By late July, the thyme and oregano in the garden have grown more than a hand's width taller with no help from you. Snip a handful of stems, rinse them, shake off the water, and your kitchen fills with their scent. On that same day, though, run a finger around the rim of the jar of cheong (Korean fruit syrup) you made last week, and you may feel a slick film. You can wash the jar and dry it, and a few days later the slime is back.
Yeast Survives by Clinging to Surfaces
Beverage factories run into the same problem. A team of researchers took seven strains of yeast from beverage production lines and studied how they behave. Two of them, Wickerhamomyces anomalus and Pichia kudriavzevii, stuck to smooth surfaces especially well. The researchers kept the yeasts at 25°C (77°F) and measured them after 24 and 48 hours. The longer they waited, the more cells had attached.
Once yeast takes hold, rinsing with water won't remove it. Even after cleaning agents are applied, a thin layer of cells can survive on the surface and spoil the next batch. That's why stored juice sometimes smells sour or a lid starts to bulge. Homemade cheong and pickles face much the same problem. Once yeast settles on the inside of a glass jar, the trouble can come back even after you fill the jar with something new.
When Oregano and Thyme Work Together
The researchers tested oregano oil and thyme oil on the yeasts, first on their own and then mixed together. All seven strains reacted to both oils. The one that stopped growth at the lowest concentration was the blend of the two. The smallest amount needed to stop the yeasts from growing came to 0.4 to 1.5 grams per liter.
Even at lower doses too weak to kill the yeast, less of it stuck to surfaces. The team measured this two ways. They stained the cells left on the surface, and they collected and counted the live cells. Both methods gave the same result. After 48 hours, the treated surfaces also looked noticeably thinner under a fluorescence microscope.
Oregano oil is rich in a compound called carvacrol, and thyme oil in a compound called thymol. The two have similar structures, but each attacks the cell membrane in a slightly different place. The researchers think that's why the two oils together stopped the yeast at lower doses than either one alone.
These tests, however, were done on lab culture plates. It isn't yet known whether the oils would work as well on real production equipment, or whether their aroma and flavor would end up in the drinks. The study doesn't claim that essential oils can replace standard cleaning agents.
Putting Your Garden Herbs to Work in the Kitchen
One distinction matters here. An essential oil is a concentrate distilled from several kilograms of leaves. It is far stronger than fresh leaves steeped in water. So sterilize your jars with boiling water or disinfecting alcohol, and don't use an herb infusion instead.
Use the herbs for flavor and aroma. Thyme and oregano are most fragrant just before they flower. Cut whole stems in the morning once the dew has dried, and spread them out to dry in the shade. When they're completely dry, strip off the leaves and store them in an airtight glass jar. They'll keep their fragrance well into winter.
- Dried oregano: tomato sauce, roasted eggplant, pickling brine
- Dried thyme: roast chicken, roasted potatoes, sautéed mushrooms
- Fresh leaves: a sprig in lemon water or sparkling water, added just before drinking
Today, cut a few sprigs of thyme and spread them out in the shade to dry. Sterilize your jars in boiling water, and let the herbs handle the flavor.
Source: A study published in the Journal of Applied Microbiology, 2026
