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6 Existing Technological Options and Future Research Needs THE NEED TO MODIFY THE NUTRITIONAL ATTRIBUTES OF ANIMAL PRODUCTS Research on foocl-proclucing animals has led to decreased production costs, improved product quality, and advances in under- stancling human biological needs. Figure 1 provides a schematic Illustration of some of the interactions that occur between live- stock research and production, animal prod- ucts, life-styles, ant] human health. It is important to note that all interactions occur in both directions. In fact, the committee's major purpose is an example of this namely, to determine what technological options can be used to alter animal products to enhance human nutrition. The following questions must be taken into consideration: · What components of animal products are important to human nutrition and health? · What components of animal products can be altered with current technologies or through additional research? · What effect does altering the compo- nents of animal products have on shelf life, 115 visual appeal, flavor, texture, safety, nu- trient content, and stability of different retail products? · Is there sufficient consumer demand to justify the research en c] product develop- ment efforts necessary to generate new products? · Are there standards of identity or reg- ulatory aspects that preclude or seriously impede the development of new or altered animal products? The last question is of particular impor- tance, for in addition to health-related and marketplace needs, there must also be in place the appropriate technology and reg- ulations needed to develop wholesome, nu- tritious, and palatable products. The marketplace is changing in relation to consumer needs an(l the variety of food products that can be selected. Each year, about 6,000 to 8,000 "new" products appear that are either newly packaged, newly for- mulatecl, or newly fabricated. Many are in direct response to consumer concern about the link between nutrition and diet. The wide variety of (different dairy products on the market reflects this.
116 Animal Research 1 ~ Livestock Production \ - Food Processing ~ Food Research and Product Development Life-styles Quality of Life DESIGNING FOODS Human Nutrition and Health FIGURE ~1 Schematic of interactions among animal, food, and human dimensions affecting human health. It seems likely that animal products or their components will be increasingly al- tered, fractionated, ant] formulated to ad- dress consumer needs and market oppor- tunities, but this will require additional inputs in research and technology as well as reexamination of some current regulatory policies such as standards of identity. It is important to recognize who does research on animal products and how it is funded. Food product clevelopment can be clivicled into three distinct phases. First, the components of the foot] ingredient or raw material (such as an agricultural com- mo(lity) must be described. It may be de- sirable to separate these components for uses in other applications. In this case, the processes for separation and reformulation must be clevelopec3, ant! the characteristics of and potential applications for the individ- ual components must be determined. Sec- ond, it must be determined how the various components interact to give the food prod- uct its d~i~erent characteristics. Finally, the commodity, its individual components, or a partially modified product must be con- vertecI into a retail product that is whole- some, palatable, and in demand. In addi- tion, the product must have a reasonable shelf life, conform to all labeling and regu- latory standards, and, ideally, be nutritious. The first part of this research is usually conducted! by the public sector university or U.S. Department of Agriculture (USDA) laboratories. Likewise, much of the second phase is clone in the public sector, but, depending on the need and the product, a significant amount may be done in the private sector (for instance, by a food in- dustry firm). Some of the technologies d~e- velope(1 will be patented to protect invest- ments since the food product per se is generally not patentable. The third (limen- sion is primarily the responsibility of the private sector, mainly because of the market orientation of these firms. A variety of sources fund these research phases. Typical sources inclucle · State and federal government funcling of agricultural experiment stations (all three phases); · Commodity check-oE funds (all three phases); · Competitive government agency grants (limited amount in the first and second phases); · Industry-funded public research (first and second phases); and · Private industry, in-house research and development (primarily the second and third phases).
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS All these research efforts would benefit somewhat from a more systematic approach, especially in terms of product development. There is also a need to better coordinate work between the public and private sec- tors. A systems approach baser! on major topic areas, such as animal products, would help link some of the public and private sector programs that are contributing to similar goals. CURRENT STATUS OF TECHNOLOGY MANAGEMENT Before identifying ~. potentially useful changes in technology, the maturity of the technologies currently in use must be ex- amined. A too} commonly used for this purpose in strategic planning and technol- ogy forecasting is the S-curve, shown in Figure 6-2 (Becker ant! Speltz, 1986~. In a young technology (few agricultural production technologies are young), exten- sive long-term research is neecled to pro- duce technical progress. As the technology grows, significant advances can be made with smaller and smaller increments of ef- fort. But as technology matures, each effort produces smaller and smaller increments of progress. This is illustrates] by the top curve in the figure. At the midpoint of the curve, research productivity declines (see the bot- tom curve in the figure) and the research manager must decide whether sufficient gains can be maple to justify continued effort (research resources) or whether a new tech- nology must be ctiscovered, developed, or perfected to ensure continued technical progress and product growth or acceptance. As an example, if one uses a performance index for the modern broiler chicken that includes reproductive capacity, hatchabil- ity, growth rate, feed conversion, body composition, and the like and plots that index against time, an S-curve like that shown in Figure 6-3 might be constructed (hypothetically, since it is difficult to accu- rately reconstruct an index). The technolo o. o cat - - ._ 3 o a: / - Effort - 117 - Effort FIGURE ~2 The S-curve of technical progress versus effort. As technology matures, each effort produces smaller increments of progress (top curve); at the midpoint of the curve, research productivity declines (bottom curve). Source: R. H. Becker, and L. M. Speltz. 1986. Working the S-curve: Making more explicit forecasts. Res. Manage. 29:21. gies involved in shifting this index included nutrition, genetics, disease resistance and control, and management; but it is clear that some new technology was needed clur- ing the late 1960s or early 1970s. In fact, a new technology (dotted line in the figure) was being clevelope(1 recombinant DNA technology- but it was largely ignored by poultry scientists and other animal scientists and is only now, in the late 1980s, appearing on the food production scene. The research recommendations discussed in this section should be useful to research
