Chemistry
Rancidity
Rancidity refers to the development of off-flavors and odors in fats and oils due to the breakdown of their chemical structure. This process is primarily caused by oxidation, which can be accelerated by exposure to light, heat, or air. Rancidity can negatively impact the taste, smell, and nutritional quality of food products containing fats and oils.
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6 Key excerpts on "Rancidity"
eBook - ePub- Dominic Man(Author)
- 2015(Publication Date)
- Wiley-Blackwell(Publisher)
−1 for the first and second steps, respectively) and is therefore not stopped by lowering the temperature of food storage. It can occur by one of the two mechanisms: the classical free radical mechanism involving a catalyst, for example copper ions, which can operate in the dark, or a photo-oxidation mechanism in the presence of a sensitizer, for example myoglobin, which is initiated by exposure to light (Hamilton, 1994). The first product in both cases is a lipid hydroperoxide, which itself is odourless, but which breaks down to smaller molecules that produce the Rancidity. There is yet a third mechanism of oxidation, which is the lipoxygenase route. The enzyme lipoxygenase is believed to be widely distributed in foods from the plant and animal kingdoms, for example cereals (wheat, barley, maize), oilseeds (soya, groundnut) and fish. The oxidative deterioration again involves a hydroperoxide intermediate that eventually gives rise to off-flavour products.Oxidation Rancidity decreases the nutritional quality of food because the free radicals and peroxides generated destroy polyunsaturated fatty acids and fat-soluble vitamins A and E in food. These intermediates can also react with sulphydryl bonds in proteins. As sulphur amino acids are often the nutritionally limiting amino acids in many proteins, a reduction in their content will invariably lead to a decrease in protein quality. Several classes of material in oxidised fat are known to have some effects. They include peroxide fatty acids and their subsequent end products, polymeric material and oxidised sterols. Polycyclic aromatic hydrocarbons produced from the pyrolysis of fats on grilling and roasting meat and fish are known carcinogens (Sanders, 1994).
- Leif H Skibsted, Jens Risbo, Mogens L Andersen(Authors)
- 2010(Publication Date)
- Woodhead Publishing(Publisher)
Part I Understanding and measuring chemical deterioration of food and beverages
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Oxidative Rancidity in foods and food quality
J. Velasco, C. Dobarganes and G. Márquez-Ruiz, Consejo Superior de Investigaciones Científicas (CSIC), SpainAbstract:
Oxidative Rancidity, one of the major causes of quality deterioration in foods, is caused by the oxidative deterioration of lipids by atmospheric oxygen. Lipids oxidize through a complex series of reactions giving rise to a myriad of non-volatile and volatile compounds that are responsible for off-flavours even at concentrations in the parts-per-billion range. This chapter provides a general overview of lipid oxidation in foods by presenting the main aspects of the oxidative development in general and, because oxidation of lipids is a complex process, for particular foods. The reaction mechanism and the principal factors affecting the rate of lipid oxidation are described. Then, the analytical methods normally applied to determine the extent of lipid oxidation, as well as accelerated methods to determine oxidative stability are presented. Finally, some recommendations are given on how to prevent or retard lipid oxidation.Key words lipid oxidation oxidative stability quality deterioration1.1 Introduction: oxidative Rancidity and food quality
Oxidative Rancidity in foods refers to the perception of objectionable flavours and odours caused by oxidation of the unsaturated fatty acid chains of lipids by atmospheric oxygen. Because of the ‘spontaneous
- Michael EJ Lean(Author)
- 2006(Publication Date)
- CRC Press(Publisher)
Different types of oil and fat show varying degrees of resistance to spoilage; thus, most vegetable oils deteriorate only slowly, whereas animal fats deteriorate more rapidly and marine oils, which contain a relatively high proportion of combined highly unsaturated fatty acids deteri-orate so rapidly that they are useless for edible purposes unless they have been refined and hydrogenated. Spoilage may occur in many ways, but two import-ant types of Rancidity may be distinguished, namely hydrolytic Rancidity and oxidative Rancidity. Hydrolytic Rancidity Hydrolytic Rancidity occurs as a result of hydrolysis of triglyceride molecules to glycerol and free fatty acids and it is brought about by the presence of moisture in oils. The rate of hydrolysis in the pres-ence of water alone is negligible, but it is hastened by the presence of enzymes and microorganisms. Oils and fats that have not been subjected to heat treatment may contain lipases which catalyse hydrolysis. They may also contain moulds, yeasts and bacteria present in the natural oil or they may become contaminated with them during processing. Such microorganisms hasten hydrolytic breakdown. The nature of the unpleasant flavours and odours produced by hydrolysis depends upon the fatty acid composition of the triglycerides. If the triglycerides contain combined fatty acids of low molecular weight containing 4–14 carbon atoms, hydrolysis yields free acids having characteristically unpleasant odours and flavours. For example, hydrolysis of but-ter yields the rancid-smelling butyric acid, while palm kernel oil yields considerable amounts of lauric and myristic acids. Oils containing combined fatty acids with more than 14 carbon atoms are not liable to hydrolytic Rancidity as the free acids are flavourless and odourless. Oxidative Rancidity Oxidative Rancidity is the most common and import-ant type of Rancidity and it results in the produc-tion of unpleasant rancid or ‘tallowy’ flavours.
