lecture 5: lactic acid fermentation

What is fermentation?

The chemical breakdown of a substance by bacteria, yeasts, or other microorganisms

What is lactic acid fermentation?

A metabolic process where glucose or other sugars are converted into cellular energy and the metabolite lactate

How does lactic acid fermentation occur?

Occurs in the absence in oxygen (anaerobically) and is commonly used by certain bacteria, fungi, and animal muscle cells when oxygen is scarce

What kind of process is lactic acid fermentation considered?

Metabolism

How does lactic acid fermentation work?

Through glycolysis, glucose is broken into pyruvate and makes a small amount of ATP

Pyruvate is reduced to lactic acid by lactate dehydrogenase, regenerating NAD+ so glycolysis can continue

Why does lactic acid bacteria convert sugars into lactic acid?

To produce energy (ATP) in the absence of oxygen and create an acidic environment =

What are the two types of lactic acid fermentation?

Homofermentative and heterofermentative

What is the main product of homofermentative lactic acid fermentation?

Lactic acid

What is the main pathway of homofermentative lactic acid fermentation?

Almost all glucose converted to lactic acid

What are some examples of microorganisms that undergo homofermentative lactic acid fermentation?

Lactobacillus delbrueckii, Streptococcus thermophilus

What are the main products of heterofermentative lactic acid fermentation?

Lactic acid + CO2 + ethanol or acetic acid

What is the main pathway of heterofermentative lactic acid fermentation?

Glucose partially converted to lactic acid plus other products

What are some examples of microorganisms that undergo heterofermentative lactic acid fermentation?

Leuconostoc spp, Lactobacillus brevis

First step of lactic acid fermentation

Sugars from milk, plants, or other ingredients are available to lactic acid bacteria

Second step of lactic acid fermentation

Lactic acid bacteria transport sugars into the cell

Third step of lactic acid fermentation

Sugars are broken down to pyruvate, producing a small amount of ATP

Fourth step of lactic acid fermentation

Pyruvate is the key intermediate produced

Fifth step of lactic acid fermentation

Pyruvate us reduced to lactic acid, regenerating NAD+ so glycolysis can continue

Sixth step of lactic acid fermentation

Lactic acid accumulates in the fermentation medium, lowering pH

Seventh step of lactic acid fermentation

Lactic acid bacteria gain energy (2 ATP per glucose) and grow under anaerobic conditions

What is homolactic fermentation?

Process of glycolysis that breaks glucose into only lactic acid as an end product

What are some common products associated with homolactic fermentation?

Dairy: yogurts, sour creams, cheeses

What are some common microorganisms associated with homolactic fermentation?

Lactobacillus acidophilus, Lactococcus lactis, Streptococcus thermophilus

What is heterolactic fermentation?

Process of glycolysis that breaks glucose down into lactic acid along with other compounds like ethanol and CO2

What are some common products associated with heterolactic fermentation?

Fermented veggies: sauerkraut, kimchi, pickles

Beverages: beers and wines

What are some common microorganisms associated with heterolactic fermentation?

Leuconostoc mesenteriodes, Lactobacillus brevis, Weissella species, Oenococcus oeni

What microorganism is a genus of lactic acid bacteria and is widely used in the food industry due to fermentation metabolism, probiotic benefits, and ease of use?

Lactobacillus

What does Lactobacillus's morphology look like?

Rod-shaped, non-spore forming, non-motile, gram-positive

What does Lactobacillus's metabolic pathway look like?

Both homofermentative and heterofermentative, produces lactic acid, ethanol, acetic acid, and CO2, ferments carbohydrates

What does Lactobacillus's growth conditions look like?

Facultatively anaerobic/aerotolerant, acid tolerant (pH 4-6), species dependent: mesophilic temperature 25-40 C, competitive

Common Lactobacillus species

Lactobacillus acidophilus (in a lot of dairy products)

Lactobacillus plantarum

Lactobacillus casei

Lactobacillus rhamnosus

Common foods associated with Lactobacillus

Yogurt, kefir, sourdough, fermented pickles, cheeses, fermented sausages, sauerkraut, kimchi

What microorganism is a genus of lactic acid bacteria and is important in fermented vegetables and some dairy fermentations and contributes to acidification, flavor development, CO2 production, and microbial competition?

Leuconostoc

What does Leuconostoc's morphology look like?

Coccus, chains or clusters, non-spore forming, non-motile, gram-postive

What does Leuconostoc's metabolic pathway look like?

Obligately (strict) heterofermentative, sugars produce lactic acid + CO2 + ethanol and/or acetate, many species tolerate salt and are important for vegetable fermentation, produces important flavor/aroma compounds, including diacetyl in some strains

What does Leuconostoc's growth conditions look like?

Facultatively anaerobic/aerotolerant, acid tolerant (pH 5.5-6.5), mesophilic temperature 20-30 C, competitive

Common Leuconostoc species

Leuconostoc mesenteroides

Leuconostoc citreum

Lenconostoc kimchii

Common foods associated with Leuconostoc?

Sauerkraut, kimchi, pickles

What microorganism is a genus of lactic acid bacteria and is widely used in the fermented meat industry due to the development of tangy flavors?

Pediococcus

What does Pediococcus's morphology look like?

Coccus, pairs, triads, or clusters, non-spore forming, non-motile, gram-postive

What does Pediococcus's metabolic pathway look like?

Homofermenatative, sugars produce primarily lactic acid, rapid acidification, some strains produce bacteriocins (pediocin)

What does Pediococcus's growth conditions look like?

Facultatively anaerobic/aerotolerant, acid tolerant (pH 5.5-6.5), mesophilic temperature 25-35 C, synergistic

Common Pediococcus species

Pediococcus acidilactici

Pediococcus pentosasceus

Pediococcus damnosus

Common foods associated with Pediococcus

Salami, pepperoni, fermented chorizo, summer sausage, some fermented vegetables

What microorganism can have species that occur naturally in foods, can contribute to fermentation and flavor development, and is unusual among food-associated lactic acid bacteria because some strains are opportunistic pathogens and may carry clinically important antibiotic-resistance or virulence traits?

Enterococcus

What does Enterococcus's morphology look like?

Cocci, pairs or short chains, non-spore forming, non-motile, gram-positive

What does Enterococcus's metabolic pathway look like?

Homofermentative, converts sugar into lactic acid, some species/strains may contribute to food spoilage, some strains produce bacteriocins (enterocins)

What does Enterococcus's growth conditions look like?

Facultatively anaerobic/aerotolerant, tolerates a broad pH range, many strains grow at pH 9.6, many species can grow from 10-45 C (Optimum: 35-37 C), hardy/stress tolerant

Common Enterococcus species

Enterococcus faecalis

Enterococcus faecium

Enterococcus durans

Common foods associated with Enterococcus

Traditional/artisanal cheeses

Raw milk cheeses

Mediterranean-style cheeses

Fermented sausages

Some fermented vegetables