overnight at 4 °C, centrifuged for 20 min at 11 °C (600× g), and then left at room temperature for 2 h. The dry sample was weighed, and the WBC was calculated as the difference in weight between the initial and final product.

2.3. Formulation Development

The initial formulation used for the milk liqueurs included the following ingredients: buffalo milk, water, sugar, brandy, glucose syrup, maltodextrins, caseinate, and sodium citrate. These were collected in the local market (

Table 1. Formulation tests with pectin: C control; P0.05 pectin 0.05%; P0.10 pectin 0.10%; P0.15 pectin 0.15%; and P0.20 pectin 0.20%.

Ingredients 1	C	P0.05	P0.10	P0.15	P0.20
Buffalo milk	45.00	45.00	45.00	45.00	45.00
Water	17.40	17.40	17.40	17.40	17.40
Sugar	17.00	17.00	17.00	17.00	17.00
Brandy	16.00	16.00	16.00	16.00	16.00
Glucose sirup	2.00	2.00	2.00	2.00	2.00
Maltodextrins	1.66	1.61	1.56	1.51	1.46
Pectin	0.00	0.05	0.10	0.15	0.20
Caseinate	0.83	0.83	0.83	0.83	0.83
Sodium citrate	0.11	0.11	0.11	0.11	0.11
Total	100	100	100	100	100

¹ the ingredients are reported as a percentage.

). Various quantities of pectin (0.05%, 0.10%, 0.15%, and 0.20%) were tested by replacing corresponding amounts of maltodextrins, maintaining the company formulation. For each formulation, as shown in Table 1, 5 samples were prepared, for a total of 75 samples. Each sample was analyzed in triplicate.

2.4. Rheological and Shelf-Life Testing

The rheological analysis technique used was viscosity measurement. This was performed with a rotational viscometer (VISCO STAR R 1000-983 JP Selecta, Barcelona, Spain) equipped with spindle no. 5. The spindle was submerged in 50 mL of liqueur milk, and the viscosity was assessed by measuring the resistance of the sample at a pre-set speed ranging from 10 to 50 rpm. The test conditions included maintaining the sample in a water bath at a constant temperature of 8.0 ± 0.1 °C after an equilibrium time of 10 min. Viscosity was expressed in centipoise (cP). Sampling was performed for analysis after 24 h of production. Shelf-life tests were conducted both in the laboratory and at the Antica Distilleria Petrone (Mondragone, Caserta, Italy) production site.

To assess the stability of the hydroalcoholic mixture obtained, a straightforward and reproducible analytical method was developed. Specifically, the quantity of the lipophilic phase emerging from the hydroalcoholic mixture was determined using a graduated cylinder and by leveraging the unitary operations of flocculation and creaming. This quantity was expressed as a percentage relative to the fat content in the mixture.

Given that the fat content in the mixture is exclusively derived from the milk component, it was determined that the fat content amounts to approximately 3.7 g per 100 mL of the final liqueur. This value was taken as a reference, accounting for the seasonal variability of fat content in buffalo milk, with an average of 8.2 g per 100 g over a full lactation year.

The sample, prepared following the company’s formulation, was subjected to shelf-life testing in the laboratories of the University of Palermo.

Accelerated shelf-life tests were performed at varying temperatures (35 °C, 45 °C, 55 °C) with different illumination conditions (12 h or 24 h of light exposure), observing the phase separation (lipid phase separation) over time. The separation of the fat phase was measured using graduated cylinders, and the shelf-life was modelled using the Arrhenius equation to predict the stability of the formulations under normal conditions for this liqueur (at ambient temperature of 25 °C).

Reaction kinetics were evaluated, and a predictive shelf-life model was developed using the Arrhenius equation (Equation (1)):

ln(k) = ln(A) − Ea/R⋅T

(1)

A total of 72 liqueur samples were divided into six experimental groups, with each group consisting of 12 bottles. These groups were subjected to the specified temperature and illumination conditions.

