Hazelnuts as Source of Bioactive Compounds and
Health Value Underestimated Food
MATTIA DI NUNZIO
Department of Agri-Food Sciences and Technologies (DISTAL),
University of Bologna, Cesena, Italy.
Abstract
Hazelnut (HN) has found its way into nontraditional foods due to the
recognition of its nutritional and nutraceutical properties. Among nut
species, hazelnut plays a major role in human nutrition and health
because of its special composition of fat (mainly oleic acid), dietary
fibre, vitamins (vitamin E), minerals, phytosterols (mainly β-sitosterol),
and antioxidant phenolics.
In particular, lipids represent 60% of its dry weight and are mainly
represented by triacylglycerols where the main fatty acids are oleic
and linoleic acids. Furthermore, HN oil is an exceptional source of
specific bioactive compounds as tocopherols, mainly α-tocopherol.
Besides a favourable fatty acid profile and high tocopherols content,
HNs are also a source of minerals and phytosterols, where potassium
and β-sitosterol are the major ones. In addition, the presence of several
phenolic antioxidants such as mono- and oligomeric flavan 3-ols has
been reported.
HNs represent a very interesting food, and their nutritional and health
value need to be further evidenced in intervention trials. In addition,
the use of HN by-products as new functional ingredient represents an
important challenge for the sector and the food industry.
Current Research in Nutrition and Food Science
www.foodandnutritionjournal.org
ISSN: 2347-467X, Vol. 07, No. (1) 2019, Pg. 17-28
CONTACT Mattia Di Nunzio mattia.dinunzio@unibo.it Department of Agri-Food Sciences and Technologies (DISTAL), University
of Bologna, Cesena, Italy.
© 2019 The Author(s). Published by Enviro Research Publishers.
This is an Open Access article licensed under a Creative Commons license: Attribution 4.0 International (CC-BY).
Doi: doi.org/10.12944/CRNFSJ.7.1.03
Article History
Received: 04 March 2019
Accepted: 25 April 2019
Keywords
Hazelnut;
Lipids;
Dietary Fibre;
Minerals;
Tocols;
Squalene;
Phytosterols;
Phenols.
Introduction
Corylus avellana L., the European HN, is the second
most popular nut worldwide just after almonds and
production ranges from North Africa and Europe
to the Asia Minor and Caucasus region. Countries
around the Black Sea account for the majority
of production in the world: Turkey (610,264 tons,
average for the period 2009–2011), Azerbaijan
18NUNZIO, Curr. Res. Nutr Food Sci Jour., Vol. 7(1), 17-28 (2019)
(28,564 tons), and Georgia (20,567 tons). Other
important producers are Italy (114,991 tons), the
USA (35,079 tons), and Spain (16,988 tons).1
In Turkey, the principal HN producer providing
around 72% of the HN production in the world, the
major HN cultivar is Tombul, followed by Çakıldak,
Mincane and İncekara, which are located mainly
in the provinces of Ordu and Giresun.2 Italy, the
world's second largest producer, possess numerous
traditional cultivars, which are mostly growth in the
regions Campania, Latium and Piedmont. Recently,
some of the major cultivars (Tonda Romana from
Latium, Tonda di Giffoni from Campania and Tonda
delle Langhe from Piedmont) obtained the European
Community quality stamp for their traditional
peculiarity.3 Today exists a variety of almost 400 HN
cultivars, but only about 20 of them represents the
basis of world production. Nut chemical, physical and
morphological characteristics are highly dependent
on interactions with the environment and genotype,
postharvest management and cultural techniques.4
Thanks to their sensory properties, HNs are
consumed not only as a ripe or “green” fruit but also
in a variety of manufactured food such as chocolate
spread, cereal bar, cookie, nougat, pastry, ice cream
and cooking oil production. HNs are consumed
roasted or raw, chopped, intact, or processed into a
praline paste; they are typically processed integrally
into food products, although HN oil is also frequently
used for cooking.5,6
Among nuts, HN plays a key role in human nutrition
and health because of its special content of
macronutrients (lipids and fibre), micronutrients
(minerals and vitamins), fat-soluble bioactives
(tocols, phytosterols, phytostanols and squalene)
and phytochemicals (flavonoids and phenolic and
hydroxycinnamic acids).7-9 A list of each class of
nutrients and their quantity is reported in table 1.
