2. Fermentation

Different types of processes by secreted enzymes acting on substrate

• Proteolysis: digestibility, flavor and taste

• Lipolysis: flavor

• Amylolysis: digestiblity, acid formation

• Pectinolysis: digestibility

• Cellulolysis: digestibility

What kind of metabolites do microorganisms produce?

• Flavor compounds

  • acids, alcohol, esters, acetoin

• Coloring substnaces

  • Carotenoids

• Vitamins

  • B-vitamins

  • Yeast extract

• Antimicrobial compounds

  • Acids, alcohols, carbon dioxide

  • Hydrogen peroxide

  • Reuterin, bacteriocins, killer-toxins

Conversion of carbohydrates into lactic acid

Produces:

• yoghurt

• cheese

• vegetables

• sausages

not efficient; not all the substrate is used

Types of lactic acid bacteria

1. Homofermentative: can switch between the following types of fermentation

   • Homolactic fermentation → only produces lactate as an end product.

   • Mixed acid fermentation

2. Heterofermentative

Homo-fermentation vs. heterofermentation

no CO2 in homo

How to ferment alcohols

Use yeast

Steps of alcoholic fermentation

1. EM pathway: common glycolysis.

   • EM pathway is found in: bacteria & yeast.

   • Net yield: 2ATP, 2NADH, 2pyruvate

2. EtOH production: pyruvate reduction

   • Net yield: 2CO2, 2NAD+, 2ethanol

Fusel alcohols

• Alcohols with a higher number of C atoms

• Whisky, brandy, and rum rely on fusel alcohols (and their esters) for depth, body, and fruity/spicy notes.

• Isoamyl alcohol → used to make isoamyl acetate (“banana oil”) for flavorings and fragrances.

How are fusel alcohols made?

• Following the Ehrlich pathway

  • Transamination
    → The amino acid loses its nitrogen (–NH₂) group.
    → This turns it into a keto acid.

  • Decarboxylation
    → The keto acid loses a carbon (as CO₂).
    → What’s left is an aldehyde.

  • Reduction
    → The aldehyde is reduced (adds hydrogen) using NADH.
    → That turns it into a fusel alcohol.