399
1 INTRODUCTION
The knowledge and understanding of moisture
sorptionisothermsforfoodsisofgreatimportancein
food science and technology for many uses such as
the design and optimization of processing as for
instance in drying, for assessing the packa ging and
transporting problems, for modeling moisture
changes which occur
during drying, for predicting
shelf life stability and for ingredient mixing
predictions[SinijaandMishra,2008].
Tea leaves (Camellia sinensis L.) are the source of
the world’s most popular beverage, and can be
processed and fortified with different fruits, flowers
and spices to fulfill specific features desired by the
consumer.Teaisgrownincountrieswithwarmand
humid climate mainly in China, India, Japan, Sri
Lanka,butalsoinRwandaandArgentina.Teaisone
of the most popular beverages in the world, which
caninhibitthedevelopmentofcancer,lowertherisk
of cardiovascular disease, and improve cognitive
healthinhumanhuman[Chi‐Weietal.,2017,Chung
et al., 2010]. It is a plant rich in polyphenols,
flavonoids such as flavanols (catechins,
procyanidins), flavonols (rutin, quercetin,
kaempferol), and phenolic acids (gallic, caffeic).
Polyphenolic compounds in tea leaves make up to
30%ofgreentea,butonly
10%ofblacktea.Themajor
catechins in tea are: (+)‐catechin (C), (‐)‐epicatechin
(EC), (‐)‐gallocatechin (GC), (‐)‐epicatechin gallate
(ECG), (‐)‐epigallocatechin (EGC), and (‐)‐
epigallocatechingallate(EGCG).However,themajor
antioxidantintealeavesisconsideredtobeEGCG,a
Equilibrium Moisture Content Importance in Safe
Maritime Transport of Black Tea
P.Dmowski&M.Ruszkowska
GdyniaMaritimeUniversity,Gdynia,Poland
ABSTRACT:Inseatransport,averyimportantthingisanassignmentofcargotoaparticularclassofstorage
climate conditions and it is carried out on the basis of the requirements that cargo places upon its storage
atmosphere.Thewatercontentofblackteamust
notfallbelow2%,astheproductotherwisebecomeshay‐like
anditsessentialoilsreadilyvolatilize,whileontheotherhand,itmustnotexceed9%asitthenhasatendency
to grow mould and become musty. Therefore, tea requires particular temperature, humidity/moisture and
possiblyventilationconditions.In
thiscontext,teaisahygroscopicmaterialthathastheabilitytoabsorbor
desorbwaterinresponsetotemperatureandrelativehumidityoftheatmospheresurroundingit.Themoisture
contentofteaisoneofthemostimportantvariablesaffectingitschemicalandsensoryproperties.Therefore,to
exploreandpredict
thebehaviourduringtransportoftea,itsequilibriummoisturecontentmustbedetermined
forarangeoftransporttemperaturesandrelativehumidities.Thepresentpaperfocusesontheevaluatethe
hygroscopicpropertiesofteafromRwandawithdifferentdegreeoffragmentationbasedonisothermsofwater
vapour sorption and characteristics
selected parameters of the surface microstructure determining transport
conditionsandthereformicrobiologicalstabilityofteas.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 12
Number 2
June 2018
DOI:10.12716/1001.12.02.22
400
catechin compound with eight free hydroxyl groups
(OH)which are decisive for its high antioxidant
activity [Gramza‐Michałowska et al., 2016]. In
products’ (such tea, coffee) storage and transport,
polyphenols are unstable under various conditions,
such as presence of oxidative enzymes, high
temperatures, pH level, moisture content, and
presence of light and oxygen. Wang and Helliwell
[2000] they were demonstrated that time of tea
storage played a significant role in reducing the
content of catechins. The contents of catechins in
samples of tea after 60 days of storage were about
20% lower than in fresh leaves of tea –
respectively
from363mg/100mlto317mg/100mlfor( ‐)‐EGCand
from 87.3mg/100ml to 71.5mg/100ml for (‐)‐EC and
after 120 days of storage the content the catechins
have moved to the level of 266mg/100ml and
62.3mg/100ml–respectivelyfor(‐)‐EGCand(‐)‐EC.
