• Biochemistry is Your Friend. For Reals!

    by  • September 16, 2011 • Science • 6 Comments

    If you mention fermentation to the average person, it usually conjures up images of beer. However, in the science world fermentation is the process by which organisms get energy from the environment. These biochemical pathways existed long before we took advantage of their waste products: alcohol and lactic acid. In cheesemaking, we’re interested in the production of lactic acid to coagulate our milk to form curds.

    Fermentation is the process that breaks down the nutrients taken in to the cell and converts them to ATP, which is the most common and valuable molecular form of energy used by all living organisms. The typical starting products for fermentation are sugars like glucose, sucrose, and in the case of cheese, lactose. However, cells can also make ATP from fatty acids and other organic molecules too.

    Glucose is the cash of the molecular energy world. It’s a six-carbon sugar that is used as the starting material for ATP generation, and it requires no further processing in order to get it going down the path of fermentation. Since it’s only composed of one molecule, it’s called a monosaccharide. Sucrose and lactose, on the other hand, need to be broken down first because they are disaccharides. That is, they are composed of two sugar subunits:

    When sucrose is broken down, you get one molecule of glucose and one molecule of fructose. Lactose is broken down in to one molecule of galactose and one molecule of glucose. Since glucose is the preferred starting material in the generation of ATP, we’ll focus on that first.

    In order to make ATP, glucose must first be cleaved in to two molecules of pyruvate in a process called glycolysis.

    Isn't this rad?The answer is yes.  Yes it is rad.

    This process occurs in humans, too! However, human cells are also capable of a process called aerobic respiration, of which pyruvate is the starting material. It requires oxygen and produces a lot of ATP and other molecular energy currency such as NADH.

    Our little cheesemaking bacteria are not so lucky. They rely entirely on the fermentation process for making ATP. The production of lactic acid (in the case of lactic acid fermentation) or alcohol (from alcohol fermentaion) from pyruvate is done entirely to make NAD+, which we need for the fermentation process.

    So how is the same process able to deliver us delicious alcohol on one side and tasty cheese on the other? It’s all in the biochemistry.

    Lactic Acid Fermentation
    Lactic acid fermentation is the most basic type of fermentation. When we break glucose in to the two molecules of pyruvate to generate ATP, we transfer electrons to NAD+ in order to make NADH. However, there isn’t a lot of NAD+ hanging around in the cell. Luckily, with the help of the enzyme lactate dehydrogenase, pyruvate accepts electrons directly from NADH in order to generate the NAD+ that we need to break down more glucose. So you see, the balance of NADH and NAD+ can remain relatively constant in the cell while we crank out as much ATP as the incoming glucose allows. Rad!

    In the simplest form of lactic acid fermentation, called homolactic fermentation, the two molecules of pyruvate are converted in to two molecules of lactate (lactic acid). Some organisms are also capable of heterolactic fermentation, which results in the production of one molecule of lactate, one molecule of ethanol, and one molecule of CO2.

    Lactic acid fermentation occurs in bacteria and also in our cells as well. You might be familiar with the burning sensation you get when you work out really hard. That’s a result of your muscles not getting enough oxygen. So instead of using the aerobic respiration seen above that generates tons of ATP, your muscles pack it in and can only use glycolysis. The byproduct – lactic acid – is what causes the burn. If our muscles instead chose to undergo alcohol fermentation, we would get drunk exercising. Science!

    In cheesemaking, the production of lactic acid by bacteria is one of the ways in which we get curds to form. This is the primary method of coagulation for chèvre and cream cheese, for instance. It’s also the reason that we say milk goes “sour” when we’ve kept it in the fridge for too long. Not that any of us has ever done that, of course.

    Alcohol Fermentation
    Some bacteria and yeasts can undergo alcohol fermentation. While production of lactic acid relies on only one enzyme, lactate dehydrogenase, alcohol fermentation requires two: pyruvate decarboxylase and alcohol dehydrogenase. Pyruvate decarboxylase converts pyruvate in to an intermediate, called acetaldehyde, which is then converted in to alcohol (ethanol) and carbon dioxide by alcohol dehydrogenase.

