Abstract

General Background: Nuts are widely recognized for their nutrient density, offering a rich source of beneficial fatty acids and antioxidant phytochemicals. Specific Background: Among these, macadamia and hazelnuts are popular for their unique compositions, yet direct comparisons of their nutritional and bioactive profiles remain limited. Knowledge Gap: Despite their recognized health benefits, comparative analyses of the fatty acid profiles, bioactive compositions, and physicochemical characteristics between these nuts are scarce. Aims: The study compared macadamia and hazelnut oils' fatty acid content, lipid-soluble vitamins, phenolic compounds, and physicochemical properties to enhance their functional applications. Results: The findings revealed that hazelnut oil exhibited a higher polyunsaturated fat content, particularly linoleic acid (22.8% vs. 18.25%), while macadamia oil contained significantly more vitamin E, contributing to superior antioxidant potential. Both oils demonstrated the presence of vitamins A, D, and K1, with notable concentrations of quercetin and rutin among other flavonoids. Novelty: This study is novel in its comprehensive comparative analysis of these two nuts, particularly in identifying the unique phenolic compounds in the nutshells, which are often discarded but show potential as nutraceuticals. Implications: Macadamia and hazelnut oils offer significant nutritional benefits, with compositional variations allowing for personalized bioactivity and culinary applications. Further research is recommended for human health promotion.

Highlights:

1. Hazelnut oil has more polyunsaturated fats; macadamia oil is richer in vitamin E.
2. Both nuts contain vitamins A, D, E, K1, and flavonoids.
3. Phenolic compounds in nutshells have potential as nutraceuticals.

Keywords: macadamia nuts, hazelnut oil, fatty acids, antioxidant phytochemicals, bioactive compounds

Introduction

Tree nuts like almonds, cashews and walnuts have gained increasing recognition as health-promoting foods, largely attributable to their unique nutrition profiles consisting of “good” unsaturated fats, protein, fiber, vitamins, minerals, and a range of bioactive phytochemicals (Pradhan et al., 2020). The distinctive fatty acid composition rich in monounsaturated and polyunsaturated fats may be particularly beneficial, as frequent nut consumption has been associated reduced risk of cardiovascular disease, aberrant blood lipids, diabetes, and metabolic syndrome (Arnesen et al., 2023). Nuts also provide essential micronutrients like fat-soluble vitamins A, D, E and K that support physiological processes. Additionally, the phenolic compounds, carotenoids, phytosterols and other plant chemicals found in nuts and seeds offer antioxidant, anti-inflammatory, and anticancer activities that further contribute to their functional food status (Pinela et al, 2012).

While almonds, walnuts, cashews and pistachios have dominated nut research and commercial markets to date, other “niche” nuts remain less scientifically explored. Macadamia nuts are produced by trees of the genus Macadamia, consisting of four species indigenous to Australia (Hu et al., 2022). There are approximately 10 types, but two species are primarily cultivated commercially for their edible nuts: M. integrifolia and M. tetraphylla. The nuts are prized for their flavor and nutrition, containing high amounts of fats (75%), particularly monounsaturated fats, as well as vitamins, minerals, carbohydrates and dietary fiber (Carrillo et al., 2017). Macadamia oil extracted from the nuts has unique properties like high smoke point and long shelf life that make it suitable for cooking applications (Kochhar & Henry, 2009). The oil is also rich in skin-nourishing fatty acids and can have cosmetic uses (Lin et al., 2017). Kernels of the hard-shelled nuts exceed 75% fat composition, with prior literature confirming over 75% total monounsaturated acids - mainly oleic acid - that may support healthy blood lipid profiles similarly to olive oil (Venkatachalam and Sathe, 2006; Talcott and Peterson, 2012). Additionally, past analyses have revealed an array of flavonoids like quercetin, phytosterols like β-sitosterol, fat-soluble vitamin E, and squalene that could impart antioxidant and other beneficial biological effects (Mustafa et al., 2022; Feng et al., 2022). Hazelnuts or filberts demonstrate similar potential based on nutritional status, although published research on the composition and attributed health properties are more limited compared to other popular tree nuts. Hazelnuts are rich in unsaturated fatty acids like oleic acid and phenolic compounds analogous to those found in macadamia nuts that could promote positive health outcomes (Rondanelli et al., 2023; Wojdyło et al., 2022).