In
sea transport, a very important thing is an
assignment of cargo to a particular class of storage
climateconditionsanditiscarriedoutonthebasisof
the requirements that cargo places upon its storage
atmosphere. Depending on the classification of the
product, different parameters have to be set for
the
risk factors temperature, humidity/moisture and
ventilationtopreventareductioninproductquality.
Tea requires particular temperature,
humidity/moisture and possibly ventilation
conditions(SCVI).Thisgroupincludesgoodswitha
low water content (WCC 2 – 1.5 – 30%), which is
constantly determined by the humidity and
temperatureconditions
oftheambientmedium.This
groupincludesgoodswithalowwatercontentthatis
constantly determined by the humidity and
temperatureconditionsof theambient medium. The
watercontentofblackteamustnotfallbelow2%,as
the product otherwise becomes hay‐like and its
essential oils readily volatilize,
while on the other
hand,itmustnotexceed9%asitthenhasatendency
to grow mold and become musty. The sorption
isotherms for these hygroscopic goods exhibit a
continuous profile without sudden jumps.
Undesirable changes occur as a function of relative
humidity and temperature, in particular due
to
dampening (mold, rot, mildew stains, fermentation,
deliquescence, self‐heating) or desiccation
(solidification, jamming/caking, fragmentation,
drying‐out).Thegoodsinthisgroup(alsotea)donot
have any particular requirements as to ventilation
conditions,sincetheyaredryforshipmentanddonot
respire(BA 3‐goodsin whichrespirationprocesses
(external respiration) are suspended, but in which
biochemical, microbial and other decomposition
processesstillproceed,suchasmeat,fish,processed
grainproducts,dried fruits, spices, cocoa and coffee
beans, tea, tobacco, expellers, fish meal). Other
examplesofgoodsinthisgrouparefoodstuffs(riskof
dampening, drying‐out and loss
of aroma), green
coffeebeans, raw cocoa, green tobacco(risk of post‐
fermentation), dried fruits (risk of syrup formation
andfermentationduetoeffectofhumidity/moisture),
furs, hides, packaging materials, natural fibers and
fibrous materials, wood and wooden articles,
furniture and musical instruments. Ventilation may
possiblybenecessary,iftherequired
temperatureand
relative humidity parameters are to be achieved.
Favorabletraveltemperaturefortearangedfrom5to
25°C.Teamustbestowedawayfromsourcesofheat
inorder toavoidtherisk of desiccationanddrying.
Tea requires also particular humidity/moisture
conditions in the range from 60 to
65%, because
mouldmayeasilydeveloponsamplesinthetoohigh
relativehumidityenvironment.Teaispredominantly
transported in standard containers, also known as
general purpose containers. Containers intended for
loading have to be watertight and must not be
contaminated in any way. Containers which floors
release a foreign odor,
are contaminated by any
substancesoraretoodampshouldberejected.Below
deckstowageisrequiredtoruleoutthepossibilityof
exposuretorainorseawaterorofoverheatingbyday
and cooling at night. Tea in containers should be
stowedawayfromsourcesofheat.Indamp
weather
(rain, snow), the cargo must be protected from
moisture, since moisture may lead to mold growth
and mustiness. [www.tis‐gdv.de;Ładunki okrętowe,
1994;Sharnow,2000].
In this context, tea, as other plant materials (for
examplewood),isahygroscopicmaterialthathasthe
ability to absorb or desorb water
in response to
temperatureandrelativehumidityoftheatmosphere
surroundingit.Thisaffinityofteaforwateriscaused
bythehydroxylgroupsaccessibleinthecellwallsof
tea.Asaconsequence,themoisturecontentofteais
one of the most important variables affecting its
chemical(catechins)
andsensoryproperties.
Generally, there are two types of water in tea
leaves. The “water of constitution” is the water
includedinthechemicalstructureofwoodanditis
inherent to the organic nature of the cell walls. It
cannot be removed without modifying the chemical
compositionof tea leaves.
