foods
Article
Stinging Nettles as Potential Food Additive: Effect of Drying
Processes on Quality Characteristics of Leaf Powders
Swathi Sirisha Nallan Chakravartula
1
, Roberto Moscetti
1,
* , Barbara Farinon
1
, Vittorio Vinciguerra
1
,
Nicolò Merendino
1
, Giacomo Bedini
1
, Lilia Neri
2
, Paola Pittia
2
and Riccardo Massantini
1,
*
Citation: Nallan Chakravartula, S.S.;
Moscetti, R.; Farinon, B.; Vinciguerra,
V.; Merendino, N.; Bedini, G.; Neri, L.;
Pittia, P.; Massantini, R. Stinging
Nettles as Potential Food Additive:
Effect of Drying Processes on Quality
Characteristics of Leaf Powders.
Foods 2021, 10, 1152. https://doi.org/
10.3390/foods10061152
Academic Editor: Hans Verhagen
Received: 9 April 2021
Accepted: 18 May 2021
Published: 21 May 2021
Publisher’s Note: MDPI stays neutral
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iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1
Department for Innovation in Biological, Agro-Food and Forest Systems, University of Tuscia,
01100 Viterbo, Italy; [email protected] (S.S.N.C.); [email protected] (B.F.);
2
Faculty of Bioscience and Technologies for Food, Agriculture, and Environment, University of Teramo,
01100 Viterbo, Italy; [email protected] (L.N.); [email protected] (P.P.)
* Correspondence: [email protected] (R.M.); [email protected] (R.M.); Tel.: +39-0761-35-75-39 (R.M.);
+39-0761-35-74-96 (R.M.)
Abstract:
Stinging nettle (Urtica dioica L.) is a ubiquitous, multi-utility, and under-utilized crop with
potential health benefits owing to its nutritional and bioactive components. The objective of the work
is to produce powders by drying wild stinging nettle leaves as a storable, low-cost functional additive
to be used in bakery and ready-to-cook products. Convective drying (CD) and freeze-drying (FD)
were applied on unblanched (U) or blanched (B) leaves, which were then milled to nettle powders
(NPs). The obtained NPs were evaluated for selected physicochemical (moisture, color), techno-
functional (flow indices, hygroscopicity), and phytochemical (pigments, phenols) characteristics as
well as mineral contents. Blanching improved mass transfer and reduced the oxidative degradation
of pigments during drying, but it caused a loss of total phenols content, antioxidant activity, and
potassium content. As for the drying method, CD resulted in better flow properties (i.e., Carr Index
and Hausner Ratio), while FD retained better the color, pigments, magnesium content, phenolic, and
antioxidant parameters. Overall, the evaluated processing methods resulted in different technological
properties that can allow for better evaluation of NPs as a food additive or ingredient. Among the
NPs, blanched and freeze-dried powders despite showing inferior technological properties can be
recommended as more suitable ingredients targeted f or food enrichment owing to better retention of
bio-active components.
Keywords: Urtica dioica L.; drying; powder; technical properties; functional properties
1. Introduction
The rising consumer interest in functional foods and demand for ‘clean label’ food is
pushing the food industry to rediscover the use of wild plants and potherbs. Their bioactive
constituents and various physiological benefits at the molecular level has propelled research
not only in the direction of medicine but also technologically to have industrially adaptable
raw and auxiliary materials.
Among the vast selection of wild plants, Urtica dioica L., commonly known as stinging
nettle of family Urticaceae, is a spontaneous, ubiquitous, and perennial plant and a known
weed in intensive farming. It is traditionally used as medicine and is a multi-purpose
commercial crop used for pharmaceutical extracts, textile fibers, and as colorant (chloro-
phyll, E140) [
1
–
3
]. Moreover, nettle leaves and extracts were found to contain various
phytochemicals such as organic acids and phenolic compounds (e.g., flavonoids) that
render them with diuretic, anti-diabetic, and anti-inflammatory activities [
4
,
5
]. However,
water extract showed very little anti-hyperglycemic and antiglycation activities in induced-
diabetic rats [
6
]. In fact, the major contributing constituents of nettle were found to be cyclic
hydrophilic proteins [
6
], which are usually absent or in very low quantity in water extract.
Foods 2021, 10, 1152. https://doi.org/10.3390/foods10061152 https://www.mdpi.com/journal/foods
Foods 2021, 10, 1152 2 of 15
In addition, nettle leaves are good sources of lutein, ß-carotene, vitamins (A, C), minerals
(Ca, K, Fe), and phenols in general [
7
–
9
], especially the well-documented chlorogenic acid,
caffeic acid, and kaempferol-3-rutinoside [
2
], which can inhibit the glycation process [
10
].
These attributes make nettle leaves a potential source of bio-active components for utiliza-
tion as a potherb and especially as whole ingredient in functional foods [
11
,
12
] as leaves,
stems, and powders thereof.
Despite the various advantages mentioned, nettles remain under-utilized in the food
sector due to seasonality and other reasons such as market stigma i.e., market resistance
due to a consumer perception of nettle as famine/poor man’s food with low sensorial qual-
ity and undesirability due to stinging, as duly pointed by Shonte and de Kock (2017) [
13
].
As most vegetables and fruits, nettles are also perishable, requiring post-harvest oper-
ations to extend the storability and consumption period. Among various preservation
technologies, an effective, viable, and widely used industrial process for seasonal foods to
ascertain their economic value is drying. Drying with hot air or convection is a widely used
method, even though it is energy intensive and detrimental to the product’s nutritional
quality [
14
,
15
]. As for other available methods, freeze drying, although expensive, has
been gaining commendable interest among food producers as an efficient technology that
retains better nutritive and functional quality owing to the sublimation process at low
temperatures [15,16].
