Comparative evaluation of physical characteristics and volatile flavor components of Bangia fusco-purpurea subjected to hot air drying and vacuum freeze-drying

Abstract

Bangia fusco-purpurea is an economically important seaweed with Fujian characteristics. Given that its harvest is seasonal, drying is often used to remove moisture, extend storage time, and facilitate further processing. Hence, the current study sought to explore the impact of different drying processes on the quality and volatile fingerprints of Bangia fusco-purpurea. To this end, the effects of hot air drying (HAD) and vacuum freeze drying (VFD) on the drying characteristics, microstructure, rehydration, and volatile components of dried B. fusco-purpurea were investigated. The results showed that the water removal efficiency of HAD was significantly higher than that of VFD. However, VFD better preserved the skeletal structure of B. fusco-purpurea than HAD, with a faster rehydration rate and a more uniform cell structure after rehydration. Using electronic nose and comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC × GC-TOF MS), significant differences in the volatile profiles of fresh, HAD, and VFD B. fusco-purpurea were assessed. E-nose analysis revealed that both HAD and VFD treatments significantly reduced sulfides, aromatic compounds, and nitrogen oxides in fresh B. fusco-purpurea. However, the alcohol, aldehyde, and ketone contents were lower in the VFD samples compared with HAD and fresh samples, whereas the content of methyl flavor substances was significantly higher. GC × GC-TOF MS analysis revealed that the most abundant volatile categories in HAD and VFD were hydrocarbons, alcohols, and esters. The number of volatile components in the HAD samples was significantly lower than in the VFD and fresh samples. As drying progressed, hydrocarbons and alcohols were formed in dried B. fusco-purpurea due to the thermal degradation of carbohydrates, lipids, amino acids, and the Maillard reaction. There were also significant flavor differences between HAD and VFD B. fusco-purpurea. Thus, although HAD exhibits better drying efficiency, VFD has more significant advantages in terms of product quality.

Full text

Current Research in Food Science 7 (2023) 100624
Available online 20 October 2023
2665-9271/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Comparative evaluation of physical characteristics and volatile avor
components of Bangia fusco-purpurea subjected to hot air drying and
vacuum freeze-drying
Jingna Wu
a
,
*
, Nan Pan
b
, Xiaoting Chen
b
, Debiao Shan
a
, Huifang Shi
a
, Yingshan Qiu
a
,
Zhiyu Liu
b
, Yongchang Su
b
, Junfa Weng
c
a
Xiamen Key Laboratory of Marine Medicinal Natural Products Resources, Fujian Universities and Colleges Engineering Research Center of Marine Biopharmaceutical
Resources, Xiamen Medical College, 361023, Xiamen, PR China
b
Fisheries Research Institute of Fujian, 361013, Xiamen, PR China
c
Aquatic Science Research Institute of Putian, 351100, Putian, PR China
ARTICLE INFO
Handling Editor: Professor Aiqian Ye
Keywords:
Bangia fusco-Purpurea
Microstructure
Rehydration capability
e-nose
GC×GC-TOF MS
ABSTRACT
Bangia fusco-purpurea is an economically important seaweed with Fujian characteristics. Given that its harvest is
seasonal, drying is often used to remove moisture, extend storage time, and facilitate further processing. Hence,
the current study sought to explore the impact of different drying processes on the quality and volatile nger-
prints of Bangia fusco-purpurea. To this end, the effects of hot air drying (HAD) and vacuum freeze drying (VFD)
on the drying characteristics, microstructure, rehydration, and volatile components of dried B. fusco-purpurea
were investigated. The results showed that the water removal efciency of HAD was signicantly higher than
that of VFD. However, VFD better preserved the skeletal structure of B. fusco-purpurea than HAD, with a faster
rehydration rate and a more uniform cell structure after rehydration. Using electronic nose and comprehensive
two-dimensional gas chromatography-time-of-ight mass spectrometry (GC ×GC-TOF MS), signicant differ-
ences in the volatile proles of fresh, HAD, and VFD B. fusco-purpurea were assessed. E-nose analysis revealed
that both HAD and VFD treatments signicantly reduced suldes, aromatic compounds, and nitrogen oxides in
fresh B. fusco-purpurea. However, the alcohol, aldehyde, and ketone contents were lower in the VFD samples
compared with HAD and fresh samples, whereas the content of methyl avor substances was signicantly higher.
GC ×GC-TOF MS analysis revealed that the most abundant volatile categories in HAD and VFD were hydro-
carbons, alcohols, and esters. The number of volatile components in the HAD samples was signicantly lower
than in the VFD and fresh samples. As drying progressed, hydrocarbons and alcohols were formed in dried
B. fusco-purpurea due to the thermal degradation of carbohydrates, lipids, amino acids, and the Maillard reaction.
There were also signicant avor differences between HAD and VFD B. fusco-purpurea. Thus, although HAD
exhibits better drying efciency, VFD has more signicant advantages in terms of product quality.
