Advancing Circular Economy in Olive Oil Production: Comparing Maturation Systems for Vermicompost Creation from Olive Pomace

Abstract

The production of extra virgin olive oil (EVOO) creates by-products like olive pomace, which brings environmental issues due to its strong odors and the challenges involved in storage. To address this within a circular economy framework, this study explores the potential of olive pomace as a nutrient source for earthworms, aiming to transform it into a beneficial soil amendment. Key nutrients in the pomace, such as polyphenols, sugars, and organic matter, were examined for their effectiveness in nourishing earthworms. Four distinct treatments were applied to the pomace: mechanical mixing, aeration, a combination of both, and no treatment. For a period of 30 days, chemical parameters including pH, polyphenol levels, and moisture content were monitored, while earthworm preferences were assessed at Centro Lombricoltura Toscano (CLT). The study revealed significant differences in the chemical composition of the pomace depending on the treatment, especially regarding polyphenol and total sugar content. These changes influenced the palatability for earthworms, with the combined treatment producing the most appealing pomace, likely due to the increased nutrient availability. Ultimately, olive pomace has promising potential to be repurposed into a nutrient-dense soil amendment, alleviating environmental concerns and contributing to more sustainable waste management within the olive oil industry.

1. Introduction

Olive trees (Olea europaea L.) are widely cultivated globally, primarily for the production of virgin olive oil. The olive oil sector holds immense importance in the Mediterranean region for multiple reasons. Traditional, intermittent pressing lines have been replaced with continuous systems that use decanters (screw conveyors) for oil extraction. In these continuous systems, olives undergo milling, followed by malaxation for a flexible duration, which facilitates the coalescence of olive oil droplets and the solvation of nutraceutical molecules [1,2].

Olive cultivation carries deep cultural and economic significance, playing a crucial role in local traditions, dietary habits, and the livelihoods of communities that rely on this industry. Beyond its symbolic value, olive farming also significantly contributes to the regional economy [3,4]. The processes of harvesting, pruning, and oil production result in the generation of substantial biomass, comprising mainly leaves, branches, pomace, stones, and mill wastewater [5]. Managing this waste has become increasingly important, as the extraction process produces approximately 800 to 850 kg of by-products per ton of olives, depending on whether a two- or three-phase extraction system is used [6]. This large output of biomass highlights the need for effective waste management strategies to ensure the sustainability of olive cultivation.

Virgin olive pomace consists of a mixture of liquid and solid waste, including olive pulp, skin, stones, and water. Historically, it has been valued by oil mills as a raw material for pomace oil extraction [7].

However, this usage has progressively declined, especially in Italy, due to growing consumer preference for high-quality virgin oils, particularly extra virgin olive oil (EVOO). EVOO is favored for its superior organoleptic properties and health benefits [8,9].

The increasing water content in pomace, resulting from new extraction technologies, complicates and raises the cost of its management. Disposing of pomace poses environmental challenges, such as the risk of groundwater contamination from leachate and increased pollution concerns due to microbial growth. Additionally, the direct application of untreated pomace to soil poses risks due to its high concentrations of antioxidants like polyphenols. While these compounds are beneficial to human health, they can disrupt soil microbial balance and hinder natural decomposition processes [10]. The advent of new extraction technologies has led to a reduction in profit margins for oil mills, making the extraction of pomace oil extremely disadvantageous. Simultaneously, pomace contains a wealth of high-value molecules, including phenolic compounds like secoiridoids, lignans, flavonoids, phenolic acids, and simple phenolic alcohols. As a result, they represent a substantial reservoir of bioactive molecules [11].

The circular economy (CE) constitutes a production and consumption model wherein resources are maximally utilized, emphasizing the recycling of products and the recovery of resources over mere disposal [12].

Implementing CE in the olive oil sector has become increasingly important globally due to the large volumes of by-products generated, such as pomace and wastewater, and the costs associated with their management and disposal. Countries like Spain and Italy have successfully embraced CE principles by converting by-products into bioproducts. For example, olive pomace has been transformed into biomass for energy production or fertilizers, turning waste into valuable resources. Adopting CE offers businesses significant competitive advantages by encouraging innovation in business models, digital technologies, and sustainable engineering solutions. Consequently, the circular economy has the potential to reshape traditional production and consumption patterns, promoting both environmental sustainability and economic growth [13,14].

