researchportal.bath.ac.uk · Web viewBlack tea liquor ultrafiltration: effect of ethanol pre-treatment upon fouling and cleaning characteristics Iain S. Argyle1, Arto Pihlajamӓki2,

Black tea liquor ultrafiltration: effect of ethanol pre-treatment upon

fouling and cleaning characteristics

Iain S. Argyle1, Arto Pihlajamӓki2, Michael R. Bird1*

1Membrane Applications Laboratory, Department of Chemical Engineering, University of

Bath, Bath, BA2 7AY, UK 2Laboratory of Separation Technology, Group of Membrane Technology, Lappeenranta

University of Technology, Lappeenranta, Finland

*corresponding author - [email protected]

Keywords: ultrafiltration; pre-treatment; black tea; fouling; cleaning

Abstract

This paper reports on the use of polymeric ultrafiltration (UF) membrane ethanol pre-

treatment as a strategy to improve filtration performance in terms of both flux increase, and

membrane water flux recovery following cleaning. A 4-fold pure water flux (PWF) increase

was observed for a 100 kDa polysulfone membrane. Marked increases in permeate flux were

recorded for ethanol treated UF membranes over a range of molecular weight cut-off values.

Ethanol treatment also aided fluxes over multiple foul-clean cycles, and enabled the enhanced

transmission of polyphenols during UF for the clarification of black tea liquor. Following tea

fouling and NaOH cleaning repetitions, PWF values of treated membranes were returned to

values of >150% of the untreated virgin membrane PWF over 4/5 consecutive cycles,

indicating that the ethanol pre-treatment strategy adopted had a prolonged effect upon

subsequent performance.

1

1. Introduction

Fouling of membranes leads to reductions in performance and thus process efficiency. For

membrane systems this is manifested in flux losses and often inefficient membrane

regeneration through expensive and time-consuming cleaning operations. Returning

membranes to their pristine condition and achieving a performance similar to that of a new

membrane is the over-arching aim of the cleaning process, with recovery typically being

measured as percentage of original membrane permeability [1].

There is a general requirement for technological improvements in tea product manufacturing

processes due to the growing worldwide popularity of iced tea, or more generally, RTD tea

products, evident in the fact that they occupied 8.7% of the ca $500 billion soft drinks market

in 2012-13 [2, 3]. Application of UF in black tea processing has received specific focus for

its potential to remove the natural tea cream which forms upon cooling of tea liquors [4].

Membrane processes offer distinct advantages over other haze removal methods such as

alkali solubilisation, enzymatic treatments, centrifugation or fining by eliminating or reducing

chemicals addition, energy consumption and processing time [5, 6], over the aforementioned

methods. Membrane processing can additionally offer a continuous processing option and

integrated pasteurisation process [7].

Tea haze is noticeable by the mottled appearance which develops on the surface of a tea brew

as cooling commences. This haze forms an unpleasant haze characteristic in tea; the haze

agglomerates having been shown to include tea constituents which contribute to the taste and

colour of black tea and thus loss through hazing (through the consequent phase separation/

precipitation) and result in a perceived lowering of quality [8]. The formation of haze has

been attributed predominantly to the interaction of polyphenols with themselves or other

components e.g. proteins, caffeine and metal cations [8-10] and is a function of pH,

concentration and temperature-time history [11] through interactions by homo-association of

polyphenolic galloyl groups (π – π stacking interactions) or hetero-association of species

through H-bond, hydrophobic or [10, 12, 13]. 85% of haze particulates in the 0.1 – 100 µm

have been shown to be below 1.03 µm suggesting removal of these particulates through

physical size exclusion is a feasible strategy using microfiltration (MF) or UF [5]. Solubility

experiments on tea cream moieties have shown that it is stable below 40 °C and remains

partially formed up to 90 °C [11], indicating that formation begins from the instant an

2

infusion begins to cool (at ca 90 °C) and that removal of haze initiators (i.e. early stage haze

agglomerates) can be carried out using membranes prior to haze maturation.

