Harnessing and Evaluating Almond Hulls and Shells for Bio-Based Products

Abstract

Almond hulls and shells, the byproducts of the almond industry, were analyzed to assess their potential valorization pathways. Shells showed a higher content in lignin and polysaccharides, but very low levels of extractives and inorganics. Hull’s high polar extractives fraction showed poor phenolic preponderance and antioxidant activity, but high sugar and mineral contents, and its lipophilic extracts were highly enriched in triterpenes (from 73.5% to 91.3%), while shells presented more fatty acids (27.4% to 34.2%) and sterols (17.4% to 29.1%). Shells exhibited much higher S/G ratio (syringyl to guaiacyl units) in their lignin polymer (1.0 to 1.4), compared to hulls (0.5 to 0.6). After mineral analyses, hulls showed high amounts of potassium (3.7–4.9%). Fixed carbon content was similar for both materials, but shells showed a higher energetic content, ~20 MJ/kg. Finally, both hulls and pellets increased the water holding capacity (WHC) of the soil by 50%, when added in weight percentages of 6.25% (hulls) and 25% (pellets). With these results, the range of possibilities for these waste materials is exciting: shells could be used to obtain hemicellulose oligosaccharides, while hulls could be used in sugar extraction for biotransformation or as a soil amendment.

1. Introduction

The almond tree (Prunus dulcis) is the second most widely planted nut-bearing tree in Portugal, covering approximately 74 thousand ha and ranking the country as the 5th world producer [1], with approximately 52 thousand tons of almonds produced in 2023 [2]. Almonds can be eaten raw or roasted, or even used to produce almond milk and flour. They are high in monounsaturated fats and fiber and their regular consumption is associated with numerous positive health benefits, including a reduced risk of cardiovascular diseases and cancer, as well as improved blood sugar regulation [3]. The almond processing yields several by-products: (i) Almond hulls, about 52% of the total fresh weight of the almond fruit, consisting of the fleshy mesocarp and pericarp that split open upon maturity; (ii) almond shells, about 33% of the total fresh weight, consisting of the hard and lignified endocarp removed to extract the kernels; (iii) and almond testa, or skins, representing 4–8% of the fruit and consisting on the outer layer removed during blanching [4]. These by-products are typically disposed of or used for low-value applications, such as biomass fuel or animal feed [5,6,7,8]. With the growing global demand for almonds, the accumulation of by-products in significant amounts prompts the necessity to investigate creative methods for repurposing almond hulls and shells into valuable materials. This shift can foster a more circular economy, benefiting both the environmental and economic aspects of sustainability. Several studies have already examined the chemical composition and potential applications of both the hulls and shells of almonds. Queirós et al. [9] found that shells (P. dulcis (Mill.) D. A. Webb) from two Portuguese farms (Chaves and Campo Maior) had the following chemical composition: 0.7% ashes, 5.7% total extractives, 28.8% lignin, and 56.1% polysaccharides. On the other hand, almond hulls have a very different chemical composition: 7.5% of ashes, 33.6% of ethanol soluble carbohydrates, 5.1% of crude protein, 2.3% of soluble proteins, 7.6% of lignin, 18.8% of neutral detergent fibers, 13.0% of acid detergent fibers, 11.0% of crude fiber and 0.4% of starch [10]. Hulls present a promising opportunity as a sustainable biomass resource. They can be applied in numerous contexts, such as antimicrobial and antioxidant solutions, bioenergy generation, and enhancements of organic matter in agriculture. For example, Kiani et al. and D’Arcangelo et al. [11,12] found that almond hull extract (Prunus dulcis) exhibits antimicrobial activity, particularly against Staphylococcus aureus and Escherichia coli. The polyphenols present in this extract are responsible for its antimicrobial effects, and it is non-toxic to human cells. Sang et al. [13] isolated protocatechuic acid and catechin, compounds with strong antioxidant capabilities. Esfahlan et al. [6] found that almond hulls and shells from Prunus amygdalus L. can be used as adsorbents for heavy metals or dyes, or in the preparation of activated carbons. Shaikhiev et al. [14] reviewed how hulls from Prunus dulcis can be utilized to facilitate the removal of pollutants from water. Furthermore, almond hulls contain a high percentage of fermentable sugars, which can be extracted and converted into biofuels, such as levulinic acid (building block) [15]. The sugars can be fermented to produce ethanol and biomethane, with high conversion efficiencies, making almond hulls a viable bioenergy resource [16]. Additionally, almond hulls and shells have also been evaluated as a potential sustainable alternative to synthetic materials in agriculture. They can be used as a natural soil cover or mulch [17,18,19], as substrates [20,21], in the production of biochar for soil conditioning [22], or as amendments needed for biosolarization [23,24,25]. All these will result in the improvement of the soil physical structure through the creation of an organic layer on the soil surface that could improve important factors for crop systems, such as nutrient cycling, pest suppression, soil fertility, and resilience to extreme temperatures and moisture conditions [26,27,28,29,30,31]. Almond hulls and shells can benefit the soil microbial community in the organic layer by enhancing microbial biomass and stimulating activity [32,33,34]. Andrews et al. [6] found that applying a hull and shell (Prunus dulcis) amendment to the soil can create higher tree root biomass. Additionally, almond hulls and shells have also been found to aid in the recycling of nutrients and carbon within almond crop systems [25,33,34]. Lastly, Silva et al. [4] found that substrates based on almond hulls and shells can enhance the mixture’s water retention capacity, which is crucial for plants susceptible to drought or in areas with low annual precipitation.

