Reconstructing something that Viking Age Scandinavians would recognize as “beer/ale”
Worldwide, a fermented drink made from grains (beer or ale, in this document we use these terms interchangeably, as in Nelson 2014) is an ancient drink, with evidence going back to the Stone Age, and some suggest it was part of the drive to domesticate grains (Hayden et al 2012, Homan 2004, Unger 1988). Here, we are looking only at Scandinavia.
In Northern Europe, beer was the most common alcoholic drink (Nelson 2005, Nelson 2014, Strickland 2007). Prior to the Viking Age, we have some written accounts which say that beer or ale was a common drink among humans living in northern Europe (Pliny, Tacitus, etc). Within Scandinavian written tradition, there are mentions of beer and ale in the Havamal, Alvissmal (an Eddic poem), and in various laws. The Gulathing Law requires farmers to brew ale to contribute for consumption at large festivals during the year, and it was also common for household and local celebrations (Riseley 2014). There are also mentions of ale in mythology, such as in the Hymskvitha (Codex Regius). Alcoholic drinks, including ale, were of great cultural importance to Viking Age Scandinavians, and of nutritional importance to many people in early medieval northern Europe (Buckley 2018, Pearson 1997, Hauge 1996). Serving beer, along with other alcoholic drinks, was considered a sign of prestige and wealth (Zori et al 2013).
Beer can vary greatly, from individual brewer, to community. In this experiment, we are looking at a specific ethnographic tradition of a medium color, juniper flavored, farmhouse ale that is found in areas surrounding the Baltic Sea. There are still strong home-brewing traditions of making an alcoholic drink that meets these descriptions in Norway, Gotland, Estonia, Latvia, and Lithuania (Laitenen 2018, Ekberg et al. 2015, Nordlund 1969, Garshol 2013, Mosher 2004, etc). http://www.garshol.priv.no/blog/305.html
Grain.
The most essential ingredient for brewing beer is grain. Barley was the most common grain used for beer-brewing in Europe across most time periods, including the Viking Age (Nelson 2014). Six-row, hulled barley is the most common grain found in Viking Age Scandinavia, with oats, rye, and wheat being found in smaller amounts (Hjelmqvist 1990, Larsson 2018, Wennberg 1986, Sveinbjörnsdóttir, 2004, Church et al 2005, Grabowski, 2013). Given this was the most commonly grown grain, we conclude that six-row, hulled barely would have been the primary grain used for brewing in the Viking Age.
Some of the traditional beers in the Baltic/Scandinavian region include a mix of grains. Additionally, grain is often found stored and processed together, so it is unclear how much effort VA Scandinavians took in carefully sorting the grains apart from one another.
The majority of our grain for this project was Full Pint, a hulled, two-row barley bred for our local growing conditions.
http://www.territorialseed.com/product/full-pint-barley-grains-seed/grains
Malting.
Malting is a process where grains are cued to begin the germination process, resulting in the formation of enzymes able to convert starch into sugar. Malted grains are then heated to stop the germination process, resulting in a shelf-stable product that can be converted into sugar at any time.
There is evidence from pre-Viking Age Scandinavia for the process and equipment of malting. For example, a 5th -7th century malting kiln at Uppakra Sweden. This kiln was a low temperature kiln, approximately 2 by 1.5 meters in size, and was found surrounded by sprouted grains, many of them charred, which the authors interpret as a malt-drying kiln (Larsson et al 2018). Malted grains are also found away from kilns, they are both germinated and charred. There is evidence that as early as the first century, malt was being stored as malted grains (Helbæk 1938). There are also finds of malted grains from Sweden in the 6th and 9th centuries (Helbæk 1966, Viklund 1989).