eBook - PDF- B Baigrie(Author)
- 2003(Publication Date)
- Woodhead Publishing(Publisher)
6 Oxidative Rancidity as a source of off-flavours R. J. Hamilton, formerly of Liverpool John Moores University, UK 6.1 Introduction The major components of food are carbohydrates, proteins and lipids. Off-flavours can be produced from the carbohydrate and the protein part of the food, but it is mainly the lipid portion of the food which gives rise to off-flavours. Rancidity is sometimes defined as the subjective organoleptic appraisal of off-flavours in foods. It is subjective because the ability to per-ceive an off-flavour varies from person to person. The flavour threshold of a compound can vary greatly so that it is possible for a small amount of an off-flavour to give an unpleasant taste to a large quantity of a foodstuff.The flavour threshold is defined as the minimum quantity of a substance, which can be detected by 50% of the taste panel (Hamilton, 2001). This chapter will describe the three main chemical mechanisms involved in the oxidation of lipid molecules. Irrespective of the oxidation mechanism, it is recognised that the lipid molecule is oxidised to an intermediate which has no odour or flavour in itself. However, it is the breakdown of this lipid intermediate as in Eqn [6.1] which gives the off-flavour: [6.1] Unsaturated lipids can be oxidised in the dark and at room temperature. These conditions are associated with the production of free radicals and the mechanism is known as autoxidation (see Section 6.3). Depending on the type of food, the unsaturated nature of the lipid portion and the amounts of antioxidants present in the food, these reactions exhibit a lag phase (induction period). If some measure of the oxidation, for example perox-ide value or oxygen uptake, is measured with time, a graph like that shown Lipid Intermediate Secondary reaction products Æ Æ . in Fig. 6.1. can be produced which is sometimes called the induction curve (Simpson, 1999).
eBook - PDF- H A Bremner(Author)
- 2002(Publication Date)
- Woodhead Publishing(Publisher)
14.1 Introduction Rancidity is a perennial problem in oily fish associated primarily with the frozen and dried storage state. Indeed, the shelf-life of frozen oily fish is usually terminated by the onset of rancid flavours. Although there are studies investigating off-lavour development during chill storage, this is not generally considered to be as much of a problem because the susceptibility of fish to microbiologically induced spoilage during extended chill storage will precede any perceivable changes caused by lipid oxidation. In canned fish, the total elimination of oxygen during processing is sufficient to give these products a shelf-life of many years. This chapter discusses the factors affecting the development of rancid off-flavours in fish, particularly on frozen storage, and the measures that are reported to reduce the problem. The approaches discussed include the addition of antioxidants and chelators, the rise in interest in natural herb extracts, their efficacy and the legislation involved in the addition of antioxidants to fish products. The storage stability of products with elevated antioxidant content achieved via dietary supplementation (particularly tocopherol), modified atmosphere and vacuum packaging are also discussed. In the living animal the ingestion and regeneration of antioxidants prevents excessive oxidative deterioration of important biological components. Post-mortem, the protective systems become depleted and are unable to regenerate. Thus the edible muscle tissues of fish are liable to react with oxygen when exposed to air (oxidation). Oxygen may react with many of the biochemical components of animal tissues including lipids. It is the oxidation of polyunsaturated fatty acids (PUFAs)-containing lipids that causes the 14 Understanding lipid oxidation in fish I. P. Ashton, Unilever R&D, Sharnbrook development of off-flavours and aromas, often referred to as ‘Rancidity’.
eBook - PDF- Servet Gulum Sumnu, Serpil Sahin, Servet Gulum Sumnu, Serpil Sahin(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
4.3.1 H YDROLYTIC R ANCIDITY Hydrolysis leads to hydrolytic Rancidity and involves hydrothermal hydrolysis to FFAs and other products. Hydrolysis is the major chemical reaction that takes place during deep-oil frying. It occurs when food is fried in hot oil. Water, which is present as steam, attacks the triglycerides, which are hydrolyzed to FFAs, monoacylglycerols, diacylglycerols and glycerol (Roth and Rock 1972). Hydrolysis can also take place by the action of lipase enzymes. Low levels of FFAs are not necessarily objectionable, particularly if they are 16C- or 18C-fatty acids as commonly found in soybean or corn oil. However, for other fats such as coconut oil, low levels of shorter carbon-chain fatty acids may be quite objectionable. Hydrolytic Rancidity is not as important in oils containing pre-dominantly fatty acids of longer chain-length except when they are used as frying media (Robards, Kerr, and Patsalides 1988). During frying, when heat and water are present, FFAs may develop rapidly if poor processing techniques are adopted. Moreover, there is no additive that effec-tively prevents formation of FFAs. Antioxidants will not prevent formation of FFAs owing to chemical hydrolysis. 4.3.2 O XIDATIVE R ANCIDITY Oxidative Rancidity results from more complex lipid oxidation processes. According to Belitz and Grosch (1987), the autoxidation mechanism has several fundamental steps. The oxidation process is essentially a radical-induced chain reaction divided into initiation, propagation, branching and termination steps. During the initiation phase, molecular oxygen combines with unsaturated fatty acids to produce hydro-peroxides and free radicals, both of which are very reactive. For this phase to occur at a meaningful rate, some oxidative initiators must also be present, such as chemi-cal oxidizers, transition metals (i.e., iron or copper) or enzymes (i.e., lipoxygenases). Heat and light also increase the rate of this and other phases of lipid oxidation.
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