Unlocking he Po en ial o Lignocellulosic Biomass: Mic owa e and
Hyd o he mal P e ea men o Imp o e he P oduc ion o High
Value-Added Bio e ine y Compounds
Ame Rouabhia, Ca los JoséAl a ez-Gallego, Luis Albe o Fe nández-Guel o,*Ma iana Valdez Cas illo,
and An onio A alos Rami ez
Ci e This: h ps://doi.o g/10.1021/acs.ene gy uels.5c03953
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ABSTRACT: This s udy is ocused on he pe o mance o a
hyd o he mal eac o (HTR) and mic owa e-assis ed (MW)
p e ea men s o suga bee pulp (SBP), o ange peel (OP), b ewe
spen g ain (BSG), and ice husk (RH) o e alua e he ex ac ion o
high- alue bio e ine y compounds. The in luence o empe a u e,
du a ion o ea men , and ene gy consump ion on hyd olysis
e iciency was e alua ed by quan i ying o al educing suga s
(TRS), p o eins (PR), polyphenols (TP), and ola ile a y acids
(VFA). MW p e ea men a 180 °C o 30 min yielded 18% TRS
and 24% PR om OP, espec i ely. In con as , HTR a 200 °C, o
60 min, achie ed highe yields o 32% TRS and 22% PR o OP.
BSG showed highe esponsi eness unde HTR, eaching 25% TRS
and 20% PR a 220 °C a e 120 min. The highes VFA p oduc ion was 16 g H−Ac/L (BSG, HTR) and 3.2 g H−Ac/L (SBP, MW)
a e 120 and 5 min a 220 °C, espec i ely. F om he poin o iew o ene gy consump ion, MW p e ea men consumed
signi ican ly less ene gy (40.1 kJ/g) han HTR (70.85 kJ/g) unde equi alen condi ions (120 min a 220 °C). In addi ion, he MW
p e ea men p o ed o be mo e ene gy-e icien o simple subs a es (SBP, OP), whe eas HTR was op imal o complex biomasses
(BSG, RH). The e o e, ailo ed p e ea men s a egies based on subs a e ype a e c ucial o op imize ene gy consump ion and
maximize biop oduc eco e y.
1. INTRODUCTION
In ecen yea s, bio e ine y has a ac ed signi ican global
in e es . Many coun ies ha e s a ed o adop bio e ine ies as
an economic al e na i e o adi ional ossil-based indus ies
o sus ainable p oduc ion and esou ce managemen .
1
The
pla o m concep in ol es he con e sion o biomass in o
se e al ypes o aluable biop oduc s
2
including bio uels,
biochemicals, bioplas ics, biopha maceu icals, biocosme ics,
bionu ien s, bio e ilize s, and bioma e ials.
3,4
A p e ea men
s ep is ypically in eg a ed in o he bio e ine y p ocess o
enable he e icien ac iona ion o biomass, which is c ucial in
he con e sion o complex biomasses such as he lignocellu-
losic ma e ials used in his s udy. This is o en he i s s age in
a lignocellulosic bio e ine y and i is essen ial o b eaking
down he complex s uc u e o biomass.
5
Physicochemical and
biological p e ea men s ha e been applied o e icien
biomass con e sion in o aluable biop oduc s,
6
and hei
selec ion is based on he speci ic a ge p oduc s.
7
Among
o he p e ea men s, hyd o he mal p e ea men (HTP)
gene a es g ea in e es because o i s simplici y, mode a e
ene gy consump ion, ela i ely sho p ocessing imes and cos
e ec i eness.
8,9
In HTP, empe a u e and p essu e play a
c i ical ole, ypically exceeding 180−200 °C and 15−20 ba .
Unde hese high- empe a u e and high-p essu e condi ions,
wa e unde goes inc eased au oioniza ion, gene a ing hyd o-
nium (H3O+) and hyd oxide (OH−) ions.
10
This ac ion
mechanism is analogous o he dilu e acid p e ea men ,
enhancing biomass depolyme iza ion and imp o ing he
e iciency o downs eam p ocesses such as enzyma ic
hyd olysis and anae obic diges ion.
11
Unde op imal condi ions, HTP e ec i ely dissol es hemi-
cellulose and pec in, leading o he eleasing aluable
byp oduc s and mic obial inhibi o s such as xylo-oligosaccha -
ides (XOS), u u al, 5-hyd oxyme hyl u u al (HMF), ace ic
acid, le ulinic acid, and o mic acid.
12
Indeed, empe a u es be ween 220 and 230 °C a e e ec i e
o HTP bu hey pose he isk o deg ading eleased suga s
in o u u al and HMF, which can be po en ially ad e se o
Recei ed: July 26, 2025
Re ised: Sep embe 25, 2025
Accep ed: Sep embe 26, 2025
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la e biological p ocesses in a bio e ine y app oach.