A drawback of membrane filtration of tea is the preferential rejection of polyphenols over

other constituents as indicated by Chandini et al. (2013) [6]. The phenolic species have also

been identified as being prolific foulants of membranes [14]. Evans et al. (2008) showed that

negatively charged fluoropolymer (FP) membranes fouled by tea inherited additional

negative charges associated with tea constituents, before cleaning returned them to a charge

between that of the virgin membranes and fouled membranes. In further work, repetitive

fouling and cleaning of an FP 30 kDa cut-off membrane showed progressive membrane

surface hydrophilisation upon cleaning, the contact angle being reduced from 65° to 52° after

23 fouling and cleaning cycles [15]. The study highlighted the synergy of tea fouling and

NaOH cleaning and the resulting modification in surface chemistry. Also apparent from

previous work is the importance of membrane material with particular relevance to surface

hydrophilicity and through pore charge with respect to fouling propensity and species

transmission [16].

There are three main strategies for enhancing the performance of commercial membranes:

i. Optimising the hydrodynamic conditions or changing physicochemical variables

(temperature, ionic strength, pH) to either prevent foulant deposition or to induce

changes to solution or solute properties.

ii. Improving foulant removal by optimisation of detergent concentration and cleaning

thermo-hydraulics to improve membrane regeneration.

iii. Pre-treating membranes to modify membrane performance by changing the chemical

or physical nature of the membrane surface and/or underlying porous matrix.

Pre-treatment of UF membranes with alcohols prior to aqueous filtration is an under-

investigated area of research, with little work being reported in the literature [17, 18].

Kochan et al. (2009) [18] reported near 3-fold increases when treating polysulfone flat sheet

membranes with 80 wt.% ethanol solutions, the treatment showing an improvement in

performance for up to 5 days when permeability was tested with pure water. The authors

showed a flux uplift during sludge supernatant filtration when the same membrane was pre-

treated with 80 wt.% ethanol for 2 hours. This shows that this apparently simple pre-

3

treatment method can have positive and lasting effects on membranes. Little attention has

been given to the effects of surface chemistry as a result of alcohol modification. Further, the

application of modified membranes to the filtration of an industrially relevant feed (tea),

whose filtration performance has been shown to be influenced by surface chemical

parameters such as charge and hydrophobicity is made here. Tea as a surface modifier also

shows interesting phenomena relating to membrane surface chemistry, when considering

fouling and cleaning synergy.

This paper reports on results of a standard fouling and cleaning protocol applied multiple

times to polymeric UF membranes which have been subjected to pre-treatment using ethanol.

An emphasis on filtration performance, PWF recovery and transport mechanisms is made, as

well as connections between these factors and the underlying surface chemical changes

associated with ethanol pre-treatment and subsequent tea fouling and cleaning. Flux uplifts

during sub-limiting flux operation are reported. A lasting effect of this treatment method is

seen over multiple fouling and cleaning cycles, and influences to species transmission are

apparent. Overall, improvements in the membrane filtration of tea is documented, which has

relevance to the broader food process engineering sector in the context of developing

adjusted processes, improving process efficiency and improving product quality.

4

2. Materials and methods

2.1. Membranes and filtration system

UF was performed using a DSS Lab-Unit M10 (DSS, Denmark). Three different flat sheet

polysulfone membranes (PS50, PS100, MFG1) with nMWCOs of 50 kDa and 100 kDa, and

one with 0.1 µm nominal pore size (Alfa Laval, Denmark) were used for experimentation. An

effective filtration area of 336 cm2 was available within the plate and frame module used.

After being subjected to hot water conditioning to remove anti-humectant, membrane PWFs

were measured at 22 °C and 1.0 bar transmembrane pressure (TMP).

2.2. Pre-treatment

Following rinsing and hot water conditioning, membranes were used in their untreated state

or subjected further to ethanol treatment. Ethanol (50 wt.% or 100 wt.%) (ACS grade,

obtained from Sigma Aldrich) was used to purge and fill the membrane module. Nitrogen gas

was used to pressurise an ethanol solution reservoir and subsequently force ethanol through

the membrane at 1.0 bar (in the manner of dead-end filtration). When saturated the

membranes remained immersed and under 1.0 bar of hydrostatic pressure for 24 hours before

purging with reverse osmosis (RO) water and re-measurement of PWF. PWF after treatment

was divided by PWF prior to treatment to give relative flux change (equation 1).