Olive pomace, though lower in quality than extra virgin olive oil, can be used for extracting olive pomace oil through solvent methods, suitable for cooking and frying [15]. It can also be repurposed for animal feed or for extracting antioxidants used in cosmetics or pharmaceuticals [16].

Many oil mills face challenges managing large extracted pomace volumes and often rely on biogas companies to collect it for free [17].

Extracted pomace can be used as a sustainable energy source through processes like torrefaction or pyrolysis, producing biochar, bio-oil, and biogas [18].

Agronomically, when applied properly or composted, pomace can enhance soil nutrient content and structure. It can also support vermicomposting, producing humus for agricultural use [19,20].

While the management of olive mill wastewater has been extensively investigated [21], the aim of this study is to test various treatment methods on the wet pomace obtained from a two-phase olive mill to utilize the pomace as food for earthworms, facilitating the vermicomposting process and obtaining humus that can be distributed in olive groves or other contexts.

However, there exist several potential issues and obstacles that may surface throughout the olive pomace maturation process. For instance, insufficient aeration of the olive pomace can result in the emergence of anaerobic conditions, which, in turn, may lead to the production of malodorous compounds that are not only unpleasant but also potentially harmful to plants, like hydrogen sulfide and volatile fatty acids. These compounds can be unpleasant and also potentially harmful to plants, as they disrupt soil microbial activity and could hinder plant growth by creating a toxic environment for root systems [22]. Another challenge pertains to inadequate moisture levels, which can hinder the maturation of olive pomace, potentially creating anaerobic conditions that promote the growth of mold and fungi. This issue arises when moisture levels are either too high or when insufficient aeration occurs, depriving the pomace of necessary oxygen. Both of these factors can significantly slow down the maturation process. These challenges must be carefully managed within the maturation timeline required by producers, ensuring an economically efficient process while optimizing the spatial requirements needed for storage and decomposition.

Different treatments were applied to the virgin olive pomace in separate tanks to investigate the potential for accelerating the maturation process through the introduction of physical/mechanical processes during storage operations at the mill. The goal is to produce earthworm humus within a shorter timeframe.

In a broader context, this study aims to promote environmental sustainability in managing the pomace by-product.

2. Materials and Methods

2.1. Experimental Conditions and Monitored Parameters

For this experimentation, the olive pomace was found at the Olivicoltori Colline Arno Sieve -O.L.C.A.S. company (Carbonile, Florence, Tuscany, Italy) for a quantity of 50 kg. The pomace was taken in November 2022 directly out of the stoner without any contact with the oxidized pomace in the storage tank.

The experimental plan was developed around 4 treatments, for each of which 3 replications were performed. Each replica consisted of 4 kg of pomace placed in a black rectangular tray (IKEA UPPSNOFSAD, 35 × 25 × 14 cm) for a total of 12 trays.

The experiment was conducted in a controlled lab setting, with environmental conditions (temperature and humidity) continuously monitored using a sensor, set to measure every 10 min (INKBIRD Tech. C.L. Company Address: 18/F, Zhimei Guowei electronic building, No. 68, Guowei Road, Luohu District, Shenzhen, China). Additionally, all the tanks remained in darkness throughout the experiment to control light exposure.

The treatments tested were as follows:

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No treatment, serving as the control.

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Mechanical mixing of the pomace was performed twice daily using a manual auger, with each mixing lasting 30 s, reversing the direction every 15 s.

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Air circulation, achieved by insufflating air from below using a timer set for 4 daily cycles, each lasting 10 min at 6 h intervals. The air was delivered via pumps (QZ air pump HQ-602, Xiaolan, Guangdong Province, China 528415) connected to porous plates (Nano Bubbler AMZLAB Gmbh, Laubenhof 23, 45326 Essen, Germania), with a flow rate of 2.5 L/min, resulting in a total of 100 L of air daily.

