WO1999003559A1

WO1999003559A1 - Sintered metal fiber for use in the preparation of beverages

Info

Publication number: WO1999003559A1
Application number: PCT/BE1998/000113
Authority: WO
Inventors: Marc Arnaut
Original assignee: N.V. Bekaert S.A.
Priority date: 1997-07-18
Filing date: 1998-07-17
Publication date: 1999-01-28
Also published as: AU8427398A; ZA988567B; BE1011287A3

Abstract

This invention relates to a filter for preparation of fruit beverage comprising a filter medium on the basis of metal fibres, whereby the filter medium comprises a possibly multilayered, sintered, isostatically cold pressed metal fibre web. The metal fiber web may also comprise metal powder or an organic or unorganic coating layer.

Description

SINTERED METAL FIBER FOR USE IN THE PREPARATION OF BEVERAGES

The invention relates to a filter medium for the filtration of fruit beverages during their preparation. With fruit beverage is meant here, amongst others, beer, wine, fruit juice such as apple-, pear-, orange-, grapefruit-, grape-, apricot- juice and vegetable oils. Nowadays kieselguhr is often used as filter aid at the filtration steps during the preparation of these beverages.

Beer is typically filtered in the final stage of the production process and wine in various production phases to remove any suspended particles, such as any carried-along yeast cells. The liquid remaining in the fermentation and matura- tion tanks after the usable wine has been pumped off, the so-called tank bottom, is essentially also wine. It is true it contains a large quantity (typically 5 X or more for beer and 10 X or more for wine) of suspended solid particles. A certain quantity of usable consumption wine resp. beer usually arises from the filtration of these tank bottoms. Often a preliminary filtration process is carried out for beer and a primary and secondary filtration for wine, using a filter aid, such as kieselguhr or diato ite. Final filtration or sterile filtration then ensures removal of the carried-along filter aid and any entrained yeast cells and the micro-organisms which are harmful for wine.

In what follows, the term "beer", resp. "wine" is intended to include potable fermented beer, resp. potable wine with a normal suspended solids content (typically 0.1 X) and the tank bottoms and similar liquids with a much higher suspended solids content. A filter medium for fruit beverage, in particular for beer and wine, must meet a number of very divergent requirements. In the first instance the medium must give a filtrate that satisfies the various quality requirements, such as purity (complete yeast retention), clarity, chill haze, bitterness and other specific quality standards in respect of potable beer and wine. In addition, the filter medium must have considerable mechanical and thermal resistance. It must be able to withstand not only the high throughputs and pressures but also the high temperatures attained, for example, during steam sterilization of the installations at more than 120°C.

It is also very important that the filter media could be recovered. The aforementioned conventional filter media are often, if not invariably, intended for once-only use. However, replacing a filter medium can be labour-intensive and cannot as yet run fully automatically. The frequency of replacement of the medium therefore has a direct influence on the process or operational costs of production of fruit beverage.

Conventionally, different types of filter media are used in the filtration of fruit juice, for example for beer and wine, such as organic and anorganic membranes, candles and planar media comprising synthetic or ceramic fibres.

Use of ceramic filter membranes in single- or multi-stage filtration is described in European Patent No. EP 0208450. Use of a heat-sintered filter medium on the basis of metal fibres, i.e. including metal fibres and possibly metal powder is also known from European Patent No. EP 0457611. Cellulose filter media or candle filters with a basis of (synthetic) resins are also often used. Furthermore, cellulose filter media, or media on the basis of cellulose and kieselguhr are frequently used. Many problems are attached to the use of these conventional filter media. For instance, such media are not recoverable or, if so, only with difficulty, they display only limited mechanical strength and service life and give a filtrate (consumption product) that does not always satisfy the relevant requirements.

A further consideration is that kieselguhr, used in large quantities as a filter aid, is potentially carcinogenic, difficult to handle, abrasive and expensive, and therefore to be avoided as far as possible.

It is therefore an object of this invention to provide a filter medium for the preparation of fruit beverages that guarantees total yeast retention, gives a filtrate that satisfies all the relevant quality standards and is readily recoverable. Apart from that, a guarantee for the complete retention of micro-organisms is required for wine. It is another object of the invention to provide a filter medium having a high mechanical and thermal resistance and a long service life.

It is a further object of the present invention to eliminate as much as possible the use of kieselguhr and diatomite in the preparation of fruit juices.