Hazelnuts belong to the Betulaceae family and are one of the major tree nut crops, widely used by the food industry for their distinctive flavor and oil content. They are considered an important source of beneficial fats, protein, vitamins and minerals in the human diet (Seyhan et al., 2007). Over 90% of harvested hazelnuts are processed into kernels, with properties like size, shape, taste, and oil content being factors for evaluating quality and relationships to environmental conditions (Romero-Aroca et al., 2021).

This study aimed to conduct side-by-side comparative analysis of macadamia nuts versus hazelnuts sourced from nearby geographical regions. Specific objectives were four-fold: 1) Quantify and qualify the fatty acid distribution (% composition) in oils extracted from the nut kernels using gas chromatography (GC) techniques, 2) Identify and quantify the lipid-soluble and water-soluble vitamin contents in the nut oils using reversed-phase high-performance liquid chromatography (HPLC), 3) Screen and characterize the phenolic compounds and other phytochemicals present in the crude nutshell extracts using HPLC methods, and 4) Evaluate relevant physicochemical properties of the pressed nut oils.

Methods

Reagents and Chemicals

HPLC-grade solvents including acetonitrile, methanol, acetone, hexane, ethyl acetate, chloroform, 0.01% trifluoroacetic acid, formic acid and Folin-Ciocalteau phenol reagent were used. Additionally, chromatography standards for oleic acid, linoleic acid, stearic acid, palmitic acid, gallic acid, catechin, chlorogenic acid, caffeic acid, p-coumaric acid, ellagic acid, quercetin, apigenin and rutin (> 98% purity) as well as butylated hydroxytoluene (BHT) were used. Vitamin standards including retinol (A), cholecalciferol (D3), α-tocopherol (E) and phylloquinone (K1) were used. All solvents and chemicals were analytical or HPLC grade. Deionized water was prepared in the laboratory.

Plant Materials and Processing

Whole raw hazelnuts and macadamia nuts were purchased from local wholesale markets. Initial quality analysis verified the nuts met respective Codex standards for conforming cultivars. Upon delivery to the laboratory, nuts were manually cracked, deseeded and separated from the hard shell fragments. Hazelnut and macadamia kernels were subsequently dried under vacuum at 50±5°C for 10-12 hours then sealed and maintained at -20°C before analysis. The nut shells were discarded in commercial applications but retained for phytochemical screening in this research. Shells were rinsed, sliced, and dried at 60 °C for 8 hours. Dried shells were pulverized to a coarse powder using a commercial mill and stored in amber jars away from light.

Oil Extraction

Oil was mechanically expelled from the hazelnut and macadamia kernels (200 g) using a Komet screw press. Crude oils were passed through a filter before bottling in amber glass and stored frozen until analysis. The extracted oils were clear and absent visual hazes or particulates.

Solvent Extraction of Phenolics from Nut Shells

Phenolic compounds were solvent extracted from the dried, ground hazelnut and macadamia shells based on an established protocol with minor adjustments. Briefly, hazelnut or macadamia shell powder (20 g) was weighed into a flask then mixed with aqueous acetone (500 mL, 70:30 v/v) containing 0.1% w/v BHT as antioxidant. Extractions were carried out under reflux for 120 minutes at 70±5°C. After cooling to room temperature, crude extracts were filtered under vacuum and acetone removed using a rotary evaporator. The remaining aqueous filtrates were lyophilized then reconstituted in HPLC-grade methanol at 10 mg/mL concentration (w/v). All samples were filtered through 0.45 μm membranes before analysis.

Determination of vitamins

Solution preparation: Lithium perchlorate solution (LPS) was prepared in aqueous media (0.4M, pH2.4), while butylhydroxytoluene solution (BHTS) in methanol. To determine vitamins, an amount of 0.1 g of the tested sample were mixed with 5ml of LPS, incubated in tightly closed vessel (2h, at RT and darkplace). Stirred for 20 min at RT afterwards and samples were centrifuged to be ready for HPLC assay.