With respect to the drying processes of nettle leaves, the available literature focused
on drying and its optimization based on chemical and nutritional characteristics. Of these,
Adhikari et al. (2016) [
7
] have comparatively characterized the proximate composition and
selected functional components (tannins, polyphenols, and antioxidants) for solar dried
leaves to that of wheat and barley flours as a potential ingredient. Shonte et al. (2020) [
5
]
and Shonte and de Kock (2017) [
13
] in different studies observed that oven drying in general
adversely affected the nutritional, functional, and sensorial properties of dried nettle leaves
and their infusions, respectively. Similarly, Alibas (2007) [
17
] reported longer drying
times in convection and microwave drying negatively affected the color of the dried nettle
leaves. Movagharnejad et al. (2019) [
18
] found that at higher microwave power, shorter
infrared lamp distance nettle leaves had shorter drying cycles and better release of phenolic
compounds. Among different drying methods such as convection, freeze, microwave, air,
and solar drying, Branisa et al. (2017) [
16
] recommended freeze drying as a preferable
method for the optimal retention of pigments, antioxidant content, and phenolic content in
nettle leaf powders. These results from different studies highlight that the drying processes,
particularly those with high temperatures, may deteriorate the nutritional and functional
quality of nettle products. A well-known pretreatment used to reduce the degradation
of quality during drying is blanching, which as a stand-alone treatment was observed by
Rutto et al. (2013) [
19
] to retain beneficially the mineral and vitamin contents in nettle
leaves. Furthermore, utilizing a pretreatment such as blanching was found to improve the
product quality by inhibiting enzymatic activity as well as the drying efficiency, facilitating
mass transfer [
20
,
21
]. However, no studies were found to systematically characterize the
effects of pretreatment in combination with drying conditions on the techno-functional
properties in relation to processing factors.
The present work aims to evaluate the feasibility of using blanching pretreatment in
combination with two types of drying to obtain nettle powder as a potential additive or
functional ingredient. The paper wants to contribute to the existing studies by enriching
the available knowledge with information on some physicochemical, techno-functional,
and nutritional properties of the obtained nettle powders.
2. Materials and Methods
2.1. Nettle Powder Preparation
Wild stinging nettle leaves were collected from Latium region (42.7477
◦
N, 11.8630
◦
E),
Italy, in multiple harvests (3 lots) during the period of June–July 2019. Each set of harvested
leaves was manually separated from the stem, cleaned, and split into two batches; one was
Foods 2021, 10, 1152 3 of 15
not blanched (unblanched, U), while the other was blanched (B). The leaves were blanched
using a sous-vide cooker (Model 10030542, Klarstein, Berlin, Germany) at 90
◦
C for 1 min,
cooled in ice-cold water for 1 min, and drained. The optimal combination of blanching
temperature and time was chosen from preliminary tests in agreement with Moscetti et al.
(2017) [
22
]. Each batch of leaves were minced using a domestic blender and split in two
equal parts on weight basis. One part was convective dried (CD) at 40
◦
C for about 12–14 h
in a cabinet drier (Innotech, Leonberg, Germany), with an air flow of 1 m s
−1
. The other
part was frozen at
−
80
◦
C and freeze-dried (Modulyo, Edwards) at
−
48 to
−
55
◦
C for 28 h.
The dried leaves after each treatment were ground in a laboratory scale mill (MF 10,
IKA
®
-Werke, Staufen, Germany) using a 0.50 mm sieve with minimal heat production. The
obtained nettle powders, namely UCD (unblanched, convective-dried), UFD (unblanched,
freeze-dried), BCD (blanched, convective-dried), and BFD (blanched, freeze-dried) were
weighed and stored in airtight glass bottles in dark at ≈4
◦
C until further analyses.
The nettle powder yield was calculated as the amount of powder obtained after
milling to that of the non-blanched or blanched material prior to drying and expressed as g
NP/100 g of material used.
2.2. Physicochemical and Techno-Functional Characteristics
The moisture content of both fresh leaves and differently dried powders were de-
termined by the hot-air oven method (AOAC, 2000) [
23
] at 105
◦
C. Water activity (a
w
)
was measured using a benchtop water activity meter (Acqualab, Decagon devices Inc.,
Pullman, WA, USA). The bulk density (
ρ
β
, g cm
−3
) was read as the loose volume of 5 g of
NP weighed into a 50 mL graduated cylinder. The cylinder was tapped repeatedly from a
standard height as described by Caparino et al. (2012) [
24
], and the leveled volume was
used to calculate the tapped bulk density (
ρ
T
, g cm
−3
). The Carr Index (CI, Equation (1))
and Hausner Ratio (HR, Equation (2)) to evaluate the flowability and cohesiveness were
calculated according to the equations given by Koç and Dirim (2018) [25]:
Carr Index
(
CI
)
=
(
ρ
T
− ρ
B
)
ρ
T
× 100, (1)
Hausner Ratio
(
HR
)
=
ρ
T
ρ
B
, (2)
Hygroscopicity was determined according to Koç and Dirim (2018) [
25
] with pre-
weighed samples (0.5 g each) placed over a saturated solution of NaCl (R.H. 75%) at
20 ± 1
◦
C
, weighed after 8 days. Results were expressed as moisture adsorbed in g per
100 g of NP.
Water-holding capacity (WHC) and water solubility index (WSI) were determined
according to Ahmed et al. (2014) [
26
] on 0.5 g samples added with 10 mL of deionized water
with few modifications. The samples were agitated mechanically (CertoMat, Germany) for
6 h at 20
◦
C, followed by centrifugation (NEYA 16R, Remi Elektrotechnik Ltd., Vasai, India)
at 8000
×
g, 15
◦
C for 15 min. The residue in centrifuge tubes (50 mL) was weighed, and the
WHC was calculated as weight of water held per gram of sample (g water g
−1
NP, dw).
The supernatant collected was dried at 105
◦
C until constant weight, and the dried residue
weight was used to calculate the WSI (%).
The color of fresh leaves and powders was measured in the CIELab color space using
a colorimeter (CM-2600d, Konica Minolta, Japan) with a D65 illuminant. The chromatic
parameters were expressed as lightness (L*), redness (a*), yellowness (b*), chroma (C*),
and hue angle (h) as well as CIELab color difference (∆E*) of dried powders against fresh
leaves [22].