    An overview of both processes, collectively known as anaerobic metabolism looks like this:

    Interestingly enough, alcohol dehydrogenase is the same enzyme that your body uses to break down alcohol when you drink it.

    Now if we put together the entire process, from glucose to lactic acid, in to one figure it looks something like this:

    Biochemistry Truly is Your Friend
    When all is said and done, the same processes that lead to the making of cheese and beer are the same as those that lead to making energy for you to run – or in my case, make cheese. I take that as a sign that we should be drinking lots of beer and eating lots of cheese. Do it for science!

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    6 Responses to Biochemistry is Your Friend. For Reals!

    1. Chef Danahy
      July 23, 2013 at 10:50 pm

      This was a great article! I loved the humor and the diagrams were fantastic. I know this was more about the biochemistry, but I wish it had continued further into the processes of various cheese making techniques/procedures….basically I wanted to keep reading.
      I’m sharing this with my students whether they like it or not! (haha)

      • Angel
        July 25, 2013 at 3:04 pm

        Thanks so much! I’ve been on quite a long hiatus (since I’m writing my dissertation), but rest assured I will have some new content coming along soon. Are there any particular aspects of cheesemaking that you’d be interested in learning? I’m never really sure what people would like to learn about, so I do my best to guess. Your feedback would be greatly appreciated!

    2. Samantha
      February 1, 2014 at 6:34 pm

      I would love to use this article to do a lab with my students about macromolecules and the funtion of enzymes but I’m trying to understand the cheese making process enough to figure out where it fits in. My understanding is when the milk is denatured by heating and then something happens to the casein micelles so that the fat and proteins bond in the curd and sugars (lactose) and water are released in the whey. Is the “lactic acid fermentation” what happens to the casein micelles or am I connecting things incorrectly?

      Thanks for your help and for writing this article!

      Samantha

      • Angel
        December 5, 2014 at 6:09 pm

        Hi Samantha,

        I’m sorry this took SO long to reply – I’ve had a lot of spam!

        Lactic acid fermentation occurs inside bacteria and yeast, and it is mediated by the enzymes listed above. Fermentation is what produces the acid that causes the fat and proteins to “bond” in the curd. I cover this in my article on coagulation, so you should take a look. Basically, a high enough acid content will make the micelles associate with each other instead of remain suspended in the milk – at which point you get curds. You can also make curds by heating up the milk so that the proteins on the micelles denature, but at that point you’ve killed all the microorganisms in the milk as well.

        If you want to discuss this in more detail, please let me know! I’d love to help you put together a lab.

    3. Emily
      August 21, 2014 at 4:34 pm

      Hi there! I know this is a bit of a delayed response but I’ve been reading and enjoying your posts tremendously! I was curious, do you know anything about reduced fat cheese biochemistry? I’m interning in a cheese factory, I’m a food science major with a fermentation focus, and I’ve heard a lot of talk about how the sensory quality is very bad in low/reduced fat cheeses. Just curious as to how a lack of fat affects the ripening process, figured it was worth an ask!

      • Angel
        December 5, 2014 at 5:48 pm

        Hey Emily,

        Sorry for the delay – I didn’t see this comment buried in all the spam! The reason that the sensory quality is low in low fat cheeses has to do with the process of lypolysis, or the breakdown of the fat. As they break down, they produce tons of fatty acids. Some cheeses are capable of having hundreds of different fatty acids produced from the different bacteria that inhabit them. As a cheese ages, you get further breakdown of the fatty acids and generation of an amazingly complex array of scents and flavors. When there’s little fat to begin with, there will be less of these fatty acid byproducts in the cheese compared to a whole-fat variety. It won’t make much difference in quick-aged cheeses like ricotta or direct-acid mozzarella (except for yield), but for aged cheeses there with be a huge, cumulative difference.

        I hope that answers your question!

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