Changes

''Lactobacillus'' generally prefers glucose, fructose, and maltose, and does not ferment maltotriose. Some species may prefer certain types of sugars over others, for . 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 . 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]].

Both primary and secondary metabolites play a large role in the flavor and aroma profile of wort fermented with ''Lactobacillus''. Secondary metabolite metabolites 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 flavorsand aromas (compare this to food grade lactic acid in which none of these secondary metabolites exist).

An example from one study showed that ''L. plantarum'' produced significantly more diacetyl, acetoin, and acetaldehyde than ''L. reuteri'' and ''L. brevis''. These three compounds were associated with dairy-related notes of "buttery", "lactic", and "yogurt" flavors identified during sensory testing <ref name="Peyer"></ref>. Some LAB can release these compounds through the catabolism of citric acid, which is found in wort. Ester production is generally low, although significant ester formation has been found during malolactic fermentation in red wines. Some strains may also produce fusel alcohols and other off-flavors. For example the referenced study found an accumulation of the fusel alcohol n-Porponal in the sample of ''L. reuteri'', and a small decrease of isovaleric acid coupled with a small increase of [https://en.wikipedia.org/wiki/Hexanoic_acid hexanoic acid] by ''L. brevis'', ''L. plantarum'', and ''L. reuteri'' (To doonly 0.25-0.32 mg/L was found, and the flavor threshold of hexanoic acid is 5.4 mg/L <ref>[http://www.leffingwell.com/odorthre.htm Leffingwell & Associates website. Odor Thresholds. Retrieved 12/30/2015.]</ref>) <ref name="Peyer"></ref>. Heterofermentative species can also produce [[Tetrahydropyridine]], which is the cause of "mousy" off-flavors <ref name="Costello">[http://pubs.acs.org/doi/abs/10.1021/jf020341r Mousy Off-Flavor of Wine:  Precursors and Biosynthesis of the Causative N-Heterocycles 2-Ethyltetrahydropyridine, 2-Acetyltetrahydropyridine, and 2-Acetyl-1-pyrroline by Lactobacillus hilgardii DSM 20176. Peter J. Costello and Paul A. Henschke. 2002.]</ref>. Aging has an a large impact on the aromas and flavors produced by ''Lactobacillus'' fermentation over time and is typically influenced by temperature of the environment, oxygen exposure, and the byproducts of fermentation . Generally, fermentation has a positive effect on preserving some aroma and flavor compounds. Other compounds may change, causing aroma and flavor changes. For example, one study characterized wort freshly fermented with ''L. plantarum'' as "butter" and honey", and when aged as "yogurt" and "sour". In the same study, ''L. reuteri'' was characterized as "sour" when fresh, and "honey" and "pungent" when aged. ''L. brevis'' was characterized as "soy sauce" when fresh, and "yeasty" and "cider" when aged <ref name="Peyer"></ref>.