2.3. Phytochemicals
Chlorophyll a (Chl a), chlorophyll b (Chl b), and total carotenoids (TC) were deter-
mined by procedure adapted from Wellburn (1994) [
27
] with some modifications: briefly,
100 mg of sample was weighed, added with 10 mL of acetone (99% pure), and extracted
Foods 2021, 10, 1152 4 of 15
with ultrasound for 2 min (1st extraction). The extraction was repeated, increasing the
time to 5 min for 2nd and 3rd extractions and 10 min for 4th and 5th extractions. The
supernatant was pooled, made up to 50 mL, and centrifuged at 1000
×
g for 5 min at
15
◦
C
. Subsequently, aliquots were read for absorbance in range of 380 to 750 nm. Pigment
concentrations (
µ
g mL
−1
extract) and contents (mg 100 g
−1
of NF, dw) were calculated
according to Lichtenthaler and Buschmann (2001) [
28
] using the equation for acetone and
Ðurovi´c et al. (2017) [8] using Equations (3)–(6),
Chlorophyll a
µg
mL o f extract
= 11.24 A
661.6
− 2.04 A
644.8
, (3)
Chlorophyll b
µg
mL o f extract
= 20.13 A
644.8
− 4.19A
661.6
, (4)
Total Carotenoids
µg
mL o f extract
=
(
1000 A
470
− 1.9 c
a
− 63.14c
b
)
/214, (5)
m
mg o f pigment
g o f NP
=
C × V × D
f
G1000
, (6)
In Equation (6), C = concentration of chlorophyll a (c
a
), b (c
b
), or total carotenoids
calculated using formulas (3)–(5); V = volume of acetone; D
f
= dilution factor; G = initial
mass of the NP sample. Final values were presented as mg pigment/100 g nettle powder.
2.4. Antioxidant Capacity
One hundred milligrams of samples were added with 10 mL of methanol:water (95:5
v/v) solution in an orbital shaker (mod. 711, Tecnochimica Moderna S.r. l, Italy) in dark at
ambient temperature for 24 h. The extract was centrifuged (mod. PK121R, ALC, Italy) at
8000
×
g for 5 min at 15
◦
C. The supernatant was collected and stored at
−
80
◦
C until further
analyses. In the case of TPC, only one extraction was used: preliminary tests demonstrated
that additional steps did not significantly improve phenols extractability.
Total phenolic content (TPC) was determined by the Folin–Ciocâlteu standard method
with modifications [
29
]. The assay was conducted by mixing 4 mL of deionized water,
0.25 mL
of diluted extract, 0.25 mL of diluted Folin–Ciocâlteu reagent (1:1), and 0.5 mL of
30% (w/v) Na
2
CO
3
. The mixture was stored in dark at room temperature for 30 min, and an
aliquot was measured for absorbance at 725 nm using UV-spectrophotometer (UVikon-942,
Kontron Instruments, Ztirich, Switzerland). The results were expressed as mg of gallic acid
equivalents (GAE) per gram of sample, dw.
Antioxidant capacity was assessed by both ferric reducing antioxidant power (FRAP)
and Trolox equivalent antioxidant capacity (TEAC) assays.
The FRAP assay was performed according to the method described by Benzie and
Strain (1999) [
30
] adapted for 96-well plates and automatic reader (Infinite 2000, Tecan,
Salzburg, Austria): 160
µ
L of FRAP assay solution (consisting of 20-mM ferric chloride
solution, 10-mM TPTZ solution, and 0.3-M acetate buffer, pH 3.6) was freshly prepared and
mixed with 10
µ
L of diluted sample (varied dilutions among samples, ratios not shown),
standard or blank and dispensed into each well of a 96-well plate. The absorbance was
measured at 595 nm after an incubation of 30 min in dark at 37
◦
C.
The TEAC assay was carried out using the OxiSelectTM TEAC Assay Kit (Cell Biolabs
Inc., San Diego, CA, USA) according to the manufacturer instructions: 150
µ
L of ABTS
reagent diluted 1:50 times in 75% ethanol freshly prepared was added to 25
µ
L of diluted
sample (varied dilutions among samples, ratios not shown) and pipetted into each well of a
96-well plate. After 5 min incubation on an orbital shaker, the absorbance was measured at
405 nm. Results were expressed as mM of Trolox equivalents (TE) per gram of
sample, dw.
Foods 2021, 10, 1152 5 of 15
2.5. Mineral Content
Monovalent and divalent cations (i.e., Na
+
, K
+
, Mg
2+
and Ca
2+
) were determined by
Ionic Chromatography equipped with a conductivity detector (IC-CD) according to the
method described by Cataldi et al. (2003) [
31
]. Briefly, 4.0 mL of 5.0-mM HCl was added to
≈
15 mg of sample, agitated on a shaker for 15 min, and centrifuged at 3000 rpm for 10 min.
The resulting supernatant was diluted, filtered, and injected in the ionic chromatography
system consisting of a LC-10ADvp solvent delivery pump and a CDD-10Avp conductivity
detector (Shimadzu Corporation, Japan). The cation separation was carried out with a
Universal Cation HR column (4.6 mm
×
100 mm; Alltech Associates Inc., Deerfield, IL,
USA) and eluted with aqueous 1.5-mM H
2
SO
4
at a flow rate of 1 mL min
−1
.
Total iron quantification was performed using an inductively coupled plasma optical
emission spectrometer (ICP-OES, Optima 8000 DV, PerkinElmer, Waltham, MA, USA)
with an axially viewed configuration, equipped with an ultrasonic nebulizer, quartz torch,
and quartz detector operating in the following conditions: RF-power of 1450 W, auxiliary
gas flow rate of 0.3 L min
−1
, plasma gas flow rate of 10 L min
−1
, nebulizer flow rate of
0.65 L min
−1
, and sample aspiration flow rate of 1.5 mL min. The external calibration
solutions were prepared from standard certified elemental solutions (CaPurAn) and Milli-
Q water containing 3% HNO
3
to get a range of concentrations (0.5 to 40 mg L
−1
). The
sample (200 mg) was subjected to microwave-assisted (Mars plus CEM, Bergamo, Italy)
acid digestion by the addition of 7.5 mL HNO
3
, 0.5 mL HCl, and 2.0 mL H
2
O
2
(30%). A
one-step heating program was used for 37 min from 25 to 180
◦
C and 15 min at 180
◦
C at
1200 W. After cooling, the digested sample solutions were carefully transferred into 25 mL
volumetric flasks to quantify prior to analysis by ICP-OES. Reagent blanks were prepared
containing the same reagents as the samples.
