Although the pH value in food processing is an important control parameter, it is usually measured in the laboratory and not in-line in the process
pH is one of the key parameters in food production.
An optimally adjusted pH value determines the degree of enzyme activity. Endopeptidase, which is responsible for decomposing protein in mash when brewing beer, requires a pH environment of pH5.
Rennin, which thickens milk in cheese production, requires a value of pH6.2.
In the production of acidified milk products, the pH must be maintained precisely to ensure the quality of the final product.
In the production of yoghurt, the pH must not drop below 4 - 4.2.
Therefore, the product is rapidly cooled down at a pH of 4.4 - 4.6 during yoghurt maturation to prevent over-acidification.
Although the pH value in food processing is an important control parameter, it is usually measured in the laboratory and not in-line in the process.
Laboratory measurements are firstly expensive, and secondly inefficient compared with an in-line pH control system, which can display the current product pH values and start or stop process steps directly.
It could also create documentation of pH curves for an entire bottled product batch.
Of course, the issues relating to cleanability of the sensors and assemblies integrated in the process are extremely high.
Contrary to laboratory measurements, a glass electrode is subjected to various continuous stresses in in-line measurement.
Chemical influences as well as cold acids and alkalis occurring during CIP cleaning dehydrate the pH-sensitive coating on the glass membrane.
High temperatures during steam sterilisation age the glass prematurely and lead to a loss of slope.
On the reference side, the diaphragms (mainly made of ceramic) can become blocked by fat and proteins.
This can lead to seriously false measured variables.
If the electrode connectors and cable connections are not spraywater-proof, it can lead to repeated faults.
Modern pH process electrodes, such as the gel-filled Orbisint or the Ceraliquid filled with liquid KCl, have a pH-sensitive process glass membrane which supply excellent measurement results in cold, weak-acid and acid pH ranges of the product (eg, beer: pH4.4 at 0C).
Attack by acids and alkalis in CIP systems on the glass membrane is negligible, ie, the acid or alkaline error after CIP cleaning is max.
0.1pH.
This special glass recipe ensures long life, even when exposed to extremely differing process conditions.
Of course, these electrodes can be sterilised in steam and in autoclaves.
Orbisint, a gel-filled pH combination electrode, is equipped with a large-area ring diaphragm made of special sintered PTFE.
PTFE is dirt-repellent and prevents the deposit of fats and proteins on the diaphragms. The enormous increase in area also ensures long-term defect-free function of the reference system.
Not always is the gel-filled electrode the best solution for the process.
A pressurised, liquid-filled electrode is better suited to fast-changing pH jumps or media containing high levels of protein or fat. Ceraliquid, the liquid-filled combination electrode, is fitted with a zirconium dioxide diaphragm which has a constant pore size due to a patented production process.
It therefore ensures a constant KCl flow rate (0.65ml/d).
This flow rate is vital since it rinses the diaphragm free and allows a drift-free pH measurement.
The escaping electrolyte, 3-molar KCL, poses no risk of contamination for the foodstuff produced since its concentration is high and the volume used is low.
Every electrode is provided with a diagram which shows the pressure settings to obtain the optimal electrolyte flow through the diaphragm at various process pressures and temperatures.
In order to determine the pH value in the process with absolute accuracy, it is vital to measure the temperature and perform temperature compensation with the currently measured value at the same time.
This is why Orbisint and Ceraliquid electrodes have an integrated Pt100 temperature sensor which is located directly at the tip of the electrode behind the pH-sensitive glass.
The advantage here is that the actual temperature is detected both in measurement and calibration mode, thus ensuring precise measurement and calibration.
Moisture bridges immediately lead to short circuits in the high-impedance pH measurement.
THe measuring systems in breweries and dairies are constantly sprayed with water by the cleaning systems.
TOP68 is a pH electrode plug-in system that is protected to IP68 and is therefore more than just spraywater protected.
An O-ring on the electrode head forms the radial seal with the coupling.
The plug and the coupling are made of rigid, highly resistant PPS.
The insulator, in which two, four, or six contact pins are embedded, is made of soft, hydrophobic (water-repellent) PFA plastic.
The contacts can thus be embedded without gaps and the electrode can then be cleaned in an autoclave in open state.
Since PFA repels water and only microfine droplet traces remain on the surface, there are no moisture bridges if the plug is wetted during assembly or disassembly.
The electrode need only be wiped to make it immediately functional again.
Retractable assemblies are used to immerse pH sensors in the process.
These assemblies can be cleaned during the process, immersed in water during idle times, retracted during CIP phases or the pH sensor can be automatically cleaned and calibrated.
The Probfit CPA 465pH retractable assembly is made of polished stainless steel and is designed to fit all process adapters typically used in the food industry, such as the DN 50 sanitary tube union, 2" clamp adapter, APV or the Varivent adapter.
The electrode is inserted in a metal tube and sealed with an O-ring.
The pH-sensitive glass membrane is additionally protected against mechanical forcess and damage by fours webs.
For cleaning or calibration, the electrode guide together with the electrode are retracted from the process pneumatically and a shaped seal wipes off any product rests still adhering to the electrode.
The shaped seals made of EPDM are integrated in the assembly with no undercuts and have much greater strength than other O-rings normally used.
They therefore have much longer service intervals than O-rings.
If the seal is damaged, it is immediately signalled by a leak detector.
In the USA, the Probfit CPA 465 is approved as 3-A Sanitary Equipment; in the Netherlands, the TNO has granted its approval for steam sterilisation according to the regulations fo the European Hygienic Equipment Design Group (EHEDG).
The pH transmitters of the Mycom and Liquisys series designed for the food industry are fitted with several functions which allow easier and more reliable pH measurements: Simple two-point calibration with plausibility check.
The measuring point is calibrated by only pressing one button on the instrument and involves no complicated menu navigation; Safety functions provided by sensor and process check systems such as sensor life check alarm which checks continuously whether the sensor signal is a live measuring signal; Sensor check system for the pH glass detects hairline cracks and glass breakage; Sensor check system for the reference system detects soiling and blockages in the reference electrode. This can also trigger sensor cleaning; All instruments are available with Profibus-PA protocol as well as the conventional 4 - 20mA output signal for the pH and temperature measured variable.
Finally, two different examples of quality monitoring with pH in-line systems: Before bottling, liquid mustard must not exceed a pH range of 3.5 - 3.7, otherwise quality is drastically impaired.
Therefore, for safety reasons, a pH measuring system is installed comprising a gel-filled electrode, a CPA 465 retractable assembly fitted with pneumatic manual control and a transmitter with limits.
The system is cleaned manually after every bottling operation and is calibrated once a week.
This solution gives the user a high degree of measuring reliability, allows simple cleaning and minimises maintenance costs.
At the milk acceptance point, milk collected from various farms is transferred from the tanker to the dairy.
The key criterion is that the pH value of the incoming milk should not be less than 6.65 or more than 6.85.
To ensure high measuring accuracy, a pressurised, liquid-filled Ceraliquid pH electrode is installed. The retractable assembly is controlled by a CPC 200/210 automatic calibration and cleaning system.
This system controls all the cleaning and calibration processes, and moves the wetted assembly to standby position during service breaks.
Automatic cleaning takes place ten times a day with water and a special cleaning agent.
Calibration takes place once a day.
