Food Functional Powders with Redox Capacity and Antioxidant Properties Obtained from Food Losses and Waste of Olive Oil Industry

Abstract

Food powders were produced from olive pomace (Coratina, COP, and Arbequina, AOP) using freeze-drying with maltodextrin (MD) and native corn starch (NS) as wall materials in varying proportions. Optical microscopy revealed that OP was partially recovered by MD and NS. COP presented a total polyphenol content (TPC) of 53.8 g GAE/kg; meanwhile, AOP was 31.9 g GAE/kg. Accordingly, powders obtained from COP presented higher TPC than those from AOP. NS allowed obtaining powders with higher TPC and antioxidant activity. The greatest encapsulation efficiency was achieved by using 15% MD, achieving values of 94.9% for COP and 89.4% for AOP. Hydroxytyrosol was identified as the predominant polyphenol in the powders. It was demonstrated that powders could be added to food formulations and withstand cooking temperatures up to 220 °C without suffering a significant thermal degradation. Chemometric analysis of MIR and NIR spectra evidenced that they are analytical techniques capable of differentiating OP varieties and wall material types, besides variations in OP concentration. Results derived from this study demonstrated that it is feasible to give added value to olive pomace, obtaining powders rich in antioxidants to be used as ingredients of functional foods.

1. Introduction

Plant cells are capable of producing various phytochemical compounds through metabolism, which can be classified into primary and secondary metabolites [1,2]. Primary metabolites are those involved in the growth, development, and reproduction of plants, such as proteins, lipids, and carbohydrates. Secondary metabolites are chemical compounds that plants synthesize to perform functions in response to environmental stress, potential pathogens that affect plants, as well as possible predators [3]. Depending on the biosynthetic pathway, secondary metabolites can be classified into terpenes, nitrogen-containing compounds, and phenolic compounds. Phenolic compounds, which feature an aromatic ring with one or more hydroxyl groups, are known for their ability to scavenge reactive oxygen and nitrogen species associated with non-communicable diseases [4], making them highly valuable for various industries, including pharmaceuticals, food, cosmetics, and nutraceuticals [5]. However, these compounds can undergo chemical degradation when exposed to changes in pH, light, heat, and oxygen [6]. Literature suggests that encapsulation may be an effective strategy to overcome these limitations [7,8]. Encapsulation of bioactive compounds improves their solubility, masks unpleasant flavors, improves storage stability, bioavailability, and bioaccessibility, and protects against oxidation and degradation that these compounds may suffer by external agents, thus extending their shelf life [9]. This is achieved through the formation of bonds or the coating of bioactive compounds with wall materials [10] and can be carried out with various equipment, encapsulants, and methodologies. The most commonly used encapsulation procedures are extrusion, emulsion, spray-drying, and freeze-drying [11]. For obtaining dry biomass, in which it is essential to preserve the bioactive compounds that could be sensitive to high temperatures, freeze-drying is presented as a solution due to its simplicity [12]. Freeze-drying is a drying method that allows separating water or another solvent from the sample; for this purpose, the sample undergoes freezing followed by a sublimation process at reduced pressure [9]. Wall materials can be added with the aim of increasing the stability of the powders during storage and minimizing possible chemical degradation of the bioactive compounds [13,14]. Examples of wall materials include proteins (such as vegetable protein isolates, casein, and whey) [15,16], pectin, inulin, gums (guar, arabic), and biopolymers such as maltodextrins and starches [17]. Starch from various botanical sources can be used as a wall material, either in its native or modified form, as reported in the literature [18]. Native starch has good processability as microcapsule wall materials. Maltodextrin, a white powder produced by enzymatic or partial hydrolysis of starch, is considered a modified form of starch [18]. Among its advantages, it can be mentioned the capacity to form films and achieve greater stability of the bioactive compounds when used as a wall material [13,19].

Food safety and environmental protection are increasingly important in today’s world. One major issue facing agriculture-based and food production industries in both developed and developing countries is waste management, including both liquid and solid waste [20]. Approximately 30% of the global food supply is produced but not consumed and is instead discarded. Therefore, the valorization of food losses and waste has become a significant research focus aimed at promoting sustainability in the food chain. These residues are often rich in nutrients and bioactive compounds, making them suitable for the production of food powders [21].

Olive is the fruit of the olive tree (Olea europaea), botanically classified as a drupe. In the cold extraction of olive oil, approximately 800 kg of olive pomace are produced from 1000 kg of processed olives [22]. This pomace is primarily composed of dietary fiber, proteins, and phenolic compounds [23]. This pomace is part of food losses and waste and does not have many applications; its final disposal generates environmental and economic problems. In this context, it is essential to give added value to these residues since they have a high content of bioactive compounds, and several industries, mainly those related to functional food, are currently interested in their revalorization. Therefore, the production of food powders from natural sources, such as olive pomace, with a high content of phenolic compounds is of great interest [24,25]. In this sense, the evaluation of the total phenolic content and the antioxidant capacity of this residue are relevant. In addition, it is of interest to identify the varieties of plants from which the polyphenols were extracted, as well as the type of polyphenols, which are related to their botanical source [26,27,28].

The objective of this work was to obtain food powders with a high content of phenolic compounds from olive industry waste. Olive pomace from Coratina and Arbequina olive varieties was encapsulated using maltodextrin and native corn starch as wall materials through freeze-drying. The food powders were morphologically characterized, and their moisture content, hygroscopicity, total polyphenolic content, individual polyphenolics, antioxidant capacity, and encapsulation efficiency were determined. Additionally, the study proposed the use of non-destructive, innovative, and sustainable analytical methods to evaluate the formulations throughout the production process.

2. Olive Pomace

Olive pomace of Coratina (COP) and Arbequina (AOP) olive varieties was provided by Estilo Oliva (Finca Don Nicolás, Coronel Dorrego, Buenos Aires, Argentina).

3. Methodology

3.1. Food Powders

3.1.1. Freeze-Drying

Food powders were obtained from the freeze-drying of the olive pomace samples (COP and AOP) in the presence of two different wall materials: maltodextrin (MD, dextrose equivalent DE12, Parafarm, Saporiti S.A.C.I.F.I.A., Buenos Aires, Argentine) and native corn starch (NS, Arcor, Tucumán, Argentine). Olive pomace samples were mixed with the wall agents at different concentrations and were freeze-dried using a lyophilizer (L-A-B3-C, Rificor, Buenos Aires, Argentine). In

Table 1. Formulations of food powders based on olive pomace of Coratina and Arbequina olive varieties (COP and AOP) and wall agents (maltodextrin, MD, and native starch, NS).