2.6. Statistical Analysis
All measurements were carried out in triplicates except for density, which was dupli-
cated. The data were elaborated by two-way analysis of variance (ANOVA) with Tukey’s
honest significant differences (HSD) as post hoc test (
α
= 0.05) using R-software (R-studio,
version 3.6.2) and the ‘agricolae’ R-package. Results were arranged in tables and expressed
as mean
±
standard deviation for both main effects (i.e., pretreatment and drying) and the
interaction effect (i.e., pretreatment × drying).
3. Results and Discussion
3.1. Physicochemical and Techno-Functional Characteristics
3.1.1. Moisture Content and Water Activity
The nettle powders obtained (Section 2.1) were visually uniform without aggregates
and ranged from a dark green to an olive-green color. The yields ranged from 11.29
to
14.81 g 100 g
−1
and were lowered by blanching pretreatment as expected due to the
leaching of solids and lower moisture content post-processing (Table 1). The moisture
content lowered from 87 to 89 g 100 g
−1
in fresh nettle leaves to 3 to 11 g 100 g
−1
in the
dried powders and was apparently affected by the interaction of pretreatment and drying
conditions (p < 0.05, Table 1). The use of freeze drying, particularly in combination with
blanching, lowered the moisture content as opposed to that observed by Shonte et al.
(2020) [
5
], wherein freeze-dried (6.4 g 100 g
−1
) nettles registered higher values than the
oven-dried (3.4 g 100 g
−1
) nettles. This difference observed might be attributed to the size
reduction of the nettle leaves prior to drying and the drying conditions used in this study.
The water activity was in accordance with the moisture content with values below 0.60
indicative of microbial and bio-chemical stability.
Foods 2021, 10, 1152 6 of 15
Table 1.
The interaction of pretreatment and drying factors on the physicochemical and technological properties of nettle
powders. Data are mean
±
standard deviation of the mean. Mean values belonging to the same factor without common
letters are statistically different according to the Honestly Significant Difference or HSD (p ≤ 0.05).
Factor
Yield
(%)
Moisture Content *
(g 100 g
−1
)
Water Activity
(a
w
)
Bulk Density
(g cm
−3
)
Tapped Density
(g cm
−3
)
Pretreatment (PR)
Unblanched (U) 13.99 ± 1.93a 8.06 ± 2.28 0.42 ± 0.12 0.30 ± 0.13 0.44 ± 0.10
Blanched (B) 11.67 ± 1.14b 6.79 ± 3.13 0.34 ± 0.17 0.30 ± 0.14 0.45 ± 0.13
p value ≤0.05 ≤0.05 ≤0.05 ns ns
HSD 0.035
Drying (DR)
Convective drying (CD)
13.05 ± 2.24 9.89 ± 0.29 0.51 ± 0.02 0.41 ± 0.02a 0.54 ± 0.02
Freeze drying (FD) 12.61 ± 1.77 4.96 ± 1.12 0.24 ± 0.07 0.18 ± 0.01b 0.35 ± 0.02
p value ns ≤0.05 ≤0.05 ≤0.05 ≤0.05
HSD 0.001
PR × DR
U × CD 14.81 ± 1.27 10.13 ± 0.14a 0.53 ± 0.01a 0.41 ± 0.02 0.53 ± 0.02b
U × FD 13.18 ± 2.38 5.98 ± 0.02c 0.30 ± 0.01c 0.19 ± 0.01 0.36 ± 0.01c
B × CD 11.29 ± 1.30 9.64 ± 0.10b 0.49 ± 0.01b 0.42 ± 0.01 0.56 ± 0.01a
B × FD 12.04 ± 1.07 3.93 ± 0.07d 0.18 ± 0.02d 0.18 ± 0.01 0.34 ± 0.01c
p value ns ≤0.05 ≤0.05 ns ≤0.05
HSD 0.001 0.001 0.02
ns = no significant difference; * nettle leaves initial moisture content 86.55 ± 0.84 (U) and 88.52 ± 1.03 (B) g 100 g
−1
.
3.1.2. Flow Properties
Table 1 reports the flow properties affecting the product reconstitution, packaging,
handling, and storage properties of the NPs. The bulk density (
ρ
β
) was significantly
influenced by the drying method irrespective of blanching factor, whereas the tapped bulk
density (
ρ
T
) by the interaction of factors with FD nettle powders having lower values than
the CD nettle powders (p < 0.05).
The differences in
ρ
β
can be attributed to the drying method, wherein the sublimation
of ice at temperatures lower than initial product’s glass transition temperature in freeze-
drying resulted in a porous structure. This was primarily due to the absence of liquid
transfer from the layers to the surface of the plant tissues as in CD, thereby preventing the
tissue shrinkage and structural collapse. Similar observations were made in FD black plum
powders [
32
] and freeze-dried mango powders [
24
] with
ρ
β
values of 387 and
≈400 kg m
−3
,
respectively, which were higher than those of freeze-dried NPs (
≈200 kg m
−3
)
. As for the
tapped bulk density (
ρ
T
), the values were in general higher than
ρ
β
due to compact
arrangement of particles as also observed in dried maple syrup by Bhatta et al. (2019) [
33
].
The values were double those of their respective
ρ
β
values in case of FD powders indicative
of voids and a higher amount of occluded air that pack loosely in comparison to CD
powders (Table 1).
Furthermore, the handling properties namely, flowability and cohesiveness given
by Carr Index (CI) and Hausner Ratio (HR) behaved in a similar fashion to that of
ρ
T
and
ρ
β
values, respectively (Table 2). Freeze-dried NPs, irrespective of the pretreatment,
exhibited high cohesiveness (HR > 1.4, poor) and high compressibility (CI > 45%, poor)
despite low moisture contents, which can be attributed to their lower density and increased
inter-particle forces as compared to that of CD powders [
33
]. This indicates that that pure
FD nettle powder can be less efficient in production and processing than CD nettle powder,
potentially leading to additional load on the sieves, downtime due to clogged conveyor
lines, improper discharge from bins, and process downtime [
34
]. A good understanding of
the flowability of an ingredient is fundamental to adapt processing conditions and, thus,
obtain a high-quality product [
34
]. In addition, Bhatta et al. (2019) [
33
] observed better
Foods 2021, 10, 1152 7 of 15
flow characteristics for spray-dried maple syrup powders with higher moisture, wherein
the moisture was hypothesized to act as a lubricant, thereby improving the flowability.
Table 2.
