Wort collection

Wort collection describes that part of the brewing process in which wort is delivered from the brewhouse to the fermenting vessels. This must be done in such a way that after the process is complete appropriate initial conditions are established within the filled fermenting vessels. At first sight this may appear a relatively straightforward operation; however, it involves several distinct steps which if performed incorrectly will have adverse effects on both fermentation performance and the quality of the resultant beer.

The wort parameters that have to be controlled during the collection process are: volume, specific gravity, temperature, sterility and clarity. In addition, at some point in the process oxygen and yeast must be added. The total time taken to fill the fermenter and the sequence and timing of oxygen and yeast addition has to be considered. Finally, it is usually necessary to make some additions to the wort in fermenter, to achieve the desired composition. The technical complexity of the methods used to control the parameters listed above tends to reflect the sophistication of the brewery.

6.1.1 Wort cooling and clarification

The final stage of wort preparation is the copper boil. Before the wort can be added to the fermenting vessel it must be cooled. Both the boiling and cooling stages are accompanied by precipitation of solid materials. These are referred to as 'hot' and 'cold break', respectively, or 'trub', the German for sediment or clouding. Trub is a heterogeneous complex formed from a coagulation reaction between wort proteins and other nitrogenous components with polyphenols. In addition, insoluble minerals may be present, together with a lipid fraction derived from malt and hops and some hop resins. Specific process plant is provided for the separation of hot break. This may be of several types depending on the brewery in question but in modern plants the whirlpool is the most commonly used method (see Section 2.3.6).

In traditional breweries, the hot wort was cooled in a shallow open vessel often referred to as a 'coolship'. Cooling was achieved simply by exposing the wort to the ambient temperature of the room. Occasionally ceiling mounted fans were used to accelerate the process, or more efficient cooling could be obtained by allowing ingress of refrigerated air into the room. Open wort coolers had the advantages that aeration was achieved naturally by exposure to air and the shallow design of the vessel facilitated sedimentation of cold break such that relatively clear wort could be decanted, leaving the sludge behind. However, these advantages were far outweighed by poor hygienic design. In addition, the manner of cooling made it impossible to avoid some oxidation of wort components since inevitably exposure to air commenced when the wort was still hot. The risks of wort contamination could be minimised by controlling the atmosphere of the room in which the coolships were placed. Thus, the rooms were designed to avoid the possibility of condensation forming on walls, ceilings and other internal structures such that it could drip back into the vessels. Furthermore, the supply of cool air was filtered to minimise levels of airborne contamination. Nevertheless, this operation was fraught with risk and could only be managed by restricting the period that the wort spent in the coolships to no longer than a couple of hours.

An alternative to the open coolships was the use of closed wort receiving vessels. These took several forms but were generally deeper than coolships (1-2 metres as opposed to less than 20 cm). Cooling could be accelerated by the provision of internal coils through which cold water or brine was circulated. The design allowed for settlement of solid material and subsequent separation from the clarified wort during run-off. The enclosed design presented a barrier to contamination with air-borne micro-organisms. However, there was less opportunity to ensure that the wort became saturated with air.

In the interests of efficiency, the static cooling vessels have given way to in-line methods of temperature control. Originally, especially in the United Kingdom, coolships were replaced by horizontal refrigerators. These consisted of slightly inclined shallow vessels containing coils fed with a coolant. The hot wort was allowed gently to pass over the coils, thereby lowering the temperature and allowing cold break to form and settle out. Such coolers were extravagant in usage of floor space, and subsequently they were replaced with vertically oriented refrigerators. In these, the wort was allowed to trickle in a thin stream down a series of plates through which a suitable coolant was circulated. Such coolers could be quite efficient and allowed natural aeration of worts to occur. However, no means of cold break separation was provided. If the latter were required then additional plant downstream of the cooler was needed. Of course, any open cooling system required careful control of the atmosphere of the surrounding room to minimise the risk of microbial contamination.

To maintain the efficiency benefits of in-line cooling and provide adequate microbiological control most modern breweries now use enclosed heat exchangers for wort cooling. Typically these are plate heat exchangers or 'paraflows', as originally introduced by Seligman of A.P.V. Limited (Dummett, 1982). They are made up of arrays of elements each of which consist of a stainless steel plate which has holes in each corner and a series of grooves (Figs 6.1 and 6.2). Individual plates are joined together within an enclosing frame. The cavities between each plate are connected by the four corner holes, thereby providing channels for liquid flow. The grooves in the plates are compressed together to form serpentine channels for liquid flow. A series of gaskets between the plates provide watertight seals and control the direction of liquid flow such that coolant and wort circulate through alternate cavities in a counter-current fashion.