Sample	Olive Pomace Variety (%, w/w)		Wall Agent (%, w/w)
	Coratina Olive Pomace (COP)	Arbequina Olive Pomace (AOP)	Maltodextrin (MD)	Native Starch (NS)
COP	100	-	-	-
AOP	-	100	-	-
MD	-	-	100	-
NS	-	-	-	100
COP50-MD50	50	-	50	-
COP65-MD35	65	-	35	-
COP75-MD25	75	-	25	-
COP85-MD15	85	-	15	-
COP90-MD10	90	-	10	-
COP50-NS50	50	-	-	50
COP65-NS35	65	-	-	35
COP75-NS25	75	-	-	25
COP85-NS15	85	-	-	15
COP90-NS10	90	-	-	10
AOP50-MD50	-	50	50	-
AOP65-MD35	-	65	35	-
AOP75-MD25	-	75	25	-
AOP85-MD15	-	85	15	-
AOP90-MD10	-	90	10	-
AOP50-NS50	-	50	-	50
AOP65-NS35	-	65	-	35
AOP75-NS25	-	75	-	25
AOP85-NS15	-	85	-	15
AOP90-NS10	-	90	-	10

, the assayed formulations are listed. Freeze-dried samples were milled using a coffee grinder and sieved using a 40-micron mesh sieve.

3.1.2. Characterization

• Morphology, Particle Size Distribution (PSD), and Color

The morphology of powder samples was analyzed by optical microscopy, as suggested by Rodriguez et al. [29]. This study was carried out using an optical microscope (Primo star model, Carl Zeiss, Oberkochen, Germany) equipped with a color camera (Axiocam 105, Carl Zeiss, Oberkochen, Germany), under transmission and polarized modes at 10× magnification.

The PSD of powder samples was analyzed by laser diffraction (LD) using a particle size analyzer (LA-950, Horiba, Kioto, Japan). Samples were analyzed similarly to the methodology reported by Hammoui et al. [30] for different types of olive pomace.

In accordance with Lammi et al. [31], the color of powder samples was evaluated by colorimetry using a colorimeter (NR110 Precision, Shenzhen ThreeNH Technology Co., Guangzhou, China) in transmittance mode. Color parameters L, a, and b were recorded according to the CIElab scale. Color parameters range from L = 0 (black) to L = 100 (white), −a (greenness) to +a (redness), and −b (blueness) to +b (yellowness).

In each case, the samples were tested in triplicate.

• Moisture Content and Hygroscopicity

The moisture content of samples was determined gravimetrically [32]. Powder samples (~500 mg) were dried at 105 °C in a moisture analyzer (MB45, Ohaus, Greifensee Switzerland) until no weight change (WC) was detected for 90 s. Assays were carried out in triplicate and expressed as a percentage (wet basis) (Equation (1)).

$$\mathrm{M}\mathrm{o}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t} (\mathrm{\%})=\left({\displaystyle \frac{\mathrm{W}\mathrm{C}}{\mathrm{M}}}\right)\times 100$$

(1)

where M is the initial weight of the sample.

Powders were characterized in terms of their hygroscopicity by evaluating their equilibrium moisture content (EMC) (Equation (2)) according to Callahan et al. (1982) [33]. Samples (about 100–200 mg) in watch glasses were placed at 25 °C in an airtight plastic container comprising a NaCl saturated solution (75.29% RH). After one week, each sample was weighed, and EMC was expressed as g of total water (initial + gained water) per 100 g of sample. The change in moisture content of the powder depends on the initial moisture content and the relative humidity of the environmental chamber. Samples were weighted at day 10 to ensure equilibrium was attained by day 7. The samples were analyzed in duplicate, and the coefficient of variation (CV %) for all samples was below 4%.

$$\mathrm{E}\mathrm{M}\mathrm{C} (\mathrm{\%})=\left({\displaystyle \frac{\mathrm{P}}{\mathrm{P}+100}}\right)\times 100$$

(2)

where P is the moisture content in dry basis (%) of the powder sample at equilibrium.

• Total Phenolic Content (TPC) and Antioxidant Activity (AOA)

The TPC of samples was determined by using the Folin–Ciocalteu method [34]. First, the antioxidants were extracted using the traditional maceration method with 10 mL of hydroalcoholic solution (50% v/v ethanol) and 1.0 g samples. After 30 min maceration at room temperature, samples were centrifuged, and an aliquot (250 μL) was taken to determine TPC. To the 250 μL of extract, 12.5 mL distilled water and 1.25 mL Folin–Ciocalteu reagent (Anedra, Buenos Aires, Argentine) were added. After 4 min, 5 mL of 20% w/v sodium carbonate (Anedra, Buenos Aires, Argentine) solution (5 mL) was added, and the final volume was adjusted to 25 mL with distilled water. After 20 min, the absorbance was measured at 765 nm using a UV/Vis spectrophotometer (Lambda 265, Perkin Elmer, Cambridge, UK), and TPC was expressed as mg of gallic acid (GA, Sigma–Aldrich, Merck KGaA, Darmstadt, Germany) per g of dried olive pomace (DOP). A calibration curve was carried out in the range of 0.5–24.0 mg GA/g DOP, yielding a linear regression equation TPC (mg GA/g DOP) = 0.0753 [GA] + 0.0679, with a correlation coefficient of 0.998 (detection limit 0.020 and quantification limit of 0.030).