The interaction effect of pretreatment (PR) and drying (DR) factors on the physicochemical and technological
properties of nettle powders. Data are mean
±
standard deviation of the mean. Mean values belonging to the same factor
without common letters are statistically different according to the Honestly Significant Difference or HSD (p ≤ 0.05).
Factor
Carr Index
(%)
Hausner Ratio
Hygroscopicity
(g H
2
O 100 g DW
−1
)
WHC
(g H
2
O g DW
−1
)
WSI
(%)
Pretreatment (PR)
Unblanched (U) 36.00 ± 15.03 1.63 ± 0.38 7.49 ± 2.90 6.18 ± 0.32 11.90 ± 1.47
Blanched (B) 36.00 ± 12.73 1.60 ± 0.35 8.59 ± 4.27 6.14 ± 1.19 5.97 ± 0.57
p value ns ns ns ns ≤0.05
HSD
Drying (DR)
Convective drying (CD) 24.00 ± 1.15 1.30 ± 0.01b 4.85 ± 0.42b 5.49 ± 0.49 8.14 ± 2.73
Freeze drying (FD) 48.00 ± 1.63 1.93 ± 0.05a 11.23 ± 1.66a 6.83 ± 0.43 9.72 ± 3.82
p-value ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05
HSD 0.001 0.001
PR × DR
U × CD 23.00 ± 0.01b 1.30 ± 0.01 4.86 ± 0.61 5.92 ± 0.12c 10.62 ± 0.58b
U × FD 49.00 ± 1.41a 1.95 ± 0.07 10.12 ± 0.14 6.45 ± 0.17b 13.17 ± 0.39a
B × CD 25.00 ± 0.01b 1.30 ± 0.01 4.84 ± 0.24 5.06 ± 0.20d 5.67 ± 0.06c
B × FD 47.00 ± 1.41a 1.90 ± 0.01 12.34 ± 1.78 7.22 ± 0.01a 6.27 ± 0.73c
p value ≤0.05 ns ns ≤0.05 ≤0.05
HSD 0.05 0.001 0.01
ns = no significant difference; WHC = water-holding capacity; WSI = water solubility index.
3.1.3. Hygroscopicity, Water-Holding Capacity and Water Solubility Index
Hygroscopicity, as a critical parameter affecting the flowability and storage stability,
ranged from 4.8 to 12.5 g 100 g
−1
and was significantly influenced by the drying method
(Table 2). The observed values were only slightly higher than the cut-off values (5.13 to
9.38 g 100 g
−1
) arbitrarily used by Caparino et al. (2012) [
24
] for mango powders (16 to
20 g 100 g
−1
)
. Freeze-dried NPs presented higher values consistent with their lower water
activity and higher moisture gradient in test environment resulting in higher moisture
adsorption. This hygroscopic behavior of FD nettle powders further confirms their poor
flowability and higher cohesiveness.
WHC and WSI, affecting the rehydration capacity and formulation characteristics (e.g.,
the amount of water added to a formulation), were influenced by the interaction of factors
with blanching and freeze drying synergistically increasing the values (Table 2). This can
be attributed to the fact that although blanching degrades the cell structure, freeze drying
induces a higher porosity of the cells with respect to that of convective drying. Ahmed
et al. (2020) [
35
] observed similar trend in banana powders with FD powders (3.2 g g
−1
, db)
having notably higher values due to their finer particle size in comparison to the tray-dried
samples (2.73 g g
−1
, db).
As for the Water Solubility Index, CD powder had a lower value than FD powder only
for unblanched samples. It can be explained as a result of interaction among nutrients due
to exposure to a higher temperature, as seen in Correa et al. (2011) [
36
]. No differences be-
tween drying methods were noted for blanched samples, although blanching significantly
reduced WSI as already observed by Fombang et al. (2017) [37] in Moringa leaves.
3.1.4. Colorimetric Parameters
The colorimetric parameters of fresh nettle leaves used both as reference and to
calculate total color difference (
∆
E*) were L
0
* = 37.10
±
2.71, a
0
* =
−
9.65
±
0.98 and
b
0
* = −19.41 ± 2.12
. The chromatic parameters of the NPs were different from those of
Foods 2021, 10, 1152 8 of 15
fresh leaves and significantly affected by the interaction of factors (Table 3). The
∆
E* value
was lowest for BFD powder, whereas UFD powder showed highest values indicating that
blanching limited the color changes during drying.
Table 3.
The interaction effect of pretreatment (PR) and drying (DR) factors on colorimetric parameters of the nettle powders
obtained from different treatments. Data are mean
±
standard deviation of the mean. Mean values belonging to the same
factor without common letters are statistically different according to HSD (p ≤ 0.05).
Factor
Luminance
(L*)
Redness
(a*)
Yellowness
(b*)
Chroma
(C*)
Hue Angle
(h)
∆E*
Pretreatment (PR)
Unblanched (U) 43.59 ± 2.87 −3.76 ± 0.70 18.77 ± 1.12 19.15 ± 1.23 101.27 ± 1.42 9.14 ± 1.46
Blanched (B) 41.14 ± 0.90 −7.21 ± 1.82 18.50 ± 2.59 19.87 ± 3.07 111.01 ± 2.24 5.61 ± 0.88
p value ≤0.05 ≤0.05 ns ≤0.05 ≤0.05 ≤0.05
HSD
Drying (DR)
Convective drying (CD) 40.89 ± 1.13 −4.34 ± 1.33 16.96 ± 0.91 17.56 ± 0.58 104.48 ± 4.92 7.14 ± 0.97
Freeze-drying (FD) 43.85 ± 2.46 −6.64 ± 2.45 20.32 ± 0.64 21.46 ± 1.34 107.80 ± 5.76 7.62 ± 3.05
p value ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05 ns
HSD
PR × DR
U × CD 41.15 ± 1.48b −3.13 ± 0.02a 17.76 ± 0.06c 18.03 ± 0.05c 99.99 ± 0.10d 7.93 ± 0.70b
U × FD 46.02 ± 0.75a −
4.40
±
0.10b
19.78 ± 0.22b 20.27 ± 0.21b 102.55 ± 0.34c 10.36 ± 0.66a
B × CD 40.62 ± 0.90b −5.55 ± 0.17c 16.16 ± 0.40d 17.09 ± 0.43d 108.97 ± 0.22b 6.35 ± 0.16c
B × FD 41.67 ± 0.61b −
8.87
±
0.11d
20.85 ± 0.38a 22.66 ± 0.39a 113.05 ± 0.14a 4.87 ± 0.53d
p value ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05
HSD 0.01 0.001 0.001 0.001 0.001 0.001
ns = no significant difference; ∆E* = CIELab color difference.
