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80 Grapegrower & Winemaker www.winebiz.com.au November 2014 – Issue 610

Understanding filterability index:an overview and some new insights

Dr Paul Bowyer, BHF Technologies

(Blue H2O Filtration) regional manager,

and Greg Edwards, Vinpac group

technical manager, offer their insights

into the complex topic of filterability

index and how important this

information can be when judgements are made on

wine impact on membranes during sterile filtration

ALTHOUGH it is common for wine packaging facilities

to measure nephelometric turbidity units (NTU) in order to

evaluate wine suitability for particular filtration grades, not

all measure filterability index (FI). Part of the reason for this

lies in the commonly-held falsity that NTU correlates with FI,

but also perhaps because filterability is not a topic currently

taught to winemakers in our education system. NTU is a

measurement of light scattering in a sample, and so provides

an approximation of particulate loading. The measurement of

NTU cannot, however, provide an indication of wine colloidal

status; hence the situation can arise where a very clean wine

with low NTU can still foul filtration media, as was extensively

observed for 2011 South Australian reds. Filterability index

determination is a direct challenge of the wine to be filtered

to the membrane, and so provides an indication of all fouling

components in the sample, whether particulate or colloidal. We

have published previously on this topic and here provide the

most recent developments in our understanding of this rather

complex topic.

FI MEASUREMENTThe measurement of FI is quite straightforward, nevertheless

many laboratories avoid this measurement since, without

appropriate methods and equipment, and it can be cumbersome

and time consuming for laboratory staff. There are several

different methods for determining filterability. Some are quite

complex and some are relatively simple. One universal fact is

that if the method and equipment used are not simple and easy

to work with, this measurement is often neglected and viewed

only as an annoyance.

The method that we advocate is perhaps the simplest. A

sample is delivered to a membrane disc at a constant 2 bar, with

the time recorded to pass 200 and 400mL (or g, in the case of

the BHF automated FI unit). A simple calculation provides the

filterability index as indicated in equation 1:

FI = T400 - 2T200 (1)

Any fouling of the membrane slows the flow of sample,

increasing T400, and so when the formula is applied an

index is generated. Sometimes a constant of 1.67 is applied

but, since it is a constant, it can be omitted. The assay can be

performed manually, but it is far easier to use an automated

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82 Grapegrower & Winemaker www.winebiz.com.au November 2014 – Issue 610

unit. Critically, the membrane discs used for FI determination

should be the same as those membranes used at bottling. This is

sometimes overlooked, but a comparison of relative membrane

performances reveals the importance of this simple point (see

below).

The measurement of FI provides an evaluation of the

fouling that is likely to occur during bottling, however it is

not a replication of the bottling filtration process. Typically,

a sterile bottling task will see the wine pass through tight

lenticular (depth) onto 0.65um membrane pre-filters onto

0.45um final membranes. There are several permutations of

this configuration, but this is the most commonly used. FI

measurement does not replicate this process, since no pre-

filtration is applied before the measurement. Wine destined

for sterile filtration must be within specification (e.g. < 1 NTU)

before it is accepted, in which case minimal loading should be

applied to any of the sequenced filters, yet this is not always

the case. FI measurement is used to detect problem wines,

specifically those that will cause unacceptable loading of

the final membranes, and represents a way of avoiding both

production delays for the packager and elevated filtration

costs for the customer. These wines are typically found to

have low NTU and high FI. We generally advise an FI threshold

of 20 sec.

COLLOIDSThere are two types of colloids found in wine: associative

colloids and macromolecular colloids. Association colloids are

aggregates of small molecules, driven to associate through weak

intermolecular forces such as van der Waal’s forces, hydrogen

bonding and hydrophobic interactions, and are typically

produced by fining processes. Macromolecular colloids are

comprised of one major species, such as polysaccharides,

proteins, mannoproteins, carboxymethylcellulose (CMC) etc.

Generally these compounds possess a molecular charge, and

they can be used to stabilise association colloids, an example

of which would be the addition of gum arabic to stabilise

pigmentation in reds. Colloids can present a problem for

filtration in that, unless the colloid is of sufficient size to be

observed by NTU, it lies latent and undetected in the wine until

it is pushed through a membrane, at which point it can cause

fouling. Since filterability measurement employs the membrane

filtration process, the true filterability nature of a wine sample

can be discerned.