The capacity of the antioxidant compounds to scavenge radicals was measured by the DPPH• bleaching method [35]. This widely used assay measures the antioxidant to assess the antioxidant power of both lipophilic and hydrophilic substances [36]. Briefly, a DPPH• (Sigma–Aldrich, Merck KGaA, Darmstadt, Germany) solution in methanol with an absorbance of approximately 1.000 was prepared, and its exact absorbance at 517 nm was measured. Immediately, a 30 μL aliquot of extract was added to the cuvette containing 3 mL of the DPPH• solution. The mixture was shaken manually, and absorbance was measured at 517 nm against methanol as blank in 30-s cycles for 10 min using a UV/Vis spectrophotometer (Lambda 265, Perkin Elmer, Cambridge, UK), in order to obtain the absorbance value at infinite time through mathematical extrapolation (exponential decay). A mathematical fitting was performed with the solver tool in Microsoft Excel software (Microsoft office LTSC Standard 2021), yielding r² ≥ 0.9880 for all samples. This value, along with the initial absorbance value, allows the calculation of the percentage of antioxidant activity, according to Equation (3).

$$\mathrm{A}\mathrm{O}\mathrm{A} (\mathrm{\%})=\left(1-{\displaystyle \frac{{\mathrm{A}}_{\mathrm{s}\mathrm{s}}}{{\mathrm{A}}_0}}\right)\times 100$$

(3)

where A₀ is the absorbance before the aliquot was added (approximately 1.000) and A_ss is the absorbance at steady state (infinite time). The calibration curve was prepared with Trolox authentic standard (Sigma–Aldrich, Merck KGaA, Darmstadt, Germany) to express AOA in mmol of Trolox equivalents (TE)/kg of dried olive pomace (DOP) (0.03–2 mmol/L; AOA (%) = 21.89 [TE] (mM) + 0.0791; r² = 0.9994). The samples were analyzed in triplicate.

• Encapsulation Efficiency (EE)

The EE was determined following the methodology reported by Zanoni et al. [37]. First, superficial phenolic content (SPC) was determined by mixing 500 mg of sample with 10 mL of ethanol. The solution was stirred for 2 min in an orbital shaker and then centrifuged at 4500 rpm for 10 min. The supernatant was filtered using a syringe filter (0.45 µm). To determine the total polyphenol content (TPC), 500 mg of sample were placed into a 15.0 mL centrifuge tube, and 3 mL of distilled water was added. The sample was sonicated for 30 min at 25 °C to disintegrate the encapsulate. Ethanol (10.0 mL) was added, stirred for 2 min in an orbital shaker, and centrifuged at 4500 rpm for 10 min. Finally, the supernatant was filtered using a syringe filter (0.45 µm). The SPC and TPC were determined by the Folin–Ciocalteu method as previously described. The EE was calculated using Equation (4).

$$\mathrm{E}\mathrm{E}\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{T}\mathrm{P}-\mathrm{S}\mathrm{P}}{\mathrm{T}\mathrm{P}\mathrm{C}}}\times 100$$

(4)

• Determination of Phenolic Compounds (PC) Using High Performance Liquid Chromatography

Determination of PC was carried out by HPLC-PDA. The HPLC analysis of the two lyophilized olive pomace varieties and the encapsulated ones was performed on a high-performance liquid chromatography (HPLC) equipment (Waters 600, Waters Inc., Milford, MS, USA) with a photodiode array (PDA) detector. A column (150 mm × 4.6 mm, 5 µm, Zorbax Eclipse C18, Agilent, Santa Clara, CA, USA) was used to separate the compounds. An isocratic mobile phase composed of acetonitrile, ACN (≥99%, Sigma–Aldrich, Merck KGaA, Darmstadt, Germany), and ultrapure water in an 80:20 (v/v) ratio was used. An injection volume of 50 µL, a flow rate of 1 mL min⁻¹, and room temperature were used in all the experiments. The detection wavelength was set at 280 nm, and the runtime was set to 6 min. Standard stock solutions of tyrosol (TYR), hydroxytyrosol (HTYR), and oleuropein (OLE) (Sigma–Aldrich, Merck KGaA, Darmstadt, Germany) were prepared by diluting appropriate amounts in methanol (≥99%, Sigma–Aldrich, Merck KGaA, Darmstadt, Germany), to prepare 360 mg L⁻¹, 1370 mg L⁻¹, and 883 mg L⁻¹, respectively, which were kept at 4 °C. Working standard solutions were prepared by diluting suitable volumes of each stock solution in acidified water (1% acetic acid) and acetonitrile 90:10 (v/v).

For sample preparation, 500 mg of the lyophilized pomace samples (COP and AOP) were mixed with 5.00 mL of acidified water (1% acetic acid) and acetonitrile 90:10 (v/v) solution. The samples were then filtered to remove the non-dissolved solids. For the determination of polyphenols in the encapsulated powders, the methodology proposed by González et al. [38] was followed with modifications. Briefly, 500 mg of powder was dispersed in Milli-Q water (5.0 mL), vortexed for one minute, and sonicated (10 min). Ethanol (10.0 mL) was added, vortexed (1 min), and sonicated (10 min). The samples were centrifuged at 2500 rpm for 10 min, and the supernatant was collected. The supernatants were filtered through a 0.22 μm filter and injected into the HPLC.

Calibration curves for quantification of TYR and HTYR were prepared, and the phenolic compound contents were expressed as mg of PC/100 g food powder.

• Thermal degradation

Thermal degradation was evaluated by thermogravimetric analysis (TGA) using a thermogravimetric balance (TGA5500, TA Instrument Discovery Series, New Castle, DE, USA). Samples (~10 mg) were heated from 25 to 350 °C at 10 °C/min under nitrogen atmosphere (20 mL/min). Curves of mass percentage as a function of temperature were recorded, and the onset and maximum decomposition temperature were obtained from the first derivative curves.

3.1.3. Classification of Olive Pomace/Wall Material Mixtures and Quantification of Olive Pomace in Food Powders

Mid-infrared spectroscopy (MIR) spectra were obtained using a spectrophotometer (Thermo Nicolet 6700 FTIR, Thermo Electron Corporation, Madison, WI, USA), recorded at a resolution of 4 cm⁻¹ over the range of 4000–400 cm⁻¹ (2500–25,000 nm), with an accumulation of 16 scans and air as background. Samples were prepared by mixing food powder samples as fine powder with potassium bromide (99%, Sigma–Aldrich, Merck KGaA, Darmstadt, Germany) at a concentration of 5%, w/w.