2.5. Analytical Methods

Total polyphenol analysis was carried out according to the method of Todaro et al. from 2010 [43]. Aliquots of lemon peel (5 g) were homogenised using an Ultra-Turrax T25 homogeniser, set at a maximum speed, and extracted with 50 mL of methanol under continuous stirring for 1 h at room temperature. Samples were then centrifuged at 4000× g for 20 min at 4 °C and filtered through Whatman No. 42 paper under vacuum conditions via a Buchner funnel. Total phenolics content was determined according to the Folin–Ciocalteau method [44], using catechin as a standard. Results were expressed as l g catechin equivalents per gram of dry weight.

Sugars (sucrose, fructose and glucose) were determined according to the method of Hundie and Abdissa 2021 [45] by High-Performance Liquid Chromatography (HPLC). Samples (10 mL) of centrifuged juices (15,000× g for 20 min at 4 °C) were purified through a Sep-Pak C18 cartridge (Waters Corporation, Milford, MA, USA), diluted in water, filtered through a 0.45 μm filter, and injected directly into the column. The separation and identification of sugars were performed using a Waters 600-E HPLC system (Waters Corporation, Milford, MA, USA) equipped with a Waters 410 refractive index (RI) detector and a Luna-NH2 column (250 × 4.6 mm i.d., 5 μm; Phenomenex, Torrance, CA, USA). The elution was performed with an acetonitrile–water (80:20 v/v) solution at a flow rate of 1.8 mL min⁻¹. Sugars were identified by comparing their retention times with those of pure standards and confirmed by co-injection. The quantification of each compound was performed using an external standard calibration curve.

The water binding capacity (WBC) of the lemon peel powder was also measured to assess its potential as a stabilizing agent in cream formulations.

2.6. Sensory Analysis

Sensory analysis was conducted on liqueur samples with and without the addition of pectin. The sensory analyses were conducted according to the guidelines, defined by the ISO standards, for defining panels and training methods for judges [46,47].

The sensory panel consisted of 16 trained persons (10 males and 6 females) with ages between 35 and 60 years. Participants were asked to refrain from smoking, eating, or drinking (except water) during the three hours before the test sessions.

The participants gave their written consent after receiving full information about the sensory test. The subjects did not experience any risk as a result of the sensory test.

The sensory analysis was conducted by examining 3 visual descriptors (colour intensity; creamy; presence of suspensions), 3 aroma descriptors (milk aroma; alcohol aroma; intensity of aroma), 6 taste descriptors (sweet; sour, bitter, milk taste, pungent/alcoholic; taste persistence, and 1 overall judgement. The intensity of the descriptors was defined using a scale from 1 to 9, where “1” indicated no perception and “9” a very intense perception. During each evaluation session, the judges analyzed two samples, restoring the palate with water between each tasting. All samples were evaluated in three replicates.

2.7. Statistical Analysis

The XLStat software, version 2014.5.03 (Addinsoft Incorporated, New York, NY, USA), was used to statistically elaborate the data. The report of the data was given as mean ± SD.

The one-way ANOVA test was performed to check for significance between the mean values of the formulations analysed. A significance level of p < 0.05 was considered statistically significant.

3. Results

3.1. Pectin Extraction Yield

The pectin extraction from lemon peel waste yielded approximately 20% of the dry weight of the peels, which is consistent with the literature values [47,48]. Based on the annual production of 30,000 kg of lemon peel waste in a medium, it was estimated that 4000 kg of pectin could be extracted per year, which could be used to stabilize the milk liqueurs.

3.2. Rheological Properties and Formulation Optimization

Viscosity measurements were used to evaluate the impact of pectin addition on the texture and stability of the milk liqueur formulations. The optimal viscosity was observed in the formulation with 0.10% pectin (P0.10), which showed similar viscosity to the control sample (

Table 2. Viscosity of different samples after 15 days from production: C control; P0.05 pectin 0.05%; P0.10 pectin 0.10%; P0.15 pectin 0.15%; P0.20 pectin 0.20%.

Sample	Viscosity (mPa·s) ± SD
C	79.3 ± 3.2 b 1
P0.05	68.5 ± 2.7 c
P0.10	81.1 ± 3.6 b
P0.15	92.9 ± 5.1 a
P0.20	94.7 ± 5.5 a

¹ different letters indicate significant differences at p < 0.05 among the samples.

