Post-Souring Gravity, Multi-Batch Brew, Priming a Keg, and Fermenting Lemon Juice

I have been getting into sour beers lately with two of my latest being Irish reds kettle soured with GoodBelly mango-flavor probiotic juice drink. I stopped the first at a pH of 3.8 and the second at 3.4. Both are great, with the 3.4 the best. Will let the next batch go a bit lower. This started me thinking, obviously the souring bacteria eats sugar to make the lactic acid, so how much of the fermentable sugar is used up? The Irish reds were already expected to result in a low abv. I lost my starting gravity readings on both so I can’t get any numbers but the taste tells me the finished products are lower than expected. Also, would the lactic acid have messed up the readings anyway? I’m thinking the next batch I may use 50–100% more fermentables. Does that sound reasonable?

Duncan K. Burns
Charleston, South Carolina

Thanks for the interesting question, Duncan. The product you are using as your source of bacteria lists Lactobacillus plantarum as the only bacterial ingredient in this beverage. Lactobacillus plantarum is a facultative heterofermentative lactic species. This means that under anaerobic conditions, Lactobacillus plantarum behaves like a homofermentative lactic species and produces lactic acid as its sole metabolic by-product, which then switches to a heterofermenter under aerobic conditions and also produces ethanol, acetic acid, and carbon dioxide. Not all Lactobacillus plantarum strains behave the same, but they generally ferment a wide array of carbohydrates ( and wort is a great growth media for these bacteria. Sounds like you are concerned that these little critters are going to consume too much fermentable sugars and cut into the production of ethanol by yeast?

The hydrometer is a pretty handy brewing tool and you can certainly use it to monitor the change in wort density before and after your souring phase. The specific gravity of lactic acid solutions vary by concentration, but over the range seen in wort/beer the specific gravity is not much higher than water. This means that as carbohydrates are metabolized the specific gravity of the wort will drop, and that the wort density at the time of yeast pitching can be used like the original gravity (OG) of non-sour brews. This is not an exact measurement, but it is a reasonable approximation.

The hydrometer is a pretty handy brewing tool and you can certainly use it to monitor the change in wort density before and after your souring phase.

One thing to keep in mind about fermentations, in general, is that fermentation by-products often regulate fermentation. In the case of lactic acid fermentations, environmental pH affects metabolism. This is why yogurt contains lactose, even after prolonged storage in the presence of lactic acid bacteria. Kettle sours tend to bottom out around pH 3.2, empirically suggesting that the environment is regulating the rate of fermentation. The other practical consideration relevant to your question is the kettle sour process. Most brewers monitor pH during the souring phase and use pH as a control point; for example, if the goal is to reduce pH to 3.4, the brewing process advances to the next step when this goal is achieved. This means that even if lactic acid bacteria are able to consume all fermentable carbohydrates, the brewing process control would prevent that from occurring. The brewers I know who are commercially brewing kettle sours all carefully monitor pH because many of these brewers are looking for flavor consistency as well as efficient use of their equipment; as soon as the wort pH falls to the target level, they are moving on to wort boiling.

I am not even going to touch attempting to calculate alcohol content of kettle sours because there are simply too many variables to consider. The best calculation tools used to approximate alcohol content in beer based on OG and FG are based on lots of data from “normal beer.” In order to develop the same sort of model for sour beers requires a matrix of process measurements in addition to lab analysis of the ethanol content of the finished beer. Sounds like a great project for an eager brewing chemist!

Something to consider about these beers is how they stand up on the palate and if there is a real need to boost OG to offset the loss of fermentables during the souring phase. Most kettle sours being produced commercially have average to below-average alcohol concentrations and generally are made from worts in the 10–12 °Plato (1.042–1.050 SG) range, and sometimes lower. Berliner weisse is a great example of a low-alcohol beer that does not leave my palate asking for the missing experience that is often the case with reduced-alcohol, non-sour styles. If you like what you find in the market and want to brew these sorts of beers at home, skip the additional fermentables. Alcohol consumption is dropping globally and brewers are being challenged to produce lower-alcohol alternates to traditional beers. Kettle sours, with their refreshing acidity and high drinkability, are really a style that fits in with this shift. Focus your process and recipe on how your beer stands up to tasting, and make changes from a flavor standpoint first and ABV targets second.