Alcoholic extraction of shell

Dry powder prepared from milling 3g of dry sample (Macamedia or hazelnut), then missed with hydroalcoholic cosolvent (40 methanol:60water), incubated overnight at RT. Next day, the mixture were vacuum filtered and filtrate was hydrolysed by NaOH (2N, 30min) concentrate were buffered to pH 7 using HCl (2N). Followed by liquid/liquid extraction of phenolic acid using ethyl acetate. Finally, the ethylacetate removed under reduced pressure. The leftover or reminannt dissolved in 10 ml methanol to be ready for injection to HPLC

Gas Chromatography Analysis of Nut Oils

The fatty acid composition of the nut oils was analysed by gas chromatography with flame ionization detection (Shimadzu GC-2010) after derivatization to fatty acid methyl esters (FAMEs) using boron trifluoride (BF3) according to AOAC Official Method 996.06. Briefly, ~35 mg oil was dissolved in hexane then methylated with 14% BF3-MeOH reagent at 100°C for 1 hour. FAMEs were extracted in hexane, dried with sodium sulphate then filtered for injection. FAMEs were separated on a SUPELCO SPTM-2560 fused silica capillary column (100m×0.25mm i.d, 0.2-μm film thickness) with helium carrier gas at 1.0 mL/min flow. The detector and injector temperatures were 230°C and 250°C, respectively. Oven temperature was held at 140°C for 5 min then increased to 240°C at 4°C/min for 10 minutes. A 1 μL sample split injection (1:50) was used. Individual FAME peaks were identified by comparison to authentic FAME standards and peak areas integrated for percentage composition.

High Performance Liquid Chromatography Analysis

Reverse phase HPLC was conducted using a Shimadzu prominence system equipped with photodiode array detector, CBM-20A controller and LC Solution software. separated using either a C18 or C30 column (250 x 4.6 mm I.D; 5 μm) depending on the analysis. For vitamin analysis of oil samples, a Zorbax Eclipse XDB C18 column was employed with methanol-water (97:3 v/v, 1% acetic acid) isocratic mobile phase at 1 mL/min flow rate per optimized methods. Lipid-soluble vitamin peaks were detected at 292 nm and identified versus standards. Water-soluble vitamins were not assessed. For phenolic screening, hazelnut and macadamia nutshell extracts were separated on a YMC Pack C30 column using aqueous formic acid and acetonitrile mobile phases at 1 ml/min flow under a mixed isocratic-gradient program for 0 to 60 minutes runtime at ambient temperature. Phenolics and organic acids were detected at 278 nm wavelength and identified by matching HPLC spectra and retention times against pure standards.

Physicochemical Analysis of Nut Oils

Quality indices and physicochemical properties were determined for the extracted hazelnut and macadamia nut oils. Specific gravity, refractive index (RI), colour, free fatty acids, peroxide value, p-anisidine value, Totox value and viscosity were quantified using standard IUPAC methodologies. Moisture contents were evaluated by automated Karl Fischer titration.

Result and Discussion

Result

Gas Chromatography Analysis of Nut Oil Fatty Acids

Gas chromatography of the nut oils confirmed six primary fatty acids in both the hazelnut and macadamia lipids (Figure 1). The fatty acids detected were palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2), alpha-linolenic acid (C18:3) and arachidic acid (C20:0). As shown in Table 1, oleic acid (C18:1) was the predominant fatty acid in both nuts’ oils, followed by linoleic (C18:2) and palmitic (C16:0) acids as next major fractions. Of note, hazelnut oil contained significantly greater linoleic acid than macadamia oil (22.8% vs. 18.25%). Alternatively, macadamia oil demonstrated relatively higher oleic and palmitic acid contents. Alpha-linolenic acid (C18:3) was detected in minor amounts (<2%) in both oils.

