Rancidification

Rancidificationis the process of complete or incompleteautoxidationorhydrolysisoffatsand oils when exposed to air, light, moisture, or bacterial action, producing short-chainaldehydes,ketonesand freefatty acids.^[1]

When these processes occur in food, undesirable odors and flavors can result. In processed meats, these flavors are collectively known aswarmed-over flavor.In certain cases, however, the flavors can be desirable (as inaged cheeses).^[2]

Rancidification can also detract from the nutritional value of food, as somevitaminsare sensitive to oxidation.^[3]Similar to rancidification, oxidative degradation also occurs in other hydrocarbons, such aslubricating oils,fuels,and mechanicalcutting fluids.^[4]

Pathways

Five pathways for rancidification are recognized:^[5]

Hydrolytic

Hydrolyticrancidity refers to the odor that develops whentriglyceridesare hydrolyzed and free fatty acids are released. This reaction of lipid with water may require acatalyst(such as alipase,^[6]or acidic or alkaline conditions) leading to the formation of freefatty acidsandglycerol.In particular,short-chain fatty acids,such asbutyric acid,aremalodorous.^[7]When short-chain fatty acids are produced, they serve as catalysts themselves, further accelerating the reaction, a form ofautocatalysis.^[7]

Oxidative

Oxidativerancidity is associated with the degradation by oxygen in the air.

Free-radical oxidation

Thedouble bondsof anunsaturated fatty acidcan be cleaved byfree-radicalreactions involving molecular oxygen. This reaction causes the release of malodorous and highlyvolatile aldehydesandketones.Because of the nature of free-radical reactions, the reaction is catalyzed by sunlight.^[7]Oxidation primarily occurs with unsaturated fats. For example, even though meat is held under refrigeration or in a frozen state, the poly-unsaturated fat will continue to oxidize and slowly become rancid. The fat oxidation process, potentially resulting in rancidity, begins immediately after the animal is slaughtered and the muscle, intra-muscular, inter-muscular and surface fat becomes exposed to oxygen of the air. This chemical process continues during frozen storage, though more slowly at lower temperature. Oxidative rancidity can be prevented by light-proof packaging, oxygen-free atmosphere (air-tight containers) and by the addition ofantioxidants.^[7]

Enzyme-catalysed oxidation

A double bond of an unsaturated fatty acid can be oxidised by oxygen from the air in reactions catalysed by plant or animallipoxygenaseenzymes,^[6]producing ahydroperoxideas a reactive intermediate, as in free-radical peroxidation. The final products depend on conditions: the lipoxygenase article shows that if ahydroperoxide lyaseenzyme is present, it can cleave the hydroperoxide to yield short-chain fatty acids anddicarboxylic acids(several of which were first discovered in rancid fats).

Microbial

Microbialrancidity refers to a water-dependent process in which microorganisms, such as bacteria ormolds,use their enzymes such aslipasesto break down fat.^[6]Pasteurizationand/or addition ofantioxidantingredients such asvitamin E,can reduce this process by destroying or inhibiting microorganisms.^[6]

Food safety

Despite concerns among the scientific community, there is little data on the health effects of rancidity or lipid oxidation in humans.^[8]^[9]Animal studies show evidence of organ damage, inflammation, carcinogenesis, and advanced atherosclerosis, although typically the dose of oxidized lipids is larger than what would be consumed by humans.^[10]^[11]^[12]

Thefree radicalpathway for the first phase of the oxidative rancidification of fats.

Antioxidantsare often used aspreservativesin fat-containing foods to delay the onset or slow the development of rancidity due to oxidation. Natural antioxidants includeascorbic acid(vitamin C) andtocopherols(vitamin E). Synthetic antioxidants includebutylated hydroxyanisole(BHA),butylated hydroxytoluene(BHT),TBHQ,propyl gallateandethoxyquin.The natural antioxidants tend to be short-lived,^[13]so synthetic antioxidants are used when a longer shelf-life is preferred. The effectiveness of water-soluble antioxidants is limited in preventing direct oxidation within fats, but is valuable in intercepting freeradicalsthat travel through the aqueous parts of foods. A combination of water-soluble and fat-soluble antioxidants is ideal, usually in the ratio of fat to water.

In addition, rancidification can be decreased by storing fats and oils in a cool, dark place with little exposure to oxygen or free radicals, since heat and light accelerate the rate of reaction of fats with oxygen. Antimicrobial agents can also delay or prevent rancidification by inhibiting the growth of bacteria or other micro-organisms that affect the process.^[1]

Oxygen scavengingtechnology can be used to remove oxygen fromfood packagingand therefore prevent oxidative rancidification.

