Acid tolerance starter
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An acid tolerance starter (also known as an "acid shock starter") is a special kind of starter that allows yeast to acclimate to an acidic medium. Acid-tolerant yeast is added at bottling to increase the speed and consistency of carbonation in bottle-conditioned aged sour beers. This is also regarded as the most effective technique to reduce or eliminate the formation of the THP off flavor. If yeast added at bottling has NOT developed acid tolerance, it suffers from "acid shock", which results in poor performance.[1][2][3]
Original methods for creating acid-tolerant yeast have involved using sour beer in a yeast starter, which has some notable disadvantages such as introducing oxygen to the beer that will be bottled. We are developing a modern method of acclimating the yeast to acid, one which does not require the use of sour beer.
Modern protocol[edit]
These instructions are for a 5 gallon (20L) batch, with a target pitch rate of >2 million cells per mL (assuming 20B cells/g).
Rehydration medium materials:
- 500-1000mL beaker or flask
- 2g EC-1118 yeast
- 2.5g Go-Ferm (yields 334ppm YAN in 250mL)[4]
- 50mL filtered tap water
Acidic medium materials:
- 200mL or larger beaker
- 20g dextrose or sucrose (whichever will be used for priming)
- 3mL 88% lactic acid (yields 1.06% lactic acid in 250mL)
- 3mL 5% acetic acid (yields 0.06% acetic acid in 250mL)
- Top up to 200mL with filtered tap water (about 181mL is needed)
Recommended equipment: Stir plate with stir bar, thermometer, digital scale, scoopulas®, stir rods, watch glass, and syringes or pipettes.
Water bath:
- A container to hold warm water and the rehydration beaker/flask.
Instructions:[5]
- Create the rehydration medium:
- Heat 50mL water in a beaker.
- Dissolve the Go-Ferm by stirring.
- Allow it to cool to 104°F (40°C).
- As that is cooling, heat a larger container of water to 104°F (40°C) to act as a water bath to help hold temperature.
- Sprinkle in the yeast and put the beaker in the bowl (water bath) to help maintain temperature. Do not stir.
- Wait 20 minutes. During this time, create the acidic medium.
- Remove the yeast from the water bath and begin atemperation with the acidic medium:
- Slowly over the course of 5 minutes add 50mL of the growth medium. Stir gently.
- Wait 10 minutes.
- Slowly add the remaining 150mL.
- Cover and place on a stir plate on low speed somewhere warm for at least 12 hours. Use within 48 hours. Do not refrigerate before use.
- Add the yeast to the beer halfway through racking (do not add it directly to priming sugar solution).
Science and Rationale[edit]
Proper rehydration of dry wine yeast is extremely important to maintain high viability. One of the major hurdles to proper yeast rehydration faced by home brewers is maintaining the proper temperature on such a small scale. A water bath is an easy solution to combat excessive heat loss during the rehydration period.
The inhibitory effect of lactic and acetic acids depends largely on their concentration (in the undissociated state), and not solely on the pH.[6][7][8] Furthermore, the inhibitory effect of these acids is synergistic. The media tested by Rogers et al. (2016) did not contain acetic acid, and the concentration of lactic acid in their growth media was not measured.[2] Our protocol corrects both of those potential flaws by using both lactic acid and acetic acid at levels above what are generally considered to be inhibitory.
Tap water should be used in order to provide calcium. You may use RO purified or distilled water instead, but you must add about 0.05g calcium chloride to the "acidic medium" in order to give 50ppm calcium.
There is nothing detrimental about using simple sugar in the acclimation medium, and it may even be preferred.[9]
Competitive factor produce by Saccharomyces is not active against Brettanomyces,[10] so flavor development during bottle conditioning is not inhibited.
How do the yeast adapt to acidic media?
Adaptation may involve building up a sufficient concentration of the acid to create a buffer pair internally to resist pH change. It has been reported that anions formed through dissociation of carboxylic acids are retained within the cells while protons are ejected through an energy (ATP)-coupled reaction involving plasma membrane ATPase.[8] Rogers et al suggest that up-regulation of stress-response genes may also play a role.[2]
References[edit]
- ↑ Blair, J. "Terminal Acidic Shock and Sour Ale Bottle Conditioning." The Rare Barrel. December 2017.
- ↑ a b c Rogers, CM., et al. "Terminal acidic shock inhibits sour beer bottle conditioning by Saccharomyces cerevisiae." Food Microbiology, vol. 57, 2016, pp. 151-158.
- ↑ "Packaging - Acid Shock Starter." Milk the Funk wiki. Accessed May 2020.
- ↑ "Approximate YAN Contribution for the Important Yeast Nutrients." Vancouver Amateur Winemakers Association. Accessed May 2020.
- ↑ "Easy Steps for Optimal Yeast Rehydration." Scott Labs. Accessed May 2020.
- ↑ Narendranath, N., et al. "Effects of acetic acid and lactic acid on the growth of Saccharomyces cerevisiae in a minimal medium." J. Ind. Microbiol. Biotech., vol. 26, 2001, pp. 171–177.
- ↑ Casal, M., et al. "Mechanisms regulating the transport of acetic acid in Saccharomyces cerevisiae." Microbiology, vol. 142, 1996, pp. 1385–1390.
- ↑ a b Thomas, KC., et al. "Influence of Medium Buffering Capacity on Inhibition of Saccharomyces cerevisiae Growth by Acetic and Lactic Acids." Appl. Environ. Microbiol., vol. 68, 2002, pp. 1616–1623.
- ↑ Van Landschoot, A., et al. "Effect of pitching yeast preparation on the refermentation of beer in bottles." Cerevisia, vol. 29, no. 3, 2004, pp. 140–146.
- ↑ Mehlomakulu, N.N., et al. "Non-Saccharomyces killer toxins: Possible biocontrol agents against Brettanomyces in wine?" S. Afr. J. Enol. Vitic., vol. 36, no. 1, 2015, pp. 94–104.