Rejuvelac is essentially a natural (yeast and lactic) fermentation of sprouted grains.  One person quoted something to the effect of at its peak it is a pleasant tonic, but fermented too long or fermented wrong, it is simply putrid.  If the grains were rye, it could be called Kvass (Russian or Slavic origin). There are three general steps:

Soaking Sprouting Fermenting

The soaking and sprouting steps are essentially the same as “sprouting” in the food code or in FDA guidance.  Sprouting grains should be carried out in acidified water OR at refrigeration temperatures.  Ambient temperatures are not needed for seeds or grains to sprout.

Use canning citric acid powder to acidify tap water to pH ≤ 4.2.  Sprout grains in the acidified water at ambient temperatures.  Alternatively: sprout grains in plain tap water under refrigeration ≤ 41F.  Both of these methods will prevent ALL foodborne illness bacteria from growing.  Yes, Listeria monocytogenes can grow at ≤ 41F, but it grows VERY slowly.  Please see this LINK for an inexpensive and recommended pH meter.

After sprouting, the sprouts and husks are rinsed in potable water.  The acidified water is not needed for the rinse.  Fermentation is the next step.  Water is added to the sprouted seeds.  An ACTIVE culture should be added.  A wild fermentation is not recommended.  One concern with this process is the possible presence of bacterial pathogens.  Salmonella has been implicated in foodborne illnesses in sprouts and Bacillus cereus in grains and rice.  Note that Kvass and sourdough bread processes have a heating step and baking step respectively.  Rejuvelac does not.  In fact it is advertised as ‘raw”.  The control, therefore, is to have a culture that is active and ferments rapidly.  An active culture provides competitive inhibition, reduces available nutrients and lactic acid bacteria fermentation reduces the pH.  The concern then is that if one relies on the natural wild biota to grow from a few cells to many, during this time, pathogens can also grow from a few cells to many.

Add an ACTIVE fermentation culture.  Choose one of three methods.  1. Use a commercial dried or freeze dried culture.  2. Use a culture from a very recent successful previous batch.  3- Create a mother culture from wild microorganisms.  No 3. is essentially making a mini batch solely to grow an active culture.  An active culture is a necessity to minimize foodborne illness bacterial growth until fermentation reducing the pH of the Rejuvelac first to pH ≤ 4.6, then preferably to pH ≤ 4.2.  generally, Rejuvelac is fermented in 1-2 days at ambient temperature.  If it takes longer than this, it is a sign that the culture was not ACTIVE and the batch should be discarded.

A natural sprout biota (wild) culture will likely be a mixture of yeasts and lactic acid bacteria (think sourdough bread). The natural culture is allowed to ferment the sprouted grain water producing a yeasty (bready) smell along with a lactic acid sourness.  A purchased culture is more likely to be a lactic acid bacteria that will produce a yogurt like smell that has less of a bready smell.

Quality Control - Refrigeration
Rejuvelac that has been fermented to pH ≤ 4.2 is no longer TCS (temperature control for safety).  It does NOT have to be refrigerated for food safety.  Refrigeration is therefore solely for quality.  This means that if you were to transport or sell Rejuvelac at ambient temperature (e.g. at a farmer’s market), it could remain outside of refrigeration safely.
Soaking All pathogens Refrigeration ≤ 41F
Sprouting All pathogens Refrigeration ≤ 41F or acidify to pH ≤ 4.2
Fermenting All pathogens Ferment with an active culture dropping the pH to ≤ 4.2 as rapidly as possible
Aging At pH ≤ 4.2 no pathogens can grow* Refrigeration ≤ 41F for quality (not safety)
*it is possible that if E. coli O157 or Salmonella were present they could survive.