Question

Briefly describe one microbial fermentation. What does fermentation recycle to keep glycolysis functioning?

Answer 1

Ans: Fermentation is a catabolic process that makes a limited amount of ATP from glucose without an electron transport chain and that produces a characteristic end product, such as ethyl alcohol or lactic acid i.e., fermentation is the incubating raw materials with microbes for food production and preservation.

Essential components of fermentation: microbial growth, need abundant carbon and energy sources that are accessible to microbes, appropriate environment (pH, temp, water etc)

In homolactic fermentation one molecule of glucose is ultimately converted to two molecules of lactic acid.

Glucose --> 2 pyruvate --> 2 lactate (lactic acid)

In heterolactic fermentation: glucose --> pyruvate, ethanol, CO2 ---> lactic acid

The main role of microbes in food fermentation is to produce chemicals that limit the growth of other organisms in food.

Yeasts and lactic acid bacteria are used for fermenting foods. They are able to grow at a low pH. Lactic acid bacteria and facultative yeasts will also grown under anaerobic conditions. Lactic acid bacteria and facultative yeasts will often occur together in fermented foods. Lactic acid bacteria are gram positive, non-spore forming rods or cocci. Tend to be aerotolerant anaerobes.

The two pathways used by lactic acid bacteria to ferment carbohydrates are:

1. Homofermentation - results in the single product of lactic acid.

2. Heterofermentation - equimolar amounts of lactic acid, ethanol or acetate and CO2 are produced.

Homofermentation occurs via the Emden-Meyerhof-Parnas (EMP) glycolic pathway. 6-carbon glucose is phosphorylated and isomerized, cleaved by aldolase and converted to pyrucate. To regenerate NAD+, pyruvate is reduced to lactate. Lactate is the single end product. 2 ATP molecules are produced per molecule of glucose

Heterofermentation uses the 6P-gluconate and phosphoketolase pathways. The 6-C glucose is transformed into a 5-C pentose by oxidation and decarboxylation. Pentose is then cleaved into glyceraldehyde phosphate and acetyl phosphate. Acetyl phosphate is reduced to ethanol and CO2 is produced in the absence of oxygen. Lactate, ethanol or acetate and CO2 are produced in equimolar amounts. One ATP molecule is produced per molecule of glucose in the absence of oxygen. Acetate plus ATP is formed from acetyl phosphate in the presence of oxygen.

Also buttermilk is a type of fermented milk produced by the fermentaion of skimmed or partially skimmed milk. Utilizes a mixture of starter cultures including Lactococcus lactis which acts as a lactic acid producer, Lactococcus lactis subsp Lactis and Leuconostic mesenteroides subsp. Cremoris which produces the diacetyl which gives the buttery flavour found in buttermilk. Citrate is often added as it can be metabolized to form diacetyl to ensure buttery flavour.

The goal of fermentation is to recycle NADH and enables glycolysis to keep functioning. Fermentation keep Glycolysis functioning: produces NAD+ which is what glycolysis needs to function. Fermentation removes electrons from NADH and recycles NADH+ molecules for Glycolysis

Fermentation is important because glycolysis like cellular respiration, needs a molecule that picks up electrons. Glycolysis needs molecules of NAD+

The steps of Fermentation: Pyruvate and NADH from glycolysis enter the fermentation process. 2 NADH molecules provide energy to provide pyruvate into Lactic Acid . As the NADH is used, it is converted back into NAD+. 2 molecules of NAD+ are recycled back to Glycolysis. The recycling of NAD+ allows glycolysis to continue.