Lacto converts lactic acid?

[aeviaanah]

Most of us know yeast convert sugars to alcohol. What exactly is going on when lactobacillus produce lactic acid?

 

[Gnomebrewer]

They break down glucose molecules in to lactate (lactic acid when in solution), which releases energy. It's the same reaction that our muscles can use when there's a higher energy need than respiration can provide (i.e. not enough O2).
C6H12O6 --> 2C3H6O3 + energy

 

[aeviaanah]

They break down glucose molecules in to lactate (lactic acid when in solution), which releases energy. It's the same reaction that our muscles can use when there's a higher energy need than respiration can provide (i.e. not enough O2).
C6H12O6 --> 2C3H6O3 + energy

Interesting, would this result in a drop of OG?

 

[Gnomebrewer]

Interesting, would this result in a drop of OG?

Yes and no.

Short answer: maybe a point to a few points of gravity, nothing significant.

Long answer:
With fermentation, CO2 is given off which makes the resulting beer less dense. With lactic acid fermentation, no gas is given off so mass of the beer stays the same. But lactate has a slightly lower density than glucose so there would be an extremely minimal gravity (density) change, which should be an increase! It doubt it would be measurable with a hydrometer.

But, because sugar is consumed, a refractometer might register a change in 'gravity' which is actually a change in sugar content. I'm not sure how or if lactic acid affects refractometers.

Also, some lactic acid bacteria can produce small amounts of ethanol, which might register a drop of up to a few points.

 

[VikeMan]

Interesting, would this result in a drop of OG?

My kettle soured worts tend to drop about 2-3 gravity points. This is with Lactobacillus plantarum, a "facultative" heterofermentative species, which means that it can produce alcohol/CO2 under certain conditions. But because beer wort contains a lot of sugar, it normally doesn't make alcohol/CO2, which is a mode used when sugar is scarce.

Even "obligate" heterofermentative species, which "always" produce alcohol/CO2, tend not to produce very much in beer wort. That may be because brewers tend to halt the process (by boiling) once the pH has dropped into the desired soured range, but also because the drop in pH slows down the bacteria. I suspect the latter is true, because attempts to do complete fermentations with "obligate" heterofermentative" species have failed. The only attempts that seem to have succeeded were strongly suspected to have (inadvertently) also contained wild yeast.