Lipids and Fatty Acids
The main nutrient of the HN kernel is the lipid portion,
which has the biggest impact on kernel flavour,
especially after roasting. For many years the edible
vegetable oils composition has been evaluated
with the objective to obtain knowledge to improve
product quality in terms of flavour, taste, nutrition,
storage stability and guaranteeing the legitimacy of
the material.10
Lipids may constitute more than 60% of the HN
kernel dry weight and are constituted of 98.8%
triacylglycerols (TAG) and 1.2% polar lipids (PL). Within
PL, phosphatidylcholine, phosphatidylethanolamine
and phosphatidylinositol are present at 56.4%,
30.8% and 11.7%, respectively.11 Among fatty acids,
oleic acid (C18:1n9) is by far the most predominant
ranging from 76.7% to 82.8%, followed by linoleic
(C18:2n6), palmitic (C16:0), stearic (C18:0) and
vaccenic (C18:1n7) acids with mean values of
9.2%, 5.6%, 2.7% and 1.4%, respectively.12 This
is very similar in composition to fatty acids of olive
oil and generally recommended for a healthy diet.13
Moreover, due to the high level of mono-unsaturated
fatty acids (MUFA) and tocopherols/tocotrienols
content, HN oils have an oxidative stability similar
to the value of olive oil, and higher compared to
rapeseed oil.14 As consequence, it is present only
a minor increase of the possibly harmful trans fatty
acids during the thermal treatment of nuts (roasting)
and, although some minor changes occurred in the
TAG and fatty acid compositions, the corresponding
profiles basically remained identical to that of raw
HNs.12
Moreover, various studies reported as lipids content
increased continuously during the development of
the kernel, from 6.38 g/100g dry matter to 68 g/100g
dry matter.13,14 Regarding fatty acids, from early to
harvest stage a reducing and an increasing trend in
the amount of polyunsaturated fatty acids (from 31
to 10.3 g/100g of oil) and MUFA (from 22 g/100g oil
to 79.2 g/100g oil) was detected, respectively. No
significant changes were observed in total saturated
fatty acids at different maturation stage.14,15
From the nutritional viewpoint, various studies have
confirmed that a diet with a low amount of saturated
fatty acids and high content of MUFA can effectively
reduce the risk of coronary heart disease amending
blood lipid levels and blood pressure ameliorating
metabolic syndrome and insulin sensitivity.18-21
Dietary Fibre
In the simple terms, dietary fibre can be considered as
a ‘roughage’ material of carbohydrates (beta-glucans,
19NUNZIO, Curr. Res. Nutr Food Sci Jour., Vol. 7(1), 17-28 (2019)
lignin, cellulose, pectin and hemicellulose) resistant
to small intestine digestion, requiring microbiota
fermentation situated in the large intestine.22,23 Types
of dietary fibre may be categorized according to
their sources, solubility, fermentability, physiological
effects, and they can be obtained from cereals,
legumes, fruit and vegetables.24-25
After cereals, nuts are the vegetables most reach in
fibre. Among tree nuts, the highest content in dietary
fibre were measured in almonds (9.2%), followed by
HNs (8.7%), walnuts (6.8%), macadamia nuts (5.5%)
and pistachios (4.2%).26 Moreover, Silva et al.27
compared the fibre in six cultivars of HNs harvested
in Portugal. The fibre content, expressed as g/100g,
ranged from 12.07 to 8.05 for Butler and Merveille de
Bollwille varieties respectively, indicating consistent
variations of dietary fibre among HN cultivars.