Jrel=J final

J initial (1)

Where Jrel is the relative flux change, J final is the post-treatment water flux and J initial is the

original PWF.

2.3. Feed and cleaning regimes

Soluble spray-dried black tea powder was supplied by Unilever, UK. Tea powder was

reconstituted with RO water at 90 °C to make 8 L of 0.5 wt. %. Temperature was adjusted to

50 °C before 60 minute filtration. For cleaning, 0.5 wt. % NaOH at 60 °C was used at 1.0 bar

TMP for 10 minutes following ambient rinsing for 15 minutes at 1.0 bar TMP. Cross flow

velocities (CFV) for PWF measurement, fouling, and cleaning were fixed at 1.0 ms-1, rinsing

was carried out at 1.5 ms-1. Between each phase of the foul-clean cycle, PWF was measured

at 1.0 bar. See Table 1 for full protocol.

5

Table 1. Pre-treatment and filtration protocol

Phase of operation # Fluid (concentration) Conditions (T, TMP, CFV, t)

PWF 1 RO water (-) 22 °C, 1.0 bar, 1.0 ms-1, 10 min

Pre-treatment 0/1 None/ethanol (50/100 wt. %) 22 °C, 1.0 bar, static, 24 hours

PWF 0/1 RO water (-) 22 °C, 1.0 bar, 1.0 ms-1, 10 min

Filtration 4/5 Black tea (0.5 wt. %) 50 °C, 1.0 bar, 1.0 ms-1, 60 min


Rinse 4/5 RO water (-) 22 °C, 1.0 bar, 1.5 ms-1, 15 min


Clean 4/5 NaOH (0.5 wt. %) 60 °C, 1.0 bar, 1.0 ms-1, 10 min


2.4. Solute analysis

Permeate samples were collected at 15 minute intervals during each filtration with retentate

samples taken at filtration commencement. All collected permeate (except samples) was

returned to the feed tank to maintain constant conditions. Samples were analysed for total

solute concentration via dry weight measurement by dehydration at 60 °C and for total

polyphenols via the method as described by Singleton and Rossi (1965) [19]. For total

polyphenols, measurements were carried out in 96 well micro-titre plates at 765 nm

wavelength using a Synergy HT multi-mode plate reader (BioTek, Winooski, USA). Gallic

acid standards were used as a reference. Apparent rejection (Rapp) of total solids and

polyphenols was made by calculation using both sample concentration (C p) and

feed/retentate concentration (C r) measurement using equation 2.

Rapp=1−C p

C r (2)

2.5. Turbidity

Degree of haze was measured by way of turbidity in consistently diluted samples. Permeate

sample concentration was approximated by use of refractive index indicating a °Brix value.

Permeates were then diluted to 0.3 wt.% based on this measurement and turbidity recorded

using an HI 93703 turbidimeter (Hanna Instruments, Woonsocket, USA) against turbidity

standards (0, 10 and 100 NTU) at room temperature within 2 hours of filtration.

6

2.6. Tea colour

Samples were loaded into cuvettes before L* (lightness), a* (redness) and b* (yellowness)

parameters (CIELAB colour space) were measured and analysed using a Shimadzu UV-1601

spectrophotometer over a 380 – 770 nm wavelength range and interpreted using UVPC Color

Analysis software v.3.0 (also Shimadzu).

2.7. Contact angle

Water contact angles were measured via the sessile drop technique using an OCA 15 Pro

goniometer (Dataphysics GMBH, Germany). Samples dried in room temperature air and

stored in a desiccator prior to loading with a 1.0 µL drop of pure water. OCA 15 software

was used to calculate the contact angle.

2.8. Apparent zeta potential

Apparent zeta potential (ZP) was calculated by measurement of streaming potential through

membranes pores as per the method described by Nyström et al. (1994) [20]. 1.0 mM KCl

solution was made using Milli-Q water and was fluxed through 10.4 cm2 membrane samples

held in a custom module housing two Ag/AgCl electrodes; one positioned near the membrane

surface at the retentate side, and one near the support layer at the permeate. Streaming

potential was measured over a range of TMPs (typically 0.5 – 1.0 bar) before pH adjustment.