In some cases wines have been ultrafiltered (cross-flow) and

pass FI testing, only to fail a retest within a relatively short time

period. We speculate that examples of this type demonstrate the

impact of associative colloids, which are likely to be disrupted

during ultrafiltration through the application of shear forces,

enabling a pass during FI measurement, but which then

Filterability index measurement is used to detect problem wines, specifi cally those that will cause unacceptable loading of the fi nal membranes, and represents a way of avoiding both production delays for the packager and elevated fi ltration costs for the customer.

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November 2014 – Issue 610 www.winebiz.com.au Grapegrower & Winemaker 83

reassociate in short time to once again provide a mechanism for

fouling membranes.

The impact of macromolecular colloids on FI is more

straightforward, as FI can be measured pre- and post-addition

to determine filterability impact. In some cases a transient

elevation of FI can be observed, such as with the application

of CMC.

FI AND FImOccasionally wines are found that pass FI but present a

differential pressure increase across the membrane during

bottling at a faster than expected rate, so what is going on here?

The basic measurement of FI (Equation 1) gives a filterability

snapshot of the wine up to 400mL, but this has a limitation in

that it is a small measurement (0 – 400mL) and we are seeking

to extrapolate this FI value to the filtration of a large volume of

wine with filters of a much greater surface area. We have found

that by extending the measurement to 600mL and using three

data points, as in Equation 2, to calculate a modified filterability

index (FIm), a better understanding of the wine impact on the

filtration membrane is possible.

FIm = (T600 – T200) – 2(T400 – T200) (2)

In many cases, FI and FIm are very similar values, indicating

that over time the sample is unlikely to have a deleterious effect

on the filtration performance, but on occasion the two values

are sufficiently different in magnitude to raise some suspicions.

It may even be that FI represents a pass value (typically <

20), but FIm does not. Since FIm better reflects membrane

performance over time, perhaps we should be giving this value

proportionately more weight when evaluating the filterability

of a sample.

In Table 1 we can see two data sets for the same wine,

a 2012 Malbec, which has been filtered through different

grades of Becopad. The filtration grades are the same in

each case except for the final grade applied, with 170 being

tighter than 220 (see data in the caption to Table 1). In this

example, the value of the tighter 170 grade as the final

filtration stage becomes obvious when one examines the FI and

FIm data: with tighter final filtration these values are almost

identical, but with a slightly looser grade of final filtration

FIm becomes significantly greater than FI, indicating that

this wine would load the 0.45um membrane to a much

Table 1: FI and FIm data for a 2012 Malbec with different filtration grades applied

in sequence. Becopad 580: Nominal 3.0 - 4.0 um; Becopad 450: Nominal 1.0 -

2.0 um; Becopad 220: Nominal 0.3 - 0.5 um; Becopad 170: Nominal 0.2 - 0.4 um.

FI FIm

2012 Malbec post Becopad 580/450/170 7.80 7.24

2012 Malbec post Becopad 580/450/220 12.0 34.9

Figure 1: A comparison of the FI plots for the wines from Table 1.

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84 Grapegrower & Winemaker www.winebiz.com.au November 2014 – Issue 610

greater extent than the previous wine, and may cause premature

blockage.

In conjunction with this, there is a strong advantage in using

an automated FI unit, such as the BHF-supplied units at all

Vinpac facilities, in terms of ease of use and laboratory time

allocation, and also in that a real-time plot of the filterability

curve is generated by the software during the measurement.

A sample report is easily generated that can be provided to

the customer to help them understand the filterability status

of their wine. The filterability plot also allows closer scrutiny

of the membrane performance during a filtration run, and this

is not evident when using a manual unit since only a single

numerical value is obtained, constituting only a pass or fail

result. If we examine the plots from the wines used in Table

1, the difference between them becomes immediately obvious

(Figure 1). The influence of the tighter filtration applied through

the Becopad 170 is clearly evident through both the greater

linearity of the plot and the faster flow rate, in addition to

the similarity of the FI (7.80) and FIm (7.24) values. The tailing

evident in the plot for the wine filtered through Becopad 220

indicates higher membrane loading, which is also reflected in

the greater disparity between FI (12.0) and FIm (34.9).