The degree of toasting gives beer its color, and much of the taste of the malt. There are extensive finds of charred grains, indicating that different degrees of toasting was happening (Viklund 1989). This may or may not have been intentional, as experiments with a prehistoric malting kiln produced malt that was smokey, with some of the resulting malt remaining pale, but due to the uneven heating, other parts became a dark malt (Sitka 2011). There are also some indirect written descriptions from the time period, for example, an Irish poem from approximately AD 1000 describes different beers as being different colors, including “red like wine” (Binchy 1963).
Another line of evidence for toasted and smoked malt comes from the ethnographic evidence. Many of the beers in the circum-Baltic tradition are described as smoky. Some varieties of Sahti (from Finland), Gotlandsdricka, and Norwegian Farmhouse all have varieties or are generally described as having a smoky quality. These beers vary from pale to dark brown in color, with most falling in the modern range known as amber to brown ale (Mosher 2004, Nordlund 1969, Laitinen 2018, Garshol 2013, http://www.rekolanpanimo.fi/sahti-in-english/, etc).
As we said above, we chose to use mostly (2.5 lbs) Full Pint (unsmoked), a local pale barley, with ¼ lb each of cherry smoked barely, chocolate malted barley, and malted oats. We chose this because we were assuming that most malt from the Viking Age kilning process would be successfully pale, with a few areas of the malt that was a little over toasted, and that the entire process would involve enough smoke to impart a smoky flavor to the malt. We chose to include some oats as that is a traditional ingredient in some of the beers we had been reading about (see Nordlund 1969).
Grinding the malt.
Malted grains are very coarsely ground in order to expose the interior starch and enzymes evenly to one another during the mashing process (Papazian 2003).
In Viking Age Scandinavia, household querns were turned by hand, water mills were introduced at the very end of the Viking Age, and most household still had hand querns (Baug 2015). Viking Age Scandinavians would have used a hand turned quern to grind the malt by passing it through the quern only once, instead of several times, as to grind fine flour.
We used the electric grain grinder at our local brewing store. As a note, my local brewing store is AMAZING. They let us bicker for a half hour over the malts, only vaguely roll their eyes at our “outdated” methods, think our experiments are interesting, and help us with the complex switch-arrangement on the grinder. Plus they are great people, with amazing supplies for really competitive prices (that means cheaper than the internet). YOU should go talk to YOUR local brewing store!
Mashing.
Mashing is the final step in the enzymatic process begun during malting. Water is added, and the resulting wet mash kept at specific temperatures in order to facilitate the most efficient conversion of starch to sugar by the enzymes (Papazian 2003).
We have exactly zero information about Viking Age Scandinavian mashing. Modern mashing is often bottom heated in a well insulated, and very large, pot, with a visible temperature control (Papazian 2003). Obviously that isn’t an option for Viking Age brewing. Much of the heating of various stages of brewing seem to have been accomplished with the addition of very hot rocks, but we haven’t QUITE worked ourselves up to that (stay tuned! In the meantime, see Laitinen 2013, Garshol 2018, Nordlund 1969). The addition of hot rocks means that mashing can be done in any container, such as a stone or wooden tub (Nordlund 1969, Dineley and Dineley 2013). However, there is also another option. This is to heat the water in small amounts, and keep adding very hot water to keep the temperature up. We tried the latter. We added only small amounts of water (less than a gallon) that would be easy to heat in an average sized Viking Age cooking pot (see above).
Now, we understand more about the chemistry and biology of mashing than Viking Age people likely did, so our mash schedule is quite complex. The ethnographic evidence indicates that there was not a single consensus about how best to mash a hundred years ago, let alone a thousand (Nordlund 1969). We had the advantage of being able to look up a very efficient step mash procedure that worked wonderfully, but instead of just being able to turn the burner on and off, we were determined to do this with small amounts of hot water. That was difficult, but ultimately successful. We heated the initial mix of water and crushed grains (in a large metal pot) to 122F on the burner, and then placed it into an insulated box. This was allowed to work for 30 minutes, then we stirred it, and added water to bring it up to 150F. We held at that temperature for 15 minutes, then added hot water to bring it up to 158 F for 20 minutes. At this point we used iodine to test (on a plate) if the starch had all been converted to sugar. Our mash passed with flying colors, and so we heated it up to 170 F to end the reaction (mash schedule adapted from Papazian 2003). We had some trouble with this, the water needed to be hotter than we thought, it didn’t hold as well as we hoped, and we had to use more water than we had anticipated. Even with all those adventures, it still passed the test great, so we proceeded to the next step.