Oxidative stability measurement

Oxidative stability is a measure of oil or fat resistance to oxidation. Because the process takes place through achain reaction,the oxidation reaction has a period when it is relatively slow, before it suddenly speeds up. The time for this to happen is called the "induction time", and it is repeatable under identical conditions (temperature, air flow, etc.). There are a number of ways to measure the progress of the oxidation reaction. One of the most popular methods currently in use is the Rancimat method.

The Rancimat method is carried out using an air current at temperatures between 50 and 220 °C. The volatile oxidation products (largelyformic acid^[14]) are carried by the air current into the measuring vessel, where they are absorbed (dissolve) in the measuring fluid (distilled water). By continuous measurement of the conductivity of this solution, oxidation curves can be generated. Thecusp pointof the oxidation curve (the point where a rapid rise in the conductivity starts) gives the induction time of the rancidification reaction,^[15]and can be taken as an indication of the oxidative stability of the sample.

The Rancimat method, the oxidative stability instrument (OSI) and the oxidograph were all developed as automatic versions of the more complicated AOM (active oxygen method), which is based on measuring peroxide values^[15]for determining the induction time of fats and oils. Over time, the Rancimat method has become established, and it has been accepted into a number of national and international standards, for exampleAOCSCd 12b-92 and ISO 6886.

References

^^a ^bLück, Erich; von Rymon Lipinski, Gert-Wolfhard (2000). "Foods, 3. Food Additives".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a11_561.ISBN 3527306730.
^Thomas, Alfred (2000). "Fats and Fatty Oils".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a10_173.ISBN 3527306730.
^Termes, Waldemar (1990).Naturwissenschaftliche Grundlagen der Lebensmittelzubereitung.Hamburg: Behr's Verlag. pp. 50–37.ISBN 978-3-925673-84-9.
^Klemchuk, Peter P. (2000). "Antioxidants".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a03_091.ISBN 3527306730.
^Freeman, I. P. (2000). "Margarines and Shortenings".Ullmann's Encyclopedia of Industrial Chemistry.doi:10.1002/14356007.a16_145.ISBN 978-3-527-30673-2.
^^a ^b ^c ^dRobin Koon (4 August 2009)."Understanding rancidity of nutritional lipids".Natural Products Insider.Retrieved7 April2019.
^^a ^b ^c ^dSergey, Bylikin (January 2014).Chemistry: course companion.Horner, Gary; Murphy, Brian; Tarcy, David (2014 ed.). Oxford.ISBN 978-0-19-839212-5.OCLC 862091138.{{cite book}}:CS1 maint: location missing publisher (link)
^Cameron-Smith, David; Albert, Benjamin B.; Cutfield, Wayne S. (23 November 2015)."Fishing for answers: is oxidation of fish oil supplements a problem?".Journal of Nutritional Science.4:e36.doi:10.1017/jns.2015.26.ISSN 2048-6790.PMC4681158.PMID 26688722.
^EFSA Panel on Biological Hazards (2010)."Scientific Opinion on Fish Oil for Human Consumption. Food Hygiene, including Rancidity".EFSA Journal.8(10): 1874.doi:10.2903/j.efsa.2010.1874.
^Albert, Benjamin B.; Cameron-Smith, David; Hofman, Paul L.; Cutfield, Wayne S. (2013)."Oxidation of Marine Omega-3 Supplements and Human Health".BioMed Research International.2013:464921.doi:10.1155/2013/464921.PMC3657456.PMID 23738326.
^Kanner, Joseph (2007)."Dietary advanced lipid oxidation endproducts are risk factors to human health".Molecular Nutrition & Food Research.51(9): 1094–1101.doi:10.1002/mnfr.200600303.PMID 17854006.
^Falade, A. O.; Oboh, G.; Okoh, A. I. (2017)."Potential health lmplications of the consumption of thermally-oxidized cooking oils – a review".Polish Journal of Food and Nutrition Sciences(in Polish).67(2): 95–105.doi:10.1515/pjfns-2016-0028.
^Rahmawati S, Bundjali B (2009)."Kinetics of the oxidation of vitamin C".Prosiding Seminar Kimia Bersama UKM-ITB.VIII(9–11): 535–546.
^Allen, J.C.; Hamilton, R.J. (1994).Rancidity in Foods.Springer-Verlag GmbH. p. 47.ISBN 978-0-8342-1287-9.
^^a ^bMiraliakbari, H. (2007).Tree nut oils: chemical characteristics, oxidation and antioxidants.Library and Archives Canada. p. 31.ISBN 978-0-494-19381-5.^{[permanent dead link‍]}