Lecture 5 (W6) : Biosynthesis of flavour in fruits and vegetables

Question 1

Fruits

What metabolic process are flavour compounds biosynthesised in fruits?

Answer 1

Flavour compounds are synthesised in fruits during respiration, where substrates are catabolised into products such as volatiles.

Question 2

Fruits

What 4 subtrates can be metabolised into flavour compounds in fruits?

Answer 2

1. Lipids (fatty acids) -> lipoxygenase rxn, beta oxidation, lipolysis
2. Protein (amino acids) : -NH₂ and -CO₂ to give aldehydes
3. Sugars
4. Glycosylated aromatic compounds

Question 3

Fruits

What 3 reactions can fatty acids undergo to generate aroma compounds in fruits?

Answer 3

1. Lipoxygenase reactions : generates lipid hydroperoxides → aldehydes / ketones / carboyxlic acids → alcohols
2. Beta oxidation of fatty acids : generates (short chain) fatty acyl-CoA, precursors for esters
3. Alcoholysis : fatty acyl CoA + alcohol → esters

Question 4

Fruits

In lipoxygenase reaction of fatty acids, list out the key steps and key enzymes involved.

Answer 4

1) Unsaturated fatty acids → fatty acid hydroperoxide (R-O-OH)
- catalysed by lipoxygenase

2) Lipid hydroxperoxide, ROOH → aldehyde / ketone / carboxylic acid
- catalysed by hydroperoxide lyase (cleaves C-C bonds in ROOH).

3) Aldehyde / ketone/ carboxylic acid → alcohols
- catalysed b alcohol dehydrogenase, in the presence of NADH (NADH is oxidised into NAD⁺ while reducing the substrate

recall from LSM: dehydrogenase catalyse transfer of e- from electron carrier to substrate

Question 5

Fruits / veg - fatty acids

What flavour compounds (volatiles and non-volatiles) can be formed from lipoxygenase reactions? [4]

Answer 5

1) Aldehydes / ketones
Aldehydes can be subsequently converted to….
2) Alcohols [R]
3) Carboxylic acids [O]
4) Esters (reaction of alcohols and acetyl CoA)

Note : esters are only for fruits

Question 6

Fruits / veg - fatty acids

What are the 2 common types of lipoxygenases in fruits, and what aldehydes / alcohols do they produce (C chain length)?

Answer 6

• 9-lipoxygenase : produces C9 aldehydes / alcohols
• 13-lipoxygenase : produces C6 aldehydes / alcohols

Question 7

Fruits / veg - fatty acids

9-lipoxygenase produces C9 aldehydes/alcohols, present in what kind of fruit / vege? [1]

Answer 7

Cucumber

Question 8

Fruits / veg - fatty acids

13-lipoxygenase produces C6 aldehydes/alcohols, present in what kind of fruit / vege?

Give examples of C6 aldehydes/alcohols, and what is their odour characteristic?

Answer 8

Apple/tomato
- C6 aldehydes / alcohols : hexanal / hexanol ;; responsible for green flavours

Question 9

Fruits - fatty acids

Beta oxidation of fatty acids involves removal of ____ carbon atoms from the fatty acid chain.

Answer 9

2

Question 10

Fruits - fatty acids

Beta oxidation can produce shorter chain carboxylic acids, but its intermediate, fatty acyl CoA, can undergo what reaction to produce what flavour compound?

Answer 10

Alcholysis with alcohols to form esters.

Question 11

Fruits - amino acids

What 3 main flavour compounds are synthesised from amino acids in fruits?

Answer 11

1. Alcohols
2. Carboxylic acids
3. Esters - alcohol + fatty-acyl CoA

Derived from aldehydes (unstable intermediate)

Question 12

What are the 2 main routes / pathways for synthesis of aldehydes from amino acids in fruits?

Answer 12

1. Decarboxylation of AA to give amines → oxidative deamination of amine to give aldehydes
2. Transamination of AA to give α-keto acids → decarboxlation to get rid of -COOH group to form aldehydes

AA → aldehyde requires removal of NH₂ and C atom (decarboxylation), its j a matter of which comes first

Question 13

What are glycosides and glucosides?

