The Science Behind Kimchi

My attempts at making homemade kimchi have led to an interesting question: how long can it be aged, and why?

The origin of fermented food was dependent upon a few key factors: nutritional value and shelf life.  Kimchi itself is from Korea and comes in hundreds of styles.  It is categorized in several ways, of which there are a few important designations: base produce (ie. cabbage, radish, cucumber), season (ie. winter kimchi, summer kimchi) and region (ie. northern Korea, southern Korea).

To maximize the value and flavor of kimchi it is necessary to understand some of the contributing factors that determine how kimchi is preserved and at what point it becomes inedible.  Within a batch of kimchi, a unique environment is created based on the acidity, pH level, sodium level and temperature.  This environment creates a very restrictive ecosystem, which is preferred by Lactic Acid Bacteria (LAB). In the case of kimchi the primary bacteria family is Lactobacilius, a bacteria already present in the human digestive system.  A single batch of kimchi may have hundreds of different strains of LAB, each contributing certain characteristics to their environment, but most importantly producing lactic acid, which is the main preservation agent.

(http://www.humanecologyreview.org/pastissues/her151/scottandsullivan.pdf)

So what happens when kimchi is made?  Each of the unique ingredients which make up kimchi have important roles: salt regulates the speed of fermentation, sugar and starch provide the food for the bacteria to consume, the base produce provides the body of the kimchi, the ginger and garlic provide nutrition and antibacterial qualities that regulate the freshness and fermentation of the mix.  Each of these also carries their own set of ambient yeasts and bacteria.

The warmer the storage temperature, the faster the metabolism of the bacteria and the faster the fermentation takes place.  The best results are achieved when the bacteria begins to ferment the kimchi quickly, but are then transferred to conditions that allow for the slowest ripening.  This allows less time for other, less desirable bacteria, to affect the quality and flavor of the kimchi.  There have been studies on using sherry yeast or a starter from previous batch of cold fermented kimchi (41*F) to jump start fermentation, much like a sourdough bread starter.

The initial fermentation of kimchi takes place between hetero LAB strains.  These strains of bacteria primarily produce organic acids and carbon dioxide as byproducts.  After the first fermentation, the flavor profile is at its peak, with a target pH level of 4.2-4.5 and an acidity level of 0.6-0.8% (http://cms.daegu.ac.kr/sgpark/microbiology/김치발효젖산균.pdf).  To maximize shelf life, the goal is to quickly bring the kimchi through this first fermentation and maintain the pH and acidity levels by monitoring temperature and having the right balance of salt and antiseptic/ antibacterial ingredients (ginger, garlic and optional green tea).

As kimchi ages, the pH slowly drops and the acidity level rises, this change happens quickly when the kimchi reaches its second fermentation between homo LAB strains.  These strains of bacteria produce excessive amounts of lactic acid.  This fermentation brings the kimchi out of the desired pH and acidity levels and closer to inedible acidity levels, introducing less desirable flavor profiles.  So, ideally kimchi goes quickly through its first fermentation and is then introduced to an environment that delays the secondary fermentation as long as possible.

If prepared and stored properly, a batch of kimchi may remain edible for as many as 3 years, although at this point it is well beyond the target flavor profile, pH and acidity levels.  All that’s left to do is make a hot and sour kimchi soup.  Let's take a quick look at the perfect environment for a batch of extended shelf-life kimchi.

First we need a very even and accurate level of sodium to initially dehydrate the base produce.  Applying granulated salt by hand tends to be less accurate, so a different technique serves very well.  A 15% saltwater brine (a ratio of 1:5 salt to water) provides even distribution and an accurate level of salt.  A batch of kimchi begins with a 6-hour brine.

After the ingredients have been put together and the kimchi is put in jars, preferably with a starter from a previous batch of cold fermented kimchi (41*F), the kimchi needs to spend less than 18hours in a cool (60*F), dark place and then be moved to the refrigerator.  This will allow the slow fermenting hetero LAB strains to get a head start without over acidifying the kimchi and then move into a cold environment that will allow the fermentation process to slow down and delay the second fermentation.

This is what you need to make Kimchi:

Equipment:

3 Large mixing bowls

Rubber Gloves

2 two-quart jars

Ingredients:

Raw Ingredients:

2 Napa Cabbages

1 Korean Radish

¼ c. Green Onion

Brine:

                        5 c. Salt

30 c. Water

Kimchi Paste:

1 c. Sweet Rice Flour (Tobiko)

3 c. Water

½ c. Sugar

1 tbs. Fish Sauce (I'll make this myself sometime soon)

1-3 c. Red Pepper Powder (grind yourself if you're up to it)

1 Large Onion

1 c. Garlic

3 tbsp. Ginger

Optional Additions:

1 c. Raw Oysters

1 c. Dried Shrimp

1 tbsp. Matcha green tea powder

First, create a 15% (1:5) salt-water brine by combining your salt and water and stir until completely dissolved.  Halve your napa cabbage through the heart and chop your radish into 1" cubes and rinse, retaining 1/5 of uncubed radish for the paste.  Separate the cabbage and radish into separate bowls.  Pour enough brine over the cabbage and radish to cover and let them sit for 6 hours, stirring once.  When this is complete, rinse the radish and cabbage thoroughly between each leaf at least 3 times and strain to dry.

Meanwhile, begin your kimchi paste: add your sweet rice flour and water to a small saucepan until fully dissolved.  Add the sugar and stir until the mixture just begins to bubble.  Transfer mixture to a large mixing bowl.

In a food processor, combine garlic, ginger and onion and process into a paste.  Add to flour mixture with fish sauce, red pepper powder, green tea powder, chopped green onions and the rest of the julienned radish.

At this point you may add your optional seafood.

Mix the kimchi paste ingredients together and you are ready to start putting your kimchi together.  It is advisable to use some gloves for this step.  With your hands, apply the kimchi paste in between each of the leaves of the cabbage and on the outside.  Squeeze out any excess paste and you are ready to bottle your cabbage kimchi.  Add the radish to the remainder of the paste, stir and this is ready to bottle as well.