Three pH values were measured in the decreasing pH direction before purging and rinsing of

the electrolyte solution. Streaming potential for three further pH values in the increasing pH

direction were then conducted. All measurements were in the range of between pH 3 and pH

7. Zeta potential was calculated by assessing the rate of change streaming potential with

respect to TMP using the uncorrected form of the Helmholtz-Smoluchowski equation

(equation 3).

ζ = ΔETMP

μkε0 εr

(3)

Where ζ is the zeta potential, TMP is transmembrane pressure, μ is electrolyte solution

viscosity, k is conductivity, ∆ E is streaming potential change, ε 0 and ε r are permittivity of

free space and the dielectric constant of water respectively.

7

3. Results and discussion

3.1. Effect of alcohol pre-treatment

Figure 1 shows the relative flux increases observed following treatment of the membranes

used. For the PS50 membrane (untreated average virgin PWF of 110.1 ± 5.2 Lm-2hr-1),

treatment with 50 wt.% and 100 wt.% ethanol resulted in relative increases of 1.32 ± 0.01 and

2.32 ± 0.04. For the PS100 membrane (untreated average virgin PWF of 254.7 ± 7.2 Lm-2hr-

1), treatment gave increases of 3.75 ± 0.11 and 3.84 ± 0.06, this membrane giving the most

prolific flux uplifts. For MFG1, with PWF of 309.4 ± 1.5 Lm-2hr-1, the relative uplifts were

2.09 ± 0.04 and 2.13 ± 0.03.

PS50 PS100 MFG1

Relative flux (

J rel) (-)

0

1

2

3

4

5No treatment 50 wt.% ethanol 100 wt.% ethanol

Figure 1. Relative PWF for membranes following 24 hours of static pre-treatment under 1.0

bar pressure in various ethanol-water mixtures.

The polar nature of water influences its organisation at the solid-fluid interface and is

dependent on the charge interactions between the two phases. Considering that the negative

charge is reduced to a more neutral value relative to the untreated state (as later shown in

figure 8), the adherence of water molecules to the wall would be reduced. The effect of

reduced charge would thus have the effect of increasing the slip at the wall. Normally it

would be assumed that fluid flowing through pores would observe a no slip boundary

8

condition where the outermost molecules are stationary due to this adherence. The results

indicate that if a charge modification to the pore brings about such evident permeability

increases, generally assuming a no slip condition for membranes with similar properties

(moderate surface charge and hydrophobicity) would thus be insufficient and that

incorporation of slip due to surface charge (such as for cases involving the modelling of fluid

flow in porous structures) should be a general consideration.

The exact mode of action of the alcohol remains unclear at this stage though a number of

hypotheses can be suggested. Either ethanol molecules are adhering in a film with their

orientation such that their polar end is attached to the wall and the remaining non-polar

hydrophobic tail protrudes into the pore space reducing the adhesion of water. The charge

modification could thus be a result of charge masking given its lower conductivity. Another

theory is that alcohol is partially dissolving the polymer matrix (e.g. swelling) which, as well

as potentially modifying the membrane morphology, could bring about changes in its

chemical nature. The action of swelling, and any morphological changes which occur as a

result, could increase the effective pore diameter of some membranes. Swelling in three-

dimensional porous structures can lead to two opposing effects: 1) macroporous swelling

would lead to a dilation of the entire structure causing expansion of pores and 2) microporous

swelling would result in the polymer swelling though the effective total space occupied by

the membrane would change negligibly. Since both effects will occur simultaneously, the

relative proportions of these two phenomena may differ between membranes offering

differing degrees of uplift in pure water flux following treatment.

The PS50 membrane shows a greater rise in flux between 50 wt.% and 100 wt.% ethanol

treatment, whereas the PS100 shows its most significant rise in flux between being untreated,

and treated with 50 wt.% ethanol. If adsorption of ethanol molecules to the pore walls is

considered, the initial magnitude of charge would influence the amount of ethanol adsorbed

since ethanol must displace water at the solid-fluid interface in order to adsorb to the wall.