Today, recent and persuasive evidences confirmed
that high dietary fibre intake promotes overall
health and associates with lower mortality through
preventing and mitigating of cardiovascular disease,
colon cancer and type 2 diabetes mellitus,28
suggesting an adequate intake for the Italian adult in
the amount of 25 g/day.29 Although the mechanisms
that underline the described effects of dietary fibre
on health are not well-known, it is supposed to be
a consequence of changes in nutrient absorption,
production of short chain fatty acids, gut hormones
secretion and intestinal viscosity.30-32
Minerals
Minerals are normally divided into macro-minerals
and micro-minerals. Major minerals include Ca, Mg,
K, Na, Cl, P and S; while trace minerals are I, Zn,
Se, Fe, Mn, Cu, Co, Mo, F, Cr and B. Different plant
and animal sources can be consumed to obtain a
number of essential minerals for a healthy nutrition.33
In HN, at least a total of 24 minerals have been stated
so far with an extremely high variability depending on
genotypes, geographical origin, year of harvesting,
climate, soil composition, irrigation, use of fertilizer
and method of cultivation.34,35 Generally, K is the most
present mineral with a concentration ranging from
147 mg/100g to 761 mg/100g, followed by P (from
256 mg/100g to 458 mg/100g), Ca (from 65 mg/100g
to 328 mg/100g) and Mg (from 34 mg/100g to 335
mg/100g). HNs serve also as an excellent source
of trace minerals as Cu (from 0.94 mg/100g to 3.47
mg/100g), Mn (from 1.4 mg/100g to 19 mg/100g),
and Se (from 5.5 μg/100g to 60 μg/100g).35-40 With
regard to the trace minerals, a standard portion of
HN (30g) supplies, as percentage of the Population
Reference Intake (PRI) or Adequate Intake (AI) for
Italian adult males (aged 30-59 years), 31-116% of
Cu, 16-211% of Mn, and 3-33% of Se.29
Even though each essential mineral has its own
health benefits, Se in particular is an essential trace
mineral of central importance to human health. As
part of L-selenocysteine, selenium is needed for the
Fig. 1: Natural forms of vitamin E.44
20NUNZIO, Curr. Res. Nutr Food Sci Jour., Vol. 7(1), 17-28 (2019)
synthesis of selenoproteins, a class of proteins with
important functions including skeletal and cardiac
muscle function, T-cell immunity, thyroid hormone
metabolism and antioxidant defense.41
Tocols
Tocopherols and tocotrienols are monophenols
having the identical main chemical structure
constituted by a long chain attached at 2-position
of a chromane ring. Tocotrienols diverge from
tocopherols because they have a farnesyl rather than
a saturated isoprenoid C16 side chain42 and exist as
four homologues (α, β, γ, δ) which differ from each
other by the number and location of methyl groups
in their chemical structures.43
Various nuts have been reported to show significant
tocopherols and tocotrienols differences, ranging
from approximately 1.6 mg/100g of kernel for
macadamia to 32 mg/100g of kernel for black walnut.
Among them, α- and γ-tocopherol are the most
represented isoforms.45 HN oil is an exceptional
source of vitamin E, where α-tocopherol being the
dominant form with a content up to 41.9 mg/100g
extracted oil,8,34 corresponding at 96% of total
tocols.46 Differences in vitamin E content in HN
oil depend on the variety and geographical origin,
where the Tombul variety grown in Turkey seems to
have the highest tocols content.41 Moreover, roasting
and removal of the pellicle (peeling) have shown to
reduce considerably tocopherol content.46
The high α-tocopherol content represents a peculiar
characteristic, since of the eight naturally occurring
forms, α-tocopherol is the most active homologues
retained in human plasma with the highest antioxidant
activity.47 In addition to its activity as an antioxidant
in the prevention of potentially harmful phospholipid
oxidation events at plasma membrane,48 vitamin E is
also involved in various metabolic processes such
as regulation of gene expression, cell signalling
and immune function.49 Moreover, vitamin E
forms suppress pro-inflammatory signalling such
as STAT3/6 and NF-κB and inhibit eicosanoids
catalysed by cyclooxygenase- and 5-lipoxygenase.44
Consistent with mechanistic findings, assumption
of vitamin E contributes to the prevention of various
diseases as cardiovascular, neurodegenerative,
non-alcoholic fatty liver diseases and some kind of
cancer.50
Phytosterols and Phytostanols
HNs are also rich in plant sterols (phytosterols
and phytostanols). Phytosterols are comparable in
structure to cholesterol, possessing the same basic
cyclopentanoperhydrophenanthrene ring structure
but differentiating in the side chain at C24 and/or
the position and configuration of unsaturated double
bonds and the optical rotation at chiral carbons.51,52
Phytostanols are produced by hydrogenating
phytosterols.
Sterols include a major percentage of the unsaponi-
fiable matter of most vegetable oils and they exist as
free sterols and sterol esters of fatty acids.
