Oregano – Still a Loaded Culinary Herb?
Report from a day in the lab
Dr. Florian Hägele, Thomas Kapp
High levels of harmful pyrrolizidine alkaloids, pesticide residues, and myriad cases of adulterated plant material – dried oregano has had several problematic areas in the past, as our analyses in 2019 clearly revealed. But what is the current situation for oregano? Has it taken a turn for the better in the meantime? Our investigations into dried oregano from the past year signalize a definite yes! Only one sample of oregano was adulterated with rockrose leaves. The levels of pyrrolizidine alkaloids sunk on average by approximately 80 % compared to the previous reporting period, even when individual oregano samples still exhibited increased amounts. With an above-average violation rate of 22 %, the only area that remains largely unchanged is that of pesticide residues.
Illustration 1 (stock photo): Fresh herbs with intact structures can easily be identified by their leaf forms (here: oregano). For crushed herbs, however, it isn’t possible for the naked eye to recognize whether foreign plant parts are included.
The Initial Situation
Previous analyses conducted by CVUA Stuttgart on dried, crushed oregano (Origanum vulgare L.) revealed an urgent need for action regarding the general state of quality for this popular herb. Firstly, conspicuously high levels of pyrrolizidine alkaloids (PA), a group of hazardous substances, were consistently detected in oregano samples in 2019, 70 % of which were reported as unsafe for consumption. Secondly, approximately 20 % of the oregano samples were judged to be fraudulent because they had been cut with substantial amounts of olive tree leaves and/or rockrose, in order to maximize on profit. In fact, one sample labeled as “oregano“ consisted of only 35 % oregano; the remaining 65 % were identified as olive leaves. Moreover, about one out of three oregano samples (29 %) contained levels of pesticides and contaminants above the legally established maximum.
In response to the dissatisfactory situation, we followed up in 2021 by analyzing a total of 36 samples of dried, crushed oregano from retailers and wholesalers as well as food-processing companies for the presence of pyrrolizidine alkaloids (PA) and for adulteration with foreign plant material. Eighteen of these samples were also analyzed for residues of over 750 different pesticides and contaminants.
Analytical Results
Pyrrolizidine alkaloids
Pyrrolizidine alkaloids (PA) are naturally occurring substances that serve to protect some types of plants from pests. PAs are problematic and thus unwanted in food, however, due to their chronic liver-damaging effect. The subgroup of unsaturated PAs and their N-oxides are especially troubling, as they are suspected of being genetically harmful and have been found in animal testing to be carcinogenic.
No formation of PA in oregano (Origanum vulgare L.) has been thus far described in specialist literature. It can be assumed, therefore, that the PA findings in the oregano samples resulted from a co-processing with PA-forming foreign plants during the harvest.
In 2019 levels of PA detected in the analyzed oregano samples were significantly above average, as high as 32,400 µg/kg. The average PA amount was 6,160 µg/kg (median: 5,430 µg/kg) (see "Oregano – an Aromatic but Loaded Culinary Herb. Part I: Pyrrolizidine Alkaloids").
Fortunately, the current analyses from 2021 show a significant reduction of PA in oregano. The average amount detected in the 36 analyzed samples of oregano was 1,206 µg/kg, a reduction of 80 %. A look at the median value of 822 µg/kg shows an even larger reduction, of 85 %. The highest level of PA observed in the current oregano samples was 8,600 µg/kg.
As depicted in Illustration 2, the majority of analyzed oregano samples from 2021 (22 samples; 61 %) contained less than 1,000 µg/kg of PA, an amount that is not considered to be a health risk. In contrast, only 25 % of oregano samples from 2019 were at this level. Even though this is a very positive development, there is still a high amount (14 samples; 39 %) of oregano sold in supermarkets that contain PA levels above 1,000 µg/kg. PA levels of over 10,000 µg/kg, as were found in 24 % of samples in 2019, were no longer observed in this current investigatory time frame.
Illustration 2: Quantities of pyrrolizidine alkaloids in dried oregano from 2021; 36 samples
In 2019 maximum levels for the sum of pyrrolizidine alkaloids or individual pyrrolizidine alkaloids had not yet been established for Germany or the EU. An assessment of PA findings was only possible in terms of toxicological guidelines (cancerous effects of PA (0.024 µg PA/kg body weight per day)) or of not cancerous, but chronic liver effects of PA (HBGV of 0.1 μg/kg body weight), derived from the German Federal Institute for Risk Assessment (BfR). In the meantime, law makers have established maximum levels for PA for relevant products EU-wide which, after a transitional period, will take effect in July 2022. The future legal maximum level for PA in oregano will be 1,000 µg/kg.