This study examines the chemical and thermal properties of almond hulls and shells from five varieties grown in Portugal, namely Avijor, Marinada, Guara, Soleta, and Belona, to evaluate preferential valorization routes for developing new biobased solutions. To our knowledge, this is the first time these five varieties have been assessed in such an extensive manner. We also examined how pellets from hulls and shells can produce a natural Superabsorbent Polymer (SAP) to increase the soil’s water retention capacity.

2. Materials and Methods

2.1. Sampling

Almond hulls and shells from five different varieties—Guara, Avijor, Soleta, Marinada, Belona—were supplied by Vera Cruz, located in Covilhã, Portugal. Both materials were air-dried, and hulls were further put in an oven at 60 °C for 24 h and at 100 °C for 1 h. A knife-mill, Retsch SM 2000, was used to mill the samples until 1 mm in diameter.

2.2. Chemical Analysis

A summative chemical analysis was conducted using Tappi standard protocols. All analyses were made in triplicate. Ash content was determined gravimetrically following TAPPI standard T211 om-93. Approximately 1 g of sample was weighed in ceramic crucibles, combusted at 525 °C overnight, and the residue obtained was weighed. The total extractive content was determined using the Soxhlet method. 5 g of sample was weighed in cellulose thimbles and extracted successively with dichloromethane (6 h), ethanol (16 h), and water (16 h). After extraction, each liquid fraction was dried, and the residues were weighed.

The total lignin content was obtained by adding the Klason lignin and soluble lignin. Klason lignin was obtained according to Tappi T222 om-88 by acid hydrolysis, and soluble lignin was obtained from the hydrolysis liquid of the Klason lignin by measuring the absorbance at 205 nm using a UV/VIS spectrophotometer, as described in UM250 om-83.

HPLC was used to measure the total sugar content in the hydrolysis liquid from Klason lignin. Neutral monosaccharides, including glucose, galactose, xylose, arabinose, rhamnose, and mannose, as well as galacturonic and galacturonic acids, were identified using a Dionex ICS-3000 High-Pressure Ion Chromatographer. The analysis employed a Carbopac SA10 plus Aminotrap column (4 mm × 250 mm) and a NaOH + CH₃COONa gradient at a flow rate of 1 mL/min and a temperature of 25 °C. To quantify xylose and mannose that co-eluted in the previous method, a Carbopac PA10 plus Aminotrap column (4 mm ×  250 mm) was utilized with the following conditions: NaOH (1 mM) as the eluent at 40 °C and a flow rate of 1.2 mL/min. Lastly, acetic acid was quantified using a Waters 600 HPLC system equipped with a UV/Vis detector set at 210 nm and a Bio-Rad Aminex 87H HPX column (300 mm × 7.8 mm). The eluent in this case was H₂SO₄ (10 mM) at a flow rate of 0.6 mL/min and a temperature of 30 °C.

2.3. Polar Extracts Composition

Ethanol/water extracts were analyzed for sugar composition, total phenolics, flavonoids, condensed tannins, and antioxidant activity. A sample of approximately 1 g was extracted using 20 mL of ethanol/water (50% v/v) in an ultrasonic bath for 1 h at 40 °C. After extraction, the liquid fraction was filtered, diluted to 50 mL, and set aside for analysis.

Twenty milliliters of the liquid were utilized to determine the sugar composition. Ethanol was evaporated using a rotary evaporator, followed by hydrolysis of polymeric sugars with 0.7 mL of sulfuric acid in an autoclave at 120 °C for 1 h. The liquid fraction was again filtered, and the sugars were quantified using the same approach as in the chemical analysis Section 2.2.

The total phenolic content was measured using the Folin–Ciocalteu method, which assessed absorbance at 765 nm and reported the results as gallic acid equivalents. Flavonoid content was determined using the aluminum chloride method, with absorbance readings taken at 510 nm. The condensed tannin content was assessed using the vanillin-H₂SO₄ method, measuring absorbance at 500 nm. Results for flavonoids and condensed tannins were presented as (+)-catechins.

The antioxidant activity was determined via the FRAP and DPPH methods. The FRAP method measures the extract’s ability to reduce Fe(III) to Fe(II), with absorbance read at 595 nm and results expressed as Trolox equivalents. The DPPH method measures the extract’s potential to reduce the 2,2-diphenyl-1-picrylhydrazyl (DPPH˙) radical, with results expressed as the amount of extract required to inhibit 50% of DPPH (IC50). The antioxidant activity index (AAI) for DPPH was also calculated, as described by Scherer et al. [35]. AAI is classified as: weak AAI ≤ 0.5; moderate 0.5 < AAI ≤ 1; strong 1 < AAI < 2; very strong when AAI ≥ 2.

