there are many different ways that bacteria can undergo fermentation we're going to focus on two main types and this slide does a nice job comparing the two types of fermentation so let's take a quick look at this and then we'll focus on lactic acid fermentation now here's our glucose molecule going through glycolysis to form pyruvate or pyruvic acid and each time it does that it's generating lots of lovely ATP that the cell can use for energy but it's also producing NADH and remember that if you have oxygen present and the cell can undergo aerobic metabolism the NADH is terrific because you can use it in the electron transport chain to make ATP but when you're undergoing fermentation and there's no oxygen this NADH really is just a waste product of glycolysis and the cell needs to regenerate NAD+ or it won't be able to keep doing glycolysis if it doesn't have any Deni d-plus available so the process of fermentation is really just a mechanism by which cells add a step on to glycolysis to take NADH and convert it back to nad plus one of the ways it can do that is it can take pyruvate or pyruvic acid and it can reduce pyruvic acid to lactate and so when one molecule gets reduced another molecule gets oxidized and voila we have nad plus that can go back and allow the cell to continue to undergo glycolysis and make all these ATP for the cell the other major mechanism we're going to look at is alcoholic fermentation or alcohol fermentation and this is where you take pyruvate and it's actually two steps the first thing we're gonna do is decarboxylate pyruvate to a molecule called acetaldehyde and then we're going to reduce acetaldehyde to ethanol and when we reduce acetaldehyde to a smell we oxidize NADH back to a nad plus and now nad plus is available to let us undergo more fermentation me more glycolysis and produce more ATP for the for the cell I stole this picture from the Khan Academy website I really like it here's glucose it's forming pyruvate and they do a really nice job just showing that this entire step from pyruvate to lactate is really just to regenerate NAD+ and then the nad plus goes back and it can participate in repeated rounds of glycolysis lactate is lactic acid fermentation for example in your muscles it's the final product it still contains a lot of energy so in your body your liver will actually the lactate will go into the bloodstream it'll go back to the liver and the liver will convert it back to pyruvate and then you'll be able to use the pyruvate again in cellular respiration this usually happens after you've cut up on your oxygen debt and you've done breathing hard and there's plenty of oxygen for the cells to to undergo aerobic metabolism lots of bacteria used lactic acid fermentation streptococcus lactobacillus bacillus and we see this in fermented foods if you've ever had yogurt one of the ways people make yogurt is they add a lactobacillus culture to warm milk and that allows the yogurt to to become acidified from the lactic acid it actually helps preserve it it gives it a little bit of a sour taste people don't like the sour taste so the tons of sugar to yogurt but that makes it not healthy unfortunately because it really is yummy the other fermented foods are things like sauerkraut and pickles chocolate is a fermented food you know who knew when you when you add when you add acid or you ferment food so that it produces the bacteria produce acid it's actually a way of preserving the food refrigeration is a fairly new phenomenon maybe 100 years old but people have been using fermentation as a way to preserve food for centuries and if you think back you know people used to have these beautiful gardens in the summer and then the winter their grounds froze and they can't grow any food you know you would you would take all the extra food you made in this in the summertime and you would pickle it right you can it jar it so that you have a you don't starve in the wintertime now fermentation can also spoiled foods so remember Louis Pasteur and he used pasteurization to prevent wine from being spoiled by vinegar producing bacteria when you apply heat and when you pasteurize wine or milk what you're doing is you're killing the microorganisms that would ferment and this stops the fermentation process and kills the bacteria that might add this acid and spoil the wine so Custer was a big hero to the wine industry as you can probably imagine all right so I'm going to put this video into the next little video section and then we'll come back and talk about alcoholic fermentation