I am currently brewing with one of the Grainfather-like all-in-one brewing systems to produce between 5–6 gallons (19–23 L) of beer with each batch. One of the beers is a big imperial stout and is absolutely delicious! The only problem is that I don’t have enough of it.

I was wondering if it would be possible to produce a larger volume of the beer by adding more wort in batches. Let’s say on day one I produce a 5-gallon (19-L) batch and add it to the fermenter with yeast and the next day brew another 5-gallon (19-L) batch and add it to the actively fermenting beer? This would increase the volume of what I could produce without buying new equipment, which I don’t have the space for at the moment.

Bob Schepers
Leiden, Netherlands

The answer is yes! This is a great way to produce more beer volume from a brewhouse of a fixed size and is commonly used by commercial brewers around the globe. The economics of this are simple: Two single-batch fermenters cost 2 units of currency, one double-batch fermenter costs about 1.3 units of currency, and one quadruple-batch fermenter costs about 1.7 units of currency. The commercial economics become even more pronounced when the cost of cleaning, labor, utilities, instrumentation, and automation are considered. Multi-batch fermenters have a multitude of real advantages. There are a few general tips to consider when putting more than one batch into a single fermentation vessel.

Multi-Batch Tip #1:

Most breweries fermenting multiple batches of wort in a single fermenter fill the fermenter within about 18 hours because periods longer than this can result in an interruption in the way yeast grow and transform wort to beer. Without getting in the weeds of this topic, you will be a happy and successful multi-batch brewer if your first brew fills into the fermenter towards the end of day one and the second brew fills into the fermenter before noon on the following day. If you brew on the weekend, this means doing a late afternoon/early evening brew on Saturday and a morning brew on Sunday (or Friday evening and Saturday morning). Your first brew may just be beginning to ferment when your second brew is added. Brewing both brews in the same day is preferred, but this can seem more like work than having fun!

Multi-Batch Tip #2:

Add all of the yeast pitch, enough for the combined volume, to the first brew. As you become more experienced with this method you can reduce the yeast pitch rate because there is yeast growth in between initial pitching and the second batch of wort flowing into the fermenter, but beginning conservatively is a good plan. Ideally the two brews are produced in the shortest timeframe possible and the yeast population is likely in the lag phase (still no growth — reproduction has not yet commenced) when the second batch of wort is added to the fermenter.

Multi-Batch Tip #3:

Commercial breweries aerate in-line between the wort cooler and the fermenter. A two-brew fermentation is usually aerated on both fills when the two brews fill into the fermenter before yeast growth begins. Many breweries will skip aeration after yeast growth takes off. You can aerate your fermenter twice if the two batches are filled in quick succession, but if the two batches take longer than ~8 hours to produce consider skipping the second aeration. Why aerate twice? If the second batch comes in without aeration, the oxygen content of the wort is diluted and may cause fermentation issues. If you are using dried yeast, aeration is not a concern because dried yeast does just fine in non-aerated wort. This is another topic for another day!

Multi-Batch Tip #4:

Limit your multi-batch brewing to two batches per fermenter. Things become a bit more complex when fermenters contain more than two batches. Some brewers add yeast with alternating wort fills and also alternate how wort is aerated. This can all be done at home, but you are looking to solve a practical problem that is easily addressed without adding too much complexity.

Multi-Batch Tip #5:

Another method that works very well at home is to simply ferment your two brews as independent batches and blend the two batches together after primary fermentation is complete. If you normally rack your beer into a secondary, this would be an ideal time to conduct the blend. If you use a single-vessel process, you can rack one of the batches into the other provided your fermenter is large enough.