In HN, the total phytosterols content ranges from
133.8 mg/100g to 263 mg/100g of oil. Among them,
Fig. 2: Structures of common dietary phytosterols and cholesterol.53
21NUNZIO, Curr. Res. Nutr Food Sci Jour., Vol. 7(1), 17-28 (2019)
β-sitosterol is the major one with a mean percentage
of 83.6%, while ∆5-avenasterol and campesterol
are the second and the third components of
the group with mean values of 6.1% and 5.8%,
respectively.12 Other minor phytosterol/phytostanol
found in HN are sitostanol, stigmasterol, cholestenol,
campestanol, ∆7-campesterol, ∆5,23-stigmastadienol,
∆5,24-stigmastadienol, clerosterol, ∆7-stigmastenol,
and ∆7-avenasterol.10 The HN oil sterol composition
is influenced by agronomic and environmental
conditions, crop season, cultivar as well as storage
conditions and oil extraction methods.2 In particular,
the percentage of sterol esters ranged from 11% to
75%, mainly depending on the refined process and
geographical origin. It is remarkable that sterol esters
of Turkish HN oils (either crude or refined) included
more than 40%, while they were less than 35% in
HN oils from Franch, Italy and Spain, the lowest
values being for roasted crude HN oils (11–16%).16
Phytosterols are well-known for their ability of
reducing blood cholesterol. In fact, many studies
have demonstrated that phytosterols induce clinically
significant reductions in low-density lipoprotein
cholesterol (LDL-C) levels.54 In particular, daily
dose of 1.5 g - 3 g of phytosterols, phytostanols
and their esters have been suggested for lowering
total cholesterol (TC) and LDL-C concentration
significantly.34 One of the most suggested
mechanisms of action of phytosterols in lowering
plasma cholesterol concentration is their capacity
to reduce cholesterol absorption at intestinal
level. In fact, phytosterols are structurally similar
to cholesterol and are assimilated into micelles in
the intestinal tract. Since plant sterols are more
hydrophobic than cholesterol, they possess a higher
inclination for micelles than they have for cholesterol.
Consequently, they displace cholesterol from mixed
micelles and determine a reduction in the duodenal
cholesterol absorption and a higher fecal excretion
of cholesterol.55 Furthermore, in vitro and in vivo
studies suggest that phytosterols content in diet
promotes a decrease in various cancers including
colon, breast and prostate cancer by slowing cell
cycle progression, inducing apoptosis, and inhibiting
tumor metastasis.53,56-58
Squalene
Squalene is a highly unsaturated all-trans linear
terpenoid hydrocarbon which comports as the
biochemical precursor of terpenoids and sterols
with their central role in human, animal and plant
functions.59,60
It is extensively present in nature, and considerable
quantities are found in oil from shark and whale liver,
wheat-germ, palm, rice bran, olive and amaranth.61
Among nuts, squalene content was higher in HN >
macadamia > peanuts > almonds > walnut,62 with
a value in HN ranging from 93 mg/kg to 885 mg/kg
oil, depending by cultivar, environmental conditions,
geographical origin, fruit development and the
method of squalene extraction.2,63-65 In particular for
the latter, squalene contents appeared higher in HN
oil extracted with solvent compared to cold pressed
one, probably due to higher squalene solubility in
hexane.2
From a nutritional perspective, squalene has
important beneficial effects on health, mainly
related to its hypolipidemic, anticancer, antioxidant
and detoxifying activity.66 Enriched squalene diet
significantly increased paraoxonase 1and high-
density lipoprotein cholesterol (HDL-C) and reduced
oxidative damage in animals.67
In parallel to its plasma lipids lowering effect,
experimental studies have revealed that squalene
may efficiently prevent chemically-induced skin, lung
Fig. 3: Structure of squalene in coiled form.60
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and colon tumorigenesis in rats. The mechanisms
implicated for the chemopreventive action of
squalene can comprise modulation of carcinogen
activation, anti-oxidative activities and inhibition
of Ras farnesylation.68,69 Furthermore, various in
vivo and in vitro studies suggested that squalene
possesses an antioxidant activity, principally acting
as radical scavenger, and may protect different
biological molecules as DNA, lipids and protein
against oxidative stress.70-74
Phenols
The health effects of diets rich in fruits and vegetables
are due not only to minerals, vitamins and fibre but
also to a variety of plant secondary metabolites
referred collectively as polyphenols,75 to which
many biological effects have been attributed.76-78
The preponderance of polyphenols in plants exist
as glycosides with diverse sugar units at different
positions of the polyphenol skeletons and have
been categorized by their biological function, source
of origin and chemical structure.79-80 According to
the chemical structures of aglycones, polyphenols
may be classified in flavonoids, stilbenes, lignans,
flavonoids, hydrobenzoic and hydroxycinnamic
acids.81
The presence of several phenolic and hydroxycinnamic
acids (sinapic acid, gallic acid, p-coumaric acid,
caffeic acid, vanillic acid, protocatechuic acid, ferulic
acid,), and flavonoids (catechin, quercetin, myricetin,
kaempferol) have been reported in HNs. In particular,
the main polyphenolic subclass comprises of mono-
and oligomeric flavan 3-ols, which accounts between
34.2 and 58.3% in HN kernels, with a total phenolic
content ranging from 491.2 to 1700.4 mg of gallic
acids equivalent/kg.82
Moreover, roasting increase the phenolic content
in a time and temperature dependent manner
compared to raw HNs.83 Numerous epidemiological
and nutritional evidences suggest that natural
polyphenols play a key role in prevention of cancer,84
and in particular Li & Parry have shown that extract
of HN roasted skin cultivated in Oregon significantly
reduce the proliferation of a human colon cancer
cell line.85
Table 1. HS Nutrient Composition
Total lipids > 60% d.w.