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A combination of both mechanical mixing and air circulation.

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In addition, measurements were conducted on the olive pomace before the 30-day storage period.

Each tank had a capacity of 8 kg of pomace but was filled with only 4 kg for the experiment.

An HOBO brand probe (ONSET 1-800 LOGGERS) was placed in each tank to record temperature and humidity at predetermined intervals. The experiment lasted 30 days, with an initial sample of olive pomace collected before the start for comparison with the final treated pomace.

Thanks to the support of the CLT (Centro Lombricoltura Toscano agricola SRL Migliarino—Vecchiano, PI-Italy), which also had experience in treating pomace, the quantities of water to be added for proper treatment of the pomace and the timing were agreed upon. The water dose established was 200 mL to be administered every 3 days to each tank.

Parallel to this operation, with the same 3-day interval, the following parameters were collected for each individual tank: pH using a pH meter (Hanna Edge pH), Brix degree using a refractometer (Hanna HI96813 Wine Line), and color using a colorimeter (PCE RGB2). An RGB colorimeter is a device used to measure the intensity of red, green, and blue (RGB) light in a sample, providing a way to quantitatively assess color.

In Section 3 and Section 4, the authors have reported the sum of the intensity of the three colors measured.

The internal temperature of the samples inside the tanks was recorded using a digital thermometer (DELTA Phm HD 21071), total mass was recording using a digital dynamometer (VALEX NL191220), VOC and HCHO concentrations (volatile organic compounds, formaldehyde) were recording using an Air Quality Monitor (LIFE BASIS), and a centrifuge (HERMLE Z206 A) was used to separate the phases within the falcon tubes.

Throughout the duration of the experimentation, humidity and external temperature were also monitored using a Bluetooth probe “INKBIRD Smart Sensor IBS-TH1 Plus”. Similarly to the probes placed inside the tanks, the measurement interval for this probe was also set to measure every 10 min.

2.2. Chemical Analysis on the Pomace

For the chemical analyses conducted on the olive pomace samples, the following methods were used to determine each parameter:

Moisture and Volatile Substances: The moisture content was determined by differential weighing. Samples were heated at 105 °C for 24 h, and the loss in weight was recorded as moisture and volatile substances.

Oil content on dry basis and on fresh matter: The total oil content was assessed using an automated Randall extractor (model 148, Velp Scientifica, Milan, Italy) with hexane as a solvent. This method follows a Soxhlet extraction principle, where oil content is extracted from the dry matter of the pomace. The results were expressed as a percentage of both dry matter and total sample weight.

Polyphenols: Polyphenols, which are antioxidants present in olive pomace, were measured following the guidelines established by the International Olive Council (COI). This typically involves an extraction process followed by spectrophotometric analysis to quantify the total phenolic content.

Total Sugars: The total sugar content was analyzed using standard spectrophotometric techniques, often involving colorimetric methods, such as the phenol–sulfuric acid assay. This technique quantifies the sugars present by measuring absorbance at specific wavelengths.

C/N Ratio (Carbon/Nitrogen Ratio): The C/N ratio, important for understanding the decomposition potential and nutrient balance, was measured using an elemental analyzer, which burns the sample and determines the amount of carbon and nitrogen present. This ratio is crucial for composting and soil application studies.

Organic Matter: The organic matter content was determined by incinerating the sample in a muffle furnace at high temperatures (around 550 °C). The loss of mass due to combustion of organic material gives the percentage of organic matter.

The moisture content was determined by differential weighing after 24 h at 105 °C. All other analyses were performed following the COI directive [23].

2.3. Feeding Test of Treated Pomace on Eisenia Fetida Earthworms

At the end of the treatment stage, the pomace was tested by CLT for an assessment of its attractiveness to the earthworms. The vermicomposting was carried out in dedicated tanks measuring 60 × 40 cm each.

In each experimental tank, a comparison was established between the experimental mixture and the CLT control mixture (a mixture typically used to feed earthworms, consisting of cow manure and starchy additives). Approximately 3 kg of earthworms was introduced into each tank.

The application of the experimental mixtures was carried out in 2 distributions, spaced about 15 days apart. The first evaluation was conducted simultaneously with the second feeding.

