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Stainless steel body _

-Detachable base

Fig. 6.14 A dropping can.

(1991) lists a number of attributes that sample cocks should possess. In the case of fermenting vessels they should be suitable for removal of both bulk samples for analyses such as present gravity and smaller aseptic samples for microbiological assessment. This requires that the sample valve can be cleaned in place with the vessel and there should be no dead spaces to harbour sources of contamination. If necessary, it should be possible to sterilise the valve prior to sample removal, either by application of steam and/or flooding with alcohol. The design should facilitate easy and rapid operation. They should be relatively maintenance free and have a long operating life.

The location of sample valves is important. A fundamental assumption of off-line analysis for monitoring fermentation progress is that the sample is representative of the whole of the contents of the fermenter. This requires that the fermenting wort is homogeneous, a condition that is probably achieved only during active primary fermentation. Some stratification is likely towards the end of fermentation when mixing due to natural convection currents is limited. Thus, at the end of fermentation some errors are an inevitable consequence of off-line analysis, wherever sample points are located. This may be of small significance with regard to monitoring specific gravity. However, measurement of vicinal diketone concentration is performed when primary fermentation is nearing completion. This requires accuracy since a certain minimum concentration must be achieved before the fermentation is judged to be finished (see Section 6.3.9). It is better, therefore, to perform analyses on samples which may overestimate the true concentration. This is best done by avoiding locating sample points close to areas where the yeast concentration may be very high, for example, in the cone of a cylindroconical fermenter. Here the high yeast density is capable of reducing vicinal diketone concentrations at a more rapid rate than that in other parts of the vessel where the yeast count is more sparse. This may lead to significant underestimates of analysis.

Moll et al. (1978) described an automatic sampling system suitable for removing small volumes of wort from large-capacity cylindroconical vessels during fermentation. The device was capable of removing sequential samples of fermenting wort from three different locations in the vertical section of the vessel. Samples were degassed automatically and refrigerated prior to being fed into instrumentation for measurement of yeast count, dissolved oxygen, temperature, pH and ethanol.

6.3.2.2 Off-line gravity measurement. Samples of fermenting wort are removed periodically from fermenting vessels and used for measurement of specific gravity (or a related parameter). Typically, readings are taken every 8 hours and plotted on a chart together with a record of the temperature. Frequently each chart has a predrawn 'standard' gravity versus time profile so that actual and 'ideal' data can be compared directly. Each fermentation is given a unique chart which may also contain information such as identifier of wort batch, wort volume, quantity of yeast pitched, yeast viability at pitch, identity and source of pitching yeast and details of postcollection additions. In addition, the chart may be used to record details of any nonstandard process changes or breakdowns. The charts are retained to form the basis of an archive, which allows tracking of individual batches of beer. The charts may be maintained as 'hard copy' or in electronic form.

Measurement of the specific gravity of samples removed from fermenting vessels is commonly made using a hydrometer (or 'saccharometer') calibrated in one of the units described earlier. The sample must be de-gassed before readings are made to avoid errors due to carbon dioxide break-out. It is essential to record the temperature of the sample as measurements are made to allow appropriate compensation to be made. Specific apparatus has been designed for making measurements with a saccharometer. This consists of a cylinder, usually made from copper or stainless steel, which has an outer jacket through which water is circulated to maintain a constant temperature. The cylinder is filled to the brim and after allowing a period for attemperation and gas dispersal the saccharometer is suspended in the liquid and the reading made. Saccharometers are designed for measurement of gravity over a limited range of sugar concentrations. Where worts of widely different strengths are used, a complementary range of saccharometers is required.

The specific gravity of off-line samples may be determined automatically. For example, the apparatus based on the oscillating U-tube method (Jiggens, 1987). Here the sample is pumped into a glass U-tube that is clamped at its open end. The chamber in which the U-tube is located is attemperated to 20° C. The specific density of the sample is derived from the degree of damping to the oscillation (the time period of the oscillation) caused by the liquid contained within it. Measurement is based on the natural frequency (f) of oscillation of the U-tube which my be related to density in the following relationship:

where f is the natural frequency of the system; c is the elasticity constant; m is the mass of the tube, v the volume of the tube and p the density of the liquid in the tube.

The time period (T) of the oscillation is equal to the reciprocal of the natural frequency. Therefore, from the relationship above:

Design of the instrument allows insertion of two constants, A and B, which are related to the parameters discussed already as shown:

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