The elevated oleic acid levels classify both as “high-oleic” oils, conferring improved oxidative stability for cooking applications compared to more polyunsaturated rich oils like safflower or sunflower (Mahmud et al., 2023). However, the roughly 10-15% higher polyunsaturated fatty acid (PUFA) concentrations, predominantly linoleic acid, found in hazelnut oil would impart superior anti-inflammatory bioactivity (Simopoulos, 2016). Ultimately from a lipid biochemistry standpoint, both nut oils provide beneficial support for cardiovascular, metabolic and immune health.

Figure 1.Representative GC chromatogram of FAMEs in A) hazelnut oil versus B) macadamia nut oil.

HPLC Analysis of Vitamins

HPLC chromatograms of the nut oil samples detected four lipid-soluble vitamins over a 20-minute analysis, including vitamins A, D3, E, and K1 (Figure 2). The identities and quantities of the vitamins are presented in Table 2. In both oils, vitamin E was the most abundant nutritionally-relevant vitamin observed, found at concentrations exceeding >100 mg per 100 g oil. However, macadamia oil contained significantly greater vitamin E than hazelnut oil (150.2 mg per 100 g versus 117.9 mg per 100 g, respectively), conferring superior potential antioxidant value. Vitamin K1 was the predominate form of vitamin K detected, while vitamins A and D were present at lower yet relevant nutritional levels that would satisfy percentages of recommended daily intakes (RDIs). Overall, the presence of all four fat-soluble vitamins highlights the excellent nutritional status of both hazelnut and macadamia nut oils. The oils could individually provide the daily requirements needed for reducing risk of deficiency.

Figure 2.HPLC traces showing lipid-soluble vitamin detection in A) hazelnut oil compared to B) macadamia nut oil. Peak identities: 1. Vitamin A (3.71), 2. Vitamin D3, 3. Vitamin E, 4. Vitamin K.

†Recommended Daily Intake (RDI) values based on FDA guidelines for adults.

Phenolic Composition of Nut Shell Extracts

HPLC analysis of the reversed-phase phenolic profiles identified and confirmed diverse polyphenols present in the crude hazelnut and macadamia nut shell extracts (Figure 3). Using pure reference standards for qualification, key phenolic acids, flavonoids and hydrolysable tannins were detected, as summarized in Table 3. Most prominent peaks matched gallic acid, chlorogenic acid, p-coumaric acid and ellagic acid based on the overlaid HPLC spectra. Quercetin, rutin and quercetin spectra were verified among the observed flavonoids. Though concentrations were not fully quantified, the arrays of nut shell phenolics may impart beneficial antioxidant and anti-inflammatory bioactivities in vivo when consumed via whole nuts or potentially isolated as by-product ingredients (Parejo et al., 2004). Further isolation, identification, and bioassay testing is warranted to fully characterize the phytochemical profiles and functionality.

Figure 3.Overlaid HPLC phenolic profiles of (a) hazelnut [denoted compounds: Gallic acid 3.18, Chlorogenic acid 4.15, P-coumaric acid 5.40, Rutin 6.12, Ellagic acid 7.20] and (b) macadamia [denoted compounds: Quercetin 2.08, Gallic acid 3.11, Caffeic acid 3.69, Apigenin 4.79, Rutin 6.12] nutshell extracts.

Nut Oil Physicochemical Properties

Select physicochemical characteristics were measured for the pressed hazelnut and macadamia nut oils to evaluate quality (Table 4). Key criteria assessed included specific gravity, refractive index, color, viscosity, free fatty acids, peroxide value and p-anisidine values. Both oils were clear yellowish liquids at room temperature. Refractive index values ranged from 1.457-1.470 for the oils. As expected, based on the higher unsaturated fat content, hazelnut oil registered a lower refractive index than macadamia oil. Viscosity analysis also showed hazelnut oil to be more viscous (61.2 mPas·s) compared to macadamia (46.5 mPas·s). The elevated viscosity could impact high-heat cooking applications. Acid value titrations revealed low free fatty acids (~0.3% as oleic) and peroxide numbers below 10 mEq/kg, suggesting minimal hydrolysis or oxidation had occurred post-production. Overall, the fresh oils were highly stable with quality parameters equalling or exceeding respective Codex thresholds. Based on the density and viscosity differences, macadamia oil may be preferential for high-temperature cooking without smoking or matrix interference.