1. Introduction
Seaweeds are highly esteemed raw materials used in the food and
medical industries due to their abundance of biologically active sub-
stances (St´
evant et al., 2018). However, seaweed chemical composition
is not fully understood when compared to that of land plants. Indeed,
numerous factors signicantly affect their composition, including the
species, geographical location, maturity stage, and environmental con-
ditions (Pe˜
na-Rodríguez et al., 2011). Bangia fusco-purpurea (Dillwyn)
Lyngbye is a Bangia species member that belongs to the family Bangia-
ceae, order Bangiales, class Protoorideophyceae, and phylum Rhodo-
phyta. This species is primarily found in rocky intertidal zones, where it
experiences signicant variations in external salinity due to ebb and
ow tides. Furthermore, it is subjected to extended periods of desicca-
tion and prolonged exposure to high irradiance levels (J.N. Wu et al.,
2021). B. fusco-purpurea contains signicant quantities of proteins, fatty
acids, and polysaccharides. In fact, the B. fusco-purpurea nutrient con-
centration surpasses that of Porphyra (Xu et al., 2022). Research con-
ducted on the biological properties of metabolites derived from
* Corresponding author. Xiamen Key Laboratory of Marine Medicinal Natural Products Resources /Fujian Universities and Colleges Engineering Research Center of
Marine Biopharmaceutical Resources, Xiamen Medical College, Xiamen, 361023, PR China.
E-mail address: wjn@xmmc.edu.cn (J. Wu).
Contents lists available at ScienceDirect
Current Research in Food Science
journal homepage: www.sciencedirect.com/journal/current-research-in-food-science
https://doi.org/10.1016/j.crfs.2023.100624
Received 27 June 2023; Received in revised form 4 October 2023; Accepted 18 October 2023

Current Research in Food Science 7 (2023) 100624
2
B. fusco-purpurea has revealed various benecial effects on human
health, including antioxidant activity (Wu et al., 2015), pro-angiogenic
(Jiang et al., 2021), anti-inammatory (Zheng et al., 2022), and
anti-tumor (Wu et al., 2021) properties.
A key challenge associated with utilizing seaweed in food and me-
dicinal sectors is its seasonal harvest, which requires preservation and
storage methods to ensure continuous year-round supply. Typically,
seaweeds are harvested over a period of 2–3 months and subsequently
processed in a systematic manner throughout the year (Obluchinskaya
and Daurtseva, 2020). To facilitate their use in nutritional or industrial
applications, fresh seaweeds are typically subjected to a drying process
due to their substantial moisture content (ranging from 75% to 85%)
and vulnerable character (Neoh et al., 2016). Various preservation
techniques effectively prolong the seaweed lifespan, including conven-
tional drying (L´
opez-P´
erez et al., 2020). The dehydration process plays a
vital role in eliminating a signicant quantity of seaweed’s aqueous
content, leading to a decline in the water activity level. This effectively
hinders microorganism proliferation while also diminishing the overall
product bulk and mass. Such benets have a favorable impact on storage
and transportation expenses (Uribe et al., 2019). However, seaweed
nutrient loss and structural deformation are natural consequences of the
drying process (St´
evant et al., 2018). Therefore, it is imperative to
thoroughly investigate the impact of drying techniques on seaweed
quality.
Sun drying is a prevalent and economical method. Nevertheless, the
product quality may be impacted by weather and microbial intrusion
and dust, insect, and bird contamination. Moreover, this process pre-
sents certain challenges regarding adequate quality control (Fudholi
et al., 2014). Over time, articial drying methods have become preva-
lent due to their controllable drying conditions, faster drying rates, and
superior material quality as compared with non-articial drying
methods. Several drying technologies have been formulated, encom-
passing hot air drying (HAD), vacuum freeze-drying (VFD), infrared
drying, and microwave drying (Ling et al., 2015). HAD utilizes hot air as
the drying medium and relies on heat exchange through air convection
to accomplish drying. Generally, heat-sensitive constituents, including
bioactive components, undergo irreversible degradation or oxidation
when subjected to prolonged, high-temperature dehydration procedures
(Tan et al., 2021). The drying process can also modify the volatile
organic compound (VOC) ratio in the product, including those with fruit
and green aromas, including (E)-2-hexenal, hexanal and (E)-2-nonenal
(Liu et al., 2022). VFD is currently considered the optimal approach for
producing premium dried products. However, its disadvantages such as
elevated production expenses, heightened energy consumption, and
limited throughput (Hsu et al., 2003) tend to hinder its development.
Ling et al. (2015) evaluated seven drying techniques to assess their
impact on Kappaphycus alvarezii phytochemical composition and anti-
oxidant activity; the highest levels of phytochemical compounds and
improved abilities for free radical scavenging and reducing were ob-
tained through oven-drying at 40 ◦C. Meanwhile, Chilean green seaweed
(Ulva spp.) convective drying at 70 ◦C for 120 min is considered the most
effective method for retaining physicochemical parameters and antiox-
idant capacity, when compared to other drying techniques including
freeze-drying, vacuum-drying, and solar-drying (Uribe et al., 2019).
Chenlo et al. (2018) investigated and compared the dehydration and
rehydration characteristics of Ascophylum nodosum and Undaria pinna-
tida, two brown seaweed types, over a wide range of temperatures
relevant in industrial settings. The study concluded that conventional
air-drying methods are suitable for A. nodosum; however, alternative
drying techniques should be investigated to improve U. pinnatida
rehydration properties.
Notably, insufcient information is available concerning B. fusco-
purpurea drying properties. Moreover, there have been limited in-
vestigations conducted regarding HAD and VFD impacts on B. fusco-
purpurea VOC. Hence, comprehensive study of the various drying tech-
niques is warranted.
Developing a rapid, convenient, and energy-efcient drying method
is key to realizing the B. fusco-purpurea economic value. Accordingly,
here, we examined HAD and VFD effects on B. fusco-purpurea drying
characteristics, microstructure, rehydration, and volatile compounds,
subsequently selecting the most appropriate dehydration parameters.
