The quality of essential oils of common thyme (Thymus vulgaris) and spearmint (Mentha spicata) during long storage and following experimental (25 weeks) period were studied. Samples of both plants were hydrodistilled periodically during the experimental period and evaluated for their composition, as well as for antioxidant activity. The quality of the oils was evaluated by analyzing the percentage composition of the constituents, but also by quantification of the major compounds via calibration method using authentic standards. Although the percentage composition did not show any significant changes during the experimental period, results from quantification of the major compounds revealed that the quality of both oils significantly decreases during this period. Consequently, the antioxidant activity of the oils also decreased. It was found that the first 15 − 20 weeks of the post-harvesting period are crucial for the quality of the essential oils. Presented results show that percentage content is not necessarily reliable way of presenting the quality of the essential oil as a crop product that will be used in the food and pharmaceutical industry.

百里香（Thymus vulgaris）和留兰香（Mentha spicata）精油的质量) 在长期储存期间和随后的实验 (25 周) 期间进行了研究。在实验期间定期对两种植物的样品进行水蒸馏，并评估它们的成分以及抗氧化活性。油的质量是通过分析成分的百分比组成来评估的，但也可以通过使用真实标准的校准方法对主要化合物进行量化。虽然百分比组成在实验期间没有显示出任何显着变化，但主要化合物的量化结果表明，在此期间，两种油的质量都显着下降。因此，油的抗氧化活性也降低。发现收获后的前 15 - 20 周对精油的质量至关重要。呈现的结果表明，百分比含量不一定是表示精油作为将用于食品和制药行业的作物产品质量的可靠方式。