2.4. Mineral Analysis

Given the high ash content, the mineral composition analysis was conducted on hull samples, as the ash content of the shell samples was below 1.5%. Elements were extracted first using aqua regia soluble extraction. Subsequently, major mineral elements (P, S, K, Ca, Mg, and S) and minor elements (Fe, Cu, Zn, Mn, B, Cr, Pb, Ba, and Li) were quantified using inductively coupled plasma optical emission spectroscopy.

2.5. Lipophilic Extracts Composition

The extractives soluble in dichloromethane were dried in aliquots under nitrogen and in a vacuum oven. One mg of extract was weighed in a vial, dissolved in pyridine (120 µL), and trimethylsilylated with bis(trimethylsilyl)trifluoroacetamide (80 µL), and reacted at 60 °C for 30 min. The derivatized extracts were injected into an Agilent 7890A gas chromatograph coupled to a mass detector (Agilent 5975C). The column used was a Zebron 5HT Inferno (30 m × 0.25 mm i.d. × 0.25 µm film thickness). The oven heating program began at 100 °C (held for 1 min), then rose to 150 °C at a rate of 10 °C/min, followed by an increase to 200 °C at 5 °C/min, then to 300 °C at 4 °C/min, and finally to 380 °C at 10 °C/min (held for 5 min). The injector temperature was maintained at 280 °C, with helium serving as the carrier gas at a flow rate of 1 mL/min. The MS source temperature was set at 220 °C, and the electron ionization energy was maintained at 70 eV. Compounds were identified as TMS derivatives based on comparisons with authentic standards, literature mass spectra, data from Wiley 6 and NIST libraries, and the interpretation of mass spectrometric fragmentation patterns. Semi-quantification of the identified compounds was achieved by calculating the relative abundance of each peak area concerning the total area of the total ion current chromatogram (TIC), with their relative proportions expressed as a percentage of the total chromatographic area.

2.6. Analytical Pyrolysis

Extractive-free samples were dried under vacuum at 35 °C overnight. Subsequently, 0.10 mg of each sample was weighed and subjected to pyrolysis at 550 °C for 1 min using a platinum coil pyrolyzer (Pyroprobe), connected to a CDS 5150 valved interface and coupled to a GC-MS system. The system was equipped with a fused-silica capillary column (ZB-1701; 60 m × 0.25 mm i.d., 0.25 μm film thickness), operating with helium as the carrier gas at a constant flow rate of 1 mL/min. The GC oven temperature was programmed as follows: held at 40 °C for 4 min, increased to 100 °C at 20 °C/min, then ramped to 270 °C at 6 °C/min and held for 5 min. The injector and MS interface temperatures were set at 270 °C and 280 °C, respectively. Electron ionization was performed at 70 eV. Compound identification was carried out by comparing the obtained mass spectra with the Wiley and NIST2014 libraries, as well as with literature [36]. The relative abundance of each identified compound was expressed as a percentage of the total chromatogram area. Lignin monomeric composition was determined by summing the peak areas corresponding to products derived from p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units. The H/G/S ratio was then calculated from these values.

2.7. Thermal Properties

Ultimate analysis (C, H, N, and S) was performed according to the ASTM standard method D5373-08 using a Thermo Fisher Scientific Flash EA 1112 CHNS series elemental analyzer. The oxygen percentage was calculated by subtracting the total C, H, N, S, and ash content from 100%. Proximate analysis was performed following the ASTM E870-82 standard according to Costa et al. [37]. The higher heating value (HHV) was determined using an adiabatic bomb calorimeter, in accordance with the ABNT NBR 8663 standard.

2.8. Pelletization Feasibility

HRV process solutions produced pellets used for WHC tests. For this, 1 ton of shells (20%) and hulls (80%) of equal amounts of all five varieties were milled to 2 mm, and their humidity was adjusted to 12.5%. The quantity of hulls and shells used in the mix was calculated to use as many hulls as possible (this is the SAP) while still obtaining cohesion (given by the shells). Using the industrial pelletizer M6 from Andritz (Esbjerg, Denmark) with a 55 kW motor and a Ø5 × 60/15 matrix, 820 kg of pellets were produced in 50 min. The machine had an energetic consumption of 49 kW and operated between 115 and 120 A.

2.9. Water Holding Capacity

To determine the Water Holding Capacity (WHC), a series of mixtures with soil was achieved, starting at 100% (by weight) of hulls or pellets and consecutively dividing this percentage in half until 1.6%. Three replicates were used for each treatment, with soil serving as the control. The soil or the mixtures of soil/hulls or soil/pellets were thoroughly wetted until saturation, then drained for 48 h and dried for 48 h at 60 °C, followed by 1 h at 100 °C. The WHC was calculated by dividing the mass of water (saturated sample minus oven-dried one) by the mass of oven-dried soil (weight after oven drying).

3. Results and Discussion

3.1. Chemical Analysis

The chemical summative analysis (

Table 1. Chemical summative analysis of almond hulls and shells. Mean values of three replicates ± STDEV. Bold values corresponding to main components of lignocellulosic biomass. Percentages refer to g/100 g dry sample.

