Filter sheets sterile

The properties of flat-sheet filters may be defined (Section 11.4.1) by a nominal cutoff expressed in ftm. It is also possible to determine the maximnm quantity of microorganisms in suspension likely to be retained per cm of filter surface under specified operating conditions. Bacteria are mnch less efficiently trapped than yeast cells. The flow rate of the finest sterilizing filter sheets is naturally lower than that of clarifying sheets and they are also more snsceptible to clogging. 

Table 11.7. Successive stages in the clarification of a sweet white wine until almost total sterility is obtained, nsing sterilizing filter sheets (Serrano, nnpublished data)...

Untreated wine, before filtration Filtration through diatomaceous earth precoat Filtration throngh sterilizing filter sheets... 

Before filtration Clarifying filter sheet no. 3 Clarifying filter sheet no. 5 Clarifying filter sheet no. 7 Clarifying filter sheet no. 10 Sterilizing filter sheet... 

It is vital to sterilize all equipment, and especially the filter and filter sheets, every morning prior to starting filtration and bottling. Table 11.9 shows the importance of sterilization. In particular, it is necessary in order to achieve perfect yeast retention, which is indispensable for sweet wines. If the system has not been sterilized, the first... 

At the end of the operation, the water is drained from the filter and, at the same time, it is filled with wine. However, after flushing with the quantity of water necessary for sterilization, a 40 cm x 40 cm filter sheet retains approximately 0.85 1 of liquid. It is, therefore, essential to eliminate the first wine that is filtered (at least one liter per sheet), as it is highly diluted and may have slight organoleptic defects. 

The standard filter sheet in breweries is 60 x 62 cm (approx. 23 6 X 24 4 in) and the largest is normally 100 x 100 cm (39 4 x 39-4 in). Some large single-ended filter presses have 240 filter-plates which permit an hourly flow rate of about 120 hl/hr (73 brl/hr). Such small throughput capacity means that sheet filtration has lost popularity except for second stage filtration in which they are designed to polish and in some cases sterile-filter. Considerable... 

It is convenient to compare performances of powder filters on the basis of (/) filtration efficiency, ( ) ease of sterilizing the unit, ( 7) ease of automation, (/v) avoiding beer dilution, (v) ease of spent powder removal and (vi) dryness of the spent powder discharged [12]. Table 20.3 gives details of such a comparison. Flow rates are usually about 4-5 hl/m but slower rates ensure more efficient particle removal. The average pore size of filter sheets is 4-6 (xm, that of leaf septa is 45-70 (xm and the slits in a candle are 50-90 (xm. 

Sterile filtration. A filter sheet of suitable grade, although a depth filter, may enable effective sterile filtration to the satisfaction of the brewer. 

In 1916 the sterile filter sheet became available for sterilizing filtration for the beverage and dairy industries. Other applications were soon found for this material, including vaccines, cosmetics, sugar, ink, film and paint processing. Today, however, asbestos is no longer used in the manufacture of filter sheets because of its toxicity. Considerable research has resulted in the availabihty of asbestos-free filter sheets that utihze cellulose fibres and mixtures of natural (uncalcined) kieselguhr and perlite. Other types of sheet also incorporate an advanced resin system. 

Vinegar is normally filtered at the bottling stage. Sterilizing filter sheets are reqnired for malt and wine vinegars. 

In some breweries it is of extreme importance that the beer be absolutely sterile, and therefore it is necessary to use a sheet filter filtration in addition to the primary filtration. 

The number of sheets required in the filter depends on the throughput of the bottle filler. Calculations show that 23 clarifying sheets or 45 sterilizing sheets (40 cm square) are required for a bottling line with a capacity of 3000 bottles/h. 

The system mnst be sterilized before filtration starts every morning, as described in the Section on flat-sheet filters (Section 11.7.4). The sterilizing flnid, either steam or hot water at 90°C, is circulated at low pressure in the same direction as filtration. Water mnst be preflltered to avoid damaging the membranes. Once the equipment has been sterilized, it is cooled with filtered cold water. 

Neither clarifying nor sterilizing flat-sheet filters cansed any more noticeable changes than fine earth filters. A reduction in fermentation esters was noted, althongh the terpenols in Muscat wines were unaffected. No significant differences were identified when the wines were tasted. 

---