FAN: It's What Beer Yeast Craves!

 

Free amino nitrogen (FAN) is a term used to describe the amount of nitrogen-containing  compounds found in wort that may be metabolized by yeast during the fermentation process. FAN includes amino acids usually found in wort along with ammonia and small peptides (Hill & Stewart, 2019). Most brewers have a vague notion that FAN is important for yeast, but we feel that this is an important topic worth explaining further, as mastering the match between a yeast and its nutrition needs is one the keys to mastering fermentation. 

In beer, an ample supply of FAN is extracted from the malts we use to make wort. Yeast use the nitrogen available in the wort for synthesizing cellular compounds such as proteins. At minimum, 130mg/L (ppm) of FAN is needed for proper yeast growth to attain optimal fermentation. Below this, yeast growth begins to lag and incomplete fermentation can occur along with sulfury off-flavours (Hill & Stewart, 2019).

FAN consumption between yeast groups 

The figure above is data from a recent experiment we did, comparing many of our yeasts in the same wort in a "yeast cage match". As you can see, most of the lager strains follow the ~130ppm FAN consumption rule, but many of the ale strains gobble up significantly more than that. 

It is worth noting that most beer yeast fermentation research has historically been done with lager yeasts, which means that the textbook rules may not be the same ones that apply in modern craft breweries who mostly use ale yeasts. For example, most Belgian ale strains consume quite a lot of FAN (>175ppm).

When we break this down into strains, we see that some strains consume nearly double that of others (compare Dry Belgian Ale to Mexican Lager, for example).  

The optimal FAN concentration in your wort varies by yeast strain. Overall the amount of FAN necessary is roughly proportional to yeast cell growth. An increased FAN content directly correlates with an efficient reduction of wort gravity, also the amount of FAN utilized relates to the decrease in wort pH during fermentation (Hill & Stewart, 2019).

In general, a wort containing 180ppm of FAN will be sufficient for the vast majority of brewing yeasts you use.

FAN and flavour

In terms of flavour, FAN also has an impact on the flavour and aroma compounds created during fermentation. FAN directly impacts the formation of aldehydes, esters, diacetyl, sulfur compounds, and higher alcohols (Hill & Stewart, 2019). Small differences in the FAN content in the initial wort can have a large impact in the flavour outcome of the final beer. Excess FAN levels have also been known to cause off-flavours like diacetyl and higher alcohols like isoamyl alcohol, propanol and isobutanol, whereas low FAN can reduce ester yields (Hill & Stewart, 2019).

Excess FAN is why high-gravity, all-malt beers like DIPAs and Barleywines can be prone to issues with diacetyl and fusel alcohols where beers with sugar additions such as Belgian Tripels and Quads don't have this issue. 

Getting enough FAN to your yeasts is especially important with some of the most aromatic yeasts, such as the Saison, Belgian and Kveik groups - all with high FAN requirement, and all which will produce muted, sulfury beer if they don't get enough FAN. 

Insufficient FAN causes stalled ferments

Insufficient FAN is a common cause of stuck ferments, especially in non-beer fermentations such as wine, cider, and hard seltzer.  Adding utilizable nitrogen is often enough to resume fermentation of a stuck ferment.

However, there's still more detail. Not all amino acids have the same roles in promoting yeast growth. Glycine is known to inhibit yeast growth and fermentation, whereas others help to promote growth. Precisely which amino acids promote growth is not fully known, although some amino acids are preferred by yeast over others. (Thomas & Ingledew, 1990). Addition of methionine was shown to inhibit yeast growth and slow fermentation whereas addition of lysine was seen to enhance yeast metabolic activity and shorten fermentation times (Lekkas et al., 2005).

Low gravity worts have lower FAN

If you asked brewers the question "Which wort should you add more nutrients to", the typical answer to this question would be "high gravity beers". But, let's work through the logic: 

1. FAN is extracted from grain 
2. FAN extraction is roughly proportional to concentration of grain
3. Therefore, higher gravity worts are likely to have higher concentration of FAN. 

 As a result, we reckon that it is in fact *low gravity* worts that would benefit the most from FAN/nutrient additions. This probably also explains why we troubleshoot many more stalled, sulfury ferments when the brewer has brewed a low-gravity wort versus high-gravity worts. There's just not enough food in low-grav worts for the yeast to be happy. 

At the end of the day, that's all we're really doing: cooking enough food for our yeast to be happy. Nourishing the yeast. 

Since we already have some data on FAN consumption by our house yeasts (above), we did a small mini-mash experiment to test the malt side of things. 