	Hulls					Shells
	Guara	Avijor	Soleta	Marinada	Belona	Guara	Avijor	Soleta	Marinada	Belona
Ashes	11.1 ± 1.7	13.3 ± 1.7	9.6 ± 2.0	11.1 ± 2.9	9.2 ± 1.7	1.4 ± 0.01	0.8 ± 0.07	0.7 ± 0.17	1.1 ± 0.03	1.0 ± 0.06
Total extractives	53.3 ± 1.9	48.1 ± 2.2	64.1 ± 0.6	48.1 ± 3.4	58.4 ± 4.3	5.3 ± 0.3	4.2 ± 0.2	3.7 ± 0.1	4.8 ± 0.2	4.5 ± 0.3
Dichloromethane	1.9 ± 0.01	1.8 ± 0.21	1.8 ± 0.04	1.8 ± 0.01	1.9 ± 0.05	0.2 ± 0.01	0.2 ± 0.01	0.3 ± 0.01	0.4 ± 0.05	0.2 ± 0.07
Ethanol	29.0 ± 1.3	23.1 ± 0.3	42.0 ± 2.3	22.0 ± 0.6	37.9 ± 3.6	3.1 ± 0.1	2.2 ± 0.1	1.9 ± 0.2	2.5 ± 0.2	2.9 ± 0.1
Water	22.5 ± 1.9	23.1 ± 2.0	20.3 ± 2.9	24.3 ± 3.6	18.6 ± 0.8	2.0 ± 0.2	1.8 ± 0.2	1.5 ± 0.3	1.9 ± 0.2	1.5 ± 0.3
Total lignin	15.5 ± 0.2	16.4 ± 0.1	11.0 ± 0.2	15.3 ± 0.1	12.8 ± 0.1	32.7 ± 0.3	33.6 ± 0.6	34.3 ± 0.6	36.3 ± 0.8	34.0 ± 0.5
Klason lignin	15.2 ± 0.2	16.2 ± 0.1	10.8 ± 0.2	15.1 ± 0.1	12.7 ± 0.1	30.9 ± 0.1	31.5 ± 0.6	32.5 ± 0.6	34.4 ± 0.8	32.2 ± 0.4
Soluble lignin	0.3 ± 0.03	0.3 ± 0.02	0.1 ± 0.02	0.3 ± 0.01	0.1 ± 0.02	1.9 ± 0.25	2.1 ± 0.10	1.8 ± 0.07	1.9 ± 0.12	1.8 ± 0.07
Polysaccharides	21.7 ± 0.2	23.3 ± 0.2	16.6 ± 0.3	26.2 ± 0.2	19.6 ± 0.1	59.7 ± 0.5	61.7 ± 0.8	61.9 ± 1.3	58.3 ± 1.3	59.8 ± 0.6
Rhamnose	0.4 ± 0.01	0.4 ± 0.01	0.3 ± 0.01	0.4 ± 0.01	0.3 ± 0.01	0.4 ± 0.01	0.4 ± 0.01	0.4 ± 0.01	0.4 ± 0.01	0.4 ± 0.01
Arabinose	3.4 ± 0.25	3.9 ± 0.10	3.0 ± 0.06	4.5 ± 0.13	3.4 ± 0.10	0.7 ± 0.01	0.8 ± 0.02	0.8 ± 0.04	0.7 ± 0.01	0.6 ± 0.02
Galactose	2.1 ± 0.11	2.1 ± 0.04	2.2 ± 0.09	2.5 ± 0.06	2.5 ± 0.02	1.0 ± 0.02	1.0 ± 0.01	1.0 ± 0.02	0.9 ± 0.01	0.9 ± 0.02
Glucose	10.0 ± 0.2	11.1 ± 0.2	7.3 ± 0.2	11.7 ± 0.1	8.7 ± 0.1	23.2 ± 0.3	25.1 ± 0.5	25.2 ± 0.4	24.6 ± 0.6	24.5 ± 0.4
Xylose	3.1 ± 0.1	3.0 ± 0.1	2.0 ± 0.1	3.7 ± 0.1	2.5 ± 0.1	26.6 ± 0.2	26.8 ± 0.2	26.9 ± 0.9	24.5 ± 0.6	26.0 ± 0.3
Galacturonic acid	1.4 ± 0.04	1.3 ± 0.03	1.0 ± 0.02	1.8 ± 0.13	1.1 ± 0.01	0.7 ± 0.02	0.7 ± 0.01	0.7 ± 0.02	0.7 ± 0.01	0.6 ± 0.01
Glucuronic acid	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01
Acetic acid	1.2 ± 0.07	1.5 ± 0.09	0.8 ± 0.04	1.4 ± 0.08	1.0 ± 0.04	6.9 ± 0.01	6.7 ± 0.14	6.8 ± 0.02	6.4 ± 0.05	6.8 ± 0.05

) revealed significant differences between the hulls and the shells. With higher similarity to wood, the shells presented low quantities of total extractives, high content of total lignin and polysaccharides, and low inorganic content. The lignin content found in these almond shells is comparable to that in other lignocellulosic materials used in various industries to produce materials such as adhesives, encapsulants, coatings, sunscreen formulations, antioxidant agents, and bioadsorbents [38]. Accordingly, the quantities of polysaccharides, particularly glucose and xylose, are higher than those of other lignocellulosic materials [38,39]. Hemicelluloses can be utilized in various industries to produce a range of materials, including natural adsorbents, smart hydrogels employed in tissue engineering, as a carbon source for energy, and for packaging and nanoparticles [40]. Several studies have demonstrated that almond shell-based materials can be utilized as adsorbents for removing hazardous pollutants from water [15].