The quantities distributed were as follows:

Four liters per square meter for each experimental mixture and five liters per square meter for the CLT mixture.

The evaluation of attractiveness was carried out according to the following methods, performed by a single observer for all assessments: an observation of the experimental tank with a direct comparison between the experimental mixture and the CLT mixture and an assignment of a value from 0 to 3 (with 4 possible ratings) based on the scale described in

Table 1. Scale for evaluating the degree of earthworm attractiveness.

Value	Material Description
0	Repellent material causing the departure of the earthworm population.
1	Non-appetizing material: no adult or young individuals are found within the material.
2	Slightly appetizing material: few adult individuals (<50%) are found within the material.
3	Appetizing material: many (>50%) adult individuals and also young forms are found within the material.

.

2.4. Statical Analysis

A one-way ANOVA was conducted to determine if there were significant differences among the treatments applied to olive pomace. The significance threshold was set at p < 0.05. Where significant differences were found, a post hoc Tukey’s Honestly Significant Difference (HSD) test was applied to compare the mean values. All trials were conducted in triplicate, resulting in a total of 15 samples. The results are presented as mean values along with their corresponding standard deviations.

All statistical computations were carried out using R software (version 4.4.1) and with the XLSTAT software (Addinsoft 2024. XLSTAT statistical and data analysis solution. Paris, France).

3. Results

3.1. Results of Monitoring Data During the Trial

In the first analysis, data related to the monitored significant parameters during the 30-day treatment of olive pomace are reported in Figure 1.

The internal temperature monitored every 3 days during the experiment showed no significant variation across the different treatments, averaging around 18.85 ± 0.85 °C. Similarly, the environmental conditions, including a temperature of 19.5 ± 0.82 °C and humidity of approximately 62.4 ± 3.5%, remained consistent throughout the trial.

Shown in Figure 1 are the results of the monitoring of some parameters followed during the treatment of the pomace. Regarding pH, each treatment followed a similar trend, although with different values. At the beginning of the experimentation, the pH averaged between 4.85 and 4.75. Over the next 15/20 days from the start of the experimentation, there was a decrease in pH values, reaching 4.69. This decrease in pH can be attributed to microbial activity, which produces organic acids during the decomposition of organic matter.

Regarding the trend of volatile compound concentrations, it appears to decrease, suggesting that treatment and decomposition processes likely occurred during the experimentation, possibly due to microbial activity and oxidative processes. The trend of decreasing volatile organic compound (VOC) concentrations over time, as depicted in Figure 1, suggests that the processes of treatment and decomposition were successfully occurring throughout the experimentation period. This decrease in VOCs can be attributed to several factors, notably microbial activity and oxidative processes.

During the early stages of pomace treatment, microbial populations increase as they begin to decompose organic matter. This activity initially results in the release of various volatile compounds, including alcohols, aldehydes, and organic acids. As decomposition progresses, however, these compounds are either further broken down by microorganisms or volatilized into the atmosphere, leading to a reduction in their concentration. The flexure observed around 15–20 days mirrors the trend seen with pH, indicating that this time frame marks a critical point in the treatment process.

As for the total sugar content, measured in Brix, normalized after the addition of water during the experimentation, a decrease is observed over time, more evident for the Air circulation treatment compared to the others.

3.2. Results Obtained at the End of the Experiment

The collected analytical data were processed by applying an ANOVA, considering the treatments as main factors and the average values over three replications at the end of the experimentation, for a total of 15 samples. The significance level was set at p < 0.05. The table reported (

Table 2. Means and standard deviations of the parameters examined at the end of the experiment as a function of the treatment carried out on the pomace. Different letters correspond to significantly different values (p ≤ 0.05).