For a more strongly charged virgin membrane (-15 mV for the PS50 membrane) this

saturation effect (or equilibrium) would thus be favoured towards the molecule with greater

relative polarity (water) and would thus require a higher presence of ethanol molecules to

effect a change over the more neutrally charged surfaces (PS100).

3.2. Tea filtration

9

The benefits of the treatment protocol can be seen when performing filtration with 0.5 wt.%

black tea. As for the post-treatment PWFs, it is the difference between 50 wt.% and 100 wt.%

treatment that made the greatest difference in filtration fluxes for PS50 (Figure 2a). For the

PS50 without treatment, the relative terminal filtration flux was 0.255 ± 0.010. On the fourth

filtration, this rose to 0.304 ± 0.011. Contrastingly to the untreated operation, repetitive

fouling of conditioned membranes showed a decrease in terminal filtration fluxes; falling

from 0.455 ± 0.016 to 0.380 ± 0.013. Quantifying these values in terms of percentage change,

the PS50 with 50 wt.% ethanol treatment shows a 21% improvement falling to a 6%

improvement after 4 consecutive cycles. For 100 wt.% ethanol treatment, a 79% increase

during the first filtration is observed falling to a 25% enhancement after 4 cycles. The PS50

membrane generally showed large and prolonged increases when treated with ethanol, with

ethanol concentration increases correlating positively with maintained flux improvements

over multiple cycles.

Figure 2b and 2c show similar results for PS100 and MFG1 respectively. The increases are

less significant than for the PS50 membrane though the similar trends emerge in terms of

depleted performance after multiple cycles for treated membranes, and an improved

performance for untreated membranes. Another observation is that the pure water

permeability values mirror the changes during filtration, with 50 wt.% and 100 wt.% ethanol

treatment giving more closely allied terminal flux values compared to the untreated case,

whereas the PS50 treated with 100 wt.% ethanol gave by far and away the best performance

over untreated and 50 wt.% ethanol treatment.

10

Total filtration time (s)0 3600 7200 10800 14400

Relative filtration flux (-)

0.0

0.2

0.4

0.6

0.8

1.0No treatment50% ethanol100% ethanol

Total filtration time (s)0 3600 7200 10800 14400 18000


0.0

0.1

0.2

0.3

0.4


Total filtration time (s)0 3600 7200 10800 14400


0.0

0.1

0.2

0.3

0.4


a

b

c

Figure 2. Relative tea filtration fluxes for 0.5 wt.% feed (a) PS50, (b) PS100, and (c) MFG1

over successive foul-clean cycles

11

3.3. Cleaning and flux recovery

The increase in flux is also observable over 4 foul and 4 cleaning (4F4C) cycles by

determination of the percentage flux recovery compared to the virgin untreated PWF (Figure

3). For PS50, the performance after cleaning multiple times mirrored the initial pre-treatment

flux uplifts, though degradation of the benefits of treatment was apparent. For 100 wt.%

ethanol treatment, membrane regeneration was 188% ± 12% after F1C1, falling to 166% ±

9% after 4F4C and for 50 wt.% ethanol, 116% ± 5% to 110% ± 2% for the same membrane

ages respectively. Considering the PS100 (Figure 3b); the untreated membrane is less well

regenerated than the PS50 membrane, and the MFG1 membrane (Figure 3c) is even less well

returned to its original condition. The PS100 offered ca 80% return to the virgin PWF, and

the MFG1 when cleaned using the experimental protocol described herein, only offered a 77

– 79% regeneration to the virgin water flux performance. When observing the regeneration of

the membrane after initial ethanol treatment, it is seen that the effect of 50 wt.% ethanol

treatment is diminished after F1C1, even when the substantial PWF gains after treatment and

prior to fouling are in evidence, although the effect can still be seen.

For the PS100 membrane with 100 wt.% ethanol treatment, the largest regeneration of the

PWF is seen, this showing a return of 203% ± 24% and representing a two-fold gain over the

untreated PWF following F1C1 and maintaining a 133% ± 2% figure after 5F5C. The MFG1

membrane also showed improvements over the untreated membrane for 4F4C. For the 50%

ethanol treatment case, this membrane offered a 118% ± 12% regeneration improvement after

1F1C, and 98% ± 8% recovery after 4 cycles, in comparison to an untreated membrane flux

recovery of 78% ± 2% after the same number of cycles. This pattern is also mirrored for the

100 wt.% ethanol treated membranes of similar specification.