13
In
addi ion, c ys alline cellulose unde goes depolyme iza ion a
empe a u es abo e 220 °C, while p o eins hyd olyze in o
amino acids be ween 250 and 400 °C.
14
These condi ions also
acili a e lignin depolyme iza ion, p oducing phenolic com-
pounds such as sy ingols and ca echols.
15
Pa ame e s such as empe a u e, p essu e, sol en - o- eed
a io, low a e, sol en ype, and ope a ion ime play c i ical
oles in de e mining he e iciency o biomass solubiliza ion
and biop oduc eco e y.
16
Fu he mo e, he choice o hea ing
me hods, including elec ical hea ing, mic owa e adia ion,
s eam injec ion, o he mal oil sys ems, can signi ican ly a ec
he homogenei y and e icacy o hyd o he mal p e ea men .
Ruiz e al. (2017) demons a ed he impac o di e en
hea ing ans e mechanisms (i.e., conduc ion, con ec ion, and
adia ion) on p ocess e iciency.
17
Among hose, adi ional
hea ing eac o (HTR) and mic owa e-assis ed hyd o he mal
eac o (MW) sys ems a e he mos equen ly applied.
Mic owa e-assis ed p e ea men o e s se e al ad an ages
including low ope a ional cos , as p ocessing, and e icien
olume ic hea ing. I equi es minimal sol en use and
dec eases he likelihood o side- eac ions.
18,19
This app oach
aligns wi h g een chemis y p inciples, u ilizing wa e as a
sol en and biomass as a enewable eeds ock, he eby
unde sco ing i s po en ial as a sus ainable and e ec i e
al e na i e o adi ional hea ing me hods.
20
The biomass used in his s udy was chosen based on i s
global ele ance and la ge a ailabili y. O ange peel (OP) is one
o he mos abundan ag o-indus ial was es wo ldwide, wi h
Spain being he leading ci us p oduce in Eu ope gene a ing
abou 2.65 million ons annually.
21,22
The o ange peel is he
was e wi h he highes olume in he ci us indus y. I is
es ima ed ha a ound 20% o he o ange is o ange peel.
23
I s
s ong seasonali y can be mi iga ed by d ying and s o age.
Suga bee pulp (SBP) is ano he key byp oduc in Spain.
App oxima ely hal o he ube (0.5 kg/kg) is ejec ed du ing
he indus ial p ocess in he o m o SBP.
24
Du ing he 2023/
24 ag icul u al campaign, 3.02 million ons o suga bee s we e
p oduced; al hough seasonal, i is o en pelle ized o yea -
ound use. B ewe spen g ain (BSG), a byp oduc o bee
p oduc ion, is con inuously gene a ed bu unde goes as
mic obial deg ada ion because o i s high mois u e con en . I s
p oduc ion was a ound he 75% o he o al byp oduc and can
be used o bio echnological p ocesses because o i s
composi ion.
25
Finally, ice husk (RH) is p oduced in mo e
han 150 million ons wo ldwide (20 kg husk pe kg ice). Rice
husk con ains aluable bioma e ials wi h ex ensi e applica ions
in a ious ields, and is abundan and ela i ely easy o s o e
because o i s d y and s able na u e.
26
Al oge he , hese a ailabili y pa e ns and p ese a ion
op ions gi e po en ial o he s udied biomasses as sus ainable
eeds ocks. This ensu es hei sui abili y o indus ial-scale
bio e ine y applica ions.
F om all o he abo e, his esea ch is ocused on a
comp ehensi e compa ison be ween mic owa e-assis ed
(MW) and hyd o he mal eac o (HTR) p e ea men s,
aiming o clea ly dis inguish hei e ec s on p oduc yield,
selec i i y, and ene gy consump ion. I sys ema ically in es-
iga es how di e en lignocellulosic biomass esponds o
a ia ions in empe a u e and ope a ional ime, ocusing on
how hese ac o s in luence he dis ibu ion and selec i i y o
aluable biop oduc s in he hyd olysa e phase. The s udy also
includes a de ailed ene gy consump ion analysis, exp essed pe
kilog am o biomass (kJ/g), o e alua e he e iciency and
sus ainabili y o each p e ea men me hod. Fu he mo e, a
s a is ical analysis was pe o med o op imize he expe imen al
pa ame e s and alida e he signi icance o he obse ed ends.
To ou knowledge, his is he i s wo k o in eg a e hese
objec i es ac oss mul iple biomass ypes wi hin a single
e alua i e amewo k, o e ing aluable insigh s in o op imizing
p e ea men s a egies o imp o ed high- alue biop oduc
p oduc ion and esou ce e iciency in bio e ine y applica ions.
