How to Clean and Sanitise (Sterilise) PET Plastic Demijohns

Sterilising and cleaning your PET demijohn between every brew is the single most important step for producing consistent, high-quality fermentations. The process is simple, but for beginners it can feel a bit overwhelming but don’t worry, this guide walks you through everything safely, with PET-suitable products only.

Note: If you are looking for the guide on how to clean Glass Demijohns Click Here

Image Source: AlcoFermBrew.com

Why You Must Clean and Sanitise

A quick rinse isn’t enough. To properly prepare your PET demijohn for the next batch, you need to follow two steps in order:

  1. Cleaning removes residue, sugars, yeast deposits, grime.
  2. Sanitising kills bacteria, wild yeasts, mould, and microbes.

This ensures predictable, contamination-free fermentation every time.

What You’ll Need

For Cleaning (PET-Safe)

  • Lukewarm water (never hot or boiling)
  • PBW (low-temperature safe) or Sodium Percarbonate or OxiClean Free (unscented)
  • A soft bottle brush (no very stiff bristles as plastic can scratch easily)

For Sanitising

  • Star San or Chemsan (no-rinse, PET-safe)
  • OR a very mild bleach solution (only if you have no other option)
  • Drying rack or clean draining surface

How to Clean & Sanitise a PET Demijohn

1. Clean the Inside (Remember to be Gentle!)

Use:

  • PBW
  • OxiClean Free
  • Sodium Percarbonate

Steps:

1.       Rinse the demijohn with lukewarm water.

       ·       Don’t use boiling or very hot water because PET and plastic can warp or weaken.

2.       Add 1–2 teaspoons of your chosen cleaner.

3.       Fill halfway with lukewarm water, shake gently, then top up fully.

4.       Let soak for 30–60 minutes.

      ·       For stubborn buildup, leave it overnight.

5.       Use a soft PET-safe brush to gently scrub the neck or any residue.

      ·       Avoid stiff or metal brushes, which can scratch the plastic and create bacteria-harbouring grooves.

6.       Rinse thoroughly with cool or lukewarm water until none of the cleaner remains.

2. Sanitise the Demijohn

Using Star San/Chemsan

1.       Mix the sanitiser according to the manufacturer’s ratio.

  1. Pour it into the demijohn and swirl to coat all internal surfaces.
  2. Allow at least 30 seconds of contact time.
  3. Pour out the remaining liquid (No need to Rinse)  
  • The foam is harmless and will not affect fermentation.

Bleach Method (Only if Necessary)

Bleach is safe for PET at low concentrations, but not ideal.

  1. Mix 2 tbsp (30 ml) of plain bleach per 5 litres of water.
  2. Fill the demijohn and let stand for 20 minutes.
  3. Rinse 3–4 times with cooled boiled water or filtered water to avoid off-flavours.

3. Let the Demijohn Drain

Place the demijohn upside down on a sanitised rack or a clean, raised surface.
Allow it to drip-dry fully before use.

Final Thoughts

Cleaning and sanitising a PET demijohn is straightforward once you get the hang of it. The key difference from glass is treating the plastic gently is not to use hot water, harsh scrubbing, or strong chemicals.

Once your demijohn is ready, don’t forget to sanitise your bungs, airlocks, and other equipment as well. For a Guide on that Click Here