Figure 1. A box of blankets, for insulating the mash-tun while the enzymatic reaction occurred. The mash needed to be held at specific temperatures, this made that possible on a cold winter day.
Filtering/lautering.
After mashing, the sugar is dissolved in water, but it is also still within a thick soup of now-spent grains. These must be separated so the spent grains can be disposed of, and the sugar water fermented.
There is a circum-Baltic tradition of brewing beer with juniper as a filter or flavoring component (Madej et al 2014). There is no evidence that this tradition can be traced to the Viking Age with certainty, but the “traditional” brewing of the region seems to involve this particular step or flavoring (eg. Ekberg et al. 2015, Nordlund 1969, etc.).
http://www.garshol.priv.no/blog/345.html
THIS is where things get a little different from modern home-brew. But before we dive in here, we have to have a QUICK aside.
(this is Kada speaking, btw. I grade alllllll my students down for using Wikipedia and individual blogs. “Follow the sources! Use something peer reviewed! Put in the work to find sources you can actually trust!”. So, all of you can now point and laugh, because in the next section, I am going to cite BOTH personal blogs and Wikipedia. WHY? Let’s go one by one. The personal blogs have the best ethnographic information I can find in this area. I’ve searched, I’ve read the accounts from the 19th century and the early 20th, they are also cited here. I have been unable to find academics who are currently visiting the traditional breweries and making observations. However, there are a half-dozen amazing personal blogs where people are going into great detail to describe exactly what they are observing in, in multiple countries, and in many different brewing traditions. But why Wikipedia? Language barrier. I can use Google translate with the best of them, but when I need detailed diagrams and a SUMMARY of how to do a complex task, that trick is often not sufficient. I tried. I followed all of the links and references, and made my best attempt, but could make neither head nor tail of them. However, I understand the wiki article, so if you feel like “grading me down”, go for it, this was the best I could do. Okay, back to brewing!)
We went to a friend’s house and picked a big pile of juniper branches. What species? NO CLUE. Definitely NOT Juniperus communis, like we wanted. I was unable to accurately identify the species, as there were no berries present, and it is some kind of commercialized cultivar. And yes, I did try, quite extensively. No clue.
Figure 2. The scrubby juniper bush that we used to filter our beer.
After reading about traditional filtering plans, we built a structure known in Gotlandsdricka brewing as a “bunn”, (a filtering structure https://en.wikipedia.org/wiki/Gotlandsdricka). We stripped the scales (you’d call them needles) off a few boughs so we just had juniper branches. Since we picked this from the side of a street, we decided to blanch the juniper prior to brewing with it. First we rinsed them in cold water. Then we boiled a large pot of water, and briefly (about 30 seconds per branch) immersed the branches in the boiling water, then placed them in a large pot to cool. I don’t know if they had a lot of stuff on the surface, or if we made some kind of tea (?) from them, but the water after this step was distinctly yellow, and smelled terrible.
Figure 3. Pouring off the GROSSSSSSS water we’d used to clean off the juniper branches. YUCK, smeelllllllleddd baaaaaaaaad!
There are no Viking Age Scandinavian finds of lautering tuns, though Dineley and Dineley (2013) propose that the large stone troughs previously identified as being for laundry or bathing were used as various brewing vessels, including for lautering. Traditional circum-Baltic lauters are usually one of two forms, both based on a coopered wooden vessel. First are a barrel or large tub on three legs with a central hole in the bottom (Nordlund 1969, Garshol 2013, Laitinen 2018). This hole can be closed via a stick that comes up through the mash. The second is a barrel with a tap at the bottom. Either way, a filtering device is constructed at the bottom of the vessel, usually consisting of some combination of small sticks, straw, fabric, and almost always, juniper branches (Nordlund 1969, Garshol 2013, Laitinen 2018).