Near-infrared (NIR) spectra were obtained using a Luminar 5030 handheld miniature AOTF-NIR analyzer (USA) equipped with a cone-top handheld unit. Spectra were recorded in absorbance mode between 1100 and 2300 nm (Δλ = 2.0 nm). The NIR spectra were collected using a rotating disk, with the sample placed on it for analysis. The cone-top handheld unit was positioned approximately 1 cm above the sample surface. Fifty spectra were taken while the disk rotated at approximately 5 rpm. The average spectrum for each sample was calculated prior to analysis.

3.2. Statistical Analysis

Analysis of variance (ANOVA) was performed in order to evaluate differences between samples and followed Tukey’s post hoc test for mean comparison. Differences were considered significant at a 0.05 significance level. All statistical analyses were conducted using Infostat 2014.

Data analysis of NIR and MIR spectra was carried out using the Unscrambler 9.7 software, employing chemometric techniques such as principal component analysis (PCA) and partial least squares (PLS). Prior to model development, spectral preprocessing techniques, including standard normal variate (SNV), multiplicative scatter correction (MSC), and second derivative (SD), were applied. Various spectral regions were also assessed during the analysis to optimize model performance. The evaluation of the models was based on the coefficient of determination (r²) of calibration, explained variance (EV), and the required number of principal components, which together provided insight into the model accuracy and efficiency of the models.

4. Results and Discussion

4.1. Food Powders Characterization

• Morphology, PSD, and Color

Figure 1 presents micrographs of the food powders produced, specifically from COP and AOP, with 15% MD and NS. The powders exhibit irregular shapes, consistent with previous findings on powders produced via freeze-drying, in contrast to the spherical shape of powders obtained through spray drying methods [39,40]. Additionally, typical morphologies of the wall materials can be observed, with some identifiable pomace particles and signs of encapsulation. Corn NS particles are clearly visible in Figure 1b,d, characterized by their spherical and polyhedral structures, and are approximately ten times smaller than the pomace particles [41,42]. In contrast, the MD powder (Figure 1a,c) has a more irregular form and larger size compared to NS. The pomace particles are distinguishable by their unique yellowish color, shape, and size relative to the wall agents. Moreover, it is possible to differentiate between the two pomace types: AOP particles are generally larger than COP particles, while COP has a darker, more intense color than AOP. Micrographs of COP-MD, COP-NS, AOP-MD, and AOP-NS reveal particles that vary in shape and size, allowing for clear differentiation between olive pomace and wall material particles. Similar observations were reported by González–Ortega et al. [43] for microencapsulates obtained of olive leaf extract by freeze-drying. Some encapsulation signs in all four combinations studied (COP-MD, COP-NS, AOP-MD, and AOP-NS) could be detected, as indicated by arrows showing carrier particles partially covering the pomace ones. COP-MD appears to exhibit the highest degree of coverage, forming a more continuous layer on the surface of pomace granules. This could be attributed to some degree of MD solubilization in fresh OP prior to freezing, given that MD has higher water solubility compared to NS [44,45].

Particle size is a critical parameter of powders, as it impacts their physical properties, including stability, handling, and storage. Figure 2 represents the PSD of olive pomace samples (COP and AOP), wall materials (MD and NS), and food powders with an olive pomace-to-wall materials ratio of 50:50 and 90:10. The olive pomace samples display a bimodal particle size distribution (Figure 2a), with COP exhibiting two distinct populations with peak values around 70 μm and 200 μm, while AOP shows a primary population around 230 μm and a secondary, smaller population. Wall materials demonstrate a unimodal PSD (Figure 2b), with MD particle sizes ranging from 40 to 300 μm and NS particles measuring between 8 and 67 μm. The mean particle size of freeze-dried powders containing MD ranged from 102 to 142 µm, while those containing NS ranged from 50 to 131 µm, indicating greater size variation in samples containing NS (Figure 2c,d). Aliakbarian et al. [46] reported particle size up to 25 μm for microencapsulates of olive pomace extract with maltodextrin as carrier material. The larger particle size of COP-MD and AOP-MD than those reported by Aliakbarian et al. [46] is likely due to the fact that their microencapsulates were derived from olive pomace extracts, while those in this study are based on whole olive pomace. In addition, the greater uniformity in PSD of samples containing MD may be due to the wall material role as a water-soluble film-forming polymer [47], which can form a film around the olive pomace particles during freeze-drying, reducing size variation in the powder particles.

A notable characteristic of olive varieties is the color, which is related to the chemical composition and influences on fruit sensory properties [48]. Olive color depends on harvest time, ripeness, and the proportion of chloroplast pigments. The olive pericarp comprises an external epicarp and a mesocarp with high chloroplast content, contributing to its natural green chlorophyll pigment [49,50]. Additional pigments, such as carotenoids and anthocyanins, provide the distinctive color of olive fruits. For instance, the Coratina variety not only has the highest polyphenolic content but also high levels of chlorophyll and carotenoids, which contribute to its intense color [51]. Therefore, evaluating the color of olive pomace powders and relating it to chemical composition is essential. After dehydration, visual inspection revealed that Coratina pomace yielded greenish powders, while Arbequina pomace produced brownish powders. To verify these observations, color parameters (L*, a*, and b*) were measured instrumentally. Figure 3a shows the a* and b* parameters, indicating a tendency for powders with COP to display yellow and green tones, while those with AOP ranged between red and yellow. Similar to findings by González–Ortega et al. [43], higher concentrations of olive leaf extract intensified powder color. Notable differences in the a* and b* parameters were also observed between powders with NS and MD wall materials. Regarding luminosity (L*), COP-MD samples exhibited values between 64 and 79, COP-NS ranged from 74 to 81, AOP-MD ranged from 57 to 72, and AOP-NS ranged from 70 to 82. For freeze-dried olive pomace without wall material, luminosity was 66 for COP and 48 for AOP. Although a clear trend in luminosity values was not observed, both olive varieties and wall materials influenced the color of the olive pomace powders. Figure 3b presents photographs of the powders, illustrating their color. These results suggest that freeze-drying does not significantly alter the composition of olive pomace samples, allowing them to largely retain their inherent color characteristics.

Overall, the PSD and color parameters of the powders align well with the optical micrographs.

• Moisture Content and Hygroscopicity

Moisture is a key parameter related to the physicochemical stability of powders, as it acts as a plasticizer in wall materials, reducing their glass transition temperature [52]. This reduction may lead to detrimental phenomena such as caking and agglomeration [53], which in turn decrease powder flowability and complicate handling and storage. Additionally, chemical stability is compromised by increased molecular mobility, enhancing the rate of deteriorative reactions such as oxidation [53,54].