Despite the improved overall situation, if the newly established maximum PA level had been applied to the recently analyzed dried oregano samples, about 40 % would have been above the maximum level and, therefore, not marketable.
From a toxicological point of view, only one out of the total 36 samples was conspicuous, with a peak level of 8,600 µg/kg. According to the BfR, long-term consumption of products with high levels of PA especially by children, pregnant or nursing women, presents a health risk due to the toxic effect on the liver caused by 1,2 unsaturated PA (see Info Box). Given the extreme degree of contamination from PA, this sample was classified as unacceptable for human consumption and thereby judged to be an unsafe food. In comparison, more than 70 % of the oregano samples analyzed in 2019 contained toxicologically inacceptable levels of PA and were even judged in some cases to be hazardous.
All of the currently examined, contaminated samples still presented a matching alkaloid pattern, although mainly the alkaloids europine, europine-N-oxide, lasiocarpine and lasiocarpine-N-oxide are responsible for PA contamination in dried oregano. This indicates that the source of the detected contamination can be traced to an invasive foreign plant from the borage family (Boraginaceae) growing around the oregano cultivation.
Illustration 3: Structure formulae of the pyrrolizidine alkaloids and their N-oxides most frequently found in oregano. Crucial for the toxicity of the compounds is the unsaturated 1,2 position of the basic structure (numbered in blue).
Info Box
Pyrrolizidine alkaloids
Pyrrolizidine alkaloids (PA) are a group of several hundred structurally tightly related individual compounds. They serve to protect the plants from predators, and occur naturally in over 6,000 plant species. These can be divided into three main families:
- composite (Asteraceae),
- borage (Boraginaceae) and
- legume (Fabaceae or Leguminosae).
PAs are problematic, however, due to their chronic liver-damaging effect. The subgroup of unsaturated PAs and their N-Oxides are especially troubling, as they are suspected of being genetically harmful and have been found in animal testing to be carcinogenic.
The Federal Institute of Risk Assessment (BfR) has therefore recommended not exceeding a daily dose of 0.024 µg unsaturated PA/kg body weight (BW). For an adult weighing 70 kg, this means consuming no more than the extraordinarily small amount of 1.68 µg PA per day, and even less for children and toddlers.
A non-cancerous health effect resulting from exposure to PA is especially liver damage, which can lead to liver necrosis. To assess these effects, the BfR derived a Health Based Guidance Value (HBGV) of 0.1 μg/kg body weight. Non-cancerous damage is not to be expected with doses under this level. [1]
The investigative spectrum at CVUA Stuttgart currently includes 42 toxic individual compounds (including N-Oxides). In order to better present and compare PA contamination of the analyzed samples, the measured individual values are summarized as aggregates.
PA-forming plants, with the exception of borage, are not normally consumed as food. Therefore, elevated PA amounts generally point to the presence and further processing of foreign plants together with those to be harvested. Given its pronounced toxicity, food businesses must take measures to avoid contamination from PA as much as possible. The EU has a minimization imperative for contaminants, including PA. The quantities must be limited to the same extent that the ALARA principle (As Low As Reasonably Achievable) requires, from production to market. This is especially necessary for genotoxic-carcinogenic substances, because even small amounts, especially when taken regularly, are associated with higher risks of unwanted health effects.
Adulteration of dried oregano with foreign plant material
Typically, dried oregano refers to the dried yellowish to greenish leaves, as well as the petals and upper stems of the Origanum vulgare L. from the labiate family (labiates or lamiaceae). As our analyses in 2019 revealed, however, the contents of the oregano samples were not limited to oregano. In the previous investigations, apart from rockrose leaves, the oregano had been supplanted with, in some cases, considerable amounts of olive leaves (Olea europaea L.). As a result, 20 % of the so-called “oregano“ samples were judged to be fraudulent in terms of their identity and composition.
Our current investigations have yielded, in contrast, much more positive results. Among the 36 oregano samples analyzed for adulterations, only one (2.5 %) was detected with foreign plant material in the form of rockrose leaves (Cistus sp.), comprising 10 % of the sample. In 2021 no falsifications with olive leaves were observed.
