Pork, lipids

Four aspects of research involving the use of SFE for the improvement of quality of muscle food products are briefly discussed. These include supercritical CO2 extraction of lipids fi om fresh ground beef and from dried muscle foods the extraction and separation of lipid and cholesterol from beef tallow supercritical CO2 extraction of flavor volatiles from beef and pork lipids for use as additives in synthetic meat flavors and identification and quantitation of flavor volatiles extracted with SC-CO2. 

Fatty Acids Composition of Beef and Pork Lipids... 

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). 

Lipid characteristics. If the lipid becomes too unsaturated, the meat is not suitable for, for example, sausage production. Furthermore, products become oxidative unstable, accelerating rancidity problems, especially in many preheated catering products with an increased incidence of the development in warmed-over-flavour. Therefore, the anti-oxidative status of pork, for example content of vitamin, is an important technological quality criterion. 

Oxidatively stable raw materials are necessary to obtain a profitable shelf life of the products (Sheard et al., 2000). When fed above requirement levels, vitamin E increases the oxidative stability in fresh pork and pork products considerably (Jensen et al., 1998). Selenium is also involved in reducing lipid oxidation there is no evidence, however, that supplying additional selenium above the requirements improves pork quality (NRC, 1998). 

Fats and other lipids are poorly soluble in water. The larger the accessible surface is—i. e., the better the fat is emulsified—the easier it is for enzymes to hydrolyze it (see p. 270). Due to the special properties of milk, milk fats already reach the gastrointestinal tract in emulsified form. Digestion of them therefore already starts in the oral cavity and stomach, where lipases in the saliva and gastric juice are available. Lipids that are less accessible—e.g., from roast pork—are emulsified in the small intestine by bile salts and bile phospholipids. Only then are they capable of being attacked by pancreatic lipase [4] (see p. 270). 

Homstein and Crowe 18) and others (79-27) suggested that, while the fat portion of muscle foods from different species contributes to the unique flavor that characterizes the meat from these species, the lean portion of meat contributes to the basic meaty flavor thought to be identical in beef, pork, and lamb. The major differences in flavor between pork and lamb result from differences in a number of short chain unsaturated fatty acids that are not present in beef. Even though more than 600 volatile compounds have been identified from cooked beef, not one single compound has been identified to date that can be attributed to the aroma of "cooked beef." Therefore, a thorough understanding of the effect of storage on beef flavor and on lipid volatile production would be helpful to maintain or expand that portion of the beef market. 

Supercritical COj (SC-CO2) was used to reduce the lipid of meat and the cholesterol of meat and beef tallow. Lipids can be removed quantitatively from dried muscle foods by SC-CO2, but relatively high temperatures are needed. The use of SC-CO2 in conjunction with ethanol, adsorbents and multi-separators also reduced the cholesterol of beef tallow. SC-CO2 was also used to concentrate volatile flavor compounds from beef and pork fat. The volatile components in various extraction fractions were identified and quantitated. 

Most studies of SC-CO2 quantitative extraction of lipids from muscle foods have been limited to dried materials with moisture levels below 10%. King et aL (26), Froning (27) and Hardardottir and Kinsella (28) were able to quantitatively remove lipid from muscle foods and data for Aese studies are shown in Table I. This table also includes data from study of dried pork. Maximum removal of lipid from dried beef and luncheon meat was obtained at 300-345 bar/45-80 C (26, 27) and at 275 bar/80 C for trout (28). 

Lactones are minor constituents in pork fat compared to beef fat, but 5-decalactone was highly concentrated in the FI fraction of the 207 bar/50 C extract The furans, furanones and thiazoles were undoubtedly formed from Maillard reaction precursors and this type of volatQe would be more prevalent in the lipid fraction of cooked pork compared to pork fat alone. 

CLA has also been identified in the tissue lipids of other nonruminant animals. The CLA concentration in nonruminant animals (chicken and pig) is considerably lower than ruminant animals (Table 1). The exception among nonruminants is turkey (2.51 mg CLA/g fat), which is about five fold higher than chicken or pork. More than 76% of the CLA isomer found in nonruminant tissues is cis-9, trans-11 isomer. 

Olsen, E., Vogt, G., Veberg, A., Ekeberg, D., Nilsson, A. (2005), Analysis of early lipid oxidation in smoked, comminuted pork or poultry sausages with spices. J. Agric. Food Chem. 53 7448-7457. 

This group evaluated the impact of changes in fatty acid composition on headspace aldehyde content of fresh pork and demonstrated the utility of these saturated aldehydes as an indicator of lipid oxidation. 

Younathan, M.T. and Watts, B.M. 1960. Oxidation of tissue lipids in cooked pork. FoodRes. 25 538-543. 