The primary aim of this study was to determine how to preserve B. fusco-
purpurea physical characteristics and avor quality while providing
practical drying information that can be scaled up for industrial use.
Based on economic efciency and product quality principles, our eval-
uations suggest that HAD remains a viable option for B. fusco-purpurea
dehydration.
2. Material and methods
2.1. Materials
Fresh B. fusco-purpurea was collected from the coastal area of Putian,
China, and provided by Putian Hai Dao Ren Jia Aquatic Products Co.,
Ltd. The samples were transported to the laboratory in seawater at 4 ◦C
and cleaned to remove impurities.
2.2. Drying methods for B. fusco-purpurea
2.2.1. HAD
Fresh B. fusco-purpurea weighing 100 ±0.5 g with a 5-mm thickness
were spread on gauze and placed in a hot-air drying oven (BGZ-240,
Shanghai Bosen Industrial Co., Ltd. Medical Equipment Factory, China).
The drying temperature was 70 ◦C, and the wind speed was 2 m/s. The
B. fusco-purpurea initial weight was measured, and the sample was
weighed every 0.5 h thereafter until the nal product reached a constant
weight.
2.2.2. VFD
Fresh B. fusco-purpurea weighing 100 ±0.5 g with a 5-mm thickness
were spread on gauze and pre-frozen at -80 ◦C for 1 h. Subsequently, the
samples were placed in the drying chamber (Alpha2-4 LD Plus, Christ,
Germany) when the cold trap temperature reached -60 ◦C. The primary
drying was performed at -30 ◦C and a 0.37 mbar vacuum pressure; the
sample weight was measured every hour until the nal product achieved
a constant weight.
2.3. Determination of dry basis moisture content
The dry-basis moisture content during drying was determined using
Abbreviations
e-nose electronic nose
GC gas chromatography
GC £GC-TOF MS two-dimensional gas chromatography-time-of-
ight mass spectrometry
VOC volatile organic compound
HAD hot air drying
HS headspace
HS-SPME headspace solid-phase microextraction
MR moisture ratio
MS mass spectrometry
PCA principal component analysis
PUFAs polyunsaturated fatty acids
RR rehydration ratio
SEM scanning electron microscopy
SPME solid-phase microextraction
VFD vacuum freeze-drying
J. Wu et al.

Current Research in Food Science 7 (2023) 100624
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Equation (1):
Mt=mt−md
md
(1)
where Mt represents the B. fusco-purpurea dry basis moisture content at
time t (g water/g dry basis), m
t
is the B. fusco-purpurea weight at time t
(g), and m
d
is the B. fusco-purpurea weight after drying to a constant
weight, which is the dry basis weight (g).
2.4. Determination of moisture ratio
The moisture ratio (MR) represents the amount of water in a material
that has not been removed under certain drying conditions and can be
used to indicate the drying rate. During the drying process, MR was
calculated using Equation (2):
MR =Mt
Mo
(2)
where Mt is the moisture content on a dry basis at time t (g water/g dry
basis), and Mo is the initial moisture content on a dry basis (g water/g
dry basis).
2.5. Determination of effective diffusivity coefcient
The effective diffusivity coefcient (Deff) is a measure of moisture
removal ability through diffusion. This can be calculated using Fick’s
second law (Equation (3)):
MR =8
π
2exp(−
π
2Deff
L2t)(3)
Taking the logarithm of both sides of Equation (3), we obtain
Equation (4):
ln MR =ln(8
π
2)−
π
2Deff
L2t(4)
where MR is the moisture ratio, t is the drying time (h), L is the sample
thickness (m), and Deff is the effective diffusivity coefcient (m
2
/h).
Plotting lnMR on the y-axis and the drying time t on the x-axis yields a
linear equation, the slope of which can be substituted into Equation (5)
to determine Deff for moisture (Vega-G´
alvez et al., 2010):
Deff = − L2
π
2k(5)
2.6. Observation with scanning electron microscopy (SEM)
A dry sample of B. fusco-purpurea was xed onto a conductive sample
holder using conductive adhesive tape. The sample was then subjected
to vacuum and coated with a layer of gold via sputtering to ensure
conductivity. The sample holder was then positioned in a low-
temperature scanning electron microscope (SU8100, Hitachi, Ltd.,
Japan), and its microstructure was observed under an acceleration
voltage of 5 kV.
2.7. Determination of rehydration ratio
The rehydration ability of the sample was represented by the rehy-
dration ratio (RR), determined by soaking the dried B. fusco-purpurea
samples in distilled water at 25 ◦C for rehydration testing. The sample
was removed every minute, and the surface was wiped dry with absor-
bent paper, weighed, and placed back into the water for rehydration.
This process was repeated until the weight of the sample stabilized and
remained constant. RR was calculated as follows (Equation (6)):
RR =Wt−WO
WO
(6)
where, W
0
represents the weight of the sample before rehydration and
Wt is the weight of the sample at time t during rehydration.
2.8. Observation with inverted uorescence microscopy
To visualize the effects of different drying techniques on the rehy-
dration performance of B. fusco-purpurea, an inverted uorescence mi-
croscope (ECLIPSE Ts2R-FL, Nikon Corporation, Japan) was used to
observe the microstructure of fresh and rehydrated samples. Fresh and
dried B. fusco-purpurea were placed in water, and intact single-layer
B. fusco-purpurea was selected and placed on a glass slide. Character-
istic microstructure images of the samples were captured and photo-
graphed at 10 ×40 magnication.