). This formulation was selected for further shelf-life testing and sensory analysis.

3.3. Shelf-Life Predictions

To evaluate and predict the shelf-life of the newly developed formulations, two distinct sets of tests were conducted by subjecting the samples to light and temperature conditions designed to accelerate the potential separation of the lipophilic phase. Specifically, three temperatures (35 °C, 45 °C, 55 °C) and two illumination durations (12 h, 24 h) were employed.

Phase separation between the aqueous and fat phases was monitored visually across the different mixtures analyzed.

Samples were collected every 24 h to measure phase separation. The results are presented in Figure 1, which shows the percentage of fat separation relative to the total mixture.

In Figure 1a, the kinetics of fat phase separation are shown under the most extreme illumination conditions (24 h of continuous light), while Figure 1b illustrates the separation kinetics under standard illumination conditions (12 h of light).

Accelerated shelf-life testing showed that formulations with 0.10% pectin (P0.10) demonstrated significantly reduced phase separation compared to the control formulation. As observed, the fat phase begins to visibly separate starting from the 60th day after production.

To enhance the stability of the formulation, varying percentages of pectin were added, as detailed in Table 1, with a corresponding reduction in maltodextrin content. Viscosity tests were conducted using a rotational viscometer to evaluate the characteristics of the modified formulations compared to the original. The optimal viscosity was observed in sample P0.10, as shown in Table 2. Since sample P0.10 exhibited viscosity behavior closely resembling that of the control sample, it was selected for further accelerated shelf-life testing alongside the best-performing samples, specifically samples C and P0.10.

The separation of the fat phase in the control formulation began to be visible after 60 days, while the P0.10 formulation remained stable for a longer period.

The Arrhenius model was applied to predict the shelf-life of the milk liqueurs at an ambient temperature (25 °C), with results indicating an approximate shelf-life of 15 months (Figure 2). This was based on the threshold of 5% fat separation as the acceptable limit for product quality.

Based on the reaction kinetics, the following graph was constructed to illustrate the behavior of the Arrhenius equation. This analysis enabled the prediction of the product’s shelf-life under ambient temperatures and standard light conditions (12 h illumination).

As mentioned above, the fat component was explained by the milk in the formulations. Figure 3 illustrates the percentage of fat separation determined during shelf-life. From the study of the degradation kinetics, particularly focusing on texture, it can be concluded that the product’s shelf-life is approximately 15 months. This estimation uses the fat separation index as the quality parameter, with a maximum acceptable limit of 5% fat separation relative to the total mixture.

The results obtained from the addition of 0.10% pectin derived from lemon by-products demonstrate a significant reduction in phase separation. This finding is particularly important as it shows that the study successfully achieved a substantial extension of the product’s shelf-life. The original formulation, lacking pectin balancing, exhibited a shelf-life of only a few months, whereas the improved formulation offers markedly enhanced stability.

3.4. Sensory Analysis

Sensory analysis was performed on the control sample and the sample with 0.10% pectin, selected previously. Figure 4 shows the spider plot representation of sensory analysis of the Buffalo Milk Liqueur samples.

In detail, the visual descriptors have a higher intensity of colour and creaminess in the P0.10 sample, while the control had a high number of suspensions due to the aggregation of fats that condition its stability.

Both samples showed a low alcohol aroma and a similar milk aroma intensity. However, the sample with added pectin had a higher overall intensity. The taste descriptors showed statistically significant differences for pungent/alcoholic taste and taste persistence, respectively, being lower and higher in the sample with 0.10% pectin. Finally, the panel expressed an overall judgement on the analysed products, with the sample with pectin being rated higher.

Thus, the results showed that the addition of 0.10% pectin to the formulation improved the Buffalo Milk Liqueur by enhancing its stability and visual characteristics, which influence purchasing decisions, as well as its aroma and taste, which make it pleasant to consume.

