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All metabolism by Lactobacillus, including growth, will require sugar to be consumed and lactate (lactic acid) to be produced. Two categories of metabolism exist, '''homolactic''' and '''heterolactic'''. ''Homolactic'' metabolism is described as the cell catabolizing one molecule of glucose to yield two molecules of pyruvate, which is then further reduced to two molecules of lactate (lactic acid). ''Heterolactic'' metabolism is described as the cell catabolizing one molecule of glucose into one molecule of CO2, one molecule of glyceraldehyde phosphate, and one molecule of acetyl phosphate. The molecule of glyceraldehyde phosphate is reduced to one molecule of lactate, and the acetyl phosphate is reduced to one molecule of ethanol (or one molecule of acetic acid instead of ethanol, depending on its growing environment <ref name="Raunak">[https://raunakms.wordpress.com/2011/01/30/lactic-acid-bacteria/ Lactic Acid Bacteria. Raunak Shrestha. Retrieved 6/7/2015.]</ref>). ''Homolactic'' fermentation only allows the fermentation of hexoses (glucose), while ''heterolactic'' fermentation allows the fermentation of hexoses and pentoses <ref>[https://books.google.com/books?id=eZjIfud742wC&pg=PA33&lpg=PA33&dq=facultative+heterofermentative&source=bl&ots=QQYpzpsrvC&sig=kkyP7wUjgWiE2UV2qkIaRyxMMGA&hl=en&sa=X&ei=K_d0VYDJPLX9sATv_IXgAQ&ved=0CDIQ6AEwBDgK#v=onepage&q=facultative%20heterofermentative&f=false Handbook of Dough Fermentations. Karel Kulp, Klaus Lorenz. CRC Press, May 20, 2003. Pg 33.]</ref>. In summary, ''homolactic'' fermentation produces only lactic acid, while ''heterolactic'' fermentation produce lactic acid, CO2, and ethanol/acetic acid <ref>[http://textbookofbacteriology.net/lactics_2.html Todar's Online Texbook of Bacteriology. Kenneth Todar, PhD. Retrieved 05/06/2015.]</ref>.
There are three categories of LAB ''Lactobacillus'' based on the type of fermentation they are capable of (''homolactic'', ''heterolactic'', or both): '''obligatory homofermentative''', '''obligatory heterofermentative''', and '''facultatively heterofermentative'''. ''Obligatory homofermentative'' bacteria only perform ''homolactic'' fermentation, and thus only produce lactic acid. ''Obligatory heterofermentative'' bacteria only perform ''heterolactic'' fermentation, and thus produce lactic acid, CO2, and ethanol (or sometimes acetic acid instead of ethanol). ''Facultatively heterofermentative'' bacteria generally are ''homolactic'' when there is an abundance of carbohydrates, but can also perform ''heterolactic'' fermentation when carbohydrates are not abundant <ref>[https://books.google.com/books?id=6SLB1j0rX78C&pg=PA76&lpg=PA76&dq=what+is+obligatory+facultatively+heterofermentative&source=bl&ots=6E4DB8Px_X&sig=PXZTNO4fB3ifCF_qAl5_I4euKmk&hl=en&sa=X&ei=Z-10VZqvFIWyggTInIHYBw&ved=0CCsQ6AEwAw#v=onepage&q=what%20is%20obligatory%20facultatively%20heterofermentative&f=false Fermentation: Effects on Food Properties. Bhavbhuti M. Mehta, Afaf Kamal-Eldin, Robert Z. Iwanski. CRC Press, Apr 12, 2012. Pg 76.]</ref>.
Other factors can determine if a ''facultative heterofermentative'' species uses ''homolactic'' or ''heterolactic'' fermentation. For example, L. plantarum, which is a ''facultative heterofermentative'' species, is ''homolactic'' without the presence of oxygen. In the presence of oxygen, however, it performs ''heterolactic'' fermentation, and produces acetic acid <ref>[https://microbewiki.kenyon.edu/index.php/Lactobacillus_plantarum_and_its_biological_implications Lactobacillus plantarum and its biological implications. Microbe Wiki. Retrieved 6/7/2015.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1083842231643990/?comment_id=1084922688202611&offset=0&total_comments=28&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Lance Shaner about L. plantarum on MTF. 6/7/2015.]</ref>.