The second type of water
comes in three forms: “bound” or “hygroscopic”
water which is adsorbed by sorption sites in
amorphous areas of cellulose and hemicelluloses
presentinthecellwalls;“free”waterwhichisliquid‐
likewaterin the celllumen and voids; and “vapor”
waterwhichispresent
invoidsandcelllumen.
As leaves of tea begin to lose moisture when
exposedtoambientconditionsinthetransport,water
leaves from voids and cell lumen first while the
boundwatercontentremainsconstant.Themoisture
contentlevelwhichcorrespondstolumencontaining
nofreewater(butwillcontain
watervapor),whileno
bound water has been desorbed from the cell wall
material, is known as the fiber saturation point. For
mosttealeaves,fibersaturationpointisintherange
8±3%.Asthemoisturecontentoftealeavesdecreases
belowfibersaturationpoint,boundwaterwillbegin
to
leavethecellwallmaterialanddifferentsituation
is observed when the moisture content of tea leaves
increasesmorethanfibersaturationpoint.Mostofthe
chemical and sensory properties vary with moisture
contentbelowandabovethispoint.Atacertainpoint,
equilibrium is attained between moisture in the leaf
and that in the surrounding atmospheric
environment,andthissituationisthebestforthetea
transportconditions.Thisiswherethetealeaveswill
not gain or lose any moisture with time and it is
knownastheequilibriummoisturecontentofthetea
[Hartley and Hamza, 2016]. Simulation
models for
dryer design, dryer optimization and control for
several agricultural products use the difference
between the actual moisture content and the
401
equilibrium moisture as a measure for the driving
forcefordrying.Duringthemanufactureofblacktea,
the macerated leaf (termed “dhoolʹʹ) which has
undergone “fermentation ʹʹ in tea terminology
(actually enzymic oxidation) is then dried from
around 70% w.b. moisture content to a target
moisture content of 3% w.b.
Some sorption of
moisture during sorting and packing takes place so
the packed product remains below 7% w.b. As the
moisture content is reduced to such a low level
comparedtomostagriculturalproducts,equilibrium
moisture content (EMC) plays a particularly
importantrole at the end of drying andat
transport
condition[TempleandvanBoxtel,1999].
Knowledgeofthemoisturesorptioncharacteristics
oftea is importantfor predicting itsstability during
storage, transport and selecting appropriate
packaging materials. Different materials have
different equilibrium moisture contents (EMC). The
EMCisdependentuponthetemperatureandrelated
humidity of the environment as
well as species,
variety and maturity of the grain. The equilibrium
moisture of tea can be used in modeling the
transporting process [Sinija and Mishra, 2008;
Ghodakeetal.,2007].
The measurement of physical properties of tea is
important because they intrinsically affect its
behaviour during storage, handling, processing and
transport.Thedegreeoffragmentationofteaandits
behaviourunderpressure,temperatureandhumidity
areimportantinhandlingandprocessing operations,
suchasstorage,transportation,formulationblending
andpackaging.
The present paper focuses on the evaluate the
hygroscopic properties of tea from Rwanda with
differentdegreeoffragmentationbased
onisotherms
of water vapor sorption and characteristics selected
parametersofthesurfacemicrostructuredetermining
transportconditions and therefor microbiological
stabilityofteas.
2 METHODOLOGY
The research material was five black tea samples
originated from Rwanda and differing by degree of
fragmentation.Samplesoforthodoxblackteawereas
follows: FOP
– Flowery Orange Pekoe, OP‐Orange
Pekoe, OPA – Orange Pekoe grade A, BOP – Broken
OrangePekoeandP–Pekoe.
The sorption method used was the static
gravimetrictechnique, based ontheuseofsaturated
salt solutions to maintain a fixed relative humidity
whentheequilibriumis
reached.Thewateractivityof
the food is identical to the relative humidity of the
atmosphere at equilibrium conditions and the mass
transfer between the product and the ambient
atmosphere is assured by natural diffusion of the
watervapor.
Tested black teas (I‐V) were subjected to a
preliminary analysis
by the physico‐chemical
determination of the initial water content.
Determinations of water content were taken by
drying at 103°C [ISO 1573‐1980 (E) Tea –
Determinationoflossinmassat103°C].