2. MATERIALS AND METHODS
2.1. Feeds ocks and Cha ac e iza ion. In his s udy, ou ypes
o lignocellulosic biomasses we e used: o ange peel (OP), suga bee
pulp (SBP), b ewe ’s spen g ain (BSG), and ice husk (RH). The OP
was sou ced om he can een o he Facul y o Science a he
Uni e si y o Cadiz (Cadiz, Spain). The collec ed OP was washed
se e al imes wi h dis illed wa e and d ied in an o en a 40 °C o 48
h. The SBP was p o ided by an indus ial suga ac o y belonging o
he AB-Suga Company loca ed in Je ez de la F on e a (Cadiz, Spain).
The RH was ob ained om a ice p ocessing plan in Se ille (Spain).
Bo h he SBP and RH we e ob ained as o igin-d ied ma e ial. The
BSG was collec ed om a local c a b ewe y in Pue o Real (Cadiz,
Spain), and he mix u e was d ied a 60 °C in an o en o 24 h.
Following a milling and sie ing p ocess, he d ied biomass was
educed o a pa icle size o 1.7 mm. Then, hey we e s o ed in a
eeze a 4 °C un il use. To cla i y he ole o hese p epa a o y s eps,
hey we e pe o med exclusi ely o s anda dize he pa icle size and
mois u e con en while p ese ing he chemical s uc u e o
biomasses.
2.2. Mic owa e P e ea men . A Miles one Flexiwa e de ice
wi h a maximum powe o 1900 W was used o ca y ou he
mic owa e hyd o he mal p e ea men . The ini ial d ied biomass
concen a ion was 8% (w/ ), and he expe imen s we e de eloped in
a 50 mL Te lon essel wi h con olled hea ing. The se empe a u es
and ope a ion imes we e 150, 180, 200, and 220 °C o 5, 15, 30, and
60 min. Tempe a u e con ol was managed by a noncon ac in a ed
senso , which accu a ely egula es empe a u es up o 300 °C,
depending on he essel ype. To ensu e consis en mic owa e ene gy
deli e y h oughou he p ocess, he eac o sys em is also equipped
wi h a high-e iciency ai -cooled magne on.
2.3. Hyd o he mal P e ea men . The hyd o he mal p e ea -
men was ca ied ou using an acid diges ion essel (Pa Ins umen ,
Model 4744, USA) ea u ing a 45 mL Te lon poly e a luo oe hylene
(PTFE) cup housed wi hin a s ainless-s eel jacke , wi h a wo king
olume o 30 mL. The sys em was equipped wi h a Scien i ic Fishe
Iso emp acuum o en (Model 282A) 3500 W ape u e o
empe a u e con ol, in which a e age empe a u e e o was
main ained wi hin ±5°C.
The subs a e was added in a d ied biomass was ewa e /wa e a io
o 8% (w/ ). The suspension was hea ed a 150, 200, and 220 °C,
wi h ope a ion imes o 30, 60, and 120 min.
2.4. Analy ical Me hods. The analy ical me hods used we e
ca ied ou in acco dance wi h p e iously published wo k.
21,27
They
we e employed o de e mine o al solids (TS), ola ile solids (VS),
soluble chemical oxygen demand (sCOD), dissol ed o ganic ca bon
(DOC), ola ile a y acids (VFAs), o al educing suga s (TRS), pH,
o al polyphenols (TP), and o al p o eins (PR). Fo all analyses,
samples we e cen i uged a 4,000 pm o 15 min o emo e
suspended solids, hen il e ed h ough 0.45 μm o sCOD and DOC
and 0.22 μm o TRS, VFAs, TP, and PR. All measu emen s we e
conduc ed in iplica e.
The solubiliza ion e iciency o he p e ea men p ocess was
de e mined using eq 1, whe e he OM and OM0 ep esen he inal
and ini ial concen a ions o solubilized o ganic ma e in he samples,
exp essed as COD, DOC, TRS, PR, TVFA, and TP, espec i ely.
= ×Y(%) 100 (OM OM )
OM
0
0
(1)
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B
Addi ionally, he yield on a weigh basis (%) was calcula ed using
he ollowing o mula:
= ×Yield (W %) Mass o P oduc
Mass o Feeds ock 100
i
k
j
j
jy
{
z
z
z
(2)
3. RESULTS AND DISCUSSION
Table 1 p esen s he physicochemical cha ac e iza ion o he
ou lignocellulosic biomasses. All he esul s ha e been
exp essed in % (w/w) on a d y ma e basis, excep o o al
solid (TS) and ola ile solid (VS), which is epo ed on a we
basis.
The o al polyphenols (TP) and o al p o eins (PR) we e
quan i ied in he aqueous phase o calcula e hei concen-
a ions a e he p e ea men s.
The selec ion o hese ou biomasses OP, BSG, SBP and
RH was d i en by hei ibe composi ions, mois u e con en ,
and po en ial o solubiliza ion. P ima y ocus was o ep esen
a ange o subs a es wi h a ying complexi ies in ibe
s uc u e o unde s and he e iciency o di e en p e ea men
me hods. Fibe con en analysis, which esul s a e exp essed as
a pe cen age o he biomass in d y weigh , was pe o med
acco ding o he Van Soes me hod.
