A rubber cap does not leak between glass demijohns and the fermentation lock. However:, note that rubber caps leak if they are used on plastic containers. Even if they are sealed with wire or jubilee clips leakage will occur. It is suggested to use food grade bungs if possible.
The fermentation lock should be of plastic. It contains a water trap that allows the venting of carbon dioxide but prevents the fermentation coming into contact with air. When fermenting with Turbo yeast or other rapid fermenting yeast a fermentation lock should not be used for the first few days. Fermentation will be so violent that the water will be forced out. The carbon dioxide, which is heavier than air, protects the fermentation from air. The fermentation lock should be fitted when the violent fermentation has subsided.
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