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Tips for efficient filtration
in food and beverage applications
Jorg Bettschen, Dipl. Chem. Eng. HTL, Application Filtermedia, Filtrox
AG
Filtration of beverages, such as wine, cider or beer, requires special
attention as to the effects of regeneration, sterilization, rinsing and
maintenance. Some general recommendations for an economic and safe
filtration.
Filter sheets are -as a rule -non-woven products
which are permeated with inorganic granulates, used for the physical
separation of insoluble particles from liquids. Most of filter sheets are
made out of cellulose fibers, kieselguhr and perlite granules. Filter
sheets are placed in a filter press, which gives the necessary mechanical
support. It is important to consider the combination of both, the sheet
and the filter machine for achieving economical and safe filtration.
Sheet filtration is a typical example of a depth filtration method, where
the solid particles are mainly retained in the interior of the matrix. The
dimensions and the surface structure of fibers and granulates are not
uniform. The interior of a sheet is a very complicated, irregular
labyrinth, through which the liquid must find its way.
Filter sheets are manufactured from cellulose or synthetic fibers from
different provenience, and kieselguhrs, perlites of various granulometric
fractions. A sheet is usually between 2,5 and 4,5mm thick, the specific
density is between 0,2 and 0,35g/cm³ and the weight ranges between 800 and
1500 g/m2. These sheets are available in all common sizes, rectangular or
circular, with central hole or peripherical holes or assembled to
modules.
Based on the enormous interior surface, approx. 8’000m²/m³, suspended
particles can be restrained from the liquid. In addition, retention of
very fine particles, such as micro-organisms, toxins, pyrogens, is
achieved by electro-kinetic adsorption effects (Zeta-Potential).
A general forecast for an economic and safe filtration is not easy. The
kind and volume of turbid matters is of great importance, since suspended
particles are retained accordingly to their size, shape and nature (electro-kinetic
energies). Also the parameters of filtration have a great influence to the
results. It is clear, that only small quantities of turbid matter -0,8 to
1%, measured as damp centrifugate - can be retained economically in
practice by depth filtration.
Principle of regeneration
During the filtration of beverages, such as wine, cider or beer, a
considerable amount of tannins, pectins, proteins and colorants, fine
crystals of tartaric acid, micro-organisms as well as colloids such as
polysaccharides, are deposited within the inner volume of the filter
sheets. Since not all colloids are real turbids, they reduce the
filtration speed considerably. By rinsing the filter sheets with water,
some of the above mentioned substances will go into solution and the
filter sheets are partially cleaned. This procedure is also known as
regeneration of filter sheets.
Filter sheets are cleaned by means of water. The aim is to change the
chemical-physical properties of the liquid and therefore to dissolve
substances which are insoluble in the unfiltered product. In practice it
is recommended to rinse for 10 -15 min with cold water. The pH will drift
to the neutral point, adsorptive bonding therefore will be removed.
Continue rinsing for 10-15 minutes with warm water of about 45°C. Proteins
and salts of the tartaric acid will be dissolved, the viscosity of the
colloids will be reduced.
Never use water at this stage warmer than 50°C! Otherwise proteins will
coagulate, that means, they will become insoluble and block up the filter
sheets. If you wish f.i. to remove the red dyes of previously filtered red
wine, you may now rinse for 10 more minutes with hot water of 80-85°C.
This will enable you to filter through the same filter sheets. Continue
the preparation of the filter equipment for either sterilization or
cooling to room temperature with cold water.
Sterilization
We limit the description of the sterilization process here to the
filter press and the filter sheets only. Remember that, in order tto
perform a successful sterile filtration, all parts which come into contact
with the product will have to be sterilized also, e.g. tubings, fittings
and the filling device. The sterilization of the filter press is done
always in the direction of filtration.
Chemical disinfecting: is recommended to use only products which are
regarded as “safe” in the foodstuff industries, e.g. sulfuric acid (SO2)
at 0,2% dissolved in water, or 0,2% peracetic acid. Do not use
disinfecting products which contain active chlorine or chlorine dioxides.
Those products attack resp. will eventually destroy the silicon rubber
sealing of the filter elements.
Steam: low pressure steam at 60 kPas (0,6 bar) and 112°C is most suitable.
Steam> 60 kPas is usually too hot and affects sealings and fittings.
Preference should be given to the sterilization with hot water instead of
steam because hot water at 80-85°C is safer and more gentle for filter
presses, filter sheets and gaskets. Depending on available infrastructure,
the sterilization of filter equipment can be done with circulating hot
water or with a heat exchanger. Recirculating saves energy.
A frequently asked question is or repeated sterilization can be made.
Should sterilization of the filter system be done once a week, or every
evening? This is individual for every plant and product to be filtered.
E.g. for the bottling of sweet wines more care for sterilization of the
equipment is required, than for dry white wines. If the filter is packed
over a long period of time, e.g. over the week-end, it is advisable to
sterilize the filter prior to the next filtration. Eventually the filter
system will be kept in a disinfecting agent, under light pressure, to
minimize bacterial contamination.
Quality of rinsing water
The preparation of filtration starts with the rinsing of the new
sheets. The regeneration of sheets is performed by rinsing with cold and
warm water and the sterilization with hot water or steam.
All these operations have a great influence to the filtration result.
Therefore a good water quality is essential. In general, drinking water
will be adequate; for pharmaceutical processes distilled water is
preferable. In any case the water must be free of suspended solids to
avoid clogging of the filter sheets. The microbiological contamination
should be as low as possible. Water containing active chlorine or chlorine
dioxide should be filtered through charcoal filter sheets.
Recommendations for an economical filtration
Several recommendations should be observed:
§ Any filtration process with depth filter sheets (sheet filtration) is
most efficient if done in several stages
§ Start with coarse filter sheets, then step-wise filtration to next two
finer grades.
§ The unfiltered liquid should not contain substances which restrain the
filterability.
§ Products containing j3-Glucanes have to be treated by enzymes and by
fining.
§ The instructions to rnax. flow rate and max. differential pressure have
to be followed. Low flow rates and low differential pressure, result in
higher capacity. Remark that the lower the hourly throughput per filter
area in L/m² *hr, the longer the capacity (= life time) of the filter
sheet will be.
In case that it is not possible to filter one batch with one filter
packing of standard sheets, it is recommended to use filter sheets which
are specially manufactured for high capacity. Another possibility is the
enlargement of the filter area. To protect and to extend life time of
membrane filter cartridges, it is recommended to use the filter sheets
upstream to retain particles within the depth filter matrix. Membrane
filters are used in many cases as the final stage in the filtration
process prior to the filling system.
Maintenance of filter press
In order to provide a good functioning filtering procedure the filter
press has of course to be maintained too. Some recommendations. Steam and
hot water make gaskets getting hard and inflexible. This results in higher
drip losses of the filter. For this reason gaskets have to be exchanged
regularly. Drip losses are also resulting from insufficient sealing of the
sheets. This can be avoided by transferring the contact pressure
completely on the sheets. Movable part like threaded bush, spindle, the
touching point of the spindle on the movable endplate should be regularly
and carefully cleaned and greased. Check from time to time the pressure
gauges on inlet and outlet. Clean after each filtration the filter
elements and inspect all elements for proper operation.<<
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