Ripan Goswami
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Shallow frying is cooking process in highly heated oil, promoting
browning of battered or non battered foods
Deep Fat frying is a cooking process that involves submerging a food in
extremely hot oil in the presence of air at a high temperature of 150°C to
190°C.
Par-frying is a industrial process, food is partially dehydrated in a
industrial fryer and flash frozen at -20°C and packaged par-fried food
stored at -5°C to -10°C before distribution in freezer trucks
Frying oil plays a great role in quality of the fried product, acts as a heat
transfer medium and contributes to texture and flavor of fried food.
Frying oil role in frying
 Texture
 Fried food flavor
 Mouthfeel
 Aftertaste
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Product flavor
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Product texture
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Product appearance
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Mouthfeel
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Aftertaste
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Shelf life of the product
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Availability of the oil
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Cost
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Nutritional requirements
To meet current market desire, frying oil must be
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Low in saturated fat
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Low in linolenic acid
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High oxidative and flavor stability
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Not hydrogenated (trans-fat free)
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In Food
 Loss of moisture
 Development of dark color, firm texture and fried flavor
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In Oil
 Bland flavor at initial frying, later develops fried food flavor in oil
 Changes due to chemical reactions
 Dark and viscous
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Oil and food both passes through an optimum stage after which quality of oil and
product flavor declines
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Unsaturated fatty acids decreases with increasing number or time of frying
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Oil and food both undergo severe changes due to frying, difficult to relate every change
in different frying process, because of the variables of oil, food and process conditions
Understanding new compounds and their dependence on the main variables of the
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frying process - Important
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Fresh oil - Fried food appears bland
Reused oil – Fried flavor
New compounds formed in oils during frying process
Causative agent
Change effect
New compounds
Moisture
Hydrolysis
Free fatty acids
Diaclyglycerol
Monoaclyglycerol
Air and Metals
Oxidation
Oxidized monomers
Oxidized dimers & polymers
Volatile compounds (aldehydes, ketones,
hydrocarbons)
Sterol oxides
High temperature
Polymerization
Dimers and non-polar polymers
Cyclic monomers
Trans isomers and position isomers
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Frying is a complex process to understand completely and thoroughly..
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Hydrolysis in fats and oil due to lipolytic enzymes or moisture
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In Frying
 High moisture in foods to be fried
 Hydrolysis end products
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TAG + H2O
Free fatty acids (FFAs) + di or monoacylglycerol
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More in oil with short-chain unsaturated fatty acids than long chain saturated fatty
acids
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FFAs - Off flavors and decreases smoke point of the oil
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Di- and monogylcerides - Foaming of oil
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Frequent replacement of frying oil with fresh oil slows down the hydrolysis of frying
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oil.
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Large amount of water in food being fried hydrolyze the oil rapidly
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Maximum recommended free fatty acids content for frying oil is 0.05% to 0.08%
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Free fatty acid value is used to monitor the quality of frying oil during a frying
process
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In a study by Dobarganes et al., well controlled frying operations of potatoes,
hydrolytic end products lower than other degradation compounds.
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Oil and water do not mix except at high temperature under high pressure or due to
surfactant during frying
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Presence of surfactant is considered majorly responsible to facilitate formation of oil
and water solution during frying causing hydrolysis reaction
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Sources
 Fresh oil can have may be due to chemical refining process, poor bleaching,
washing
 Foods – metal ions reacts with FFAs forms soap
 Poor rinsing of the fryer after cleaning
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Lipid oxidation during frying highly complex, since both oxidative and thermal
reaction occur simultaneously
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Oxidation during frying causes most significant changes in flavors of fried foods
both desired and undesired
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Thermal oxidation mechanism similar to autoxidation, but faster
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Autoxidation of unsaturated fatty acids initiated by free radicals in the presence
of
 Oxygen – during frying, air pulled into oil
 Metals (iron, copper, nickel etc.) – food or oil
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As the temperature increases, solubility of oxygen in oil decreases, although the
