Here is how each gas functions as a freshness indicator:
- The Role: Oxygen is the fuel for respiration. As fruit "breathes," it consumes O � and produces CO �.
- Freshness Indicator: O � levels are too high, the fruit ripens too fast. If they are too low (hypoxia), the fruit switches to anaerobic respiration (fermentation), which ruins the flavor and texture.
- Solution Logic: Maintaining O � at a specific low threshold (typically 1–3%) in storage is the key to "sleeping" the fruit and extending shelf life by months.
- The Role: While less common as a primary ripening indicator than Ethylene, H 係 is a signaling molecule in plants that helps regulate stress responses.
- Freshness Indicator: In the context of spoilage, H 係 is often a byproduct of protein breakdown or bacterial activity (especially in high-protein produce or if the fruit is heavily contaminated with fungus).
- Detection Note: If you detect rising H 係, it usually indicates that the produce is no longer just "ripe" but is actively rotting or hosting microbial growth.
- The Role: Ammonia is not typically produced by healthy fruit. Instead, it is a byproduct of nitrogen metabolism and the breakdown of amino acids.
- Freshness Indicator: Rising NH � levels are a strong indicator of advanced decay and tissue breakdown. It is particularly relevant for monitoring certain vegetables and berries that have higher nitrogen content.
- Safety Secondary Use: In many industrial cold storage facilities, Ammonia is used as the refrigerant. Detecting NH � in the air can also serve as a critical safety alert for a refrigerant leak, which would contaminate and kill the produce.
- The Role: This is a broad category including alcohols, aldehydes, and esters.
- Freshness Indicator: VOCs are the "aroma" of the fruit.
- Early stage: "Sweet" esters indicate peak ripeness.
- Late stage: High levels of Ethanol or Acetaldehyde indicate fermentation (the fruit is turning "boozy" or sour).
- Solution Logic: Monitoring the ratio of different VOCs allows an IoT system to predict the "Days of Remaining Shelf Life" (DRSL) with high accuracy.
| Gas | Baseline Condition | High Reading Signal | Target Sensor Type |
| O � | High (20.9%) | Rapid Respiration / Aging | SPE |
| H 係 | Zero | Bacterial Decay / Rot | SPE (Low ppm range) |
| NH � | Zero | Protein Breakdown / Leak | SPE |
| VOCs | Low | Over-ripening / Fermentation | PID (Photoionization) |
