Precise pH measurement

The perfect chocolate challenge: Dr Michael Kogej explains how best to conduct pH measurements in cocoa butter

Measuring pH levels in sticky and viscous media is a demanding task. When processing cocoa butter, there are also the exacting hygiene and safety requirements in the food industry to consider. A leading chocolate manufacturer uses measurement technology from Knick for this purpose.

Making chocolate is a complex task and involves numerous production steps before the finished product can reach the consumer. The first key step takes place immediately after the cocoa beans are harvested. During fermentation – a biochemical process – the harsh, bitter taste of the seeds changes to produce the beginnings of the desired aromas, which then fully develop during drying and roasting.

pH value as a quality characteristic

Due to the fermentation, the pH value of cocoa beans is relatively acidic at around 5.0 to 5.5. The beans are first shelled and ground, after which the cocoa butter is separated by pressing. The cocoa butter’s pH value is an important indicator of product quality and must therefore be continuously monitored during the production of chocolate. Liquid cocoa butter, however, is also viscous above its melting temperature of 60°C and tends to be sticky. As a medium, then, it represents a genuine challenge for the measurement technology applied. Consequently, one leading German chocolate manufacturer used to have samples regularly checked for their pH values in its own quality assurance laboratory, a time-consuming and labour-intensive process that staff within quality assurance were keen to optimise.

A pH measurement integrated directly within the process is required to meet the exacting demands of the food industry. All the components need to be suitable for CIP and SIP, for example, and foreign objects such as glass splinters must under no circumstance be allowed to enter the cocoa butter unnoticed. In this context, one solution is provided by retractable fittings, where the sensor only ever comes into contact briefly with the medium and can then be cleaned. Retractable fittings that operate with push rods were used during the customer’s initial trials with this technology. However, these fittings proved unsuitable because the mechanical system quickly became clogged with cocoa butter, making further measurements impossible.

Inline measurement

Undeterred, the company managed to find a solution in a system from Knick, comprising a combination of the Ceramat retractable fitting and the Unical automatic sensor maintenance system. This retractable fitting operates with a patented ceramic seal circumventing the problem of mechanical blockage caused by hardening cocoa butter. The fully automatic, programmable Unical 9000 electro-pneumatic controller activates the retractable fitting and can remove the sensor from the process and clean it. To do so, the controller pumps the required cleaning and rinsing solutions from the connected storage tanks into the Ceramat chamber – entirely automatically. If necessary, the sensor can then be calibrated with suitable buffer solutions. All the required steps are carried out fully automatically by the controller at freely configurable intervals, with no need for any user intervention.

An SE555 pH sensor with glass electrode was used for the measurement. Since the pH sensor is only ever in contact with the medium for a short time, the use of a glass electrode is acceptable in this application. Digital Memosens technology is used for data transmission to the evaluation circuits, which practically eliminates measurement errors due to interference (including glass breakage) on the signal line. At the same time, this technology enables galvanic isolation between the measuring electrode and the evaluation circuit.

To record the measured values, the sensor is connected to a Protos 3400 process analysis system. Profibus PA is used for the connection to the higher-level process control system (PCS). For this purpose, the flexible Protos system is simply fitted with the relevant communication module. Measurement of the pH value can be directly triggered by the PCS.

The sensor is then immersed in the cocoa butter, the process analysis system transmits the measured value to the PCS, and the sensor is then cleaned and possibly calibrated in the Ceramat retractable fitting. The entire process is both fully automatic and very fast. Using this method, the time lost during measurement in the quality assurance laboratory is reduced to a minimum. With the bus connection via Profibus PA, error messages and alarms can also be transmitted directly to the PCS.

Fast, extremely precise measurement

The fully automatic inline measurement of pH values in chocolate production is working very reliably, and staff in quality assurance are pleased with the outcome. The new system is delivering substantial savings, as it has eliminated the need for a great deal of manual steps relating to sampling and pH measurement in the laboratory.

The chocolate manufacturer is also pleasantly surprised by the level of precision that is now available, with a deviation of just 0.01 between the pH measurements obtained during inline measurement and measurement in the laboratory.