The present invention meets the said requirements by providing a filter medium for preparation of fruit beverages with a basis of metal fibres, the filter medium being a possibly multi -layered, sintered, isostatically cold pressed metal fibre web. The principle of isostatic cold pressing is known, and is described in the applicant's European Patent No. EP 0329863. It is a method for the homogeneous compacting of a sintered metal web, the pressure being applied to the web via a flexible medium. The same pressure acts on all points of the web surface. The pressure transfer medium may be a gas, a liquid, a flexible membrane or any felty material. Isostatic cold pressing gives a product having homogeneous permeability over the entire surface. An isostatically pressed material is typified by having a rough surface on the side to which the pressure is applied.

The beer filter medium according to the present invention is obtained by the isostatic cold pressing of a sintered, and possibly multi -layered, metal fibre web.

The metal fibre web, preferably based on stainless steel fibres (type 316L or, for example, AISI 300 or 400 series materials, FeCralloy^®, AlloyHR, Aluchrome^® ) having an equivalent diameter of between 2 and 3 microns and a weight between 300 and 1,500 g/m , is isostatically cold pressed at a minimum of 2,000 bar (2.10⁸ Pa) and preferably at a minimum of 2.5 x 10⁸ Pa.

The equivalent diameter of a fibre is the diameter of an imaginary round fibre of the same cross-sectional dimension as the actual fibre concerned.

The absolute filter fineness of the material thus obtained is less than 1.2 microns, even less than 1 μm. Retention capa- city of dust and service life are more than 20 X higher than those for conventional cellulose filters. The porosity of the isostatically pressed filter medium is 30 X to 60 X. The air permeability of the medium is at least 1 l/dm²/min at a pressure drop of 200 Pa over the medium.

The suitability of the proposed filter medium will be further demonstrated on the basis of a concrete embodiment and the corresponding test results.

Example 1 : beer filtration

The example assumes a multi -layered metal fibre web with a basis of bundled drawn stainless steel fibres of steel type AISI 316L.

Depending on the filter orientation, the following metal fibre webs are stacked: 6.5/75 - 4/300 - 2/450 - 6.5/75. Using x/y notation, the intended result is a metal fibre web with a basis of metal fibres having an equivalent diameter of x μm and a weight of y g/m².

The multi -layered metal fibre web is then heat sintered and isostatically cold pressed at 2,500 bar (2.5.10⁸ Pa) according to a method described in the applicant's patent EP 0329863. In the above example the isostatic pressing was done by exerting the force of the pressure medium upon one side of the sintered web. The product now has one rough and one smooth surface. It is used as a filter medium with the rough surface as the outflow side.

The filter medium thus obtained has a porosity of 48.7 X and an air permeability of 1 l/dm²/min at a pressure drop of 200 Pa over the medium. The filter aid has a filter fineness of 1 micron, guaranteeing complete yeast retention and, in addition for wine, guaranteeing a complete retention of micro-organisms and retention of potassium tartrates. The strength of the filter medium according to the present invention ensures easy recovery by back flush (back wash), even at increased pressure. Medium cleaning appeared to run even more smoothly using pressure impulses (approx. 2 bar) of limited amounts of water or filtered beer, resp. wine. Several dozen recoveries could be realized.

The filter medium according to the present invention can be used in cylindrical candle filters, disc filters or other filter embodiments.

The results of the beer filtration tests will be discussed in what follows. The beer filter according to the present invention will be compared with a conventional, frequently used sterile cellulose filter, Strati Depural K10R^®.

Even after several dozen recoveries, filtrate turbidity is 0 to 0.15 EBC, in other words within the generally accepted standard.

This is a very important advantage compared to the conventional cellulose filters. Indeed, back flush through a cellulose medium was observed to cause fresh cracks each time, thereby rendering recovery totally impossible.

A filtration under constant pressure with incompressible filter cake is described mathematically by Ruth's equation :

t = K_X.V + K₂

V with 75(l-e)²hμ

Kι = e³d² (l-e-h)Δp

and u

Δp

where t time (s), V filtrate volume per unit filter surface (m³/m²),

€ filter cake porosity ( ³ pore volume/m³ filter cake), h volume fraction of suspended particles (m³ particles/m³ suspension) , μ : viscosity of the fluid (Ns/m²), dp particles parameter (m) , Δp pressure drop (Pa) ,

R_m measure for the mechanical throughflow resistance of the filter holder and the filter itself (1/m).

K_j may therefore be taken as an indicator for the congestion of the filter. A high K_: value also implies the reduction of the total filter capacity.

Since μ and Δ_p are virtually constant, K₂ may be regarded as a measure for the mechanical throughflow resistance of the filter material .

Experimentally, t/V can be represented as a function of V. K_α was determined on the basis of the directional coefficient of the straight line, K₂ from the intersection with the ordi- nate. The following results are given by way of example :

From this it appears that the filter medium according to the invention clogs up much more slowly than the conventional cellulose filter media. The mechanical throughflow resistance of the filter medium according to the invention, too, is appreciably lower than that of the conventional cellulose filter media. This once again illustrates some of the advantages of the beer filters according to the invention.