Isotherms of adsorption were determined by the
static method, based on a moisture equilibrium
between the tested
product samples and the
atmosphere of defined a relative humidity of, a
regulated by means of the salt solutions [Gondek et
al.,2013].Thedeterminationofadsorptionisotherms
carriedoutat25°C±1°Cinarangeofwateractivity
a
w=0.07÷0.98. Timefixtureequilibriumwas 30 days.
Thesampleforthestudyconsistedofabout2 gofthe
product tested. The samples were weighing were
placed in a dish measuring 15 mm in diameter to
formabedwithaheightof2‐3mmanddeployedin
desiccators.
Thymol was placed in the desiccators
with water activity >0.7 in order to protect samples
againstthe growth of microorganisms. Based on the
initialweightoftheproductandthegrowthorlossof
water content equilibrium water content was
calculatedandplottedsorptionisotherms.
In order to describe empirically
designated
sorptionisothermshavebeenrestatedtheequationof
Brunauer, Emmett and Teller (BET) (1) [Brunauer et
al.,1938],therangeofwateractivity0.07≤a
w≤0.33.The
equationwascharacterizedonthebasisofthevalue
ofthecoefficientofdetermination(R
2
),standarderror
(FitStdErr)andthevalueofstatisticsF.
(1 )[1 ( 1) ]
mw
ww
ca
a
aca
(1)
where:
a‐ adsorption,(g/g);
m‐ watercontentinthemonolayer(g/g);
c‐ constantenergy(kJ/mol);
a
w‐ wateractivity(‐).
Resultsachievedinrespectof sorption properties
were analysed with the computer software Jandel‐
TableCurve2Dv5.01.,whichenableddetermination
of such parameters of the sorption process as:
capacity of the monomolecular layer and energetic
constant.
On the basis of water content estimated in
the
monolayeradsorbedatatemperaturelowerthanthe
boiling temperature and the so called “water cross‐
section”, the specific surface area of adsorbent was
calculatedaccordingtotheequation(2),[Paderewski,
1999]:
m
sp
V
aN
M
(2)
where:
a
sp–sorptionspecificsurface,(m
2
/gd.m.),
N–Avogadronumber,(6.02310
23
molecules/mol),
M–watermolecularweight,(18g/mol);
ω–watersettingsurface,(ω=1.05∙10
‐19
∙m
2
/molecule).
3 RESULTSANDDISCUSSION
Thetealeavesareveryhigroscopicplantmaterialand
very susceptible to water. As during transportation
process of tea, the relative humidity of the
surroundingaircouldbeincreased,leavesofteatend
to absorb water. On the other hand, as the relative
402
humidity decreases, the leaves of tea tend to desorb
water. The basis for the assessment of the
hygroscopicityofteascharacterizedbyvarydegreeof
fragmentation transported from Africa was the
determinationofthewatercontent(Table1).
Table1.Thewatercontentofthetestedproducts
_______________________________________________
Samplesoftea/degree Watercontent(g/100gd.m.)
offragmentationmean±SD
_______________________________________________
I/FloweryOrangePekoe 6.88±0.03
II/OrangePekoe6.99±0.04
III/BrokenOrangePekoe 7.01±0.04
IV/OrangePekoegradeA 7.13±0.05
V/Pekoe7.12±0.06
_______________________________________________
Abbreviation:SD–standarddeviation
Source:Ownstudy.
Based on the assessment, it was found that the
obtained values of the initial water content of all
samples of tea, satisfied the requirements of the
standard[PN‐ISO1573:1996]anddid notexceedthe
recommended content of 8%, which was consistent
with the literature determining the water content in
teaatthelevelof4–18%[Góreckaetal.,2004]similar
test results obtained by and Dmowski and
Ruszkowska[2016]insamplesofleafteafromChina
andSriLanka.Thehighestcontentofwateroccurred
in OPA, Pekoe and BOP samples of tea (oldest,
brokenandfanningsleavesof
tea)‐7.13g/100gd.m.;
7.12g/100gd.m.and7.01g/100gd.m.respectively.