4. Discussion

4.1. Current Formulation

The increased percentage of fat separation reduces the shelf-life of milk or cream liqueurs [49]. Fat aggregation can cause fatty acids to react more strongly with alcohol, leading to the formation of high amounts of ethyl esters. These compounds determine the development of fruity notes in milk liqueurs, limiting their sensory quality [50].

During conservation, the main problem with milk liqueur is the physical instability of the emulsion, which leads to the formation of two distinct phases. The gelling of the milk liqueur causes syneresis and thus the separation of the whey layer from the product. The stability of these products is determined by several production and compositional factors. One of the important factors determining the acceptability of milk or cream products is viscosity, which is associated with the visual assessment of the ‘body’ [49]. Viscosity is influenced by various factors, including alcohol content [29], caseinate content [51], storage temperature [52], and the use of carbohydrates and polysaccharides [53]. In this work, the variables considered are the use of pectin, a polysaccharide extracted from lemon peels, and storage temperature.

Studies showed that the addition of pectin to milk-based drinks resulted in changes in viscosity until a threshold concentration was reached. Further increasing the amount of pectin causes an unpleasant decrease in viscosity. This change is influenced by the presence of calcium ions and caseinate [17].

Different authors suggested that viscosity evaluation can be used as a valid method for evaluating the gelling level during accelerated shelf-life tests of milk liqueurs [54,55]. The viscosity data of the samples obtained 15 days after production allowed the P0.10 sample (with 0.10% pectin added) to be defined as the sample most similar to the control. Thus, the addition of pectin derived from lemon peel waste proved to be a successful method for stabilizing the liquor milk formulations.

It has been proven that pectins form complex coacervates with milk proteins (e.g., caseins and whey proteins) through electrostatic and hydrophobic interactions. Specifically, these associations modulate the stability of oil-in-water (O/W) emulsions by preventing flocculation and coalescence phenomena, resulting in a stabilizing effect that is particularly relevant in dairy systems and protein-enriched beverages [56,57,58]. The decreased viscosity observed can be attributed to chemical reactions occurring during conservation. These results are in agreement with those of Heffernan et al. (2019) [59].

Viscosity data confirm that higher viscosity can reduce the absorption rate of pectin molecules [60], with consequent negative effects on the product under investigation.

This data allowed the selection of sample P0.10 for the successive accelerated shelf-life evaluations.

Accelerated shelf-life tests showed that the sample with 0.10% pectin remained stable up to 60 days after production. Thus, the moderate addition of pectin and corresponding reduction of maltodextrin significantly extended the shelf-life of the product compared to the control formulation.

Owing to their hydrophilic and polyanionic nature, pectins decelerate the kinetics of destabilization through steric and rheological effects. The increase in viscosity reduces droplet collision frequency, thereby lowering the rates of aggregation, flocculation, and coalescence. This effect is particularly pronounced in emulsions within dairy systems, contributing to the regulation of the physical stability of the dispersed phase [61,62,63].

During the shelf-life tests, the presence of cream rings or collars on the top of the bottles or an extensive aqueous serum at the bottom of the bottles are signs of creaming and indicative of phase separation [64]. The speed of phase separation was most evident with increasing temperature under continuous 24 h illumination. In contrast, under intermittent illumination for 12 h, phase separation occurred almost similarly for samples subjected to temperatures of 35 °C and 45 °C.

The stability of the Buffalo Milk Liqueur may be associated with the interaction between pectin and the globular protein called β-lactoglobulin, derived from buffalo milk. The interaction between these substances leads to the formation of polymeric particles that are stable to oxidative phenomena [16,65]. Furthermore, as suggested by Ekene et al. (2022), the conformation and concentration of proteins influence the gelling of the product [64]. Ibanoglu [66] reported that the addition of a polysaccharide, such as pectin, acts against heat-induced whey protein aggregation by interacting with the hydrophobic patches deployed during heat treatment.

