Like all silicone valve lids, these are not 100% one-way valves. During fermentation, negative pressure can sometimes form inside the jar, causing the lid to cave in slightly. In some cases, a small amount of air may enter the jar, or you may need to pinch the valve to equalize the pressure. While this small increase in risk compared to rigid airlocks is not ideal, our testing shows it has minimal impact on the overall success of the ferment. Reduced Risk of Contamination: These lids help greatly reduce the chances chances of mold or kahm yeast developing during fermentation. Tested and Trusted: In real-world use, these lids perform on par with traditional airlock systems. Mold issues are no more frequent than with other types of lids, and many fermenters have reported excellent results using similar silicone valve designs. Convenient and Low-Maintenance: These lids offer significant advantages over traditional airlocks. There’s no need to monitor water levels, they are easier to clean and assemble, and they take up less space, Durable: While there may be a small increase in risk compared to rigid airlocks, these are made with unbreakable and dishwasher safe materials, making these lids an excellent choice for everyday fermenting.