The next step for us was to arrange our lautering vessel. We were not 100% confident in our filtering capacity, and so we placed a large chunk of modern mesh (window screen) into our lautering tun. I should, at this point, mention that our lautering tun is a deep-fryer meant for a turkey, and that cleaning all the second-hand grease out of it was NOT fun. However, possibly one of the better $7 investments we’ve made. Okay, so, deep fat fryer, lined with mesh (we’ve since replaced this with a brewing bag). We read that we should put an extra filter thing to keep the fine stuff off the outlet, so we invented an awesome arrangement. We had a broken vegetable steamer, so we put one petal of that in front of the tap, and kept it in place with a set of magnets. Oh, and the sacred rock. Some of the ethnographic research indicated we needed a sacred rock, so.
We placed the bare juniper sticks in the bottom, curling them around to make a sort of lattice. Once that was arranged, we added the juniper branches. Yeah, all of them. It ended up being about…6 inches deep? Too much, as it turned out, but that’s why it’s an experiment, right? We made sure they were fairly tightly pushed down, and evenly distributed over most of the bottom, with a slightly higher bunn where the tap is. Now that our bunn was set, it was lautering time!
We poured the mash over the branches (note, it works better if you put the solids in with a slotted spoon first), let it sit a moment (which was too long!) and then opened the tap at the bottom. We were delighted when this worked perfectly, and wort drained out the bottom with no problems. We drained the wort out into ½ gallon canning jars. We sparged with hot water, continuing until the total volume was what we’d figured on at the start (3 gallons). As with the mashing, we only added water at less than one gallon intervals, as that was a reasonable amount to be able to heat at one time in a Viking Age cooking pot.
Figure 4. Top view of the lautering tun, with the bunn set and the mash filtering through it. The wort is draining into the canning jar on the floor.
Figure 5. From left to right: the insulating box from mashing, the enamel bin for rinsing juniper branches, the lautering tun, currently draining svagdricka-wort into the mash tun, a partial bucket of svagdricka-wort.
The second pull, sparge-drink: svagdricka (a weak, fermented beverage, see Laitinen 2018). In the circum-Baltic, some traditions include a second sparging, resulting in a weak wort which is fermented into a small beer. So, we did this.
We were happy to be able to pass on our spent grains to be chicken feed for a friend, as is traditional (Nordlund 1969).
Figure 6. Most of the grain has been removed, and the bunn lifted out of the lautering tun and put in a basin for further grain extraction from the branches.
Additives, for flavor and preservation.
Modernly, beer is considered to be a combination of grain, water, yeast, and hops (and other flavorings in some areas). Hops in historical brewing were first added as both a flavoring and a preservative agent. There were many other potential additives used during brewing, but we are concentrating on only a few during THIS round (Nelson 2014).
The only additional flavoring we added was hops. Hops found in Viking Age Scandinavia at a variety of sites: Elisenhof, Ribe, Haithabu, Birka, and Lund (Behre 1999, Hansson 1996, Hjemquist 1990). These may have been grown locally, but the Graveny boat, carrying a cargo of hops, was sunk in England in the 10th century, indicating that trade in hops was occurring in Northern Europe in the Viking Age (Wilson 1975). There is debate on what hops were specifically used for during the early middle ages, but there are records of hops specifically for brewing in monanstic and tax records beginning in the 7th century in mainland Europe (Wilson 1975). We added a scant handful of hops we grew and gathered ourselves, to the lautering tun. This did absolutely nothing, as the hops were too old, and there was far too little of them to impact anything at all.
We added juniper not as a flavoring, but as a mechanism to filter the mash, but it definitely imparted a great deal of taste. The juniper used in the lautering step does have antimicrobial properties as well (Zheljazkov et al 2017).