ThelowestlevelwasfoundinFloweryOrangePekoe
tea (young leaf tea harvested from top of Camellia
sinensis tree). The oldest samples of tea contained
morewaterthanyoungest,leavestea.Onthebasisof
theobtainedresultsoftheinitialwatercontentinthe
assessedtea,itcanbeassumedthatthewatercontent
was determined primarily by the size of tea leaves,
associated with the stage of collective maturity,
resultingfromthelocationofleavesonthebushand
the technological process, that
in the case of BOP
sample of tea included the process of mechanical
cuttingoftealeavesduringproduction.
Atthesametime,basedontheconductedtests,it
wasfoundthatarelativelyhighcontentofdrymatter,
in the assessed morphological forms of teas with
different degree of disintegration,
indicated the
appropriate quality of the raw material and a high
contentofingredientswithvaluableflavours.
Ensuringhightransportandstoragedurabilityof
teas characterized by different degree of
disintegration (morphological form), requires
understanding the hygroscopic properties of raw
materials. The source of information on the water
content in
the product are sorption isotherms.
Equilibrium moisture content is defined as the
moisture content of a hygroscopic material in
equilibrium with a particular environment
(temperature and relative humidity). Values from
equilibrium moisture studies are important for
knowinghowa materialabsorbsandloses moisture
during storage and for defining the transport
and
storageconditionsinordertoobtainthebestquality
product [Temple and van Boxtel, 1999; Kędzierska
andPałacha,2011].
Hygroscopic properties of all samples of teas are
determined by comparing the relative positions of
watervaporadsorptionisotherms(Fig.1).
Figure1.ThesorptionisothermofwatervaporproductI‐
IV,determinedafter30daysofstorageinanenvironment
witha
w=0.07÷0.98.
Sorption isotherms determined empirically in
tested teas I–V, were characterized by sigmoidal in
shapeand,accordingtotheclassificationofBrunauer,
showed similarity to the isotherms of type II. The
sigmoidalshapeofthesorptionisothermsisrelatedto
theoccurrenceoftherangeofmonolayeradsorption
in the low water
activity (aw<0.3), the multilayer
adsorption range (0.3<a
w<0.7) and the capillary
condensation area (a
w>0.7). Designated sorption
isotherms in tested teas, characterized by continuity
in the course of the entire range of water activity
(0.07÷0.98) with increasing water activity increased
watercontentintheanalyzedteas.
In the area of water activity 0.07<a
w<0.44,
corresponding to the range of monomolecular
desorption, the highest hygroscopicity (sorption
capacity)wascharacterizedbysamplesofFOP,BOP,
OPAandPteaandforOPtea.Thesamesituation,it
canbeseeninOPsamplesofteaintherangeofwater
activityfrom0.07to0.55.
After
exceedingthewateractivityrespectively0.44
and 0.55, the upper limit of the range of multilayer
adsorptionandcapillarycondensationtheallsamples
of tea were characterized by the higher
hygroscopicity.Itcanbeassumedthataboveoflevel
a
w>0.75,thecapillarycondensationphenomenonwas
observedinthetestedsamples,whichcanbeequated
with exceeding the critical moisture level. It can be
observed at the inflection point of the sorption
isothermsandprobablyindicatedtheinitiationofthe
capillary condensation phenomenon. This
phenomenon is identified by exceeding the level
of
criticalhumidity,thatcandeterminedthelossofthe
productʹsabilitytocontinuetransportandstorage.
Designated sorption isotherms in tested teas,
characterizedbycontinuityinthecourseoftheentire
range of water activity (0.75–0.93) with increasing
wateractivityincreasedwatercontentintheanalyzed
OPAtea.
Theuseofadsorptionisothermsasanindicatorof
thestoragestabilityoftheproductisbasedinteralia
onthebasisofacalculationcapacitymonolayer(vm),
thatcorrespondstoasinglelayerofadsorbedwater
403
vapour molecules. The theoretically defined water
content in the product corresponding to this
monolayer it is the optimum water content in this
product. The excess water, in relation to the
monomolecular layer, leads to critical humidity,
which can cause undesired changes in the product,
for example: undesirable taste and flavor,
loss of
aroma and microbial growth [Mathlouthi, 2001;
OcieczekandKłopotek2013].