Treatment	pH	Temperature	Brix	VOC	R + G + B
Before storage	4.8 ± 0.03 b	19.1 ± 0.06 a	191.47 ± 20.25 a	1.21 ± 0.11 a	35.25 ± 10.53 a
No treatment	4.82 ± 0.05 b	19.16 ± 0.13 a	166.72 ± 20.19 a	0.15 ± 0.03 c	29.45 ± 11.21 a
Mechanical Mixing	4.81 ± 0.01 b	18.83 ± 0.06 a	155.55 ± 15.8 ab	0.09 ± 0.04 c	65.25 ± 13.87 a
Air Circulation	4.9 ± 0.03 a	19.12 ± 0.08 a	121.81 ± 6.93 b	0.16 ± 0.09 c	31.61 ± 18.94 a
Mixing + Air	4.85 ± 0.01 ab	18.98 ± 0.29 a	167.87 ± 5.3 ab	0.37 ± 0.07 b	46.48 ± 17.2 a

) shows the means and standard deviations of the measured values at the end of the experimentation, specifically after 30 days.

From the analysis of the results combined with the statistical analysis, we obtained the following values. The pH value fluctuated between 4.81 and 4.90, resulting in the formation of two statistical groups.

Regarding temperature, as previously observed, we can state that it did not show significant variations, with an average around 19 °C. After normalization due to continuous water addition, the Brix degree exhibited higher values for the samples that underwent mixing, as well as for the untreated ones.

The concentration of volatile organic compounds (VOCs) varied among the samples. The Mixing + Air sample showed higher VOC values, although these were lower than those recorded in the initial stage, while the other treatments yielded similar results.

Finally, the colorimeter value, calculated as the vector sum of the R, G, and B values, generated different statistical groups. Notably, the mechanical mixing treatment displayed higher values. Observations made during the experimentation indicated that the color of the samples treated with aeration appeared darker.

The mean values and standard deviations of the parameters measured at the end of the experiment, compared to the measurements taken before the storage of pomace, are shown in

Table 3. Means and standard deviations of the chemical parameters examined at the end of the experiment as a function of the treatment carried out on the pomace. Different letters correspond to significantly different values (p ≤ 0.05).

Treatment	Moisture	Oil Content	Oil Content	Polyphenols
	(%)	(DM %)	(%)	(mg/kg SS)
Before storage	89.82 ± 0.25 a	24.11 ± 1.16 a	3.01 ± 0.22 a	97,948.01 ± 8978.43 a
No treatment	88.27 ± 0.12 a	22.69 ± 2.54 a	2.76 ± 0.27 a	99,849.11 ± 7948.80 a
Mechanical mixing	84.70 ± 0.53 ab	20.55 ± 1.55 ab	3.14 ± 0.23 a	88,250.03 ± 9608.04 ab
Air circulation	83.17 ± 2.95 b	18.59 ± 1.16 b	3.12 ± 0.64 a	61,527.67 ± 9806.98 bc
Mixing + Air	84.21 ± 0.67 b	15.95 ± 1.72 b	2.18 ± 0.25 b	37,984.33 ± 6757.13 c
Treatment	Sugars	Ratio C/N	Organic Substance
	(g/kg)		(Van Bemmelen 1.724)
Before storage	7.87 ± 0.32 ab	59.28 ± 13.3 a	206 ± 23.1 a
No treatment	8.16 ± 0.16 a	26.68 ± 15.8 b	86.1 ± 12.6 b
Mechanical mixing	6.46 ± 0.19 ab	53.52 ± 12.6 a	132 ± 12.9 ab
Air circulation	5.72 ± 1.28 b	42.88 ± 17.8 ab	172 ± 60.3 a
Mixing + Air	7.24 ± 0.46 ab	52.79 ± 26.3 a	134 ± 13.2 ab

.

As shown in Table 3, the treatments exhibit significant variation across each parameter considered. The untreated samples display notable differences compared to those subjected to aeration and mixing with air.

The moisture content is highest in the before storage sample and lowest in the air circulation treatment.

The oil content decreases with different treatments, with Mixing + Air showing the lowest content.

The percentage of oil content shows a similar trend to the oil content in DM%, with the Mixing + Air treatment having the lowest percentage.

The polyphenol content decreases significantly with more intensive treatments, especially mixing combined with air.