Relative performances of media typesSome laboratories not only use different methods for FI

analysis, but also different membrane discs, disc sizes (25mm

or 47mm) and porosities. For example, some laboratories use

cellulose acetate discs in either 0.45um or 0.65um, yet very

few wine membranes are made of this material. Some facilities

measure FI using nylon membrane discs in either 0.45um or

0.65um, but use polyethersulfone (PES) wine membranes at

bottling. There is no specific standard for FI analysis at this

stage, so it is not possible to transpose data obtained from two

different sources.

The main common element in Australia in terms of sterile

filtration is that 0.45um final membranes are used, and

increasingly PES is gaining prominence over nylon as the

material of choice. The differences between the performances

of nylon and PES membranes are quite significant, in terms

of membrane symmetry, colour binding, FI data obtained,

membrane loading capacity and flow rates. To demonstrate

this, a sample of RO water was analysed, the data for which are

presented in Table 2 and Figure 2. The significantly decreased

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Membrane material FI FIm Test duration (s)

RO waterNylon 0.45 um 4.6 0.6 178

PES 0.45 um 0.0 0.0 59

Table 2: FI and FIm data for RO water passing through nylon and Parker-

domnick hunter PES 0.45 um discs.

Figure 2: A comparison of the FI plots for RO water using nylon 0.45 um and

a Parker-domnick hunter PES 0.45 um disc. Both membranes generate quite

linear plots (in this example) but exhibit different flow rates and FI data.

November 2014 – Issue 610 www.winebiz.com.au Grapegrower & Winemaker 85

flow rate of the nylon disc is a result of both the membrane

composition and symmetry.

In a similar way, cellulose acetate (or ‘ester’) discs are

commonplace in laboratories, and are often used for cell counts

on growth media, although their use for generating FI data

is debatable given that most wine membranes are either PES

or nylon. In Table 3 and Figure 3 we can see data for a wine

analysed using 0.65um cellulose acetate discs (as was laboratory

practice at the site in question) and PES 0.45um discs (which

was the membrane type in the bottling hall filters). The 0.65um

cellulose acetate discs, having the greater porosity, allow a

greater flow rate than the 0.45um PES discs, and even though

the FI data obtained are similar the FIm data are quite different.

The discrepancy is perhaps more obvious in the FI plots

depicted in Figure 3.

CONCLUSIONThere is, as yet, no specific standard for filterability analysis

in terms of method applied, membrane disc size and membrane

composition. These parameters can influence filterability

analysis outcomes, and this should be borne in mind before

seeking to draw comparisons between data sets. Our advice is

to use 25mm membrane discs for filterability analysis that are

identical to the final wine membranes in terms of porosity and

composition. In this way filterability information of the greatest

value can be used to make judgements regarding wine impact

on membranes during sterile filtration.

For more information, contact:

Paul Bowyer

P: 61 3 9564 7029

E: paul@blueh2o.com.au

REFERENCESBowyer, P. K., Edwards, G. and Eyre, A. (2012) NTU vs wine filterability index – what does it mean for you? The Australian and New Zealand Grapegrower and Winemaker, October issue (585), 76-80.

Bowyer, P. K., Edwards, G. and Eyre, A. (2013) Wine filtration and filterability – a review and what’s new. The Australian and New Zealand Grapegrower and Winemaker, October issue (599), 74-79.

Bowyer, P. K. (2003) Molecular polarity – it’s behind more than you think, The Australian

and New Zealand Grapegrower and Winemaker, November issue, 89-91.

Table 3: FI and FIm data for a 2013 Shiraz analysed using 0.65 um cellulose

acetate and 0.45 um Parker-domnick hunter PES discs. Note the large

difference in FI and FIm data for the cellulose acetate disc.

Figure 3: A comparison of the FI plots for a 2013 Shiraz using a cellulose

acetate 0.65 um disc and a Parker-domnick hunter PES 0.45 um disc.

The flux characteristics here are quite different, with the cellulose acetate

membrane leading to significant tailing, which in turn elevates FIm.

Membrane material FI FIm Test duration (s)

2013 ShirazCellulose acetate 0.65 um 4.3 27.2 104

PES 0.45 um 4.7 2.4 131