Lipids in foods

Because of their similarity to the composition of human bile, which consists mainly of bile salts, phospholipids, and cholesterol, of interest for pharmaceutical studies are mainly binary bile salt micelles (BS/PL) (32,33). The function of the bile is to emulsify lipids in food and to dissolve the fission products of lipids as well as fat-soluble vitamins. The spontaneous formation of micelles is a necessary prerequisite to a contact of the lipophilic fission products with the intestinal wall. For affinity measurements, micellar sys-... 

Table 19.1 reports a summary of application of HPLC dealing with analysis of different classes of lipids in foods. 

Westesen K., Drechsler M., and Bunjes H., Colloidal dispersions based on solid lipids, in Food Colloids Fundamentals of Formulation, Dickinson E. and Miller R., eds.. Royal Society of Chemistry, Cambridge, 2001, 103. 

Figure 9.28— ll NMR instrument used for routine analyses. Automatic analyser based on pulsed NMR used to quantify water and lipids in food technology. (Reproduced with authorisation of Bruker.)... 

Accurate determination of lipids in foods is required for nutritional labeling, certification, or for evaluation of standard of identity and uniformity, as well as examination of their effects on functional and nutritional properties of foods. Following lipid extraction and precise quantitative analysis, lipids so obtained may be used for analysis of other lipid characteristics and properties provided that nondestructive and mild extraction procedures are employed that retain the integrity of lipids. Thus, determination of lipid classes, fatty acid composition (unit du), and oxidative state of lipids (Chapter D2), amongst others, may be pursued following the extraction process. 

Procedures for isolation and measurement of lipids in foods include exhaustive Soxhlet extraction with hexane or petroleum ether (AOAC, 1995 see Basic Protocol 1), chloro-form/methanol (Hanson and Olley, 1963 Ambrose, 1969), chloroform/methanol/water (Folch et al., 1957 Bligh and Dyer, 1959 see Basic Protocol 2 and Alternate Protocol 2), acid digestion followed by extraction (see Basic Protocol 4), or, for starchy material, extraction with n-propanol-water (e.g., Vasanthan and Hoover, 1992 see Basic Protocol 3). Each method has its own advantages and disadvantages and successful measurement of lipid content is often dictated by the type of sample and extraction medium employed. Commercial extraction and preparation of edible oils are explained in the literature (Williams, 1997). 

Pomeranz, Y. and Meloan, C.E. 1994. Lipids. In Food Analysis Theory and Practice, 3rd ed. pp. 678-732. Chapman Hall, New York. 

Gunstone, F.D. and Norris, F.A., Lipids in Foods. Chemistry, Biochemistry and Technology, Pergamon Press New York, 1983. 

The carbohydrates, proteins, and lipids in foods are mostly in complex forms. For example, the carbohydrates are present as disaccharides, such as sucrose, or polysaccharides, such as starch. The first step in digestion is the breakdown of the larger, insoluble molecules into smaller, soluble forms that can be transported across the intestinal wall into the blood for delivery to the tissues. 

Of the different types of lipids in foods, the phospholipids, being more unsaturated, are particularly important in relation to aroma formation in meat.151 The aroma of cooked meat was not affected by the prior extraction of triglycerides with hexane, but the use of a more polar solvent (chloroform-methanol), which extracts all lipids, including phospholipids, resulted, after cooking, in the replacement of the meaty aroma by a roast or biscuit-like one. This was reflected in the volatiles, the dominant aliphatic aldehydes and alcohols being replaced by alkylpyrazines. This implies that the participation of the lipids in the Maillard reactions inhibited the formation of heterocyclic compounds. 

Lipids in foods vary from traces as in cereals to 30-50% as in nuts. The physical state and distribution of lipids vary considerably among food items. In each item lipid distributions affect its flavor as it undergoes chemical reactions and act as a flavor components vehicle or partitioning medium. Furthermore, lipids have a pronounced effect upon the structure of food items. Fatty acids of neutral (triglycerides) and polar lipids of beef and pork are tabulated in Table III. 

Van den Tempel, M. 1968. Surface-Active Lipids in Foods. S.C.I. Monograph No. 32, pp. 22-36, Society of Chemical Industry. London. 

Jeon, I.J. 1994. Flavor chemistry of dairy lipids review of free fatty acids. In Lipids in Food Flavors (C.T. Chang, T.G. Hartman, eds.), American Chemical Society, Washington, DC, pp. 196-207. 

Eriksson, C.C. 1987. Oxidation of lipids in food systems. In Auto-oxidation of Unsaturated Lipids (H.W.S. Chan, ed.), pp. 207-231, Academic Press, London. 

Although autoxidatlon of lipids in foods is generally considered as unwanted, certain products of lipid autoxidatlon at low concentrations are necessary to the characteristic odor and aroma properties of meats from different species (8.9.28 >. Therefore, the concentration and relative abundance of these chemicals in meat volatiles determine whether they play a desirable or an undesirable role in flavor characteristics of cooked meats. Thus, the origin of flavor and off-flavors developments, which are somewhat species-specific, are perhaps the same. So, in freshly cooked meats the specific flavor of meat which is species-specific develops and progression of autoxidatlon results in the formation of undesirable warmed-over flavor in cooked meats upon storage. 

Controlling lipid crystallization in foods has proven to be a technical challenge over the years. Despite a considerable amount of study, controlling the complex interactions between the various lipid components during crystallization remains essentially an empirical process of studying the effects of various operating parameters on crystal formation. Further work on the fundamental principles of lipid nucleation, growth, and polymorphic transformation is needed to truly control crystallization of lipids in foods. 

P. D. Gunstone and R A. Norris, Lipids in Foods Chemistry, Biochemistry, and Technology, Pergamon Press, Oxford, United Kingdom, 1983. 

Federal Regulations, Food and Drug Administration). In foods, much of the work on antioxidants has emphasized retardation of lipid oxidation, which eventually triggers and transforms to the oxidation of other macromolecules such as proteins. It is the intention of this chapter to summarize the available information on the chemistry, technology, and regulatory aspects of compounds that can delay oxidation of unsaturated fats and lipids in food. 

For examples of separation of lipids see general references. For tabulated examples, see Ref. B1 for separation of molecular species of phospholipids by HPLC, Ref. I for separation of lipids in food by HPLC, Ref. H for HPLC of phosphatidic acid, and Ref. B2 for preparative HPLC of lipids. 

The lipid-.soluble vitamins include vitamins A. D. E. and K. These compounds possess other characteristics in common besides solubility. They are usually as.sociatcd with the lipids in foods and are absorbed from the intestine with these dietary lipids. The lipid-soluble vitamins arc stored in the liver and. thus, conserved by the organism, whereas storage of the water-soluble vitamins is usually not significant. 

Hwang, H.I. et al., Aroma generation in extruded and heated wheat flour, in Lipids in Food Flavors, Ho, C.T. and Hartman, T.G., Eds., American Chemical Society, Washington, D.C., 1994, p. 144. 

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