118 100 90 80 70 - 60 o <~, 40 50 30 20 10 TradlUonal / Techn°l°gY / /e / Blotechnology . ! I ~1 1 1950 1960 1970 1980 1990 2000 Year FIGURE ~ A hypothetical Secure for broiler chicken growth per- formance. Source: R. H. Becker, and L. M. Speltz. 1986. Working the S-curve: Making more explicit forecasts. Res. Manage. 29:21. administrators in selecting the most appro- priate technological options for improving the nutritional attributes of animal products. ASSESSING CURRENT AND FUTURE TECHNOLOGIES The committee organized two workshops to assess (1) the knowledge that is currently available and that can be implemented im- mecliately to modify the composition of animals ant] animal products and (2) the new technologies that may eventually be useful for modifying the composition of animals and animal products. Both work- shops were held at the National Academy of Sciences' Woocis Hole Study Center dur- ing summer 1986. The objective of the first workshop was to document current knowledge related` to the measurement of intact body and carcass composition; the influence of genetics, nu- trition, ant! management on the composition of animal food products; and the influence of processing technology on the composition DESIGNING FOODS of foods made from animal products. The second workshop was convened to identify new technologies offering promise for in- creasing the nutritional quality of animal products. Special emphasis was given to identifying those technologies that influence growth particularly the repartitioning of fat to muscle. Papers presenter! at these workshops ap- pear in the Appendix and are cited through- out this chapter. TARGET LEVELS OF NUTRIENTS AND RELATED RESEARCH PRIORITIES Determining the Level of Fat in Live Animals and Carcasses More than 30 techniques exist to estimate live animal and carcass composition. Equip- ment costs range from $1 to over $1 million (Topel and Kauffman, this volume). For commercial use, accuracy must be consid- ered as well as cost ant! practicality. Re- search is needed to improve certain methods
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS ant! to make them less expensive ant] more practical. Economic imperatives to use these techniques are also necessary. This calls for marketing incentives that favor trim, mus- cular animals, which, at present, are re- ceiving only minor premiums in the mar- ketplace. There is considerable variation in body composition among animals of the same species and between different species, de- pending on growth stage, nutritional his- tory, en cl genetic base. Pork ant! beef car- casses average 30 to 35 percent fat and 35 to 50 percent muscle (Topel and Kauffman, this volume). Increased muscularity should become important to the livestock industry as consumer demand for leaner animals increases and economic pressures mount in favor of more efficient livestock production. Many indirect methods of varying degrees of complexity are available to estimate body fat. Most ofthe methods have been valiclatecl for predictability and precision by other indirect methods but rarely by direct carcass analysis of an animal. Therefore, the final choice of an indirect method ultimately depends on cost, the objective of the meas- urement, and the physical conditions under which the method is to be used. Survey of Method Older methods of determining fat levels include linear measurement of live animals ant! carcasses and the back fat probe for live animals. Linear measurement is not satis- factory for live animals but floes provide good (though not excellent) information about carcasses. The back fat probe is reasonably accurate, easy to standardize ant] use, and inexpensive; but it is slow for large numbers of animals. While the back fat probe is considered commercially practical at this time, it is not widely used (Topel and Kauffman, this volume). Other simple techniques include the re- flectance probe, live weight, ant] visual assessment. The reflectance probe is widely 119 used in Europe but not in the United States. It is simple and fast and also indicates some meat quality characteristics. Growth curves developed from the live weight of animals can be used to estimate body composition, if genetic history is known. However, the correlation of live weight with fatness can also be influenced by feeding, environment, health status, ant] digestive tract contents. Visual assessment and subjective evaluation is the most common technique used to estimate composition, but because of diffi- culties in distinguishing muscle from fat, it is of limited value (Topel and Kauffman, this volume). Newer methods of fat measurement use sophisticates] physical and chemical tech- nologies. Ultrasonic measurement is based on the principle that high-frequency sound waves pass through tissue but are reflected back at the interface between two different types of tissue. Time variations for return of reflected signals measure distances be- tween tissue boundaries. Of the many non- destructive evaluation techniques, ultra- sounc] may have the greatest immediate practical potential (Tope! and Kauffman, this volume). Video image analysis could replace or supplement subjective visual assessment for grading carcasses. The technique uses a video camera to create an image that is then processed by an analog/cligital converter ant] analyzed by a computer. While application is not simple, its benefits point toward future adoption by the U.S. beef industry (Topel ant] Kauffman, this volume). Whole-body potassium counting of a live animal relies on the direct relation of po- tassium to lean body mass and its indirect relation to fat. It is a useful research tool, but the bulky and expensive equipment and the time required, as well as some uncer- tainties in measurement, restrict commer- cial application (Topel en cl Kauffman, this volume). Body (lensity methods treat the body as a two-component system fatty tissue and
120 fat-free body each component having a different and constant density. The propor- tions of the components are estimated from the density of the whole body. Problems arise in measuring the volume of live ani- mals, and the methoc] is slow; therefore, its use is limited mostly to research (Topel and Kauffman, this volume). The Anyl-ray technique utilizes x-ray at- tenuation as an index of tissue fatness and is used commercially for ground meat. The tissue-saw~ust technique for frozen car- casses is used only as a research tool. Di- lution techniques introduce a known amount of tracer that becomes uniformly distributed in the boc3`y's water; when equilibrium is reached, the tracer's concentration is meas- urecI. Soluble, short-lived radioactive gas tracers are halogenated gases with an affinity for fatty tissue. The amount of these gases taken up is user! in research to estimate body composition. Urea dilution may be applicable to both research and industry (Topel and Kauffman, this volume). Computerized tomography (CT) presents body areas by computed synthesis of an image from x-ray transmission data. The CT scan is widely used in human medicine and has great potential as a research too! and also in genetic selection of breeding stock. European researchers have adopted com- puterized tomography faster than Ameri- cans (Tope! and Kauffman, this volume). In nuclear magnetic resonance (NMR) imaging, strong magnetic fields and pulsed radio waves incluce resonance of protons within the bo(ly; these protons return to their original orientation in a measured time and an image is produced. NMR is being used in human medicine and has great potential for application to the livestock industry, but it is expensive ant! complex (Topel and Kauffman, this volume). Near-infiared reflectance is currently used to predict the composition of plant materials and may be adapted for analysis of carcass composition. It is simple and inexpensive, DESIGNING FOODS but research is needed to develop it for commercial use (Topel and Kauffman, this volume). Total body electrical conductivity (TO- BEC) utilizes the principle that muscle conducts electricity more readily than fat because of its higher water and electrolyte contents. In practice, the animal is sur- rounded by a coil to which a current is applied, generating an electromagnetic field. The animal absorbs heat energy, perturbing the field. The loss of energy detected in the coil measures the animal's conductive mass. The theoretical basis of TOBEC has been confirmed, and the method has been applied to both human and animal subjects. TOBEC technology is promising, but more research is needed to determine its accuracy (Bo- ileau, this volume). Influencing the Level of Fat in the Growing Animal An animal's belly composition results from its cumulative growth. Altering the propor- tion of fat to lean therefore requires regu- lation and modification of growth. Lipid composition presents the greatest source of muscle tissue variation (Allen, this volume). The primary lipid fraction contributing to this variation is the triglyceride fraction that is stored in adipocytes within the muscle. These deposits are commonly referred to as marbling, ant] within the range of marbling found in the longissimus dorsi muscle of beef, the ether-extractable lipid (primarily triglyceride) varies from 1.77 to 10.42 per- cent (mean values for marbling scores) on a wet tissue weight basis (Savell et al., 1986~. In the present and near future, the most promising approach to enhancing the rate and efficiency of muscle growth (increasing lean tissue, decreasing fat tissue) is the administration of recombinant hormones (Allen, this volume). Recombinant growth hormone has been shown to have impressive effects on growth, feet! efficiency, and car