Wort, to boil or not to boil?
The Viking Age Scandinavian cooking containers that have been discovered to date are not of sufficient size to boil the entire wort (see for a summary of the copper alloy cauldrons discovered http://www.vikingage.org/wiki/wiki/Copper-alloy_Cauldrons_%26_Pans). The wort could be heated with stones, but ethnographic evidence does not indicate that this step is a required part of the household brewing tradition in the area (Nordlund 1969, Laitinen 2018). During modern brewing, the wort is boiled, and proteins coagulate and precipitate out of the final (clear) beer (Delcour and Vanhamel 1988, Papazian 2003). This reduces the worth of the final product as a nutritional source.
We chose to not boil the wort. We put it straight from the lautering tun into the fermenting vessels. We think this was responsible for the result being opaque and strangely filling when drunk. We have not performed a chemical analysis, but believe this drink has a high protein content.
Fermenting vessels.
Beer can be fermented in any container that will hold water, though we do not know what containers were used in Viking Age Scandinavia. We know that beer vessels in Viking Age Scandinavia could be large, as during Orkneying Saga, a man hides behind one. Large stone structures have been interpreted as brewing vessels in several areas which were occupied by Viking Age Scandinavians (Dineley and Dineley 2013). There are many wooden barrels and buckets from Viking Age Scandinavian graves and settlement finds (see http://www.vikingage.org/wiki/wiki/Buckets_%26_Barrels for summary). Traditionally, beer in Scandinavia and the Baltic has been brewed in large vats, usually wooden (Nordlund 1969, Garshol 2013, Laitinen 2018).
We fermented the wort in previously sterilized ½ gallon glass canning jars and a carboy. The carboy was very small (64 oz) and fitted with an airlock (the svagricka was in a clean brewing bucket, we ran out of jars). The vessels other than the carboy were fermented open-vat. We tied a double layer of cotton quilting fabric (clean and dry) over the neck of each jar with a piece of string. The jars were intentionally placed next to the radiator because A. it was the middle of winter, and B. we were in a hurry. That was likely a mistake. If Dinsley and Dinsley (2013) are correct, then brewing in Viking Age Scandinavia may have occurred away from the main living building, making consistent heating unlikely. However, we also have that story of hiding behind brewing in the main hall, and climate, to take into account. Perhaps beer could be brewed in stone tubs in an unheated building during the summer, but that’s unlikely in winter in Scandinavia. Since we were also brewing in the coldest season here, we chose to put the brewing near the heating source, as we likely would have then. Since our containers were so small, and modern heating is quite efficient, we think we may have “pushed” the yeast with too much heat.
Yeast and pitching.
There is only ethnographic evidence for intentional introduction of yeasts to brewing, but much of pre-modern brewing leads to spontaneous fermentation, or storage of yeast colonies in the brewing equipment. Sterilization was obviously unknown in the Viking Age, and if beer was brewed in the same container, then yeasts and lactobateria would both be in small gaps and even in a wooden container. We do have ethnographic evidence of using the yeast from one batch of brewing to start the next, and also for using yeast from baking to brew farmhouse ales (Ekberg et al. 2015, Mosher 2004). There is also a tradition of preserving some of the yeast on a carved stick or ring which is then hung in the brewhouse, and stirred into the next brew (Jackson 1997, Garshol 2013 see http://www.garshol.priv.no/blog/264.html).
We purchased dry bread yeast and ale yeast in suspension and pitched the jars differently, but because of the open-vat fermentation and not boiling the wort, they all tasted the same. We think the yeasts moved between the jars. We put just ale yeast in one jar, just bread yeast in one jar, and a mixture of both in several jars. They all fermented at the same rate, had comparable amounts of precipitate at the bottom, and were indistinguishable by taste or sight.
Lacto bacteria.
