Dried lipids oxidation

Lipid hydroperoxides are either formed in an autocatalytic process initiated by hydroxyl radicals or they are formed photochemically. Lipid hydroperoxides, known as the primary lipid oxidation products, are tasteless and odourless, but may be cleaved into the so-called secondary lipid oxidation products by heat or by metal ion catalysis. This transformation of hydroperoxides to secondary lipid oxidation products can thus be seen during chill storage of pork (Nielsen et al, 1997). The secondary lipid oxidation products, like hexanal from linoleic acid, are volatile and provide precooked meats, dried milk products and used frying oil with characteristic off-flavours (Shahidi and Pegg, 1994). They may further react with proteins forming fluorescent protein derivatives derived from initially formed Schiff bases (Tappel, 1956). 

Dissolve 40 mg of DPPE in a mixture of 16ml dry chloroform and 2 ml dry methanol containing 20 mg TEA. Maintain under nitrogen to prevent lipid oxidation. 

Dried whole milk is prepared by conventional spray or roller drying, with some modifications of the preheat treatment of the milk. The product is usually stored under nitrogen to delay lipid oxidation and off-flavor development. In spite of the processing changes, flavor defects and short storage life have limited the markets for dried whole milk most of it is used in the confectionery and baking industries. Federal and FAO standards require a minimum of 20% but less than 40% milk fat and a maximum of 5% moisture. 

Bassette and Keeney (1960) ascribed the cereal-type flavor in dry skim milk to a homologous series of saturated aldehydes resulting from lipid oxidation in conjunction with products of the browning reaction. The results of Parks and Patton (1961) suggest that saturated and unsaturated aldehydes at levels near threshold may impart an off-flavor suggestive of staleness in dry whole milk. Wishner and Keeney (1963) concluded from studies on milk exposed to sunlight that C6 to Cn alk-2-enals are important contributors to the oxidized flavor in this product. Parks et al. (1963) concluded, as a result of quantitative carbonyl analysis and flavor studies, that alk-2-4-dienals, especially... 

For many years, in connection with certain food products, a barrier to freeze-drying has been the problems associated with the storage stability of foods that are susceptible to lipid oxidation. In order for such foods to have a reasonable shelf life and acceptable flavor characteristics, protective additives, which retard oxidation, are often added before dehydration. Such antioxidants must carry through the process and not be lost because of volatilization. For these applications, BHA, BHT, and tert-butylhydroquinone (TBI1Q) have been found quite effective. 

Dry Oil Oxidation Fluorescence may be measured within or over an oil or within or over an emulsion of lipid in water providing the pH is below 5.5. When plate fluorescence is to be measured, solid sample fluorescence spectrophotometry is necessary (23). A Hitachi Solid Sample Holder Attachment for Model MPF-2A Hitachi-Perkin Elmer Fluorescence Spectrophotometer (Figure 4) was... 

Some of the studies on salt are, no doubt, complicated by the fact that salt may contain metal contaminants, which could serve as catalysts of lipid oxidation. Nevertheless, rancidity may still develop in the fat of dry cured hams, even though salt with a low metal content is used (27). The use of an antioxidant in combination with sucF salt, however, did inhibit rancidity and improve flavor scores. Further work to clarify the role of salt in lipid oxidation is needed before its mechanism is fully understood. 

Morphine is extracted from the dried latex (opium) from the seed capsule of the poppy. Incisions, abont 1 mm deep, are made in the seed capsnle approximately two weeks after the petals have fallen from the flower and the seed head is mature, and the latex exndes from the seed head, dries and oxidizes in the sun and is collected the following day. With successive harvestings, a pod may produce anywhere between 10 and 100 mg of opium. The latex contains 10-20 wt% alkaloids, with the remainder of the latex being sugars, proteins, lipids, gums and water. 

