Drying oregano: effects on yield and essential oil composition

The main finding is that drying did not produce an oil with a consistent composition. Microwave treatment at 460 W for 35 minutes yielded the highest result, 2.20%, while fresh material yielded 0.20%. However, that sample retained approximately 12.3% moisture: the authors themselves consider it only sufficiently suitable for microbiological preservation. Therefore, a higher yield does not automatically equate to an optimal product for every use.

Carvacrol remained prevalent in all oils, ranging from 56.2% to 81.4%. Compared to the fresh sample, it increased in all treatments except for the osmotic one; the differences indicated by the authors were statistically significant, with P < 0.0001. Conversely, alpha-pinene, p-cymene, and gamma-terpinene were generally lower after drying. In simple terms, drying can shift the balance between the molecules that contribute to the aroma, without transforming oregano into a standardized product.

For those who use oregano as a spice, the most useful takeaway is that “dried oregano” does not necessarily identify aromatically identical products. Time, temperature, exposure to air, and the method by which water is removed can leave different proportions of volatile substances. This helps explain why aroma and intensity can vary between seemingly similar products.

This work is also relevant for those interpreting studies on essential oils. The percentage of carvacrol indicates the relative share in the analyzed oil, not the amount ingested in a culinary serving, nor is it a measure of health efficacy. The data support a relationship between the technological process and chemical composition; they do not allow for the attribution of clinical benefits to dried oregano, its oil, or any single component.

This is an experimental laboratory study on a collection of Origanum vulgare carried out in July 2020. It did not compare cultivars, seasons, cultivation areas, or post-drying storage conditions. These factors can influence the oil profile, making it prudent not to extend the results to all available products.

Furthermore, the percentages describe the oil obtained through distillation, not the complete composition of the spice nor what happens after human consumption. The study did not evaluate taste, final food safety, bioavailability, culinary doses, or clinical outcomes. The relative increase in carvacrol does not prove an absolute increase in its quantity on the plate and does not demonstrate a health benefit. Comparison with other research further clarifies this context-dependency.

Initial direct confirmation comes from Özer and colleagues, who studied O. vulgare subsp. hirtum at three stages of development and using sun, shade, or oven drying. They identified 37 compounds and observed differences in both yield and the proportions of carvacrol, gamma-terpinene, and p-cymene. The maximum yield, 2.8%, appeared in material at full bloom dried in an oven at 45 °C. This supports the statement that “the process and state of the plant influence the oil,” with direct confirmation, though limited to another subspecies and specific conditions [1].

Novák and colleagues obtained a convergent result using natural drying or dryers at 30, 40, and 45 °C on O. vulgare subsp. hirtum. Carvacrol varied from 82.75% with natural drying to 89.74% at 30 °C. The work confirms that even moderate temperature differences can modify certain oil parameters; however, it does not measure human exposure or the nutritional value of the spice [2].

A further experimental study on O. vulgare subsp. hirtum compared shade, sun, oven, and microwave drying. It reported the maximum oil content with an oven at 50 °C, at 2.5%, and the maximum carvacrol with a microwave at 560 W, at 81.9%. The result reinforces the conclusion that there is no single best method: the outcome depends on the chosen parameter, such as yield, a single compound, or activity measured in a test tube [3].

Jerković and colleagues analyzed wild O. vulgare subsp. hirtum harvested in the same area during different seasons. Drying at room temperature only slightly reduced the yield and did not change the qualitative composition—that is, the set of main compounds identified; however, the harvest season strongly influenced both quantity and composition. This evidence does not invalidate the results of Caputo and colleagues, but it indicates that the harvest and chemotype—the plant's characteristic chemical profile—can be as significant as the drying method [4].

Figiel and colleagues observed that all the methods studied reduced the total concentration of volatile substances compared to fresh oregano, although with lower losses during vacuum-microwave drying. This finding suggests a distinction must be made between an increase in the relative percentage of a compound and the preservation of the overall amount of aroma. The research is relevant but uses different protocols and plant material, therefore it does not allow for a direct numerical comparison with Caputo and colleagues [5].