Illustration 4: Stereomicroscopic recordings (20x magnification) of: a) dried oregano leaves with oil droplets (Origanum vulgare L.), b) fragments of olive leaves (Olea europaea L.), and c) fragments of rockrose leaves (Cistus sp.)
maximize
Residues of Pesticides and Contaminants
A total of 18 samples of dried oregano were analyzed for residues of over 750 different pesticides and contaminants (perchlorate). Residues from a total of 24 different pesticide substances and perchlorate were determinable in all (100 %) of the samples. Multiple residues were detected in 12 (67 %) of the analyzed samples, meaning that they contained more than one pesticide substance per sample. One such sample was detected with as many as 7 different substances. On average, each sample contained 3.3 different substances. The relevant residue findings are presented in Table 1.
| Parameter* |
No. Samples with Residues (%)
|
Min (mg/kg)
|
Max (mg/kg)
|
MRL** (mg/kg)
|
No. > MRL**
|
|---|---|---|---|---|---|
| Azoxystrobin (F) |
9 (50 %)
|
0.016
|
0.089
|
70
|
-
|
| Cypermethrin (I) |
9 (50 %)
|
0.015
|
2.2
|
2
|
-****
|
| Fluazifop (H) |
6 (33 %)
|
0.046
|
1.1
|
0.02
|
4
|
| Pendimethalin (H) |
4 (22 %)
|
0.017
|
0.044
|
0.6
|
-
|
| Perchlorate (K) |
18 (100 %)
|
0.044
|
0.21
|
0.20***
|
-****
|
| Tebuconazole (F) |
3 (18 %)
|
0.009
|
0.013
|
0.05
|
-
|
The substances azoxystrobin and cypermethrin were found especially often (50 % of the samples). Moreover, the contaminant perchlorate was detected in all of the samples. The herbicide fluazifop showed up in 6 of the 18 (33 %) oregano samples, 4 of which also lay above the legally set maximum (MRL) under consideration of drying factors (DF 6); see Table 1. Even if the determined pesticide quantities are not considered a health risk for consumers from a toxicological point of view, this food is not marketable. In all, the rate of violations for dried oregano due to MRL exceedances for pesticides was 22 %, a high value compared to other vegetables or fruit.
In contrast to the positive development regarding PAs and adulterations, the situation for pesticide residues and contaminants has remained largely unchanged compared to our investigations in 2019.
Conclusion
Somewhat better, but not yet perfect – this sums up the development of dried oregano‘s qualitative characteristics in a short but succinct way.
Although this popular culinary herb garnered a lot of negative attention in 2019 due to its extremely high levels of harmful pyrrolizidine alkaloids, current analyses signal a significant easing of the problem. In the meantime, the average quantities of pyrrolizidine alkaloids in oregano have sunk by 80 % and toxicologically critical levels are no longer the rule, but rather rare exceptions. If the newly established maximum levels of 1,000 µg/kg for PA in oregano that will be valid from July 2022 were applied to the current investigations, however, approximately 40 % of the samples would not be marketable. This shows continued room for improvement. All levels of the food industry, including farmers, producers and retailers, must make goal-oriented efforts to reduce PA levels in oregano so that only safe goods are marketed to consumers from July 2022.
The myriad cases of adulterated oregano from two years ago have also greatly improved. Only one of the currently analyzed samples had been cut with 10 % rockrose leaves. Dried oregano consisting mostly of worthless olive or rockrose leaves, contrary to consumer expectations, is now the rare exception.
Only in terms of pesticide residues does the situation for oregano remain largely unchanged. Above-average numbers of oregano samples containing levels of PA that exceed the legal maximum continue to be observed. The determined levels of pesticide do not pose a health risk to consumers, but they show that good agricultural practices are not being conducted and that improvements are needed.
CVUA Stuttgart will continue to focus on this product group and will report on future developments.
References
[1] Stellungnahme Nr. 020/2018 des BfR vom 14. Juni 2018: „Aktualisierte Risikobewertung zu Gehalten an 1,2-ungesättigten Pyrrolizidinalkaloiden (PA) in Lebensmitteln“
[2] Stellungnahme Nr. 030/2016 des BfR vom 28. September 2016: „Pyrrolizidinalkaloide: Gehalte in Lebensmitteln sollen nach wie vor so weit wie möglich gesenkt werden“
[3] Stellungnahme Nr. 017/2019 des BfR vom 13. Mai 2019: „Pyrrolizidinalkaloidgehalt in getrockneten und tiefgefrorenen Gewürzen und Kräutern zu hoch“
Translated by Catherine Leiblein
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