28
Table 1. Composi ion o Biomasses
a
Pa ame e SBP
b
BSG
b
RH
b
OP
b
VS (g/kg) 739.03 ±0.2 261.06 ±0.2 772.04 ±0.1 195.06 ±0.1
TS (g/kg) 833.05 ±0.5 280.01 ±0.4 915.00 ±0.0 206.02 ±0.8
sCOD (g/kg) 10.60 ±0.2 21.90 ±0.7 1.29 ±0.0 41.10 ±0.4
DOC (g/kg) 4.09 ±0.1 7.93 ±0.1 0.52 ±0.1 10.10 ±0.0
TVFA (g H−Ac/kg) 0.87 ±0.0 1.39 ±0.0 0.03 ±0.0 0.43 ±0.0
To al p o ein (g/kg) 1.40 ±0.0 1.30 ±0.0 0.30 ±0.0 1.40 ±0.0
To al polyphenols (g/kg) 0.09 ±0.0 0.08 ±0.0 0.05 ±0.0 1.03 ±0.0
pH (pH uni s) 4.37 ±0.1 5.44 ±0.2 5.54 ±0.5 4.17 ±0.3
DOC/sCOD (%) 38.58 ±1.2 36.20 ±1.2 40.31 ±7.8 24.57 ±0.2
VS/TS (%) 88.72 ±0.0 93.23 ±0.1 84.38 ±0.0 94.68 ±0.4
NDF-Soluble ibe s
c
(%) 42.20 ±1.4 38.00 ±1.1 16.50 ±1.2 66.80 ±1.2
Cellulose (%) 21.10 ±1.4 16.30 ±0.4 32.85 ±0.4 15.70 ±2.2
Hemicellulose (%) 22.50 ±0.4 33.70 ±0.5 22.20 ±0.6 9.11 ±0.8
Lignin (%) 3.50 ±0.0 7.01 ±0.9 14.00 ±1.0 1.26 ±0.1
Res (%) 10.70 ±1.4 4.99 ±1.1 14.5 ±0.3 7.13 ±0.3
a
VS: ola ile solids; TS: o al solids; sCOD: soluble chemical oxygen demand; DOC: dissol ed o ganic ca bon; TRS: o al educing suga s; TP:
o al polyphenols; PR: o al p o eins.
b
OP: o ange peel; SBP: suga bee pulp; BSG: b ewe spen g ain; RH: ice husk.
c
NDF Soluble ibe s:
p ima ily composed o p o eins, pec in, s a ch, and mucilages.
Figu e 1. DOC and sCOD o SBP, BSG, OP, and RH by mic owa e-assis ed p e ea men (MW). No e: IN�ini ail ime.
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Ini ially, i was no iced ha OP and he BSG ha e high
mois u e con en (p e iously o he lab d ying p ocedu e),
close o 80% and 30% espec i ely. On he con a y, SBP and
RH p esen low mois u e con en since hey ha e been
p e iously d ied in he indus ial plan s whe e hey we e
gene a ed. Rega ding hei s uc u al composi ion, RH was
cha ac e ized by high concen a ion o lignin and cellulose,
14.0% and 32.8% espec i ely. Meanwhile, SBP has a low lignin
con en o 3.50%, bu i is ich in (NDF) soluble ibe o
42.2%. BSG displayed a complex s uc u e wi h 33.7%
hemicellulose, 7.0% lignin, 16.3% cellulose, and 38.0% NDF
soluble ibe , which includes s a ch and pec in. Howe e , OP
biomass, as a esh subs a e wi h no indus ial p ocessing,
e ains i s na u al composi ion wi h a ema kably high pec in
con en o 66.8% and a minimal lignin con en o 1.3%.
Polyphenols we e p esen a concen a ions ac oss all
biomasses, excep OP, which con ained 1.0 g/kg. The o al
p o ein con en was ela i ely high in OP and SBP, 1.4 g/kg
and 1.3 g/kg, espec i ely.