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS cass composition in pigs (Etherton, this volume). Research has also shown that re- combinant-derived bovine growth hormone dramatically increases milk production and mammary growth in dairy cattle (Gorewit, this volume). Transgenic animals, whose genes are transmittable to subsequent gen- erations, may have a place in livestock production systems, although reproduction has sufferer] in some early studies (Hammer, et al., 1985~. It may also be possible to construct and perpetuate important hor- mone genes that can be regulatecl at will by coupling them to promoters that can be turned on or off at critical periods through nutritional, pharmacological, or environ- mental manipulation (Allen, this volume). Technologies can be used to reduce fat deposition in the growing animal, which should facilitate production of animals with the appropriate amount of fat, thereby pre- clucling the need for extensive trimming of fat from carcasses after slaughter. The con- tributions of genetics, nutrition, and man- agement to fat reduction in cattle, swine, poultry, and milk products are reviewed next. Cattle Efficient production of palatable lean beef must be a primary objective of the beef industry if it is to maintain its competitive position over the long term. Traditionally, production of lean beef has been increased by breeding cattle of a larger frame size. These cattle produce beef that contains more protein ant] less fat than the beef produced by earlier-maturing (smaller frame size) strains or by breecis that were favored in the past (Byers, Cross, and Schelling, this volume). However, it would! be cost- effective to modify cattle growth so that lean beef could be produced regardless of the animal's frame size. In the future, ge- netic engineering may be applied to this problem, but for now, growth management 121 strategies offer immediate application. These require scientific knowledge of genetics, nutrition, ant! growth regulation. An animal's genetics establishes the pat- terns, limits, and types of growth that can be obtained. Nutrition affects the rate of deposition of fat and protein in the growing animal. As the growth rate increases, the proportion of protein decreases while the proportion of fat increases. Thus, animals manager] in (referred feeding programs will be leaner at any slaughter weight and will also be heavier when typical slaughter end points are reached (Byers, Cross, ant] Schelling, this volume). Integrated growth management programs seek to regulate growth by synchronizing nutrient supplies and nutrient needs to support the type of growth desired. The use of growth hormones, growth hormone releasing factors, beta-acirenergic agonists, and immunization strategies to remove neg- ative feedback on growth may later prove useful in these programs (Schelling and Byers, this volume). For the present, ana- bolic implants are effective as growth pro- moters, shifting nutrients from fat cleposi- tion to protein accretion ant! also enhancing growth rates (Byers, Cross, and Schelling, this volume). Current technologies to optimize tissue growth include synchronization of nutrition with the animal's needs for protein growth, continuous clelivery of repartitioning agents in all phases of growth from birth to slaugh- ter, and use of intact male animals, which provide leaner cuts than do cows or castrated bulls (Byers, Cross, and Schelling, this vol- ume). Desired results are reduction of fat deposition; generation of leaner beef through production rather than trimming; mainte- nance of desirable beef quality, flavor, an(l taste; and establishment of beef as a "lean" product. Research programs should be tar- geted to yield beef products that meet consumer preferences, to implement avail- able technology, ant] to develop new tech
122 DESIGNING FOODS nologies that allow more precise regulation odor in the meat). Other potential applica lions might result from research showing that immunization of lambs against soma tostatin can improve growth and that im munization of rats against differentiation of preadipocytes into fat cells can result in a 30 percent reduction in carcass fat (Speer, this volume). This last technique has been extended experimentally to sheep, and theoretically could be applied to any spe cies, including swine, cattle, and poultry. Overall, a number of options are currently available to the producer to change carcass composition in the market hog, an(l several other experimental products or procedures hoicl promise for reducing fatness en cl in creasing muscularity. However, the pork industry requires guidance on desirable lev els of fat in lean tissue to ensure consumer acceptance of its products. Ot growth in animals to meet market needs. S. wine Dramatic changes in swine carcass com- position have occurred during the past 15 years of genetic selection, yielding the mod- ern lean-type hog. More options are avail- able to further reduce back fat and increase muscling such as breeding, nutrition, man- agement, and endocrinology (Speer, this volume). The percentage of fat in market hogs differs among sex classes with intact males (boars) being lowest, females (gilts) inter- mediate, and castrates (barrows) highest. The percentage offal also varies with weight; above 90 kg, lean generally plateaus and fat increases. Nutrition has some influence; increasing protein intake can reduce fat deposition, while increasing fat intake has the opposite effect. Restriction of the ani- mal's overall feed intake increases the pro- portion of lean tissue in the carcass. In addition, the fatty acid composition of die- tary fats directly correlates with fat clepo- sition in the animal. Thus, increasing the percentage of unsaturated fatty acids in the pig's cliet will cause an increase in unsatu- rated fatty acids in the carcass tissue (Speer, this volume). A number of hormones can be adminis- tered to improve carcass composition in favor of lean tissue, including methyTtestos- terone, epinephrine, ant] the beta-acirener- gic agonists (Speer, this volume). Porcine somatotropin, administered by daily injec- tion, has been shown to improve daily gain, feed efficiency, and carcass measurements (Etherton, this volume). It can now be manufactured in large quantities via genet- ically engineered bacteria, thus expanding possibilities for its field application. A new application of immunology to swine production may come from recent work on immunization against androstene steroids (those compounds that cause boar or sex Poultry Fat content varies in ciressed, ready-to- cook broilers. As the percentage of fat in- creases, the percentage of protein, minerals, en cl vitamins (lecreases. Thus, the fat con- tent of poultry affects its nutritional value more than does any other factor. Broilers currently have 2 to 3 percent of their live body weight as abclominal fat, which is often cliscarcled before cooking. The total body fat of broilers ranges from 15 to 20 percent of live weight ant! is mostly subcutaneous. Muscle fat varies less than skin or abclominal fat (Gyles, this volume), but intramuscular fat is higher in reel muscle (leg and thighs) than in white muscle (breast). Several genetic options exist to recluce fat in broilers. Strain selection against fat is practiced commercially. Canclidate breeil- ers can be chosen on the basis of fat content of spent (lams, but this methoc] is not currently used. Selection for improved feet] efficiency is effective both in reducing fat deposition ant! in improving growth and carcass yiel :1 and is wiclely used in the poultry industry. Selection directed against