Similarly, both water and sugar content show a reduction following the application of the different treatments. These data illustrate the effects of various treatments on sugar concentration, C/N ratio, and organic substances in pomace compost. Here is an analysis of the results:

Before storage, this sample had the highest organic substance content compared to the other treatments and a high C/N ratio, indicating a significant amount of carbon relative to nitrogen. This could be attributed to the presence of a larger amount of undecomposed organic matter.

Untreated samples displayed higher sugar levels but a lower C/N ratio compared to the other treatments. The reduced C/N ratio indicates greater nitrogen availability relative to carbon, which likely accelerated organic matter breakdown. However, the total organic substance was significantly lower compared to samples with mechanical mixing or aeration, which may have impacted the earthworms’ preference, as the availability of certain nutrients could be limited in the untreated pomace.

Mechanical mixing seems to reduce sugar levels while maintaining a high C/N ratio, with a higher organic substance content compared to the no-treatment scenario but lower than before storage.

Air circulation, on the other hand, resulted in greater sugar consumption and a decrease in the C/N ratio. This suggests a more rapid decomposition process, as air circulation increased oxygen levels, accelerating microbial action and organic matter breakdown. Despite the faster decomposition, the organic substance remained elevated, implying that air circulation helps maintain sufficient organic matter for earthworm nourishment.

The combination of mixing and air circulation resulted in moderate sugar content and a balanced C/N ratio, indicating a more evenly decomposed substrate. The organic substance levels were comparable to those achieved with mechanical mixing alone, which might indicate that combining these two treatments created a substrate with improved decomposition dynamics, likely contributing to the higher palatability observed in earthworms.

3.3. Earthworms’ Palatability

Table 4. Means and standard deviations of the earthworm palatability test performed at the end of the experiment. The results are expressed on a rating scale ranging from 0 (lowest palatability) to 3 (highest palatability), t0 = end of trial, t1 = 9 days from the end of the trial. Different letters correspond to significantly different values (p ≤ 0.05).

Treatment	Palatability
	t0	t1
No treatment	1.33 ± 0.58 b	1.00 ± 0.00 b
Mechanical Mixing	1.00 ± 0.00 b	1.33 ± 0.58 b
Air Circulation	1.33 ± 0.58 b	2.00 ± 0.00 a
Mixing + Air	2.33 ± 0.58 a	2.67 ± 0.58 a

shows the results of the earthworm palatability tests. Values were measured two times.

At t0, the untreated, mechanical mixing, and air circulation samples were found to be unattractive to earthworms, as the presence of few to no adult earthworms was observed within the matrices. On the contrary, the Mixing + Air sample demonstrated moderately higher palatability evaluations. In the latter case, the presence of eggs and juvenile forms within the analyzed material was observed. Earthworms that were born a few days ago, appearing white in color, were an indicator of the substrate’s suitability for vermicomposting [24].

After 9 days (t1) for the samples subjected to air, there was progress regarding the palatability of these samples, which achieved a higher score together with the samples with the combined treatment.

4. Discussion

The study focuses on technological innovation in the treatment of organic matter for vermicomposting, specifically targeting olive pomace from two-phase olive mills. The aim is to enhance the pomace’s appeal to earthworms by experimenting with different storage conditions to accelerate maturation and facilitate faster vermicomposting. The vermicompost produced by earthworm activity is rich in macro and micronutrients, vitamins, and growth hormones, as well as enzymes such as protease, amylase, lipase, cellulose, and chitinase. These enzymes persist in breaking down organic matter even after being secreted by the worms [25]. Limited data exist on these processes, making this research crucial for introducing new technological solutions to improve waste disposal sustainability and establish a circular economy. Treatments were selected based on practical feasibility in existing facilities, allowing easy modification of operational tanks in olive mills. pH appears to be an important parameter for assessing the state of evolution of the pomace. Therefore, it is advisable to regularly monitor the pH and make any necessary corrections to maintain optimal conditions for decomposition and the production of high-quality vermicompost [26,27]. The pH value fluctuates between 4.81 and 4.90, and two statistical groups have been generated. Factors such as aeration and mechanical mixing may significantly influence this parameter. Aeration promotes oxygen availability, which accelerates aerobic microbial processes and potentially stabilizes pH fluctuations by preventing anaerobic conditions. Mechanical mixing, on the other hand, enhances the uniformity of decomposition and may influence microbial distribution, thus affecting pH. Regular pH monitoring is recommended to ensure optimal decomposition conditions and high-quality vermicompost production.