27
Fu he mo e, he VS/TS a io,
ep esen ing he p opo ion o o ganic ( ola ile) solids ela i e
o o al solids, p o ides insigh in o he biomass po en ial o
biocon e sion. Among he ou biomasses, OP shows he
highes o ganic con en a app oxima ely 94.65%, ollowed
closely by BSG a 93.23%. SBP and RH ha e sligh ly lowe
o ganic ac ions, 88.69% and 84.34% espec i ely. This
indica es ha OP and BSG possess a highe amoun o
biodeg adable o ganic ma e , making hem mo e amenable o
p e ea men and subsequen biop ocessing. On he o he
hand, he DOC/sCOD a io e lec s he quali y and
biodeg adabili y o he soluble ac ion a e p e ea men , as
(DOC) co esponds o bioa ailable ca bon, while (sCOD)
includes all soluble oxidizable subs ances. RH p esen s he
highes DOC/sCOD a io (40.31%), indica ing ha a
ela i ely la ge po ion o soluble ca bon is bioa ailable
despi e i s lowe o al solubiliza ion. SBP and BSG show
simila a ios (38.58% and 36.20%), sugges ing a mode a e
bioa ailabili y o he soluble ac ion. In con as , OP, despi e
ha ing he highes sCOD alue, has he lowes DOC/sCOD
a io (24.57%), implying ha a signi ican po ion o i s soluble
compounds may be less eadily biodeg adable o mo e
e ac o y. These di e se ibe and biochemical p o iles
highligh he signi icance o selec ing a p e ea men me hod,
he eby b oadening he applicabili y o s udy indings o
bio e ine y applica ions.
3.1. S a is ical Analysis. The s a is ical analysis me hod as
well as a ho ough desc ip ion o he co esponding me hod-
ology can be ound in he Suppo ing In o ma ion. In Table S1
a e summa ized he s a is ical esul s, which showed ha ime
and empe a u e had s a is ically signi ican e ec s on all
solubiliza ion pa ame e s, including sCOD, DOC, TRS, VFAs,
TP, and PR.
3.2. O ganic Ma e Solubiliza ion. 3.2.1. Mic owa e
Assis ed Solubiliza ion. The solubiliza ion yield o he
biomasses du ing mic owa e-assis ed (MW) p e ea men
was analyzed h ough sCOD and DOC measu emen s (Figu e
1).
MW p e ea men is pa icula ly e ec i e o achie ing apid
hyd olysis, especially o subs a es such as SBP and OP. I s
e iciency lies in i s abili y o accele a e he b eakdown p ocess,
making i ideal o scena ios equi ing he quick elease o
soluble o ganic compounds.
Figu e 2. DOC and sCOD o SBP, BSG, OP, and RH by hyd o he mal p e ea men (HTR). No e: IN�ini ial ime.
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Figu e 3. To al educing suga (TRS), exp essed in e ms o g/L, unde a mic owa e-assis ed (MW) and hyd o he mal eac o (HTR). No e: IN�
ini ial ime.
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SBP solubiliza ion eached i s highes alues a 180 °C a e
10 min, wi h sCOD and DOC concen a ions o 47 g O2/L
and 15.5 g C/L, espec i ely, bu declined a 220 °C (39.7 g
O2/L, 12.6 g C/L). P olonged exposu e u he educed he
e iciency. The sCOD yield o 58.75% ob ained in his s udy
su passes 13.7% o sCOD ob ained by Ozkan e al. (2011),
unde MW p e ea men condi ions (700 W, 170 °C) o 30
min. Mo eo e , i closely aligns wi h hei 58% sCOD yield
om he mal alkaline p e ea men a 121 °C.
29
These
compa isons unde sco e he impac o op imizing he mic o-
wa e powe and empe a u e condi ions o enhance he
solubiliza ion e iciency.
BSG solubiliza ion showed no signi ican changes du ing he
i s 5 min, wi h sCOD and DOC be ween 21.1−22.4 g O2/L
and 6.5−9.5 g C/L. A 220 °C a e 10 min, solubiliza ion
inc eased o 34.7 g O2/L sCOD and 11.4 g C/L DOC,
eaching maximum alues o 43.5 g O2/L and 15.4 g C/L a e
30 min. Solubiliza ion e iciency dec eased wi h ex ended
p e ea men du a ions.
OP showed as ini ial solubiliza ion ac oss all empe a u es,
wi h he highes alues a 180 °C (56.5 g O2/L sCOD, 17.5 g
C/L DOC), ollowed by 200 °C (48.9 g O2/L, 14.6 g C/L)
and 220 °C (42.7 g O2/L, 9.1 g C/L). P olonged p e ea men
pe iods educed he solubiliza ion yields, wi h he lowes alues
in e ms o sCOD and DOC obse ed a 200 °C a e 60 min
(22.7 g O2/L, 7.2 g C/L). Consis en solubiliza ion a 150 °C
achie ed maximum alues o 59.4 g O2/L and 17.6 g C/L a e
30 min, indica ing his as an op imal condi ion o sus ained
OP solubiliza ion.
RH solubiliza ion emained consis en ly limi ed, achie ing
maximum sCOD and DOC concen a ions o 15.4 g O2/L and
6 g C/L a 200 °C a e 30 min. This is consis en wi h
Kain hola e al. 2019, who epo ed 15000 mg/L sCOD o
ice s aw a 190 °C (1200 W).
30
RH showing limi ed esponse
highligh s he need o al e na i e o supplemen a y me hods
o enhance solubiliza ion e iciency.
The di e ence in ibe composi ion plays a c ucial ole in he
e iciency o solubiliza ion du ing he MW p e ea men .
