Vol.16, No. 2 March - April, 2001
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061, E-mail: firstname.lastname@example.org
Table of Contents
I. Filtration 1
Absolute Filtration 2
Dept Filtration 2
Cake Filtration 3
and Body Feeding
Modern premium winemaking generally involves minimal processing, which includes minimal filtration. If filtration is used, it is essential that it be done properly so that it does not reduce wine quality. The following is a review of filtration.
In any solution, the particles in suspension have a number of properties that affect the way in which they are retained by a filter. The smaller the particle, the more difficult it is to remove it from solution. One important characteristic of suspended particles is their size distribution.
Although the range of particle size is large, a substantial number of particles are well below 1.0 micrometer in size. The average diameter of yeast (Saccromyces) is about 1.2 micrometers. The particle size distribution in juice or wine is non-symmetrical in nature. The preponderance of particle sizes is clustered rather closely to the smallest particles in the distribution, with the population of larger particles being small.
Another important characteristic of suspended material in solution is its mechanical nature. Particles can be classified as either non-deformable or deformable. Non-deformable particles are those which retain their shape. In wine, the principle non-deformable particles are diatoms. Diatoms are often added uniformly before and/or during filtration to increase the filtration surface area. Because of their rigid nature and geometry, they themselves act as a filtration media. Using various grades of diatomaceous earth is a means of controlling the size and amount of particles which the filter will retain. (This will be further elaborated upon in a discussion of precoating and body feeding).
The other type of particles in juice or wine is the gelatinous or deformable materials. These include yeast, bacterial cells and many colloids such as fining agents, gelatin, bentonite, etc. Such materials offer more problems in filtration than do rigid materials. Because of their elastic nature, they are capable of spreading over a larger surface area. Hence they are often active in blocking filtration, due to their spreading or matting effect.
A third property of suspended particles is their tendency to agglomerate or flocculate. Many suspended particles will adhere if they come in contact with a similar particle. The result is a single larger particle, where, formerly, there were two. If it has mechanical stability, the larger particle will possess many of the properties of a single particle of that size. This tendency can be put to practical use in that larger particles may precipitate naturally or by the addition of fining agents. It is easier to remove larger particles by filtration than smaller particles. Thus, prefiltration fining is often a desirable means of increasing filterability.
Among the more useful ways of classifying filter media are as absolute or depth. These two types of filters differ fundamentally in their mechanisms of retention.
An absolute filter is a geometrically regular, porous matrix that retains particles on its surface primarily by a sieving mechanism. The pore size is controlled in the manufacturing process. Filtration through such a filter is inherently absolute, in that anything larger than the pore size is retained on the filter surface. These are the 'membrane' type filters used in the wine industry as a final filtration just prior to bottling for the removal of wine microorganisms. The advantages and disadvantages of such a filter are summarized as follows:
(1) Using absolute filtration, it is possible to derive a specific rating of membrane efficiency independent of flow rate and pressure differential. Therefore, a winemaker can be assured that no microorganisms larger than the pore diameter will travel through the filter if the filter is properly functioning. (Integrity testing is very important). Most winemakers attempting to remove yeast use a membrane filter with 0.6 to 0.8 micrometer diameter while lactic acid bacteria are generally retained by a 0.45 micrometer membrane filter.
(2) Owing to the homogenous nature of the membrane, no media migration or sloughing occurs. Thus, no particles larger than the membrane pore diameter or membrane material itself will travel down stream.
(3) Since membranes are very thin (membrane diameter = 150 m), there is only a limited possibility of microbial growth within the inner layers. Coupled with this property, there is reduced product loss.
(4) Successive layers of larger particles may act to prevent the passage of particles smaller than pore diameter.
The following points are disadvantages of the absolute type filter:
(1) Because of surface retention, membrane filters have a low 'dirt-handling capacity.' This is especially true of particles with diameters approximately equal to those of membrane pores. Therefore, only 'clean' wine should be filtered through these units for the purpose of removing microorganisms.
(2) Not all small particles (with diameters less than pore size) will pass readily through. Many may be retained in the pore passage, hence blocking the flow.
In depth filtration, the separation of solids from the liquid phase takes place inside the filtration medium only. The filtration medium consists of numerous tortuous channels of all diameters and configurations. All the channels vary in diameter from the upstream to the downstream side. The particles float at random through the channels and, at some point, impact on the walls of the channel and are retained by entrapment or adsorption.
As the particles are deposited in the depth filter, its retention capacity increases. This increases the flow resistance and the differential pressure. Eventually, this results in complete blockage.