The moisture content of powders ranged from 3.68% (AOP) to 4.88% (AOP₈₅-NS₁₅), with all values below the 10% threshold commonly considered acceptable for storage stability [55]. Hygroscopicity, which reflects a material tendency to absorb/adsorb water from the ambient [56], was measured as the equilibrium moisture content (EMC) at set relative humidity (RH) and temperature conditions. For powders containing wall materials, only the samples with the minimum and maximum concentrations were analyzed, along with the EMC for wall materials not subjected to freeze-drying (

Table 2. Moisture content and EMC (equilibrium moisture content) of food powders based on olive pomace of tina and Arbequina olive varieties (COP and AOP) and wall agents (maltodextrin, MD, and native starch, NS).

Sample	Moisture Content (%)	EMC (%)
MD	-	14.3 ± 0.4 a,b
NS	-	13.8 ± 0.2 b,c,d
COP	4.05 ± 0.04 a	12.9 ± 0.1 c,d,e
AOP	3.69 ± 0.02 b	15.2 ± 0.6 a
COP50-MD50	4.18 ± 0.04 c,d	14.0 ± 0.1 b,c
COP65-MD35	3.87 ± 0.06 e	-
COP75-MD25	4.60 ± 0.05 f,g	-
COP85-MD15	4.06 ± 0.04 d,a	-
COP90-MD10	4.81 ± 0.03 h,i	12.9 ± 0.1 c,d,e
COP50-NS50	4.04 ± 0.03 a	12.7 ± 0.3 d,e
COP65-NS35	3.69 ± 0.04 b	-
COP75-NS25	4.69 ± 0.04 i,f	-
COP85-NS15	4.25 ± 0.05 c	-
COP90-NS10	3.84 ± 0.05 e	10.7 ± 0.2 f
AOP50-MD50	4.20 ± 0.06 c	13.5 ± 0.1 b,c,d
AOP65-MD35	4.50 ± 0.04 g	-
AOP75-MD25	4.20 ± 0.04 c	-
AOP85-MD15	4.61 ± 0.03 f,g	-
AOP90-MD10	4.70 ± 0.02 i,f	12.6 ± 0.2 d,e
AOP50-NS50	3.80 ± 0.02 e,b	12.3 ± 0.3 e
AOP65-NS35	4.28 ± 0.05 c	-
AOP75-NS25	3.91 ± 0.02 e	-
AOP85-NS15	4.87 ±0.05 h	-
AOP90-NS10	3.83 ± 0.05 e	12.6 ± 0.3 d,e

Different letters in the same column indicate that there is a significant difference between samples by the Tukey test (p ˂ 0.05).

). The results align with existing literature, where Callahan et al. [33] reported an EMC value for NS of 14.4% (75% HR—25 °C), and Raja et al. [57] characterized different equivalent dextrose-based MD reporting EMC values in the range 12.47–13.13% (75% HR—28 °C). According to the classification proposed by Callahan et al. [33], all powders are moderately hygroscopic. The lowest EMC was found in COP₉₀-NS₁₀, which, alongside COP₉₀-MD₁₀, had the highest concentration of COP, reflecting that COP itself had the lowest EMC among the samples. The higher EMC of COP₉₀-MD₁₀ as compared with COP₉₀-NS₁₀ could be related to the lower molecular weight of maltodextrin as compared to starches, which favors hygroscopicity [58]. Moreover, the water solubility of NS is remarkably lower than that of MD [44,45]. Conversely, the highest EMC was observed in AOP, emphasizing the stabilizing influence of wall materials for this natural substance.

• Total Phenolic Content (TPC) and Antioxidant Activity (AOA)

As shown in Figure 4, the Coratina variety exhibited a notably high concentration of phenolic compounds, registering maximum TPC values of 53.8 g GAE/kg DOP for Coratina and 31.9 g GAE/kg DOP for Arbequina. Several authors have quantified TPC in OP samples extracted under different conditions. For instance, Cedola et al. [59] analyzed OP from the Cellina variety, reporting a concentration of 44 mg GAE/g DOP, while Gómez–Cruz et al. [60] determined a value of 39.5 mg GAE/g DOP, and Ribeiro et al. [61] found a TPC of 22 mg GAE/g DOP for the Galega Vulgar variety. In this study, powders containing wall material exhibited significantly higher TPC compared to freeze-dried OP without wall material (p < 0.05). Notably, powders with NS generally had higher TPC than those formulated with MD, with some exception where samples with 75 and 50% AOP showed equivalent TPC regardless of wall material (p > 0.05), and COP₅₀ -MD₅₀ had a higher TPC than COP₅₀ -NS₅₀ (p < 0.05). These findings suggest that NS may provide a stabilizing effect during freeze-drying, potentially due to the formation of starch-polyphenol complexes, which are thought to preserve phenolic compounds from degradation [62,63]. The optimal OP-to-MD ratio for maximum TPC was 85:15 for Arbequina and 90:10 for Coratina. For NS formulations, AOP₉₀-NS₁₀ and AOP₈₅-NS₁₅ were optimal for Arbequina, while COP₉₀-NS₁₀ was optimal for Coratina. It is known that during the freeze-drying process, the ice crystal formation stage is crucial as it generates the rupture of structures in the plant material, allowing the release of trapped active compounds, making them more available for extraction, though also for degradation. Moreover, the size of the formed crystals defines the pore pattern that will be generated upon sublimation of the ice [64]. The morphology of the resulting particles will also be affected by the sublimation rate of water during the process, which depends on the concentration of wall material [64].

Figure 5 presents the results for the AOA of freeze-dried OP, as well as mixtures of OP and wall materials. It is clearly observed that the Coratina variety exhibits significantly higher antioxidant power compared to the Arbequina variety. The maximum values obtained for both varieties were as follows: 105.3 mmol TE/kg DOP for the Arbequina variety and 363.4 mmol TE/kg DOP for the Coratina variety. Similar findings have been reported by the following authors when quantifying the AOA of olive pomaces using the DPPH radical bleaching metho: Vidal et al. [65] found an activity of 179.7 mmol TE/kg DOP (expressed as 92.7 mg TE/g dried extract, Picual variety); Ribeiro et al. [61] determined a lower value of 80 mmol TE/kg DOP (Galega Vulgar variety); and Uribe et al. [66] reported an intermediate value of approximately 100 mmol TE/kg DOP (Picual variety). These differences can be attributed not only to the inherent variability of the plant material, influenced by the degree of ripeness and olive variety, but also to the method and conditions of oil extraction from which the OP is obtained, as well as the method for obtaining the antioxidant-rich extract.

