Lipids Flashcards
what are fatty acids?
an aliphatic chain with a carboxylic acid group
what are unsaturated FA?
contain double bonds
how are saturated FA named?
based on the number of carbon atoms, with an ending of “oic” since FA contain a carboxylic group
oleic acid
18:1
linoleic acid
18:2 (omega-6)
linolenic acid
18:3 (omega-3)
arachidonic acid
20:4 (omega-6)
EPA eicosapentaenoic acid
20:5 (omega-3)
DHA docosahexaenoic acid
22:6 (omega-3)
how are double bonds, if there are several, normally found in FA?
C=C-C-C=C
what are some determinants of TAG melting points?
double bonds - symmetry on the glycerol molecule - trans/cis - polymorphic form (crystalline structure)
thermal stability of a lipid is characterized by:
smoke, flash and fire points
what are the three basic polymorphic crystal arrangements?
alpha, beta’ and beta
a melted TAG cooled rapidly will form the unstable…
alpha form!
slow heating of alpha crystals will cause
melting and resolidification into the beta-prim form. repeat this procedure to achieve the stable beta form.
the crystal form is dictated by..
FA composition and position in the TAG
solid fat content can give away information about…
emulsion stability, cloud point, melting behaviour, baking properties, spreadability
how many different polymorphic states can cocoa butter occur in?
six! melting points range from 17,3-36,4 deg C
what is conching?
stirring chocolate continuously for several hours at 50-55 deg C to get rid of undesirable flavours
what is tempering of chocolate?
cooling the chocolate to initiate crystallization, reheating and stirring to obtain very small crystals of desirable type
how is hydrogenation performed?
silica-bound Ni is used as catalyzator, it binds to and “eats up” the double bonds of UFA. after that, hydrogen replaces the Ni and a normal bond is formed.
what happens with hydrogenation if these is insufficient catalyzator, temperature or hydrogen?
double bonds may remain and there is a risk of forming trans fatty acids as the added hydrogen may leave its FA, creating a new double bond.
what are the two ways fats and oils deteriorate?
hydrolytic rancidity and oxidative rancidity
why are phospholipids surface active?
because of the polar phosphate group
what is rendering?
a thermal processing operation that breaks down cellular structures to release the TAGs from animal byproducts and oil-laden underutilized fish species
how are plant TAGs isolated?
pressing, solvent extraction or a combination. the results of these processes contains free FA, phospholipids, lipid-soluble off-flavours and carotenoids, proteins and carbs.
what are the major steps in lipid refining?
degumming, neutralization, bleaching, deodorization
why is degumming necessary?
phospholipids cause formation of water-in-oil emulsions, which makes the oil cloudy, and the water can present a hazard when the oils are heated above 100 deg C. phospholipids also contain amines that can interact with carbonyls to form browning products
how is degumming done?
by addition of 1-3% water at 60-80 deg C for 30-60 min. small amounts of acid are often added to the water to increase phospholipid solubility, because citric acid binds calcium and magnesium, thereby decreasing phospholipid aggregation and making them more hydratable. settling, filtering or centrifugation is used to remove coalesced gums.
why is neutralization necessary?
free FA must be removed because they can cause off flavours, decrease smoke point, accelerate lipid oxidation, cause foaming, interfere with hydrogenation and inter-esterification operations
how is neutralization performed?
by reacting the oil with a solution of NaOH and then removing the water containing the soaps of the free FA
why is bleaching necessary?
crude oils contain pigments that produce undesirable colours (ex carotenoids) and can promote lipid oxidation (ex chlorophyll). bleaching also removes residual free FA and phospholipids and causes breakdown of lipid hydroperoxides
how is bleaching performed?
pigments are removed by mixing hot oil (80-110 C) with absorbents such as neutral clays, synthetic silicates, activated carbon or activated earths. the absorbent is then removed by filtration.
why is deodorization necessary?
crude lipids contain undesirable aroma compounds such as aldehydes, ketones and alcohols that occur naturally in the oil or are produced from lipid oxidation reactions that occur during extraction and refining.
how is deodorization performed?
volatile compounds are removed by subjecting the oil to steam distillation at high temperatures (180-270 C) at low pressures
what is a plastic material?
it behaves like a solid below a critical applied stress known as the yield stress but behaves like a liquid above this stress
the yield stress of a fat
tends to increase with increasing SFC - higher for crystal morphologies that are able to form three-dimensional networks that extend throughout the volume of the system more readily
what is the “plastic” range?
a wide temperature range over which food TAGs melt
reactivity of unsaturated FA increases with…
their chain length, and proportionally to the number of double bonds
why are TAGs more stable than free FA?
more hydrolytic rancidity gives more free FA, in TAGs this step must be taken before oxidation
what are some factors influencing lipid oxidation/degradation rates?
light - oxygen conc - temperature - surface area (oxygen contact) - content of UFA - fat content - water activity - minor compounds/additives (antioxidants)
what are some examples of pro-oxidants?
free radicals generated by aggressive oxygen species, light, temperature - oxygen - metals like iron, copper - hemoglobin
what are some degradative reactions that take place during frying?
hydrolysis due to water in food - oxidation (aldehydes and ketones react with proteins to create Maillard reactions)
how can lipid oxidation be measured?
TBARS test, peroxide value, volatile products, free fatty acids, iodine value
how does the TBARS test work?
