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'''Flemish red-brown beer''', also known as "Flanders red brown beer" or simply "Flemish brown beer" <ref name="lostbeers_1">[https://lostbeers.com/flemish-brown-red-or-red-brown/ "Flemish brown, red or red brown? How Michael Jackson invented a beer style out of thin air," by Roel Mulder. Retrieved 08/29/2020.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/?post_id=3819410974753755&comment_id=3819670174727835&reply_comment_id=3821688484526004 Casey Wellman. Milk The Funk Facebook thread on Flemish Red-Brown beer. 08/30/2020.] </ref>, is a classic Belgian beer style that is currently produced only by a small handful of breweries in Flanders, Belgium today. Some breweries outside of Flanders and Belgium aim to produce comparable beers <ref name="snauwaert">[http://www.sciencedirect.com/science/article/pii/S0168160515301896 Microbial diversity and metabolite composition of Belgian red-brown acidic ales. Isabel Snauwaert, Sanne P. Roels, Filip Van Nieuwerburg, Anita Van Landschoot, Luc De Vuyst, Peter Vandamme. 2015.]</ref>. These beers, red to brown in color, are characterized as being sour and sometimes sweet, with malt flavors and fruity complexity from the mixed fermentation and sometimes hints of oak. They have often been described as "wine-like". Non-Belgian beer writers, beginning with Michael Jackson, have split these beers into two categories: "Flanders Red Ale" and "Oud Bruin". with The the arguable distinction between these two has arguably been them being that "Flanders Red Ales" are generally brewed in West Flanders and are aged in oak, while "Oud Bruin" is generally aged in steel fermenters and brewed in East Flanders, and the "Oud Bruin" version is less acetic and more malty in flavor (. Before beer writer Michael Jackson wrote about these styles, Belgian brewers did not make no such the same distinctions. Today, some modern Belgian brewers see the distinction between these types of beers differently from Jackson and even amongst each other <ref>[https://www.belgiansmaak.com/ep043-oud-bruin-flanders-red-brown-flemish-sours/ Belgian Smaak podcast EP043 | Roundtable Discussion — Everything Oud Bruin, Flanders Red, and Flemish Sours. Feb 2023. Retrieved 02/27/2023.]</ref>(~50 mins in). Rodenbach, Brewery Verhaeghe, Liefmans, and De Brabandere are examples of some of the most well known Belgian producers <ref name="lostbeers_1" /><ref>[http://www.beerhunter.com/documents/19133-000217.html "Belgium's Great Beers". The Beer Hunter, Michael Jackson. 1999. Retrieved 01/27/2016.]</ref><ref>[http://www.bjcp.org/docs/2015_Guidelines_Beer.pdf 23B. BJCP 2016 Guidelines. Flanders Red Ale and 23C. Oud Ruin.]</ref>. Classic Belgian examples have been flash pasteurized or sterile filtered and are not alive in the bottle<ref>[http://www.thebrewingnetwork.com/the-sour-hour-episode-9/ Rudy Ghequire from Rodenbach on the Sour Hour]</ref> (~35 minutes in) (also ref Mad Fermentationist dregs list). Belgian brewers are seeking to protect the term "Flemish Red Brown Beer"; see [[Flemish_Red_Brown_Beer#Protection_Seeking|Protection Seeking]] below.

The sour flavor of both Liefmans and Rodenbach comes from lactic acid and acetic acid. Oak aged Rodenbach contains 2500-5000 mg/L of lactic acid and around 1500 mg/L of acetic acid. Liefmans contains similar levels of lactic acid, but only around 1000 mg/L of acetic acid due to the stainless steel aging process. This results in beer that is perceived as less acidic compared to the oak aged Rodenbach beer <ref name="Dusart_2022" />. Compared to lambic, the aged red-brown beers have more lactic acid and acetic acid than younger lambic (perhaps due to a higher hop dosage in lambic production), but less than aged bottles of lambic (see [[Spontaneous_Fermentation#Aroma_and_Flavor_Production|Spontaneous Fermentation]]).