Consisting of TPC findings, for both COP and AOP samples, it was observed that, compared to MD, the presence of NS resulted in powders with equal or higher AOA. Regarding the OP/MD ratio, for both varieties, OP85-MD15 was the powder that exhibited maximum AOA (p < 0.05): 131.6 mmol TE/kg DOP for the Arbequina variety and 328.0 mmol TE/kg DOP for the Coratina variety, while for the OP/NS ratio, both varieties exhibited different behaviors. The Arbequina variety showed a maximum for the 90:10 ratio (p < 0.05) (179.3 mmol TE/kg DOP), whereas for the Coratina variety, all powders showed no statistical difference in AOA (p < 0.05), ranging from 333.9–367.4 mmol TE/kg DOP, except from COP65-NS35, which presented the lowest AOA capacity (p < 0.05): 243.6 mmol TE/kg DOP. TPC and AOA results are expressed based on the mass of DOP to nullify the dilution effect of the wall material when comparing samples subjected to different treatments, i.e., different wall material concentrations. It is evident that samples with maximum TPC based on mass of DOP are preferred, as well as samples with minimum wall material content, in order to avoid the dilution effect when consumed and to reduce production costs. Results showed that maximum AOA based on mass of DOP are exhibited by samples with minimum wall material concentration.

To assess the correlation between AOA and TPC, an ANOVA was conducted, yielding a Pearson correlation coefficient of 0.9266 (p < 0.01), indicating a strong positive relationship. The high value of this parameter indicates that the phenolic compounds extracted from the samples have a consistent free-radical scavenging ability; therefore, an increment in their total concentration implies a linear increase in free radical scavenging activity. While tocopherols, non-phenolic antioxidants in olive oil, have also been detected in OP, their concentration (typically 200–400 ppm in OP extracts [51]) is much lower than that of phenolic compounds [51]. These values are significantly lower than those for phenolic compounds, not only because tocopherols enrich the oil fraction during extraction but also due to the polar nature of the solvent mixture used for the extraction of OP. Therefore, it is reasonable that the contribution of these compounds to the measured AOA is considerably lower than that of phenolic compounds.

• Encapsulation Efficiency (EE)

Encapsulation efficiency (EE) is related to the wall material’s ability to encapsulate active compounds and could be defined as the amount of the compound on the particle surface relative to the total amount (suface + core).

Table 3. Encapsulation Efficiency (EE) of food powders based on olive pomace of Coratina and Arbequina olive varieties (COP and AOP) and wall agents (maltodextrin, MD, and native starch, NS).

Sample	Encapsulation Efficiency (EE, %)
COP50-MD50	23.9 ± 0.1 a
COP65-MD35	35.9 ± 0.1 b
COP75-MD25	56.9 ± 0.1 c
COP85-MD15	94.9 ± 0.1 d
COP90-MD10	94.4 ± 0.1 e
COP50-NS50	18.5 ± 0.1 f
COP65-NS35	23.9 ± 0.1 a
COP75-NS25	50.0 ± 0.1 g
COP85-NS15	80.4 ± 0.1 h
COP90-NS10	80.1 ± 0.1 i
AOP50-MD50	23.7 ± 0.1 a
AOP65-MD35	20.1 ± 0.1 j
AOP75-MD25	50.3 ± 0.1 g
AOP85-MD15	89.4 ± 0.1 k
AOP90-MD10	88.5 ± 0.0 l
AOP50-NS50	15.4 ± 0.0 m
AOP65-NS35	33.5 ± 0.1 n
AOP75-NS25	44.9 ± 0.1 o
AOP85-NS15	70.1 ± 0.1 p
AOP90-NS10	69.6 ± 0.1 q

Different letters in the same column indicate that there is a significant difference between samples by the Tukey test (p ˂ 0.05).

shows the EE results, highlighting the variations in EE for phenolic components depending on the wall material used. For MD, EE ranged between 20.1% and 94.9%, while for NS, it ranged between 15.4% and 80.4%. EE was observed to be higher using MD as wall material for both pomace varieties. This is because the fraction of surface polyphenols is greater for NS than for MD. This finding is supported by Loksuwan [67], who reported that surface β-carotene was higher for native tapioca starch compared to modified tapioca starch. Guo et al. [68] also noted that among different wall material combinations, including maltodextrin and starch, those containing maltodextrin had the highest EE values. In this study, increasing the concentration of MD in the wall material mixture led to a reduction in EE, aligning with findings by Yarlina et al. [69] for the encapsulation of albar bean tempeh protein concentrate. The highest EE values were obtained with COP₈₅-MD₁₅ and AOP₈₅-MD₁₅. Improving EE is a primary goal in food microencapsulation, as lower EE values result in reduced stability of bioactive compounds like polyphenols [70].

• Determination of Phenolic Compounds (PC) using High Performance Liquid Chromatography

Chromatographic separation of TYR, HTYR, and OLE was performed following the conditions found in the literature [71]. Phenolic compounds were extracted from lyophilized olive pomace varieties as described and injected into the HPLC. HTYR and TYR were detected in olive oil residues, corroborating literature findings [72]. However, OLE was absent in the pomace samples (Figure S1, Supplementary Materials), likely due to enzymatic hydrolysis during olive oil extraction [73]. Quantitative analysis of HTYR and TYR was performed using external standard calibration curves. The calibration curves were constructed with the standard solutions of the mixture of the analytes by measuring the peak areas. The calculated figures of merit are summarized in

Table 4. Analytical parameters for the determination of TYR and HTYR.

Analytes	Linear Range (mg L−1)	Slope	Intercept	R2
TYR	1.44–10.80	26,107 ± 644	−10,508 ± 3875	0.990
HTYR	5.48–41.10	50,833 ± 704	−37,434 ± 6046	0.998

.

