The monitoring of dissolved oxygen content is essential for the brewing process. If large amounts of dissolved oxygen are present in finished beer, oxidation processes take place that negatively affect the beer’s flavor and cause premature product aging. Dissolved oxygen (DO) is a relative measurement of the amount of oxygen that is dissolved or carried in a given medium.

For breweries, the measurement of DO can be as low as 8-12 ppm in the wort aeration step, 10-30 ppb in the fermentation process, 20-50 ppb in the filtration process and 100-200 ppb in the filling process. 

Dissolved oxygen content in the brewing process has for many years been monitored with electromechanical amperometric sensors. However, they have several known disadvantages, including a high sensitivity to process pressure and temperature changes, which slow response time after a cleaning-in-process (CIP) step, and add time and monetary costs associated with frequent sensor replacement. 

Hamilton Inc., a manufacturer of sensor technology headquartered in Reno, NV and Bonaduz, Switzerland, developed its VisiFerm DO B line of optical sensors as an alternative to the amperometric method. Said to be the first optical sensor designed to measure dissolved oxygen in brewing applications, it measures dissolved oxygen using a method based on oxygen-dependent luminescence, says Knut Georgy, market segment manager process analytics at Hamilton Bonaduz AG.

Georgy’s work with industrial gases began with his study at the Institute of Food Chemistry at the Westfälische Wilhelms-University Münster, Germany, where he received a PhD on the investigation of nonstarch-polysaccharides in spinach and their changes during processing. Following university, he gained sales experience as market developer for industrial gases in the food and beverage market in Bavaria and as a regional manager in southern Germany and Austria specializing in CO2. Later, he became owner of an independent company that engineered piping for liquid nitrogen and CO2 applications.  Georgy was tapped as global market segment manager for food and beverage at Georg Fischer Piping Systems, a plastic piping manufacturer. In 2011, he joined Hamilton’s Switzerland office.

FE: What issues are there with amperometric sensing?

Georgy: The amperometric sensor is a much older technology that operates according to the Clark principle, which states the content of dissolved oxygen is determined by means of a current flow in the sensor’s measuring chamber. The current is created when oxygen passes through the gas membrane of the sensor. There are an anode and cathode built into the sensor, and there is a liquid electrolyte that makes the chemical reaction possible. The chemical reaction leads to a current, and that current is what is measured. If there is no oxygen, there is no current.

The gas permeable membrane in the cap of an amperometric sensor is pressure sensitive. Pressure spikes (or pressure hammer) can occur in the line for a short time when valves are open or shut. As the pressure rises, there is a push on the membrane. The final result is more oxygen being pushed through the membrane during a surge than is experienced during processing, resulting temporarily in a higher current reading. The sensor tells you there is more oxygen in the system than there really is, and it can take a long time to adjust.

The slow response time of the amperometric sensor is especially noticeable in a brewery when a CIP cycle is run, where it may display an incorrectly higher DO value. In a brewing line, the filling only starts when the DO is at or below a defined ppb level to ensure product quality. If a line is filled with product but the sensor displays incorrect oxygen content, that product has to be discarded. During the CIP cycle, there is high DO content in the cleaning media as brewers rinse the pipeline with water. CIP is a higher-temperature process that can cause a shift in the sensing element of a DO sensor. Beer is cold, but when you run CIP cycles, it is hot.

FE: What benefits do optical sensors deliver?

Georgy: Unlike amperometric DO sensors, VisiFerm DO B is insensitive to pressure hammers, provides reliable measurement down to ppb levels and is not affected by flow, stirring or CO2. Electrolyte is not required, the sensor attains rapid startup with no polarization time, and maintenance is simplified. 

After the CIP step, the VisiFerm DO B sensor could be reused faster than an amperometric sensor. Significant reduction in the quantity of rinsing product used and discarded afterward can be realized.

The VisiFerm DO sensors proved to be less susceptible to break down and require less maintenance. The optical sensor has only one part subject to wear—the sensor cap—which must be changed every four to eight months. Replacement and recalibration take only a few minutes. 

A key differentiator is that optical DO sensors measure the partial pressure of dissolved oxygen. Partial pressure is defined as a mixture of gases where each gas has a hypothetical pressure if it alone occupied the volume of the mixture at the same temperature. According to Dalton’s law, total pressure of a gas mixture is the sum of the partial pressures of each individual gas. Gases dissolve, diffuse and react according to their partial pressure, not to their concentration. This realization allows for oxygen to be measured using an alternative approach to amperometric sensing and the Clark principle.

FE: How does oxygen-dependent luminescence work? 

Georgy: It is a fluorescent type of luminescence involving the emission of light by a substance that has absorbed light or other electromagnetic radiation. The emitted light has a longer wavelength than the absorbed light. The luminophore is a light-absorbing and emitting element (the gas permeable membrane) that gives off less fluorescence in the presence of oxygen—called fluorescence quenching by oxygen.

The VisiFerm DO B sensor is comprised of an LED that emits light in a focused (blue) wavelength. That light is absorbed by the light-sensing element. A photodiode with a red filter captures fluorescence (light emission) from the luminophore.

The luminophore is an illuminating foil—consisting of molecules embedded in part of a silicone layer —that is the sensing element. The sensor features an LED emitter. (In our case, it emits a blue light corresponding to a particular wavelength.) 

The oxygen in the luminophore causes quenching (reduction of fluorescence) leading to a phase shift in the electromagnetic waves. The photodiode measures the phase shift and the strength of the red light to determine the amount of oxygen present.

If oxygen is present, there is a quenching defect on the luminophore, so the signal gets weaker, and the time shift gets a little longer, and that is what we measure. Hamilton sets the sensor to capture just one specific wavelength to avoid any mistakes from other items that might have red light (including daylight).  

FE: What is the benefit of the CIP mode and brewery mode?

Georgy: Hamilton’s VisiFerm DO B sensor features unique CIP modes and brewery modes that enable reliable measurements in environments such as breweries, which may not allow for calibration after every CIP. 

The sensor operates in a pressure range up to 12 bar (174psi) and can be set to measure from 4 ppb to 25 ppm (DO), or from 0.1 to 600 mbar (pO2). A CIP procedure leads to a shift in the DO reading, and the offset compensation is there to eliminate this effect. The brewery mode is an automatic drift correction for long-term stability. 

FE: What real-world testing have you conducted?

Georgy: The Brewery Krombacher of Kreuztal, Germany tested the VisiFerm DO B on a filling line for several months in 2011 and in 2012 producing a pilsner brew. Hamilton installed the VisiFerm DO B sensor and kept a competitor’s amperometric DO sensor installed. The test was to determine how quickly the sensors monitored 20-50 ppb in the line after the CIP cycle, as well as the extent of changeover and maintenance tasks.

The VisiFerm sensor reported acceptable DO in the line approximately five minutes sooner than the amperometric sensor.

For that filling line, the time difference translated into 20 hectoliters of beer that could be used for filling. If several product changes are executed on the line during the week, the optical sensor could help limit the amount of wastewater the brewer has to treat. Krombacher calculated a savings in the six-digit range per year if it switches completely from amperometric to optical sensing.