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updated Sugar Utilization and Secondary Metabolites
===Sugar Utilization and Primary/Secondary Metabolites===
''Lactobacillus'' generally prefers glucose, fructose, and maltose, and does not ferment maltotriose. Some species may prefer certain types of sugars over others, for example ''L. plantarum'' ferments glucose first, and then fructose if it is available. ''L. reuteri'' ferments maltose first, while ''L. brevis'' feeds on maltose, glucose, and fructose. Peak sugar consumption without competition from yeast is typically 48 hours, and very little alcohol is produced (around 0.10-0.30%, far less than the 0.5% required for non-alcoholic drinks). Consumption of sugars occurs mainly during the 48 hour growth period, but also occurs after growth has stopped. No more than 0.5-1°P worth of sugar is consumed by ''Lactobacillus'', but rather than high residual sugar concentration being the limiting factor on growth it is thought that low pH and other metabolic byproducts weaken and finally stop the growth of ''Lactobacillus'' <ref name="Peyer"></ref>. For a chart and in depth discussion on what types of sugars are fermentable by different species of ''Lactobacillus'', as well as charts on ''secondary metabolites'', see [http://phdinbeer.com/2015/04/13/physiology-of-flavors-in-beer-lactobacillus-species/ Matt Humbard's ''Physiology of Flavors in Beer – Lactobacillus Species'' blog article]. Heterofermentative species can also produce [[Tetrahydropyridine]].
Lactic acid is the primary metabolite for ''Lactobacillus'', as well as CO2 and ethanol/acetate (acetic acid) in heterofermentative species. Acid production is at it's highest during the exponential growth phase, but continues into the stationary and decline phases. Typically just under 50% of the lactic acid produced is L-lactic acid (more nutritionally relevant) while the slight majority is D-lactic acid <ref name="Peyer"></ref>. The amount of lactic and acetic acids produced varies from species to species. For example, the referenced study showed that ''L. plantarum'' produces more than twice the amount of lactic acid than ''L. brevis'', and ''L. reuteri'' produced slightly more lactic acid than ''L. brevis''. ''L. reuteri'' produced around twice as much acetic acid than ''L. brevis'', and ''L. plantarum'' produced very little acetic acid. The small amount of acetic acid produced by ''L. plantarum'' in this study was explained by oxygen exposure during sampling, while the obligate heterofermentative species (''L. reuteri'' and ''L. brevis'') produced acetic acid as a direct result of their heterolactic fermentation <ref name="Peyer"></ref>.
Both primary and secondary metabolites play a large role in the flavor and aroma profile of wort fermented with ''Lactobacillus''. Different Secondary metabolite are compounds that are not directly related to 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 thus 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. (To do) Aging has an impact on the aromas and flavors over time and is typically influenced by temperature of the environment, oxygen exposure, and byproducts of fermentation <ref name="Peyer"></ref>.
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