The dissolved oxygen contents of air-saturated worts and sucrose solutions determined with an internally calibrated polarographic oxygen electrode show a linear decrease in the gravity range 1 -016-1 -075. The level at S.G. 1 040 is 8 5 mg/litre at 20 C.Key words: oxygen, wort, analysis-method.
IntroductionThe measurement of the absolute quantity of dissolved oxygen in wort is difficult because of the interference of organic materials in chemical methods*-7 and the problem of relating the activity or partial pressure to concentration when electrode methods are used.3 This exists whether or not the electrode is covered with a membrane."Published data on the oxygen levels of air-saturated worts vary considerably, depending upon the method of analysis used. For S.G. 1-040 worts at 20°C the following values have been reported: 5-6 mg/litre by polarography1 (corrected from the reported value of 51 at 25°C), 5-9 mg/litre by manometry,8 6-2 mg/litre by indigo-carmine,1 and 7-8 mg/litre by gas chromatography.1 Although the last value is probably the most accurate, in none of these sets of data was a linear decrease in solubility observed with increase in gravity, as would be expected from a comparison of the published data for the solubility of oxygen in saline solutions."The use in brewing of the Digox EC-3 oxygen analyser has recently been reported.0 In this instrument calibration is by the addition of a known concentration of oxygen, produced electrolytically in a separate cell, to the sample before it reaches the measuring cell.The Digox EC-3 analyser has been used to measure the dissolved oxygen content of a range of air-saturated sucrose solutions and worts.
ExperimentalSucrose solutions in the gravity range 1016 to I 075 at 2O°C/2O°C were made up in distilled water containing potas sium chloride added at a rate of 0-5% of the sucrose con centration. These were attemperatcd to 20cC, saturated with air by blowing in air through a sintered disc for 20 min, and then passed through the Digox EC-3 oxygen analyser at a flow rate of 10 litres/hour. The measuring electrode was calibrated by the electrolytic addition of 1 00 mg/litre oxygen to each sucrose solution. Solubilities were corrected to 760 mm Hg total pressure.
Results and DiscussionThe air saturation levels of oxygen in sucrose solutions at 20°C in the gravity range 1016 to 1075 arc shown in Fig. I, where it can be seen that there is a linear decrease in solu bility with gravity. A similar graph was obtained for brewery worts. The concentration of oxygen at S.G. 1040 was 8-5 mg/litre.The extrapolated value for the oxygen concentration in water (9-4 mg/litre) is higher by 0-2 mg/litre than the highest published value,8 possibly due to slight supersaturation or the samples.The effect of temperature on ihc air-saturation levels of sucrose solution S.G. 1-045 over the range I5-3O°C is shown in Fig. 2.