3.2. Phytochemical Profile and Mineral Composition

The hulls did not show as high quantities of lignin or polysaccharides; however, they did exhibit very high amounts of extractives. Although these quantities seemed promising, as they could indicate high amounts of phenolic compounds and a strong antioxidant activity, the hulls did not exhibit high total phenols, flavonoids, and condensed tannins, nor a strong antioxidant capacity (

Table 2. Phytochemical profile values (total phenolics, total flavonoids, condensed tannins) and antioxidant properties (FRAP and DPPH) from hulls. Mean values of three replicates. Percentages refer to g/100 g dry sample.

	Guara	Avijor	Soleta	Marinada	Belona
Ultrasonic Extraction Yield (%)	47.7 ± 0.1	49.4 ± 0.9	59.3 ± 0.1	51.8 ± 0.9	69.1 ± 0.6
Ash (% of total extractives)	17.4 ± 2.1	18.4 ± 1.7	13.1 ± 1.1	15.3 ± 2.2	10.9 ± 1.5
Sugars (% of total extractives)	25.6 ± 2.3	20.2 ± 0.7	25.3 ± 1.4	23.0 ± 1.1	24.7 ± 1.4
Rhamnose	0.1 ± 0.01	0.1 ± 0.03	0.1 ± 0.05	0.1 ± 0.04	0.1 ± 0.02
Arabinose	0.8 ± 0.07	0.8 ± 0.02	0.6 ± 0.07	0.6 ± 0.10	0.4 ± 0.04
Galactose	0.5 ± 0.07	0.4 ± 0.01	0.4 ± 0.08	0.3 ± 0.03	0.2 ± 0.03
Glucose	22.5 ± 2.0	17.7 ± 0.9	23.0 ± 1.2	20.7 ± 0.8	23.1 ± 1.2
Xylose	1.0 ± 0.1	0.6 ± 0.1	0.6 ± 0.1	0.7 ± 0.1	0.4 ± 0.1
Galacturonic acid	0.4 ± 0.04	0.3 ± 0.04	0.4 ± 0.02	0.3 ± 0.03	0.3 ± 0.02
Glucuronic acid	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01	0.1 ± 0.01
Total phenols (mgGAE/gExt)	82.0 ± 2.3	83.4 ± 3.1	121.7 ± 3.8	63.3 ± 3.7	177.3 ± 3.8
Flavonoids (mgCat/gExt)	75.8 ± 3.0	74.0 ± 4.9	117.9 ± 5.3	55.1 ± 1.6	162.3 ± 3.9
Condensed Tannins (mgCat/gExt)	44.1 ± 5.1	41.0 ± 3.5	69.9 ± 3.8	30.7 ± 4.2	71.2 ± 2.5
FRAP (mgTrolox/gExt)	158.1 ± 6.3	155.4 ± 5.1	223.4 ± 12.4	116.1 ± 1.8	316.4 ± 11.1
IC50 (ugExt/mL DPPH)	23.1 ± 1.1	23.0 ± 1.1	18.1 ± 1.3	27.8 ± 0.9	11.3 ±2.1
AAI	1.1 ± 0.2	1.1 ± 0.1	1.3 ± 0.1	0.9 ± 0.1	2.1 ± 0.1

). However, even though the solvent concentration and time in the ultrasonic bath differed, the total phenolic content was similar to that found by Khan et al. [41], who also found that hull extract has antimicrobial effects. These antimicrobial effects have also been observed in other studies [42,43]. It has been reported that almond hulls exhibit significant antioxidant activity; however, the methods employed differed either in the percentages of ethanol and water used [44] or in the extraction method (high-pressure homogenization analyses) [45]. Actually, it has been reported that the solvent utilized in the extraction procedure may play a crucial role in the results obtained, both for the phenolic content and the antioxidant activity [46]. Still, when comparing the total extractives obtained and the low phenolic content, the results indicate that the high percentages of extractives are probably due to other compounds. Thus, we analyzed the amount of sugars and ash present in the extract after the ultrasonic bath (Table 2). These two components do explain part of the extractives, representing from 35.6% (for Belona) to 43% (for Guara) of the amount of extractives obtained in the ultrasonic extraction. Like this, fewer compounds contribute to the phenolic content and antioxidant activity of the hulls than previously thought, and the values obtained make more sense. When removing the sugars and ash from the equation, the percentages of extractives range from 27.2% for Guara to 44.5% for Belona. Although these values align more closely with the results for total phenols and antioxidant activity, they remain high, suggesting the presence of other compounds in the extractives that are not detected in the performed assays. The almond shells, as expected from the amounts of total extractives found in the chemical summative analysis (Table 1), did not show high contents of bioactive compounds or antioxidant activity and were not considered for these analyses.