At Escarpment Labs, we keep an open mind about tools at our disposal for experiments. Here, an AeroPress coffee maker from the kitchen upstairs was used as a miniature mash filter to rapidly produce a bunch of different worts for this experiment. 

We measured the amount of grain/water used in each mini-mash, and converted all of the mini-mashes in centrifuge tubes in a water bath held at 66ºC for 1 hour followed by a 15 minute mashout step at 74ºC. Afterward, we filtered the mashes with the AeroPress, heated the worts to boiling, and measured specific gravity and FAN of the clear, cooled wort portion. 

While the data are not exactly robust (n = 1 for each treatment, and we only went up to SG 1.050), we see some trends for the different malt types we tested, including a locally-malted product (Barn Owl Pale Ale Malt). As we can see, only once the wort is up around SG 1.040 (10ºP) is there sufficient FAN (>180ppm) extracted from all three malts.

So if you're making a low ABV Berliner, Saison, Table Beer or Session IPA, it is a good idea to add nutrients to your beer. Your DIPA probably doesn't need FAN, but might benefit from micronutrients (more on that soon).

FAN and mixed ferments 

We're often asked whether brewers should add nutrients to their mixed ferments/secondary ferments when making sour beer. Our suggestion is that as long as the beer still has some yeast present, the bacteria and Brett can likely obtain what they need from the Saccharomyces kicking around. They're resourceful critters.  

Nitrogen overflow from yeast benefits lactic acid bacteria (LAB) and creates mutualism. This kind of mutualism is seen in many tasty fermentation environments, such as sourdough starters and mixed culture sour beers.

This overflow from yeast is needed to allow the survival of LAB. Overflow occurs in nitrogen rich communities where Saccharomyces cerevisiae changes its metabolism to secrete pools of metabolites such as amino acids. Amino acid release can also happen after a yeast cell dies and undergoes autolysis. This ensures the survival of lactic acid bacteria. LABs are known to be pickier eaters, with specific amino acid and vitamin requirements. Yeast can provide these nutrients to bacteria when co-fermented. 

Conclusion 

Hopefully through our work, you're able to understand a bit more about the practical needs of different yeast strains for Free Amino Nitrogen, and also what wort strengths are typically required to hit those requirements. Matching the FAN extracted from your grain (or added as an additional nutrient supplement) to your yeast will help you dial in your beer flavour and fermentation time - especially for the low-ABV crushers that we find ourselves reaching for. 

FAN isn't the cure-all of yeast nutrition though. Even in a wort with sufficient FAN, some yeasts can struggle. Stay tuned for more, as we dive into the micronutrient requirements of our fermentation friends. 

References

Brice, C., Sanchez, I., Tesnière, C., & Blondin, B. (2013). Assessing the Mechanisms Responsible for Differences between Nitrogen Requirements of Saccharomyces cerevisiae Wine Yeasts in Alcoholic Fermentation. Applied and Environmental Microbiology, 80(4), 1330–1339. doi: 10.1128/aem.03856-13

Ferreira, I. M., & Guido, L. F. (2018). Impact of Wort Amino Acids on Beer Flavour: A Review. Fermentation, 4(2), 23. doi: 10.3390/fermentation4020023

Hill, A., & Stewart, G. (2019). Free Amino Nitrogen in Brewing. Fermentation, 5(1), 22. doi: 10.3390/fermentation5010022

Lekkas, C., Stewart, G. G., Hill, A., Taidi, B., & Hodgson, J. (2005). The Importance of Free Amino Nitrogen in Wort and Beer. Technical Quarterly MBAA, 42(2), 113–116. doi: 10.1094/TQ-42-0113

Ponomarova, O., Gabrielli, N., Sévin, D. C., Mülleder, M., Zirngibl, K., Bulyha, K., … Patil, K. R. (2017). Yeast Creates a Niche for Symbiotic Lactic Acid Bacteria through Nitrogen Overflow. Cell Systems, 5(4). doi: 10.1016/j.cels.2017.09.002

Thomas, K. C., & Ingledew, W. M. (1990). Fuel alcohol production: effects of free amino nitrogen on fermentation of very-high-gravity wheat mashes. Applied and Environmental Microbiology, 56(7), 2046–2050. doi: 10.1128/aem.56.7.2046-2050.1990

Contributions 

Richard Preiss and McKenna Tosh wrote this blog post. Iz Netto organized the "yeast cage match" experiment. Richard Preiss and Shelby Stein performed the malt analysis experiment.