2.9. Electronic nose (e-nose) analysis
The e-nose sensor array (PEN3.5, AIRSENSE Analytics GmbH, Ger-
many) comprises ten metal oxide sensors (named beginning with “W”),
each with specic functions: W1C responds well to aromatic com-
pounds, W5S is sensitive to nitrogen oxides, W3C shows sensitivity to
ammonia and aromatic compounds, W6S can selectively detect hy-
drides, W5C responds to short-chain alkane aromatic compounds, W1S
detects methyl compounds, W1W is responsive to suldes, W2S detects
alcohols and aldehydes/ketones, W2W has an afnity for aromatic
compounds and organic suldes, and W3S is sensitive to long-chain al-
kanes (Wu et al., 2021).
To conduct the experiment, 0.8 g of B. fusco-purpurea dried by HAD,
0.8 g of B. fusco-purpurea dried by VFD, and 2.5 g of fresh B. fusco-pur-
purea were placed in e-nose collection bottles, sealed, and left at room
temperature (25 ◦C) for 60 min until the sample in the tube reached
equilibrium with the gas phase. The e-nose detected volatile odors, with
three replicates per group. The measurement conditions were as follows:
sensor cleaning time of 200 s, zeroing time of 10 s, sample preparation
time of 5 s, sample testing time of 200 s, and internal ow rate of 200
mL/min. Data from 150 to 152 s were selected for subsequent analyses
based on preliminary experiments to ensure the data’s stability and
accuracy. Principal component analysis (PCA) was used to cluster and
discriminate the different samples.
2.10. Headspace (HS-) solid-phase microextraction (SPME) combined
with comprehensive two-dimensional gas chromatography time-of-ight
mass spectrometry (GC ×GC-TOF MS) detection
2.10.1. Conditions of HS-SPME
The experimental conditions for HS-SPME were determined by
slightly modifying the parameters set by Liu et al. (2021). The HAD
B. fusco-purpurea (0.8 g), VFD B. fusco-purpurea (0.8 g), and fresh
B. fusco-purpurea (2.5 g) were separately weighed and placed in a 20-mL
headspace vial. Three replicates were collected from each group. Sub-
sequently, the vial was incubated at 50 ◦C for 15 min, and SPME was
performed at 50 ◦C for 30 min. Subsequently, desorption of divinyl-
benzene/carboxen/polydimethylsiloxane bers was carried out in a
splitless injection gas chromatography (GC) mode at 250 ◦C for 180 s.
2.10.2. Conditions of GC ×GC-TOF MS
The GC ×GC-TOF MS system utilized here was an Agilent Tech-
nologies 7890 B GC system, which was paired with an SSM1810 solid-
state thermal modulator, HV modulation column (Snow Scene Elec-
tronic Technology (Shanghai) Co., Ltd.), and HeXin (EI-0610, HeXin
Analytical Instrument, China) time-of-ight mass spectrometer.
GC conditions: The GC columns comprised a DB-WAX (30 m ×0.25
mm ×0.25
μ
m) as the rst dimension column and a DB-17MS (1.0 m ×
0.15 mm ×0.15
μ
m) as the second dimension column. Helium (purity
≥99.999%) was the carrier gas. The injection mode was splitless, and
the column ow rate was 1.0 mL/min. The column temperature program
J. Wu et al.

Current Research in Food Science 7 (2023) 100624
4
was as follows: initial temperature of 40 ◦C, held for 5 min, and then
increased at 4 ◦C/min to 240 ◦C.
Mass spectrometry (MS) conditions: The transfer line temperature
was 240 ◦C, and the electron ionization temperature was 220 ◦C. The
detector voltage was -1850 V, and the electron ionization energy was 70
eV. The samples were subjected to SPME. A signal of 40–400 amu was
acquired at 100 Hz.
Qualitative analysis of the VOC was performed using Canvas 2.5, a
comprehensive two-dimensional (2D) chromatography data processing
software, combined with the NIST 20 mass spectrum library. The sam-
ples were classied, aligned, and compared using the software of Album
1.0 comprehensive 2D chromatography multi-sample comparison
analysis. Feature difference analysis was performed using the Fisher
Ratio algorithm (with peak area weighting, group distance minus group
uctuation), and the top 50 compounds were compared.
2.11. Data processing and analysis
The plotting task was accomplished using OriginPro2018 software
(OriginLab Corp., Northampton, MA, USA) in conjunction with Evolu-
tionary Genotype-Phenotype Systems software (Yu et al., 2019).
3. Results and discussion
3.1. Effect of different drying methods on B. fusco-purpurea drying
kinetics
The moisture ratio change curves of B. fusco-purpurea during the VFD
and HAD processes were illustrated in Fig. 1A. As the drying time
increased, the moisture ratio of B. fusco-purpurea exhibited an expo-
nential decrease. The moisture ratio decreased rapidly in the initial
phases, whereas it gradually decreased in the later stages, owing to the
increased energy required to extract bound water (Ren et al., 2021).