When comparing Figures 2a, 2b, and 2c it is apparent that while untreated membrane

performance is improving with successive foul-clean cycles, the performance of treated

membranes stays constant, or is diminished slightly. The introduction of foulants (in the form

of tea species) adds complexity, when explaining the flux uplifts, but it is likely to be caused

by the same effects as those seen for the PWF uplifts. The greater convective mass transport

of dissolved material is a direct result of increased membrane permeability, though factors

such as surface fouling (both cake layer or by surface adsorption) and in-pore fouling present

limits to the ideal case (filtering at the PWF). Given that cleaning is not achieving PWF

regeneration to values comparable to that of the treated membrane, and the magnitude of

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regeneration falls with successive cycles, it suggests that the treatment is not effective in

permanently modifying the membrane. Further work will be needed to determine the point

where there is no additional benefit in treating the membrane, or whether such effects are

noticeable over the life time of the membrane.

Foul 1 Foul 2 Foul 3 Foul 4

Flux recovery (% of virgin P

WF)

0

50

100

150

200

250 No treatment 50 wt.% ethanol100 wt.% ethanol

Foul 1 Foul 2 Foul 3 Foul 4 Foul 5Flux recovery (%

of virgin PW

F)

0

50

100

150

200



Flux recovery (% of virgin P

WF)

0

30

60

90

120


a

b

c

Figure 3. PWF recovery measured following each foul-clean cycle for (a) PS50, (b) PS100,

and (c) MFG1.

13

3.4. Solute transmission

Ethanol treatment had no effect on solids rejection for the PS50 membrane (see Figure 4a).

This was also the case for polyphenols rejection (Figure 5a) with the exception of the first

filtration cycle, with untreated membranes showing a more substantial polyphenol rejection

(0.59 ± 0.02 for F1 and 0.48 ± 0.01 for F2). This outcome was also observed for PS100

shown in Figure 4b (F1: 0.56 ± 0.02 and F2: 0.49 ± 0.01), and MFG1 (F1: 0.48 ± 0.02 and

F2: 0.41 ± 0.02) (Figure 5c). The solids transmission of PS100 and MFG1 also showed

deviation for the first filtration cycle, in comparison to subsequent cycles. An explanation of

this could relate to fouling effects during the UF of black tea, in which membrane charge

alteration was observed after multiple fouling and cleaning cycles [16].

An interesting observation was for polyphenol rejection from the PS100 membrane (and to a

lesser extent from the MFG1) (Figures 5b and 5c). The effect of ethanol has a profound effect

upon the transmission of polyphenols species, these species contribute to the pleasant

astringency and bitterness of black tea, and thus enhanced transmission of these species is of

paramount importance; removal from the feed will cause a perceivably lower quality

beverage. For the PS100 membrane the rejection of polyphenols is drastically lowered for the

first filtration, and each subsequent filtration remains at these lower values, though the

untreated membrane is incrementally encroaching on the treated membrane values with each

foul-clean cycle; this provides a further indication that the effectiveness of ethanol treatment

reduces cycle by cycle. This implies that ethanol treatment can give a double benefit;

improved fluxes and improved transmissions of essential polyphenolics during tea

clarification.

The drop in transmission following the first to the second fouling cycle could be a result of

membrane shrinkage after use from its pre-treated swelled state. The PS100 membrane

showed the greatest uplift in pure water flux, and the sharpest reduction in pure water flux,

after cleaning cycles were completed. Another possibility is that the removal of pore forming

agents from the membrane upon pre-treatment could lead to changes in fouling propensity

and thus the effectiveness of the cleaning regimemay reduce as these species are removed.

For untreated membranes, the effect of caustic (though at relatively mild concentration) could

also induce swelling. This will be the subject of a future investigation.

