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update to flavor differences of species from Tenhovirta thesis
====Secondary Metabolites====
Both primary and secondary metabolites play a large role in the flavor and aroma profile of wort fermented with ''Lactobacillus''. Secondary metabolites are compounds that are not directly related to the growth of an organism, but often assist with survival <ref>[http://www.ncbi.nlm.nih.gov/pubmed/11036689 The natural functions of secondary metabolites. Demain AL, Fang A. 2000.]</ref>. These secondary metabolites are produced by the pathways mentioned above, and different strains probably regulate the enzymes involved in various pathways differently and produce different secondary metabolites <ref>Private correspondence with Richard Preiss from Dan Pixley. 12/29/2015.</ref>. Thus, different species and strains can produce a wide variety of flavors and aromas (compare this to food grade lactic acid in which none of these secondary metabolites exist). These secondary metabolite are the result of carbohydrate fermentation and amino acid metabolism <ref name="peyer_review"></ref>. Major secondary metabolites Different species can have varying ranges of flavor intensities, especially citrus flavor, which one study that soured wort and then fermented with US-05 two days later (not kettle soured) reported was higher for ''L. plantarum'', ''L. alimentarius'', ''L. brevis'', and ''L. buchneri'' versus ''L. rhamnosus''. Vinuous, malty, and sour flavors were also reported to vary in intensity based on species. ''L. buchneri'' was reported in the study to be the most vinuous, followed by (in descending order of intensity) ''L. brevis'', ''L. plantarum'', and ''L. alimentarius''. The sour flavor was reported highest for ''L. plantarum'' and ''L. alimentarius'', followed by (in descending order of intensity) ''L. brevis'', ''L. buchneri'', and finally ''L. rhamnosus''. ''L. rhamnosus'' had the most raspberry flavor, while the other species had a similar level of moderate raspberry flavor. All of the species tested had similar levels of apple flavor (moderate), acetic flavor (low), butyric (very low to absent, even though the fermenting vessels were never purged of oxygen), bitter aftertaste (low), and yeasty (low) <ref name="Tenhovirta_masters" />.
[http://www.sciencedirect.com/science/article/pii/S0308814617302911#t0005 Dongmo et al. (2017)] found 56 volatile flavor compounds, including various esters, alcohols, ketones, aldehydes, acids, ethers compounds, sulfur compounds, heterocyclic compounds, phenols (including guaiacol and 4-vinylguaiacol), terpenes, lactones, and several unidentified compounds. Key compounds produced by ''Lactobacillus'' include acetaldehyde (thought to be a major flavor contributor to kettle soured beers <ref name="Peyer_2017" />), β-Damascenone, furaneol, phenylacetic acid, 2-phenylethanol, 4-vinylguaiacol, sotolon, methional, vanillin, acetic acid, nor-furaneol, guaiacol and ethyl 2-methylbutanoate. Acetaldehyde was the most impactful aroma compound found followed by propan-1-ol and γ-dodecalactone. Acetaldehyde was generally produced in much higher amounts (~23-64 µg/L) by the select strains of ''L. plantarum'', while ''L. amylolyticus'' and ''L. brevis'' produced only 1.5-3 µg/L. In fact, the levels of all of these compounds differed significantly based on the species and strain. The selected strains of ''L. brevis'' were associated as having worse aromas that were dominated by methional (cooked potatoes), acetic acid (vinegar), and nor-furaneol (caramel-like). The ''L. plantarum'' strains selected were identified as producing more positive aromas from compounds such as β-damascenone (apple/fruit juice), furaneol (strawberry), 2-phenylethanol (rose/caramel) and ethyl 2-methylbutanoate (citrus). Small but significant amounts of linalool and geraniol were also found, which are normally terpenes found in [[Hops|hops]]. Vanillan is formed from ferulic acid by some ''Lactobacillus'' species as well as ''Oenococcus oeni'' <ref name="Dongmo">[http://www.sciencedirect.com/science/article/pii/S0308814617302911 Key volatile aroma compounds of lactic acid fermented malt based beverages – impact of lactic acid bacteria strains. Sorelle Nsogning Dongmo, Bertram Sacher, Hubert Kollmannsberger, Thomas Becker. 2017. doi:http://dx.doi.org/10.1016/j.foodchem.2017.02.091.]</ref>. Some strains of ''Lactobacillus'' can break down chlorogenic acids, which are esters found in plants, into their phenolic derivatives. For example, some strains of ''L. fermentum'', ''L. helveticus'', and ''L. reuteri'' were found to hydrolize chlorogenic acids into caffeic acid, which can then be converted into other phenols such as 4-vinylcatechol and 4-ethylcatechol by ''[[Brettanomyces#Phenol_Production|Brettanomyces]]'' <ref>[https://www.sciencedirect.com/science/article/pii/S0963996918303363 Site-specific hydrolysis of chlorogenic acids by selected Lactobacillus species. Elsa Anaheim Aguirre Santos, Andreas Schieber, Fabia nWeber. 2018. DOI: https://doi.org/10.1016/j.foodres.2018.04.052.]</ref>.