Ripan Goswami

Selection Criteria of Frying Oils

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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

Frying

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Selection basis

Product flavor Product texture Product appearance Mouthfeel Aftertaste Shelf life of the product Availability of the oil Cost Nutritional requirements

To meet current market desire, frying oil must be Low in saturated fat Low in linolenic acid High oxidative and flavor stability Not hydrogenated (trans-fat free)

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Changes during Frying

• In Food Loss of moisture Development of dark color, firm texture and fried flavor

• In Oil Bland flavor at initial frying, later develops fried food flavor in oil Changes due to chemical reactions Dark and viscous

• Oil and food both passes through an optimum stage after which quality of oil and product flavor declines

• Unsaturated fatty acids decreases with increasing number or time of frying

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 frying process - Important

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Chemical Changes in Oil during Frying

Fresh oil - Fried food appears bland Reused oil – Fried flavor

New compounds formed in oils during frying process

Frying is a complex process to understand completely and thoroughly..

Causative agent Change effect New compounds

Moisture Hydrolysis Free fatty acidsDiaclyglycerolMonoaclyglycerol

Air and Metals Oxidation Oxidized monomersOxidized dimers & polymersVolatile 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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Hydrolysis of Oil

• Hydrolysis in fats and oil due to lipolytic enzymes or moisture

• In Frying High moisture in foods to be fried Hydrolysis end products

• TAG + H2O Free fatty acids (FFAs) + di or monoacylglycerol

• More in oil with short-chain unsaturated fatty acids than long chain saturated fatty acids

• FFAs - Off flavors and decreases smoke point of the oil

• Di- and monogylcerides - Foaming of oil

• Frequent replacement of frying oil with fresh oil slows down the hydrolysis of frying oil.

• 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%

• Free fatty acid value is used to monitor the quality of frying oil during a frying process

• In a study by Dobarganes et al., well controlled frying operations of potatoes, hydrolytic end products lower than other degradation compounds.

• Oil and water do not mix except at high temperature under high pressure or due to surfactant during frying

• Presence of surfactant is considered majorly responsible to facilitate formation of oil and water solution during frying causing hydrolysis reaction

• 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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Hydrolysis of Oil

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• Lipid oxidation during frying highly complex, since both oxidative and thermal reaction occur simultaneously

• Oxidation during frying causes most significant changes in flavors of fried foods both desired and undesired

• Thermal oxidation mechanism similar to autoxidation, but faster

• 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

• As the temperature increases, solubility of oxygen in oil decreases, although the degradation reactions are accelerated

Oxidation of Oil

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Oxidation & Polymerization of Oil

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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

• At frying temperature (180°C), oxygen pressure reduced, initiation reaction dominates preventing the propagation reaction that involves oxygen

• Alkyl radicals (R•) concentration increases compared to alkylperoxyl radicals (ROO•)

• Polymeric compounds formed involving alkyl radicals (R•) and alkoxy radicals (RO•)

• 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

and alkoxy radicals (ROR•) by hydroperoxide decomposition in termination reactions

Oxidation & Polymerization

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• Polymerization leads to high variety of products with different polarity, stability and molecular weight

• 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

• Volatile compounds in frying oil that significantly imparts flavor quality of fried foods and oils, decreases due to Evaporation Decomposition Reaction with food components

• Formation of non radical volatile and non-volatile compounds indicate termination of oxidation

• Oils rich in linoleic fatty acids is more easily polymerized during deep fat frying than oil rich in oleic acid

Oxidation & Polymerization

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Frying Process

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

• Stability of oil for frying can be determined by relating to its inherent stability to oxidation

• 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

• Hence frying quality of oil with high unsaturated fatty acids and free fatty acids is not as good as oil with low unsaturated fatty acids and free fatty acids

Oil Stability

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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

• Depending upon the application, solid fats are sometimes blended with refined vegetable oil, hydrogenated fats and oils like shortenings

• 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 Cu <0.02mg/kg

Free fatty acids <0.05%

Peroxide value <1 meq O2/kg

Smoke point >220°C

Linolenic acid <2%

Dimethylpolysiloxane 2mg/kg

Oil specification

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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

• Any oil that is selected may compromise of at least these three factors

Ideal Frying Oil?

Nutrition

Functionality

Cost

IDEAL OIL?

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• A questionnaire for the appropriate oil chosen for any particular frying process

• Score different oils and fats for the above concerns, and you will have the best oil selected for your application with the highest score

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?

Some help?

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REFERENCES1. 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).