Additionally, various drying techniques signicantly impacted changes
in the moisture ratio of B. fusco-purpurea. The required time for VFD
B. fusco-purpurea was 3.7 times higher than that of HAD, while Xu et al.
(2020) VFD required 24 h to reach the same nal water content as hot
air drying, which took only 8.5 h to dry cabbage. Furthermore, the time
required for ginger VFD was 3.7 times greater than hot air drying (An
et al., 2016). Aware and Thorat (2011) compared the drying kinetics of
garlic using different drying methods and found that freeze-drying
required 3–5 times more time than hot drying. The VFD freezes the
material to a eutectic point or lower. After the water becomes solid, the
material enters the main drying stage. Under vacuum conditions, the
frozen water in the material changes from solid to gas through subli-
mation, removing moisture from the material (Ratti, 2001). During the
VFD process, heat was transferred to B. fusco-purpurea solely through the
difference in temperature between the zones of freezing and drying. The
ow of heat was oriented from the exterior toward the interior, whereas
the driving force of mass transfer, which occurred from the inside to the
outside, was the difference in vapor pressure between B. fusco-purpurea
and cold butane. However, as time was also required to establish a
sufcient vapor pressure difference, the drying time of VFD was
noticeably longer than that of HAD. In contrast, HAD derived its heat
source from the drying equipment, meaning that the fan blew heat to all
parts of the equipment. The heat circulated around B. fusco-purpurea and
gradually diffused from the surface toward the cell’s interior until the
moisture evaporated. Ong and Law (2011) found that heat treatment
increased the intercellular space, ruptured the cell membrane, and
reduced the tissue angle of cell adhesion, all of which contribute to
reducing the ow resistance of water and improving the drying rate.
Therefore, the hot-air drying time was shorter.
The drying process involves the transfer of momentum, mass, and
energy. When low-temperature materials absorb heat from the drying
medium, heat is transferred from the exterior to the interior of the
particles, whereas the moisture within the particles is transferred to the
exterior. After reaching the critical moisture level, the drying process
ends, and the heat inside the particles is transferred via thermal con-
duction, whereas moisture is transferred via diffusion (Komatsu et al.,
2015). The liquid diffusion theory was used in the food material drying
process without considering the driving force of diffusion to simplify the
calculation results. All of the dynamic effects were incorporated into the
diffusion coefcient using the effective moisture diffusion coefcient
(D
eff
) to characterize the average speed of moisture migration during the
drying process (Zheng et al., 2014). D
eff
was calculated using the slope
method, where ln(MR) was linearly tted to the drying time (t) (Fig. 1B),
with R
2
values >0.90, indicating good tting results. The D
eff
for VFD
was 3.7386 ×10
-10
m
2
/s, whereas that for HAD was 1.5674 ×10
-9
m
2
/s,
both falling within the Deff range of D
eff
10
-10
m
2
/s to 10
-8
m
2
/s for food
materials (Liu et al., 2014). The ndings suggest that HAD demonstrated
greater effectiveness in removing moisture compared to VFD.
3.2. Effects of different drying techniques on the microstructure of
B. fusco-purpurea
The different drying methods affected the microstructure of B. fusco-
purpurea (Fig. 2). In contrast to the wrinkled surfaces of the HAD samples
(Fig. 2A
1
), the surfaces of the samples obtained by VFD were relatively
at and smooth (Fig. 2B
1
), similar to the results of Huang et al. (2011).
Moreover, after drying by HAD, the B. fusco-purpurea appeared as a solid
column-shaped bar, therefore increasing the hardness of the sample
(Fig. 2A
2
), whereas the skeletal structure of B. fusco-purpurea dried using
VFD remained relatively intact (Fig. 2B
2
) which may be due to the
removal of moisture through sublimation under vacuum conditions
(Zhang et al., 2020). Meanwhile, any drying process involving heat
causes the sample to exhibit uneven moisture and temperature
Fig. 1. Effects of different drying techniques on the drying characteristics of Bangia fusco-purpurea. (A) Moisture ratio changes over times of Bangia fusco-purpurea
dried by vacuum freeze-drying (VFD) and hot air drying (HAD); (B) Relationship curves between lnMR (natural logarithm of moisture ratio) and drying time.
J. Wu et al.

Current Research in Food Science 7 (2023) 100624
5
Fig. 2. Effects of different drying techniques on the microstructure of B. fusco-purpurea. (A) hot air drying (HAD) B. fusco-purpurea; (B) vacuum freeze-drying (VFD)
B. fusco-purpurea. The subscript numerics 1 and 2 denote a magnication of 100 times and 2000 times, respectively.
Fig. 3. Rehydration characteristics of dried B. fusco-purpurea. (A) The rehydration ratio changes over times for B. fusco-purpurea with different drying techniques; (B)
Optical microscopy images of fresh B. fusco-purpurea magnied by 10 ×40 times; (C) Optical microscopy images of HAD B. fusco-purpurea magnied by 10 ×40
times; (D) Optical microscopy images of VFD B. fusco-purpurea magnied by 10 ×40 times. Photograph of the bottles: States of B. fusco-purpurea after rehydration.
J. Wu et al.

Current Research in Food Science 7 (2023) 100624
6
gradients, resulting in degradation, deformation, and folding (Chum-
roenphat et al., 2021). A correlation exists between the microstructure
and surface shrinkage of dried products with the water migration
mechanism and external pressure variations (An et al., 2016). During
HAD, B. fusco-purpurea was exposed to high temperatures of 70 ◦C for a
remarkable amount of time, resulting in signicant damage to the sea-
weed’s skeleton structure, dehydration caused the collapsion of the
seaweed cells, leading to severe shrinkage and hardening of the
B. fusco-purpurea’s surface. Moreover, the loss of moisture and heating
may cause an increase in pressure on the cell structure of foods, reducing
cell framework rigidity and resulting in shrinkage, collapse, and changes
in the microstructure of B. fusco-purpurea cells.