Moreover, the observed pH trend could also be associated with the formation of volatile compounds like 4-ethylphenol, which are released during drying and can cause odor problems. Aeration could mitigate these effects by facilitating gas exchange, thus reducing the accumulation of malodorous substances. The concentration of 4-ethylphenol in olive pomace produced by unmodified two-phase systems seems to reach its peak after 23 days of storage, and acidity also appears to be highest after 23 days of storage, suggesting a similar trend [28]. It has been demonstrated that lowering pH values to levels close to 2 inactivates the production of malodorous compounds. Furthermore, this rise in pH may be attributed to the decomposition of organic compounds such as proteins, amino acids, or peptides, leading to the release of ammonium ions or volatile ammonia [29].

Regarding the concentration of VOCs, this differed among the samples. The Mixing + Air sample showed higher VOC values, while the other treatments had similar results. Moreover, this value was lower than the initial stage of pomace production, and this occurred because during treatment, present microorganisms degrade and metabolize complex organic compounds, including aromatic ones, transforming them into simpler and more stable compounds. Consequently, a reduction in volatile aromatic compounds is observed in the decomposing material. This is one of the benefits of vermicomposting, as it contributes to reducing unpleasant odors associated with volatile aromatic compounds and to producing high-quality compost [30,31]. From time 3 to time 4, equivalent to 12 days from the start of the experimentation, there is a drastic reduction in the concentration of these volatile compounds.

The results at the end of experimentation (30 days) are reported in Table 2. It is worth noting that the no treatment and mechanical mixing samples maintained a significantly more acidic pH compared to the samples that underwent mechanical/physical treatments. Although the differences in pH were statistically significant between some treatments, the values were only slightly different.

Additionally, it is interesting to observe that the Mixing + Air sample belongs to both statistical groups.

Regarding the Brix degree, the lowest Brix value observed in the aerated sample, possibly due to increased oxidation leading to greater sugar consumption, can significantly influence both the palatability of the pomace for earthworms and its stability as a substrate [32]. Aeration promotes oxidative processes that not only reduce sugar content but also produce fewer malodorous compounds, which could still make it a viable option for vermiculture.

R + G + B measurements vary in function of the treatments. The difference is likely due to increased oxidation of the pomace, resulting in a dark brown to black and crusty appearance.

The Mixing + Air treatment, which theoretically provides the highest oxygenation, does not have lower Brix and R + G + B values than aeration alone. This could be explained by the balance between oxygenation and mechanical disturbance: while air alone boosts oxidation, the mechanical mixing may distribute the oxygen more evenly but not necessarily accelerate sugar consumption or oxidation at the same rate as pure aeration, leading to a different trend in color and sugar consumption.

The difference in trends is a complex interaction between oxygenation, sugar consumption, and oxidation intensity.

As shown in Table 3, the untreated samples exhibit notable differences compared to the aerated and Mixing + Air treatments, for some parameters considered. The highest moisture content was recorded in the sample taken before storage, suggesting that air circulation is the most effective treatment for reducing moisture. This reduction is likely due to enhanced water evaporation facilitated by increased airflow. Mechanical mixing and combining mixing with air reduce moisture content, as well the oil content decreases with different treatments, with Mixing + Air showing the lowest content. This reduction might be attributed to the breakdown of oil content due to the combined effects of mixing and air circulation, which might enhance microbial activity and degradation processes. The percentage of oil content shows a similar trend to the oil content in DM%, with the Mixing + Air treatment having the lowest percentage. This indicates that both mechanical mixing and air circulation contribute to the reduction in oil content, with the combination of both treatments being the most effective.