Furthermore, as hulls exhibited a higher-than-normal ash content (9.2% to 13.3%, Table 1), their mineral composition was determined and presented in

Table 3. Mineral composition (mg/kg of dry sample) of almond hulls’ ash from the five species studied.

Elements	Guara	Avijor	Soleta	Marinada	Belona
P	1473	1791	1476	2054	1098
K	44,627	48,807	40,355	42,364	37,562
Ca	2917	3476	1561	3364	1180
Mg	2515	2880	1740	2536	1595
Na	86	156	48	625	45
S	439	561	338	444	330
Fe	90	100	83	236	530
Cu	10	11	8	10	7
Zn	11	12	11	11	13
Mn	29	32	22	30	17
B	58	62	71	63	62
Mo	0.2	0.3	0.2	0.3	0.1
Cr	0.7	1.2	0.6	0.9	1.2
Ni	4.2	4.3	3.7	3.6	4.7

. Hulls have a very high mineral content compared to other lignocellulosic materials [47]. Potassium was the highest for all species (from 37,562 to 48,807 mg/kg), followed by calcium and magnesium. Being rich in potassium, hulls can be considered (and have been considered) as a low-cost organic soil amendment [48]. Andrews et al. [49] found that applying a soil amendment in orchards could increase soil potassium levels to approximately the levels required by trees. Valverde et al. [19] further demonstrated how an almond hull-based substrate can increase nutrient levels more effectively than a shell-based substrate. Furthermore, notice that if multiplying the ash percentage by the yield and dividing by 100, the result is the percentage of ash from the original material that was extracted. For Guara, for example, this gives 8.3%, whereas in Table 1 the ash content of this variety is 11.1%. This means that of the 11.1% of minerals in the hull, 8.3% is extracted: around 75% of the minerals can be easily extracted from the material and should be more easily leached upon contact with the soil, likely making it more accessible to plants.

3.3. Non-Polar Compounds Composition

Table 4. Lipophilic extract composition of the dichloromethane extracts (% of the total chromatogram peak area) of almond hulls and shells from the five varieties studied.

			Hulls					Shells
Varieties	Avijor	Belona	Guara	Marinada	Soleta	Avijor	Belona	Guara	Marinada	Soleta
Fatty acids	4.1	1.4	1.2	3.5	1.3	32.0	30.6	29.9	27.4	34.2
Alkanes and long-chain alcohols	5.0	4.0	1.9	4.4	2.5	7.2	12.5	4.7	6.4	6.8
Sugars	7.7	6.7	3.6	9.0	2.2	-	-	-	-	-
Saturated ω,α-diacids	-	-	-	0.1	-	0.7	1.6	0.7	1.8	1.9
Sterols	1.1	0.1	0.1	0.3	0.2	29.1	17.4	24.2	24.7	21.2
Triterpenes	74.6	77.5	91.3	73.5	88.8	-	-	-	-	-
Monoacylglycerols	0.3	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Aromatics	0.3	-	-	0.1	-	3.9	10.8	4.1	4.6	3.2
Others	-	-	-	-	-	9.0	8.6	12.0	10.0	9.6
Identified	93.4	89.9	98.2	91.1	95.2	82.9	82.4	76.6	75.4	77.8
Non-identified	6.6	10.1	1.8	8.9	4.8	17.1	17.6	23.4	24.6	22.2
Total	100	100	100	100	100	100	100	100	100	100

shows the different families identified in the dichloromethane extracts of almond hulls and shells from the five varieties studied. The lipophilic content in almond hulls and shells includes various compounds useful for different purposes. The hulls are rich in triterpenes (mostly Oleanolic, Betulinic, and Ursolic acid), with values ranging from 73.5% in the Marinada variety to 91.3% in the Guara variety, where oleanolic and betulinic acids are the main triterpenes present. To our knowledge, the composition of the lipophilic compounds from hulls is provided for the first time in this article. Although Meshkini [50] has carried out extraction studies using a 70:30 (v/v) acetone-water solution, they have not identified the compounds present in the extract.

Conversely, the shells are rich in fatty acids (30.8%), with chain lengths ranging from C9:0 to C29:0, as well as sterols (23.3%). The main fatty acid is palmitic acid (C16:0), averaging 19.5%, and stigmasterol, at 11.6%, is the dominant sterol. Aromatics are present in small amounts, mainly in the shells, ranging from 3.2% to 10.8%. The lipophilic profile found in the shell varieties is similar to that reported by Queirós et al. [9].

3.4. Pyrolysis Analysis

Almond hull and shell samples were also characterized using PY-GC/MS, a technique particularly useful for better understanding lignin composition. While the shells have been previously analyzed by Queirós et al. [9], this is the first time hulls have undergone such characterization. Pyrolysis was performed on extractive-free samples to reduce interference from non-lignin phenolic compounds. The identities and relative molar abundances of lignin and carbohydrate-derived compounds are presented in

Table 5. Relative quantification of the pyrolysis-derived compounds of hulls and shells.