It appears, then, that there is a general, meaty aroma, common to cooked beef, pork, and lamb (and probably poultry), attributable to the pyrolysis of the mixture of low molecular weight nitrogenous and carbonyl compounds extracted from the lean meat by cold water. But the aromas of roast beef, roast pork, roast lamb, and roast chicken are unmistakably different. The chemical composition of the muscular fat deposits of these animals differ appreciably, and it is to these lipid components that we must look to account for the specific flavor differences. Heating the carefully separated fat alone does not give a meaty aroma at all, much less an animal-specific one. It is the subsequent reactions of pyrolysis products of nonlipid components that give the characteristic aromas and flavors of roasted meats (20). 

Lipid decomposition volatiles. Reactions of sugar and amino acids give rise to odor profiles that are, at best, common to all cooked or roasted meats. The water soluble materials extracted from chicken, pork, or beef give reasonably similar meat flavor. To develop a species specific aroma one needs to study the lipid fraction and the volatiles produced from those lipids. The work of Hornstein and Crowe (10) reported that the free fatty acids and carbonyls generated by heating will establish the specific species flavor profiles. 

Many pyrazines were isolated and identified in cooked foods, especially in cooked meats (27). Pyrazines comprised over 40% of the volatile compounds found in cooked pork liver (28). Two pyrazines, 2-methyl-3(or 6)-pentylpyrazine and 2,5-dimethyl-3-pentylpyrazine were among 52 volatiles identified as lipid-protein-carbohydrate interaction products in a zein regular or waxy corn starch-corn oil model system (7). 

Esters formed during heating of lipids are contributors to pork flavor. Raw pork contains only a small number of esters, while cooked pork contains significantly more, and acetates are the most prominent volatiles. Esters of cooked pork are derived from C1-Ci0 acids, which impart a fruity sweet note to pork meat (J16). Beef contains a higher proportion of esters derived from long chain fatty acids which possess a more fatty flavor character (16). The characteristic odor of mutton is believed to be due to the evolution of 4-methyloctanoic acid, 4-methylnonanoic acid and similar compounds during heating (17). 

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. 

Pork fat contains more unsaturated fatty acids than beef and its linoleic acid content is double that in beef. Heat produced volatiles in red meat fats are listed in Table IV. The presence of pyrazines in the volatiles of beef fat is due to the presence of nitrogenous compounds in the fat amounting to 0.1-0.2%N, (Kjeldhal method). These compounds might be proteins, peptides, amino acids, and amine moities in polar lipids. Such compounds... 

Triglcerides Polar Lipids Beef Pork Beef Pork Fatty Acids % of Total Fatty Acid Content... 

Min, H.L., Nan, W.S., Ming, H.Y., Mei, L.W. and Youk, M.C. (1998) A rapid method for direct determination of free cholesterol in lipids. J. Chinese Agr. Chem. Soc., 36, 123—133. Montiel-Sosa, J.F., Ruis-Pesini, E., Montoya, J., Roncales, P., Lopez-Perez, M.J. and Perez-Martos, A. (2000) Direct and highly species-specific detection of pork meat and fat in meat products by PCR amplification of mitochondrial DNA. J. Agr. Food Chem., 48, 2829—2832. 

Our own work has shown a great decrease In the concentration of the volatiles In the cured, as compared to uncured, meats (Figure 5) (Ifi). The concentration of aldehydes originally present in cooked pork was reduced to < 12 of their original quantities (Table III, unpublished results). However, we did not Identify any new flavor active compound which could have been responsible for the cured flavor. Lipid oxidation, as measured by TBA number, was almost eliminated In cooked pork by adding nitrite at a level of 150 ppm (13.) Furthermore in preliminary evaluations, our untrained panelists were unable to differentiate amongst the flavor of nitrite-cured meats prepared from beef, chicken, mutton and pork (unpublished results). 

Lipids are biological materials that are insoluble in water but soluble in nonpolar organic solvents. Here the term will be used interchangeably with acylglycerol, the primary component of animal lipids. These are classihed as either fats or oils. The former are predominantly solid at room temperature (24°C), and the latter are liquid. The depot lipids of animals are generally fats. The major animal fats (also termed meat fats) of contemporary commerce are produced from pigs Sus scrofa), in which case they are termed lard and rendered pork fat, from the fat of cattle Bos taurus) or sheep Ovis aries) and termed tallow, or from poultry (primarily chickens. Callus gallus) and termed poultry fat. Tallow... 

Baker (98) to improve the separation efficiency by increasing the number of theoretical plates. Privett et al. (99) have used this device and were able to fractionate methyl esters of pork fiver lipids into 38 distilled fractions with chain lengths ranging from C14 to C22. Technical improvements of this distillation method were developed into molecular distillation and isolation of Vitamin D (100), esters of fish oil (101, 102), cholesterol, tocopherol (103), and fish oil to obtain high-purity DHA (104). 

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