In agreement with other authors [67,68], the Arrhenius equation has been used to predict the shelf-life of products. Oil–water phase separation and protein precipitation are normal phenomena during the storage of milk- or cream-based emulsions [69]. The P0.10 samples analyzed, with a pectin content of 0.10%, showed no evident phenomena relating to oil–water separation and flocculent precipitation. The data obtained thus enabled a shelf-life of 15 months to be indicated, keeping the liqueur at a temperature of 25 °C and under standard lighting conditions. This result was significantly better than the control and commercial samples, where the shelf-life was defined as a few months.

The results show that adding pectin improves product stability by reducing phase separation. This positive effect is also associated with the interaction that pectin establishes with the proteins and fat globules of buffalo milk [64,65,66].

There are several theories on the mechanistic interpretation of fluid flow in relation to temperature [70,71]. According to these theories, the viscous flow can explain transitions in chemical interactions between sections of the pectin polymer chain. In particular, with increasing temperature, hydrophobic interactions between the atoms of the polymer chain are favored over hydrogen bonding and van der Waals forces, reducing the viscosity of the product [72]. Studies have also shown that the viscosity of solutions containing pectin decreased with increasing temperature [73]. Kar and Arslan (1999) reported that both pectin concentration and temperature affect viscosity [74].

The results of the sensory analysis carried out on the samples showed that the addition of 0.10% pectin to the control formulation improved the Buffalo Milk Liqueur, increasing its stability and visual characteristics, influencing purchase decisions, and impacting its aroma and taste. This made it pleasant to consume. The stability of casein micelles in dairy systems is influenced by the ethanol and acid concentration [75]. However, the analysed formulations are stable in terms of alcohol and acidity. This showed that the addition of pectin improves the stability of the micelles. The colour differences observed in the samples could be attributed to an increase in turbidity due to the formation of aggregates in suspension, also affecting the creaminess of the product [76].

The aggregation of fat, visible as suspensions in the control sample, has a significant effect on the distribution of volatile compounds, with lipophilic compounds being more involved as they remain bound to the fat aggregates. This distribution also influences the volatilization of compounds and thus the aroma profile of the liqueur, with the more polar compounds perceived first and the more non-polar ones perceived last [55].

Studies have shown that the addition of pectin to milk products positively affects syneresis, improving the visual characteristics and not reducing the products’ aroma and taste [77].

4.2. Future Perspective

Several studies report that the structure of pectin can influence the stability of the emulsion [78,79,80]. This suggests that we should determine the structure of pectin extracted from lemon peels in the future, paying attention to possible differences due to the variety of lemons used.

Studies report that the rheological, sensory, and shelf-life characteristics of dairy products, including milk liqueurs, can be influenced by the intrinsic factors, i.e., the ingredients, and extrinsic factors, i.e., the production process [29,49,50,59,76]. On the basis of these studies, the authors intend to develop further investigations to optimize a new milk liqueur formulation that can be launched onto the market.

The results demonstrated that pectin not only improves product stability but also offers sustainable use of lemon peel waste, reducing environmental impact.

Moreover, the potential for commercializing lemon peel-derived pectin opens up new avenues for the valorization of waste by-products in the food and beverage industry. Further research could explore the use of other fruit waste materials and the optimization of pectin extraction methods for larger-scale industrial applications.

5. Conclusions

This study successfully developed an innovative milk liqueur formulation, incorporating pectin extracted from lemon peel waste, providing both functional and environmental benefits. The inclusion of pectin effectively addressed the critical issue of phase separation, a major factor limiting product stability and shelf-life. Accelerated shelf-life testing, modelled using the Arrhenius equation, predicted a significantly extended shelf-life of 15 months for the optimized formulation compared to the shorter shelf-life of the original product. This improvement demonstrates the ability of pectin to stabilize the emulsion by preventing fat separation, as confirmed by the quantification of separated fat over time.

Furthermore, the study highlights the dual benefit of this approach, improving product performance while promoting sustainability by valorising food industry by-products, such as lemon peel waste, as functional ingredients. Future validation under ambient storage conditions will further confirm the robustness of the model predictions and ensure applicability to real-world scenarios. These findings not only advance the formulation of milk liqueurs but also open avenues for commercial scalability and the broader integration of sustainable practices in the food and beverage industry.