	Hulls					Shells
	Guara	Avijor	Soleta	Marinada	Belona	Guara	Avijor	Soleta	Marinada	Belona
Total Identified Area	83.6	84.7	86.4	82.9	83.9	79.3	80.6	80.3	81.3	80.7
Total carbohydrates	62.0	64.1	68.0	64.0	62.9	41.7	42.3	40.0	41.3	42.8
Total Lignin	16.3	15.5	14.6	15.2	15.4	37.0	37.8	39.6	39.5	37.0
H	2.4	3.2	2.6	2.3	3.3	1.1	1.0	1.6	1.0	1.7
G	8.6	8.3	7.7	8.4	8.3	16.5	16.2	17.3	16.2	17.6
S	5.4	4.0	4.3	4.5	3.9	19.4	20.6	20.8	22.4	17.7
S/G	0.6	0.5	0.6	0.5	0.5	1.2	1.3	1.2	1.4	1.0

and Table S1 in the supplementary material.

Lignin from hull samples had a moderate amount of phenolic compounds derived from G units (between 7.7 and 8.6%), including guaiacol (peak 21 from Table S1), 4-vinylguaiacol (peak 31 from Table S1), and trans isoeugenol (peak 39 from Table S1). S units were present in amounts between 3.9% to 5.4%. Syringol (peak 36), 4-methylsyringol (peak 40), and 4-vinylsyringol (peak 49) accounted for the major compounds. Lastly, H units presented a minor amount (1.0 to 1.7%), derived from phenol (peak 20) and p-cresol (peak 22). Hulls also presented moderate amounts of catechol (peak 35) and small amounts of m-cresol (peak 23), o-cresol (peak 24), and resorcinol (peak 41). In most studied lignins, p-cresol is a marker of p-hydroxyphenyl units, while m- and o-cresols are present in lower quantities. In the case of hull samples, the opposite was found. This suggests that most likely m- and o-cresol are from non-lignin sources, such as tannins, as well as catechol and resorcinol. Regarding lignin-derived compounds from shell samples, H units derived compounds were present in small amounts (between 1.0 and 1.6%), and with higher amounts of G units (16.2 to 17.6%) and S lignin units (19.4 to 22.4%). On average, the monomeric composition of lignin in shells was 3.3, 44.0, and 52.7% for H, G, and S units, respectively. Queirós et al. [9] reported lower molar abundances for the H, G, and S units, with monomeric compositions of 2.8%, 37.6%, and 59.6%, respectively.

Pyrolysis also releases compounds from the carbohydrates, accounting for approximately 62 to 68% of the hulls and 40 to 43% of the rest. Levoglucosan, a compound derived from cellulose, was found in moderate amounts in hull samples (≈6 to 7%), while in shell samples, it accounted for only 1%. This suggests that, although almond shells contain a high amount of glucose according to the summative chemical analysis, the glucose is primarily associated with hemicelluloses rather than cellulose.

3.5. Proximate Analysis

Both hulls and shells were thermally evaluated, and the results can be found in

Table 6. Proximate analysis, ultimate analysis, and higher heating value of hulls and shells. Mean values of three replicates for proximate analysis. Ultimate analysis was made in duplicate. Percentages refer to g/100 g dry sample.

	Hulls					Shells
	Guara	Avijor	Soleta	Marinada	Belona	Guara	Avijor	Soleta	Marinada	Belona
Proximate analysis
Total Volatiles (%)	71.6 ± 1.5	72.3 ± 0.8	71.8 ± 0.9	71.9 ± 0.5	71.6 ± 0.04	79.3 ± 0.8	81.5 ± 0.2	80.6 ± 0.4	79.9 ± 0.6	80.7 ± 0.6
Fixed Carbon (%)	17.3 ± 1.5	14.4 ± 0.8	18.6 ± 0.9	17.0 ± 0.5	19.3 ± 0.04	19.3 ± 0.8	17.7 ± 0.2	18.7 ± 0.4	19.0 ± 0.6	18.3 ± 0.6
Ash (%)	11.1 ± 0.2	13.3 ± 0.2	9.6 ± 0.04	11.1 ± 0.3	9.2 ± 0.1	1.4 ± 0.01	0.8 ± 0.1	0.7 ± 0.2	1.1 ± 0.03	1.0 ± 0.1
Ultimate analysis
C (%)	40.4	39.7	43.8	42.7	43.6	45.4	45.0	45.5	46.1	45.3
H (%)	5.4	5.2	5.5	5.4	5.6	6.1	6.1	5.9	5.9	5.9
N (%)	0.8	1.0	0.6	0.8	0.6	0.2	0.2	0.2	0.2	0.1
S (%)	-	-	-	-	-	-	-	-	-	-
O (%)	42.3	40.8	40.6	39.9	41.0	47.0	47.9	47.8	46.7	47.6
H/C	0.13	0.13	0.13	0.13	0.13	0.13	0.14	0.13	0.13	0.13
O/C	1.05	1.03	0.93	0.93	0.94	1.04	1.06	1.05	1.01	1.05
HHV (MJ/Kg)	17.25	17.15	17.32	16.94	17.33	20.11	20.1	20.06	20.34	20.23

