How to Make Soy Sauce from Sweet Potato

Soy sauce or toyo is a popular Filipino seasoning. It is used almost daily in preparing popular Filipino seafood and meat recipes or as a condiment to enhance the flavor of cooked food.

The main ingredient of soy sauce is soybean and flour fermented with the Aspergillus sojae microorganism. Instead of wheat flour, you can use sweet potato flour in making soy sauce to save on costs.

Making soy sauce with sweet potato has two phases.The first phase is called the koji stage where the Aspergillus sojae mold is cultured in a medium composed of flour, soybeans and rice starter. The koji starter culture is derived during this stage. The second phase involves the fermentation of the koji in a brine solution. It is during the second stage that the taste and aroma of the soy sauce is developed and enhanced.

Preparing the Starter Culture (Koji Stage)

Step 1: Culturing the Starter Medium


  • 2 pcs sweet potato
  • 2 grams agar
  • 10 grams dextrose
  • 1 liter water

Utensils and Materials

kitchen knife spoon pressure cooker
chopping board weighing scale cylinder
cotton cooking pan & stove test tube holder
test tubes strainer with net test tube tray
mixing bowl rubber band pipette


  1. Slice the sweet potato into small pieces and put them in a pan.
  2. Add a glass of water and boil for 30 minutes.
  3. Meanwhile, prepare 2 grams of agar in a mixing bowl. Add 10 grams of dextrose to the agar.
  4. Strain the broth from the boiled sweet potato to remove solid particles. Put the broth in a cylinder.
  5. Add water to the broth to make a 500 ml solution.
  6. Add the broth and water solution to the agar and dextrose mixture. Mix, stirring throughly.
  7. Boil the mixture for30 minutes. Set aside to cool.
  8. With a pipette, transfer the cooled solution to test tubes, each test tube containing 8 ml of the solution.
  9. Sterilized solution:
    • a. Cover the test tube opening with cotton. Put several tubes together with a rubber band; put each test tube bundle in a test tube holder.
    • b. Boil the test tubes in a pressure cooker for 15 minutes at 15 psi, to sterilize.
    • c. Remove test tubes from pressure cooker. Set aside to cool.
  10. The sterilized tubes are now ready to be injected with the Aspergillus sojae mold.

Step 2: Inoculating the Culture Medium

Materials and Utensils

  • burner
  • sprayer with alcohol
  • wire loop
  • Aspergillus sojae mold in a test tube
  • test tube with sweet potato starter medium


  1. Sterilize the wire loop by heating it the over flame of a burner.
  2. With the sterilized wire loop, get Aspergillus sojae mold from the test tube.
  3. Inoculate the sweet potato starter medium in test tubes with the Aspergillus sojae mold. Set aside for 2-3 days or incubate for 2 days at 32°C. The Aspergillus sojae mold will start to grow and will grow in 2-3 days.

Step 3: Preparing the Rice Starter

Prepare for the rice starter while waiting for the mold to grow.


200 gm rice burner
200 ml water wire loop
bottles as containers Aspergillus sojae mold
funnel pressure cooker
net stove


  1. With a funnel, fill several bottles with 50 grams of rice each.
  2. Add enough water to soak the rice. Wait for 10 minutes.
  3. Drain the water from the bottle using a net to keep the rice grains from spilling out with the water.
  4. Cover the bottle openings with cotton. Put bottles in pressure cooker and sterilize for 15 minutes at 15 psi. Set aside to cool.
  5. Meanwhile, sterilize the wire loop to be used for inoculation.
  6. Inoculate the rice starter with the Aspergillus sojae mold. Set aside the inoculated rice starter for 2-3 days or until the molds start to grow in the rice starter.

Fermentation of the Koji in Brine Solution


rice starter inoculated
with Aspergillus sojae mold
cartolina and
masking tape
2.4 kilos rice alcohol
3 liters water fermentation box
katsa cloth steamer


  1. Soak the rice in water for 30 minutes. Wash with clean water.
  2. Wrap the rice in a piece of katsa cloth.
  3. Steam rice in katsa for 1 hour. Set aside to cool.
  4. Disinfect the fermentation box with alcohol.
  5. Remove the steamed rice from the katsa and place in fermentation box together with the rice starter inoculated with Aspergillus sojae. Cover the box with cartolina and tape so that no light penetrates the inside of the box.
  6. Mix the contents of the box twice a day for 3-4 days. Green mold will grow on the mixture.

