Monitoring fermentation progress

The progress of fermentation must be monitored throughout. This is necessary to confirm that performance is as expected, to provide a means of early identification of non-standard behaviour and to verify as rapidly as possible that the end-point has been reached. Historically, these tasks were the prerogative of the skilled brewer whose experience with particular fermentations was sufficient to gauge progress based on simple visual observation. As measuring devices became available, visual observation was augmented by physical monitoring. In all but the most primitive of breweries, such empirical approaches have now been entirely superseded by real measurements.

This change was a consequence of the widespread adoption of closed fermenting vessels. Thus, where open fermenters are used, the progress of the process can be seen, for example, by the formation of a yeast head. The timing of formation, the size and appearance of such yeast heads gives much valuable information regarding the vigour and progress of fermentation. In closed vessels, alternative indirect monitoring methods are required. Temperature may be measured using a thermometer located inside the vessel. Other measurements are most commonly made, off-line, on samples of fermenting wort removed from vessels. In recent years, several in-line sensors have been introduced which allow automatic monitoring of fermentation progress. These devices provide continuous output such that any deviation from the norm is quickly identified allowing rapid remedial action to be taken. In addition, they provide an output, which is suitable for regulating the activity of some other device, thereby providing the possibility of introducing systems for the automatic control of fermentation.

Provision of monitoring devices offers an opportunity to maintain records of individual fermentation performance. These data can be used to set up a permanent archive, which can be an invaluable aid to fermentation management. With access to such data, long-term drift in performance can be identified. The effects on fermentation performance of changes in process or plant can be judged.

There are three aspects to monitoring fermentation progress. First, the rate of fermentation is controlled by application of cooling and it is therefore necessary to monitor temperature. Second, the progress of fermentation is gauged, usually by periodic or continuous measurement of wort specific gravity, or a related parameter. Third, in lager fermentations it may be necessary to define the end-point by measurement of the concentration of vicinal diketones (see Sections 3.7.4.1 and 6.3.9).

6.3.1 Monitoring temperature

Although temperature may be measured off-line it is most usually and conveniently monitored using thermometers located in pockets placed in the walls of fermenting vessels. The readings can simply be used to control temperature via manual appli cation of cooling but in most cases output is linked directly to a controller which provides automatic attemperation. In large capacity cylindroconical vessels, particularly those used for both fermentation and conditioning, the location and number of temperature transmitting probes is of crucial importance to the maintenance of proper attemperation. This subject is discussed in Section 5.4.2.

Platinum resistance probes, conforming to BS 1904 class A or B, are most commonly used for temperature measurement because of their good hygienic design, accuracy and linearity of output. Care must be taken to ensure that the chosen instrument is suitable for the application. Thus, Davies (1992) reported that the BS 1904 standard does not stipulate a response time and that testing of 12 commercially available models revealed a wide range of response times to changes in temperature, one being as slow as 22 seconds.

Where large numbers of fermenting vessels are in simultaneous use, it is necessary to have some type of microprocessor-driven supervisory system. This monitors and controls the temperature of the entire tank farm - for example, the ACCOS system described by Wogan (1992). Typically, the tank farm comprises a few tens of vessels in which all stages of the fermentation cycle will be represented at any one time. Thus, vessels will be empty, being filled, cleaned, in the middle of primary fermentation, on crash cool, or on hold after cooling. The supervisory system receives and records temperature data from each vessel. It confirms that the values are appropriate and within range for the process stage that each vessel is undergoing and applies control, as necessary. During primary fermentation temperature control would typically be ±0.05°C of the set-point. Alarms are raised if temperatures are outside pre-set limits for any particular process stage. Should an alarm condition be detected, automatic processes are put into a hold situation during which the process is held under conditions that cause the least possible compromise to the integrity of the product.

6.3.2 Monitoring wort gravity

Measurement of the reduction in wort specific gravity, or a derived unit, as sugar is utilised by yeast, is the most commonly used method of gauging fermentation progress. Several different and arbitrary scales have been used throughout the world. Some of these are given here, although at present only degrees Plato and saccharin (present gravity) are in common usage.

(1) Specific gravity (also termed relative density): this is the ratio of the density of liquid at a specified temperature (often 20°C) and the density of water at the temperature of its maximum density 4°C (39.2°F). Specific gravity is without dimension.

(2) Present gravity (degrees saccharin): the specific gravity, multiplied by 1000 and minus 1000, measured at 20°C and expressed in degrees.

(3) Degrees Balling: a scale devised by Carl Joseph Napoleon Balling in 1843, from tables relating density of wort to sucrose concentration measured at 17.5°C (63.5°F). A Balling saccharometer is a hydrometer calibrated to give a measure of wort sugar as percent weight. Thus, 1 "Balling is equal to 1 g sugar per 100 g wort. One degree Balling is equal to 3.8°saccharin.

(4) Degrees Plato: Balling's tables were slightly erroneous and were later corrected by the German chemist Plato. The tables bearing his name have an identical basis to that of Balling. The difference brought about by the correction is illustrated in Fig. 6.13.

(5) Degrees Brix: a specific gravity scale also based on sugar concentration expressed as percentage weight to weight but in this case measured at 15°C (59°F).

(6) Régie: a measure of wort density used in France. 'Legal density' is defined as the ratio of the mass of 50 cm3 liquid at 15°C and the mass of an equal volume of pure water at 4°C. One degree Régie (R°) is equal to: (Legal density —1000) x 100. One degree Régie is equal to 2.6°Balling.

(7) Belgian degree: a value derived from the specific gravity by subtracting 1 and multiplying by 100.

Degrees Plato

Fig. 6.13 Relationship between wort density measured by the Balling and Plato scales.

Degrees Plato

Fig. 6.13 Relationship between wort density measured by the Balling and Plato scales.

6.3.2.1 Sampling from fermenters. Before off-line analyses can be made it is necessary to remove samples from the fermenter. In traditional open fermenters samples may be obtained using a dropping can. This device allows samples to be removed from any desired location within a vessel (Fig. 6.14). It consists of a stainless steel container with a capacity of approximately 500 ml. The base is fitted to the body via a screw thread and gasket and is made of heavy gauge metal to ensure that the can sinks. There is a handle to which is attached a length of chain. The can aperture is closed with a rubber bung, which is also attached to a chain. The latter runs through a ring in the handle, which retains the bung when it is removed from the neck of the bottle. Samples are obtained by lowering the can into the fermenter with the bung in place. When the can is submerged to a desired depth the bung is pulled out by a sharp tug on the appropriate chain. The can and sample may then be retrieved. Dropping cans may be used to sample from deep vessels although in the experience of this author with difficulty.

Modern closed fermenting vessels are fitted with sample valves to facilitate off-line analyses (see Section 8.3.2.1). It is essential that these are of good hygienic design and are properly maintained. This important aspect of brewery management is frequently neglected. Thus, poorly maintained or badly designed sample cocks can provide a source of contamination to both the sample and the fermenter contents. Perryman

Chain attached to rubber bung

Handle with chain attached

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