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS very-low-density-lipoproteins in sera re- duces final carcass fat and is used to some extent in the poultry industry (Gyles, this volume). Nutrition options are considered short term and palliative compared with genetic solutions, but many nutritional components can be manipulated to reduce fat content in poultry. Changing the energy to protein ratio in favor of protein; attention to protein quality in feed; restricting feed intake during early life or, alternatively, shortly before marketing; ant! formulating special feeds for males versus females to precisely meet nu- tritional requirements can all reduce final carcass fat percentage. In additions the type of dietary fat determines the chemical com- position of carcass fat: a diet rich in unsat- uratec] fatty acids results in an increased proportion of unsaturated fatty acids in the carcass (Gyles, this volume). Management options include marketing broilers at younger ages and at a smaller size and weight to reduce fatness, growing males and females separately to address their different feed requirements, and al- lowing marketing of younger females ant! older mates (Gyles, this volume). The most practical of these options to reduce fatness, subject to the needs of a particular poultry organization, may be ge- netic-strain selection against abdominal fat, selection against very-Iow-density lipo- proteins in blood sera, and selection for improved feed efficiency-and nutri- tional- manipulation of the energy to pro- tein ratio and restriction of feed energy shortly before marketing. Milk Milk fat, lactose, and proteins are syn- thesized in the mammary gland cells from precursors absorbed from the blood. They are releaser] in the milk by apocrine, mer- ocrine, or holocrine secretion. Many phys- iological and environmental factors can in- fluence milk secretion; among those related 123 to increases in yield are increased body weight, advancing age, increased level of nutrition, fall or winter calving, and mocI- erate or cool environmental temperatures (Gorewit, this volume). Fat content in milk can vary, subject to a variety of factors. Natural variation among breeds of dairy cows ranges from 3.4 to 5.6 percent milk fat (Bonner, 1974~. Total milk yield! ant] percentage composition of milk constituents have a negative genetic corre- lation, making it difficult to breed to im- prove both traits simultaneously (Linn, this volume). Milk fat ant! protein content are positively correlated (Bonner, 1974~; thus, genetic selection for lowered] fat content should also decrease protein content. Cur- rent dairy industry incentives are geared toward the maximum production of milk that contains the maximum content of both fat and protein. During a normal lactation of the dairy cow, the milk yield starts at a high level, peaks 3 to 6 weeks after calving, and then gradually declines toward the end of lacta- tion (Gorewit, this volume). Milk fat an(l protein percentages are inversely related to milk yield (Gorewit, this volume); in adcli- tion milk fat percentage can be affected by environment/management and health/phys- iology. Variations occur with stage of lac- tation, season, and the milking process. Mastitis can also affect fat content, as can hormones. However, one of the most im- portant means for causing variation appears to be diet (Linn, this volume). Cows can be made to produce milk with a lowered fat content by feeding on a high- concentrate/low-roughage diet (Gorewit, this volume). This diet also increases the pro- portion of unsaturated fatty acids in the milk. However, high-concentrate/low- roughage diets can cause health problems in cows, notably rumenitis and liver ab- scesses, and therefore have not been used commercially. It has been shown, though, that milk fat percentage can be lowered from the normal 3.5 percent to 1.0 percent
124 in severe cases of"milk fat depression" (Linn, this volume). Other dietary changes can also cause milk fat depression, inclucling heat-treatec] or pelleted feeds, the physical form of the ~ ~ ~ ~ ~ ~ ha ~ preferences. feed, the amount of dietary fat, and the lushness of pasture. However, high-grain/ low-roughage is the most important type of fat-depressing diet (Bonner, 19741. It may speed up nutrient passage, allowing less time for absorption of milk fat precursors, and alter rumen fermentation to increase the proportion of propionate, causing changes in physiological pathways that lead to de- creased milk fat synthesis. Furthermore, insulin levels may rise, inhibiting mobili- zation of fat from adipose tissue (Bonner, 1974; Linn, this volume). Little research has been performed on the long-term health effects of fat-clepressing diets in cows. Dietary fats themselves can alter milk fat composition. They can appear in milk fat without being changed during digestion and absorption, or they can be hydrogenate by rumen microorganisms or dehydrogenated before their incorporation into milk fat. They can also affect lipid metabolism in the animal. It is possible to increase the pro- portion of polyunsaturated fatty acids in milk fat by increasing their proportion in dietary fat through the use of oilseect sup- plements (Linn, this volume). A variety of dairy products have been test manufactured from such milk. However, increaser] po- lyunsaturated fatty acids reduce shelf life via faster oxidation, which also changes product flavor, aroma, and color. There are conflicting reports on the direction of change in total milk fat content when the proportion of polyunsaturated fatty acids in the diet is increased. Altering the Level of Fat in Animal Products Various processing technologies exist that can alter the fat level or change fat com- position in animal products. Whether these DESIGNING FOODS will be used commercially depends on such factors as product safety, economics of man- ufacture, storage life, and effects on sensory characteristics and product identity, as well as on ~overnrnent regulations and consumer Processed Beef, Lamb, and Pork Commercial production of"95 percent fat-free" hams has been a notable success. The technique of"restructuring" a product probably represents the ultimate in fat re- duction, since muscle with all visible surface and seam fat removed still contains about 0.5 to 5.0 percent fat as intramuscular fat and extractable intra- and intercellular lipids (Rust, this volume). The commonly accepted level of 25 to 30 percent fat in cooked sausage is difficult to reduce without causing the meat to have a rubbery, tough texture. This can be offset by added water, but current USDA regu- lations restrict this practice (Rust, this vol- ume). It might be better to regulate sausage composition based on minimum protein instead of the current fat and water maxi mums. It is also possible to substitute a non- binding protein for some of the fat in sau- sage. For instance, 10 percent cooker! pork skins can be substituted for 10 percent pork fat in (Iry sausage. USDA labeling require- ments for identifying "mechanically sepa- rated meat" may discourage processors from adopting this technology and using this product because of fear of consumer resist- ance (Rust, this volume). Fat can be modified in processed meat products by substituting vegetable fats and oils for animal fat. For example, vegetable oil preemulsified with milk proteins can be substitutes] for two-thirds of the animal fat in bologna. Stabilizer! preemuisions can be used to reduce visible fat in meat products. However, current USDA labeling require- ments prevent commercial applications of either of these proceclures.