3.3. Effect of different drying methods on the water rehydration properties
of dried B. fusco-purpurea
Drying can cause irreversible damage to the tissue structures of
samples. Thus, the water rehydration ratio is an essential indicator of the
degree of damage to the internal tissue structure during the drying
process. The different drying methods were found to have different ef-
fects on the water-rehydration properties of B. fusco-purpurea (Fig. 3).
After 1 min of rehydration, the water rehydration ratio of B. fusco-pur-
purea dried by VFD was 87.64%, which remained stable. In contrast, the
water rehydration ratio of B. fusco-purpure dried by HAD was only
72.77%, and 4 min were required to reach stability at a water rehy-
dration ratio of 86.12%. The HAD samples turned brown to purplish-red
(Fig. 3C), whereas VFD samples remained closer to fresh B. fusco-pur-
purea (Fig. 3D). This may be due to the HAD samples being subjected to a
high temperature (70 ◦C) for a long time, thereby damaging the internal
tissue structure and causing the collapse of micro-pore channels, leading
to the loss of contents. Furthermore, the intensication of shrinkage and
surface hardening may hinder water penetration during rehydration
(Rojas and Augusto, 2018). In contrast, the temperature during VFD was
lower, resulting in less damage to the cells and preservation of the
original color to the greatest extent possible (Zhang et al., 2020). The
VFD samples presented a porous network structure lled with air, as the
rehydration process comprised the exchange of gas from inside the
sample space with moisture from the outside (Feng et al., 2020). Thus,
the water rehydration rate of the VFD samples was higher than that of
the HAD samples.
The microscopic structure of the post-rehydration samples was
observed using an optical microscope, and images of the characteristic
microscopic structures of the samples were captured at 10 ×40
magnication to visualize the impact of different drying techniques on
the rehydration ratio of B. fusco-purpurea. The cell tissue of fresh B. fusco-
purpurea was relatively dense (Fig. 3B), whereas the dried and rehy-
drated cells became separated (Fig. 3C and Fig. 3D). Drying treatment
will cause varying degrees of damage to the cells, and during the
rehydration process, the B. fusco-purpurea continuously absorbs water,
causing internal pressure to increase and some cell structures to deform,
leading to increased spacing between the cells. Different drying treat-
ments resulted in varying degrees of cell damage, with the VFD tech-
nique better preserving the original appearance of B. fusco-purpurea
(Zhang et al., 2020), while the HAD technique induced apparent damage
and deformation. Therefore, the cell structure of the VFD sample was
more uniform than that of the HAD sample after rehydration.
3.4. Determination of VOC by e-nose
Electronic nose analysis showed that each odor produced a specic
group of organic and inorganic gases. The response values of the ten
sensors in the e-nose detection system for fresh, HAD, and VFD B. fusco-
purpurea odors were depicted in Fig. 4A. The analysis revealed that the
response values from the W1C, W3C, W6S, W5C, and W3S sensors to
fresh and dried B. fusco-purpurea odors did not differ signicantly.
However, prominent differences in response values were observed for
the W5S, W1S, W1W, W2S, and W2W sensors. The response values of the
W1S and W2S sensors differed signicantly between HAD and VFD
B. fusco-purpurea, whereas those of the other sensors were similar. HAD
and VFD remarkably reduced the levels of suldes, aromatic compo-
nents, and nitrogen oxides in fresh B. fusco-purpurea odors. Moreover,
the levels of alcohol and aldehyde compounds in HAD samples were
signicantly lower than those in other samples, whereas the levels of
methyl compounds in VFD samples were signicantly higher than those
in other samples. Cluster discrimination analysis was conducted on
various samples utilizing PCA (Fig. 4B). Notably, the rst (PC1) and
second principal components (PC2) had variance contribution rates of
53.80 % and 44.40 %, respectively, amounting to a cumulative variance
contribution rate of 98.20 %, indicating that these two factors held most
of the sample information. The PCA revealed signicant differences in
odor among the three samples.
Fig. 4. Analysis of volatile proles by electronic nose (e-nose). (A) Radar plot of e-nose data from fresh and drying B. fusco-purpurea. Regarding the type of response
of electronic nose sensors to substances, W1C responds to aromatic compounds, W5S responds to nitrogen oxides, W3C responds to ammonia and aromatic com-
pounds, W6S responds to hydrides, W5C responds to short-chain alkane aromatic compounds, W1S responds to methyl compounds, W1W responds to suldes, W2S
responds to alcohols and aldehydes/ketones, W2W responds to aromatic compounds and organic suldes, and W3S responds to long-chain alkanes. (B) Principal
component analysis (PCA) of e-nose output data.
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Current Research in Food Science 7 (2023) 100624
7
3.5. Determination of VOC by HS-SPME coupled with GC ×GC-TOF MS
Fig. 5 displays the 2D chromatographic prole and three-
dimensional (3D) view of the GC ×GC-TOF MS of the VOC of
B. fusco-purpurea manufactured using different drying techniques. The
horizontal axis of the 2D spectra represents the rst-dimension retention
time, whereas the vertical axis represents the second-dimension reten-
tion time. The signal intensity of the total ion chromatogram is illus-
trated in color, with blue to red indicating signal intensities from weak
to strong. The detection method uses a reverse column conguration
with a polar column in the rst dimension and a moderately polar col-
umn in the second. Hence, the more polar the component in the sample,
the shorter its retention time on the second dimension column. In the
graph, the component peaks are arranged from top to bottom according
to their polarity, with the compound families in the order of alkanes,
alkenes, aldehydes/ketones, heterocyclics, alcohols/esters, and acids,
based on their similar 2D retention times.