Answer 13

Glycosides are compounds where a non-sugar group is attached to a sugar group ;; while glucosides is a specific type of glycoside, where a non-sugar group is attached to glucose

Question 14

Sugars (simple sugars, glycosylated compounds) in fruits are used to synthesise what kinds of flavour compounds?

Answer 14

Monoterpenes (2 isoprene units, 10C)
- Terpenes are volatile flavour compounds

Question 15

What is the difference between monoterpenes, sesquiterpenes and diterpenes? Which type of terpene is most commonly found in fruits?

Answer 15

Monoterpenes - made up of 2 isoprene units (2 x 5C = 10C)

Sesquiterpenes - made up of 3 isoprene units (15C)

Diterpenes - made up of 4 isoprene units (20C)

• Monoterpenes is most commonly found in fruits

Question 16

Terpenes are most commonly found in what kinds of fruits?

Answer 16

Citrus

Question 17

Give examples of common terpenes [3]

Answer 17

1. Limonene - most common terpene in nature
2. Linalool
3. α-pinene

Question 18

What is the difference between terpenes and terpenoids?

Answer 18

Terpenes consist of only hydrogen and carbon (hydrocarbons), while terpenoids are oxgenated terpenes, with additional oxygen atoms (e.g. terpene aldehyes, alcohols, esters…)

Question 19

Both terpenes and terpenoids are made up of ____ units, with the chemical forumla____.

Answer 19

Isoprene ;; C₅H₈

Question 20

What is the building block for isoprene?

Answer 20

Isopentenyl diphosphate / isopetenyl pyrophosphate (IPP)

IPP must first be isomerized into dimethylallyl diphosphate (DMAPP), and then through a series of enzymatic reactions, the DMAPP molecule loses a diphosphate group to produce isoprene

Question 21

How does :
1. Glycosylated compounds (glucosides / glycosides)
2. Glucose

form monoterpenes in fruits?

Answer 21

1. Hydrolysis of monoterpene glycosides / glucosides. (The glycosylated compounds are monoterpnes covalently bonded to sugars, so the glycosidic bond is cleaved to release the monoterpenes.)
2. Glucose is converted into isopentenyl diphosphate (IPP) through various pathways. IPP is used to synthesise isoterpenoids, which are then converted into terpenes. (glucose → acetyl CoA → … → IPP →… → monoterpenes)

Question 22

Glycosidically bound aromatic compounds are found in fruits such as? [3]

Answer 22

Grapes, mango, pineapple

Question 23

Aromatic compounds can be released from glycosidically bound aroma compounds by what 3 methods?

Answer 23

Hydrolysis by…
1. Enzymes (glycosidase / glucosidase)
2. Acid (chemical hydrolysis)
3. Heat

Question 24

What are the 2 purposes of hydrolysing glycosidically bound aroma compounds?

Answer 24

1. To increase aroma strength / yield
   2. To accelerate the maturing / ageing of fruit wine

Point 2 : controlled hydrolysis can spped up the release of aromatic compounds in fruit wine, allowing them to develop their full aromatic profile quickly instead of relying on slow, natural ageing

Question 25

What is an example of the release of aroma compounds from **glucosidacally** bound aroma compounds? What enzyme can catalyse this reaction?

Answer 25

Vanillin is released from the hydrolysis of glucosidic bond from glucovanillin, hydrolysed by β-glucosidase. ## Footnote **Note : vanilla beans / pods can also be cured with heat to release vanillin**

Question 26

# **Vege** What action leads to biosynthesis of flavour compounds in vegetables?

Answer 26

Aroma compounds in vegetables mainly develop only after cellular disruption (because enzymes and substrates are in separate compartments) ## Footnote **Vegetables do not develop flavour through respiration like fruits, as vegetables have low metabolic activity after harvest**

Question 26

All vegetables are **non-odourous** before cellular disruption (through chopping / cooking etc). True or false?