The beer filter medium according to the invention was also tested for yeast penetration. To measure penetration, the filtrate was passed through a Sartorius (0.45 microns) filter which was then incubated on wort agar. The complete system was sterilized in an autoclave (20 minutes at 121°C) before filtration. The first 100 ml of filtrate were sent through a first Sartorius filter, and the next litre of filtrate through a second one. Then, the Petri dishes were incubated at 25°C. Subsequently, the number of yeast colonies were counted. Slight yeast penetration was observed on the second Petri dish only after filtration at a pressure drop exceeding 2 bar (2.10⁵ Pa). Conventional filters already display yeast penetration (yeast colonies proliferating on the entire surface of the Petri dish) after filtration at a pressure drop of 0.5 bar (5.10⁴ Pa) over the filter medium. Tests to evaluate the foam retention on the beer after filtration via the filter medium according to the invention produced positive results. The stability, the colour and the bitterness of the beer were also evaluated positively after filtration.

An alternative embodiment of the beer filter may involve the use of a metal fibre web provided before or after isostatic pressing with a water-repellent coating to prevent the adsorption of bitter substances to the filter medium, which could enhance the bitter value of the filtered beer. For example, a layer of DYLYN^® might be applied to the upstream surface of the filter medium. The application of a coating of this kind is described in the applicant's European Patent application EP 97201867.5. If required, instead of a layer of DYLYN^® another organic or anorganic coating layer functioning as micro- or nanomembrane filter may be applied. As anorganic coating layers, sol/gel dispersions comprising A1₂0₃-, Ti0₂- and/or Zr0₂-parts, for example, are deposited on the (isostatically pressed) surface of the metal fibre web and sintered thereto.

Example 2 : wine filtration

As mentioned before a wine filtration process comprises a primary and a secondary filtration. The primary filtration, after the clarifying, conventionally comprises two subsequent steps. In the first step a roughly sintered metal fibre filter (Bekipor^®) guaranteeing a filter fineness of 5 μm can be used. In the second step a metal fibre filter (Bekipor^®) which has been sintered to a finer degree and with a multi - layer building up or stacking as described for the beer filtration and guaranteeing a filter fineness of 3 μm can be used. For the secondary filtration, after the cold stabilisation process, the cold isostatically pressed filter medium as described in EP 0329863 and with the multi -layer building up as used for the in example 1 described beer filter can be used. If required, the said cold isostatically pressed filter medium can also be used here (which may comprise metal powder next to fibres) and/or where a DYLYN^® or other organic or anorganic coating layer has been applied, as described before for the beer filter.

The filter medium according to the invention thus gives a filtrate that satisfies all the quality requirements made upon the beer.

If required rougher media with a filter fineness 5 μm and/or 3 μm may be connected upstream to these cold isostatically pressed filter media.

After this secondary filtration a final sterilisation process follows (before the eventual bottling). If required, a cold isostatically pressed filtermedium according to the invention can again be used during this process. This filtermedium may or may not be coated with a tightening coating layer functioning as membrane so that an eventual filter fineness of 0.45 μm - 0.60 μm is reached.

Claims

1. Filter for preparation of fruit beverage comprising a filter medium on the basis of metal fibres, characte- rized by the fact that the said filter medium is a possibly multi -layered, sintered, isostatically cold pressed metal fibre web.

2. Filter according to Claim 1, whereby the metal fibre web consists of stainless steel fibres of an equivalent diameter between 2 and 30 microns.

3. Filter according to Claim 1, whereby the metal fibre web weighs between 300 g/m² and 1,500 g/m².

4. Filter according to Claim 1, whereby the sintered metal fibre web is isostatically cold pressed at a minimum pressure of 2.5 x 10⁸ Pa).

5. Filter according to Claim 1, whereby the filter medium has an absolute filter fineness of less than 1.2 μm.

6. Filter according to Claim 2, whereby the filter medium has a porosity between 30 X and 60 X.

7. Filter according to Claim 1, whereby the filter medium has a permeability of more than 1 l/dm²/min at a pressure drop of 200 Pa over the filter medium.

8. Filter according to Claim 1, whereby metal powder is present in the metal fibre web, next to the fibres.

9. Filter according to Claim 1 or 8, whereby the metal fibre web at the upstream surface is coated with an organic or anorganic coating layer functioning as a micro- or nanomembrane filter layer.

10. Filter according to Claim 9 whereby a dispersion comprising A1₂0₃-, Ti0₂- and/or Zr0₂-parts has been deposited on the web surface and sintered thereto.