The VOC in B. fusco-purpurea were identied using the Canvas2.5
comprehensive 2D chromatographic data processing software, which
utilized the NIST 20 mass spectrum library. Nine compounds were
identied: esters, alcohols, ketones, hydrocarbons (aromatic hydrocar-
bons, alkanes, alkenes, dienes, cyclic alkenes, substituted hydrocarbons,
cyclic hydrocarbons, and alkynes), aldehydes, ethers, phenols, acids,
and heterocyclic compounds. The results (Fig. 6) show that an average of
399 compounds were detected in fresh B. fusco-purpurea samples, 449
compounds in VFD samples, and 352 compounds in HAD samples,
indicating a reduction in the total number of volatile compounds,
possibly due to the degradation of avor precursors caused by exposure
to high temperatures (Zhang et al., 2019).
During high-temperature drying, alcohols undergo esterication re-
actions (Carrapiso et al., 2002), resulting in a signicant decrease in
alcohol compounds after being subjected to 70 ◦C HAD, while
B. fusco-purpurea treated with VFD remained relatively unchanged. Ester
compounds primarily use amino acids, sugars, and lipids as biosynthetic
precursors, resulting in the emergence of new ester compounds during
the drying process, primarily through lipid oxidation and esterication
reactions of free fatty acids and alcohols (Chen et al., 2017). Although
HAD and VFD have been found to reduce the quantity and diversity of
ester compounds in Ganoderma lucidum, few studies have indicated that
heating promotes the formation of ester compounds, including the
abundant production of 1-propen-2-ol acetate after HAD (Liu et al.,
2021). Moreover, B. fusco-purpurea exhibited a signicant increase in the
variety of ester compounds after VFD whereas the variety increased only
slightly after HAD.
It is hypothesized that aldehydes originate from the auto-oxidation
or enzymatic degradation of polyunsaturated fatty acids (PUFAs). Al-
dehydes in the range of C6–C9 exhibit a fatty-green aroma, while those
in the range of C15–C17 have a distinct marine seaweed scent (Fran-
cezon et al., 2021). In addition to aldehydes, PUFAs can be converted
into ketones via degradation (Francezon et al., 2021). After drying, the
types of ketone compounds were reduced, and the VFD samples dis-
played notably elevated concentrations compared to other samples.
Sulfur compounds exhibit distinctive algal scents due to their strong
odoriferous properties, although these compounds may exist in small
amounts and possess extremely low odor thresholds (S´
anchez-García
et al., 2021; Yao et al., 2022). When subjected to HAD or VFD, there
were minimal alterations in the variations and amounts of aldehyde- and
sulfur-containing compounds. This suggests that the typical avor
associated with algae remained intact following the drying procedure.
Hydrocarbon compounds generally tend to possess a high threshold and
exhibit a strong pungent odor, which typically makes the sample less
avorful (Zhang et al., 2019). The number of hydrocarbon compound
types was higher in the VFD samples than in the HAD samples. There-
fore, VFD samples possessed a stronger aroma when compared with
HAD samples, which is consistent with the sensory evaluation result.
According to the compound peak list, the samples were classied,
Fig. 5. GC ×GC-TOF MS (two-dimensional [2D] gas chromatography-time-of-ight mass spectrometry) spectra of B. fusco-purpurea VOC obtained by different drying
methods. (A) Comprehensive 2D chromatography-ight mass spectra. (B) Three-dimensional topographical visualization.
J. Wu et al.

Current Research in Food Science 7 (2023) 100624
8
Fig. 6. Chemical species in fresh, HAD, and VFD B. fusco-purpurea.
Fig. 7. Thermal image analysis and hierarchical clustering dendrogram of B. fusco-purpurea volatile compounds obtained by different drying techniques. The
numbers correspond to the identied signals.
J. Wu et al.

Current Research in Food Science 7 (2023) 100624
9
aligned, and compared using Album 1.0 comprehensive 2D chromato-
graphic data processing software. The top 50 volatile compounds with
the most signicant differences were selected for heatmap and cluster
analyses (Fig. 7). The relative content of each volatile substance after
different drying treatments is marked with different colors in the heat-
map, with red and blue indicating high- and low-content areas,
respectively. The deeper the red or blue color, the higher or lower the
relative content, respectively. Cluster analysis revealed good parallelism
within each set of samples, with minor differences between HAD and
VFD samples but signicant differences between fresh samples.
As observed in Fig. 7, the content of VOC, namely 1-heptadecene,
benzaldehyde, 2,6,6-trimethyl-1-cyclohexene-1-carboxaldehyde, trans-
β-Ionone,
α
-Ionone, 1-penten-3-ol, (Z)-2-penten-1-ol, 3,5,5-trimethyl-2-
hexene, cyclohexanol, ethylbenzene, diethyl-acetic acid, dimethyl-
silanediol, propyl-benzene, acetic acid ethenyl ester, (Z)-2-penten-1-ol,
1-octen-3-ol, 1-pentanol, heptadecane, 3-carene, toluene, p-xylene, and
acetic acid, were decreased after HAD and VFD.