In order to maximise rates of heat transfer the plates are constructed from thin stainless steel, typically 0.5 mm, and the grooves are designed to maximise turbulent flow. Paraflows are sized with respect to the desired wort flow rate and the degree of

Coolant in
Fig. 6.1 (a) Side view schematic of a two-stage paraflow. (b) End view of two adjoining plates of wort paraflow.

cooling required. Usually the temperature of the wort issuing from the paraflow is controlled automatically by varying the coolant flow rate. The temperature of the coolant depends on the required cooling duty. In the case of relatively warm ale fermentations a single-stage water-cooled system is adequate. For lager worts it is usual to supplement the water-cooling stage with additional plates through which a refrigerant such as brine, ethanol or glycol is circulated.

Paraflow wort coolers are highly efficient and provide very accurate control of the temperature of wort delivered to the fermenter. They are easily cleanable and totally

Fig. 6.2 Image of a paraflow (supplied by Harry White. Bass Brewers).

enclosed design provides a barrier to microbial contamination and avoids uncontrolled exposure to atmospheric oxygen. In addition, as a by-product of their operation they may generate hot water that may be used elsewhere.

Paraflows have a major disadvantage in that there is no provision for wort clarification. Consequently, it may be necessary to have process plant for solids removal located between the paraflow and fermenter. Several methods are used, which differ in the extent to which solids, both actual and potential, are removed. For example, the use of intermediary holding tanks where solids are simply allowed to settle out. In-line methods include continuous centrifugation or filtration through kieselguhr (diato-maceous earth). In European lager breweries a trub flotation process is sometimes used that involves placing the cooled wort into a tank where it is subjected to forced aeration. This pushes suspended trub upwards, where, under the influence of surface tension, it forms a pellicle, which can be skimmed off. Another method, used by New Zealand brewers in conjunction with continuous fermentation (see Section 5.6.2.2), is to chill the wort to the point at which ice crystals begin to form, hold for a period and then filter.

Lager fermentations, using bottom-cropping yeast, are perhaps less tolerant of the presence of trub. A characteristic of these beers is their excellent foaming properties and the presence of any head-negative materials would be most undesirable. In addition, wort solids in fermenter will tend to settle and contaminate the yeast crop, possibly necessitating a subsequent cleansing step. Compared to ales, lager beers require a high degree of carbonation and excessive loss of carbon dioxide during fermentation due to the presence of nucleating materials would require re-adjustment of gas levels at the conditioning stage.

The requirement to clarify wort, the stage at which it is performed and the manner in which it is accomplished is dependent on the type of fermentation. It is also a somewhat contentious issue. The presence in fermenter of appreciable wort solids has potentially both negative and positive influences. It has been suggested that unsaturated fatty acids in the lipid fraction of cold break may contribute to yeast nutrition and reduce or supplement the requirement for wort oxygenation (see Section 3.5). Siebert et al. (1986) noted that turbid worts produced more rapid fermentations than clear worts. However, these authors concluded that this was due to the particles providing nucleation sites for formation of carbon dioxide bubbles. In the opinion of the authors this alleviated inhibition of yeast growth by high concentrations of dissolved carbon dioxide. This view was supported by the observation that the stimulatory effects of trub particles could be replicated by the addition of activated carbon or kieselguhr. Lentini et al. (1994), also concluded that acceleration of fermentation rate due to trub particles was a physical effect. However, they also reported influences of trub on beer flavour. Thus, levels of linolenic acid in the lipid fraction of trub were shown to vary inversely with acetate ester concentrations in beer. It was also shown that trub had the ability to bind added zinc, such that it may not be available to the yeast. The presence of excessive trub may have other undesirable effects. The lipids may have deleterious effects on beer foam, and other components may produce undesirable flavours and decreased flavour stability (Olsen, 1981; Schisler et al., 1982; Carpentier et al., 1991).

Traditional ale fermentations performed in square fermenters are probably the most tolerant of the presence of cold break. These fermentations are managed in such a way that there is an opportunity to remove trub. Thus, much solid material rises to the surface with the first yeast head, at which point it may be skimmed off before the formation of the second clean yeast head. The latter is retained for subsequent re-pitching. Other unwanted solid material forms a sediment which is separated when the vessel is emptied. In the dropping system (see Section 5.3.3) a similar solids separation is accomplished by starting fermentation in one vessel, then, after an interval to allow sedimentation of trub, transferring the clarified fermenting wort to another vessel. Ales have relatively low carbon dioxide contents, and therefore efficient removal of this gas during fermentation is an advantage. In addition, ale yeasts tend to have quite low oxygen requirements, typically air saturation being sufficient to produce profuse yeast growth. This may be partially a result of some unsaturated fatty acid being provided by the wort.