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS Poultry Poultry products have a relatively high nutrient to calorie ratio. Even so, poultry meat is the current focus of fat-recluction technologies seeking to increase preference for poultry in the consumer market. Be- tween 1965 and 1985, per capita U. S. poul- try consumption increased 72 percent; how- ever, this reflected a 54 percent decrease in whole poultry consumption and a 575 percent increase in further processed poul- try consumption. Three-quarters of the poultry consumed in 1985 was cut up or further processed (Mast and Clouser, this volume). Thus, the growth potential in this industry lies in increasing the demand for poultry convenience foods rather than in- creasing purchases of whole bircis. The fat content of skinless, uncooked poultry is low, ranging from 1.6 to 4.9 percent, depen(ling on the type of bird and the type of meat (light versus dark). These amounts of fat increase four- to sevenfold for meat with the skin intact (Mast ant! Clouser, this volume). As with most other meats (beef, veal, pork), less than half of the fatty acids in poultry are saturated, but the proportion of polyunsaturated to satu- rated fatty acids is higher in poultry than in other meats. When total lipids are decreases] in poultry, the proportion of phospholipicls and cholesterol rises en cl the proportion of triglycericles clecreases. There is slightly more cholesterol ant! a higher overall fat content in dark versus light meat because of the fat depots between muscles. The depot fat, however, has more triglycericles than does the intramuscular fat (Mast and Clouser, this volume). Consumption of fat from poultry has in- creased more than threefold since the early l900s. While this mainly reflects an overall increase in poultry consumption, chicken (80 percent of the poultry consumed) has been higher in fat since the 1960s owing to changes in breeding and feeding (Mast and Clouser, this volume). The demand for 125 larger and faster growing birds has led to production of carcasses with 10 to 15 percent more fat, most of which lies in the bird's abclominal fat pad. The fat pad averages 40 grams and is 2.5 percent of total carcass weight and 10 percent of total body fat. Consumers remove it before cooking, now processors are removing it prior to market- ing. Current poultry production practice necessitates removal of excess carcass fat at the processing level, thereby increasing costs to both processors and consumers. Poultry frankfurters contain 18 to 22 per- cent fat versus the 25 to 30 percent fat found in beef and pork franks. Some pro- ducers have lowered the fat content of poultry franks to 13 to 16 percent by using mechanically deboned poultry from the breast and neck sections, which contain less fat than the backs or legs (Mast ant! Clouser, this volume). However, as with low-fat beef and pork franks, such products tend to be rubbery, tough, and less acceptable to con- sumers. Fat can also be recluced in fried poultry products. The four standard] cooking meth- ods for battered and breaclecl, fries] com- mercial products all yield a final meat with similar fat content. Breacled chicken prod- ucts with reduced fat ant] caloric content can be manufactured, however, by remov- ing the skin from the meat prior to breacling and then hot-air cooking instead of deep fat frying (which recluces fat by 23 to 31 percent and calories by 13 to 15 percent), for a total caloric decrease of 42 to 65 percent and a final fat content of 27 percent of total calories (versus 60 percent in conventional cooking) (Mast and Clouser, this volume). Such cook- ing systems are likely to become widely used as consumer demand accelerates for processed poultry products with lower cal- ories and fat. Dairy Products There is an increasing demand for low- fat milk products, which are clerived by
126 processing whole milk. Processing technol- ogies can also be used to exploit surplus milk fat and to separate and concentrate it for the manufacture of other dairy products. The cheese-making process concentrates the protein and fat components of milk, reduces the water, ant! eliminates the car- bohydrate. The whey derived from cheese manufacture can be further processed to concentrate the highly nutritional proteins lactalbumin and lactoglobulin. UltrafiItra- tion is now being used to concentrate whey proteins, to manufacture cheese base for further processing, and to concentrate milk fat and protein for other cheese manufacture (Hettinga, this volume). Ultrafiltration is a high-pressure microfiltration process that selectively segregates components of var- ious molecular weights. Milk-processing membranes have been cleveloped with vary- ing pore sizes to retain fat and protein while letting lactose, water, ant] salts pass through. While the United States has a surplus of butterfat, it is still relatively expensive and therefore often substituted for, rather than used in, food formulation. Methods of Altering Cholesterol Levels in Animal Products Milk The concentration of cholesterol in bovine milk ranges between 10 and 15 mg/100 ml, or 0.2 to 0.4 percent of total milk lipid. Milk cholesterol is 95 percent unesterified; the balance is esterified to long-chain, usu- ally saturated, fatty acids. Seventy-five per- cent of milk cholesterol is dissolved in milk fat, 10 percent is in the fat globule mem- brane, and 15 percent is in the skim milk (Hettinga, this volume). The effects of com- mercial processing on the concentrations and distribution of milk cholesterol are poorly defined, but this information is needec] so that technologies can be applied to decrease the cholesterol content of milk. DESIGNING FOODS A cholesterol reductase from species of Eubacterium might have use in converting milk cholesterol into coprostanol and cho- lestanol, which are poorly (or not at all) absorber! by humans. Supercritical carbon dioxicle extraction also holds promise for reducing the level of cholesterol in milk. However, it will be necessary to penetrate the milk fat globules, which contain most of the cholesterol, without destroying the globules themselves (Hettinga, this vol- ume). In general, supercritical fluid extrac- tion works by penetrating the structure of a material to be separated, dissolving soluble components, and carrying them away. Ad- vantages of this method compared with conventional extraction techniques include reducecl energy costs, higher yields, lower operating temperatures (yielding better quality products), and elimination of explo- sive or toxic solvents. At present, this tech- nology is too expensive and its technical feasibility for removing lipids an(l choles- terol is questionable. . Eggs Annual egg consumption has declined consistently since the 1940s, from 400 to 260 eggs per capita (Mast and Clouser, this volume). This is largely attributable to health concerns about cholesterol, which is present at the level of 545 mg/100 grams of whole egg, or about 270 mg per large egg. Much past research focused on how to reduce egg cholesterol by altering hens' diets or by genetic selection; these approaches met with varying degrees of success (or failure). Over- all, the nutrient composition of eggs has not changed greatly in response to modern industry practices. Eggs from hens fed the usual commercial diets differ little in the amount of cholesterol they contain. While unusual diets can increase or (lecrease cholesterol, they also tend to have cleleterious effects on the nutritional value of the egg or the hen's performance. Drugs