. In the proximate analysis, volatile matter was higher in shells compared to hulls. Shells displayed volatile contents ranging from 79.3% (Guara) to 81.5% (Avijor), while hulls ranged from 71.6% (Guara and Belona) to 72.3% (Avijor). However, eliminating the ash component, the ratio between volatile matter and fixed carbon is practically the same, with an average of 81% volatility and 19% fixed carbon for hulls. This analysis reveals little difference, except that one type of material contains significantly more ash, making it less suitable for burning, which is reflected in its higher heating value (HHV). The high ash content found in hulls poses challenges in combustion processes that can lead to operational issues such as slagging, fouling, and clinker formation, which may accelerate wear and reduce the lifespan of combustion equipment [51]. Fixed carbon content was generally higher in hulls (14.4 to 19.3%) than in shells (17.7 to 19.3%). Typically, most woody materials exhibit values of 50% or less [52,53]. Fixed carbon contributes to char formation and long-term combustion, indicating that both fractions may offer benefits in solid fuel applications depending on the desired outcome. These results are in line with the ones found by Salgado-Ramos et al. [54].

The ultimate analysis further confirmed the superior fuel quality of shells. Carbon content was higher in shells (45.0–46.1%) compared to hulls (39.7 to 43.8%), with Marinada shells having the highest carbon content. This aligns with their higher energy content. Hydrogen content was also slightly higher in shells (5.9 to 6.1%) than in hulls (5.2 to 5.6%), while nitrogen content was greater in hulls (0.6 to 1.0%) than in shells (0.1 to 0.2%). Sulfur was undetected in all samples, which is favorable for clean energy production. Oxygen content, calculated by difference, ranged from 39.9% to 42.3% in hulls and 46.7% to 47.9% in shells. These trends are clearly reflected in the higher heating values (HHV). Shells exhibited significantly higher HHVs (20.06 to 20.34 MJ/kg) than hulls (16.94 to 17.33 MJ/kg). These results are comparable to the values obtained for other forest (17.6 to 20.8 MJ/kg) and agricultural (15.4 to 19.1 MJ/kg) biomasses [55]. Among hulls, Belona and Soleta had the highest HHVs, whereas Marinada hulls had the lowest. The higher HHVs of shells are consistent with their elevated carbon and hydrogen contents and reduced ash and nitrogen levels.

3.6. Water Holding Capacity

To evaluate the effect of hulls or the densified combined biomasses (hulls and shells) as pellets on the water holding capacity of soil, several trials were conducted with results shown in

Table 7. Water holding capacity (g/100 g dry sample) of increasing percentages of almond hulls or pellets produced with almond hulls and shells. The control is the soil used to create the mixtures. For each sample, three replicates were made, and the result shown is the mean of the three.

Sample	WHC (Hulls)	WHC (Pellets)
Soil	24	22
1.6%	24	21
3.1%	29	21
6.3%	37	27
12.5%	58	31
25%	96	35
50%	135	44
100%	285	105

. Both materials showed promising results, with increasing percentages of hulls or pellets having a positive effect on the WHC. By adding 6.3% hulls to the soil, the WHC increases by 50% (from 24% to 37%), with the same increase being reached from pellets only after adding 12.5 g/100 g of soil. When pellets substitute 50% soil, the WHC doubles (from 22% to 44%), while for hulls, the increase is above 5 times the WHC of the soil (from 24% to 135%). To double the WHC of the soil with hulls, the amount added to the soil would be around 10%. The differences in the increase in WHC for hulls and pellets could be for one of two main reasons: either pellets require time to absorb water and, thus, need longer to soak in the water than the time allowed (around 4 h); or in the process of densification of the material, the space between particles is diminished and, due to that, so is the capacity to absorb water by mass unit.

Additionally, as pellets are a densified material, it was also expected that the WHC would be lower than that of the milled hulls. Several studies have investigated the potential use of almond hulls and shells as an organic soil amendment, yielding promising results [56]. Vida et al., Bonilla et al., and Andrews et al. [33,49,57] have demonstrated the significant impact of almond hulls and shells on the soil’s microbial community, in terms of biomass, biodiversity have shown the considerable impact of almond hulls and shells on the soil’s microbial community, in terms of biomass, biodiversity and multifunctionality. Fernandez-Bayo et al. and Shea et al. [25,58] further found that almond hulls and shells, when used as soil amendments, can produce organic acids with pesticide activity. A future study should evaluate the feasibility and benefits, especially in reducing the need for watering and synthetic pesticides, of incorporating hulls or pellets into the soil.

4. Conclusions

Almond hulls and shells are a waste by-product that originates from the almond industry, with significant importance in Portugal and worldwide. It is essential to understand the potential uses of these materials to help upgrade and diversify the product portfolio of these industries while reducing residues. In this study, we analyzed and compared in detail, both chemically and thermically, these two materials from five different varieties produced in Portugal and were able to suggest possible applications for them: almond shells can serve as a source of hemicelluloses, lignin, and/or carbon for energy production, while almond hulls have been found to have a high potassium content, while substantially increasing the water-holding capacity of soil, making their use beneficial for soil amendment and reducing the need for watering. Further analysis should include ecotoxicity tests for both plants and soil organisms, as well as field tests with pellets and/or hulls incorporated into the soil.