HTYR and TYR contents in lyophilized olive pomace varieties (COP and AOP) were quantified. Since MD had the highest encapsulation efficiency, MD-encapsulated samples were selected for HPLC analysis.

Table 5. Hydroxytyrosol (HTYR) and tyrosol (TYR) content in freeze-dried powders.

Sample	Polyphenol Content (mg/100 g of Food Powder)
	HTYR	TYR
COP	123.2 ± 0.3 a	27.1 ± 0.5
AOP	103.1 ± 0.3 b	-
COP50-MD50	51.9 ± 0.4 c	-
COP90-MD10	111.3 ± 0.3 d	-
ACO50-MD50	50.1 ± 0.4 c	-
ACO90-MD10	91.2 ± 0.3 e	-

Different letters in the same column indicate that there is a significant difference between samples by the Tukey test (p ˂ 0.05).

displays the results expressed in mg of the polyphenol per 100 g of sample powder. Figure 6 shows the chromatograms for the analyte standard solution (concentration of 27 mg L⁻¹ for HTYR and 8 mg L⁻¹ for TYR), compared to COP and AOP samples as well as to pomace powders employing the highest and the lowest MD concentration. HTYR and TYR were both identified and quantified in the Coratina variety pomace sample, whereas only HTYR was detected in the Arbequina variety. The HTYR levels in the powders are consistent with literature values [51,74]. As expected, encapsulated powders containing 90% OP and 10% MD had the highest HTYR values.

• Thermal Degradation

Thermal degradation of carbonaceous samples is typically studied using thermogravimetric analysis (TGA), which helps to understand thermal decomposition under inert or reactive conditions. Figure 7 displays DTGA curves obtained from TGA assays under nitrogen, showing results for COP and AOP samples as well as freeze-dried food powders with the lowest and the highest wall agents’ concentrations. COP and AOP primarily consist of hemicellulose, cellulose, and lignin [75], with DTGA curves showing weight loss associated with thermal degradation of these components. Initial weight loss steps up to 100 °C are attributed to dehydration, consistent with prior moisture content reports (~4%). The second event presents a maximum degradation rate at 176 and 178 °C for COP and AOP, respectively, which indicates hemicellulose decomposition. Subsequent thermal events between 197 °C and 315 °C (COP) and 188 °C and 213 °C (AOP) correspond to cellulose degradation. Maximum degradation rates at 363 °C for COP and 354 °C for AOP suggest cellulose depolymerization, with a shoulder at >400 °C indicating lignin decomposition, which is more thermally stable. A similar thermal degradation profile was reported by García–Ibañez et al. [76] for olive oil residues. Food powders with the highest COP and AOP concentrations showed similar degradation profile to those corresponding to the olive pomace samples, while food powders containing 50% MD or NS presented some differences due to the wall materials degradation. In the case of samples containing MD, it was observed in DTGA curves a peak located at 304 °C for COP₅₀-MD₅₀ and at 300 °C for AOP₅₀-MD₅₀ associated to cellulose decomposition and MD degradation [77]. On the other hand, the weight loss events observed at 294 °C for COP₅₀-NS₅₀ and at 292 °C for AOP₅₀-NS₅₀ are the consequence of the cellulose and starch decomposition [78]. Higher onset degradation temperatures in powders with 50% MD or NS suggest increased thermal stability, which is significant as olive pomace has potential in high-value food production [79]. In accordance with this, Simonato et al. [80] proposed to fortify pasta by replacing durum wheat semolina with freeze-dried olive pomace to achieve a food product with higher TPC and AOA. Similarly, Padalino et al. [81] partially replaced durum wheat semolina with olive paste powder to prepare spaghetti enriched in phenolics and flavonoids. This kind of food products are cooked at around 100 °C, and, as it was shown in DTGA curves, none of the food powders degrade at this temperature. Nutritional and functional properties of fish food products also improved with olive pomace. Cedola et al. [82] reported that the addition of olive pomace to fish burgers increased TPC and AOA. The cooking temperature of these products is around 180 °C, and, at this temperature, olive pomace food powders resulted almost unchanged; weight losses registered were lower than 10%, including the sample dehydration that accounts for around 4%. At this temperature it was also baked pastry products such as puddings and cakes. Another mass-consumption food product in the world that could be functionalized by adding olive pomace is bread. Cedola et al. [59] demonstrated that incorporating only 10% olive pomace to bread significantly increased AOA, attributed to the presence of phenolic compounds. Analyzing TGA results, olive pomace powders only lost between 8 and 20% weight at 220 °C, being powders with 50% MD or NS those that are more stable at this cooking temperature. Another food product that is baked at 220 °C and could be enriched with OP powders are biscuits and snacks. In this sense, Lin et al. [83] studied the effect of adding Chinese olive pomace to the sensory and nutritional properties of a biscuit, and Ying et al. [84] enriched rice-oat flour and maize-oat flour-based extruded snacks with olive pomace. In DTGA curves, typical cooking temperatures for food are indicated in order to better visualize the weight changes of studied samples. In conclusion, the use of MD and NS as wall materials allowed to enhance the thermal stability of olive pomace, making them a good alternative to functionalize different food products.

4.2. Classification of Olive Pomace/Wall Material Mixtures and Quantification of Olive Pomace in Food Powders

During pomace/wall material mixture production, industries may generate blends with different wall materials or pomaces from various olive varieties, yielding visually similar samples. Thus, a quick, cost-effective, and efficient method to differentiate mixtures by composition is valuable.

Furthermore, as the pomace/wall material ratio is crucial for achieving the desired product properties, implementing a reliable control method to accurately and simply evaluate this proportion during production is essential.

In this context, near-infrared (NIR) and mid-infrared (MIR) spectroscopies offer promising alternatives due to their rapidity, minimal sample preparation, environmental friendliness, and suitability for in-line process applications, aligning with process analytical technology (PAT) [85,86]. PAT methods are gaining attraction due to their ability to enable continuous process monitoring in manufacturing, with the advantage of detecting process errors and allowing rapid corrective actions [87].

Monitoring the pomace content in mixtures during production would facilitate adjustments to ensure optimal product quality, aligning with the quality-by-design approach. Additionally, post-production classification of mixtures could prevent packaging and labeling errors. To this end, a study was conducted to evaluate and compare the feasibility of using MIR and NIR spectroscopies for these quality control applications.