TBARS reacts with malondialdehyd (a stable product of PUFA oxidation) to form a pink complex. however, TBARS reacts with other substances like nitrite aswell.
what can you tell from the peroxide value?
determines the amount of peroxides, which are the primary product of oxidation. however as oxidation goes on the reaction speed towards secondary products increases and peroxide decreases which could be interpreted wrong with this test.
how do you measure volatile products and what can it say about lipid oxidation?
volatile substances which are the final oxidation products of lipid oxidation can be identified and quantified by GC-MS, this makes it possible to study oxidation processes more in-depth
what can free fatty acid content say about lipid oxidation?
it is actually a measurement of hydrolytic rancidity, but it can give a measure of the risk of oxidation
what can you tell frmo the iodine value?
gives an indication of the unsaturation of the fat. if this decreases oxidation has taken place.
when is it efficient to add vitamin E and C to animal products?
vitamin E works best when added to the feed. Vitamin C when added post-mortem
what are some strategies to decrease oxidation?
increase antioxidants and decrease prooxidants like salt - high pH - low temperature during processing - minimal time processing - improve packaging conditions
when a liquid oil is cooled rapidly
a large number of small crystals are formed
when a liquid oil is cooled slowly
a smaller number of larger crystals are formed
TAGs exhibit a phenomenon known as polymorphism, what is this?
the ability of a material to exist in a number of crystalline structures with different molecular packing
which is the most stable polymorphic form?
beta
why do TAGs often crystallize in the alpha-form initially?
because it has the lowest activation energy for nuclei formation
the melting point of a TAG depends on the
chain length - degree of unsaturation of its constituent FA - relative positions of the FA along the glycerol molecule
if an oil of a complex mixture of TAGs is cooled rapidly
all the TAGs crystallize at nearly the same time and a solid solution is formed, which consists of homogenous crystals in which the TAGs are intimately mixed with each other
if an oil of a complex mixture of TAGs is cooled slowly
the higher melting point TAGs crystallize first, while the low melting point TAGs crystallize later, and so mixed crystals are formed. these crystals are heterogenous and consist of some regions that are rich in high-melting point TAGs and other regions that are depleted in these TAGs.
what are some ways to alter the solid-fat content of food lipids?
blending fats with different TAG composition - dietary intervention in meat animals - genetic manipulation of plants - fractionation by cooling and separating solids and liquids
what is interesterification?
a process that can change the melting profile of lipids without changing fatty acid composition, by rearranging the fatty acids on the glycerol molecule.
free fatty acids cause problems in foods because they can…
produce off-flavours, reduce oxidative stability, cause foaming, reduce smoke point
if the formation of free fatty acids results in the development of off-flavors, this is known as
hydrolytic rancidity
the centerpiece of lipid oxidation reactions is..
free radicals
what are free radicals?
molecules or atoms that have unpaired electrons
where, on the fatty acid molecule is initiation of lipid oxidation most likely to happen?
on the methylene-interrupted carbon placed between two double bonds.
how is lipid oxidation initiation affected by degree of unsaturation?
more double bonds means more methylene between double bonds. for each added methylene-interrupted carbon, oxidation rates increase and may even double.
the first step of lipid oxidation propagation involves…
the addition of oxygen to the alkyl radical (fatty acid), one of the low-energy radicals on one of the two oxygens of atmospheric oxygen forms a covalent bond with the alkyl radical. the other radical on the oxygen remains free.
the resulting radical of addition of oxygen to the alkyl radical is known as a…
peroxyl radical, LOO*. the high energy of peroxyl radicals allows them to promote the abstraction of a hydrogen from another molecule, which in the system often is another unsaturated fatty acid. thus the reaction is propagated from one fatty acid to another.
termination of lipid oxidation in atmospheric conditions occurs as
the combination of two radicals to form nonradical species, in this case between two peroxyl radicals. in low oxygen environments termination reactions can occur between alkyl radicals to form fatty acid dimers.
if one radical only can produce another radical, and thus stops being a radical itself, why is lipid oxidation almost always exponentially increasing in foods?
this is due to the presence of other reactions in lipid oxidation that produces additional free radicals.
prooxidants, found in virtually all food systems, are compounds or factors that…
cause or accelerate lipid oxidation. these can accelerate lipid oxidation by direct interactions with unsaturated FA or by promoting formation of other free radicals.
oxidation of vegetable oils that have predominantly omega-6 fatty acids will produce
grassy and beany odors
oxidation of long chain omega-3 fatty acids in marine oils will produce
fishy aromas
antioxidant mechanisms of compounds that are used to increase the oxidative stability of foods include control of…
free radicals, prooxidants and oxidation intermediates
how is lipid oxidation measured?
sensory analysis (a trained person is highly sensitive to off-aromas and off-flavors generated by oxidative reactions) - measuring primary lipid oxidation products - measure fatty acid hydroperoxides which can oxidize an indicator compound - measuring secondary lipid oxidation products - analysis of volatile secondary products - thiobarbituric acid (TBA)
oxidative rancidity occurs via…
autocatalytic free-radical reactions
oxidative rancidity occurs when
fatty acids are decomposed into low-molecular-weight aldehydes and ketones
prooxidants such as __, __ and __ are often the major cause of lipid oxidation in foods
transition metals, singlet oxygen, enzymes
antioxidants slow oxidation by…
scavenging free radicals and/or decreasing the activity of prooxidants
lipid oxidation is also influenced by factors such as…
oxygen concentrations, fatty acid unsaturation, temperature, water activity
