Chilli (hot) sauce is often made fresh on the day that it is used. However, chillis are also often fermented prior to being made into hot sauces. These sauces are in the vein of Tabasco sauce. The utility of fermenting chillis is the increase in the flavor profile of the resulting sauce imparted by the fermentation biochemistry of the predominant Lactobacillus spp. in the brew (hence, the term "lacto-fermentation").

Scanning Electron Micrograph of a Lactobacillus sp.

Lacto-fermentation of chillis is relatively easy. You need to source some iodine-free salt (hard to find these days because of food regulations which require table salt to be iodized for preventative goitre health). Iodine will interfere with microbial growth. I used Himalayan Pink Rock Salt:
 

Himalayan Pink Rock Salt
 

• The hot compound in chillis is capsaisin.
  Avoid contact with mucous membranes (e.g. eyes and nose);
  Wear gloves when needing to be in skin contact.

• Sliced or blended chillis will release irritating fumes/aerosols.
  Take precautions not to inhale these fumes or risk eye, nose and lung irritation (coughing is the usual response).

• You are making a food product.
  Exercise maximum hygiene/sanitation in all your procedures.

Lacto-fermentation

• Make a brine solution which is 6% (w/v) in strength.
  Note: The salt concentration chosen is critical because if the salinity is not high enough, pathogenic or food-fouling microorganims may flourish and make the brew dangerous to consume.
  6% is higher than what is usually used by others but I chose to err on the side of safety. The disadvantage of too high a salinity is that the fermentation may proceed very slowly.

Bird's eye chillis
The choice of chilli to ferment is up to you and
where you wish to be on the Scoville Heat Unit scale

• Slice bird's eye chillis (I used 250 g) into into halves to expose the ribs and seeds. Then further chop up the batch into smaller pieces.
  Some folk mash/grind up the chillis at this stage but my take on this is that if the chillis are broken up too finely, the juices within (containing the desired capsaicin) will be contained in the saline. Not all this saline will end up in the hot sauce later. It is better to mash up the chillis after fermentation in order to conserve the hot compounds within.

• Smash and slice 6 cloves of fresh garlic.

• Place both the chillis and garlic into a suitable container which can be sealed tightly to keep out air. I used a small Mason jar.

• Now add sufficient of your prepared brine to cover the chillis and garlic. It is not necessary to weigh the chillis/garlic down into the brine if you shake the ferment daily until the produce no longer tends to float.

Sliced bird's eye chillis and garlic  in brine at Day 0

• Allow the ferment to brew for at least 14 days.
  Shake the contents daily to dislodge entrapped gas, and "burp" the container to release the ferment gas to atmosphere.

• You may wish to ferment for longer than 14 days to obtain more flavour.

The ferment at Day 14

• Strain. At the end of the fermenting period, drain the chillis into a fine sieve and collect the solids. Retain the briny fluid as this will contain the lactobacillus for subsequent fermentations, and you will need some of the liquid for preparing the next step.

• Blend. Add some of the fermented brine to the chilli solids and blend until a very smooth mixture results. The thickness or runniness of this mixture is up to your preference. Tabasco sauce is particularly liquid.

• Acidify. You can add a vinegar of your choice to the blend to give your hot sauce some tartness but also increase its potential for longer shelf-life (acidity deters microbial growth). You can add one part vinegar to 2 parts chilli blend (for ease of procedure, do this by weight on a balance).

• Xanthan. At some stage of the blending process, you may wish to add xanthan gum to produce a mixture with a smooth viscosity which does not separate over time into liquid and solids. One quarter teaspoon to 200 mL of liquid is a good start. Add more if you want a higher viscosity. Note that you should only add xanthan to a mixture that is being vigorously agitated e.g. add the powder as you are blending. If you add it to still liquid you are likely to end up with clumps which are hard to disperse.

Molecular structure of xanthan

Xanthan gum is a polysaccharide made by the bacterium Xanthomonas campestris in fermentation. It is a highly versatile polymer  with interesting properties which make it very useful for food applications (look at the labelling of foods which are a paste or liquid - you will often see xanthan as an additive [given the code E415]). Xanthan is used as a thickener which holds solids in suspension in liquids so that they fall out of suspension only very slowly. It is stable against acid and alkali conditions, and is thermostable. It is thixotropic which means that its viscosity decreases when a shear force is applied to it - this is the property of sauces such as tomato which only comes out of their bottles easily after you give the bottle a good shake.

• Bottling. Decant the blended mixture into suitably sized bottles with caps.

Bottled (120 mL) and ready for pasteurization.
This is the quantity (2 and 1/2 bottles) arising
from 250 g of chillis at the start.

Pasteurization

This step is done to stop the fermentation (the microorganisms are rendered unviable) and to assure a sufficient shelf-life to the prepared sauce. If the fermentation is not stopped (some people do like an unpasteurized product), gases could be produced while in storage and content may spill over upon the bottles being opened (this happens with cincalok).

• Place the bottles of sauce in a saucepan filled with sufficient water to immerse the bottles up to the level of the sauces held within.

• Heat the water in the saucepan until boiling and hold it there until the sauces in the bottles reach at least 80 degrees centigrade.

• Hold 80 degrees centigrade for at least 30 minutes to complete sufficient pasteurization.
  Note that while pasteurization removes sufficient viable cells of most food spoilage and poisoning microorganisms, it is not the same as sterilization.  Pasteurized products cannot be stored indefinitely and thus, when you are consuming your pasteurized sauce, store the remainder in a refrigerator to extend its shelf-life.

You may like to decant portions for meals and further adjust the taste of the sauce prior to serving. You can cut/blend the sauce with vinegar, calamansi juice, add sugar or fish sauce. As you please.