Continuous free or immobilised yeast fermentation systems are the least tolerant of worts containing high solids contents. Clogging of bioreactors can restrict process flow, necessitating frequent repacking and consequent loss of overall productivity due to excessive downtime. With such systems (see Sections 5.6 and 5.7), production of bright wort is essential. Hence, the practice of the New Zealand brewers of chilling wort and filtering prior to delivery to the continuous fermenting vessels.

In the case of conventional batch fermentations, the advantages and disadvantages of wort clarification have to be weighed based on considerations of cost versus process performance. Whatever the decision, it is certainly true that wort solids have to be removed at some stage in the brewing process. The formation of wort solids continues, albeit at a lower rate, after the initial cooling step. Therefore, a case may be made for delaying clarification to conditioning where removal of trub, residual yeast and chill haze materials can be combined in a single process. However, in order to maintain good fermentation control and ensure superior beer quality, the weight of evidence suggests that it is preferable to remove the bulk of the trub before fermen tation commences. However, the disadvantage is that if a wort paraflow cooler is employed then there will be an on-cost due to the need for subsequent wort clarification plant. In addition, there will be a decrease in overall process efficiency because of wort losses entrained with the solids.

6.1.2 Wort oxygenation

Traditional fermentation systems rely on wort becoming saturated with air during the processes of cooling and transfer into fermenting vessel, as discussed already. Modern installations, which are largely enclosed and essentially anaerobic, require provision for forced addition of oxygen. This is accomplished by the addition of oxygen, in-line, either as a pure gas or in the form of air, to the wort as it is delivered to the fermenting vessel. Regulation of the concentration of wort dissolved oxygen is a crucial part of the strategy by which yeast growth is controlled. Therefore, it is essential that dosing of oxygen is precise and repeatable. In general, the most sophisticated wort oxygenation systems are required for large capacity fermentations, particularly those performed at high gravity. In the case of small-volume fermentations using sales gravity worts, the requirements of the oxygenation system are perhaps less demanding.

Oxygen may be added to the wort on the hot or cold side of the paraflow. Adding oxygen to hot wort has the advantage that the risk of introducing microbial contaminants with the gas is small. Furthermore, efficient solution is favoured because of the good mixing characteristics of the paraflow. However, a large proportion of the added oxygen may never become available to the yeast since it may be wasted in wort oxidation reactions. Furthermore, it is more difficult to attain high oxygen concentrations since the solubility of oxygen in wort decreases with increase in temperature. For these reasons, oxygenation of wort on the hot side of the paraflow is only suitable for those fermentations whose oxygen requirement can be satisfied by air saturation or less.

Addition of oxygen to cooled worts has the advantage that solubility is increased; however, the sterilising effect of the hot wort is lost and a suitable steam sterilisable microbiological filter must be used. In order to make use of the dissolving capabilities of the paraflow it is common to add the gas between the two cooling stages. If oxygen or air must be added after the paraflow, it is essential to improve the gas transfer characteristics of the system. This may be accomplished by introducing the gas in the form of fine bubbles via a stainless steel sinter or candle, preferably followed by an inline mixer. Plant specifically designed to promote efficient solution of oxygen in wort was described by Ringholt (1997). This consisted of a cylindrical vessel through which the wort passed during collection. The core of the cylinder contained an infusion chamber into which air was introduced. The wort was injected into the infusion chamber via perforations in the wall. The infusion chamber was divided into four sections by a series of three plates located on the long axis of the cylinder. These plates served to close some perforations in response to fluctuations in the wort flow rate such that a constant flow velocity was maintained in the aerator. In this way, a constant aeration rate was achieved. The design of the infusion chamber facilitated good turbulent mixing of wort and air. An in-line mixer down-stream of the aeration chamber further encouraged gas solution.

It is possible to use air or oxygen as the source gas although if the former is used this does, of course, limit the oxygen concentration that can be achieved. The solubility of oxygen is influenced by temperature and the concentration of the wort, as shown in Figs 6.3(a) and (b). A desired dissolved oxygen concentration may be achieved by adding air or oxygen at a given rate which takes into account the temperature and gravity of the wort, the wort flow rate and the pressure within the wort main. This may be calculated from the following relationship (assuming perfect solution of the gas):

RT CFw

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