All the recipes in BYO call for corn sugar “if priming.” There are many articles about kegging and carbonating and a few of them I read mentioned priming when kegging but did not say how much or if it is a standard practice. I am currently using 5-gallon (19-L) “corny” soda kegs and use bottled CO2 to carbonate. I brew all-grain IPAs and winter ales, normally around 6.5% ABV. Is priming for kegs recommended and if so how much corn sugar for a 5-gallon (19-L) batch?

Murray Nunn
Happy Valley, Oregon

This is a popular topic and we have run many variations on this basic question in past issues of BYO. I believe that there are some reasons to keg condition and will give some practical considerations for you to mull over about this topic. Whether carbonating in bottles or in a keg, you must measure the volume of beer you are carbonating in order to know how much priming sugar to add for a given level of carbonation. Without going into the nitty gritty detail of past material, 110 grams/3.9 ounces of sugar will increase the carbonation level of 18 L (4.75 gallons) of beer by 3 grams of CO2 per liter of beer or 1.5 volumes. You can check out this answer for the details on carbonation calculations

One common rule of thumb among brewers who routinely keg condition is to use about 75% of the priming sugar suggested in a recipe for bottle-conditioned beer. I have never been able to find the original source of this advice. Most of the explanations for this are vague and simply state that keg-conditioned beer becomes over-carbonated if the priming addition suggested for bottle-conditioned beers is not reduced. Some suggest that the problem is due to the common practice of pressurizing the headspace of the keg after filling to seal the lid. By doing this, the priming sugar has less headspace to pressurize. I am not sold on this explanation because a Corny keg usually has more total headspace volume than 5-gallons of beer filled into 53 bottles. Assuming the typical beer bottle contains 15 mL of headspace, 53 bottles have a total of 795 mL of headspace, which is greater than the headspace volume when a Corny keg is filled to 5 gallons (19 L). Volumetric details and hypothetical asides, the contemporary rule is to use less sugar for keg conditioning than you do for bottle conditioning.

The easiest and, in my opinion, best way to repeatably carbonate beer in a keg is with a spunding valve.

I spend a lot of time visiting commercial brewers as part of my job as Technical Sales Manager – Central Midwest at BSG. One thing that I am seeing and hearing more of lately is spunding. For many brewers the practice of capping a fermenting tank of beer with an adjustable pressure relief valve for the sole purpose of capturing carbon dioxide and controlling the head space pressure of the tank, in other words spunding, is a new technique and there is a lot of excitement about this method. Old school lager heads, like me, have been spunding for a long time and have wondered why other brewers have not used this handy method of carbonation. Not only does spunding make use of carbon dioxide that is produced during fermentation to carbonate beer, it also saves time, can improve foam retention when compared to methods that result in beer foaming, such as carbonation stones, and reduces gas stripping of aromatics. This latter point is a big selling-point for brewers who are adding gobs of dry hops, like the crazed haze head crowd.

The easiest and, in my opinion, best way to repeatedly carbonate beer in a keg is with a spunding valve. You don’t have to make adjustments for your batch volume or initial carbon dioxide content because the spunding technique works by releasing any excess pressure from the keg. This means that a surplus of carbon dioxide is intentionally produced. A typical carbonation level for draft beer is 5 g/L or 2.5 volumes. If a keg is being conditioned at 72 °F (22 °C), the equilibrium pressure is 31 psig. I indicated earlier in this discussion that 110 grams (3.9 oz.)of sugar are needed to increase the carbonation level of 18 liters (4.75 gallons) of beer by 3 g/L or 1.5 volumes of carbon dioxide (this takes into account the carbonation of the beer after/during fermentation).

When using a spunding valve, the exact amount of sugar is not critical as long as there is a bit more than needed. Boost the 110 grams (3.9 oz.) by 20% and add 132 grams (4.7 oz.) when you rack your brew from the primary to your Corny keg, seal the keg lid by pressuring to about 10 psig, and attach your spunding valve. Most of these valves do not have calibrated markings on them. You will need to adjust the valve to relieve at 31 psig before you are ready to walk away for a few days. This is easy to do by pressurizing your keg to about 35 psig, adjusting the valve so that it stops venting gas at 31 psig, and then reducing your headspace pressure to about 10 psig by pulling the tab on the pressure relief valve that is on top of the Corny keg.