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS adcled to hens' diets can reduce cholesterol in eggs, but they have harmful sicle effects (Gyles, this volume). Cholesterol in eggs is not affected by age of the hen, cage versus floor management, strain of commercial layer, or geographic location of feed source. Eggs from meat- type hens, turkeys, ducks, and quails con- tain greater concentrations of cholesterol than chicken eggs; however, the former are rarely consumed in the United States (Gyles, this volume). Reducing cholesterol in eggs through ge- netic selection would be desirable, but to date, increases have been obtained only through breeding. Furthermore, some ex- periments indicate that when the level of cholesterol per egg decreases, so does the number of eggs laid (GyTes, this volume). The alternative is to modify the egg yolk after the egg is laid, but only processed eggs (about 13 percent of all eggs now consumed are amenable to such tactics. Approaches have included dilution of whole liquid egg with egg white ant! removal of portions of the yolk lipids and cholesterol with "sol- vents" to reduce the cholesterol content of the final product, and complete removal of the yolk and replacement with a substitute "yolk" made from vegetable oils and other ingredients to produce a cholesterol-free product. Numerous U. S. patents have been obtained toward these ends, nine of which are discussed in detail by Mast ant! Clouser (this volume). Supercritical fluid extraction, which may be able to selectively extract cholesterol without removing the polar lipids that are responsible for functional and sensory prop- erties in egg products, might be an alter- native to solvent extraction. Supercritical fluid extraction utilizes the high-density/ low-viscosity properties of supercritical fluids, which are gases subjected to high pressures at temperatures above their critical point. Supercritical fluids can readily diffuse into and out of foods, thereby increasing extrac 127 tion efficiency. By varying the fluicl's density through changes in pressure, its solubility can be adjusted to preferentially extract certain components of interest. To date, this technology has not been used on eggs or egg products. However, research is under way to extract cholesterol from the egg yolk with supercritical carbon dioxide at various temperatures and pressures (Mast and Clouser, this volume). Poultry, Beef, Veal, Pork, and Lamb The cholesterol content of muscle tissue varies less than the lipic] content and has been found to be fairly constant across and within maturity groups (Stromer et al., 1966), among yield grades (Rhee et al., 1982), and across breed type and nutritional background (Eichhorn et al., 1986~. It is possible to fincl variation in the cholesterol content of meat, however, because adipose tissue tends to have a different concentration of cholesterol than muscle (Allen, this vol- ume). Consequently, differences in the amount of subcutaneous or intermuscular fat consumed with the lean portion can alter cholesterol intake. It has been calculated that 37 to 56 percent of the cholesterol in a cooker! rib steak of beef originates from subcutaneous and intermuscular adipose tis- sue (Rhee et al., 1982~. It is possible that supercritical fluid extraction could be aciaptec3 to remove cholesterol from meat products. Methods To Alter Sodium Levels in Animal Products Salt is an important ingredient in many food-processing techniques. However, diets containing no added salt already provide 1.0 to 1.8 grams of sodium a clay, which clearly exceeds the daily requirement of 0.5 to 1.0 grams. When salt added by consumers in cooking and at the table is considered, per capita ciaily consumption exceeds 3.6
128 grams. This does not include salt consump- tion due to the ingestion of processed foods, which can be substantial. Meat Products Salt (sodium chIoricle) has three major functions in a meat product: preservation, promotion of binding properties in proteins, and flavoring. Salt is important in preserving dry-curec! meats (for example, hams and certain sau- sages); in fact, some research points to an increased danger of toxins arising if salt in cured meats is lowered beyond a certain point. Yet, no minimum effective salt levels have been set. Clearly, it is necessary to achieve a brine concentration sufficient to inhibit growth of molds, yeasts, ant] micro- bial pathogens. Research on salt/citrate/ phosphate interactions and their effects on pathogens is needed! (Rust, this volume). The role of salt in protein-binding prop- erties is twofoIc3. First, it extracts salt- soluble myofibrfllar proteins that then en- capsulate fat particles to create a stable "emulsion" or meat batter. Second, it pro- motes swelling of these proteins, which exposes more bonding sites for water. These properties are neecled to produce stable sausages (Rust, this volume). The flavor preference for sodium chloride is an acquired taste. Consumers in general have reclucec! their sodium intake, and the meat industry has responder! by lowering the sodium content of many of their prod- ucts. Other chlorides can be substituted, but many present flavor problems. For instance, potassium chloride has a bitter flavor and can be substituted successfully for sodium chloride only at or below the 25 percent level. Furthermore, the health ef- fects of addecl dietary potassium are stfll unknown (Rust, this volume). On the other hand, flavoring agents such as spices can be used to enhance flavor in place of sodium chloride. Alkaline phosphates can be combined DESIGNING FOODS with sodium chloride to enhance sodium function in low-soclium products. Generally, though, these phosphates are mostly the sodium salts; hence, actual sodium reduc- tion is minimal. Use of a number of alkaline potassium phosphates is allowed] under USDA-Food Safety and Inspection Service (FSIS) regulations, including dipotassium phosphate, monopotassium phosphate, po- tassium tripolyphosphate, and potassium pyrophosphate. Their use is limited, how- ever, by solubility problems, lower func- tionality than their sodium counterparts, and the potassium flavor problem (Rust, this volume). Poultry Products Processing of poultry can influence the sodium content of the meat. Immersion chflling and hot-deboning both leach sodium from the tissue, the latter to a greater degree (Mast and Clouser, this volume). Further processing of poultry into various manufac- ture(1 products can also increase its sodium content. Sodium can be lowered in processed products by replacing some or all of the sodium chloride with calcium chloride, mag- nesium chloride, or potassium chloride. In poultry franldurters, for example, 35 per- cent of the sodium chloride can be replaced by potassium chloride without adverse ef- fects on flavor. On the other hancl, mag- nesium chloride at this level causes off- flavors (Mast and Clouser, this volume). Enzymatic modification could partially alleviate the neec! for salt in processed poultry products, but spices would have to be addec! to compensate for changes in flavor. Phosphate combined with salt can also serve to produce acceptable low-salt products. Currently, poultry frankfurters average 2.2 percent sodium chloride, or 860 mg of sodium/100 grams of meat. This leve! could be reduced to 1.5 percent sodium (590 mg/100 grams) by adding phosphate or even 0.5 percent sodium (197 mg/100 grams)
