1. AUTOXIDATION AND THERMAL-OXIDATIVE DESTRUCTION OF LIPIDS
OBJECTIVES
1. Autoxidation and thermal destruction of lipids;
2. Mechanism and major consequences;
3. How to prevent oxidation of lipids;
4. Frying processes.

2. AUTOXIDATION AND THERMAL-OXIDATIVE DESTRUCTION OF LIPIDS1
Lipids – major changes during thermal treatment or storage
1. Lipolysis (hydrolysis of acylglycerols)
1.1. Enzymatic – at lower temperature: both – desired and non-desired as process; phospholipases – responsible for lowering the quality in low-temperature fish storage;
1.2. Chemical – thermal treatments and presence of water; during frying, roasting, baking – lowers the lipids stability (easier oxidation of free fatty acids).

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2. Autoxidation and thermal destruction
- one of the major factor for food deterioration – obtaining of off-flavor, lowering the nutritional value; some oxidation products have toxicity;
- in some cases the controlled oxidation processes are desired – obtaining of some cheese types, fried products etc.

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Mechanism

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Mechanism – typical free radical reaction

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Mechanism – formation of singlet oxygen
RH + О2 – the reaction has high activating energy (146kJ/mol);
To take place it is necessary presence of catalyst – hydro peroxides, metal cations, photosensitizing by presence of pigments, thermal catalysis.
or presence of singlet oxygen

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Mechanism – formation of singlet oxygen
92 kJ
155 kJ

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Mechanism

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Hydro peroxides formation
1. By unsaturated fatty acids

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Hydro peroxides formation
2. By saturated fatty acids – formation of methyl ketones (rancid odor); possible attacks are α-, β-, γ- places.
β-oxydation
hydro peroxides
- СО2

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Decomposition of the hydro peroxides
Homolytic cleavage

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Decomposition of the hydro peroxides
Heterolytic cleavage

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Decomposition of the hydro peroxides

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Decomposition of the aldehydes obtained – they could participate in oxidation reactions, in the Maillard reaction, reactions of cyclization etc.
Reaction of cyclization
The unsaturated aldehydes participate more actively in decomposition reactions
The malonaldehyde formed is in the base of the ТВА
method for measuring the lipid oxidation

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Cholesterol oxidation – lead to formation of cytotoxic and carcinogenic products; found in dried eggs, meat, fried foods.
Acrolein formation
Dehydration and
ester bond cleavage
dehydration
CH2 = CH – CHO
acrolein

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Polymerization reactions – increase of viscosity, lowering the iodine value
1). Diels-Alder reaction
Linoleic acid – dimerization 2

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Polymerization reactions – increase of viscosity, lowering the iodine value
1). Diels-Alder reaction
Acylglycerols – intramolecular and intermolecular di-(poly-)merization

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Polymerization reactions – increase of viscosity, lowering the iodine value
2). Radical recombination

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Polymerization reactions – increase of viscosity, lowering the iodine value
3). Attack of free radical on a double bonds
4). Oxygen bridges formation – polymerization
Polymerization of acylglycerols

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Factors influencing the lipid oxidation
1). Type of fatty acids – unsaturated > saturated; cis > trans;
arachidonic: linolenic : linoleic : oleic = 40:20:10:1
2). State of the fatty acids – non-bound > acylglycerols (free fatty acids – could react with metals from the packings - catalysis);
3). Oxygen presence
4). Temperature
5). Physical state of the product; presence of emulsions, porosity, surface
6). аw – 0.3 is the minimum
7). Presence of other substances – prooxidants, antioxidants

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Methods for measuring the lipid oxidation
1). Peroxide value
2). TBA (thiobarbituric acid) test – measures the malonic aldehyde formed
3). Iodine value
4). Chromatography methods
5). Sensory

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3. Antioxidants
BHA - Butylated
hydroxyanisole
BHT - Butylated
hydroxytoluene
TBHQ - tert-Butylhydroquinone
PG – propyl gallate
α-tocopherol

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3. Antioxidants – mechanism of action