Figure 8 presents NIR spectra (Figure 8a) and MIR spectra (Figure 8b) of olive pomace, wall materials, and their mixtures, with the most effective preprocessing technique and spectral regions. For NIR spectroscopy, spectra were processed using the second derivative, whereas for MR spectroscopy, SNV was applied. The spectral region shown for each technique represents where the most distinct differences between the analyzed spectra were observed: 1707–1808 nm for NIR spectroscopy and 5650–6800 nm for MIR spectroscopy. All the pomace/wall material combinations showed similar trends. As an example, the NIR spectra of the AOP/MD mixtures and the MIR spectra of the COP/NS mixtures, along with the individual components, are shown.

The selected regions for analysis correspond to spectral areas previously identified as relevant by other authors for oil pomace and olive oil [88,89,90,91,92]. In the case of NIR spectroscopy, these spectral bands relate to the vibrations of –CH, –CH₂, and –CH₃ bonds found in various chemical groups within the first overtone region [86]. Furthermore, the band at 1725 nm is the strongest absorption band in the NIR spectrum of olive oil, as well as in the spectrum of triolein, the principal component of olive oil [90]. Regarding the MIR spectra, the region at 5730 nm corresponds to the characteristic C=O stretching vibration, which can be attributed to fatty acids and ester groups [91,92]. In Figure 8a,b, it is evident that both in the NIR and MIR spectra, an increase in the content of pomace in the pomace/wall material mixtures leads to more pronounced spectral bands.

For each technique, a PCA was performed using the complete set of spectra. Various data preprocessing methods and spectral regions were tested, with the most favorable results obtained from the aforementioned preprocessing techniques and spectral regions, employing in both cases two principal components. The analysis encompassed spectra from all pomace/wall material mixtures at different proportions, as well as spectra derived from the pomace of each olive species.

Figure 9a,b illustrate the score plots derived from the PCA analyses for the NIR and MIR spectra, respectively. In the analysis of the NIR spectra (Figure 9a), a clear trend is observed, showing a decrease in PC1 values as the pomace content in the mixtures increases. Additionally, the second principal component (PC2) distinguishes between mixtures of different compositions, considering both the type of wall material and the olive pomace variety. Specifically, mixtures containing MD as the carrier agent are located in the lower quadrants of the graph, while those with NS are situated in the upper quadrants. Furthermore, the analysis can differentiate the olive species from which the pomace originates, with mixtures containing COP consistently showing higher PC2 values compared to those containing AOP.

Regarding the analysis of the MIR spectra, similar to the previous case, variations in the pomace content of the mixtures lead to changes in the PC1 values (Figure 9b). In this instance, an increase in pomace content corresponds to higher PC1 values. Additionally, the MIR plot clearly separates the samples based on the carrier agent used, with those containing MD positioned in the upper quadrant and those with NS in the lower quadrants. Regarding the olive species of the pomace used, in mixtures containing MD as the wall material, samples with AOP consistently demonstrate higher PC2 values compared to those with COP. However, the analysis reveals difficulties in distinguishing between mixtures that use NS as the carrier agent with different types of pomace as no distinct trend is evident in the data. This analysis leads us to conclude that both techniques can effectively differentiate mixtures with varying pomace content and different wall materials. However, NIR spectroscopy appears to be more efficient in distinguishing between samples derived from different olive species.

On the other hand, calibration models were developed for both techniques, aimed at evaluating their capacity to predict the pomace content in pomace/carrier agent mixtures. The analyses were conducted using the chemometric technique PLS. In this case, samples with the same wall material (MD or NS) were used in each model, incorporating both types of pomace. Various spectral regions and preprocessing methods were tested, with the best models for each technique and carrier presented in

Table 6. Figures of merit for PLS calibration models.

	NIR	MIR
Spectral Region	1707–1808 nm	5650–6800 nm
Preprocessing	Second derivative	SNV
MD
R2 Cal	0.975	0.982
EV (%)	92.83	96.72
CP	3	2
NS
R2 Cal	0.971	0.982
EV (%)	86.92	96.93
CP	3	2

. The preliminary results indicate that the models generated from MIR spectra demonstrate a better fit (higher r² of calibration) for both carrier agents, exhibiting higher explained variance (EV) values and requiring fewer principal components for the model.

Both techniques are capable of detecting changes induced by variations in the concentration of pomace in the prepared mixtures. However, the models generated using MIR spectroscopy present a more promising outlook for the future development of models aimed at predicting the pomace content in pomace/carrier agent mixtures. In contrast, NIR spectroscopy shows greater potential for its application in classifying pomace/carrier agent mixtures, being able to identify different wall materials as well as pomaces derived from various olive species. Both MIR and NIR spectroscopic techniques offer an innovative and environmentally friendly approach to the classification of freeze-dried pomace/wall material system mixtures.

5. Conclusions

Antioxidant food powders were successfully developed from olive pomace of two olive varieties, Coratina and Arbequina, using freeze-drying with maltodextrin and native corn starch as wall materials. It is important to highlight that olive pomace was employed without any pre-treatment, and the used wall materials are widely available and low cost, making this approach economically feasible in terms of raw materials. Freeze-drying was selected as a dehydration process since it allows preserving the antioxidant compounds present in olive pomace. Among the studied olive pomace varieties, Coratina yielded powders with a higher content of polyphenolic compounds. Regarding wall materials, although powders with native starch showed higher total polyphenol content and antioxidant capacity, the use of maltodextrin enhanced encapsulation efficiency, making it a more suitable material for preserving the antioxidant compounds present in olive pomace. Additionally, the food powders derived from olive pomace demonstrated thermal stability at typical cooking and baking temperatures (100, 180, and 220 °C). Quality monitoring of the powders during the production process, in terms of their composition, can be carried out by non-destructive, innovative, and sustainable techniques such as MIR and NIR spectroscopy. It was demonstrated that both techniques can be used to discriminate between pomace varieties, types of wall agents, and mixture composition. In conclusion, it was possible to add value to a food loss and waste product using a simple, rapid methodology with low-cost inputs, and, in addition, its quality controls could be carried out in a sustainable manner.