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS with appropriate spice formulations (Mast and Clouser, this volume). Milk The salt system in milk appears to be regulated by the synthesis of carbohydrates, casein, and citrate and by leakage of blood constituents into milk. Sodium is present mainly as free ions in the diffusible fraction. Its total measured level in milk is 0.6 ma/ liter; mastitis increases this level. A nutri- tional regimen for the cow that includes sodium bicarbonate lowers the sodium con- tent of milk because it lowers plasma so- dium. Overall, however, genetics, health, ant] nutrition have minimal effects on mflk's sodium content (Linn, this volume). Most of the salt in dairy products is adcled during processing, as in the manufacturing of cheese. Methods To Alter Calcium Levels in Animal Products Milk Calcium is secreted by the Golgi appa- ratus. Average levels of calcium in milk are 30 mmol/liter, but vary slightly with breed of dairy cattle ant! stage of lactation. Levels decline with mastitis. Nutrition of dairy cattle has little effect on calcium content (Linn, this volume). Milk is a particularly goof! source of calcium. Its absorption and utilization by humans is facflitated by the presence of vitamin D, obtained from sunlight or forti- fied into the milk itself (Hettinga, this vol- ume). Milk can be further fortified by the addition of extra calcium. Most milk prod- ucts, especially cheese, are rich sources of bioavaflable calcium. Eggs and Poultry Two large eggs (about 100 grams total) contain about 57 grams of calcium (Table 4- 2~. This is at least twice the calcium content 129 of poultry flesh, although storage causes small increases in the calcium content of poultry meat due to leaching of calcium from the bones into the muscle. Cooking does not significantly affect the calcium content of poultry, but processing options can increase the calcium content of such products as poultry bologna and Dankfurters (Mast and Clouser, this volume). For ex- ample, turkey and chicken frankfurters can contain 88 to 104 mg of calcium. Methocls To Alter Iron Levels in Animal Products Milk Iron is present in milk at low levels, approximately 0.05 mg/100 grams. It is bound to lactoferrins, transferring, casein, fat globules, and xanthine oxidase (Linn, this volume). Its concentration is not af- fected by the cow's (lies (Hettinga, this volume). Unfortified cow's milk is a poor source of iron. Only 10 to 12 percent of the iron present in cows' milk can be absorber] by human infants, in contrast to the 50 percent adsorbability of the iron in human milk. But if cow's milk is fortified with iron sulfate or iron gluconate, infants can absorb up to four times the iron they normally get from human milk. Iron-fortified milk offers the oppor- tunity to enrich the diets of infants, children, adolescents, and pregnant women, all of whom are at risk for iron deficiency. Fortification must use chelates! iron to ensure initial transfer to the phosphoserine groups of casein; this ligand exchange re- action protects iron from reactive milk lipids and promotes effective utilization of this element (Hettinga, this volume). Eggs and Poultry Two large eggs (100 grams total) contain about 2.08 mg of iron, while 100 grams of poultry flesh (light meat, roasted contain
130 1.06 mg of iron (Table 4-101. Slightly higher values are present in processed poultry products macle from mechanically cieboned poultry (Mast and Clouser, this volume). Poultry giblets heart, gizzard, and liver are rich sources of iron. Giblets are under- utilized in the United States because of their undesirable texture and functional pro- tein characteristics. These shortcomings may be improved, though, by chemical, enzy- matic, and physical agents (Mast and Clouser, this volume). The technique with the best potential is acylation the addition of chem- ical groups to the functional Ran. on amino acid sicle chains. Beef, Veal, Pork, and Lamb O ~ These animal products contain substantial amounts of heme iron from the hemoglobin and myogiobin present in the tissues. Heme iron is unaffected by other components in the diet, resulting in consistently high ab- sorption rates. The iron content of beef ranges from about 2.0 to 3.8 mg/100 grams; for pork it is 0.8 to 2.0 mg/100 grams; for lamb it is 1.5 to 3.2 mg/100 grams; and for veal it is 0.9 to 1.9 mg/100 grams (see the composition tables in Chapter 4~. The blood from these animals would pro- vicle a concentrated, bioavailable source of heme iron, but it is rarely used in the formulation of human food products in the Uniter! States. Blood is, however, user] in nonfood products such as fertilizers and feed aciclitives. Mast and Clouser (this volume) suggest that blood is not used in foods for humans in the United States because the consumer has an unfavorable image of blood as a food ingredient. RECOMMENDATIONS Pre- ant! postharvest technologies provide insights into options that are currently avaiT- able for reducing the fat content of animal products. Even though some of these are now being applied, others have not yet been DESIGNING FOODS adopted because of high costs, lack of de- man(l, procluct-labeling standar(ls, or, in some cases, the quality stability of such products in the marketplace. These prob- lems must be addressed by both basic and applied research. In acIdition, other pre- and postharvest areas of research have been identified that hold promise for reducing the fat content of animal products. The more that is known about the basic biology offactors controlling the partitioning of nutrients into protein or fat in animals, the higher the probability of changing these processes through genetic or metabolic ma- nipulation. Just as animal biology is ad- vancing, so is our unclerstanding of food science and the postharvest research nee(ls. These research advances are the basis for improved and new foods composed of or containing animal products. The following research recommendations suggest areas that could lead to useful new technologies for addressing the reflection of fat or salt in animal products. Preharvest Technology · Recommendation Develop technolo- gies for determining carcass fat content. Current methods are time-consuming, costly, or not sufficiently accurate. · Recommendation Alter lean to fat ra- tios of meat ant] fat content in milk through breeding, nutrition, and management. These methods have long been used in response to market incentives and can result in changes that range from slow to quite rapid. · Recommendation Alter the fatty acid composition of meat, milk, and eggs through dietary or genetic manipulation. Although this is more difficult to do in ruminants, it can be accomplished at additional cost. In nonruminants, carcass fat readfly reflects the dietary fatty acid pattern. A major lim- itation is that shelf life of animal products is decreased if the fatty acid profile is shifted too far toward the polyunsaturated fatty acids.