The phenols 4-ethylguaiacol (4EG) and 4-ethylphenol (4EP) were found in the range of 54-385 µg/l, with the Rodenbach Vintage (unblended Old Ale) having the highest amounts. These levels were still less than typical lambic/gueuze levels. The 4EG levels in Liefmans was only 54 µg/l, compared to the 236 µg/l in Rodenbach Grand Cru and 384 µg/l in Rodenbach Vintage. Liefmans had a fairly high level of 4EP (123 µg/l), but it was still below the flavor threshold of 150 µg/l. In addition, Liefmans had high levels of of 4-vinyl precursors, indicating that ''Brettanomyces'' is not present during the Liefmans brewing process. The scientists suggested that the detected 4EP came from the small amount of torrefied malt used in the Liefmans brewing process, which has been shown to produce small amounts of 4EP <ref name="Dusart_2022" /><ref>[https://pubs.acs.org/doi/10.1021/jf5015654 Scholtes, C.; Nizet, S.; Collin, S. Guaiacol and 4-Methylphenol as Specific Markers of Torrefied Malts. Fate of Volatile Phenols in Special Beers through Aging. J. Agric. Food Chem. 2014, 62, 9522–9528. DOI: 10.1021/jf5015654.]</ref>. Both Rodenbach beers had detectable lactones and other phenols associated with toasting barrels (vanillin, acetovanillone, syringalde-hyde, and eugenol). Surprisingly, the Liefmans bottle also had some of these phenols (vanillin and lactones), indicating that perhaps some of these came from the specialty malts used <ref name="Dusart_2022" />. Esters and alcohols were also measured in all three bottles by the scientists. Ethyl acetate, which is a byproduct of yeast fermentation (especially ''[[Brettanomyces]]'' fermentation) and has an odor threshold of 30 mg/L in beer, was considerably high in all three beers: Liefmans had around 100 mg/L, Rodenbach Grand Cru had around 132 mg/L, and Rodenbach Vintage had 237 mg/L. By comparison, clean ales have around 8-48 mg/L, pasteurized gueuzes have around 33-67 mg/L, and unpasteurized gueuzes have around 61-167 mg/L. Noticeably, the Rodenbach Vintage had considerably high ethyl acetate, even when compared to gueuze. As expected, isoamyl acetate was below odor threshold (1.2 mg/L) in the Rodenbach beers, but above odor threshold in the Liefmans (1.73 mg/L). Isoamyl alcohol was above odor threshold in all three beers. The ester 2-phenylethyl acetate was also below threshold in the Rodenbach beers but not the Liefmans, while its alcohol precursor was high in the Rodenbach. Both esters, isoamyl acetate and 2-phenylethyl acetate, are known to be degraded into ethyl acetate by ''Brettanomyces''. Ethyl hexanoate, ethyl octanoate, and ethyl decanoate esters had levels similar to typical ''Saccharomyces'' fermentations. The Liefmans bottle reached the odor threshold level for oct-1-en-3-ol, which has an earthy/mushroom/musty odor and is considered an off flavor in beer <ref name="Dusart_2022" />. The short chain fatty acids [[Isovaleric_Acid|isovaleric acid]] and hexanoic acid were also measured. Isovaleric acid can be produced by ''Brettanomyces''. It can also be produced by the oxidation of hop acids. The levels of isovaleric acid in the Liefmans and Rodenbach Grand Cru was around the same as it is for regular ales (0.98 mg/L and 0.96 mg/L, while the Rodenbach Vintage reached the odor threshold (1.5 mg/L in beer) of isovaleric acid at 1.53 mg/L. By comparison, gueuze has been measured to have up to 2-3 mg/L of isovaleric acid. Hexanoic acid (also known as caproic acid, and is fatty/sweaty/cheesy/goaty), however, was found at much higher levels in all three beers. At an odor threshold of 8 mg/L in beer, Liefmans measured 10 mg/L, Rodenbach Grand Cru measured 11.26 mg/L, and Rodenbach Vintage measured 11.8 mg/L <ref name="Dusart_2022" />. Pentanoic, octanoic, and decnaoic acids were found below odor threshold <ref name="Dusart_2022" />.