Oleic acid 76.7% - 82.8%
Linoleic acid 9.2%
Palmitic acid 5.6%
Stearic acid 2.7%
Vaccenic acid 1.4%
Fibre 8.05 g/100g - 12.07 g/100g
K 147 mg/100 – 761 mg/100mg
P 256 mg/100g – 458 mg/100g
Ca 65 mg/100g – 328 mg/100g
Mg 34 mg/100g – 335 mg/100g
Cu 0.94 mg/100g – 3.47 mg/100g
Mn 1.4 mg/100g – 19 mg/100g
Se 5.5 μg/100 – 60 μg/100
Tocols 41.9 mg/100g extracted oil
Total phytosterols 133.8 mg/100g – 263 mg/100g
Total sterol esters 11% - 75%
β-Sitosterol 83.6%
∆5-Avenasterol 6.1%
Campesterol 5.8%
Squalene 93 mg/kg oil – 885 mg/kg oil
Total polyphenols 0.491 g/kg oil – 1.7 g of gallic acid equivalent/kg
Flavan 3-ols 34.2% - 58.3%
23NUNZIO, Curr. Res. Nutr Food Sci Jour., Vol. 7(1), 17-28 (2019)
Health Effects of HNs Consumption
Even though a significant number of clinical
studies on various tree nuts have been realized,
only few studies specifically related to HN have
been conducted. In a recent systematic review and
bayesian meta-analysis, Perna et al. evidenced that
HN-enriched diet is associated with a decrease of
LDL-C and TC equal to -0.150 mmol/L and -0.127
mmol/L, respectively, in favour of a HN-enriched
diet.86 More recently, this trend was also confirmed
by Deon et al., who reported in adolescents with
primary hyperlipidemia a significant effect on serum
LDL-C, HDL-C/LDL-C ratio and non-HDL-C.87 Similar
results in the reduction of serum LDL-C were also
observed by Santi et al. in healthy volunteers.88 At the
same time, HN consumption was able to decrease
LDL oxidation (-15.7%) in normolipidemic healthy
volunteers89 and plasma inflammatory markers
such as high-sensitivity C-reactive protein (-35.9%),
soluble vascular cell adhesion molecule-1 (-10.6%)
and soluble intercellular adhesion molecule-1
(-8.08%) in hypercholesterolemic subjects, compared
to a control diet.90
Future Directions
At present, considering the world production of HNs,
another relevant challenge for the sector could be
to turn food processing by-products and wastes
into new ingredients. In fact, during processing of
HN, by-products arise as waste materials. Among
them, none has any commercial value except
the HN hard shell, which is currently used as a
heating source upon burning. HN wastes could
represent functional ingredients to take advantage
of to improve nutritional and health value of foods.
To do it, comprehensive studies of their chemical
composition, physical structure, sensorial properties
and nutritional characteristics are necessary.
Conclusion
The expansion of studies and investigations intended
to exam the effectiveness of functional nutrients
and food components has illuminated many parts
of the multifaceted connection between nutrition
and health. Despite, we have to take in mind that
our diet is based on foods and not on individual
compounds. Consequently, it is fundamental to
demonstrate that certain nutrients have a positive
effect in the prevention of a disease, and to
recognize which foods possess them at relevant
concentration.91 Moreover, foods are complex
matrices in which those components could have
synergistic effects, and bioaccessibility may be
influenced by both gastrointestinal conditions and
chemical characteristics of the food matrix.92-97
HN is an example of synergism among nutrients that
can be transformed into a large variety of products
consumed by a wide range of population every day.
Studies reported in this review underscore the health-
promoting effects of HN nutrients and consumption.
At present, more scientific confirmations are needed
to regard HNs as functional food, but results are
auspicious and there are various elements of great
attention that push the researchers to expand the
scientific acquaintance about nuts in general and
HNs in particular.
Conflict of Interest
The Author reports no potential conflict of interest.
Acknowledgement
This work was partially supported by a Grant of
Italian MIUR (RFO M.D.N.)
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