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4. Chemistry of frying – formation of volatile compounds (aldehydes, ketones, lactones, alcohols, acids, esters); moderately volatile compounds; dimers, polymers; free fatty acids.
What is going on with the food during the frying process?
1). Dehydration of the foodstuffs – the water contacting the hot oil is evaporated and the actual process is a steam distillation of the volatile components of the foods; The water from the foodstuffs increases the lipolysis; The water layer formed on the surface of the oil (before its evaporation) to a certain extent inhibits penetration of О2 inside the oil thus slowing down thermal-oxidative destruction;
2). Formation of flavor – diverse and great number of derivatives (heterocyclic –
pyrazines, pyridines, thiazoles etc.)
3). The food absorbs part of the oil – in this way part of destruction products enters into the foodstuffs (for French fries – 35% oil absorption);
4). Into the frying oil enter part of the products formed during thermal heating of the foods – lowering with time the stability of the frying oils.

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What is going on with the food during the frying process?

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What is going on with the food during the frying process?
Parameters – temperature, duration, oxygen access (surface), presence of metal cations, presence of antioxidants, extent of oil usage.
Tests for frying oil quality
1). Substances insoluble in hexane (used in Germany) - less than 1%
2). Determination of esters-dimers (GC)
3). Changes in dielectric constant – physical method
4). Quick tests – chemical (Oxifrit, Fritest, etc.)

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What is going on with the food during the frying process?
Control and safety measurements
1). Choice of suitable and qualitative frying oil – smoke point (sunflower oil: semirefined – ºС; unrefined – 107 ºС

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Inherent stability of oils used for frying
The higher the inherent stability, the less suitable the oil is for deep frying.
Inherent stability calculated from decimal fraction of fatty acids multiplied by relative reaction rates with oxygen, assuming rate for oleic acid = 1 , linoleic acid = 10, and linolenic acid = 25

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The stability of frying oils and fats is usually measured by an accelerated test known as the active oxygen method (AOM). In this test, air is bubbled through an oil sample maintained at 95°C and the peroxide value is measured at intervals. At the end point the peroxide value shows a sharp increase, and this represents the AOM value in hours. Typical AOM values for liquid seed oils range from 10 to 30 hours; heavy-duty frying shortenings range from 200 to 300 hours.
Active Oxygen Method (AOM) Time of Several Oils and Fats as Determined by Peroxide Value and Conductivity Measurements
from J.M. deMan, et al., Formation of Short Chain Volatile Organic Acids in the Automated AOM Method, J.A.O.C.S., Vol. 64, p. 996, © 1987, American Oil Chemists' Society.

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What is going on with the food during the frying process?
2). Choice of suitable equipment
3). Choice of suitable temperature according to the product and the oil used
4). Filtration of the frying oil – removal of residues from the foodstuffs
5). Change of the frying oil – after testing
6). Addition of antioxidants

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Summery of the autoxidation and thermal-oxidative destruction
1. Lipid oxidation is not a single process that proceeds according to a strict sequence. Rather, numerous events take place in different regions of the food matrix. The different reactions usually occur with considerable overlap, and the rate of each may
vary depending on conditions in each region.
2. The “conditions” in each region (oxidation parameters) are not static. They change continuously.
3. In food, oxidation involves oxidizable lipid substrates differing in composition, chemical and physical properties, and in their sensitivity to oxidation (fatty acids, mixtures of acylglycerols, phospholipids).
4. The lipid substrates coexist in proximity with various other nonlipid, major (proteins, carbohydrates, water) and minor (trace metals, vitamins, enzymes, pro- and antioxidants) components. A multitude of interactions take place.

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Summery of the autoxidation and thermal-oxidative destruction
5. Nearly all oxidative events are not “strictly chemical.” Instead, they are largely dictated by the physical state of both substrate and medium.
6. The minor components play a major role in a critical, delicate oxidative-antioxidative balance, the mechanism of which is not fully understood.
7. The same inhibitors (or accelerators) may play opposite roles depending on the oxidative event being considered. For example, a phenolic antioxidant may block the formation of volatile oxidation products while enhancing the accumulation of
peroxides.