A gas mixing system and method designed to flush produce with a gas-air mixture of desired oxygen concentration to preserve the produce. The produce is placed in a vacuum vessel, which is then evacuated to a desired degree. Atmospheric air is provided to the vacuum vessel via an air valve, and nitrogen gas is provided to the vacuum vessel via a gas valve. A valve controller operates in a first mode wherein the air valve is a fully open master valve and the gas valve is a partially or fully open slave valve, and operates in a second mode wherein the gas valve is a fully open master valve and the air valve is a partially or fully open slave valve. The system automatically switches between the two modes to accommodate changing conditions, to maintain the desired oxygen concentration in the gas-air mixture. Multiple modes are possible with greater numbers of valves.