Toasting the Flour

Materials and Ingredients

12 kilos of sweet potato flour match
roasting drum ladle
a bag of charcoal pot holder
kerosense mixing bowl


  1. Put sweet potato flour in roasting drum over burning charcoal.
  2. Turn roasting drum for 1 hour until the flour is reddish brown.
  3. One thoroughly roasted, set aside flour to cool in a pail with cover.

Preparing the Soya

Materials and Ingredients

  • 12 kilos soya beans
  • 1/2 liter water (just enough water for soaking)
  • 12 kilos toasted sweet potato flour
  • basin lined with katsa cloth
  • steamer
  • box
  • ladle


  1. Place soya beans in basin lined with katsa.
  2. Add just enough water to wet soya beans. Stir.
  3. Wrap soya in the katsa. Tie the ends of the katsa.
  4. Steam the soya wrapped in katsa for an hour. Set aside.

Making the Koji

Materials and Ingredients

rice starter hand gloves
steamed soya beans cartolina
toasted flour masking tape
box pair of scissors


  1. Put toasted flour in the box. Add the steamed soya beans. Mix well with ladle.
  2. Add rice starter to the flour and soya mixture. Mix thoroughly. This is the koji mixture.
  3. Cover the koji mixture with cartolina and secure with masking tape to keep light from penetrating the mixture.

Making the Root Soy Sauce

This is the stage when the koji mixture produces the root soy sauce. But before the soy sauce is produced, the following steps must be followed:

A. Preparing the Brine Solution and Fermenting the Koji Mixture

During this process, the koji mixture is soaked in a brine solution for 3-6 months. Fermentation lends the sauce its unique aroma and taste.

Materials and Ingredients

  • 12 kilos salt
  • 60 liters water
  • koji
  • pail
  • wooden ladle


  1. Prepare the brine solution. Boil water in a big pot. Add salt. Let the water boil for 20 minutes. This is the brine solution with a ration of 20 parts salt and 100 parts water.
  2. Transfer the brine solution (excluding the impurities that sink to the bottom of the pot) to a clean pail.
  3. Add the koji mixture to the brine solution. Stir to mix thoroughly.
  4. Ferment for 3-6 months.
    • a. During the first 10 days, thoroughly stir the mixture daily.
    • b. From the 11th day onward, stir the mixture once every two weeks.
    • c. Open the pail after 3 months. The koji will have become a fermented mush. Bubbles will have formed on top of the mush. This is a sign that the organisms in the solution are alive.

B. Harvesting and Pressing the Fermented Koji Mush

To produce the sauce, the fermented koji should be harvested and pressed to produce the koji liquid to be made into soy sauce.

Materials and Ingredients

nylon fabric 40 ml acetic acid
presser 5 liters molasses
pail pot
scoop stove


  1. Line the pressing compartment of the presser with the nylon cloth. Fill the pressing compartment with the fermented mush.
  2. Put a wooden piece of weight over the mush. Apply enough force to the presser to squeeze the sauce from the fermented koji.
  3. Catch the sauce from the presser in a clean container. Set aside.

C. Seasoning the Koji with Molasses

The koji sauce is mixed with molasses to give it the unique soy sauce taste and color

  1. Mix acetic acid with the molasses to dissolve any solid particles and dilute the think molasses.
  2. Add the treated molasses to the koji sauce.

D. Pasteurization

  1. Heat the soy sauce mixed with molasses at 70°C for 30-45 minutes. Pasteurization destroys pathogenic microorganisms in the soy sauce. This will lengthen its shelf life and ensure safety to consume a product. Set aside to cool.
  2. Sodium benzoate maybe added to help prolong the storage life of soy sauce. Mix 5 grams sodium benzoate to 1 liter of root soy sauce.

Quality Control

To test the safeness of a consumer food item, it is subjected to quality control procedures. Soy sauce for instance is subjected to soluble solids count to determine its quality.

Steps in the Quality Control Process

  1. With a pipette, put a sample of the root soy sauce in a flask. Add 10 ml of water to the sauce sample.
  2. Get another sample of the root sauce and place in a refractometer. Check for soluble solid particles. A refractometer measures the presence of soluble solids that indicate the quality, or lack of it, of the root soy sauce. If the soy sauces passes the quality test, it is ready for bottling, sale and consumption.

For more information, contact TRC at:

TLRC Building
#103 J. Abad Santos cor. Lopez Jaena Sts.,
Little Baguio, San Juan, Metro Manila
Tel: (02) 727-6205

Leave a Reply

Your email address will not be published. Required fields are marked *

Comment moderation is enabled. Your comment may take some time to appear.