In general, the L* values were higher in NPs than fresh leaves owing to water loss as
also observed in dried curcuma leaves [
38
]. Significantly higher L* value was observed
only for UFD powder, which probably influenced the
∆
E* value. This might be due to the
scattering of the incident light from the increased particle surfaces throughout a range of
angles [39] relative to the smaller size of the powder particles [40].
As for the redness (a*) parameter, the lowest value was observed in BFD powders
followed by BCD, UFD, and UCD. This indicates that blanching, particularly in combination
with FD, reduced the a* value by limiting the enzymatic changes and thermal degradation
of the pigments [
41
]. With respect to yellowness, FD powders showed higher b* values
indicating a higher retention of carotenoids due to reduced heat exposure.
Furthermore, higher hue angle values of blanched and freeze-dried NPs confirm their
higher green tonality relative to lower a* values. As for the color intensity, the chroma had
a similar trend as that of b* values. Relatively lower color saturation was observed in UFD
with higher L* values in comparison to BFD powders, confirming their lower greenness.
3.2. Bio-Active and Nutritional Characteristics of Differently Processed NPs
3.2.1. Phytochemicals
Nettle leaves were found to be rich sources of commercially valued chlorophylls with
Chl a and Chl b ratios ranging from 2.51 to 4.48 in fresh and blanched leaves [
42
]. In the
present study, the Chl a to b ratio ranged from 2.3 to 2.6, wherein the Chl a content was
affected by both the factors (blanching and drying), and Chl b content was affected by
only that of the drying method (Table 4). In case of Chl a, blanched NPs were observed
to have higher values that can be attributed to the protective effect of blanching that was
found to limit enzyme activity and pheophytin formation during drying in mint and basil
leaves [43,44].
Foods 2021, 10, 1152 9 of 15
Table 4.
The interaction effect of pretreatment (PR) and drying (DR) factors on chlorophylls and total carotenoid content
of the nettle powders obtained from different treatments. Data are mean
±
standard deviation of the mean. Mean values
belonging to the same factor without common letters are statistically different according to HSD (p ≤ 0.05).
Factor
Chlorophyll a
(mg 100 g DW
−1
NP)
Chlorophyll b
(mg 100 g DW
−1
NP)
Total Carotenoids
(mg 100 g DW
−1
NP)
Pretreatment (PR)
Unblanched (U) 512.68 ± 105.51b 215.35 ± 38.53 128.13 ± 29.58
Blanched (B) 585.31 ± 155.52a 229.08 ± 55.07 162.49 ± 42.31
p value ≤0.05 ns ≤0.05
HSD 0.005
Drying (DR)
Convective drying (CD) 432.61 ± 21.35b 180.73 ± 3.98b 112.88 ± 13.19
Freeze-drying (FD) 665.38 ± 74.29a 263.70 ± 22.02a 177.75 ± 25.99
p value ≤0.05 ≤0.05 ≤0.05
HSD 0.001 0.001
PR × DR
U × CD 417.54 ± 19.63 180.53 ± 5.67 101.39 ± 4.97d
U × FD 607.82 ± 17.09 250.17 ± 6.50 154.87 ± 4.30b
B × CD 447.68 ± 8.53 180.94 ± 2.72 124.36 ± 3.77c
B × FD 722.94 ± 59.70 277.23 ± 24.92 200.63 ± 10.00a
p value ns ns ≤0.05
HSD 0.014
ns = no significant difference.
As for drying method, generally, both chlorophyll a and b were positively influenced
by freeze-drying method with higher values than convection-dried powders. This can
be attributed to the higher pigment retention in FD powders due to the low temperature
drying, resulting in limited damage and better extractability of the chlorophylls as conse-
quence of a more porous product, meaning that solvents can easily penetrate the matrix
and extract more phytochemicals [
16
,
44
]. Moreover, the low temperatures in freeze drying
prevented the degradation of chlorophylls to pheophytins due to heat exposure. Among
the individual chlorophylls, Chl a registered a significant loss in convective-dried NPs
due to its thermolabile nature. A similar loss of chlorophylls was observed in oven-dried
and freeze-dried nettle and kale leaves by Branisa et al. (2017) [
16
] and by Korus et al.
(2013) [44], respectively.
The total carotenoids content in previous studies ranged between 5.14 and 262 mg
100 g
−1
in nettle leaves depending on the processing methods, maturity, and other agro-
nomic factors [
8
,
9
,
16
]. In this study, the TC ranged from 101.39 to 200.63 mg 100 g
−1
NP,
dw (Table 4) with blanched powders retaining higher contents, particularly when freeze-
dried. The effect of blanching can be attributed to the increased extractability due to cell
disruption as observed in broccoli by-products [
45
] and/or to the inactivation of oxidative
enzymes potentially involved in pigments degradation during drying. Moreover, the low
temperatures and sub-atmospheric pressures in freeze drying better retained carotenoids,
which are sensitive to heat, light, and oxygen, as also observed by Branisa et al. (2017) [
16
]
and Shonte et al. (2020) [5].
Overall, the studied pigments were better preserved in the order BFD > UFD > BCD >
UCD and are in accordance with the trend of chromatic parameters (a* and b*), confirming
the positive effect of blanching and freeze-drying processes on color and pigment stability.
3.2.2. Antioxidant Activity
Nettles are good sources of phenolic compounds with contents ranging from 29 to
129 mg
GAE g
−1
of nettle (dw) and exhibit various biological activities attributed to hydro-
cinnamic acids, flavonoids, and tannins [
5
,
7
,
8
,
18
]. In this study, the phenol contents ranged
from 6.92 to 16.72 mg GAE g
−1
of NP (dw) and were significantly affected by interaction of
Foods 2021, 10, 1152 10 of 15
factors. UFD had the highest total phenols followed by BFD–UCD, with the least values in
BCD powders (Figure 1).