14


Rejection coefficient (-)

0.0

0.1

0.2

0.3

0.4

0.5

No treatment50 wt.% ethanol100 wt.% ethanol

Foul 1 Foul 2 Foul 3 Foul 4 Foul 5


0.0

0.1

0.2

0.3

0.4

0.5




0.0

0.1

0.2

0.3

0.4

0.5


a

b

c

Figure 4. Total tea solids rejection for (a) PS50, (b) PS100, (c) MFG1

15



0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7


Foul 1 Foul 2 Foul 3 Foul 4 Foul 5


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7




0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7


a

b

c

Figure 5. Total phenolics rejection for (a) PS50, (b) PS100, (c) MFG1

16

3.5. Colour

Measurement of tea colour by spectrophotometric means not only gives an idea as to the

consumer perception of a tea infusion but also an indication to the relative concentrations of

certain tea species. Redness is known to give an indication to the presence of thearubigins in

tea [21], a class of highly polymerised polyphenols offering complexity in taste and aroma.

Data collected shows the highly increased redness in the permeate stream when filtering with

a PS100 treated membrane (Figure 6). A high degree of the redness is stripped from the tea

although this loss decreases as the membrane ages. In accordance with the polyphenols

rejection data, the redness is vastly improved for the treated membrane permeate for PS100

(this was also the case with MFG1 although not shown in this paper) and the incremental

redness increase after each foul-clean cycle is again shown with treated membranes. This

shows that as the membranes age with subsequent foul-clean cycles, its ability to transmit

polyphenols improves regardless of membrane pre-treatment.

Filtration time (min)10 20 30 40 50 60

a* (-)

-2

0

2

4

6

8

Foul 1Foul 2Foul 3Foul 4Foul 5


a* (-)

-2

0

2

4

6

8



a* (-)

-2

0

2

4

6

8


No treatment 50% ethanol 100% ethanol

Figure 6. Redness of tea permeates over filtration cycles for PS100 membranes

17

3.6. Haze removal

Haze removal was highly efficient as shown in Figure 7, which shows the average of 4

permeate samples between 15 minutes and 60 minutes of filtration. Neither membrane type

nor treatment type affected the decreaming capability. The highest haze value recorded (ca

3.0 NTU for the untreated MFG1 membrane) is not statistically relevant as being any

different from all other values measured and still represents removal in the range of an order

of magnitude from the unfiltered tea (96 ± 5 NTU for 0.3 wt.% tea at room temperature).

Figure 7. Haze of tea permeates over consecutive filtration cycles for PS100 membranes

3.7. Contact angle

Table 2 displays values for the contact angles measured at various points during the filtration

cycles. The resulting contact angle after ethanol treatment lead to increases, which generally

implies a stronger fouling propensity. As was mentioned in the introduction, tea fouling has

been shown to hydrophilise membranes following cleaning. This result is again shown here

for untreated membranes. For 100 wt.% ethanol treated PS50 membranes, fouling and

cleaning returns the membrane to a hydrophilicity almost identical to the untreated state. The

returning of the membrane to their untreated state (following treatment and foul-clean cycle

repetition) did not occur for the PS100 membrane though rehydrophilisation was occurring at

a slower rate. This indicates for both membranes that incomplete foulant removal is present

with species of a hydrophilic nature remaining on the surface. This can be looked at in a

positive manner as hydrophilisation is a common strategy for providing anti-foul membranes

18

though this would classically be achieved through surface grafting of hydrophilic molecules

or plasma treatment to yield more hydrophilic functional surface groups.

Table 2. Contact angle for membranes at various points during the foul/clean cycles (all units

in degrees)

Membrane Treatment Virgin F1C0 F1C1 F4C3F4/5* C4/5*

PS50

None 74.2 ±1.9 35.5 ± 2.5 66.5 ± 0.7 38.1 ± 4.1 65.3 ± 2.1

100 wt.% ethanol

85.0 ± 1.0 28.8 ± 2.4 73.7 ± 3.7 34.4 ± 4.8 72.4 ± 1.0

PS100

None 66.1 ± 1.7 21.5 ± 4.4 70.0 ± 2.8 - *69.4 ± 2.8

100 wt.% ethanol

73.9 ± 0.4 16.1 ± 5.5 72.3 ± 2.9 - *70.4 ± 0.6

3.8. Zeta potential

Figure 8 shows that for virgin membranes (both PS50 and PS100) the negative charge is

