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Flavour Generation - Fermentation a natural route for flavour generation

Q: In which products in lysis of LAB influencing the aroma?

Cheese --> maturation because hydrolysed caseins are further hydrolysed into free amino acids that will act as substrates for subsequent catabolism by the remaining intact cells The hydrolysis of peptides by peptidases from lysed LAB --> affects the taste by reducing the quantity of bitter peptides

56 important questions on Flavour Generation - Fermentation a natural route for flavour generation

What are reasons to use fermented foods?

To increase shelf-life
To make food safer, the micro-organisms prevent the growth of other microbes
Increase the nutritional value of foods beyond that of their raw materials

Mention 5 types of fermentation

Lactic acid fermentation
Alcoholic (ethanol) fermentation
Acetic acid fermentation
Mold fermentation
Alkaline fermentation

Aroma formation consists of a complex series of conversions that can be divided into two subprocesses:

The generation of aroma precursors
The conversion of precursors into the actual aroma compounds

Where does aroma formation take place?

Inside or outside the cell (with the help of extra-cellular enzymes).

Why are starter/mother cultures used?

If fermentation occurs spontaneously (through endogenous microbes on food surfaces or the environment), it can be complex and less predictable and feasible at larger scales. The repeatability is higher when propagating a mother culture. The introduction of a starter culture technology has led to better consistency, safety and quality.

What do lactic acid bacteria do?

They convert fermentable carbohydrates into mainly lactic acid, but also other organic acids such as acetic acid, formic acid and ethanol.

LAB can be split into 2 groups, depending on the pathway they use to metabolise glucose

Homofermentative (1 way)
Heterofermentative (1 way, then split in 2 ways)

What is the difference between the homofermentative and heterofermentative LAB?

Homofermentative: perform glycolysis through the EM-pathway with the addition of lactate dehydrogenase, the lactic acid is the only end product, which generates 2 ATP
Heterofermentative: use the pentose phospohatase pathway (phosphoketolase pathway). The end products of this pathway depend on the conditions. Only generate 1 ATP per glucose molecule

Aerobic conditions: CO2, lactate and acetate are formed
Anaerobic conditions: CO2, lactate and ethanol are formed

Mention 5 metabolic activities/pathways of LAB that make biochemical changes affecting the texture, nutritional quality and aroma of the food matrix.

Proteolysis
Lipolysis
Glycolysis
(Malolactic fermentation)
(Citrate metabolism)

In which products in lysis of LAB influencing the aroma?

Cheese --> maturation because hydrolysed caseins are further hydrolysed into free amino acids that will act as substrates for subsequent catabolism by the remaining intact cells
The hydrolysis of peptides by peptidases from lysed LAB --> affects the taste by reducing the quantity of bitter peptides

What is the most common yeast in fermented beverages, based on fruits, vegetables and grains?

Saccharomyces cerevisiae

Do yeasts grow aerobically or anaerobically?

More effectively aerobically, but a number of yeasts can also grow anaerobically using fermentative metabolism.

What does S. Cerevisiae ferment?

Glucose, sucrose, maltose and raffinose, but not lactose.

What are disadvantages of physical and biological method to remove ethanol?

Physical --> effective, but equipment costs can be high and loss or modification of aroma and flavour compounds during processing is common
Biological --> able to overcome some of limitations of physical methods, but complete removal of ethanol is more difficult

Mention 2 acetic ferments

Vinegar and kombucha

Acetic ferments are produced through a 2-step process

Fermentation phase: sugars from fruits are converted into ethanol by yeast
Respiration phase: ethanol converted into acetic acid by acetic acid bateria --> sour tasting vinegar

Mention 2 major classes of mold-ripened cheese

Surface mold-ripened cheese (e.g. Camembert and brie)
Internal mold-ripened cheese (e.g. Blue-veined cheese, Roquefort and gorgonzola)

Which enzymes are important for the generation of flavour compounds in mold ripened cheese?

Enzymes for proteolysis and lipolysis

Which enzymes can molds produce?

Amylases, proteases and lipases

Which mold is used as starter culture called koji?

Aspergillus oryzae. Koji is used for many Asian ferments such as soy sauce, sake, miso etc.

When are amino acids converted by catabolic reactions to volatile aroma compounds?

The supply of amino acids exceeds the biosynthetic requirements
The amino acid composition of the protein source does not match the amino acid composition of the biomass of fermenting microbes

Which enzymes are involved in the amino acid catabolism?

Decarboxylases
Transaminases
Deaminases
Lyases
Dehydratases

How does amino acid catabolism start?