Speci ically, SBP and OP exhibi a no ably highe pe cen age
o soluble ibe s (42% in SBP and 66% in OP) along wi h
signi ican quan i ies o ca bohyd a es and low lignin con en
(1.3% o OP and 3.5% o SBP). The low lignin p opo ion
educes s uc u al esis ance, acili a ing an enhanced in e -
ac ion be ween wa e molecules and he polysaccha ide ma ix
unde MW condi ions. These a ibu es enable as e and mo e
e icien hyd olysis a ela i ely lowe empe a u es.
31,32
In ac , imp o ed saccha i ica ion a es in OP a e MW
ea men ha e been al eady epo ed in he li e a u e due o
i s ca bohyd a e- ich composi ion.
33−36
The a o emen ioned
s udies highligh he e ec i eness o MW p e ea men in
op imizing he subsequen biomass con e sion p ocesses.
3.2.2. Con en ional Hyd o he mal Reac o Solubiliza ion.
Figu e 2 shows he solubiliza ion e iciency o he ou
biomasses (SBP, OP, BSG, and RH) ea ed in a hyd o he mal
diges e eac o (HTR) a 180, 200, and 220 °C o 30, 60, and
120 min.
A 180 °C, solubiliza ion p og essi ely imp o ed, wi h SBP
and OP exhibi ing he highes e iciency. A e 120 min, SBP
and OP eached maximum sCOD and DOC alues o 46.5 g
O2/L, 16.8 g C/L, and 55.2 g O2/L, 22.1 g C/L, espec i ely.
The solubiliza ion yield o 61% achie ed o OP a 150 °C
unde HTR aligns wi h p e ious indings;
37,38
hese au ho s
epo ed yields o 46.6% o lemon peels and app oxima ely
60% o o ange peels, espec i ely, a 160 °C. Thei s udy
con i med ha his empe a u e imp o es he highes elease o
soluble o ganic compounds om biomass. In con as , BSG
and RH displayed lowe yields, eco ding sCOD and DOC
alues o 18.6 g O2/L, 7.1 g C/L, and 5 g O2/L, 1.3 g C/L,
espec i ely. A 200 and 220 °C, he solubiliza ion yield
imp o ed ac oss all subs a es. SBP and OP eached hei
highes sCOD and DOC le els a 60 min (41.2 g O2/L, 17 g
C/L) and 30 min (49.5 g O2/L, 19.7 g C/L), espec i ely,
be o e declining. BSG and RH g adually inc eased, achie ing
hei highes solubiliza ion a 120 min, wi h BSG a 51.2 g O2/
L, 9.2 g C/L, and RH a 19 g O2/L, 7.8 g C/L. O he s udies
ha e showed ha empe a u es abo e 200 °C a e pa icula ly
e ec i e in enhancing BSG deg ada ion and biop oduc
eco e y.
39−41
O e all, he empe a u e and ime signi ican ly in luenced
solubiliza ion. Op imal hyd olysis occu ed a 220 °C, wi h OP
equi ing only 30 min, and SBP and BSG 60 min, while RH
showed he slowes and leas e ec i e solubiliza ion.
The HTR me hod gene ally achie es highe solubiliza ion
e iciency o e longe ea men imes o subs a es wi h
complex s uc u es such as BSG and RH, which a e mo e
esis an due o hei signi ican lignin and lignocellulosic
con en . Speci ically, BSG and RH con ain signi ican lignin
le els (7% and 14%, espec i ely). Addi ionally, he e is a low
soluble ibe con en (38% o BSG and 16% o RH)
compa ed o SBP and OP. Lignin ac s as a ba ie o
hyd olysis, limi ing he accessibili y o hyd oly ic agen s o
cellulose and hemicellulose ac ions, while i s in e wo en
s uc u e and chemical bonds wi h hemicellulose u he
complica e deg ada ion. Unde HTR condi ions, ele a ed
empe a u es dis up hyd ogen bonds wi hin he lignocellulosic
ma ix and induce pa ial depolyme iza ion o lignin, he eby
exposing cellulose and hemicellulose o hyd olysis.
Howe e , he e iciency o solubiliza ion is highly dependen
on he ele a ed empe a u es and ea men du a ion. S udies
indica e ha high empe a u es posi i ely impac ma e ials
wi h subs an ial lignin con en , as seen in RH. Howe e , he
p olonged ea men ime may become a limi ing ac o ,
po en ially dec easing solubiliza ion e iciency due o he
deg ada ion o soluble compounds in o ecalci an byp oduc s
such as u u al and hyd oxyme hyl u u al (HMF), which
hinde u he hyd olysis.
In Mussa o and coau ho s’ wo ks i was poin ed ou ha he
di icul y o he hyd olysis o BSG is due o i s high lignin and
hemicellulose con en .
42,43
In pa allel, Lu e al. (2012) a gued
ha he di icul y in he solubiliza ion o RH is due o i s
signi ican lignocellulosic composi ion, which equi es s ic
p e ea men condi ions.