Foods 2021, 10, x FOR PEER REVIEW 10 of 15
dw (Table 4) with blanched powders retaining higher contents, particularly when freeze-
dried. The effect of blanching can be attributed to the increased extractability due to cell
disruption as observed in broccoli by-products [45] and/or to the inactivation of oxidative
enzymes potentially involved in pigments degradation during drying. Moreover, the low
temperatures and sub-atmospheric pressures in freeze drying better retained carotenoids,
which are sensitive to heat, light, and oxygen, as also observed by Branisa et al. (2017) [16]
and Shonte et al. (2020) [5].
Overall, the studied pigments were better preserved in the order BFD > UFD > BCD
> UCD and are in accordance with the trend of chromatic parameters (a* and b*), confirm-
ing the positive effect of blanching and freeze-drying processes on color and pigment sta-
bility.
3.2.2. Antioxidant Activity
Nettles are good sources of phenolic compounds with contents ranging from 29 to
129 mg GAE g
−1
of nettle (dw) and exhibit various biological activities attributed to hydro-
cinnamic acids, flavonoids, and tannins [5,7,8,18]. In this study, the phenol contents
ranged from 6.92 to 16.72 mg GAE g
−1
of NP (dw) and were significantly affected by in-
teraction of factors. UFD had the highest total phenols followed by BFD–UCD, with the
least values in BCD powders (Figure 1).
Figure 1. Bar plots of the interaction effect of pretreatment and drying factors on total phenolic
content (TPC) of nettle powders. Data are mean ± standard deviation of the mean. Mean values
belonging to the same factor without common letters are statistically different according to HSD (p
≤ 0.05).
Similar observations were made by Korus (2011) [20] in kale leaves with the highest
values of phenolic content for unblanched, freeze-dried leaves and the lowest values for
blanched, air-dried kale. The observed trend can be attributed to the combined effect of
blanching and CD lowering the phenols due to component leakage and thermal degrada-
tion, respectively. However, higher phenol contents in nettles oven-dried at 70 °C ob-
served by Shonte et al. (2020) [5] were attributed to the condensation of tannins resulting
in higher values than freeze-dried nettles.
The total antioxidant capacity as reducing capacity and scavenging ability as pre-
sented in Figures 2 and 3 varied between 2.20 to 4.40 and 3.40 to 6.67 mM TE g
−1
NP (dw)
for FRAP and TEAC, respectively.
Figure 1.
Bar plots of the interaction effect of pretreatment and drying factors on total phenolic
content (TPC) of nettle powders. Data are mean
±
standard deviation of the mean. Mean values
belonging to the same factor without common letters are statistically different according to HSD
(p ≤ 0.05).
Similar observations were made by Korus (2011) [
20
] in kale leaves with the highest
values of phenolic content for unblanched, freeze-dried leaves and the lowest values for
blanched, air-dried kale. The observed trend can be attributed to the combined effect of
blanching and CD lowering the phenols due to component leakage and thermal degrada-
tion, respectively. However, higher phenol contents in nettles oven-dried at 70
◦
C observed
by Shonte et al. (2020) [
5
] were attributed to the condensation of tannins resulting in higher
values than freeze-dried nettles.
The total antioxidant capacity as reducing capacity and scavenging ability as presented
in Figures 2 and 3 varied between 2.20 to 4.40 and 3.40 to 6.67 mM TE g
−1
NP (dw) for
FRAP and TEAC, respectively.
Subsequently, higher total antioxidant capacity was observed in UFD nettle powder
owing to its higher phenolic content. In addition, blanching coupled with CD resulted
in lower FRAP values in correspondence to the TPC values. However, no significant
differences were found between TEAC values of BCD, BFD, and UCD powders, which
can be attributed to the fact that TEAC does not consider metal chelation as well as the
different polyphenol structure and assay compound reactivity.
3.3. Mineral Content
Table 5 summarizes the contents of selected minerals calcium, iron, magnesium,
potassium, and sodium as affected by the pretreatment and/or drying factors.
Foods 2021, 10, 1152 11 of 15
Foods 2021, 10, x FOR PEER REVIEW 11 of 15
Figure 2. Bar plots of the interaction effect of pretreatment and drying factors on ferric reducing
antioxidant power (FRAP) of nettle powders. Data are mean ± standard deviation of the mean.
Mean values belonging to the same factor without common letters are statistically different ac-
cording to HSD (p ≤ 0.05).
Figure 3. Bar plots of the interaction effect of pretreatment and drying factors on the Trolox equiv-
alent antioxidant capacity (TEAC) of nettle powders. Data are mean ± standard deviation of the
mean. Mean values belonging to the same factor without common letters are statistically different
according to HSD (p ≤ 0.05).
Subsequently, higher total antioxidant capacity was observed in UFD nettle powder
owing to its higher phenolic content. In addition, blanching coupled with CD resulted in
lower FRAP values in correspondence to the TPC values. However, no significant differ-
ences were found between TEAC values of BCD, BFD, and UCD powders, which can be
attributed to the fact that TEAC does not consider metal chelation as well as the different
polyphenol structure and assay compound reactivity.
Figure 2.
Bar plots of the interaction effect of pretreatment and drying factors on ferric reducing
antioxidant power (FRAP) of nettle powders. Data are mean
±
standard deviation of the mean. Mean
values belonging to the same factor without common letters are statistically different according to
HSD (p ≤ 0.05).
Foods 2021, 10, x FOR PEER REVIEW 11 of 15
Figure 2. Bar plots of the interaction effect of pretreatment and drying factors on ferric reducing
antioxidant power (FRAP) of nettle powders. Data are mean ± standard deviation of the mean.
Mean values belonging to the same factor without common letters are statistically different ac-
cording to HSD (p ≤ 0.05).
Figure 3. Bar plots of the interaction effect of pretreatment and drying factors on the Trolox equiv-
alent antioxidant capacity (TEAC) of nettle powders. Data are mean ± standard deviation of the
mean. Mean values belonging to the same factor without common letters are statistically different
according to HSD (p ≤ 0.05).
Subsequently, higher total antioxidant capacity was observed in UFD nettle powder
owing to its higher phenolic content. In addition, blanching coupled with CD resulted in
lower FRAP values in correspondence to the TPC values. However, no significant differ-
ences were found between TEAC values of BCD, BFD, and UCD powders, which can be
attributed to the fact that TEAC does not consider metal chelation as well as the different
polyphenol structure and assay compound reactivity.