As noted above, Viking Age brewing methods would include bacteria whether the brewer intended them or not. Many of the traditional circum-Baltic beers are soured, now on purpose, but in the past, prior to wort boiling and high levels of hopping, this may have been just how the beer was (however, see: http://www.garshol.priv.no/blog/306.html).
We did not introduce a specific strain of lacto bacteria. We were preparing to attend an event, and had already started 7 jars of lacto fermented vegetables. We placed our open-vat brewing vessels next to these, and this was actually more effective than we had hoped. Specifically, instead of just getting soured beer, we ALSO got yeast in my vegetables (VERY ANNOYING!). The vegetables were airlocked, but apparently those yeasts are determined, sigh. The one airlocked beer container also was a soured beer, though less so than the open vat fermented ones. We believe that the unboiled wort, which was poured over branches, had enough lacto bacteria in it to sour itself without any help at all from us.
How long does it ferment?
We know from the Viking Age laws that there was an emphasis on preparing beer for specific dates, which indicates at least some preparation time being needed. The yeast need time to consume sugars and form alcohol. Traditional brews vary from modern methods of several weeks in each fermenter, to drinking it after only a few weeks (Laitinen 2018).
Ours tasted “young”, which is not surprising, because we only fermented it for two weeks. We had a time deadline, so that was just what had to happen. In the future, I’d like to try fermenting it for longer.
What was it LIKE?
We called this the “beer of the forest” when we tasted the wort because it was bitter, and malty, but it tasted like you had taken a large (like, a few cubic feet) of the forest floor, squashed it up, and put it in the beer. Funky, musty, gnawing on a tree, this definitely tastes like dirt, with notes of pee and decay, this drink was BAD. Somehow, the additions of bread yeast and sourness actually made it better. I think some of the more objectionable compounds got evaporated off, or just eaten by some helpful yeast or lactobacteria. In the final form, it was more…tolerable? Like, I was willing to drink more of it, and also willing to inflict it on others outside of pranking. It still tasted like there was a forest in the jar somewhere, and possibly a little dirt, but the really objectionable parts vanished during fermentation. It did not LOOK like the most appetizing thing, it was chocolate brown, totally opaque, and a little scummy. The final taste was much more interesting, sort of…rich and strangely earthy. It still had a definite “I think I might be biting a juniper twig” element, combined with a lot of interesting funkiness.
We gave it to a LOT of people. The consensus was that people who love beer HATED it, nd people who hate beer liked it. Some of them asked for seconds! Now, we served it warm, and flat (hey, more accurate that way!), so this is quite the feat the people liked it at all.
The svagdricka pulled from this mash was also intolerable when first made, but improved later. When we first tried it, it was MORE bitter than the first pull, tasted more of forest-floor, and was most succinctly described as tasting “of poverty and sadness”. After fermenting it was…better? Still not delicious. We brewed a test batch of nearly the same beer but without the juniper additions, and the svagdricka from that was DECLIOUS and we drank gallons of it.
Svagdrika, to me, is like everything good about kombucha, with none of the things I object to about that particular drink. It is light, a little sweet, often still fermenting, so lightly bubbly, tangy without being horribly sour, GREAT. I mean…if you put too much juniper in the bunn, like we did, it will taste of poverty and sadness, but IN GENERAL, second sparge drinks are a fabulous idea and delicious.
Please consult the following amazing websites on Scandinavian and Baltic beer traditions:
https://www.brewingnordic.com/
https://thedraughtsaredeep.wordpress.com/2014/06/20/an-applied-approach-to-viking-era-brewing/
https://www.duo.uio.no/bitstream/handle/10852/40697/Riseley-Master.pdf?sequence=1
http://www.garshol.priv.no/blog/
Literature Cited
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Behre, K. E. (1999). The history of beer additives in Europe—a review. Vegetation history and archaeobotany, 8(1-2), 35-48.
Binchy, D. A. (Ed.). (1963). Scéla cano meic Gartnáin (Vol. 18). Dublin Institute for advanced studies.