44
The e o e, he choice o he p e ea men me hod and
condi ions should be ailo ed acco ding o he speci ic biomass
and he desi ed esul s o solubiliza ion e iciency.
3.3. Impac o he P e ea men P ocess on Suga
P oduc ion and Beyond. The e ec o hyd o he mal
p e ea men on p oduc dis ibu ion (TRS, PR, VFA and
TP) was also assessed.
3.3.1. To al Reducing Suga (TRS). The o al educing suga
(TRS), exp essed in e ms o g ams TRS/L eleased o he
aqueous phase a e MW o HTR p e ea men , is shown in
Figu e 3.
TRS concen a ions a ied acco ding o biomass composi-
ion. Fo MW p e ea men , SBP and OP eached hei highes
TRS concen a ions a 180 °C, wi h 12 g/L a e 60 min and
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Figu e 4. To al p o ein yield (PR), exp essed in % (w/w), o SBP, BSG, OP, and RH unde mic owa e assis ed (MW) and hyd o he mal eac o
(HTR) p e ea men s. No e: IN�ini ial ime.
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Figu e 5. To al ola ile a y acids (TVFA), exp essed as g H−Ac/L, o SBP, BSG, OP, and RH unde mic owa e assis ed (MW) and
hyd o he mal eac o (HTR) p e ea men . No e: IN�ini ial ime.
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14 g/L a e 30 min, espec i ely. BSG was achie ed a 11 g/L
a 220 °C a e 60 min. RH had he lowes yield o 3.2 g/L a
220 °C a e 60 min, indica ing a lowe hyd olysis e iciency.
TRS in OP declined signi ican ly a e 30 min, sugges ing
deg ada ion o con e sion o hese monome s in o o he
biop oduc s, such as acids, o hei ans o ma ion in o new
compounds. HTR showed a signi ican impac o e TRS,
inc easing i s alue o all biomasses o e ime. The highes
concen a ions o SBP and BSG we e 22 and 23 g/L a 220
°C a e 60 min, hough yields dec eased a e wa d. A 200 °C,
he e was no signi ican e ec on TRS o SBP, while BSG
inc eased om 10 o 22 g/L a e 120 min. This esul aligns
wi h Cos a e al. (2014), who epo ed 12.15 g/L TRS om
suga cane bagasse unde simila condi ions.
45
OP exhibi ed a
no able esponse, eaching 18 g/L a 150 °C a e 120 min and
a aining a maximum o 28 g/L a 200 °C a e 60 min be o e
declining. A 220 °C, TRS s abilized a ound 22.4 g/L be ween
30−60 min, hen dec eased a e 120 min. These esul s a e
consis en wi h Ri as e al. (2008), in which 38.2 g/L TRS
om OP a 130 °C was ounded.
46
RH exhibi ed he lowes
TRS concen a ion among he es ed biomasses, eaching 12
g/L a e 120 min a 200 °C. A simila yield was obse ed a e
60 min a 220 °C. The hyd o he mal p e ea men e ec i ely
decomposes lignocellulosic biomass, enhancing suga elease
in o he liquid phase. By b eaking down hemicellulose and
pa ially deg ading cellulose a ele a ed empe a u es, his
me hod signi ican ly imp o es TRS yields.
10
3.3.2. To al P o ein Concen a ion (PR). Figu e 4 shows he
e ec o MW and HTR p e ea men on he p o ein’s
ex ac ion o he es ed biomasses.
To al p o ein concen a ion was e alua ed by using bo ine
se um albumin (BSA) as a s anda d o compa e be ween he
wo p ocesses.
Fo SBP and OP, simila p o ein yields we e obse ed. Using
he MW p e ea men , SBP yielded 21.8% PR a e 10 min a
220 °C, while a sligh ly highe yield (22.42%) was ob ained a
he same empe a u e a e 60 min. In he case o HTR
p e ea men , he maximum PR yield (24.8%) was achie ed
wi h he OP a 200 °C o 60 min. Howe e , he highes PR
eco e y o 26.3% was ob ained unde MW ea men a 220
°C a e jus 5 min.
These esul s may be explained by he biochemical
composi ion o he subs a es. SBP and OP con ain mo e
eadily ex ac able soluble p o eins and a less s uc u ally igid
lignocellulosic ma ix. This s uc u e acili a es p o ein
dena u a ion and un olding du ing he mal o mic owa e
ea men s, inc easing he ex ac abili y. P olonged HTR
exposu e u he enhances ma ix dis up ion, pa ial lignin
depolyme iza ion, and polysaccha ide hyd olysis, which
p omo es p o ein solubiliza ion and he elease o suga s ha
may con ibu e o VFA o ma ion. Howe e , MW p e ea -
men was less e ec i e o ano he biomass, such as BSG and
RH. In hese cases, he maximum PR yields achie ed we e 6%
and 7.9% a e 60 and 30 min o MW ea men a 220 °C,
espec i ely.