TECHNOLOGICAL OPTIONS AND RESEARCH NEEDS · Recommendation Improve methodol- ogies for determining the fat and protein contents of live animals and carcasses. Rapid, accurate, ant] cost-effective methodologies would greatly enhance industry's ability to determine animal or carcass composition and thus be of great economic value. Such technology would also be useful for meas- uring human body composition ant} for mak- ing humans more aware of the relationship of obesity to diet and health. · Recommendation Identify cellular and molecular mechanisms that control parti- tioning of feed nutrients into meat, milk, ant] eggs. It is well known that livestock species display considerable genetic varia- bility in their abilities to convert feedstuffs into muscle, fat, milk, and eggs. To fully utilize the tools of biotechnology, much more information is needed about the exact genes ant] cellular or molecular mechanisms that contribute to this genetic variation. With this information, the probability of being able to optimize favorable responses through bioregulation or genetic engineer- ing will be greatly enhanced. · Recommendation Determine the ex- tent of genetic variation in the cholesterol content of meat, milk, and eggs. Without this information, it is not possible to know whether genetic selection or engineering coulc! be used to develop Tower cholesterol animal products. In abolition, more research is needed on the metabolism of cholesterol in the tissues and on the quantity of cho- lesterol that is essential to the function of the cell or organelle. This research need exists for both animals and humans. · Recommendation Determine whether oxidative rancidity of animal products can be reducecl through special feeding or man- agement of animals. Some research indi- cates that beetling vitamin E to nonrumi- nants decreases the rate of oxidative rancidity in their meat products. More research is needed to determine whether other natural or approved synthetic antioxidants are ben- eficial in extending product shelf life. 131 · Recommendation Develop more cost- effective methods for producing low-fat an- imal products by integrated production management systems. Systems analysis is an elective method for examining the mul- titude of biological, physical, and economic factors that influence the cost-effectiveness of programs and processes for reducing or altering fat in animal products. · Recommendation Expand research in the area of reproductive physiology that would permit rapid selection and propaga- tion of genetically or metabolically superior animals. Examples include sexing semen and embryos, splitting embryos, and gene insertion and gene expression. Post-Harvest Technology Postharvest technologies to reduce fat in animal products can be used quite satisfac- torfly in many situations. However, these technologies are not without costs and are usually associated with some change in prod- uct characteristics such as texture, flavor, or shelf life. In abolition (1epencling on the product and the changes a variety of reg- ulatory and labeling issues must also be addressed. · Recommendation Use physical meth- ods to reduce fat at the earliest possible stage in processing. Some such methods are being used extensively, including trimming meat, centrifuging milk, ant! separating egg yolks and whites. Low-fat milk and meat products are examples of the results that can be achieved. Furthermore, use of such procedures would create by-proclucts of lower economic value that could! be used effec- tively in food or, preferably, nonfood pro(l- ucts. The recommendations ma(le in Chap- ter 5 to allow hot-fat trimming on the slaughter floor and to change the USDA grade standards to allow for uncoupling of yield and quality grades are in concert with this recommendation.
132 · Recommendation . ~. Simulate the tex- tura~ and sensory properties of fat by using nonfat or low-fat ingredients. Certain poly- saccharides and proteins might be useful for this purpose and could produce satisfactory results in a number of products if labeling standards were more flexible. · Recommendation Adopt standards of identity that reflect toclay's technology and consumer needs. In some instances stand- ards are too restrictive, and even though a technology exists that could be used to improve a product, it cannot be applied because of current regulations. · Recommendation Reduce oxidative rancidity to extend product shelf life. The occurrence of oxidative rancidity is one of the most serious limitations to adequate shelf life and optimal palatability of many animal products. Use of certain packaging tech nolo gie s an d approve c! an tioxician ts an cI control of certain processing variables help minimize this problem in some, but not all, products. For example, skim milk and fresh pork sausage have shortened shelf lives because of the incidence of oxidative ran- cidity. · Recommendation Use fat substitution to alter the fatty acid! composition of proc- essed animal products (that is, to increase the proportion of unsaturates! fatty acids). However, the potential for increasing the susceptibility to oxidative rancidity when the fatty acic] profile is shifted too far toward unsaturated fatty acids must be considered and controlled. · Recommendation Improve metho(lol- ogies for the analysis of fat ant] sensory and other quality characteristics of animal crod- ucts. Rapid, accurate, and cost-effective analyses are important to the production and monitoring of a variety of foot] charac- teristics. · Recommendation Utilize molecular genetics and other biotechnologies to im- prove fermentation processes that are im- portant in the manufacture of animal prod- ucts such as cheese, yogurt, and sausage. DESIGNING FOODS For example, the newest technologies could be used to generate new microorganisms that could reduce the cholesterol content of the end product. · Recommendation Determine how se- lective extraction of saturated fats and cho- lestero! can be used to reduce these com- ponents in animal products. The use of supercritical carbon dioxide as an extractant shows promise for this purpose. · Recommendation Search for ways to safely and organoleptically reduce or replace sodium in manufactured animal products. Sodium chloride plays a critical role in delaying microbial growth, providing flavor, and contributing to the functional charac- teristics of many processed products. There- fore, it should not be reduced or replaced without serious consideration of the conse- quences or until a satisfactory replacement for sodium chloride is found for use in products such as cheese and sausage. REFERENCES Becker, R. H., and L. M. Speltz. 1986. Working the S-curve: Making more explicit forecasts. Res. Man- age. 29:21. Bonner, J. M. 1974. Effects of 1,YButanediol in Cows with Milk Fat Depression. Ph.D. dissertation. Iowa State University, Ames. Eichhorn, J. M., L. J. Coleman, and E. J. Wakayama. 1986. Effects of breed type and restricted versus ad libitum feeding on fatty acid composition and cho- lesterol content of muscle and adipose tissue from mature bovine females. J. Anim. Sci. 63:781. Hammer, R. E., R. L. Brinster, and R. D. Palmiter. 1985. Use of gene transfer to increase animal growth. Cold Spring Harbor Symp. Quant. Biol. 50:379. Rhee, K. S., T. R. Dutson, and G. C. Smith. 1982. Effect of changes in intermuscular and subcutaneous fat levels on cholesterol content of raw and cooked beef steaks. J. Food Sci. 47:1638. Savell, J. W., H. R. Cross, and G. C. Smith. 1986. Percentage ether extractable fat and moisture con- tent of beef longissimus muscle as related to USDA marbling score. J. Food Sci. 51:838. Stromer, M. H., D. E. Goll, and J. H. Roberts. 1966. Cholesterol in subcutaneous and intramuscular lipid depots from bovine carcasses of different maturity and fatness. J. Anim. Sci. 25:1145.