Over the next several days, the headspace pressure will increase as the priming sugar is fermented. When the pressure reaches the relief set point, excess gas will vent until the priming sugar has all been fermented. The same method can be used without priming sugar as long as the fermentation is capped with the spunding valve when the beer is about 2 °Plato (0.008 SG) above terminal gravity. When the carbonation process is complete, simply remove the spunding valve, transfer the keg to your cooler, and allow your beer to cool and settle before serving. No further adjustment is needed. The last thing to check before serving is the equilibrium headspace pressure associated with the beer carbonation level. In this case the target was 5 g/L or 2.5 volumes. If the beer is stored in a 38 °F/3 °C cooler, ~11.2 psig is needed to maintain this level of carbonation.
Easy peasy!

I recently attempted a lemon alcoholic brew, because we had a full fruiting lemon tree in our backyard and had more lemons than we knew what to do with. The cultivar is Meyer lemon, which is both juicy and a little less astringent than normal Store-bought lemons.

Using a champagne yeast, this brew took many days to begin fermenting and then weeks to finish fermenting. I expected an infection but am happy to report that did not happen. In fact it never actually fermented fully; I was left with a sweet low-alcohol product. It was only drinkable if you further backsweetened.

I have been advised to carefully look at the pH as this might be partly responsible. Naturally I will defer any further experiments until I can pinpoint the probable causes of my initial result. If you have time I’d greatly appreciate any comments you think might help.

Peter Owens

It seems that people, past and present, have accidentally or intentionally fermented just about everything. The easy way to categorize alcoholic beverages is by raw material; beer is made from grains, wine is made from fruit, and mead is made from honey. One commonality with all of these beverages is that the starting point (brewer’s wort or winemaker’s and meadmaker’s must) is a relatively friendly environment for yeast. Although the modern zymurgist often adds nutrients to optimize growth conditions, wort and must can quickly be fermented by wild and cultured yeast. Lemon juice, on the other hand is not so friendly.

The pH of lemon juice is very low and is typically reported to be around pH 2.2. To put this in perspective, this is about 1,600 times more acidic, as defined by the concentration of hydrogen ions, than wort at pH 5.4, and 16 times more acidic than grape must at pH 3.4. Yeast are effected by environmental pH and don’t perform well when the pH drops below about 3. Brewers who successfully bottle condition sour beers know this step can be tricky because the combination of low pH and alcohol requires acid-tolerant yeast strains for conditioning. Lemon juice is truly an extreme environment and the very limited fermentation you experienced is typical for the descriptions of lemon juice fermentations reported by others.

If you simply want to ferment lemon juice into a mildly alcoholic beverage and not follow traditional methods, I would suggest a few things to improve the environment before adding your yeast. Start with the pH obstacle and do something to move the pH upward. One way to do this would be to dilute the lemon juice with wort. Wort is a good source of amino acids, which serve the role of pH buffer and nutrient. Not too different from a kettle sour, except here wort is added to make the juice less acidic. Dried malt extract would make this process easier and a handy method for a quick brew day. Not too interested in a lemon beer? Consider making sima, a Finnish mead made with honey and lemons. The important thing is to bring the pH into a friendlier range.

The other challenge that should be addressed when fermenting lemon juice is nitrogen; lemon juice does not have much and yeast need it for growth. Although yeast nutrients have become fairly common in brewing as brewers have pushed the envelope, all-malt worts made for traditional styles are a good source of nitrogen and many brewers don’t have much need for or experience with nutrients. Wine and meadmakers, on the other hand, are well-versed in the use of nutrients because the musts used for wine and mead are usually deficient in nitrogen. Consider using a general purpose yeast nutrient like Superfood that contains a blend of amino acids, diammonium phosphate, and vitamins.

I hope this high-level answer gives you a little insight into some of the things you may want to consider the next time you attempt fermenting lemon juice. Definitely an interesting ingredient with some fun challenges.

Issue: December 2019