With the removal of an amino group, performed by aminotransferase. A specific alpha-keto acid is produced from each amino acid.

What kind of flavours do compounds give that are derived from branched-chain amino acids (Leu, Ile, Val)?

Aldehydes: malty
Alcohols: fruity, alcoholic
Acids: sweaty, sour, rancid, rotten, fruity, buttery (depending on amino acid)

What kind of flavours do compounds give that are derived from aromatic amino acids (Phe, Tyr, Trp)?

Rose, flowers, bitter almond, chemical, putrid, faecal

What kind of flavours do compounds give that are derived from sulphuric amino acids (Met, Cys)?

Sulphur compounds: boiled cabbage, potatoes, egg, meaty, garlicy

Which amino acid group is responsible for buttery flavour?

Aspartic acid

During maturation, accumulated free fatty acids (FFAs) can oxidise into flavour precursors and react with other flavour precursors into various volatile aroma compound such as ...

methyl-ketones (and their derivative alcohols)
lactones
aldehydes (that react further into alcohols and acids)

The end product, pyruvate, is an essential intermediate compound for the production of various aroma compounds including ...

acetoin
diacetyl
acetaldehyde
2,3-pentadione

Which metabolism is related to proteolysis?

Amino acid metabolism

Which metabolism is related to lipolysis?

Free fatty acid metabolism

Which micro-organisms are used for malolactic fermentation?

The naturally present LAB and the starter culture Oenococcus oeni

In which food product is malolactic fermentation used?

In wine. Wine is primarily fermented by yeasts, but in certain wines additional malolactic fermentation is performed.

What is the malolactic fermentation?

It is performed by naturally present LAB or starter cultures of Oenococcus oeni. Malic acid, has a sharp taste. By decarboxylating magic acid, lactic acid is produced and the acidity becomes more mellow.

How do holes in cheese occur?

CO2 is produced during citrate metabolism

How are alcohols synthesised?

the synthesis of alpha-keto acids through the catabolic Ehrlich pathway OR
an anabolic pathway for the synthesis of branched-chain amino acids from glucose

Do alcohols have a negative or positive impact on flavour?

Can be both, depends on the composition and concentration. At optimal concentrations they provide a fruity character, but when they are excessively present this can result in a strong, pungent smell and taste.

When are esters produced?

During microbial lipid metabolism and acetyl-CoA metabolism.

By which micro-organisms are fruity volatile esters produced?

Several lactic acid bacteria and yeasts

What flavour descriptors are commonly used to describe esters?

Solvent-like, fruity, candy-like and perfume.

Does fresh milk contain lactones?

No. The lactones are formed either by heating of milk or during ripening of cheeses.

What kind of flavour do lactones give?

Coconut-like, cream, sweet, fruity, buttery and nutty flavours. They are desirable in dairy products.

What kind of flavour does butyric acid have?

A cheesy flavour.

How are fatty acids formed?

Through lipolysis but they are also synthesised by different lactic acid bacteria and filamentous fungi

What is the flavour of methyl-ketones?

Fruity, musty, green and herbaceous

Are sulphites in food desired?

No, they have a negative impact on the sensory quality.

What flavour descriptors are associated with sulphur?

Cabbage, rotten agg, sulfurous, garlic, onion and rubber

How can volatile sulphur contains compounds be generated?

Different pathways

degradation of sulphur containing organic compounds such as the amino acids cysteine, methionine and glutathione
metabolism of inorganic sulphur compounds such as pesticide residues or sulphites in wine
Yeast metabolism can result in the production of sulphides, sulfur containing fusel alcohols, thiols and H2S

In which food are terpenes found?

Both hops and grapes contain monoterpenoids that are important for the aroma of beer and wine respectively due to their low sensory threshold.

What are the main microbial flavour compounds?

Alcohols
Esters
Free fatty acids
Aldehydes
Diketones
Sulphur compounds

What is lactic acid fermentation?

Bacteria convert sugars into lactic acid. Lactic acid bacteria are vital to producing and preserving inexpensive, wholesome foods.

Which type of fermentation is beer?

Ethanol/alcohol fermentation

Which type of fermentation is wine vinegar?

Acetic acid fermentation

What is ethanol/alcohol fermentation?

Yeast cells convert fermentable sugars into alcohol and carbon dioxide.

What does Saccharomyces cerevisiae do?

It converted sugars via pyruvate to ethanol

How can off-flavours of fermentation be solved?

Conversion aldehydes --> alcohols (hexanal to hexanol)

The question on the page originate from the summary of the following study material:

Food Flavour Design

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