Answer 26

False, there are some vegetables which are odourous even without cellular disruption, such as celery, corrainder, bell pepper

Question 26

What are the 3 types of substrates in vegetables that can undergo reactions to form flavour / aroma compounds in vegetables?

Answer 26

1. Unsaturated fatty acids 2. Cysteine sulfoxide derivatives → sulfur-containing 3. Glucosinates (thioglycosides) → sulfur-containing

Question 27

What reaction does unsaturated fatty acids in vegetables undergo, and what kind of flavour compounds does it produce?

Answer 27

Lipoxygenation (lipoxygenase reaction) ;; to produce aldehydes / carboxylic acids / ketones → alcohols ***unlike fruits, esters are rarely produced as vegetables lack necessary enzymes**

Question 28

Unlike fruits, esters are rarely produced as vegetables lack necessary enzymes. What enzymes do these vegetables lack, and what reaction do these enzymes catalyse?

Answer 28

Enzyme : alcohol acyltransferase, catalyse the reaction : alcohol + **acetyl-CoA** → ester + CoA (alcoholysis)

Question 28

Cysteine sulfoxide derivatives are found in what genus of plants? Give examples

Answer 28

Allium genus ; onion / garlic

Question 29

What are general names of 2 types of cysteine sulfoxide derivatives?

Answer 29

1. S-alkyl-L-cysteine sulfoxide (saturated) 2. S-alk**en**yl-L-cysteine sulfoxide (unsaturated, C=C)

Question 30

What are the names of the cysteine sulfoxide derivatives (precursors) in 1. Onion 2. Garlic?

Answer 30

Onion : S-1-propenyl-L cysteine sulfoxide Garlic : S-2-propenyl-L-cysteine sulfoxide

Question 31

What is the general pathway for degradation of cysteine sulfoxide derivatives in generating flavour compounds?

Answer 31

Cysteine sulfoxide derivatives (S-alk(en)yl-L-cysteine sulfoxide) → **sulfuneic acid** → mono-,di-, tri-sulfides

Question 32

Glucosinate (thioglycoside) are compounds that are found in what genus of vegetables? Give examples.

Answer 32

Brassica genus;; Brocolli / caulliflower / cabbage / brussel sprouts / wasabi ## Footnote **All of these vegetables come from the same plant, brassica oleracea (wild cabbage), but are different parts of the plant**

Question 33

Glucosinates (thioglycosides) are converted into what 3 types of intermediates? What is the enzyme catalysing this reaction?

Answer 33

Converted to : 1. isothiocyanate ( R-N=C=S) 2. thiocyanate R-S-C≡N 3. nitrile (C≡N) ***Catalysed by thioglucosidase (myrosinase)**

Question 34

# **Thioglycosides in vege** The intermediates : isothiocynate, nitriles, thiocyanate are eventually converted into what aroma compounds?

Answer 34

- Sulfides (R-S-R), disulfide (R-S-S-R), trisulfide (R-S-S-S-R) - methyl-, butyl-, allyl- isothiocyanate

Question 36

Lipoxygenase reactions are associated with ___ odour. Volatiles that contribute to this odor include :

Answer 36

Green odour - Hexenal, hexenol

Question 37

Benzaldehyde is a key odourant in ____ and____

Answer 37

Cherry ; almond

Question 37

What are some key odourants in apples?

Answer 37

**Mainly esters** 1. Butyl (4C) **acetate** 2. Hexyl (6C) **acetate** 3. 2-methylbutyl (C,4C) **acetate** 4. Ethyl-2-methyl *butanoate*

Question 38

What are some key flavour compounds in orange? [2]

Answer 38

1. Limonene (terpene) 2. Linalool (terpene alcohol)

Question 40

Key flavour compounds in grape?

Answer 40

**Methyl anthranilate (MANT)** Linalool (monoterpene)

Question 41

Key flavour compound in banana?

Answer 41

Isoamyl acetate