degradation reactions are accelerated
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Lower frying temperatures
 Induction period of oxidation is slow
 Hydroperoxides (ROOH) are major compounds until advanced stage
 Polymers formed only at accelerated stage of oxidation – end of induction
Higher frying temperature
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At frying temperature (180°C), oxygen pressure reduced, initiation reaction
dominates preventing the propagation reaction that involves oxygen
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Alkyl radicals (R•) concentration increases compared to alkylperoxyl radicals
(ROO•)
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Polymeric compounds formed involving alkyl radicals (R•) and alkoxy radicals
(RO•)
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Hydroperoxide (ROOH) decomposition becomes higher than its formation
 Polymeric compounds formed at early stages of heating
 Triaclyglycerol dimers (R-R), alkyl radicals (RR•) at initiation reaction
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and alkoxy radicals (ROR•) by hydroperoxide decomposition in
termination reactions
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Polymerization leads to high variety of products with different polarity, stability
and molecular weight
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Compounds distinguished by mol.wt
 Oxidized monomers - similar to parent RH
 Dimers and oligomers - higher than parent RH
 TAG containing short chain components and volatiles - lower than parent RH
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Volatile compounds in frying oil that significantly imparts flavor quality of fried
foods and oils, decreases due to
 Evaporation
 Decomposition
 Reaction with food components
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Formation of non radical
termination of oxidation
volatile
and
non-volatile
compounds
indicate
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Oils rich in linoleic fatty acids is more easily polymerized during deep fat frying
than oil rich in oleic acid
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Physical and Chemical changes of oil during deep fat frying
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Polymers formed affects the quality of frying oil
 Accelerate further degradation of oil
 Increase oil viscosity
 Reduce heat transfer
 Induce foaming
 Develops undesirable color in fried food
 Increase absorption of oil in food
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Stability of oil for frying can be determined by relating to its inherent stability to
oxidation
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Inherent Stability relate to relative reaction rates of unsaturated fatty acids with
oxygen, hence oil with low inherent stability is less susceptible to oxidation
during frying
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Hence frying quality of oil with high unsaturated fatty acids and free fatty acids
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is not as good as oil with low unsaturated fatty acids and free fatty acids
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Selection of oil is dependent on factors other than their frying
 Appearance
 Fat retention
Ex. Doughnuts for serving fresh or packaged
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Depending upon the application, solid fats are sometimes blended with refined
vegetable oil, hydrogenated fats and oils like shortenings
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Comprehensive oil specification studies are needed to address any performance,
nutritional and labeling issues that may arise
Criterion
Specification
Color
Light
Flavor
Bland
Phosphorus
<1mg/kg
Trace metals
Fe<0.1mg/kg
Free fatty acids
<0.05%
Peroxide value
<1 meq O2/kg
Smoke point
>220°C
Linolenic acid
<2%
Dimethylpolysiloxane
2mg/kg
Cu <0.02mg/kg
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NO ideal oil or shortening that satisfies each and every frying application
 Types of food products
 Frying conditions
 Further processing
 Storage
 Expected shelf life
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Any oil that is selected may compromise of at least these three factors
Functionality
Ideal
Frying Oil?
Nutrition
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Cost
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A questionnaire for the appropriate oil chosen for any particular frying process
Area of Concern
Factors to be considered
Nutrition
Low Saturated FA content?
Level of trans-fats accepted?
Additives acceptable?
Process
Frying temperature?
Continuous or discontinuous frying?
Turnover rate?
Oil storage facilities?
Product
Type of product?
Fresh or stored?
Shelf life desired?
Sensory attributes?
Commercial
Cost?
Packaging type?
Environmental
Availability?
Score different oils and fats for the above concerns, and you will have the best 16
oil
selected for your application with the highest score
REFERENCES
1. Bernd Brinkmann. Quality criteria of industrial frying oils and fats. European
Journal of Lipid Science and Technology, 102, 539-541 (2000).
2. Choe E. and Min D.B. Chemistry of Deep-Fat Frying Oils. Concise Reviews in Food
Science. Journal of Food Science. 00, R1-R10, (2007).
3. Dobarganes, M.C. and Marquez Ruiz, G. Formation and analysis of oxidized
monomeric, dimeric and higher oligomeric triglycerides. In: “Deep Frying:
Chemistry Nutrition and Practical Applications”. 2nd edition. pp 87-110 (Edited by
M.D. Erickson. AOCS Press, USA) (2006).
4. Frank Orthoefer T. and Gary List A. Initial quality of frying oil. In “Deep Frying,
Chemistry, Nutrition and Practical Application”. 2nd edition. (Edited by Michael
Erickson D. AOAC Press, USA) (2007).
5. Manoj Gupta, Frying Oils. In “Bailey’s Industrial Oil and Fat Products”, 6th Ed.
Vol.6, (Edited by Fereidoon Shahidi) John Wiley & Sons, Inc. USA.(2005).
6. Marquez-Ruiz, G. and Dobarganes, M.C. Analysis of non-volatile lipid oxidation
compounds by high-performance size-exclusion chromatography. In “Analysis of
Lipid Oxidation”. pp. 40-69 (Edited by A. Kamal-Eldín and J. Pokorny, AOCS
Press, USA) (2005).
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