Biochemical Degradation Biochemical degradation is another harmful transformation that occurs with most biological products. There are four key reactions to consider lipid oxidation, Maillard browning, protein denaturation, and various enzyme reactions. These reactions are both heat- and moisture-dependent such that control of temperature and moisture profiles can be very important during drying. 

Lipid oxidation. Lipid oxidation is normally observed as a product discoloration and can be exacerbated with excess levels of bleach. It is catalyzed by metal ions, enzymes, and pigments. Acidic compounds can be used to complex the metal ions. Synthetic antioxidants, such as butylated hydroxtoluene (BHT) and butylated hydroxyanisole (BHA) can be added to the product, but are limited and coming under increased scrutiny due to toxicology concerns. It may be preferable to use natural antioxidants such as lecithin or vitamin E or to dry under vacuum or in an inert (nitrogen, steam) atmosphere. 

A suitable size of the flask should be selected during rotary evaporation as to allow for the formation of a thin and homogenous lipid layer inside the flask. Preferably all steps of nanoliposome preparation should be carried out under nitrogen to minimise risk of lipid oxidation. If required, the dried lipid film can be stored in fridge under nitrogen for a couple of weeks, after which should be discarded due to possible dete-rioration/degradation of the lipids. 

Aldehydes are by far the most numerous compounds identified as dry-cured ham odorants, with different odors (green, rancid, toasted) and thresholds in air ranging from 0.09 to 480 ng/L (Table 1). Most of them were identified in the first works focused on dry-cured ham volatile compounds (7,2). Aldehydes are essential for meat flavor (70), but large quantities in meat and meat products have been related to lipid oxidation and deterioration (77). The effect of several quality factors has been researched and it was found that the rearing system of pigs (S) and ripening conditions (7) influence on the contribution to odor and the content of some aldehydes. 

Sodium chloride content is closely related to ham quality, not only because of its direct effect on taste but also because of its effect on flavour development (75). The influence of sodium chloride content on volatile compound generation could be related to its effect on lipid-oxidation reactions, enzyme activity and microbial growth. As expected, salt content affected significantly a number of compounds described previously as dry-cured odorants (Figure 1) seven aldehydes and two acids. 

The effects of water on the destruction of the protective food structure in some specific dehydrated foods is probably involved in prevention of lipid oxidation in heated meat systems (Karel, 1986). In systems in which there are both surface lipids and lipids encapsulated within a carbohydrate, polysaccharide, or protein matrix, the surface lipids oxidize readily when exposed to air. The encapsulated lipids, however, do not oxidize until the structure of the encapsulated matrix is modified and/or destroyed by adsorption of water as shown by Simatos and Karel (1988). In some IM meats, muscle may be considered as being encased in or surrounded by a humectant matrix. However, free lipid may be left on the surfaces. The unwarranted overuse of lipids, which often happens in the indirect drying process to improve heat transfer and to prevent burning, is detrimental to the products. Prevention of this structural change is of considerable... 

Karel, M. (1986). Control of lipid oxidation in dried foods. In Concentration and Drying of Foods (D. MacCarthy, ed.), pp. 37-68. Elsevier, London. 

It would be most desirable to study lipid oxidation in intact food products without the extraction or sample preparation steps currently necessary (e.g., pelletization, freeze-drying). Perhaps improvements in techniques such as the Fourier transform infrared photoacoustic spectroscopy (Yang and Irudayaraj, 2000) will open such possibilities. 

During storage of food products, volatile compounds produced by lipid oxidation cause rancidity, especially if the lipids contain linolenic acid. Dark flours become easily rancid on storage. Crackers and other durable bakery products should be stored in an inert gas atmosphere or be protected by antioxidants. Roasted products, such as peanuts, may change their agreeable flavor if stored in air therefore, they are stored either in nitrogen or under reduced pressure. Fried products are most sensitive to oxidative rancidification, especially Med products with weak flavor, such as fried bread, French fries, or potato chips. Dry and deep-frozen products are generally rather sensitive to oxidation because of easier access of air into the inner layers of the food product. 

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