Checking frying oil: polar compounds, limits, measurement

Frying oil changes every time you use it. Heat, oxygen, water and food residues break the oil down until it affects the taste and quality of the food. Spoiled frying oil counts as unsafe food, and authorities in Austria and Germany check it regularly. This guide explains what polar compounds are, which limits apply, how to measure correctly and when to change the oil. What is binding for you is always your own HACCP plan and the requirements of your local authority. This article is not legal advice.

What polar compounds are

Fresh oil consists almost entirely of triglycerides. During frying, oxidation and polymerisation produce breakdown products that are more polar than the intact triglycerides. The German Society for Fat Science (DGF) names short-chain fatty acids, mono- and diglycerides, aldehydes, ketones and polymers as examples. Their total is called total polar compounds (TPC) or total polar material (TPM). Some of these substances add to the typical frying flavour. As their share rises, however, the smoke point drops, the oil becomes more viscous and it starts to foam.

According to the DGF, only the content of polar compounds and polymerised triglycerides allows an objective assessment of heat stress. The acid value is not suitable for this. The DGF names 10 to 18 % polar compounds as the optimal range during normal operation.

Limits in Germany and Austria

There is no EU-wide limit for polar compounds. Each country assesses used frying oil by its own criteria, and the values differ.

Germany

The 24 % value goes back to a DGF recommendation, as Bertrand Matthäus of the Max Rubner Institute explains in OFI Magazine. Food control authorities do not judge oil by a single number. LGL Bavaria and the Hesse State Laboratory check the sensory profile and compare it with the levels of polar compounds and polymerised triglycerides. Spoiled frying oil may not be placed on the market. This is no minor issue: in 2023 the Hesse State Laboratory classed 28 of 247 samples as overused and unfit for consumption. In Bavaria, about 19 % of used oils were rejected in 2006 and about 29 % in 2007 (LGL).

Austria

The Austrian Food Codex (Lebensmittelbuch), chapter B 30, lists signs that frying oil has already changed, including 27 % polar compounds, more than 12 % polymerised triglycerides, blue-grey smoke at 180 °C and clear defects in smell or taste. According to the Codex, frying oil that is unfit for consumption also makes the food fried in it unfit. In a targeted campaign, the Austrian Agency for Health and Food Safety (AGES) tested 72 samples from across Austria. Five were rejected, among other things for sensory defects, dark colour and sediment.

Frying temperature and acrylamide

High temperatures speed up oil breakdown and promote acrylamide formation. Regulation (EU) 2017/2158 requires food businesses to minimise acrylamide. Businesses at retail level, which includes restaurants and catering, apply Annex II Part A. For fried potato products this means, among other things:

• Keep frying temperatures below 175 °C and as low as possible.
• Choose oils and fats that allow shorter frying times or lower temperatures.
• Maintain oil quality by skimming off crumbs and small particles frequently.
• Display colour guides showing the best balance of browning and low acrylamide for staff.

Businesses must be able to show that they apply these measures. Recording frying temperature and oil quality helps with that.

Measuring correctly: sensory check, quick test, laboratory

Sensory check

Check smell, taste, smoke and foaming. A scratchy or rancid taste, the first signs of smoke and increased foaming are clear warnings. The DGF points out that a darker colour on its own is not a measure of spoilage.

Quick tests and measuring devices

Quick tests and electronic testers are available for daily use. The DGF lists colour reactions, redox tests and methods based on foam height, viscosity or dielectric constant. Many handheld testers show polar compounds directly as a percentage. According to the DGF, quick tests give a rough indication, and results depend on the type of oil. They work best when you use the same oil continuously. Follow the manufacturer's instructions on measuring temperature and sensor cleaning, and always measure under comparable conditions.

Laboratory analysis

Only a laboratory determines polar compounds and polymerised triglycerides precisely. An occasional lab sample can help you check your tester and your change routine.

How often to check and when to change

The guidance sheet of the Wetterau district in Germany requires a daily check of frying oil, either by sensory testing or with a suitable test system, and a dated record of every oil change. The DGF recommends changing the oil before it spoils and setting the change routine in your HACCP plan. This routine has proven useful:

• Check sensory signs every day before service and measure polar compounds if you have a tester.
• Check the frying temperature: no more than 180 °C, preferably 160 to 175 °C; below 175 °C for French fries.
• Following the DGF, replace about 20 to 25 % of the oil with fresh oil each day and top up lost oil regularly.
• Filter the warm oil through a heat-resistant filter; according to the DGF this slows spoilage by about 10 to 20 %.
• Skim off crumbs frequently during service.
• Change the oil at the latest when your own limit is reached, with a clear margin below 24 % (Germany) or 27 % (Austria).
• Empty and clean the fryer at every change and remove all oil residues.
• Record every reading and every change with date, fryer, value, action and name.

Documentation

A frying oil log is short and quick to keep. Useful columns are date, fryer, frying temperature, measured polar compounds, sensory result, action taken (filtered, topped up, changed) and name. Set your own change limit in your HACCP plan below the value used by the authorities. That leaves a margin for measuring uncertainty.

Frequently asked questions

At what percentage of polar compounds is frying oil spoiled?

In Germany, more than 24 % polar compounds indicates spoilage, based on a DGF recommendation. In Austria, the Food Codex names 27 % polar compounds and more than 12 % polymerised triglycerides as signs of change. Authorities always assess the sensory profile as well.

How hot can frying oil get?

The DGF and the Austrian Food Codex name 180 °C as the upper limit, and the DGF recommends 160 to 175 °C. For French fries, Regulation (EU) 2017/2158 requires temperatures below 175 °C.

How often do I need to check frying oil?

Authorities such as the Wetterau district require a daily check, by sensory testing or with a test system, and a record of every oil change. The exact routine is set in your HACCP plan.

A digital system such as Kernwert brings your daily self-checks together: staff scan the QR sticker, tap their name and enter the value. Deviations turn red immediately and require a corrective action, and the inspection folder is ready with one click.

This article is not legal advice and does not replace a business-specific HACCP analysis. The applicable regulations and the requirements of your local authority are decisive.