Figure 3.
Bar plots of the interaction effect of pretreatment and drying factors on the Trolox equivalent
antioxidant capacity (TEAC) of nettle powders. Data are mean
±
standard deviation of the mean.
Mean values belonging to the same factor without common letters are statistically different according
to HSD (p ≤ 0.05).
Foods 2021, 10, 1152 12 of 15
Table 5.
The interaction effect of pretreatment (PR) and drying (DR) factors on mineral content of nettle powders obtained
by different processing methods. Data are mean
±
standard deviation of the mean. Mean values belonging to the same
factor without common letters are statistically different according to HSD (p ≤ 0.05).
Factor
Ca
(mg g DW
−1
NP)
K
(mg g DW
−1
NP)
Mg
(mg g DW
−1
NP)
Na
(mg g DW
−1
NP)
Pretreatment (PR)
Unblanched (U) 16.83 ± 0.97 23.1 ± 1.26a 2.59 ± 0.35 0.28 ± 0.02b
Blanched (B) 15.99 ± 1.37 10.85 ± 0.88b 2.26 ± 0.57 0.38 ± 0.03a
p value ns <0.05 <0.05 <0.05
HSD 0.001 0.032 0.001
Drying (DR)
Convective drying (DR) 16.00 ± 0.78 17.17 ± 6.45 2.04 ± 0.30b 0.33 ± 0.07
Freeze drying (FD) 16.82 ± 1.50 16.78 ± 7.11 2.81 ± 0.27 0.33 ± 0.04
p value ns ns <0.05 ns
HSD 0.001
PR × DR
U × CD 16.25 ± 0.94 22.98 ± 1.59 2.29 ± 0.16 0.26 ± 0.02
U × FD 17.41 ± 0.67 23.22 ± 1.19 2.89 ± 0.07 0.29 ± 0.01
B × CD 15.75 ± 0.67 11.36 ± 0.56 1.79 ± 0.12 0.39 ± 0.03
B × FD 16.23 ± 2.02 10.34 ± 0.91 2.73 ± 0.39 0.36 ± 0.02
p value ns ns ns ns
HSD
ns = no significant difference.
Calcium content was not affected by either factors and was lower than that reported by
Ðurovi´c et al. (2017) [
8
] in shade-dried leaves (28.60 mg g
−1
, dw) and Shonte et al. (2020) [
5
]
in oven/freeze-dried nettles (21–23 mg g
−1
, dw). As for magnesium and potassium, the
blanched powders had lower values due to the higher susceptibility of Mg and K to
leaching and were not subjected to the chelating effects of organic matrix unlike cations
of group II. The higher quantity of Mg in FD powder might be attributed to the increased
extractability. As for Na content, it relatively increased probably due to better extractability
as also observed in nettles blanched or cooked in salt water by Rutto et al. (2013) [19].
Iron is the only trace element, and the mean values were 32.40
±
5.61 mg 100 g
−1
(dw)
in unblanched and 5.06
±
1.36 mg 100 g
−1
(dw) in blanched NPs, irrespective of the drying
method utilized with blanching adversely affecting the values [19].
In general, the mineral contents of NPs in this study are not in agreement with the data
reported in literature [
7
,
8
] owing to the variations in plant maturity and other agronomic
factors. However, considering the mineral contents it can be said that 1 g of nettle powder
intake can provide up to 2.14% of calcium, 0.75% of magnesium, 0.01% of sodium, and
0.60% of potassium of the required intakes for an average adult above 25 years as suggested
by EFSA [46].
4. Conclusions
The results of this study showed that nettle leaves can be effectively used to produce
powders by blanching as well as drying processes. The blanching and drying methods
used (CD and FD) influenced the physicochemical, technological, and functional quality of
the powders. In particular, the use of blanching resulted in an improvement of the product
physicochemical stability in terms of color and pigment retention. Regarding the drying
process, although convective drying resulted in superior flow properties, freeze drying
provided higher antioxidant potential, pigments, and color retention. Although an expen-
sive and time-consuming process, freeze-dried powders are more suitable for processing
to have higher availability of bioactive components and nutrients. Although the results
obtained can be interesting for academic or industrial uses, further studies are needed to
confirm these findings and deeply explore pros and cons in the usage of nettle powder
as an ingredient/additive for novel foods (e.g., bakery products). Additional studies are
Foods 2021, 10, 1152 13 of 15
being carried out (i) to characterize the phenolic profile of nettle powders through HPLC;
(ii) to evaluate the effects of blanching and drying on the product stability during storage;
and (iii) to evaluate the anti-diabetic effect of nettle powder after incorporation into a food
matrix by analyzing the content of cyclic proteins and performing
in vivo
tests, which were
beyond the scope of the presented study.
Author Contributions:
S.S.N.C. Formal analysis and planning, Methodology, Data interpretation
and analysis, Writing—original draft, review and editing; R.M. Conceptualization, Formal analysis
and planning, Methodology, Data interpretation and analysis, Project management, Writing—original
draft, review and editing; B.F. Formal analysis, Methodology, Data analysis, Writing—original draft,
review; V.V. Formal analysis, Methodology, Data analysis, Writing—original draft, review; N.M.
Methodology, Supervision, Writing—review and editing; G.B. Methodology, Data interpretation and
analysis, Writing—original draft, review and editing; L.N. Conceptualization, Methodology and
planning, Writing—review and editing; P.P. Conceptualization, Supervision, Funding acquisition,
Project management, Writing—review and editing; R.M. Conceptualization, Supervision, Funding
acquisition, Resource planning, Project management, Writing—review and editing. All authors have
read and agreed to the published version of the manuscript.
Funding:
The authors gratefully acknowledge (1) CORE Organic Plus consortium (ERA-NET action)
and MiPAAF (Ministero delle politiche agricole alimentari e forestali, Italy) for financial support
through the SusOrgPlus project (D.M. 20 December 2017, n. 92350), and (2) the “Departments of
Excellence-2018” Program (Dipartimenti di Eccellenza) of the Italian Ministry of Education, University
and Research, DIBAF-Department of University of Tuscia, Project “Landscape 4.0—food, wellbeing
and environment”.
Acknowledgments:
Sincere thanks to the student Giovanni Ciano for his valuable work during the
laboratory analysis and Gianpaolo Moscetti for the English revision of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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