Discussion

This research conducted comparative analysis of extracted oils and isolated phytochemicals from hazelnut and macadamia nuts as well as examined physicochemical characteristics relevant to functional applications. While both nuts demonstrated excellent nutritional profiles, some compositional differences were illuminated that could impact health-promoting activities.

Gas chromatography of the pressed nut oils verified primarily oleic acid composition, ranging 71-79%, classifying them as high-oleic acid oils. However, hazelnut oil showed roughly 5% greater concentrations of polyunsaturated fats, particularly omega-6 linoleic acid. The 15-20% total PUFAs align with recognized ratios relative to monounsaturated fats for balancing anti-inflammatory eicosanoids, thrombosis factors, blood pressure regulation and oxidative reactions (Simopoulos, 2010). Therefore, the lipid profile of hazelnut oil could confer added preventative effects against chronic inflammation underlying disorders like cardiovascular disease, arthritis, diabetes and cancer (Perna ey al., 2016). Both oils would still benefit blood cholesterol levels based on overwhelming oleic acid amounts that can downregulate LDL oxidation while elevating HDL levels for cholesterol clearance (Perna ey al., 2016; Lu et al., 2019). Hazelnut oil also demonstrated significantly greater vitamin E quantities that could impart improved antioxidant capacity to protect against lipid peroxidation or reactive oxygen species. On the other hand, as evidenced by the viscosity evaluations, the lower polyunsaturated fat content of macadamia oil renders it more thermally stable, ideal for high-heat cooking without smoking.

Though not fully quantified, the arrays of phenolic acids, flavonoids and tannins detected in both the hazelnut and macadamia nutshell extracts suggest bioactive potential as botanical bioproducts or nutraceuticals. For instance, the identified ellagic acid, gallic acid and catechins have exhibited anticancer, antiviral and anti-inflammatory activities in vitro, attributed to potent antioxidant and gene expression modulation mechanisms (Ismail et al., 2004). Their metabolic fate and bioavailability from complex nut matrices still requires investigation. As the hard shells are typically wasted in commercial applications, isolation and concentrated of the phytochemicals could allow sustainable utilization of the by-product, adding revenue streams for producers. Enrichment of those fractions as supplements could further potentiate the health functionality of the nuts themselves.

Overall, both hazelnuts and macadamia nuts provide valuable nutritional and potential pharmacology value, but with some differentiation that could tailor usage. The elevated monounsaturated fat, vitamin E and possibly phytochemical contents of hazelnuts seem to confer advantages supporting cardiovascular health, antioxidant status and fighting systemic inflammation. However, the unique oil quality characteristics of macadamia nuts better enable high heat cooking without compromising bioactive degradation. Further studies are still needed to substantiate the bioactivities of the nuts’ bioactive constituents in people as well to optimize processing applications.

Conclusion

In summary, this research provided comparative analysis of macadamia and hazelnut nutritional compositions, functional bioactives like vitamins and polyphenols, as well as relevant physicochemical properties of the extracted oils. Both nuts demonstrated robust nutrition profiles rich in unsaturated fatty acids like oleic acid as well as detectable levels of fat-soluble vitamins and arrays of antioxidant phenolic compounds. Hazelnut oil contained significantly greater concentrations of heart-healthy PUFAs and vitamin E that could impart added cardiovascular and anti-inflammatory benefits. Unique phenolics were also isolated from the discarded shells, revealing value-added potential as nutraceuticals. Key criteria assessments showed higher density and viscosity values for hazelnut oil that may impact high heat cooking stability. Ultimately, routine consumption of both macadamia nuts and hazelnuts can promote wellness, with some compositional differences allowing tailored bioactivity or culinary applications. Further research should explore confirming the health functionality, bioavailability and metabolism of the quantifiable food and phytochemical constituents in people.

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