Unit 8 - Enzymatic and Thermal degradation products of lipids: Formation of flavors and off-flavors Flashcards
T or F: Enzymatic reactions of lipids can create flavors and off-flavors
T
Lipids occur in all _____________ in varying concentrations, and are associated with ___________ and ____________ functions, In addition they function as long-term storage _____.
living cells
metabolic
structural
fuel
T or F: With a few exceptions lipids occur in all types of food
T
Oils and fats are pure lipids which in practice contain one lipid class, the ______________.
triglycerides
Major components of lipids are _________ and ________ fatty acids
free
bound
Lipids can undergo 3 types of reactions that lead to the formation of compounds able to modify flavor properties of food.
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)
What is the main fatty acid found in olive oil:
oleic acid
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
T
T or F: Most of the fatty acids present in butter are saturated so it is harder to make it react with an oxygen
T
Sugars cannot undergo ____________as quickly as fatty acids with allylic groups. Fatty acids are the only ones that can do that at room temperature.
oxidation
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
T
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
unsaturated fatty acids
non-enzymatic
odor and taste
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.
radicals
hydroxy- and oxo- fatty acids
T or F: If we do not have free radicals initially, no chain reaction will occur
T
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
autoxidation of unsaturated fatty acids
The ______________ produces small numbers of highly reactive free radicals, molecules with unpaired electrons
initiation stage
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.
propagation stage
fatty acid hydroperoxides
free radicals
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)
termination reactions
- aldehydes
- ketones
- alcohols
Cis-hex-3-enal, butanedione, pentane-2,3-dione, and penta-2,4-dienal give the the characteristic odor of ______________________
reverted soybean oil
When a lower dissociation energy is needed, the initiation step will occur more _________
easily
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
cooking
rancid flavor formation
In food the futur of hydroperoxides depends on the______________
temperature
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.
labile
decomposition
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
lactones
coconut
dairy
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
iron
Thermal oxidation =
quick degradation
lipid derived flavors are often associated with _______ products
meaty
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
methyl ketones
Hydroperoxide formation at the beta-position is thought to lead to the formation of ___________ after a decarboxylation step
methyl ketone
The aldehydes and ketones produced by lipid oxidation can also undergo secondary reactions in the presence of amino acids to make ____________________
other flavor compounds
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
Warmed-Over-Flvor phospholipids oxidation trace metal ions heam pigments
T or F: Any food ingredient exposed to bery high temp will degrade
T
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
ROOH peroxides
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
chain length of the fatty acids
alkanes
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
volatile components
nature and the properties of the enzymes
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.
aldehydes
alcohols
esters
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.
odor thresholds
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.
cracking of the cell structure
off-flavors
hydroperoxidation
Acyl hydrolases or lipases also play a role in supplying the lipoxygenase with a substrate which only oxidizes __________________
free fatty acids
Hydroperoxides break down to give aldehydes, alcohols and esters with the help of ______________, _________________, ________________ and esterases
hydroperoxide lyases
alcohol dehydrogenases
isomerases
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
lipoxygenases
lactones
When fatty acids have double bonds, it accelerates the rate of oxidation because the ______________ is reduced which facilitates C-H bond cleavage
dissociation energy
Hexanal:
green, leaf-like
(2E,4E)-Decadienal:
fatty, deep-fried
3Z-Nonenal:
cucumber-like
2E-Nonenal:
cardboard-like
