I have been meaning to run some experiments for Year of the Sour Mash to determine the optimal medium to grow Lactobacillus, but it seems I have been beaten to the punch! Sam over at Eureka Brewing recently published an excellent writeup on evaluating starter media to propagate Lactobacillus that is far more comprehensive than anything I would have been able to produce. Take a look and I will share my thoughts below.

What I want to share today are the results of an evaluation experiment I performed a couple of months ago to look for MRS media alternatives to propagate lactobacillus at home. MRS media is kind of the golden standard used to propagate lactobacillus. It works very well but with the disadvantage of being a quite expensive media. I therefore tested a couple of cheaper alternative media and compared the growth/propagation efficiency with MRS.

Source: Evaluate starter media to propagate Lactobacillus sp. | Eureka Brewing

Optimizing Lactobacillus Growth

The challenge with growing Lactobacillus is that if you are not careful LAB (lactic acid bacteria) can produce toxic levels of acidity. To counteract this tendency we will need to add something to neutralize the acidity produced as the Lactobacillus grows. The key finding that Sam shared in his post is that adding calcium carbonate (CaCO3) at a rate of 20 grams per liter neutralizes enough acidity to accomplish growth rates similar to an MRS agar solution.

The apple juice mentioned in the article I imagine is a good source of simple sugars for the bacteria. Though it is interesting to note that a pure apple juice solution is not the optimal medium I have heard it made out to be; my hunch is that apple juice’s pH is too low (frequently ~3.4) and it also lacks many of the nutrients needed for bountiful growth.

A Two Step Process

To recreate a starter with the optimal media of 10°P DME + 10% apple juice + CaCO3 (20 g L-1) + yeast nutrients as mentioned in the article, I recommend the following two-step process. The goal in step 1) is to nurture a culture of wild Lactobacillus to the point where it is the dominant organism and ready to be stepped up. If you are pitching a pure culture please skip to step 2).



In a 500 mL flask make a lacto starter, following the directions for the wild starter (at 1/4 the amounts). In a 1000 mL flask, make an “optimal” starter: 900 mL of “standard” starter (900 mL water, 90 g DME, pinch of yeast nutrient). Add 20 g CaCO3 (aka chalk). Boil for 15 minutes. Chill to ~120ºF. Add 100 mL of apple juice. At this point, we have a mostly full “optimal” starter and a full mini wild starter. You can either dump a small amount of the optimal starter to make room, or just top off with as much wild starter as will fit in the flask, there should be plenty of Lactobacillus to inoculate the optimal starter. Strain the grains from the wild starter and pitch into the optimal starter. Finally, cap the flask with an airlock and keep the starter as warm as you can (~110ºF is best) for 1-2 days. Pitch into whatever you want to sour (my guide).

There you have it, a one-two punch that will ensure that wild the critters you grow will be healthy, vigorous, and, most importantly, produce the clean sour character you are looking for.

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