Unit 8 - Enzymatic and Thermal degradation products of lipids: Formation of flavors and off-flavors

Question 1

T or F: Enzymatic reactions of lipids can create flavors and off-flavors

Answer 1

T

Question 2

Lipids occur in all _____________ in varying concentrations, and are associated with ___________ and ____________ functions, In addition they function as long-term storage _____.

Answer 2

living cells
metabolic
structural
fuel

Question 3

T or F: With a few exceptions lipids occur in all types of food

Answer 3

T

Question 4

Oils and fats are pure lipids which in practice contain one lipid class, the ______________.

Answer 4

triglycerides

Question 5

Major components of lipids are _________ and ________ fatty acids

Answer 5

free

bound

Question 6

Lipids can undergo 3 types of reactions that lead to the formation of compounds able to modify flavor properties of food.

Answer 6

1) Autoxidation and thermal oxidation (catalyzed by metals in the presence of air)
2) Thermal degradations at elevated temperatures
3) Enzymatic oxidation (catalyzed by lipoxygenase and associated enzymes at physiological conditions)

Question 7

What is the main fatty acid found in olive oil:

Answer 7

oleic acid

Question 8

T or F: Saturated fatty acids require much more energy to react with oxygen to break the carbon - hydrogen bond to replace the hydrogen with oxygen

Answer 8

T

Question 9

T or F: Most of the fatty acids present in butter are saturated so it is harder to make it react with an oxygen

Answer 9

T

Question 10

Sugars cannot undergo ____________as quickly as fatty acids with allylic groups. Fatty acids are the only ones that can do that at room temperature.

Answer 10

oxidation

Question 11

T or F: Once fatty acids are oxidized, they will start breaking down and this is why we should have enough antioxidants in our diet to prevent such oxidation from happening in our bodies

Answer 11

T

Question 12

Among the major components of the food, the ______________________ are the only class of compounds which even in physiological conditions are rapidly degraded by a _________________ reactions to a great number of volatile compounds which, in part, exhibit very low ___________________ thresholds

Answer 12

unsaturated fatty acids
non-enzymatic
odor and taste

Question 13

In saturated and unsaturated lipids, all positions of the hydrocarbon can be attacked by ____________ with the formation of a large number of volatile compounds. The minor components of lipids such as _____________ and _________ fatty acids and sterols must not be forgotten when considering lipids as flavor precursors. In various foods such as tomatoes, butter, peaches, it is from these that the volatile substances arise which are important for the typical aroma.

Answer 13

radicals

hydroxy- and oxo- fatty acids

Question 14

T or F: If we do not have free radicals initially, no chain reaction will occur

Answer 14

T

Question 15

The familiar rancid odors of deteriorated fats and oils result from ____________________________________. The course of the reaction is the same whether the acids are esterified or free and occurs in 3 steps: initiation stage, propagation stage and termination reactions

Answer 15

autoxidation of unsaturated fatty acids

Question 16

The ______________ produces small numbers of highly reactive free radicals, molecules with unpaired electrons

Answer 16

initiation stage

Question 17

In the _________________ oxygen reacts with the free radicals produced in the initiation stage to form fatty acid _____________. These hydroperoxides break down to generate more __________ which can either attack more fatty acids and/or cycle with more oxygen to maintain a chain of reactions.

Answer 17

propagation stage
fatty acid hydroperoxides
free radicals

Question 18

When sufficiently high concentrations of free-radical species are formed, they tend to react together in ____________________ to give polymers or decompose to form stable end-products characteristic of rancid fat: (3)

Answer 18

termination reactions

• aldehydes
• ketones
• alcohols

Question 19

Cis-hex-3-enal, butanedione, pentane-2,3-dione, and penta-2,4-dienal give the the characteristic odor of ______________________

Answer 19

reverted soybean oil

Question 20

When a lower dissociation energy is needed, the initiation step will occur more _________

Answer 20

easily

Question 21

The free radical lipid oxidation reactions involved both during ___________ and in ______________ follow the same basic pathways (described above) and result in similar types of volatile products

Answer 21

cooking

rancid flavor formation

Question 22

In food the futur of hydroperoxides depends on the______________

Answer 22

temperature

Question 23

Hydroperoxides are extremely heat _________. At lower temperatures, they are more stable, and more may build up before ___________ occurs. This leads to different proportions of the various radical intermediates, resulting in different levels of volatile products in heated vs stored foods.