reduced upon ethanol treatment, which would normally suggest a greater fouling tendency

towards negatively charged phenolate ions (dissociated polyphenol species). The PS50

membrane shows the largest charge alteration, and after fouling and cleaning, this reduction

is still apparent. The fouled ZP for the untreated membrane is less negatively charged than for

the virgin, F1C1 and F4C4 membranes. The fouled ZP for the treated PS50 measures

similarly to that of the untreated equivalent suggesting the charge of the fouling layer is

consistent and not influenced heavily by the membrane. This is also true for PS100. Upon

cleaning, the reduced charge of the treated membranes is likely to reduce the adhesion of

negatively charged cations to the membrane and is a possible explanation for the enhanced

flux recovery seen, especially for the PS100 membrane which displayed the lowest

magnitude of charge following cleaning. The untreated PS100 membrane (Figure 8c) shows

19

little change after F1C0, F1C1 and F4C4 whereas the charge on the treated membrane

(Figure 8d) is not returned to the lower magnitude of negative charge of the virgin treated

membrane. This suggests that foulants that are less easily removed from the treated

membrane following cleaning are potentially aiding the transmission of phenolic species.

This would seem logical, as lower negative charge repulsion would occur with a lower

magnitude of negative ZP.

pH (-)3 4 5 6 7 8

Apparent ZP

(mV

)

-20

-15

-10

-5

0virginfouled 1fouled 1 cleaned 1fouled 4 cleaned 4

pH (-)3 4 5 6 7 8

Apparent ZP

(mV

)

-20

-15

-10

-5

0

virgin membranefouled 1fouled 1 cleaned 1fouled 4 cleaned 4

a b

pH (-)3 4 5 6 7 8

Apparent ZP

(mV

)

-20

-15

-10

-5

0

virginfouled 1fouled 1 cleaned 1fouled 4 cleaned 4

pH (-)3 4 5 6 7 8

Apparent ZP

(mV

)

-20

-15

-10

-5

0

virgin membranefouled 1fouled 1 cleaned 1fouled 4 cleaned 4

c d

Figure 8. Apparent zeta potentials calculated from streaming current measurements through

membrane pores for PS50. (a) untreated, and (b) 100 wt.% ethanol treated. PS100 (c)

untreated, and (d) 100 wt.% ethanol treated

20

4. Conclusions

Ethanol treatment of polymeric membranes has been shown to be effective in both improving

filtration fluxes and prolonging the performance of membranes over multiple foul-clean

cycles. Ethanol treatment has also shown that performance modification facilitates enhanced

transmission of polyphenolic species and the normally disproportionate rejection of

polyphenolic species is mediated somewhat by modifications in surface charge, this also

being apparent over multiple foul-clean cycles. Whilst negative charge reduction of the

membrane following treatment would normally be seen as detrimental to performance, the

propensity for fouling with species which act to beneficially modify the membrane is

improved. Tea filtered through modified membranes has been shown to be of superior quality

to the unmodified membrane permeates (and improves cycle on cycle), shown through

measurement of total phenolics and colour. Additional experiments to examine the ageing

over longer time scales would be of interest to determine whether ethanol could affect the

membrane through chemical degradation, and whether re-treatment after a given number of

cycles can recharge the optimal performance observed after the first fouling cycle in this

study. The likelihood is that re-treatment with solvents could also enhance the cleaning of the

membranes and help to return them to a near pristine state. Another requirement when

assessing the commercial benefit of such an operation would be to match the choice of

solvent to the subsequent filtration performance of the particular feed in question, as such an

optimal solvent choice may be feed specific.

5. Acknowledgements

The authors would like to thank Dr Frank Lipnizki of Alfa Laval for kindly supplying the

membranes used in this study. Spray dried tea powder was kindly supplied by Unilever R&D

Colworth, UK. We thank the EPSRC for supporting this project through a Doctorial Training

Account.

21

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Documents

researchportal.bath.ac.uk · Web viewBlack tea liquor ultrafiltration: effect of ethanol pre-treatment upon fouling and cleaning characteristics Iain S. Argyle1, Arto Pihlajamӓki2,