New VOC (dodecane, 1-decene, 4,5-dimethyl-nonane, 3-methyl-non-
ane, 4-methyl-nonane, 1-nonanol, 3-methyl-tridecane, (S)-(+)-6-
methyl-1-octanol) were produced in the B. fusco-purpurea samples sub-
jected to HAD and VFD. Only VFD samples contained 2-(diisopropyla-
mino)ethyl methacrylate and 1-ethenyl-aziridine. Moreover, during
HAD, a signicant decrease was observed in the acetoin and hexadecane
content. However, the levels in VFD and fresh B. fusco-purpurea
remained relatively constant.
Using PCA for cluster discriminant analysis of different samples,
Fig. 8 shows the score and loading plots of the samples. The PC1 and PC2
contributed to the variance rates by 82.20% and 13.70%, respectively,
with a cumulative variance contribution rate of 95.90% (Fig. 8A). The
three samples were divided in the score plot, similar to the results for the
e-nose. Regarding PC1, there were signicant differences among the
HAD, VFD, and fresh samples. Regarding PC2, there were signicant
differences between the HAD and VFD groups, and the fresh sample was
located between them. In the loading plot (Fig. 8B), each component
was divided into four groups according to the quadrant, with attributes
in the same group having a close positive correlation. Approximately all
variables were within the 95% condence interval, meaning the model
can explain these variables well.
4. Conclusions
Here, the drying characteristics and volatile proles of B. fusco-pur-
purea were investigated in detail using HAD and VFD. It was revealed
that the drying efciency of HAD was superior to that of VFD, evidenced
by the notable differences in drying time, with VFD taking 3.7 times
longer than HAD. Adversely the continuous high temperature of 70 ◦C
during HAD led to the shrinkage, collapse, and deformation of the
microstructure of B. fusco-purpurea. After a rehydration period of 4 min,
the sample achieved stability, with a rehydration ratio of 86.12%.
Signicant differences between HAD samples and fresh B. fusco-purpurea
were observed in cell structure damage after rehydration. The surface of
the VFD sample was relatively complete and smooth, and after a brief
rehydration period of 1 min, the sample achieved stability with a
rehydration ratio of 87.64%; the sample was closer to fresh B. fusco-
purpurea after rehydration, indicating that VFD can better preserve the
physical and chemical characteristics of B. fusco-purpurea.
E-nose results indicated that the response values of the HAD and VFD
B. fusco-purpurea were notably distinct for the W1S and W2S sensors,
which were sensitive to methyl, alcohol, aldehyde, and ketone, respec-
tively. The response values of the other sensors were relatively similar.
Both drying methods signicantly reduced the sulde, aromatic com-
ponents, and nitrogen oxide compounds in fresh B. fusco-purpurea;
conversely, the content of methyl avor substances in VFD samples was
signicantly higher than in the others, and the HAD, VFD, and fresh red
hair algae samples were well distinguished via PCA.
GC ×GC-TOF MS analysis of fresh, HAD, and VFD B. fusco-purpurea
revealed the presence of 399, 352, and 449 volatile compounds,
respectively. Nine volatile categories were identied in the three types
of B. fusco-purpurea, with hydrocarbons, alcohols, and esters being the
most abundant chemical classes. Thermogram and cluster analyses
demonstrated that the drying process signicantly affected the volatile
components of B. fusco-purpurea. Substances including benzaldehyde,
2,6,6-trimethyl-1-cyclohexene-1-formaldehyde, and
α
-ionone were
signicantly reduced in both HAD and VFD samples. As drying pro-
gressed, hydrocarbons and alcohols were generated in the HAD and VFD
samples owing to the thermal degradation of carbohydrates, lipids,
amino acids, and Maillard reactions. The content of acetoin and hex-
adecane in HAD samples was signicantly lower, while that of 1,3-dime-
thylbenzene was signicantly higher. The contents of 1-methoxy-2-
propanol, decanal, 1-ethenyl-aziridine and 2-(diisopropylamino)ethyl
methacrylate in VFD samples were signicantly higher.
HAD has a better drying efciency than VFD. However, VFD can
greatly preserve the quality of products. Different drying methods for
B. fusco-purpurea resulted in signicant differences in the types and
amounts of volatile aroma constituents. The initial exploration into the
impact of the drying procedure on the quality and avor of B. fusco-
purpurea has provided preliminary theoretical and technical support for
the further development of high-quality dried B. fusco-purpurea. Based
on this, future research should be conducted to shorten the drying time
of VFD, improve the drying efciency, and elucidate the formation
mechanism of key avor substances during the drying process.
CRediT authorship contribution statement
Jingna Wu: Conceptualization, Methodology, Investigation,
Writing, Visualization. Nan Pan: Methodology, Writing, Formal anal-
ysis, Visualization. Xiaoting Chen: Writing, Formal analysis. Debiao
Shan: Formal analysis. Huifang Shi: Formal analysis. Yingshan Qiu:
Fig. 8. PCA of the volatile components of fresh, HAD, and VFD B. fusco-purpurea. (A) PCA score plot; (B) PCA loading plot.
J. Wu et al.

Current Research in Food Science 7 (2023) 100624
10
Formal analysis. Zhiyu Liu: Funding acquisition. Yongchang Su: Re-
sources. Junfa Weng: Resources.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
This work was supported by the Youth Science and Technology
Innovation Program of Xiamen Ocean and Fisheries Development Spe-
cial Funds [grant number 23YYST079QCA12] and Fujian Provincial
Science and Technology Plan [grant number 2022N3004].
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