Answer 23

labile

decomposition

Question 24

Hydroperoxide formation at the gamma or delta positions is believed to be responsible for the formation of gamma and beta _________ via cyclization of the resulting hydroxy acids. Most gamma and delta lactones have flavor thresholds in the 7-500 ppb range. These compounds are typically used as flavorings in __________, hazelnut and other nut flavor formulations and in a number of ________ formulations

Answer 24

lactones
coconut
dairy

Question 25

Meat products are rich in _______ which will accelerate the oxidation process, once the meat is cooked, don’t keep it like this for too long because the taste will change

Answer 25

iron

Question 26

Thermal oxidation =

Answer 26

quick degradation

Question 27

lipid derived flavors are often associated with _______ products

Answer 27

meaty

Question 28

The fungi in blue cheeses has all the enzymes necessary to transform the fatty acids into a ______________ and this is what gives the characteristic taste and aroma to blue cheeses

Answer 28

methyl ketones

Question 29

Hydroperoxide formation at the beta-position is thought to lead to the formation of ___________ after a decarboxylation step

Answer 29

methyl ketone

Question 30

The aldehydes and ketones produced by lipid oxidation can also undergo secondary reactions in the presence of amino acids to make ____________________

Answer 30

other flavor compounds

Question 31

Lipid autoxidation also occurs during long-term storage of meats, producing undesirable flavors. ____________________ (WOF), a flavor that becomes apparent when cooked meat is kept at refrigerator temperatures for a few days and is then reheated, has been attributed to lipid oxidation products. it is believed to be due to the autoxidation of _____________, which are usually more unsaturated than triglycerides. During the initial heating, disruption and dehydration of meat cell membranes, the phospholipids become more susceptible to ___________. This reaction is catalyzed by _______________ in particular iron, made available by the breakdown of ___________ during cooking

Answer 31

Warmed-Over-Flvor 
phospholipids
oxidation
trace metal ions
heam pigments

Question 32

T or F: Any food ingredient exposed to bery high temp will degrade

Answer 32

T

Question 33

When we use deep-frying oils as a cooking medium, we can use the same oil for 1-2 cycles but not more than that because it can produce __________ type of products, heterocyclic compounds can even form and can be toxic / have negative health effects

Answer 33

ROOH peroxides

Question 34

A number of studies have been reported on the thermal degradation of saturated triacylglycerol under strictly non-oxidative conditions. in general, the different types of reactions known to occur include dehydration, decarboxylation, hydrolysis of ester bonds, polymerization, double bond conjugation, dehydrogenation, degradation by C-C cleavage, etc. The specific compounds formed by these reactions depend on the _________________ and the intensity of the thermal treatment during processing. At temperatures above 200C __________ are the main hydrocarbon compound formed with other minor constituents such as alkenes, fatty acids, symmetric ketones, oxopropyl esters, propene, propanediol esters and acylglycerol formed with prolonged thermal degradation

Answer 34

chain length of the fatty acids

alkanes

Question 35

Only 2 unsaturated fatty acids, linoleic and linolenic acids which occur in practically all plant tissues, are the precursors of a wide range of ____________________, although in fruit and vegetables considerable differences occur in the major breakdown products. The differences, which can be decisive for the aroma of the foodstuff, are caused by the _______________________ involved in the oxidative degradation of unsaturated lipids

Answer 35

volatile components

nature and the properties of the enzymes

Question 36

Incubation experiments with homogenates from foods of plant origin and linoleic and linolenic acid as substrates have demonstrated that enzyme systems that convert these unsaturated fatty acids in the presence of oxygen to volatile __________, _________ and ________ are widely distributed in all plants. The contribution of such compounds to a flavor can be beneficial or detrimental depending on their concentration and composition as well as on the type of plant material where they occur.

Answer 36

aldehydes
alcohols
esters

Question 37

In fresh plants the concentrations of the volatile breakdown products from fatty acids are in the range of _____________. Such low amounts can only influence positively the aroma.

Answer 37

odor thresholds

Question 38

Storage of the fresh non-heated material or various processing procedures that are associated with _____________________ can promote the enzymatic reactions in such a way that the volatile compounds accumulate in concentrations sufficient to cause _________. The key reaction of this process is often the ______________ of unsaturated fatty acids by the enzyme lipoxygenase.

Answer 38

cracking of the cell structure
off-flavors
hydroperoxidation

Question 39

Acyl hydrolases or lipases also play a role in supplying the lipoxygenase with a substrate which only oxidizes __________________

Answer 39

free fatty acids

Question 40

Hydroperoxides break down to give aldehydes, alcohols and esters with the help of ______________, _________________, ________________ and esterases

Answer 40

hydroperoxide lyases
alcohol dehydrogenases
isomerases

Question 41

Each type of plant has its own set of _______________, pH and medium conditions. These factors influence the path of break down resulting in different volatile products from the same starting polyunsaturated fatty acids. When the lipid oxidation forms 4- or 5- hydroxy acids, _____________ are usually obtained

Answer 41

lipoxygenases

lactones

Question 42

When fatty acids have double bonds, it accelerates the rate of oxidation because the ______________ is reduced which facilitates C-H bond cleavage

Answer 42

dissociation energy

Question 43

Hexanal:

Answer 43

green, leaf-like

Question 44

(2E,4E)-Decadienal:

Answer 44

fatty, deep-fried

Question 45

3Z-Nonenal:

Answer 45

cucumber-like

Question 46

2E-Nonenal:

Answer 46

cardboard-like