To the estimate the surface microstructure and
phenomen of sorption, the course of sorption
isothermsin the water activity rangeof a
w=0.07÷0.33
enableddeterminingparametersoftheBETequation
(v
m) by assaying the degree of its fit (R
2
, FitStdErr,
Fstat) to empirical data. Respective results were
presentedinTable2.
Table2.TheBETequationparameters
_______________________________________________
Product
v
m
c
e
R
2
FitStdErr Fstat Specific
Surface of
sorption
[m
2
g
-1
]
_______________________________________________
I/FOP 4.154 119.22 0.951 0.251 38.706 145.96
II/OP 4.164 168.87 0.955 0.229 42.354 146.30
III/BOP 4.226 149.29 0.967 0.202 58.899 148.47
IV/OPA 4.451 152.17 0.944 0.254 34.035 156.39
V/P 4.497 171.50 0.940 0.285 31.517 157.99
_______________________________________________
where: R
2
- determination coefficient;
FitStdErr- standard error;
Fstat –statystyka F;
Source: Own correlation
ThestudiedteasI–Vweredifferentintermsofsize
monolayer. The tea V (Pekoe) was characterized by
the highest capacity monolayer. It can be assumed
that water was bonded stronger in this type of tea
than in other types. Similar capacity of monolayers
was characterized by the remaining samples
of
Orange Pekoe tea. It can be assumed that the most
defragmentation of tea (V / Pekoe), more strongly
boundedwatercomparedtootherproducts.
Probably, it resulted from differences in the
surface structure determined by the degree of
defragmentation of this sample of tea, that was the
fanningstype.
The
monolayermoisture content v mis recognized
as the optimum moisture content for good storage
stability.Thisv
mvalueisthecriticalmoisturecontent
forteatokeepflavourandquality[ChenandWeng,
2010].Theaveragevalueofv
m,rangedfrom4.15%to
4.49%, was recommended as the optimum moisture
content. The corresponding ERH at this monolayer
moisturecontentvalueisnearly60%.
This situation was confirmed by the estimated
values of monolayer capacity (v
m) based on the BET
model, which formed the basis for calculating the
specificsurfaceareaofsorption,thehighestvalueof
which was determined in fannings tea (V)‐157.99
m
2
/g,andthesmallestintealeaftea(I)‐145.96m
2
/g.
4 SUMMARY
Based on the conducted research, it was found that
evaluatedsamples ofblackteawascharacterizedbya
different initial water content, determined by the
degreeofdisintegration.
The evaluation of hygroscopic properties of the
blackteasamplesshowedthatthesorptionisotherms
ofitswerecharacterizedby
sigmoidal inshapeand,
according to the classification of Brunauer, showed
similaritytotheisothermsoftypeII.
In the area of water activity 0.07<a
w<0.44,
corresponding to the range of monomolecular
desorption, the highest hygroscopicity (sorption
capacity)wascharacterizedbysamplesofFOP,BOP,
OPAandPteaandforOPtea.Thesamesituation,it
canbeseeninOPsamplesofteaintherangeofwater
activityfrom0.07to0.55.While,
intheareaofwater
activity 0.69<a
w<0.98 corresponding to the range of
monomolecular absorption, the highest
hygroscopicity was characterizedby all tea samples.
Inwateractivityrangefrom0.75to0.93,thehighest
level of hygroscopicity in OPA tea sample were
determined.
Designated sorption isotherms in tested teas,
characterizedbycontinuityinthecourseoftheentire
range of water activity (0.75÷0.93) with increasing
wateractivityincreasedwatercontentintheanalyzed
OPAtea.
Water activity is an alternative method of
describing equilibrium relative humidity, and is
expressedasapercentage.ThecorrespondingERHis
about60%forlong‐termtransportandstorageof all
researchedsamples
ofblacktea.
Assessing the quality and transport durability of
black teas, based on the parameters of the
monomolecular layerʹs capacity and the specific
surfaceareaofsorption,itwasfoundthatespecially
fannings tea (no. V) was the least susceptible to
changesinthetransportconditions.
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