The advantages of depth filtration include:
(1) Since the depth filter can retain particles throughout its matrix, rather than solely on its surface, it will filter many times the material that the absolute type filters can process.
(2)Further, owing to its principle of adsorption, this filter will retain particles smaller than its flow passages.
The disadvantage of depth filtration are as follows:
(1) Media migration can occur. This refers to the tendency of filter media (fragments) to slough off during filtration. This problem is increased in cases where the wine to be filtered encounters the filter as a surge, rather than at a uniform flow.
(2) Microbial growth within the filter matrix may become a problem, especially in long filter runs. Under proper conditions, organisms may reproduce within the filter and successive generations will penetrate deeper into the matrix. The result is filtrate contamination.
(3) A certain amount of product may remain within the filter matrix after filtration. In the wine industry the filter is usually 'blown out' with nitrogen and the wine transferred back to the feed tank.
Because of the nature of depth filtration, an absolute particle retention rating is difficult. These filters are assigned a 'normal rating'. This is usually a particle size, above which a certain percentage (usually 98%) of particulates will be retained. This is determined experimentally after the filter is produced. It is important to note that this rating is valid only under strictly defined conditions of flow, temperature, pressure, and viscosity. Change in any parameter will effect particle retention. The so-called sterilizing pads are depth filters especially made to have a uniform porosity. These pads, however, can only remove yeast under a specific set of conditions such as flow rate and differential pressure across the pad.
The depth filters in common use in the wine industry include the pad filter, like plate and frame filters, as well as pressure leaf or cake filters.
Cake filtration is perhaps the most widely used filtration system in the wine industry. In cake filtration, the solid material separated from the liquid accumulates on the surface of the medium so that, after a short initial period, the deposited solids form a cake through which the liquid must pass. The cake in the form of diatomaceous earth is deposited initially on a coarse screen in the case of pressure leaf filters or pads in the case of plate and frame filters. In the wine industry, filter cloths made of cotton or other suitable synthetic fiber, such as nylon, are used. The process may continue, increasing the depth of the cake until the space available is filled or until the pressure differential becomes so great that the flow is reduced to an uneconomical level.
In order to improve the filtration characteristics of this system, the wine industry uses diatomaceous earth (D.E.) for precoating of the screen or filter pads as well as for a continuous proportioned body feed throughout the filtration cycle. By selecting the particle size of the D.E. used, different fineness of filtration can be achieved from rough filtration to polish filtration.
D.E. is composed of the fossil remains of microscopic marine plants called diatoms. These plants extract silica from the water and form exoskeletons or shells. The skeleton remains after the plant dies and settles to the bottom and accumulates. Diatomaceous earths are processed at 1500-2000 degrees F to burn off all organic matter. This leaves a residue which is almost pure silica. There are various grades of D.E. depending on the fineness or particle size which range from about 2.5 to 38 microns. The finer particle size produces a more polished filtration.
The amount of D.E. needed to deposit an effective precoat depends on the flow characteristics of the filter, the type of screens and filter pads used and the pump characteristics. The most effective amount can only be determined by actual experimentation.
Plate and Frame Filters.
The plate and, frame filter consists of a number of plates and frames corresponding in size and shape which are arranged alternately and which are supported on a pair of rails. The plates have ribbed or waffle surface to facilitate the flow of filtrate. They may be constructed of stainless steel or plastic.
To set up a plate and frame filter, the filter pads are placed on both sides of the frame so that there is a repetitive arrangement of plate, filter pad, frame, filter pad, plate, closed tightly by means of a manually operated screw. The feed channel in this filter is formed by corresponding holes in each plate and frame that register together when the filter is tightened so that they form a continuous flow path. Each frame has an opening that leads from this channel into the inside space of the frame. There is another opening in the bottom of each plate that connects the down flow side of the filter cloth to an outflow channel formed in a manner similar to the feed channel and which leads to the filtrate outlet port. When the filter is in operation, liquid flows into the filter through the inlet port, frame ports, cake, filter cloths, plate ports and out through the filtrate outlet port.
II. Precoating and Body Feeding
During the initial stage of the operation, the liquid is filtered through the filter pad only. Therefore a circulation phase is needed in order to deposit a cake on the surface of the filter cloth. This is known as 'PRECOATING'. Once this is accomplished, the flow is diverted into a tank and the filtration continues while filteraid is added to the wine as a 'BODY FEED'. The body feed prevents rapid plugging of the filter by providing a continuous supply of new porous filter elements.