Unde HTR condi ions, he PR yield in BSG inc eased
signi ican ly, eaching 19.41% a e 60 min a 200 °C, while in
RH i only eached 10.55% a e 120 min a 220 °C. The lowe
MW e iciency may be a ibu ed o he complex ibe -encased
s uc u e and highe lignin con en o BSG and RH, which
limi p o ein accessibili y. HTR, wi h i s uni o m and sus ained
he mal exposu e, dis up s hese ma ices mo e e ec i ely,
allowing g ea e p o ein solubiliza ion. P olonged ea men ,
howe e , leads o a dec ease in o al p o ein, e lec ing
deg ada ion in o smalle molecules such as amino acids and
pep ides, con ibu ing o he soluble o ganic ac ion.
Li e a u e suppo s hese indings, o example Yin e al.
(2014) om 30 g o ood was es (mainly ice, mea , ege ables
and o u) epo ed 22.50 g/kg o solubilized p o ein a e 30
min o HTR p e ea men a 220 °C.
47
O he au ho s as Qin e
al. (2018) s udied he hyd o he mal p e ea men o BSG o
p o ein ex ac ion, achie ing 66% o he o al p o ein (14.91 g/
100g aw BSG) success ully ex ac ed a 60 °C a e 24 h o
ea men .
48
Acco ding o he e iew o Sche zinge and Kal schmi
(2021), he applica ion o MW p e ea men o a ious ood
was es a 175 °C has been success ul o enhanced
solubiliza ion o p o eins, suga s and humic-like subs ances.
49
O e all, he esul s demons a e ha subs a e composi ion
s ongly in luences p o ein solubiliza ion, which subsequen ly
a ec s he pe o mance o subsequen biomass hyd olysis and
biocon e sion p ocesses.
3.3.3. E ec o Hyd o he mal P ocess in VFA P oduc ion.
The o al ola ile a y acid (TVFA) p oduc ion in e ms o
ace ic acid is shown in Figu e 5. Wi hin he es ed condi ions
and selec ed subs a es, he MW p e ea men does no seem
o be e ec i e o he TVFA p oduc ion. O e all, he
con e sion o biomass was es o TVFA is lowe han 3 gH−
Ac/L. Especially wi h RH and BSG, TVFA p oduc ion is e y
low (0.5 g H−Ac/L), sugges ing ha he MW-assis ed p ocess
has in insic limi a ions.
The high silica and lignin con en in RH and BSG p obably
ende ed hese ma e ials esis an o MW p e ea men ,
he eby cons aining he con e sion o hyd olysa es o
VFA.
50
SBP and subsequen ly OP showed he highes TVFA
p oduc ion a e 5 min a 220 °C, wi h maximum alues o 3.2
and 1.4 g o H−Ac/L, espec i ely. TVFA p oduc ion
emained s eady o 30 min be o e dec easing a all
empe a u es. This sugges s ha while ini ial TVFA p oduc ion
is as , con e sion e iciency dec eases o e ime due o
possible deple ion o eadily a ailable subs a es o inhibi ion
by accumula ed byp oduc s. The lowe lignin con en and
highe ca bohyd a e accessibili y o SBP a e mo e sui able o
he MW p e ea men . Fu he mo e, pec in and essen ial oils
o OP may hinde ull hyd olysis and VFA con e sion; despi e
ela i ely high ini ial suga elease, hese componen s may slow
o e all p ocess e iciency.
51,52
Figu e 5 also highligh s he subs an ial impac o HTR
p e ea men on VFA p oduc ion, showing consis en inc eases
o e ime a 200 and 220 °C. Maximum concen a ions we e
achie ed a e 120 min a 220 °C: 16 g H−Ac/L o SBP, 8.5 g
H−Ac/L o BSG, and 4.5 g H−Ac/L o RH. These esul s
a e simila o hose ob ained 20 g/L VFA om maize s alks
unde simila hyd o he mal condi ions (hyd o he mal ea -
men se e i y HTS ac o 5.16, 220 °C, oxygen- ee).
53
Simila ly, Zieminski e al. (2014) obse ed compa able ends
o SBP, 1.6 mg/mL o VFA a 200 °C a e 20 min a e liquid
ho wa e p e ea men .
54
Fo OP, he highes TVFA
concen a ion o 8 g/L was eached a e only 60 min a 220
°C. Howe e , a e his maximum, he TVFA concen a ion
g adually dec eased, indica ing ha he op imal hyd o he mal
p e ea men ime o OP is sho e han ha o he o he
es ed biomasses. P e ea men a 150 °C was ine ec i e ac oss
all biomasses wi h VFA yields lowe han 0.5 g/L, indica ing
insu icien con e sion e iciency a lowe empe a u es. Ace ic
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