Buckley, J. (2018). Inspiration and inebriation, transformative drinks in Old Norse literature. (PhD Dissertation).
Delcour, J. A., Vanhamel, S., Moerman, E., & Vancraenenbroeck, R. (1988). Protein precipitation during wort boiling: quality aspects of diminished wort boiling times of brews prepared from proanthocyanidin‐free or regular raw materials. Journal of the Institute of Brewing, 94(6), 371-374.
Dineley, G., & Dineley, M. (2013). Where Were the Viking Brew Houses. EXARC Journal, 2(2).
Ekberg, J., Gibson, B., Joensuu, J. J., Krogerus, K., Magalhães, F., Mikkelson, A., ... & Wilpola, A. (2015). Physicochemical characterization of sahti, an ‘ancient’beer style indigenous to Finland. Journal of the Institute of Brewing, 121(4), 464-473.
Garshol, L.M. (2013). Larsblog-Beer. http://www.garshol.priv.no/blog/beer/
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Jackson, M. (1997). Odin’s Glass of Nectar. http://www.beerhunter.com/documents/19133-000103.html
Jónasson, B., Sæmundsson, M. V., & Scudder, B. (1992). Hávamál: the sayings of the Vikings. Guðrún.
Laitinen, M. (2018). Farmhouse Ales. https://www.brewingnordic.com/farmhouse-ales/
Larsson, M., Svensson, A., & Apel, J. (2018). Botanical evidence of malt for beer production in fifth–seventh century Uppåkra, Sweden. Archaeological and Anthropological Sciences, 1-12.
Larsson, M. (2018). Barley grain at Uppåkra, Sweden: evidence for selection in the iron age. Vegetation History and Archaeobotany, 27(3), 419-435.
Madej, T., Pirożnikow, E., Dumanowski, J., & Łuczaj, Ł. (2014). Juniper beer in Poland: the story of the revival of a traditional beverage. Journal of Ethnobiology, 34(1), 84-103.
Mosher, R. (2004). Radical Brewing: Recipes, Tales and World-Altering Meditations in a Glass. Brewers Publications.
Nelson, M. (2005). The barbarian's beverage: A history of beer in ancient Europe. Routledge.
Nelson, M. (2014). The Geography of Beer in Europe from 1000 BC to AD 1000. In The Geography of Beer (pp. 9-21). Springer, Dordrecht.
Nordland, O. (1969). Brewing and beer traditions in Norway: the social anthropological background of the brewing industry. Oslo: Universitetsforlaget.
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Viklund K (1989) Makrofossilanalys. Hässelby, Börje sn, Uppland. Unpublished report, Dept. Of Arch. University of Umeå
Wennberg, B. (1985). Iron Age agriculture at Trogsta, North Sweden.
Wilson, D. G. (1975). Plant remains from the Graveney boat and the early history of Humulus lupulus L. in W. Europe. New Phytologist, 75(3), 627-648.
Zheljazkov, V. D., Semerdjieva, I. B., Dincheva, I., Kacaniova, M., Astatkie, T., Radoukova, T., & Schlegel, V. (2017). Antimicrobial and antioxidant activity of Juniper galbuli essential oil constituents eluted at different times. Industrial crops and products, 109, 529-537.
Zori, D., Byock, J., Erlendsson, E., Martin, S., Wake, T., & Edwards, K. J. (2013). Feasting in Viking Age Iceland: sustaining a chiefly political economy in a marginal environment. Antiquity, 87(335), 150-165.
Additional useful websites
https://beersforbreakfast.wordpress.com/2013/12/11/gotlandsdricka/
https://www.norwegian.com/magazine/features/2013/10/a-short-history-of-beer-in-lithuanian
https://www.morebeer.com/articles/Sahti_finland
http://soribrewing.com/articles/brewing-sahti
http://www.rekolanpanimo.fi/sahti-in-english/
https://draftmag.com/kveik-the-hottest-new-centuries-old-beer-yeast-youve-never-heard-of/