Regarding polyphenols concentration, there is a significant reduction of almost 70% in polyphenol content among the treatments. Polyphenol content is highest in the no treatment and lowest in the Mixing + Air treatment. The significant reduction in polyphenol content with Mixing + Air suggests that this treatment is very effective at degrading polyphenols, which could be due to increased microbial activity and aeration. The reduction in polyphenol levels corresponds to a decrease in phytotoxicity, thereby rendering this substrate suitable for utilization as an effective soil amendment, after vermicomposting [33]. Polyphenols play a pivotal role as an indicator of the treatment state of the substrate, and an elevated concentration can significantly restrict the palatability of earthworms, consequently hindering the compost’s effectiveness in enhancing soil health [34]. Also, the other parameters change in function of the introduction of a treatment. This comprehensive understanding of the changes in different parameters emphasizes the dynamic nature of treatment processes. Composting entails the organic waste’s natural biological decomposition, which is influenced by various factors such as temperature, aeration, moisture content, and the composition of the materials [26]. Water and sugar content diminished across different treatments, highlighting the intricate and dynamic nature of this processes [35]. When delving into composting processes, the argument highlights the significance of the carbon (C) to nitrogen (N) ratio. The ratio’s role is crucial in determining the efficiency and duration of the composting process, and for this reason it has been measured. C/N ratios lower than 20 result in the consumption of all available carbon but may not achieve complete nitrogen stabilization. Conversely, C/N ratios exceeding 50 lead to prolonged composting periods due to an excess of carbon, requiring more time for the breakdown and transformation of organic materials. Striking a balance within the C/N ratio is, therefore, essential for optimizing the composting process and efficiently obtaining high-quality compost [36], but since this is a study on the treatment of a material potentially used to feed earthworms for vermicomposting, higher C/N values do not indicate a negative trend, because the olive pomace could be transformed by the earthworms in vermicompost.

The results demonstrate how different treatments influence sugar content, C/N ratio, and organic matter in olive pomace compost, affecting its decomposition and nutrient availability. In terms of palatability, untreated samples and those subjected to mechanical mixing showed lower appeal to earthworms compared to those treated with air circulation or the combined treatment of mixing and aeration. This suggests that air circulation alone, or in combination with mixing, significantly enhances the suitability of pomace for vermiculture.

Chemical analysis revealed that mixing and aeration effectively reduced polyphenols and oil content, key factors that increased palatability for earthworms. The combination of mechanical mixing and air circulation produced a well-balanced C/N ratio and moderate sugar content, resulting in a more decomposed substrate and the highest preference from earthworms. Overall, treatments involving mechanical mixing and aeration improved the nutrient composition of the pomace, making it a more suitable substrate for vermicomposting in a relatively short time.

5. Conclusions

This study is set within the context of technological innovation applied to the process of organic matter for vermicomposting, specifically focusing on olive pomace obtained from two-phase olive mills. The main objective was to optimize the storage conditions of the pomace to accelerate the process and make it more appealing to earthworms, thereby facilitating a faster and more efficient vermicomposting process.

Currently, available data on the maturation of olive pomace are limited. This research aims to fill this gap by contributing to the development of technological solutions to improve the sustainability of the disposal process and promote a true circular economy.

The selected treatments were chosen for their practical feasibility in existing facilities, allowing for simple modifications to the tanks already present in the mills. The results obtained from chemical analyses and monitoring, compared with those from the palatability test, showed significant differences among the various options. In particular, the combined mixing and aeration treatments made the olive pomace significantly more appealing to Eisenia fetida and did so in considerably shorter times compared to traditional storage without treatment.

The “Aeration + Mixing” treatment led to a significant reduction in polyphenol and oil content. Polyphenols, which are bitter substances indicative of phytotoxicity, were reduced, likely improving palatability for earthworms.

In terms of sustainability, this study suggests that transforming olive pomace from a waste product into a valuable resource is feasible. The treated pomace can be efficiently used in vermiculture, turning it into high-quality compost. While the environmental benefits of this process are clear, further research is required to evaluate its economic sustainability. Looking ahead, it is necessary to assess the long-term stability of the treated pomace under different environmental conditions and to explore the feasibility of scaling these treatments for industrial applications. A detailed cost–benefit analysis will also be crucial to determine the economic viability of these innovations, ensuring they can be widely implemented in olive mills.